01-SS7 Signaling System

Technical Manual – Signaling System HUAWEI M800 CDMA Mobile Switching Center

SS7 Signaling SystemTable of Contents

Huawei Technologies Proprietary

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Table of Contents

Chapter 1 Introduction to CDMA SS7.......................................................................................... 1-1 1.1 Concepts of SS7................................................................................................................ 1-1

1.1.1 Common Channel Signaling System ...................................................................... 1-1 1.1.2 SS7 Signaling Network ........................................................................................... 1-2 1.1.3 Signaling Transfer Mode ......................................................................................... 1-5

1.2 Architecture and Functions of SS7 .................................................................................... 1-5 1.2.1 Layered Architecture ............................................................................................... 1-6 1.2.2 Introduction to Functional Layers ............................................................................ 1-6

Chapter 2 Message Transfer Part ................................................................................................ 2-1 2.1 Introduction to MTP ........................................................................................................... 2-1 2.2 MTP Functions................................................................................................................... 2-1

2.2.1 Signaling Data Link Functions................................................................................. 2-2 2.2.2 Signaling Link Functions ......................................................................................... 2-2 2.2.3 Signaling Network Functions................................................................................... 2-4

2.3 MTP Messages.................................................................................................................. 2-7 2.3.1 Format of Signal Units............................................................................................. 2-7 2.3.2 Functions and Codes of Signal Unit Fields ............................................................. 2-8

Chapter 3 Telephone User Part.................................................................................................... 3-1 3.1 Introduction to TUP............................................................................................................ 3-1 3.2 TUP Functions ................................................................................................................... 3-1 3.3 TUP Messages .................................................................................................................. 3-1

3.3.1 Format of TUP Messages ....................................................................................... 3-1 3.3.2 Encoding of TUP Messages.................................................................................... 3-2 3.3.3 Example of TUP Messages..................................................................................... 3-4

Chapter 4 Signaling Connection Control Part............................................................................ 4-1 4.1 Introduction to SCCP ......................................................................................................... 4-1

4.1.1 TUP Signaling Transfer Based on MTP.................................................................. 4-1 4.1.2 SCCP Signaling Transfer Based on MTP ............................................................... 4-2

4.2 Services Provided by SCCP.............................................................................................. 4-3 4.2.1 SCCP Service Classes ........................................................................................... 4-4 4.2.2 Connectionless Services......................................................................................... 4-4 4.2.3 Connection-Oriented Services ................................................................................ 4-5 4.2.4 SCCP Service Procedure........................................................................................ 4-6

4.3 SCCP Addressing and Routing ......................................................................................... 4-8 4.4 SCCP Primitives ................................................................................................................ 4-9

4.4.1 Definition ................................................................................................................. 4-9 4.4.2 Structure................................................................................................................ 4-11




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4.4.3 SCCP User Primitives ........................................................................................... 4-11 4.4.4 MTP Service Primitives ......................................................................................... 4-13

4.5 SCCP Messages.............................................................................................................. 4-14 4.5.1 Format of SCCP Messages .................................................................................. 4-14 4.5.2 Encoding of SCCP Messages............................................................................... 4-17 4.5.3 Example of SCCP Messages................................................................................ 4-25

Chapter 5 ISDN User Part ............................................................................................................. 5-1 5.1 Introduction to ISUP........................................................................................................... 5-1 5.2 ISUP Functions .................................................................................................................. 5-2

5.2.1 Bearer Services....................................................................................................... 5-2 5.2.2 User Terminal Services........................................................................................... 5-2 5.2.3 Supplementary Services ......................................................................................... 5-2

5.3 ISUP Messages ................................................................................................................. 5-3 5.3.1 Format of ISUP Messages ...................................................................................... 5-3 5.3.2 Encoding of ISUP Messages .................................................................................. 5-4 5.3.3 Example of ISUP Messages ................................................................................... 5-8

Chapter 6 Transaction Capabilities Application Part ................................................................ 6-1 6.1 Introduction to TCAP ......................................................................................................... 6-1 6.2 TCAP Structure.................................................................................................................. 6-2

6.2.1 Transaction Sublayer .............................................................................................. 6-3 6.2.2 Component Sublayer .............................................................................................. 6-3

6.3 TCAP Messages................................................................................................................ 6-4 6.3.1 Encoding of TCAP Messages ................................................................................. 6-4 6.3.2 Format of TCAP Messages..................................................................................... 6-7 6.3.3 Transaction Portion ................................................................................................. 6-7 6.3.4 Dialog Portion........................................................................................................ 6-11 6.3.5 Component Portion ............................................................................................... 6-11 6.3.6 Example of TCAP Messages ................................................................................ 6-13

Chapter 7 Mobile Application Part............................................................................................... 7-1 7.1 Introduction to MAP ........................................................................................................... 7-1 7.2 MAP Functions................................................................................................................... 7-2

7.2.1 MAP Management Functions.................................................................................. 7-2 7.2.2 MAP Operations...................................................................................................... 7-4

7.3 MAP Messages.................................................................................................................. 7-7 7.3.1 Format of MAP Messages....................................................................................... 7-7 7.3.2 Encoding of MAP Messages ................................................................................... 7-7 7.3.3 Example of MAP Messages .................................................................................... 7-7

7.4 Common MAP Procedures .............................................................................................. 7-10 7.4.1 Location Registration ............................................................................................ 7-10 7.4.2 Inter-Office Call ..................................................................................................... 7-11 7.4.3 Handoff Forward.................................................................................................... 7-11




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Chapter 8 Base Station Application Part .................................................................................... 8-1 8.1 Introduction to BSAP ......................................................................................................... 8-1

8.1.1 About the A Interface .............................................................................................. 8-1 8.1.2 BSAP Functions ...................................................................................................... 8-1

8.2 BSAP Messages................................................................................................................ 8-2 8.2.1 Format of BSAP Messages..................................................................................... 8-2 8.2.2 Encoding of BSAP Messages ................................................................................. 8-3 8.2.3 Example of BSAP Messages .................................................................................. 8-6

8.3 BSAP Procedures............................................................................................................ 8-15 8.3.1 Location Update .................................................................................................... 8-15 8.3.2 Mobile Origination ................................................................................................. 8-16 8.3.3 Mobile Termination................................................................................................ 8-17 8.3.4 Call Clearing.......................................................................................................... 8-18 8.3.5 Circuit Block/Unblock ............................................................................................ 8-19 8.3.6 Circuit Reset.......................................................................................................... 8-21


SS7 Signaling SystemList of Figures


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List of Figures

Figure 1-1 Architecture of a CCS system............................................................................... 1-1

Figure 1-2 A 3-layer signaling network................................................................................... 1-3

Figure 1-3 Associated transfer of signaling messages .......................................................... 1-5

Figure 1-4 Quasi-associated transfer of signaling messages................................................ 1-5

Figure 1-5 Functional blocks of SS7...................................................................................... 1-5

Figure 1-6 Layered architecture of SS7................................................................................. 1-6

Figure 2-1 Position of the MTP in SS7................................................................................... 2-1

Figure 2-2 Function levels of the MTP................................................................................... 2-2

Figure 2-3 Signaling message handling................................................................................. 2-5

Figure 2-4 Message routing ................................................................................................... 2-6

Figure 2-5 Structure of FISU.................................................................................................. 2-7

Figure 2-6 Structure of LSSU................................................................................................. 2-8

Figure 2-7 Structure of MSU .................................................................................................. 2-8

Figure 2-8 SIO structure ........................................................................................................ 2-9

Figure 2-9 Structure of SIF................................................................................................... 2-10

Figure 3-1 Position of TUP in SS7 ......................................................................................... 3-1

Figure 3-2 TUP message structure........................................................................................ 3-2

Figure 3-3 TUP label structure ............................................................................................... 3-2

Figure 3-4 IAI format .............................................................................................................. 3-4

Figure 3-5 Calling line identity field ........................................................................................ 3-8

Figure 3-6 IAM format ............................................................................................................ 3-9

Figure 4-1 The SCCP in the SS7In the signaling network..................................................... 4-3

Figure 4-2 Connectionless transfer of signaling messages................................................... 4-4

Figure 4-3 Connection-oriented transfer with a middle node................................................. 4-5

Figure 4-4 Connectionless service in GT addressing ............................................................ 4-6

Figure 4-5 Connection-oriented service signaling flow .......................................................... 4-7

Figure 4-6 Primitives and messages of MTP and SCCP..................................................... 4-10

Figure 4-7 Structure of a primitive........................................................................................ 4-11

Figure 4-8 Structure of SCCP messages............................................................................. 4-15

Figure 4-9 SCCP message .................................................................................................. 4-25


SS7 Signaling SystemList of Figures


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Figure 5-1 ISUP position in SS7 ............................................................................................ 5-1

Figure 5-2 ISUP message structure....................................................................................... 5-4

Figure 5-3 Format of routing label in the ISUP message....................................................... 5-5

Figure 5-4 Format of the CIC in a ISUP message ................................................................. 5-5

Figure 5-5 IAM message...................................................................................................... 5-12

Figure 6-1 Position of TCAP in the SS7 network ................................................................... 6-2

Figure 6-2 Structure of TCAP................................................................................................. 6-2

Figure 6-3 Structure of TCAP IE ............................................................................................ 6-4

Figure 6-4 The format of a tag containing one octet .............................................................. 6-5

Figure 6-5 The format of a tag containing more than one octets........................................... 6-5

Figure 6-6 Length of contents -- the short form .....................................................................6-6

Figure 6-7 Length of contents – the long form.......................................................................6-6

Figure 6-8 TCAP message structure ..................................................................................... 6-7

Figure 6-9 RUIDIR message................................................................................................ 6-13

Figure 7-1 CDMA network architecture.................................................................................. 7-1

Figure 7-2 Structural relation between MAP and MTP messages ......................................... 7-7

Figure 7-3 Registration Notification message traced on a SS7 link ...................................... 7-7

Figure 7-4 Location registration procedure.......................................................................... 7-10

Figure 7-5 Inter-office call procedure ................................................................................... 7-11

Figure 7-6 Handoff forward procedure................................................................................. 7-12

Figure 8-1 Reference model of A interface protocol stack ..................................................... 8-2

Figure 8-2 BSAP message structure ..................................................................................... 8-3

Figure 8-3 Example of CM service request ........................................................................... 8-8

Figure 8-4 Location update procedure................................................................................. 8-16

Figure 8-5 Mobile origination procedure .............................................................................. 8-16

Figure 8-6 Mobile termination procedure............................................................................. 8-17

Figure 8-7 Call clearing initiated by BSC ............................................................................. 8-19

Figure 8-8 Circuit block procedure....................................................................................... 8-20

Figure 8-9 Circuit unblock procedure................................................................................... 8-20

Figure 8-10 BSC-initiated circuit reset ................................................................................. 8-21

Figure 8-11 MSC-initiated circuit reset................................................................................. 8-22

Figure 8-12 MSC-initiated circuit reset failure...................................................................... 8-22


SS7 Signaling SystemList of Tables


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List of Tables

Table 2-1 SI codes allocation ................................................................................................. 2-9

Table 2-2 SSF codes allocation............................................................................................ 2-10

Table 3-1 Coding format of H0 ............................................................................................... 3-3

Table 3-2 Calling party category............................................................................................. 3-4

Table 3-3 Message indicators ................................................................................................ 3-6

Table 3-4 Encoding of the address signals ............................................................................ 3-7

Table 3-5 Encoding of the first indicator octet ........................................................................ 3-7

Table 3-6 Address indicators .................................................................................................. 3-8

Table 3-7 Calling line identity codes....................................................................................... 3-9

Table 4-1 SCCP user primitives ........................................................................................... 4-11

Table 4-2 MTP service primitive ........................................................................................... 4-13

Table 4-3 SCCP message type and code ............................................................................ 4-16

Table 4-4 SCCP message parameters................................................................................. 4-17

Table 4-5 Structure of address indicator............................................................................... 4-18

Table 4-6 Correspondence between GT type codes and GT types ..................................... 4-19

Table 4-7 Allocation of SCCP SSNs..................................................................................... 4-19

Table 4-8 Type 1 GT............................................................................................................. 4-20

Table 4-9 Type 2 GT............................................................................................................. 4-20

Table 4-10 Type 3 GT........................................................................................................... 4-20

Table 4-11 Type 4 GT ........................................................................................................... 4-21

Table 4-12 Protocol classes ................................................................................................. 4-22

Table 4-13 Handling of messages in case of transfer failure ............................................... 4-22

Table 4-14 Coding of release causes................................................................................... 4-23

Table 4-15 Coding of return causes ..................................................................................... 4-24

Table 5-1 Encoding of ISUP messages.................................................................................. 5-5

Table 5-2 Parameters of IAM ................................................................................................. 5-8

Table 5-3 Code of the nature of connection indicators........................................................... 5-9

Table 5-4 Codes of forward call indicator ............................................................................. 5-10

Table 5-5 Codes of the calling party category ...................................................................... 5-11

Table 6-1 Structure of TCAP message tag............................................................................. 6-4


SS7 Signaling SystemList of Tables


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Table 6-2 TCAP package type identifier ................................................................................. 6-8

Table 6-3 Correspondence between TCAP package type and cell........................................ 6-9

Table 6-4 P-Abort causes..................................................................................................... 6-10

Table 6-5 IEs contained in the dialog portion ....................................................................... 6-11

Table 6-6 Correspondence between component types and component type identifiers ..... 6-11

Table 6-7 Correspondence between component types and IEs .......................................... 6-12

Table 7-1 MAP operations ...................................................................................................... 7-5

Table 8-1 Type 1 IE structure ................................................................................................. 8-4

Table 8-2 Type 2 IE structure ................................................................................................. 8-4

Table 8-3 Type 3 IE structure of type 3 (example 1) .............................................................. 8-5

Table 8-4 Type 3 IE structure (example 2) ............................................................................. 8-5

Table 8-5 Type 4 IE structure (example 1) ............................................................................. 8-6

Table 8-6 Type 4 IE structure o (example 2) .......................................................................... 8-6

Table 8-7 Complete layer 3 message..................................................................................... 8-7

Table 8-8 CM service request message................................................................................. 8-7

Table 8-9 Paging request message...................................................................................... 8-12

Table 8-10 Paging request message.................................................................................... 8-13

Table 8-11 Connect message............................................................................................... 8-14

Table 8-12 Assignment request message ............................................................................ 8-14

Table 8-13 Assignment complete messages........................................................................ 8-14

Table 8-14 SCCP messages used by BSAP........................................................................ 8-15


SS7 Signaling SystemChapter 1 Introduction to CDMA SS7


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Chapter 1 Introduction to CDMA SS7

This chapter describes the concepts, architecture, and functions of signaling system No.7 (SS7) for the CDMA system.

1.1 Concepts of SS7

This section introduces some important concepts related to SS7.

Before introducing the major concepts, this section explains the following basic terms:

Information: the content carried in a message Message: carrier of information Signal unit: entity made up of a message and some signaling information fields

necessary for applications and transferred on signaling links Signaling: information set used to interconnect different entities in

communication networks

1.1.1 Common Channel Signaling System

The signaling system helps network entities to cooperate with each other to implement particular tasks.

There are two types of signaling systems:

Common channel signaling (CCS) system Channel associated signaling (CAS) system

SS7 is a common channel signaling system.

I. Definition of CCS System

In a CCS system, signaling channels and traffic channels are separate. Signaling is transferred on common data links (signaling channels in this case) in the form of messages.

Figure 1-1 shows the architecture of a CCS system.

Switchingnetwork

Switchingnetwork

Signalingequipment

Signalingequipment

Publiccontrol

Publiccontrol

Switch A Switch BTraffic channel

Data link

Figure 1-1 Architecture of a CCS system




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II. Transfer of SS7 Signaling Messages

SS7 messages are data packets exchanged between processors of various nodes (such as exchanges) in a telecommunication network. They are transmitted in packet switching mode on signaling links.

Therefore, the SS7 network is essentially a data communication network (a special packet switching network) independent of the service switching system.

One timeslot (excluding TS 0) of each digital trunk line in a 2-Mbit/s primary group is used as the signaling channel, also called the signaling link. The transmission rate of a signaling link is 64 kbit/s.

Most of the timeslots are used as traffic channels. For example, a speech channel transmitting voice information is also a traffic channel.

SS7 messages can also be transferred on analog transmission lines. In this case, the SS7 messages are sent by using a modulator-demodulator (Modem). Typical transmission rates are 2.4 kbit/s and 4.8 kbit/s.

III. Advantages of CCS System

Compared with the CAS system, the CCS system has the following advantages:

High channel utilization High signaling transmission speed Large signaling capacity Wide applications in the integrated services digital network (ISDN) , mobile

communications network and intelligent network Easy maintenance and management due to the separation of signaling network

and communication network Adaptive to new signaling protocols for new service provisioning

To realize the above features, the CCS system must:

Maintain high reliability of signaling links. Adopt advanced signaling network functions and security measures. Possess the function of speech channel continuity check (to ensure high

performance of speech channels).

1.1.2 SS7 Signaling Network

A signaling network is dedicated to the transmission of signaling messages. It is logically independent of the communication network.

Because channel associated signaling messages are transmitted together with traffic data, the concept of signaling network applies only to the common channel signaling system.

A signaling network consists of

Signaling points (SPs)




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Signaling transfer points (STPs) Signaling links

The SS7 signaling network is a support network. It is physically integrated with the communication network.

Figure 1-2 is an example of a 3-layer signaling network.

HSTP

LSTP

SP SP: Signaling point LSTP: High-level signaling transfer point LSTP: Low level signaling transfer point

Figure 1-2 A 3-layer signaling network

I. Signaling Point

An SP is where the signaling messages are processed or exchanged. It functions as either the origination or the destination in signaling message transmission.

Generally, an SP can be an exchange, an operation and maintenance center, or a network database.

Usually, an SP corresponds to one physical node. It is represented by the symbol "O" in a network topology diagram.

In some cases, however, two logically separate signaling points are configured for one physical node. This often happens on gateway offices, for example, an international incoming and outgoing office is an SP of the national signaling network and an SP of the international signaling network.

An SP is identified by a signaling point code (SPC).

An originating SP (OSP) is identified with an originating point code (OPC); A destination SP (DSP) is identified with a destination point code (DPC).

There are two types of SPCs:

14-bit SPC 24-bit SPC

II. Signaling Transfer Point

An STP transfers the signaling messages it receives from one signaling link to another. It is presented by a " " in a network topology diagram.

An STP is identified by the SPC.




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There are two types of STPs:

Independent STP Integrated STP

An independent STP only functions to transfer signaling. It is not regarded as an SP.

An integrated STP not only to transfers signaling but also serves as an SP where signaling is originated or terminated.

As shown in Figure 1-2, in a 3-level signaling network, STPs are classified into two types:

Low-level signaling transfer point (LSTP) High-level signaling transfer point (HSTP)

III. Signaling Link

Signaling links are the physical channels that connect SPs and STPs and transmit signaling messages.

IV. Signaling Link Set

A collection of signaling links with the same attributes is called a link set.

The links to the same office may belong to one link or several link sets. The links between two adjacent SPs, however, must be configured in one link set.

V. Signaling Link Code

Each signaling link is uniquely identified within an office with a signaling link code (SLC) .

All signaling links between two adjacent SPs are uniquely numbered in the same way. The SLCs of these links must be consistent at the two SPs.

For signaling links to different offices, the SLCs may be identical.

VI. Signaling Route

A signaling route is a path along which signaling messages are transmitted from an OSP to a DSP.

The selection of signaling route depends on the signaling relations and the transfer mode.

VII. Signaling Route Set

All signaling routes that correspond to a signaling relation form a route set.

A given signaling message is transmitted along a specific route in normal cases. When this route becomes faulty, an alternative route is taken.




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1.1.3 Signaling Transfer Mode

There are two signaling transfer modes:

Associated transfer Quasi-associated transfer

I. Associated Transfer

In associated transfer, the signaling messages between two SPs are transmitted on direct signaling links. In this case, the traffic channels and signaling links are parallel.

Figure 1-3 shows the associated transfer of signaling messages.

Traffic channel

Signaling link

SP SP

Figure 1-3 Associated transfer of signaling messages

II. Quasi-Associated Transfer

In quasi-associated transfer, signaling messages between two SPs are transmitted on indirect signaling links. These signaling links are designated by data configuration for signaling transmission.

Figure 1-4 shows the quasi-associated transfer of signaling messages.

STP

SP SP

Signaling link

Traffic channel

Designated Signaling links

Figure 1-4 Quasi-associated transfer of signaling messages

1.2 Architecture and Functions of SS7

SS7 consists of several functional blocks, including a message transfer part (MTP) and a number of user parts (UPs), as shown in Figure 1-5.

User Part MTP User Part

Figure 1-5 Functional blocks of SS7

MTP is responsible for signaling message transfer.




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UPs are responsible for the generating, grammar checking, semantic analysis and processing control of signaling messages. All UPs function with the support of MTP.

1.2.1 Layered Architecture

Figure 1-6 shows the architecture of SS7.

INAP OMAP MAP ISUP TUP

ISP

SCCP

MTP-3

MTP-2

MTP-1

L 2

BSAPMAP ISUP TUP

TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLRVLR

BSAP

L 1

L 7

L 4 - L 6

L 3

Figure 1-6 Layered architecture of SS7

There is a correspondence between the SS7 and the open system interconnection (OSI) network model (seven layers).

In the layered architecture, an upper layer is the user of its lower layer, and is served by the lower layer.

1.2.2 Introduction to Functional Layers

The functional layers of SS7 follow the hierarchy of data transfer strictly. Signaling messages in SS7 are transferred transparently in a layer that is not responsible for the processing of the messages. Signaling messages are transferred between the corresponding functional levels at both sides.

This section gives a brief introduction to the functions of each functional layer of the SS7.

I. Message Transfer Part

MTP serves as a transport system providing reliable transfer of the signaling messages.

It is divided into the three levels:

MTP-1: Signaling data link function MTP-2: Signaling link function MTP-3: Signaling network function




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II. Signaling Connection Control Part

Signaling connection control part (SCCP) is based on MTP and provides additional functions to MTP.

It provides connectionless and connection-oriented network services.

The connectionless network service means that UP transfers signaling messages without establishing signal connection in advance. With connectionless network service, the data of one UP can be transferred to another UP on the signaling network. For example, authentication of subscribers in mobile networks, and account inquiry in intelligent networks are all transferred by this means.

The connection-oriented network service means that a message transport channel between the two nodes (UPs) is established after exchange of requests and responses between the UPs prior to data transfer.

III. Telephone User Part

Telephone user part (TUP) handles call-related signaling messages, such as those related to the setup, monitoring and release of calls.

TUP messages are classified into several message groups, such as forward and backward call setup messages, call monitoring messages, circuit and circuit group monitoring messages and network management messages.

TUP messages are transferred in signal units on signaling links.

IV. ISDN User Part

ISDN user part (ISUP) provides signaling functions to support ISDN basic services and supplementary services.

ISUP has all functions of the TUP. Therefore, it can also function as the TUP.

V. Transaction Capabilities Application Part

Transaction capabilities application part (TCAP) provides interfaces for various communication network services, such as mobile services and intelligent services.

TCAP provides the dialog capabilities to support information request and response for the applications of network services.

TCAP is a public protocol and does not involve specific applications. The specific applications implement the message transfer on the interfaces provided by the TCAP. For example, the MAP implements the location of roaming subscribers with the support of the TCAP. The intelligent application part (INAP) implements the service control point (SCP) database registration and data query with the support of the TCAP.




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VI. Intermediate Service Part

Intermediate service part (ISP) corresponds to layer 4 to layer 6 of the OSI model. It is not defined yet.

Together with the TCAP, it is referred to as the transaction capabilities (TC).

VII. Mobile Application Part

Mobile application part (MAP) is a functional unit used for interconnection within the public land mobile network (PLMN) and between the PLMN and other networks.

VIII. Base Station Application Part

Base station application part (BSAP) BSAP is an application part based on A interface protocols. It fulfills the functions of A1 interface between MSC and BSC.


SS7 Signaling SystemChapter 2 Message Transfer Part


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Chapter 2 Message Transfer Part

This chapter introduces the concepts related to message transfer part (MTP), as well as its functions and position in the SS7.

2.1 Introduction to MTP

MTP constitutes the bottom layers (LI, L2 and L3) of SS7, providing physical links for signaling transmission to ensure reliable message transfer. MTP also provides signaling route management and signaling network management functions.

Figure 2-1 shows the position of the MTP in SS7.


TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

L 2

BSAPINAP OMAP MAP ISUP TUP

TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAP

L 1

L 7

L 4 - L 6

L 3

Figure 2-1 Position of the MTP in SS7

2.2 MTP Functions

MTP functions are classified to three levels:

Signaling data link functions Signaling link functions Signaling network functions

These three levels correspond to the MTP-1, MTP-2, and MTP-3 shown in Figure 2-2.




2-2

Signaling Network Fuctions

Signaling Link functions

Signaling Data Link

User Part

MTP-1

MTP-2

MTP-3

Figure 2-2 Function levels of the MTP

2.2.1 Signaling Data Link Functions

Signaling data link functions are performed by the MTP at Level 1, defining the physical and electrical features and the connection mode of signaling data links.

A signaling data link is a bi-directional transmission path for signaling. It assumes a timeslot of the pulse code modulation (PCM) system with a bit rate of 64 kbit/s. An analogue signaling data link with modems may also be adopted with a bit rate typically at 2.4 kbit/s or 4.8 kbit/s.

As signaling transmission is bi-directional, a signaling point (SP) receives signaling from and delivers signaling to another SP. Therefore, full duplex operation over a 4-wire transmission link is adopted.

The operational signaling data link shall be exclusively dedicated to the use of an SS7 signaling link.

A signaling link is transparent, that is, bit integrity of the transmitted data stream must be ensured. Equipment such as echo suppressors, digital pads, or A/u law converters attached to the transmission link must be disabled.

2.2.2 Signaling Link Functions

The signaling link functions are performed by the MTP at Level 2.

Together with a signaling data link as a bearer, the signaling link functions provide a signaling link for reliable transfer of signaling messages between two directly connected SPs.

Errors may occur to the data link between directly connected SPs after long distance transmission, which are not allowed in SS7. In the case of data link errors, the signaling link functions can ensure the reliable transmission of signaling messages.

The signaling link functions comprise:

Signal unit delimitation Signal unit alignment Error detection




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Initial alignment Signaling link error monitoring Flow control Processor error control

I. Signal Unit Delimitation

The beginning and end of a signal unit are indicated by a unique 8-bit pattern "01111110", called flag.

The end flag of one signal unit is usually the start flag of the following signal unit.

Measures are taken to ensure that the pattern cannot be imitated elsewhere in the unit.

The transmitting signaling link terminal inserts a 0 after every sequence of five consecutive 1s before attaching the flags. At the receiving signaling link terminal, the 0s that directly follow a sequence of five consecutive 1s will be deleted.

II. Signal Unit Alignment

The alignment here does not refer to initial alignment. It refers to the alignment of signaling link in the delimitation procedure.

Normally, the length of a signal unit is a multiple of eight bits.

Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (more than six consecutive 1s) is received, or when a certain maximum length of signal unit is exceeded. In such cases, the received signal unit is discarded and the signal unit error rate monitor or alignment error rate monitor is incremented.

III. Error Detection

The error detection function is performed by means of 16 check bits provided at the end of each signal unit.

Two forms of error correction are provided, the basic method and the preventive cyclic retransmission method.

The basic method applies for signaling links where the one-way propagation delay is less than 15 ms.

The preventive cyclic retransmission method applies for signaling links where the one-way propagation delay is greater than or equal to 15 ms.

IV. Initial Alignment

The initial alignment procedure is appropriate to both first time initialization and alignment in association with restoration after a link failure.

There are five phases of alignment procedure. They are:

Idle Not aligned




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Aligned Proving period Aligned ready

The initial alignment involves the following four link statuses:

Status indication out of service (SIOS) Status indication out of alignment (SIO) Status indication normal (SIN) Status indication emergency (SIE)

V. Signaling Link Error Monitoring

Two signaling link error rate monitors are provided:

Signal unit error rate monitor Alignment error rate monitor

The signal unit error rate monitor is employed while a signaling link is in service.

The alignment error rate monitor is employed while a link is in the proving state of the initial alignment procedure (first time initialization and alignment in association with restoration after a link failure).

VI. Flow Control

Flow control is initiated when congestion is detected at the receiving end of the signaling link.

The congested receiving end of the link notifies the remote transmitting end of the condition by means of an appropriate link status signal unit.

The remote transmitting end indicates the link as failed if the congestion continues too long.

VII. Process Error Control

This function marks the faulty state of a processor.

2.2.3 Signaling Network Functions

The signaling network functions are performed by the MTP at Level 3 to ensure reliable transmission of signaling messages through control of signaling network route and performance.

The signaling network functions can be divided into two basic categories:

Signaling message handling Signaling network management




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I. Signaling Message Handling

Signaling message handling is to ensure that signaling messages originated by a particular UP at an SP (originating SP) are delivered to the same UP at the destination SP indicated by the sending UP.

As illustrated in Figure 2-3, signaling message handling functions are classified into:

Message discrimination Message distribution Message routing

Messagedistribution

Messagediscrimination

Messagerouting

To/From L2To/From L4

Figure 2-3 Signaling message handling

Message Discrimination

The message discrimination function is used at an SP to determine whether or not a received message is destined to the point itself.

A message destined to the SP itself is transferred to the message distribution function. Otherwise, the message is transferred to the message routing function.

Message Distribution

The message distribution function is used at each SP to deliver the received messages (destined to the SP itself) to the appropriate UP.

Message Routing

The message routing function is used at each SP to determine the outgoing signaling link on which a message (originated or transferred from the message discrimination) is sent towards its destination SP.

Figure 2-4 illustrates the message routing function.




2-6

Messagerouting

Signalingnetworkfunction

Signaling linkfunction

SP 1

SP 2

SP N

﹕

﹕

﹕

Link 1

Link 2

Link N

Figure 2-4 Message routing

II. Signaling Network Management

Signaling network management is to provide reconfiguration of the signaling network in case of failures and to control traffic in case of congestion.

The signaling network management functions are divided into three categories:

Signaling traffic management Signaling link management Signaling route management

Signaling Traffic Management

When message security and correct transmission can be assured, the signaling traffic management modifies signaling routing to transfer messages from unavailable signaling links to available links.

In the case of congestion at SPs, the signaling traffic management may need to slow down signaling traffic on certain routes.

Signaling Link Management

In the case of a signaling link failure, the signaling link management function tests the link and restores it.

Signaling Route Management

When an SP or link fails to transmit messages as a result of fault, the signaling route management function notifies the SP of the condition and allocates another route for the messages to ensure reliable transmission.




2-7

2.3 MTP Messages

Signaling messages are transferred in the SS7 in different lengths.

A signaling message is a set of information defined by the UP of the MTP. Some signaling network management and test maintenance messages can be defined by Level 3 functions of the MTP.

For the sake of reliable transmission, some signaling information fields are attached to each message to constitute the single unit (SU) actually transmitted in the signaling link.

The length of a signal unit is a multiple of eight bits. The length of signal units is described in octets. An octet consists of eight bits.

2.3.1 Format of Signal Units

In SS7, there are three types of signal units:

Fill-in signal unit (FISU) Link status signal unit (LSSU) Message signal unit (MSU)

I. FISU

An FISU contains no information. It is a null signal transferred between network nodes when the link is idle. The purpose of transferring FISUs is to ensure that the signaling link is available and the local end can receive messages from the peer.

Figure 2-5 shows the structure of an FISU.

8 16

F CK

First bit transmitted8

LIFIB

FSN

BIB

BSN

F

6 1 7 1 72 Figure 2-5 Structure of FISU

II. LSSU

An LSSU carries the information of network link status that is indicated by an SF field.

Figure 2-6 shows the structure of an LSSU.




2-8

8 16

F CK

8/16 First bit transmitted8

LIFIB

FSN

BIB

BSN

F

6 1 7 1 7

SF

2

Figure 2-6 Structure of LSSU

III. MSU

An MSU conveys the real information to be transmitted. The information is encapsulated in a signaling information field (SIF) and a service information octet (SIO) . Figure 2-7 shows the structure of an MSU.

8 16

F CK

8n, n>2

SIF

First bit trasmitted8

LIFIB

FSN

BIB

BSN

F

6 1 7 1 72

SIO

8

Figure 2-7 Structure of MSU

2.3.2 Functions and Codes of Signal Unit Fields

A signal unit comprises a number of fields, namely:

Signal unit delimitation flag (F) Check bit (CK) Length indicator (LI) Service information octet (SIO) Signaling information field (SIF) Sequence numbering and indicator bits

I. Signal Unit Delimitation Flag

The bit pattern for the signal unit delimitation flag is 01111110.

The opening F of one signal unit is normally the closing F of the preceding signal unit. For example, in Figure 2-5, the opening F is on the right, and the closing F is on the left.

Other fields in the middle can be inserted randomly to lower system processing load in case of overload.




2-9

II. Check Bit

Every signal unit has 16 check bits for error detection.

Cyclic redundancy codes (CRC) are assumed for check bits.

III. Length Indicator

The length indicator (LI) is used to indicate the number of octets following the length indicator octet and preceding the check bits. The unit of LI is octet.

The length indicator differentiates between the three types of signal units as follows:

LI = 0: FISU LI = 1 or 2: LSSU LI > 2: MSU

IV. Service Information Octet

The SIO is present only in MSUs to indicate the type of messages.

MTP Level 3 functions distribute messages to corresponding function modules according to their SIO and indicate whether the message is from an international network or a national network.

The service information octet is divided into the service indicator (SI) and the sub-service field (SSF), each taking four bits as shown in Figure 2-8.

D C B A D C B A

SSF SI

Transmit direction

Figure 2-8 SIO structure

The codes of SI and SSF are allocated as described in Table 2-1 and Table 2-2. Bits A and B are spare bits.

Table 2-1 SI codes allocation

D C B A Meaning

0 0 0 0 Signaling network management messages

0 0 0 1 Signaling network testing and maintenance messages

0 0 1 0 Spare

0 0 1 1 Signaling connection control part (SCCP)

0 1 0 0 Telephone user part (TUP)

0 1 0 1 ISDN user part (ISUP)

0 1 1 0 Data user part (DUP) (call and circuit-related messages)




2-10

D C B A Meaning

0 1 1 1 Data user part (DUP) (facility registration and cancellation messages)

1 0 0 0 to 1 1 1 1 Spare

Table 2-2 SSF codes allocation

D C Network indicator

00 International network

01 Spare (for international use only)

10 National network

11 Reserved for national use

V. Signaling Information Field

The SIF, the message intended to be delivered, consists of an integral number of octets, greater than or equal to 2 and less than or equal to 272.

In view of shortening signaling transmission delay, the maximum length of SIF used to be 62 octets, and 63 octets plus SIO. The LI value was set to 6 bits, spanning from 0 to 63.

With the development of ISDN services, larger capacity of signaling messages is demanded. As the processing capacity of processors is elevated, the maximum length of SIF can ascend and has been increased to 272 octets.

To maintain the former signal unit format, the LI code is not changed. The LI value of the signal unit with SIF of 63 or more octets is set to 63.

An SIF is composed of two parts, signal information (SI) and label, as shown in Figure 2-9.

SICIC DPC OPC

Label

Figure 2-9 Structure of SIF

SI is encoded depending on user part and its message types.

MTP Level 3 functions select a signaling route according to the label. The label contains three fields:

Originating point code (OPC)




2-11

Destination point code (DPC) Circuit identification code (CIC)

The least sign four bits in the CIC form the signaling link selection (SLS) code.

VI. Sequence Numbering and Indicator Bits

The forward sequence number (FSN) is the sequence number of the signal unit in which it is carried.

The backward sequence number (BSN) is the sequence number of a signal unit being acknowledged.

The forward (retransmission) indictor bit (FIB) indicates the current transmitted signal unit. The value is "0" or "1". When the value of FIB is inverted, the signaling message will be retransmitted.

The backward (retransmission) indicator bit (BIB) indicates the acknowledgements of the received signal unit. When the BIB is inverted, the signaling receiving terminal will notify the transmitting terminal to retransmit messages from the BSN +1.


SS7 Signaling SystemChapter 3 Telephone User Part


3-1

Chapter 3 Telephone User Part

This chapter introduces the concepts related to telephone user part (TUP), as well as its functions and position in the SS7.

3.1 Introduction to TUP

The TUP is at layer 4 of the SS7. It is one part of the user part.

Huawei CDMA system employs the TUP compliant with the ITU-T Recommendation of Blue Book Fascicle V1.8 (1988).

Figure 3-1 shows the position of TUP in the SS7.

INAP OMAP MAP

TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAPINAP OMAP MAP

TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAP

L 1

L 2

L 3

L 4 - L6

L 7

ISUP TUP

Figure 3-1 Position of TUP in SS7

3.2 TUP Functions

The TUP defines the circuit signaling function necessary in the call control signaling of SS7 (signaling messages transmitted between MSCs).

3.3 TUP Messages

This section introduces the format and encoding of TUP messages and provides some TUP message examples.

3.3.1 Format of TUP Messages

In SS7, TUP messages are carried on the signaling data link by means of MSUs.




3-2

The signaling information of each message constitutes the SIF of the corresponding signal unit.

It basically contains

The label The heading code (H0, H1) One or more signals and/or indications

The length of the message is changeable. Figure 3-2 shows the structure of TUP messages.

F CK SIF SIO LI FIB FSN BIB BSN F

8 16 8n 8 2 6 1 7 1 7 8 n>=2

First bit transmitted

Signaling Information (SI) H1 H2 Label 8n 4 4 64

F: Delimitation flag BSN: Backward sequence number BIB: Backward indicator bit FSN: Forward sequence number FIB: Forward indicator bit LI: Length indicator SIO: Service information octet SIF: Signaling information field CK: Check bit

Figure 3-2 TUP message structure

3.3.2 Encoding of TUP Messages

TUP messages are encoded by means of label and heading code.

I. Label

The label is an item of information that forms part of every signaling message. It is used by the message routing function at MTP Level 3 to select the appropriate signaling route. The UP identifies the particular transaction (e.g. the call) to which the message pertains. The length of a label must be a multiple of 8 bits.

Figure 3-3 shows the label structure.

CIC OPC DPC

First bit transmitted 4 12 24 24

Figure 3-3 TUP label structure




3-3

The label contains the following fields:

DPC OPC CIC

II. Heading Code

All telephone signal messages contain a heading consisting of two parts, heading code H0 and heading code H1 that identify each telephone signal.

Heading Code H0

The heading code H0 occupies 4-bit field following the label to identify up to 16 message groups. It is coded as in Table 3-1.

Table 3-1 Coding format of H0

DCBA Meaning

0000 Spare, reserved for national use

0001 Forward address messages (FAM)

0010 Forward set-up messages (FSM)

0011 Backward set-up request messages (BSM)

0100 Successful backward set-up information messages (SBM)

0101 Unsuccessful backward set-up information messages (UBM)

0110 Call supervision messages (CSM)

0111 Circuit supervision messages (CCM)

1000 Circuit group supervision messages (GRM)

1001 Spare, reserved for international use

1010 Circuit network management messages (CNM)

1011 Reserved for international and basic national use

1100 Successful national backward set-up information messages (NSB)

1101 National call supervision messages (NCB)

1110 Unsuccessful national backward set-up information messages (NAM) (out of service now)

Heading Code H1

The heading code H1 occupies 4 bits. It either contains a signal code or in case of more complex messages, identifies the format of these messages, when several signal codes and message indicators are contained. A message group identified by an H0 contains a maximum of 16 messages.




3-4

3.3.3 Example of TUP Messages

There are 65 kinds of TUP messages in SS7. Each kind of message has its own function and format. The following cites the initial address message with additional information (IAI) as an example to describe the format and encoding of TUP messages.

I. IAI Message

In a mobile network, an MSC connects with another MSC or a transit exchange using IAIs and forwards initial address messages (IAM) to the local office.

Figure 3-4 shows the basic format of the initial address message with additional information. The IAM message comes before the first indicator octet.

Figure 3-4 IAI format

The following codes are used in IAI in the MSC:

Heading Code H0

Coded 0001, indicating the FAM

Heading Code H1

Coded 0010

Calling Party Category

The calling party category occupies 6 bits containing 64 kinds of calling party categories. It is coded as described in Table 3-2.

Table 3-2 Calling party category

EFDCBA

000000 - 001000 Spare

001001 Operator (no interrupting function)

001010 Ordinary calling subscriber, used between the mobile office and the local office (transit exchange)




3-5

EFDCBA

001011 Calling subscriber with priority , used between mobile offices

001100 Data call

001101 Test call

001110 Spare

001111 Spare

010000 Ordinary calling subscriber, free of charge, used between the mobile office and the remote office

010001 Ordinary calling subscriber, periodic, for inter-office use (including international offices)

010010 Ordinary calling subscriber, user table, immediate (received from the local office only)

010011 Ordinary calling subscriber, printer, immediate (receiving only)

010100 Calling subscriber with priority, free following charge, used between the mobile office and the remote office

010101 Calling subscriber with priority, periodic, for inter-office use (including international offices)

010110 Spare

010111 Spare

011000 Ordinary calling subscriber, received from the local office only

011001 - 111111 Spare

Message Indicators

Table 3-3 lists the message indicators.




3-6

Table 3-3 Message indicators

Nature of address indicator

00 Local subscriber number

01 spare

10 national (significant) number

BA

11 international number

Nature-of-circuit indicator

00 No satellite circuit in the connection

01 One satellite circuit in the connection

10 Spare

DC

11 Spare

Continuity-check indicator

00 Continuity-check not required

01 Continuity-check required on this circuit

10 Continuity-check performed on a previous circuit

FE

11 Spare

Outgoing echo-suppressor indicator

0 Outgoing half echo suppressor not included

G

1 Outgoing half echo suppressor included

Incoming international call indicator

0 Call other than international incoming

H

1 Incoming international call

Redirected call indicator (related to call forwarding)

0 Not a redirected call

I

1 Redirected call

All-digital-path-required indicator (related to ISDN services)

0 Ordinary call

J

1 Digital path required

Signaling path indicator

0 Any path

k

1 All SS7 path

L Spare




3-7

Number of Address Signals

A code expressing in pure binary representation the number of address signals contained in the IAM.

Address Signals

Table 3-4 describes the encoding of the address signals.

Table 3-4 Encoding of the address signals

0000–001 Digits 0–9

1010 Spare

1101 Spare

1110 Spare

1011 Code 11 (*), used in international network connection

1100 Code 12 (#), used in international network connection

1111 ST

Filler

In case of an odd number of address signals, the filler code 0000 is inserted after the last address signal. This ensures that the variable-length field that contains the address signals consists of an integral number of octets.

First Indicator Octet (related to additional information)

The encoding of the first indicator octet various according to whether or not additional information is attached.

Table 3-5 describes the encoding of the first indicator octet.

Table 3-5 Encoding of the first indicator octet

Network capability or user facility information indicator (not in current use, set as 0)

0 Network capability or user facility information not included

A

1 Network capability or user facility information included

Closed user group information indicator

0 Closed user group information not included B

1 Closed user group information included

Additional calling party information indicator (unavailable)

0 Additional calling party information included C

1 Additional calling party information not included




3-8

Additional routing information indicator (unavailable)

0 Additional routing information not included D

1 Additional routing information included

Calling line identity indicator

0 Calling line identity not included E

1 Calling line identity included

Original called address indicator

0 Original called address not included F

1 Original called address included

Charging information indicator (unavailable)

0 Charging information not included G

1 Charging information included

H Spare

Calling Line Identity

As shown in Figure 3-5, the calling line identity consists of three parts: 4-bit address indicator, 4-bit number of address signals, and calling line identity.

Calling line

identity

Number of

address

signals

Address

indicator

First bit transmitted 8n 4 4

D C B A D C B A

Figure 3-5 Calling line identity field

Address indicators

Table 3-6 lists the address indicators.

Table 3-6 Address indicators

Nature of address indicator

00 Local subscriber number

01 Spare, reserved for national use

10 National significant number

BA

11 International number

C Calling line identity presentation indicator




3-9

0 Calling line identity presentation not restricted

1 Calling line identity presentation restricted

Incomplete calling line identity indicator

0 No indication D

1 Incomplete calling line identity

Number of address signals

A code expressing in pure binary representation the number of address signals.

Calling line identity

Table 3-7 lists the calling line identity codes.

Table 3-7 Calling line identity codes

0000-1001 Digits 0-9

1010 Spare

1011 Code 11 (*), used in international network connection

1100 Code 12 (#), used in international network connection

1101 Spare

1110 Spare

1111 ST

Original Called Address

The original called address is the same as the calling line identity except that the bits D and C of the address indicator are spare.

II. IAM Message

Figure 3-6 gives an example of IAM message.

Figure 3-6 IAM format

The meaning of the values in the message is as follows:

84 Service Indication Octet

10------ Network Indication: National network

--00---- Spare bit: Reserved




3-10

----0100 Service Indication: TUP

21 Initial address message with additional information (IAI)

00 12 Circuit identity code: 00 12

00 Signaling link selection: 00

36 66

00------ Spare: 00

--110110 OPC: 36 66

11 E3

00------ Spare: 00

010001 DPC: 11 E3

00 00 Maintenance station reserve 2 bytes: 00 00

TUP message

0F Calling party category

00------ Spare: Reserved

--001111 Calling party category: Payphone

Message indicator

00

0------- International incoming call indicator: Call other than international incoming

-0------ Echo suppressor indicator: Outgoing half echo suppressor not included

--00---- Continuity-check indicator: Continuity check not required

----00— Circuit nature indicator: No satellite circuit in the connection

------00 Address nature indicator: Subscriber number

B4

1011---- Address signal number: 0B

----0--- Collect call indicator: Not collect call

-----1— Signal path indicator: All signaling system No.7 path

------0- All-digital-path-required indicator: Ordinary call

-------0 Redirected call indicator: Not a redirected call

97 20 69 11 03 02 Address signal: 79029611302

02 of it

0000---- Filler: 0

----0010 2




3-11

10 First indicator octet

0------- Spare: Reserved

-0------ Charging information indicator: Charging information not included

--0----- Original called address indicator: Original called address not included

---1---- Calling party subscriber line indicator: Calling line identity included

----0--- Additional routing information indicator: Additional routing information not included

-----0-- Additional calling party information indicator: Additional calling party information not included

------0- Closed user group information indicator: Closed user group information not included

-------0 Network capability or user facility information indicator: Network capability or user facility information not included

Calling party subscriber line identity

72 Calling party subscriber line identity head

0111---- Number of address signal: 07

Address indicator

----0--- Incomplete calling line identity indicator: No indication

-----0— calling line identity presentation indicator: Calling line identity presentation not restricted

------10 Nature of address indicator: National number

55 05 00 F1 Address signal: 5550001

F1 of it

1111---- Filler: F

----0001


SS7 Signaling SystemChapter 4 Signaling Connection Control Part


4-1

Chapter 4 Signaling Connection Control Part

This chapter introduces the concepts related to signaling connection control part (SCCP) , as well as its functions and position in the SS7.

4.1 Introduction to SCCP

This section introduces the features of:

TUP signaling transfer based on MTP SCCP signaling transfer based on MTP

4.1.1 TUP Signaling Transfer Based on MTP

There are two types of signaling messages on the telecommunication networks:

Circuit-related message Non-circuit-related message

The following details the advantage and disadvantage of TUP when transferring these two types of signaling messages based on MTP.

I. Circuit-Related Messages

The signaling messages transferred on telephone networks are circuit-related messages. That means all signaling messages are related to call circuits, and the paths for these messages are usually in one-to-one correspondence with call connection paths.

Circuit-related messages feature real-time but small-capacity transfer.

The 4-layer signaling architecture with TUP on MTP enables efficient transfer of various call control and connection control messages. It is ideal for telephone network, especially digital switching telephone network.

II. Non-Circuit-Related Messages

Non-circuit-related messages are also called node-to-node messages. They are irrelevant to calls or call circuits.

Non-circuit-related messages feature large-capacity transfer with certain time delay.

III. Disadvantages of TUP Signaling Transfer Based on MTP

The following describes the disadvantages of TUP signaling transfer of call-related and non-call-related messages.

Transfer of Call-Related Messages




4-2

TUP is used to set up call connection paths between two exchanges. It transfers messages along the established call connection paths segment by segment. This increases resources waste and transfer delay.

Transfer of Non-Call-Related Messages

MTP selects a route and determines the terminal user according to the DPC and SI. This addressing method has the following disadvantages:

SPC is defined by the network to which the SP belongs. As a result, the code format may be different, resulting in inter-network addressing failure.

According to CCITT specifications, a SPC is composed of 14 bits. Therefore, the maximum number of SPs on a signaling network is only 16,384 due to code limitation.

An SI is composed of four bits. That is, MTP can allocate messages to a maximum of 16 user parts. This capacity is being challenged by ever-increasing service demands.

MTP allows connectionless transfer only. It does not support non-real-time connection-oriented transfer of large amount of messages between network nodes.

4.1.2 SCCP Signaling Transfer Based on MTP

SCCP and MTP together realize the functions of an OSI network. They allow transparent transfer of signaling messages directly between any two SPs. The SCCP and the MTP in this case are referred to as the network service part (NSP).

SCCP provides connectionless and connection-oriented network services between exchanges and network centers to transfer signaling messages and other types of information.

Figure 4-1 shows the relation between SCCP and other functional elements in a signaling network.




4-3


TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR


TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAP

L 1

L 2

L 3

L 4 - L6

L 7

Figure 4-1 The SCCP in the SS7In the signaling network

In SS7 hierarchical architecture, ISUP and TCAP are the users of SCCP.

ISUP, with the help of SCCP, realizes end-to-end message transfer and supports related ISDN supplementary services.

TCAP utilizes the complete network-layer services provided by SCCP and MTP to provide the following functions:

Realizes long-distance transfer of non-circuit-related messages. Supports new services and functions of wireless networks, intelligent networks,

telecom administration networks, and so on.

Operation and maintenance application part (OMAP) , mobile application part (MAP), home location register (HLR) , and visitor location register (VLR) are all referred to as SCCP subsystems. SCCP performs management on these subsystems with the support of the TCAP. When there is signaling interaction between two of these subsystems, the signaling messages to be transferred are encoded for routing in the SCCP before they are transferred to the peer subsystem. These two subsystems can be of the same SP or of different SPs. However, the signaling messages transferred between two subsystems of the same SP will not go through MTP.

The SCCP provides the following functions:

Transfers non-circuit-related signaling messages. Performs enhanced addressing and routing to realize direct signaling transfer

between different SS7 networks worldwide. Provides connectionless and connection-oriented network services.

4.2 Services Provided by SCCP

SCCP provides various connectionless and connection-oriented network services to meet different demands for data transfer quality.




4-4

4.2.1 SCCP Service Classes

The SCCP provides four classes of service:

Class 0: Basic connectionless class Class 1: In-sequence delivery connectionless class Class 2: Basic connection-oriented class Class 3: Flow control connection-oriented class

4.2.2 Connectionless Services

Connectionless services are performed similar to the transfer of datagram in packet switching. No message transfer channel needs to be set up before message transfer. Signaling data are transferred on the signaling network. Therefore, routing function is provided by the SCCP.

In signaling message transfer, SCCP converts a called address into a signaling point code that can be recognized by MTP.

In connection services, messages are transferred as a whole (as unit data UDT) instead of being segmented.

Figure 4-2 shows the transfer procedure of UDT.

SCCP1 SCCP2 SCCP1 SCCP3 SCCP4 SCCP2

MTP1 MTP3 MTP4 MTP2

UDTUDTUDT

Node 1 Node 2 Node 1 Node 3 Node 4 Node 2

(a) Logical transfer path (b) Actual transfer path

Figure 4-2 Connectionless transfer of signaling messages

Class 0 and Class 1 are connectionless services.

Class 0: Basic connectionless service

Signaling messages are transferred independent of one another. Therefore, there is no guaranteed in-sequence delivery of signaling messages to the destination signaling point.

Class 1: In-sequence delivery connectionless service

The data from the same information flow are attached with a signaling link selection code (SLS) . Data packets with the same SLS are transferred on the same signaling link. Therefore, it is guaranteed that messages can be delivered to the destination signaling point in accordance with the transfer sequence.




4-5

4.2.3 Connection-Oriented Services

Connection-oriented services are performed similar to the packet switching over virtual circuits.

Before signaling message transmission, a message transfer channel (logical connection or virtual connection) is established between the originating node and the destination node.

The connection-oriented service is suitable for transferring a large amount of data.

SCCP1 SCCP1 SCCP1

CR CR

CC CC

DATA

RLSD

RLC

DATA

DATA DATA

RLSD

RLC

Node 1 Middle node Node 2

CR: Connection request CC: Release confirm RLSD: Released RLC: Release complete

Figure 4-3 Connection-oriented transfer with a middle node

Class 2 and Class 3 are connection-oriented services.

Class 2: Basic connection-oriented class

This type of service can guarantee that signaling messages are sent in the same sequence as they are received. Therefore, a long message can be transferred in segments, and reassembled after it is received.

Class 3: Flow control connection-oriented class

The features of class 2 are complemented by the inclusion of flow control, expedited data transfer, and detection of message loss or mis-sequencing.

Connection-oriented services are classified into temporary signaling connections and permanent signaling connections.

Temporary signaling connections are always under control (for example, during establishment, data transfer, and release). They are released after the transfer is finished. It is similar to a call connection.

Permanent signaling connections are similar to permanent virtual circuits in packet data switching. They are established0 and controlled by the operation




4-6

and maintenance system of the local or a remote MSC or managed by connected nodes. Mobile subscribers cannot control these connections.

The signaling transfer procedures of temporary and permanent signaling connections are the same.

4.2.4 SCCP Service Procedure

This section describes the signaling procedures of connectionless and connection-oriented services provided by the SCCP.

I. Connectionless Services

Figure 4-4 shows the transfer of connectionless service messages in GT addressing.

USERA SCCPA SCCPC SCCPB USERB

N-UNITDATA Request

N-UNITDATA Indication

UDT

UDT

Figure 4-4 Connectionless service in GT addressing

The following explains the procedure:

1) USERA sends an N-UNITDATA Request to SCCPA to request for connectionless service data transfer.

The N-UNITDATA Request primitive contains:

Called address: GT (implicit DPC=B, SSN=USERB) Calling address: DPC=A, SSN=USERA In-sequence control: Sequence Return selection: Error return User data

2) After SCCPA receives the primitive, it analyzes the called address. After it determines that the called address is a GT, it transfers the UDT to SCCPC.

3) After SCCPC receives the UDT, SCCP translates the GT of the called address into the following parameters:

DPC=B SSN=USERB

Then It performs addressing according to the DPC and the SSN. If SCCPC finds that the DPC and SSN of the destination SCCP are available, SCCPC transfers the UDT message to the destination SCCPB.




4-7

4) After SCCPB receives the UDT, it sends an N-UNITDATA Indication to USERB, and transfers user data to USERB.

In this example, GT translation is completed by SCCPC. In actual networks, GT translation may be done by different nodes on the network, or centralized in one node.

II. Connection-Oriented Services

Figure 4-5 shows the signaling procedure of connection-oriented service provided by the SCCP.

USERA SCCPA SCCPC SCCPB USERB

N-CONNECT Request

N-CONNECT Indication

CR

CR

N-CONNECT Confirmation

N-CONNECT Response

CC

CC

Figure 4-5 Connection-oriented service signaling flow

The following explains the procedure:

1) USERA (calling party) sends an N-CONNECT Request to SCCPA to request the signaling connection with USERB (called party).

2) After SCCPA receives the N-CONNECT Request, it checks whether resources are available. If yes, SCCPA allocates a source local reference and a SLS for the connection section.

Then SCCPA establishes a correspondence between the called address and the connection section, and determines protocol class and credit. Finally it selects the route for the CR message and transfers it to SCCPC.

3) When SCCPC receives the CR message, it checks whether the called address is a local SCCP user through the routing and diagnosis function. If the SCCP user is not a local SCCP user, the SCCP requires setting up a connection section.

SCCPC checks whether the resource is available. If yes, SCCPA allocates the received source local reference and SLS for the connection section. Then it establishes a correspondence between the input connection section and output connection section and determines the protocol class and credit.




4-8

SCCPC forwards the CR message to SCCPB through the routing function without changing the addressing contents of CR message.

4) After SCCPB receives the CR message, it checks whether the called address is a local user through the routing and diagnosis function, and whether the destination monitoring has available resources to establish the connection section. If the results are positive, SCCPB allocates the received source local reference and SLS to the input connection section (between the middle node and the destination).

Then SCCPB determines protocol class and credit, and, through an N-CONNECT Indication, notifies USERB to establish a connection.

5) If the connection is approved, USERB sends an N-CONNECT Response to the SCCPB.

6) After SCCPB receives N-CONNECT Response, it allocates a protocol class and credit, and determines the local reference of input connection section. Then it sends a CC message to the source SCCPC of the connection section by using SCCP routing function.

7) After SCCPC receives the CC message, it allocates the source local reference of the CC to the output connection section. Then SCCPC determines the protocol class and credit and the local reference for the corresponding output connection section. Finally it transfers the CC message to the source SCCPA corresponding to the input connection section by using the SCCP routing function.

8) After SCCPA receives the CC message, it allocates the protocol class and credit, and allocates the source local reference in the received CC to the connection section. Finally it sends an N-CONNNECT Confirmation to USERA, notifying that USERA signaling connection is successful.

After that, the user can transfer data through the signaling connection. The connection is released after data transfer is finished.

4.3 SCCP Addressing and Routing

There are three types of SCCP address:

Signaling point code (SPC) Subsystem number (SSN) Global title (GT)

SPC refers to the address of MTP. It is valid in a SS7 network only.

The MTP designated by SPC identifies a destination SP with the received DPC and performs routing to the destination SP. In addition, it identifies the user of the destination SP according to Service Indicator (SI).

SSN is the local addressing information used by SCCP to identify SCCP users in the same node.




4-9

For example, SSNs can be used to represent TCAP, ISUP, MAP, and so on. It serves as a supplement to MTP messages in defining subscriber address. With the SSN, the local addressing range of SI is extended and thus an SS7 network can meet the requirements for future new services development.

GT applies when the destination network address is unknown to the originating node.

GT can be used to identify any signaling point and subsystem worldwide.

MTP, however, cannot perform routing according to GT. The SCCP must first translate the GT of the called party into DPC or DPC+SSN. In addition, when SCCP sends the DPC or DPC+SSN to MTP, it needs to specify the numbering plan of the GT.

The calling address and called address in a SCCP message may be any or the combination of SPC, SSN, and GT.

SCCP can perform addressing and routing according to the following two types of addresses:

DPC+SSN GT

If SCCP sends GT+DPC+SSN to MAP, it must specify whether the routing for message transfer is performed in accordance with GT or DPC+SSN.

4.4 SCCP Primitives

This section gives the definition and structure of SCCP primitives, followed by a detailed description of SCCP user and service primitives

4.4.1 Definition

In SS7 layered architecture, any layer can be regarded as Layer N except for the top and the bottom layers. The upper layer and the lower layer next to layer N is referred to as Layer N+1 and Layer N-1. Layer N+1 is the user of Layer N, and Layer N is in turn the user of Layer N-1. Services are provided by Layer N-1 to Layer N and by Layer N to Layer N+1.

To realize the information exchange between Layer N+1 and its peer end, Layer N+1 requests that Layer N communicates with the peer Layer N on the basis of the connection provided by Layer N-1 with peer Layer N-1. In this process, Layer N provides services to Layer N+1.

When Layer N+1 requests services from Layer N or Layer N provides services to Layer N+1, the service user shall interact with the service provider. The signaling data transferred between these two layers in this process are referred to as primitives.




4-10

Four types of primitives are available:

Request: A primitive issued by a service user to invoke a service element. Indication: A primitive issued by a service provider to indicate that a service

element has been invoked by the service user. Response: A primitive issued by the peer service user in response to the

request. Confirmation: A primitive issued by a service provider acknowledge the reception

of the response.

Note:

Not every primitive has all of the four types (Request, Indication, Response and Confirmation). It is subject to specific service protocol procedure.

Service interfaces from SCCP to upper layer and MTP are defined by primitives and parameters.

The lower layer of SCCP is MTP, and the corresponding primitive is the MTP-primitives. The upper layer is SCCP user, the primitive between the SCCP and its user is N-primitives (also called SCCP user primitives).

Inter-SCCP communication is performed with SCCP messages, and inter-MTP communication, with the MTP messages.

Figure 4-6 shows the primitives and messages of the SCCP and MTP.

USER

SCCP

MTP

USER

MTPMTP Message

SCCP MessageSCCP

Request Confirmation Response Indication

Request Confirmation Response Indication

Figure 4-6 Primitives and messages of MTP and SCCP




4-11

4.4.2 Structure

A complete primitive consists of three parts: generic name, specific name, and parameters, as shown in Figure 4-7.

×Genericname Specific name Parameters

Figure 4-7 Structure of a primitive

%: Stands for the functional block providing the services (M represents MTP, and N represents SCCP).

Generic name: Primitive name, indicating the service provided and the task shall be completed in addressing layer.

Specific name: Primitive type, indicating the direction of primitive flow. Parameter: Data needed for implementing this service.

For example, a signaling message is transferred to the destination in the form of UDT. When the destination SCCP transfers this data to its user, the indication primitive of the UDT is:

N-UNITDATA indication (CDA, CGA and UD)

Where:

"N" represents the network layer (the SCCP primitive). "UNIDATA" is the generic name. "CDA, CGA, and UD" are the primitive parameters, representing the called

address, calling address and subscriber data.

4.4.3 SCCP User Primitives

Table 4-1 lists the name, protocol type, and parameters of SCCP user primitives.

Table 4-1 SCCP user primitives

Protocol type Generic

name Specific

name 0 1 2 3

Parameters

N-UNITDATA Request Indication

ª ª

Calling address Called address Sequence control Return option User data




4-12

Protocol type Generic

name Specific

name 0 1 2 3

Parameters

N-NOTICE Indication ª ª

Called address Calling address Reason for return User data

N-CONNECT

Request Indication Response Confirm

ª ª

Calling address Called address Responding address Receiving response selection Expedited data selection Quality of service parameter set User data Connection identification

N-DISCONNECT

Request Indication

ª ª

Originator User data Responding address Connection identification Reason

N-DATA Request Indication

ª ª

Response request User data Connection identification

N-EXPEDITED DATA

Request Indication

ªUser data Connection identification

N-RESET

Request Indication Response Confirmation

ª

Originator Reason Connection identification

N-INFORM Request Indication

ª ª

Reason Connection identification QOS parameter set

The following describes these SCCP user primitives:

N-UNITDATA: UDT primitive, used to transfer the UDT in connectionless service.




4-13

N-NOTICE: Notice primitive, used to notify the originating end, destination or transfer point that the SCCP cannot transfer the message in case of the connectionless service.

N-CONNECT: Connection primitive, used to establish a connection. N-DISCONNECT: Disconnection primitive, used for disconnection. N-DATA: Data primitive, used to transfer data in connection-oriented service. N-EXPEDITED DATA: Expedited data primitive, used for Class 3 to transfer

expedited data. N-RESET: Reset primitive, used to transfer reset messages in Class 3 protocol

and to restart the flow control procedure from initial state. N-INFORM: Inform primitive, used to transfer relevant network or user

information during data transfer phase in connection-oriented service.

4.4.4 MTP Service Primitives

SCCP layer communicates with MTP by exchanging MTP service primitives.

Table 4-2 lists the MTP service primitives.

Table 4-2 MTP service primitive

Generic name Specific name Parameters

MTP-TRANSFER Request Indication

SCCP message

MTP-RESUME Indication Affected signaling point

MTP-PAUSE Indication Affected signaling point

MTP-STATUS Indication Affected signaling point

MTP-UPU Indication Affected signaling point

The following describes these MTP service primitives:

MTP-TRANSFER Request: Used by SCCP to access MTP signaling message handling function.

MTP-TRANSFER Indication: MTP message handling function, transferring signaling messages to SCCP.

MTP-PAUSE Indication: Sent by MTP to indicate that it fails to transfer messages to the designated destination.

MTP-RESUME Indication: Sent by MTP to notify the user that MTP is capable of providing MTP service to the designated destination.

MTP-STATUS Indication: Sent by MTP to notify the user that MTP is partially capable of providing MTP service to the designated destination. This primitive is also used to notify the user of the cause of peer user available or unavailable.




4-14

4.5 SCCP Messages

On receiving the primitive request or response from a user, SCCP encapsulates user data and necessary control and routing information into SCCP messages.

4.5.1 Format of SCCP Messages

SCCP messages encapsulated in the MSUs in MTP before transfer. For a MSU, SCCP message is its signaling information field (SIF).

I. Message Structure

Figure 4-8 shows the structure of SCCP messages.




4-15

F CK SIF SIO LI FIB FSN BIB BSN F

Routing label

Message type

Mandatory fixed part(F)

Mandatory variablepart (V)

Optional part (O)

Mandatory parameter A

¡-¡-

Mandatory parameter I

¡-¡-

Parameter M pointer

Parameter P pointer

Start pointer of optional part

Parameter M length

Parameter M

¡-¡-

Parameter P length

Parameter P

Parameter name X

Parameter X length

Parameter A

¡-¡-

Paraemter name Z

Paraemter Z length

Paraemter Z

End of optional parameter

Figure 4-8 Structure of SCCP messages

The following describes the components of SCCP messages:

Routing label: The structure is DPC+OPC+SLS. It provides routing information for the transfer of SCCP messages.

Message type: It identifies different SCCP messages. It is a mandatory byte for all messages, and determines the function and format of a message.

Mandatory fixed part (F): It includes all mandatory parameters with fixed length in this message.

Mandatory variable part (V): It includes all mandatory parameters with variable length in this message.




4-16

Optional part (O): It includes all optional parameters in this message.

II. Message Type

Table 4-3 shows the message types and their codes of various messages.

Table 4-3 SCCP message type and code

Protocol class Message type

0 1 2 3 Message type

code

Connection request (CR) √ √ 0000 0001

Connection confirm (CC) √ √ 0000 0010

Connection refused (CREF) √ √ 0000 0011

Released (RLSD) √ √ 0000 0100

Release complete (RLC) √ √ 0000 0101

Data form 1 (1DT1) √ 0000 0110

Data form 2 (2DT2) √ 0000 0111

Data acknowledgement (AK) √ 0000 1000

Unit data (UDT) √ √ 0000 1001

Unit data service (UDTS) √ √ 0000 1010

Protocol data unit error (ERR) √ √ 0000 1111

Inactivity test (IT) √ √ 0001 0000

Functions of SCCP message types are as follows:

CR and CC are used to establish a connection. CREF is sent to the originating node when the middle node or the destination

node lacks sufficient resources. DT1, DT2 and ED are messages used to transfer data after successful

connection. RLSD and RLC are release signals after data transfer. ERR is sent when protocol error is detected. IT is used to detect whether the two ends of the connection can work normally. UDT, UDTS, XUDT, and XUDTS are connectionless service messages. UDT

and XUDT are used to transfer connectionless service data and segmented data of over-length messages. UDTS and XUDTS are sent to the originating point to specify the cause if UDT or XUDT fails to reach the destination.




4-17

4.5.2 Encoding of SCCP Messages

A SCCP message contains 17 parameters in total.

Table 4-4 lists all parameter names and their codes.

In the table, M stands for "mandatory" and O stands for "optional".

Table 4-4 SCCP message parameters

Field Message Code

UDT

UDTS

CR

CC

CREF

RLSD

RLC

DT1

DT2

AK

ED

EA

RSR

RSC

ERR

IT

Message type M M M M M M M M M M M M M M M M

Destination local reference M M M M M M M M M M M M M 00000001

Source local reference M M M M M M M 00000010

Called party address M M M O O 00000011

Calling party address M M O 00000100

Protocol class M M M M 00000101

Segmenting/reassembling M 00000110

Receive sequence number

M 00000111

Sequencing/segmenting M M 00001000

Credit O O M M 00001001

Release cause M 00001010

Return cause M O O O 00001011

Reset cause M 00001100

Error cause M 00001101

Data M M O O O O M M M 00001111

Refusal cause M 00001110

End of optional parameters O O O O O O 00000000




4-18

The following describes these parameters:

I. Destination Local Reference and Source Local Reference

Destination local reference and source local reference apply to connection-oriented services. They are the internal numbers used by the destination of a signaling connection section and the source SCCP to identify this connection section. They are allocated by SCCP at both ends of a connection when the connection is set up. These two references identify the path for later data transfer.

The parameter is a three-octet field. The "all 1s" codes are reserved for future use.

II. Calling Address and Called Address

Calling address and called address identify the originating and destination signaling point and the user part.

The calling and called addresses in a connectionless service message represent the origination and destination of the SCCP message. Those in a connection-oriented service message represent the source and destination of a signaling connection (not signaling connection section) and are used for connection setup and connection acknowledgement messages.

Calling/called address codes consist of the following units in order:

Address indicator Signaling code SSN GT

The following details address indicator, SSN, and GT.

Address Indicator

Address indicator indicates the type of address information contained in the address field. The address consists of one or the combination of signaling point code, GT, and SSN.

Table 4-5 shows the structure of an address indicator.

Table 4-5 Structure of address indicator

7 6 5 4 3 2 1 0

Reserved for national use

Routing indicator GT indicator SSN

indicator Signaling point code indicator

Signaling point code indicator

0: Signaling point code not included.

1: Signaling point code included.




4-19

Subsystem indicator

0: SSN not included.

1: SSN included.

GT indicator

There are four types of GT.

Table 4-6 shows the correspondence between GT indicator codes (bits) and GT types.

Table 4-6 Correspondence between GT type codes and GT types

GT type code GT type

0000 GT not included.

0001 GT includes nature of address indicator only

0010 GT includes translation type only

0011 GT includes translation type, numbering plan and encoding scheme

0100 GT includes translation type, numbering plan, encoding scheme and nature of address indicator

0101~1110 Spare international

1110~1111 Reserved for extension

Routing indicator

0: Select the route according to the GT in the address.

1: Select the route according to the DPC and the SSN in the called address.

SSN

SSN is a one-octet code. It identifies an SCCP user function.

Table 4-7 explains the meanings of SSNs.

Table 4-7 Allocation of SCCP SSNs

SSN SCCP user

0000 0001 SCCP management

0000 0101 Reserved for compatibility

0000 0110 Home Location Register (HLR)

0000 0111 Visitor Location Register (VLR)

0000 1000 Mobile Switching Center (MSC)

0000 1010 Authentication Center (AC)

1110 1110 Message Center (MC)




4-20

SSN SCCP user

1110 1111 SCP

1111 1100 BSMAP

GT

There are four types of GT. Table 4-8, Table 4-9, Table 4-10, and Table 4-11 show the structures of these GTs.

Table 4-8 Type 1 GT

8 7 6 5 4 3 2 1

O/E Nature of address indicator

The 2nd address signal The 1st address signal

…

Filler (if necessary) The Nth address signal

Table 4-9 Type 2 GT

8 7 6 5 4 3 2 1

Translation type


…


Table 4-10 Type 3 GT

8 7 6 5 4 3 2 1

Translation type

Numbering plan Encoding scheme


…





4-21

Table 4-11 Type 4 GT

8 7 6 5 4 3 2 1

Translation type

Numbering plan Encoding scheme

Spare Nature of address indicator


…


The following describes the fields in a GT:

Nature of address indicator: Nature of the GT

0 0 0 0 0 0 1 — Subscriber number

0 0 0 0 0 1 0 — Reserved for national use

0 0 0 0 0 1 1 — National significant number

0 0 0 0 1 0 0 — International number

Numbering plan is as follows:

0 0 0 0 — Unknown

0 0 0 1 — ISDN/telephony numbering plan (Recommendations E.163 and E.164)

0 0 1 0 — Generic numbering plan

0 0 1 1 — Data numbering plan (Recommendation X.121)

0 1 0 0 — Telex numbering plan (Recommendation F.69)

0 1 0 1 — Maritime mobile numbering plan (Recommendations E.210, E.211)

0 1 1 0 — Land mobile numbering plan (Recommendation E.212)

0 1 1 1 — ISDN/mobile numbering plan (Recommendation E.214)

Encoding scheme is as follows

0 0 0 0 — Unknown

0 0 0 1 — Binary-Coded Data (BCD), odd number of digits

0 0 1 0 — BCD, even number of digits

Translation type: Coding and definition is to be specified. In present CDMA systems, all-zero codes are used.

The coding of internal interface in the network subsystem (NSS) uses type 4 GT. In the A interface SCCP message, the address information generally does not contain the GT.




4-22

III. Protocol Class

Table 4-12 lists the protocol classes defined by bits 1–4.

Table 4-12 Protocol classes

Bits

Bit 4 Bit 3 Bit 2 Bit 1 Protocol Class

0 0 0 0 Class 0

0 0 0 1 Class 1

0 0 1 0 Class 2

0 0 1 1 Class 3

When bits 1–4 are coded to indicate a connection-oriented-protocol class (class 2, class 3), bits 5-8 are spare.

When bits 1–4 are coded to indicate a connectionless protocol class (class 0, class 1), bits 5–8 are used to specify message handling as described in Table 4-13:

Table 4-13 Handling of messages in case of transfer failure

Bits

Bit 8 Bit 7 Bit 6 Bit 5 Return message in case

of transfer failure

0 0 0 0 No

1 0 0 0 Yes

IV. Segmenting/Reassembling

The parameter Segmenting/Reassembling determines whether data in the DT1 is to be transferred in several segments and reassembled at the destination.

This parameter is a one-octet field.

Bits 2–8 are spare.

Bit 1 indicates whether more data is followed. It is called M bit. It is coded as follows:

M=0: no more data M=1: more data

V. Receive Sequence Number and Credit

These two parameters are used in data acknowledgement. They apply to protocol class 3 only.

Receive sequence number indicates the sequence number of the next expected message.




4-23

Credit is used for flow control function. It contains a window size value coded in pure binary.

VI. Sequencing/Segmenting

This parameter consists of two octets. It is used for DT2.

This parameter has two functions:

Indicates the sequence number of a sent message and the number of next expected message to receive for flow control function.

Indicates whether the message is segmented.

VII. Release Cause

The Release Cause parameter is a one-octet field containing the cause for the release of a connection.

Table 4-14 shows the coding of this field.

Table 4-14 Coding of release causes

Bits

Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Release cause

0 0 0 0 0 0 0 0 End user originated

0 0 0 0 0 0 0 1 End user congestion

0 0 0 0 0 0 1 0 End user failure

0 0 0 0 0 0 1 1 SCCP user originated

0 0 0 0 0 1 0 0 Remote procedure error

0 0 0 0 0 1 0 1 Inconsistent connection data

0 0 0 0 0 1 1 0 Access failure

0 0 0 0 0 1 1 1 Access congestion

0 0 0 0 1 0 0 0 Subsystem failure

0 0 0 0 1 0 0 1 Subsystem congestion




4-24

Bits

Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Release cause

0 0 0 0 1 0 1 0 MTP failure

0 0 0 0 1 0 1 1 Network congestion

0 0 0 0 1 1 0 0 Expiration of reset timer

0 0 0 0 1 1 0 1

Expiration of receive inactivity timer

0 0 0 0 1 1 1 0 Reserved

0 0 0 0 1 1 1 1 Unqualified

0 0 0 1 0 0 0 0 SCCP failure

0 0 0 1 0 0 0 1

… … … … … … … …

1 1 1 1 1 1 1 1

Spare

VIII. Return Cause

The Return Cause parameter, used in connectionless protocol UDTS, is a one-octet field containing the cause for message return.

The coding of the Return Cause field is as shown in Table 4-15

Table 4-15 Coding of return causes

Bits

Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Return cause

0 0 0 0 0 0 0 0

No translation for an address of such nature

0 0 0 0 0 0 0 1

No translation for this specific address

0 0 0 0 0 0 1 0 Subsystem congestion

0 0 0 0 0 0 1 1 Subsystem failure




4-25

Bits

Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Return cause

0 0 0 0 0 1 0 0 Unequipped user

0 0 0 0 0 1 0 1

… … … … … … … …

1 1 1 1 1 1 1 1

Spare

IX. Reset Cause, Refusal Cause, and Error Cause

These three parameters indicate reset cause, refusal cause and error cause respectively.

X. User Data

This is a variable-length field containing SCCP user data to be transferred transparently between SCCP user functions.

4.5.3 Example of SCCP Messages

The following is an example of SCCP messages.

Figure 4-9 shows the SCCP message traced on an SS7 link.

Figure 4-9 SCCP message

The following explains the message:

83 Service indicator octet field

10⎯⎯ Network indicator: National

⎯00⎯⎯ Spare

⎯⎯0011 Service indicator: SCCP

00 Invalid

00 Circuit identifier: 00 00

00

0A Signaling link selection code: 0A

60 60 60 source code: 60 60 60




4-26

00 0B 11 destination code: 00 0B 11

0A Message type: Connection confirm

******Length fixed mandatory parameter part******

9A 0E 05 destination local reference number: 9A 0E 05

02 5E 0F source local reference number: 02 5E 0F

******Protocol class part******

02

⎯⎯0010 Protocol class: Class 2

0000⎯⎯ Spare

01 Start pointer of optional part: 01

******Optional part******

Data part

0F Data

0E SCCP user data part length indicator: 14

00 BSSMAP message indication: 00

0C Data length: 0C

For the analysis of BSAP messages, see Chapter 8 "Base Station Application Part".


SS7 Signaling SystemChapter 5 ISDN User Part


5-1

Chapter 5 ISDN User Part

This chapter introduces the concepts related to ISDN user part (ISUP), as well as its functions and position in the SS7.

5.1 Introduction to ISUP

ISUP is in the 4th functional block of SS7 architecture, corresponding to layers 4–7 function in OSI reference model.

ISUP is added with non-speech bearer service protocol and supplementary service protocol based on the TUP.

ISUP supports basic bearer services and supplementary services of ISDN users, and realizes the functions of TUP and data user part (DUP).

Figure 5-1 shows the ISUP in the SS7 architecture.

INAP OMAP MAP TUP

TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAPINAP OMAP MAP TUP

TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAP

L 1

L 2

L 3

L 4 - L6

L 7

ISUP

Figure 5-1 ISUP position in SS7

As shown in Figure 5-1, ISUP needs the support of MTP and SCCP.

ISUP has more functions than TUP, but fewer message types.

The features of ISUP are as follows:

Complete message types: Information carried in the message is abundant. Variable message length: Multiple parameters can be carried. Simple signaling program. Powerful functions: Supports various speech, non-speech, and supplementary

services.




5-2

5.2 ISUP Functions

ISUP provides bearer services, user terminal services, and supplementary services.

5.2.1 Bearer Services

Bearer service is a low-layer message transfer capability provided by a network. It only indicates ISDN communication capability, and is irrelevant to the type of terminals. Therefore, different terminals can use the same bearer capability.

ISUP supports the following bearer services:

64 kbit/s circuit switching unrestricted Speech 3.1 kHz audio 2 x 64 kbit/s 384 kbit/s unrestricted 1,920 kbit/s unrestricted

5.2.2 User Terminal Services

User terminal service is application oriented. It includes the communication capability provided by a network and that of a terminal.

For example, a videophone terminal requires that the minimum bearer capability is 2 x 64 kbit/s unrestricted. The properties of a user terminal service include low-layer, high-layer, and general properties.

Low-layer property indicates the necessary bearer capability of a network, and the property value may be identical with that of the bearer service.

High-layer property indicates the fixed capability of a terminal such as G4 facsimile machine and telephone.

General property indicates available supplementary service, QoS, and so on.

ISUP supports the following user terminal services:

Telephone Intelligent user telegraph G2 and G3 facsimile G4 facsimile Hybrid mode Videotext Videophone

5.2.3 Supplementary Services

Supplementary services are the extra functions provided by a network to complement bearer services and user terminal services.




5-3

Supplementary services must be provided together with a bearer service or user terminal service.

ISUP provides the following supplementary services:

Call forwarding–unconditional (CFU) Call forwarding–busy (CFB) Call forwarding–no answer (CFNA) Call forwarding–default (CFD) Calling number identification presentation (CNIP) Calling number identification restriction (CNIR) Calling number identification restriction over (CNIR-Over) Call waiting (CW) Call transfer (CT) Three-way calling (3WC) Conference calling (CC) Remote feature control (RFC) Subscriber PIN access (SPINA) Subscriber PIN intercept (SPINI) Do not disturb (DND) Preferred language (PL) Call forwarding to voice mailbox Voice message retrieval (VMR) Message waiting notification (MWN) Subscriber lock Feature code service

5.3 ISUP Messages

ISUP messages are transferred in the form of MSU. The structure of ISUP messages is similar to that of SCCP messages.

The following details the structure and coding of ISUP messages.

5.3.1 Format of ISUP Messages

ISUP messages are transferred in a signaling link through MTP layer.

The SIF of ISUP messages is in an octet field stack, as shown in Figure 5-2.




5-4

Routinglabel

Circuit Identification Code (CIC)

Message type code

Mandatory fixed part (F)

Mandatory fixed part Fn

End of optional parameter field (00)

12345678

Bit sending sequence

Pointer (Location of parameter V1)

Pointer (Location of parameter Vn)

Mandatory variable part V1 Parameter length indicatorParameter content

Pointer (Start location of optional parameter group)

Optional parameter O1

Mandatory variable part Vn Parameter length indicatorParameter content

Parameter contentParameter length indicator

Parameter name

Optional parameter OnParameter content

Parameter length indicatorParameter name

Comm

on pa

rtSp

ecial

part

Octet

send

ing se

quen

ce

Figure 5-2 ISUP message structure

A ISUP message consists of the following parts:

Routing label Circuit identification code (CIC) Message type code Mandatory fixed part Mandatory variable part Optional parameters

In message transmission, the system first transfers the routing label, and then the optional part. Each byte is transmitted starting from the least signification bit.

5.3.2 Encoding of ISUP Messages

Each ISUP message consists of a number of parameters. Each parameter is allocated a name and is encoded according to bytes.

The length of a parameter can be fixed or variable.

Each variable parameter contains a length indicator indicating the number of bytes in the parameter. A length indicator occupies a byte.

I. Routing Label

Figure 5-3 shows the format of the routing label in an ISUP message.




5-5

SLS OPC DPC

Bits sent first 4 4 24 24

Figure 5-3 Format of routing label in the ISUP message

DPC: Destination signaling point code.

OPC: Originating signaling point code.

SLS: Signaling link selection code used for load sharing. At present, only the least significant four bits are used.

II. CIC

CIC is used for the connection between originating and destination signaling points. At present, the least significant 12 bits are used, and the remained 4 bits are spare (0000).

Figure 5-4 shows the structure of CIC.

CIC (Least significant bit)

Spare

12345678

CIC (Most significant bit)

1

2

Figure 5-4 Format of the CIC in a ISUP message

III. Message Type Code

Table 5-1 describes the encoding of ISUP messages.

A message code defines the function and format for an ISUP message.

Table 5-1 Encoding of ISUP messages

Message type Abbreviation Code

Address Complete ACM B00000110

Answer ANM B00001001

Blocking BLO B00010011

Blocking Acknowledgement BLA B00010101

Call Progress CPG B00101100

Circuit Group Blocking CGB B00011000

Circuit Group Blocking Acknowledgement CGBA B00011010

Circuit Group Query CQM B00101010




5-6


Circuit Group Query Acknowledgement CQA B00101011

Circuit Group Reset GRS B00010111

Circuit Group Reset Acknowledgement GRA B00101001

Circuit Group Unblocking CGU B00011001

Circuit Group Unblocking Acknowledgement CGUA B00011011

Charge Information CRG* B00110001

Confusion CFN B00101111

Connect CON B00000111

Continuity COT B00000101

Continuity Check Request CCR B00010001

Facility FAC B00110011

Facility Accepted FAA B00100000

Facility Reject FRJ B00100001

Facility Request FAR B00011111

Forward Transfer FOT B00001000

Identification Request IDR B00110110

Identification Response IRS B00110111

Information INF B00000100

Information Request INR B00000011

Initial Address IAM B00000001

Loop Back Acknowledgement LPA B00100100

Network Resource Management NRM B00110010

Overload OLM B00110000

Pass Along PAM B00101000

Release REL B00001100

Release Complete RLC B00010000

Reset Circuit RSC B00010010

Resume RES B00001110

Segmentation SGM B00111000

Subsequent Address SAM B00000010

Suspend SUS B00001101

Unblocking UBL B00010100




5-7


Unblocking Acknowledgement UBA B00010110

Unequipped Circuit Identification Code UCIC B00101110

User Part Available UPA B00110101

User Part Test UPT B00110100

User-to-User Information USR B00101101

Operator Information OPR B11111110

Metering Pulse Message MPM B11111101

Calling Party Clear Information CCL B11111100

Note:

The item marked with "*" is not used at present. The code Bxxxxxxxx indicates binary Xxxxxxxx.

IV. Mandatory Fixed Part (F)

This part contains those parameters that are mandatory and of fixed length.

The position, length and order of the parameters are uniquely defined by the message type; thus, the names of parameters and the length indicators are not included in the message.

V. Mandatory Variable Part (V)

This part contains mandatory parameters of variable length in a message.

Pointers are used to indicate the beginning of a parameter.

The name of a parameter and the order in which the pointers are sent is implicit in the message type. Therefore, the message type define both the number of parameters and the number of pointers.

VI. Optional Part (O)

The optional part consists of parameters that may or may not occur in a message.

This part may include fixed length and variable length parameters.

Each optional parameter includes the parameter name and a length indicator followed by parameter contents.

"End of optional parameters" octet contains all zeros.




5-8

5.3.3 Example of ISUP Messages

Below is the description of ISUP messages by using the example of the Initial Address (IAM) .

I. Introduction to IAM

The IAM can contain at most 35 parameters, including 5 mandatory fixed parameters, 1 mandatory variable parameter, and 29 optional parameters.

The IAM contains called party address information, and other information related to call connection control.

Table 5-2 describes the parameters of the IAM.

Table 5-2 Parameters of IAM

Parameter name Type Length Remarks

Message Type F 1 B00000001

Nature of connection indicators F 1 Mandatory fixed

Forward call indicator F 2 Mandatory fixed

Calling party category F 1 Mandatory fixed

Transmission medium request F 1 Mandatory fixed

Called party number V 4-11 Mandatory optional

Transit network selection O 4-? Optional

Calling party number O 4-12 Optional

Optional forward call indicator O 3 Optional

Redirecting number O 4-12 Optional

Redirection information O 3-4 Optional

Closed user group interlock code O 6 Optional

Original called number O 4-12 Optional

User-to-user information O 3-131 Optional

Access transport O 3-? Optional

User service information O 4-13 Optional

User-to-user indicators O 3 Optional

Generic number O 5-13 Optional

Propagation delay counter O 4 Optional




5-9

Parameter name Type Length Remarks

User service information O 4-13 Optional

Network dedicated performance O 4-? Optional

Generic digits O ? Optional

Originating International Switching Center (ISC) point code O 4 Optional

Future terminal service information O 7 Optional

Parameter compatibility information O 4-? Optional

Generic notification O 3 Repeated

Transmission medium requirement O 3 Optional

Location number O 5-12 Optional

End of optional parameters O 1 Optional

II. Encoding of IAM Parameters

Message Code

00000001

Nature of Connection Indicators

This is a mandatory fixed parameter with one octet field (A–F). Table 5-3 gives the codes of the parameter.

Table 5-3 Code of the nature of connection indicators

Satellite indicator

00 No satellite circuit in the connection

01 One section of satellite circuit in the connection

10 Two sections of satellite circuit in the connection

BA

11 Spare

Continuity check indicator

00 Continuity check not required

01 Continuity check required in the circuit

10 Continuity check completed in the previous circuit.

DC

11 Spare




5-10

Echo control device indicator :

0 Outgoing half echo control device not included. E

1 Outgoing half echo control device included.

F–H Spare

Forward Call Indicator

This is a mandatory fixed parameter with the length of two octets (A–M). Table 5-4 gives the codes of the parameter.

Table 5-4 Codes of forward call indicator

National/international call indicator

0 Call to be treated as a national call A

1 Call to be treated as an international call

End-to-end indicator

00 None (only section by section forward method available) CB

01 Transfer method available

End-to-end method indicator

10 SCCP method available CB

11 Transfer mode and the SCCP method available

Interworking indicator

0 Interworking not encountered (SS7 signal in all directions) D

1 Interworking encountered

End-to-end information indicator

0 No end-to-end information available E

1 End-to-end information available

ISDN User Part indicator

0 ISDN user part not used in all directions F

1 ISDN user part used in all directions




5-11

ISDN user part preference indicator:

00 ISDN user part preferred in all directions

01 ISDN user part not required in all directions

10 ISDN user part not required in all directions

HG

11 Spare

ISDN access indicator

0 Initial accessing non-ISDN I

1 Initial accessing ISDN

SCCP method indicator

00 No indicator

01 Connectionless method available

10 Connection-oriented method available KJ

11 Connectionless and connection-oriented methods available

L Spare

P–M Reserved for national use

Note:

Bits B–F and J–K constitute a protocol control indicator.

Calling Party Category

This is a mandatory fixed parameter. Different from the ISUP message of a fixed network, the calling party category is a one-octet field (H–A).

Table 5-5 gives the codes of the parameter.

Table 5-5 Codes of the calling party category

HGFEDCBA Description

00000000 Calling party category unknown (receive only)

00000001–00001000 Spare

00001001 Operator (no insertion function)

00001010 Ordinary subscriber, used between a mobile office and local office, and between a mobile office and tandem office

00001011 Preference subscriber, used between mobile offices




5-12

HGFEDCBA Description

00001100 Data call

00001101 Test call

00001110–11101111 Spare

11110000 Ordinary, Free, used between a mobile office and toll office (including international office)

1110001 Ordinary, periodic, used between a mobile office and toll office (including international office)

11110010 Ordinary, subscriber table, immediate (receive only from the local office or tandem office)

11110011 Ordinary, printer, immediate (receive only from local office or tandem office)

11110100 Preference, free, used between a mobile office and toll office (including international office)

11110101 Preference, periodic, used between a mobile office and toll office (including international office)

11110110–11111111 Spare

Other than the above mandatory parameters (including the mandatory variable parameter Called Party Number), the IAM also includes 29 optional parameters.

Optional parameters are selected in accordance with the basic services and supplementary services supported by the IAM.

For example, if the call transfer service exists, the IAM parameters shall include redirecting number, redirection information, original called number, and generic notification.

III. Example of IAM

Figure 5-5 shows an IAM message traced.

Figure 5-5 IAM message


85 Service indicator octet (SIO)

10------ Network indicator: National network (NAT)




5-13

--00---- Spare

----0101 Service indicator: ISUP

01 Message type: Initial Address (IAM)

00 01 Circuit identification code: 00 01

01 Signaling Link Selection: 01

11

00------ Spare

--010001 OPC: 11 E3

E3

36 66

00------ Spare

--110110 DPC: 36 66

00 00 Maintenance station reserve 2 bytes: 00 00

00 Nature of connection indicators

000----- Spare: 00

---0---- Echo control device indicator: outgoing half echo control device not included

----00- Continuity check indicator: continuity check not required

------00 Satellite indicator: no satellite circuit in the connection

Forward call indicators

20

00------ ISDN user part preference indicator: ISDN user part preferred in all directions

--1----- ISDN user part indicator: ISDN user part used in all directions

---0---- End-to-end information indicator: no end- to- end information available

----0--- Interworking indicator: no interworking encountered

-----00- End-to-end method indicator: no end- to- end method available

-------0 National/international call indicator: call to be treated as a national call

01

000----- Reserved for national use: 00

---0---- collect call indicator: not collect call

----0--- spare

-----00- SCCP method indicator: no indicator




5-14

-------1 ISDN access indicator

0A Calling party’s category: ordinary calling subscriber

00 Transmission medium requirement: speech

02 Pointer to mandatory variable part: 02

06 Pointer to start of optional part: 06

******Mandatory Variable part******

Called party number

04 Length indicator of Called party number: 04

81

1------- Odd/even indicator: odd number of address signals

-0000001 Nature of address indicator: subscriber number

90

1------- Internal network number indicator (INN ind): routing to internal network number not allowed

-001---- Numbering plan indicator: ISDN (Telephony) numbering plan (Recommendation E.164)

----0000 Spare

Address signal, F indicates ST, address complete

23 32F

0F

0000---- Filler: 0

----1111 F

******Optional part******

08 Optional forward call indicator

01 Length indicator of Optional forward call indicators: 01

00 Optional forward call indicators

0------- Connected line identity request indicator: not request

-0000--- spare: 00

-----0-- Simple segmentation indicator: no additional information will be sent

------00 Closed user group call indicator: non−CUG call

0A Calling party number

07 Length indicator of calling party number: 07

03




5-15

0------- Odd/even indicator: even number of address signals

-0000011 Nature of address indicator: national number

13

0------- Calling party number incomplete indicator (NI): complete

-001---- Numbering plan indicator: ISDN numbering plan

----00- Address presentation restricted indicator: presentation allowed

------11 Screening indicator: network provided


09 92 16 31 50 Calling party number 0929611305

1D User service information

03 Length indicator of User service information: 03

80

1------- Extension indicator: last octet

-00----- Coding standard: ITU-T standardized coding, as described below

---00000 Information transfer capability: speech

90

1------- Extension indicator: last octet

-00----- Transfer mode: circuit mode

---10000 Information transfer rate: 64kibit/s

A3 Other information: A3

31 Propagation delay counter

02 Length indicator of Propagation delay counter: 02

00 00 Propagation delay counter: 00 00

3F Location number

03 Length indicator of Location number: 03

83

1------- Odd/even indicator: odd number of address signals

-0000011 Nature of address indicator: national number

97

1-------- Internal network number indicator: routing to internal number not allowed

-001---- Numbering plan indicator: ISDN numbering plan

----01- Address presentation restricted indicator: presentation restricted.

------11 Screening indicator: network provided




5-16


0F

0000---- Filler: 0

----1111 F

00 End of optional parameter


SS7 Signaling SystemChapter 6 Transaction Capabilities Application Part


6-1

Chapter 6 Transaction Capabilities Application Part

This chapter introduces the concepts related to transaction capabilities application part (TCAP), as well as its functions and position in the SS7.

6.1 Introduction to TCAP

With the development of telecommunication networks, more services are demanded. Such services include the intelligent services like freephone (FPH) and virtual private network (VPN) , as well as the operation, administration, maintenance and provision (OAM&P) and mobile application part (MAP).

These services and applications are irrelevant to call control. That is, message transfer functions are separated from call control functions. They are provided on the basis of the correlation between:

Exchanges Exchanges and network service centers Subscribers and network service centers

To address these demands, the transaction capability (TC) protocol is applied.

Transaction capabilities are functions that control non-circuit-related information transfer between two or more signaling nodes through the SS7. They serve as the interface between several applications and one particular service.

The TC protocol provides general standards for the applications as a whole instead of for a particular application.

The TC consists of transaction capability application part (TCAP) and intermediate service part (ISP).

The former corresponds to Layer 7 of the OSI model, and the latter, Layers 4–6.

In the CDMA system only TCAP is involved. That is, TCAP is directly involved in data transfer.

Figure 6-1 shows the position of TCAP in the SS7 network.




6-2


TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR


TCAP

ISP

SCCP

MTP-3

MTP-2

MTP-1

HLR VLR

BSAP

L 1

L 2

L 3

L 4 - L6

L 7

Figure 6-1 Position of TCAP in the SS7 network

Currently there are two TCAP standards:

TCAP defined by International Telecommunication Union - Telecommunication Standardization Sector (ITU-T)

TCAP defined by American National Standard Institute (ANSI)

CDMA system uses the latter standard.

6.2 TCAP Structure

TCAP is divided into

Component sublayer (CSL) : responsible for operation administration Transaction sublayer (TSL) : responsible for transaction administration

The CSL communicates with TC user over TC primitive interface and with the TSL over TR primitive interface.

Figure 6-2 shows the structure of TCAP.

CSL

TC User A

TCAP

TC User B

TCAP

SS7 NetWorks

TC-Primitive

TR- Primitive

N- Primitive

TSL

TC-Primitive

CSL

TSL

TR- Primitive

N- Primitive

Figure 6-2 Structure of TCAP




6-3

6.2.1 Transaction Sublayer

TSL is responsible for the signaling exchange between TC users and the transaction management in this process. A user at this layer is referred to as a TR user. Currently the only TR user defined is the CSL.

Communication between the CSL of the same level (also the TC users of the same level) is referred to as a session.

A session is the process of TCAP message exchange between two TC users performed in the signaling exchange to realize the provisioning of a particular service.

The initiation and termination of message exchange and the sequence of messages exchanged are controlled and explained by TC users.

TSL is responsible for the management on the initialization, proceeding and termination of sessions, as well as the detection of errors and subsequent troubleshooting during sessions.

The protocol applied is also applicable to all application service related sessions (that is, transactions). Therefore, in ANSI TCAP, transaction and session are regarded as equivalent concepts.

In ANSI TCAP, two sorts of sessions are defined: layered and non-layered sessions. This definition is made from the point of view of session management, and does not involve actual applications.

I. Non-Layered Session

A non-layered session is similar to a connectionless transfer of SCCP. It contains only a unidirectional TCAP message sent by the local end and no reply is expected.

There is no division of initialization, proceeding, and termination phases for a session of this type. That is, no TCAP transaction is established.

II. Layered Session

A layered session contains three phases: initialization, proceeding (also TCAP message exchange), and termination.

Similar to connection-oriented data transfer, these three phases are initiated by TC users.

More than one session can proceed between two TC users, and each of these sessions is marked with a unique ID.

6.2.2 Component Sublayer

CSL consists of the dialog portion and component portion, respectively responsible for the control of sessions and the processing of components.

A TCAP message exchanged in a session contains one or several components, each of which reflects the request for the execution or results of a particular operation. In




6-4

some cases such a message may contain no component and this type of messages are responsible for the control of the session only.

Each component is marked with a unique Invoke ID, and is invoked by this ID with other components.

The Invoke ID is defined only for CSL to recognize different components and perform monitoring and management on the components. That is, the Invoke ID of a component must be perceived differently from an operation code that is defined by a TC user.

The indication of an Invoke ID is determined by the actual application, and will not be analyzed or processed by TCAP.

6.3 TCAP Messages

The encoding of TCAP message complies with the specifications in abstract syntax notation one (ASN.1).

6.3.1 Encoding of TCAP Messages

All information elements (IE) are formed in the designated “Tag + Length + Contents” format, as shown in Figure 6-3.

IE

Tag

Length

Contents

Figure 6-3 Structure of TCAP IE

I. Tag

The tag identifies an IE and describes the contents of the IE.

A tag is composed of class, form, and tag code.

A tag may contain one or several octets. See Table 6-1 for the structure of a tag.

Table 6-1 Structure of TCAP message tag

BIT7 BIT6 BIT5 BIT4 BIT3 BIT2 BIT1 BIT0

Class Form Tag Code

Class

00: Universal




6-5

01: Application wide

10: Context specific

11: Private use

Form

0: Primitive

1: Constructor

Tag Code

A tag code is formed by taking out the bits 0–4 in the tag when the tag contains only one octet.

When the tag has extension octets, the tag code is formed by taking out bits 0–4 from the first octet and adding it to the extension octets.

In a tag that contains only one octet, the range for the tag code is 00000–11110, as shown in Figure 6-4.

Class Form Tag Code(00000 - 11110)

Figure 6-4 The format of a tag containing one octet

In a tag that contains more than one octets, suppose bits 0–4 in the first 8 bits are “11111” (binary), that is, “0X9F” (hexadecimal).

If the first bit (bit 0) in the second octet is “1” (for example, this octet is “0X81”), it indicates that there is another octet that follows.

If the first bit (bit 0) in the second octet is “0” (for example, this octet is “0X02”), it indicates that this is the last octet of the tag.

Class Form Tag Code1 1 1 1 1

Ext1 MSB

Ext0 LSB

Figure 6-5 The format of a tag containing more than one octets

II. Length

The length field is coded to indicate the number of octets in the contents of an IE. That is, it does not include the tag field or the length field itself.

Short Form




6-6

When the contents in an IE are shorter than 128 octets, the short form is then used. The length contains one octet, with bit 7 set to “0”. See Figure 6-6.

Length of contentsMSB LSB

0 Length of contentsMSB LSB

0

Figure 6-6 Length of contents -- the short form

Long Form

When the contents are longer than 127 octets, the long form is used.

Bit 7 of the first octet is coded “1”.

Bits 0 to 6 of this octet encode a number less than the size of the length whose most significant bit (MSB) and least significant bit (LSB) are bits 0 and 6 respectively.

The length of the contents is encoded from the second octet on, with the MSB as the bit 7 of the second octet and the LSB as the bit 0 of the last octet. See Figure 6-7.

1 Size of length - 1 MSB LSB

MSB

Length of contents

LSB

Figure 6-7 Length of contents – the long form

Indefinite Form

The indefinite form is one octet long. It has the fixed value 10000000, serving as the tag of the indefinite code instead of indicating its length.

Indefinite form is applicable to IEs of any length. The maximum length of an IE is determined by the maximum length of SCCP messages.

Indefinite form can be used to replace long or short form when the IE is a combination.

Examples

If the contents of the TCAP message are within 0x00 (hexadecimal) to 0x7F (hexadecimal) octets long, the length contains one octet.

If the contents of a TCAP message are 0x80 (hexadecimal) octets long, the length contains two octets, that is, 0x81 0x80.

If the contents of a TCAP message are 0x90 (hexadecimal) octets long, the length contains two octets, that is, 0x81 0x90.




6-7

If the length is 0x82 0x01 0x00, the contents of the TCAP message is 0x01 00 (hexadecimal) octets long, that is, 256 (decimal) octets long.

Note:

In ANSI standards, an IE with the length “0” does not have any contents. While in ITU-T standards, an IE with the length “0” is one that does not actually exist.

III. Contents

Contents are the substance of an IE, containing the primary information the element is intended to convey.

IEs are classified into atomic IE and constructor IE.

The contents in an atomic IE are inseparable.

The contents in a constructor IE contain other IE who has a similar structure. The length of a constructor IE is the integer multiple of octet.

6.3.2 Format of TCAP Messages

A TCAP message includes three portions: transaction portion, dialog portion, and component portion, as shown in Figure 6-8.

Transaction Portion

Dialog Portion

Component Portion

Figure 6-8 TCAP message structure

The transaction portion is mandatory, and the dialog portion and component portion are optional. However, either the dialog portion or the component portion (or both) must be present in a TCAP message.

6.3.3 Transaction Portion

The following introduces the elements of transaction portion.

I. Package Type Identifier

The package type identifier is used to differentiate the TCAP package type in a TCAP message. One byte is used to indicate the package type identifier.




6-8

Table 6-2 lists the TCAP package types and codes.

Table 6-2 TCAP package type identifier

Package Type Code Description

Unidirectional 0xE1This transaction package type sends information in one only direction without reply. No TCAP transaction is established.

Query with permission 0xE2

This transaction package type initiates a TCAP transaction and informs the destination node (that is, the node that receives the message) that it may end the TCAP transaction.

Query without permission 0xE3

This transaction package type initiates a TCAP transaction and it informs the destination node that it may not end the TCAP transaction.

Response 0xE4 This transaction package type ends the TCAP transaction.

Conversation with permission 0xE5

This transaction package type is the continuation of the TCAP transaction and it informs the destination node that it may end the TCAP transaction.

Conversation without permission

0xE6This transaction package type is the continuation of the TCAP transaction and it informs the destination node that it may not end the TCAP transaction.

Abort (P-Abort) 0xF6

This transaction package type informs the destination node that the source node has terminated the established TCAP transaction without sending any pending components that may be expected due to a prior message.

Abort (User-Abort) 0xF6

This transaction package type informs the destination node that the source node has terminated the established TCAP transaction without sending any pending components that may be expected due to a prior message.

The Unidirectional is adopted only by T-ANSWER and O-ANSWER in the provisioning of CDMA IN services.

Table 6-3 shows the correspondence between TCAP package type and IE.




6-9

Table 6-3 Correspondence between TCAP package type and cell

Package type IE

IE name Type Tag

Unidirectiona

l

Query w

ith perm

ission

Query

without

permission

Response

Conversation

with

permission

Conversation

without

permission

Abort

(p-abort)

Abort

(user-abort)

Transaction ID

Constructor

0xE1/0xE2/0x E3/0x E4/0xE5/0xE6/0xF6

M M M M M M M M

P-Abort Cause

Primitive

0XD7

None None None No

ne None None M None

User-Abort Cause

Primitive

0XD8

None None None No

ne None None None M

Dialog Portion

Constructor 0XF9 O O O O O O Non

e O

Component Sequence

Constructor

0XE8 M O O O O O Non

e Non

e

Note:

The dialog portion and component portion do not belong to the transaction portion. This table only describes the IEs contained by each TCAP package type.

II. Total TCAP Message Length

This length field indicates total message length.

III. Transaction ID

The transaction ID includes the originating transaction ID and responding transaction ID. They are used to realize simultaneous interaction of several transactions between two entities. If they two appear together, the responding transaction ID is always presented after the originating transaction ID.




6-10

IV. P-Abort Cause

This ID indicates the cause of a P-Abort message, as described in Table 6-4.

Table 6-4 P-Abort causes

Cause Cause description Code

Unrecognized Package Type

The package type tag is not included in Table 6-2. 0x01

Incorrect (Mistyped) Transaction Portion

The transaction portion is incorrectly tagged. 0x02

Badly Structured Transaction Portion

Basic errors such as length error occurred to the codes of the transaction portion.

0x03

Unassigned Responding Transaction ID

The transaction ID received is mismatched with the ongoing transaction.

0x04

Permission to Release Problem P-abort cause is not clear currently. 0x05

Resource Unavailable The resources at the TSL are insufficient for establishing transactions. 0x06

Unrecognized Dialog Portion ID The ID for the dialog portion is incorrect. 0x07

Badly Structured Dialog Portion

Some codes are lost for the dialog portion. 0x08

Missing Dialog Portion The mandatory dialog portion is lost. 0x09

Inconsistent Dialog PortionThe contents of the dialog portion are mismatched with the status of the transaction.

0x0A

V. User-Abort Cause

This IE indicates the cause of TC user aborting transaction.

VI. Dialog Portion

The dialog portion does not belong to the transaction portion and it is a part of component sublayer. For details, refer to section 6.3.4 "Dialog Portion".

VII. Component Sequence

Actually the component sequence is not a part of the transaction portion. It is a part of the component portion used to indicate the sequence of one or several components.

The components are processed in the sequence that they are received. ANSI-41 standard requires that each transaction correspond to an individual component only.




6-11

6.3.4 Dialog Portion

Table 6-5 lists the IEs contained in the dialog portion.

Table 6-5 IEs contained in the dialog portion

Dialog Portion Optional/Mandatory

Protocol Version Optional

Integer Application Context Optional

User Information Identifier Optional

Integer Security Context Optional

Confidentiality Identifier Optional

6.3.5 Component Portion

The following introduces the elements of component portion.

I. Component Sequence Identifier

This filed identifies the component sequence and is coded “E8”.

II. Component Sequence Length

This filed encodes the total length in octet of the component sequence.

III. Component Type Identifier

This field encodes the type of the component.

Table 6-6 gives the correspondence between component types and component type identifiers.

Table 6-6 Correspondence between component types and component type identifiers

Component Types Component Type Identifier Remarks

Invoke (Last) 0xE9 Used to request for the invocation of remote application process.

Return Result (Last) 0xEA Used to request for the return of results for particular application.

Return Error 0xEB Used to report the invocation failure of particular application.

Reject 0xEC Used to report the status of a Component or Transaction of being accepted or rejected.




6-12

Component Types Component Type Identifier Remarks

Invoke (Not Last) 0xED Used to request for the initiation of a remote application.

Return Result (Not Last) 0xEE Used to return the results for

particular application.

Table 6-7 lists the IEs contained in various types of components.

Table 6-7 Correspondence between component types and IEs

Component TypeIE

IE name IE type Tag

Invoke Return Result Return Error Reject

Component ID Primitive 0XCF M M M M

Operation Code Primitive 0XD0/0XD

1 M None None None

Error Code Primitive 0XD3/0XD4 None None M None

Problem Code Primitive 0XD5 None None None M

Parameter Constructor 0XF2 M M M M

IV. Component Length

This IE indicates the length of the component (excluding the fields of component type identifier and component length).

V. Component ID

The component ID identifies a particular component. It correlates the Invoke of an operation and the Response. It is mandatory only if an invoke ID is present in the corresponding invoke.

Invoke ID: Assigned by the initiator of an operation. Correlation ID: Received from the Invoke ID in the response to the initiation of an

operation.




6-13

VI. Operation Code

The operation code indicates the operation to be initiated by a TC user. It is application specific and is unexamined by the TCAP. It can be National or Private. For more information, refer to ANSI-T1.114.

VII. Error Code

The error code provides the reason why a specific operation could not be completed. It is application specific and is unexamined by the TCAP. For more information, refer to ANSI-T1.114.

VIII. Problem Code

This field indicates the reason the component or transaction portion was rejected.

IX. Parameter

It indicates a particular parameter. It is application specific and is unexamined by the TCAP.

6.3.6 Example of TCAP Messages

Figure 6-9 gives an example of a TCAP message: the remote user interactive directive (RUIDIR) message.

Figure 6-9 RUIDIR message

The following describes the values in this message:

83 Network Indicator

10------ Network Indicator: National

--00---- Spare

----0011 Service Indicator: SCCP

00 Invalid Bit

00 00 Circuit Identification Code

01 Signaling Link Selection Code

77 77 77 Originating Signaling Point Code




6-14

FE 55 55 Destination Signaling Point Code

09 Unit Data (UDT) Message Type

00 Return upon Absence of Class 0 Protocol (Non-Sequential Connectionless Service) Setting

03 Mandatory Variable Pointer 1

10 Mandatory Variable Pointer 2

1D Mandatory Variable Pointer 3

0D 12 08 00 61 04 64 00 03 59 5515 00 F0 Called Address 460030955551000

0D 12 06 00 60 04 64 00 03 19 11 01 00 F0 Calling Address 460030911110000

3A SCCP User Data Length Indicator (Length: 58 bytes).

E6 TCAP Message Package Type: CONVERSATION WITHOUT PERMISSION

38 TCAP Message Length

C7 Transaction ID

08 Transaction ID Length

59 05 01 38 9A 00 01 5C Transaction ID Contents

E8 Component Sequence Identifier

2C Component Sequence Length

E9 Component Type Identifier (Invoke Last)

2A Component Type Identifier Length

CF Component Type Identifier

01 Component Identifier Length

4D Contents

D1 Operation Code Identifier

02 Operation Code Length

09 0D Operation Code Contents: Remote User Interactive Directive (compliant with ANSI-41 protocol).

The rest of the parameters are MAP related. Refer to Chapter 7 "Mobile Application Part" for details.


SS7 Signaling SystemChapter 7 Mobile Application Part


7-1

Chapter 7 Mobile Application Part

This chapter introduces the concepts related to mobile application part (MAP), as well as its functions and position in the SS7.

7.1 Introduction to MAP

MAP is a specialized functional entity for a public land mobile network (PLMN) to achieve intra-network and inter-network connections. MAP specifies intersystem data transfer between the network entities in a CDMA network to support mobile roaming. These network entities include:

Mobile switching center (MSC) Visitor location register (VLR) Home location register (HLR) Authentication center (AC) Message center (MC) Service control point (SCP)

Figure 7-1 presents the interfaces between these entities.

MS BSS MSC/VLR

MC

MSC/VLR

HLRSCP

A

T1C/D

E

Q

MS: Mobile station BSS: Base station subsystem

Figure 7-1 CDMA network architecture

Except for the A interface, all interfaces in the CDMA network can transmit MAP messages.

The following describes these interfaces.

A interface

The A interface is between the network subsystem and base station subsystem. This interface carries messages related to MS management, BTS management, mobility management, call processing, and so on.

B interface




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B interface is VLR to MSC interface. Through this interface, the MSC requests location information from the VLR and notifies the VLR to update the location information of a MS. This interface also carries supplementary services operation messages.

C interface

C interface is MSC to HLR interface. In a mobile terminated call, the GMSC obtains roaming number from the HLR through the C interface. In a mobile terminated short message service, the MC obtains, over this interface, the number of serving MSC from HLR through GMSC.

D interface

D interface is VLR to HLR interface. Over this interface, VLR and HLR exchange MS location and subscriber management information to ensure that the subscribers in the serving area can make and receive calls normally.

E interface

E interface is between two MSCs, controlling the handoff of MSs between to neighbor MSCs. E interface carries messages between the MSCs to initiate and implement handoff operations.

Note:

In actual implementation, MSC and VLR are usually integrated into one physical entity. As a result, the B interface becomes an internal interface. C interface and D interface may share physical links.

7.2 MAP Functions

MAP implements intra-PLMN and inter-PLMN interworking functions and operations. This section describes the MAP functions.

7.2.1 MAP Management Functions

MAP location and data management are the basic functions of CDMA network.

The functions include:

Realizes automatic roaming and roaming restrictions. Provides user data for other services. Maintains data consistency between VLR and HLR. Protects network resources from being accessed by illegal users.

MAP enables intersystem information transfer in the following procedures in a CDMA network.




7-3

I. Location Management

MAP involves in the following operations:

Power-up registration: Used in user location registration, qualification authentication, and user data access

Power-down registration Default registration: Used in the registration in the case of no user data available

when a call is originated Location cancellation Qualification request: Obtains the subscriber service list and qualification period Qualification directive: Maintains consistency of subscriber service list between

HLR and MSC. Bulk deregistration Unreliable roaming

II. Authentication Management

The purpose of MAP authentication management is to prevent illegal subscribers from accessing the system. This includes the authentication for location registration, mobile originated calls, and mobile terminated calls. It also includes the subscriber authentication and periodic authentication performed by the authentication center (AC) using its algorithms.

III. Handoff Management

Handoff management function enables subscribers to move freely without affecting the connection quality. MAP handoff management complies with protocols, ensuring the interconnection between equipment of different suppliers and roaming in different MSCs.

Handoff management function includes:

Basic handoffs, namely, handoff forward, handoff backward, and handoff to a third party.

Transparent signaling transmission after handoff MAP circuit management

IV. Call Functions

MAP call functions include:

Origination request: Obtains calling subscriber data from the HLR or SCP. Location request: Obtains location information of the called party from the HLR. Forwarding request: Obtains the forwarding number.




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V. Supplementary Service Support

MAP supports various call-related and non-call-related supplementary services, such as conference calling. MAP can identify and support feature operations intended to the service control point (SCP) .

VI. Intelligent Services

MAP supports the following intelligent services:

Intelligent control

MAP applies for processing related to intelligent services and obtains related data such as Trig Type, TOD and TDO. The data is then forwarded by the MAP to the SCP. The processing result is transmitted from the SCP to the MAP and forwarded to the related call module.

MAP can also receive call control instructions initiated by the SCP and forward the instructions to the related call module. The processing result is passed to MAP and forwarded to SCP.

SCP-based forwarding services

Intelligent subscribers can subscribe to forwarding service at the SCP. Upon receiving instructions from the call module, the MAP sends a TBUSY or TNOANS message to the SCP to obtain the forwarding number and forwards the information to the call module to continue the forwarding operation.

Pre-paid charging (PPC)

In a PPC service, MAP restores the MSC or SCP in case of abnormality. When the MSC is recovered, the MAP initiates a recovery operation on the related SCP. It also transmits call records after the SCP sends an abnormality recovery request.

Intelligent announcement playback

MAP supports the intelligent peripherals module (IPM) in the MSC and a standalone intelligent peripheral (IP) .

7.2.2 MAP Operations

Implementation of each MAP function contains several operations. Each operation is defined by a set of elements including:

Operation name, code, and type Invoke parameter Success parameter Failure code and parameter Linked operations allowed

Table 7-1 lists the MAP operations.




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Table 7-1 MAP operations

Operation code Abbreviation Label

Handoff Measurement HANDMREQ 01

Facilities Directive FACDIR 02

Mobile On Channel MSONCH 03

Facilities Release FACREL 05

Qualification Request QUALREQ 06

Qualification Directive QUALDIR 07

Blocking BLOCKING 08

Unblocking UNBLOCKING 09

Reset Circuit RESETCKT 0A

Trunk Test TTEST 0B

Trunk Test Disconnect TTESTDISC 0C

Registration Notification REGNOT 0D

Registration Cancellation REGCANC 0E

Location Request LOCREQ 0F

Routing Request ROUTREQ 10

Feature Request FEATREQ 11

Unreliable Roamer Data Directive UNRELDIR 14

MS Inactive MSINACT 16

Transfer To Number Request TRANUMREQ 17

Redirection Request REDREQ 18

Flash Request FLASHREQ 1A

Authentication Directive AUTHDIR 1B

Authentication Request AUTHREQ 1C

Base Station Challenge BSCHALL 1D

Authentication Failure Report AFREPORT 1E

Count Request COUNTREG 1F

Bulk Deregistration BULKDEREG 22

Handoff Measurement Request HANDMREQ 23

Handoff Back HANDBACK 25

Handoff To Third HANDTHIRD 26

Authentication Directive Forward AUTHDIRFWD 27




7-6

Operation code Abbreviation Label

Authentication Status Report ASREPORT 28

Information Directive INFODIR 2A

Information Forward INFOFWD 2B

Inter System Answer ISANSWER 2C

Origination Request ORREQ 2F

Random Variable Request RANDREQ 30

Remote User Interaction Directive RUIDIR 32

SMS Delivery Backward SMDBACK 33

SMS Delivery Forward SMDFWD 34

SMS Delivery Point To Point SMDPP 35

SMS Notification SMSNOT 36

SMS Request SMSREQ 37

MAP operations are classified into four categories:

Category 1 operations: Report is required regardless of the operation result. In the case of a successful operation, the result is reported; in the case of an unsuccessful operation, the error is reported.

Category 2 operations: Report is required only in the case of operation failure. Category 3 operations: Report is required only in the case of operation success. Category 4 operations: Report is not required.

For the sake of security, when MAP originates a remote operation, the operation time limit must be specified. If no report is received in the time limit, processing is as follows:

For categories 1 and 3, it is considered operation failure For categories 2 and 4, it is considered success.

Currently, category 4 only contains OANSWER and TANSWER operations. Other operations are all classified to category 1.

Note:

The above operation codes do not cover the newly added intelligent service operations.




7-7

7.3 MAP Messages

MAP messages are transferred based on the services provided by MTP, SCCP, and TCAP.

7.3.1 Format of MAP Messages

In the SS7, MAP messages are transmitted as part of TCAP messages. Figure 7-2 shows the structural relation between MAP and MTP messages.

MAP messageTCAP messageSCCPmessage

MTPmessage

Figure 7-2 Structural relation between MAP and MTP messages

MAP messages are coded in ASN.1 format. The message type is in one to one correspondence with the operation code in the TCAP component.

In message transmission, one MAP message corresponds to one invoke ID. The invoke ID is the unique identifier of a MAP message. Thus, a TCAP component can be translated to a MAP message based on the invoke ID.

7.3.2 Encoding of MAP Messages

MAP messages are encoded in the same way as TCAP messages. For detailed information, refer to section 6.3 "TCAP Message".

7.3.3 Example of MAP Messages

Take the Registration Notification message as an example. Figure 7-3 shows the message traced on a SS7 link of the MSC.

Figure 7-3 Registration Notification message traced on a SS7 link




7-8


83

10------ Network indicator: national network

--00---- Spare

----0011 Service indicator: SCCP

00 Invalid bits resulting from signaling link tracing

00 00 Circuit identification

05 Signaling link selection

FE 55 55 Originating signaling point code (OPC)

77 77 77 Destination point code (DPC)

09 Unit data message type (UDT)

80 Class 0 protocol (Non-sequential connectionless service), report required.

03 Mandatory variable pointer 1

10 Mandatory variable pointer 2

1D Mandatory variable pointer 3

0D 52 06 00 61 04 64 00 03 59 53 03 00 01 Called address 460030953530001

0D 12 07 00 61 04 64 00 03 59 5515 00 00 Calling address 460030955551000

74 SCCP user data part length indication, 116 bits

E2 TCAP Package Type: QUERY WITH PERMISSION

72 Total TCAP Message length

C7 Transaction ID identifier

04 Transaction ID length

98 00 01 5C Transaction IDs

E8 Component Sequence identifier

6A Component Sequence length

E9 Component Type identifier (Invoke Last)

68 Component length

CF Component ID identifier

01 Component ID length

4C Component IDs

D1 Operation Code

02 Operation Code length

09 0D Operation Code: Registration Notification (employing ANSI-41 standard)

F2 Parameter Set




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5F Parameter Set length

89 ESN

04 ESN Length

53 53 00 01 ESN content

88 MIN

05 MIN length

90 35 35 00 10 MIN content

95 MSCID

03 MSCID length

64 05 01 MSCID content

91 Qualification Information Code

01 Qualification Information Code length

03 Qualification Information Code content: Require approval from the roaming subscriber and service list.

96 System My Type

01 System My Type length

00 System My Type content

9F 20 PC_SSN

05 PC_SSN Length

05 55 55 FE 07 9F PC_SSN Content

67 Sender Identification Number

0C Sender Identification Number length

00 31 61 0F 64 00 03 59 55 15 10 00 00 Sender Identification Number content

9F 22 System Access Type

01 System Access Type length

03 System Access Type content

9F 31 System Capacity

01 System Capacity length

0F System Capacity content

9F 2F Terminal Type

01 Terminal Type length

20 Terminal Type content

9F 7B Transmission Capacity

02 Transmission Capacity length




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1F 30 Transmission Capacity content

9F 68 Message Address

0C Message Address length

00 31 61 0F 64 00 03 59 55 15 00 00: Message Address content

BF 82 18 WIN Capabilities

0C WIN Capabilities length

9F 82 15 Trigger Capability

03 Trigger Capability length

FF FF 1F Trigger Capability content

9F 82 19 WIN Operation Capability

01 WIN Operation Capability length

03 WIN Operation Capability content

7.4 Common MAP Procedures

This section details the flow of MAP messages in various procedures.

7.4.1 Location Registration

Figure 7-4 shows the transfer of MAP messages in location registration procedure.

BSS MSC/VLR A HLR/AC MSC/VLR B

CC

LA_UPDATE_REQ

REGNOT

REGCANC

regcanc

regnot

LA_UPDATE_ACC

CLEAR COMMAND

CLEAR COMPLETE

a

b

c

d

e

f

h

i

g

Figure 7-4 Location registration procedure

The following describes the MAP messages only.

REGNOT: The originating MSC (MSC/VLR A in the figure) sends a location registration request to the HLR through interfaces C and D.




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REGCANC: The HLR sends a registration cancellation message to the original VLR (VLR B).

regcanc: The original VLR sends response to the HLR after registration cancellation.

regnot: The HLR notifies the VLR A of the registration success.

7.4.2 Inter-Office Call

Figure 7-5 shows the transfer of MAP messages in an inter-office call.

BSS MSC HLR MSC BSS

CM SERV REQCC

LOCREQ

routreq(TLDN)

locreq

ASSIGNMENT

ASSIGN COMP

ASSIGNMENT

ASSIGN COMP

ANC

a

b

c d

f

g

p

q

CONNECT

ACMo

n

m

CC l

PAGING RSP k

PAGING REQ j

IAI (TLDN) i

h

e

ROUTREQ

Figure 7-5 Inter-office call procedure


LOCREQ: The originating MSC sends a called party location request to the HLR. ROUTREQ: The HLR sends a routing request to the serving MSC. routreq: The serving MSC returns a routing response containing the temporary

mobile directory number (TLDN). locreq: The HLR sends a location response to the originating MSC.

7.4.3 Handoff Forward

Figure 7-5 shows the transfer of MAP messages in handoff forward procedure.




7-12

Anchor&ServingMSC Target MSC BSC

HANDMREQ

handmreq

FACREQ

facreq

Handoff Order

MS arrives on new channel

MSONCH

Handoff Complete

a

b

c

d

e

i

Call in progress

f

g

h

Figure 7-6 Handoff forward procedure


HANDMREQ: The serving MSC determines whether handoff to an adjacent MSC is required using the internal algorithm. It sends a Handoff Measurement Request (HANDMREQ) to adjacent MSCs to request a signal quality measurement.

handmreq: The adjacent MSC measures signal quality and reports the result to the serving MSC.

FACREQ: The serving MSC determines that handoff to the adjacent MSC (the target MSC) is required. It sends a FACREQ message to the target MSC to initiate the handoff forward.

facreq: If a free traffic channel is available in the designated cell, the target MSC adds "1" to the segment counter of the BillingID. The new BillingID will be employed for later billing. Notification that the request was accepted is reported to the serving MSC to start the handoff forward.

MSONCH: The target MSC sets up a traffic channel and the connection of trunk circuit to the serving MSC. It then notifies the serving MSC the arrival of the MS on the new channel and the completion of related processes. The serving MSC then connects the call to the inter-MSC trunk circuit to complete the handoff.


SS7 Signaling SystemChapter 8 Base Station Application Part


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Chapter 8 Base Station Application Part

This chapter introduces the concepts related to base station application part (BSAP) , as well as its functions and position in the SS7.

8.1 Introduction to BSAP

BSAP is an application part based on A interface protocols. It fulfills the functions of A1 interface between MSC and BSC.

8.1.1 About the A Interface

The A interface, between an MSC and a BSC, consists of:

Signaling channel A1 interface: Transfers common channel signaling. User traffic channel A2 interface: Transfers voice, data, or unrestricted digital

information processed with pulse code modulation (PCM) . Duplex data channel A5 interface between the interworking function (IWF) and

selection/distribution unit (SDU): Transfers fax service and asynchronous data service traffic.

In the signaling system, the A interface generally refers to A1 interface. Established on the basis of SS7, the A interface is divided into three layers:

L1: Physical interface between neighboring nodes L2: Transmission layer. It consists of the MTP and SCCP, and ensures effective

transmission of data at application layer. L3: BSAP above the SCCP.

8.1.2 BSAP Functions

BSAP accomplishes the functions of the MSC to BSC interface. It consists of two parts:

BS management application part (BSMAP) Direct transfer application part (DTAP)

I. BSMAP

BSMAP supports all radio resource management and facility management procedures between the MSC and BSC.

BSMAP messages are not passed to a MS. They are used only to perform functions at the MSC or the BSC. Only one type of BSMAP messages (called complete L3 information) is used together with a DTAP message to establish a connection




8-2

between a MS, BSC, and MSC. For detailed description of the complete L3 information, refer to 3GPP2 specifications.

II. DTAP

DTAP messages are used to transfer call processing and mobility management messages between the BSC and MSC. BSC does not use DTAP messages, but converts the messages into appropriate messages to be transmitted on air interface.

Figure 8-1 shows the A interface protocol model.

A InterfaceBS side MSC side

Physical Layer

TransportProtocol(s)

BSAP

DTAP BSMAP

BSAP

DTAP BSMAP

TransportProtocol(s)

BS Base station BSAP Base station application part

BSMAP Base station management application part DTAP Direct transfer application part

MSC Mobile switching center

Figure 8-1 Reference model of A interface protocol stack

8.2 BSAP Messages

This section introduces the format and encoding of BSAP messages. An example is given to explain BSAM messages.

8.2.1 Format of BSAP Messages

One or two bytes in the message head (call message discrimination flag) is used to distinguish between DTAP messages and BSMAP messages. The subsequent bytes contain the length indicator and complete L3 information.

Figure 8-2 shows the structure of BSMAP and DTAP messages.




8-3


DLCI (Always set to 0)


DTAP Message Header BSMAP Message Header

octet 1 octet 1

octet 2

Length Indicator Length Indicatoroctet 3 octet 2

octets 3 to koctets 4 tok+1

BSAPMessageHeader

Layer 3MessageLength

Layer 3Message

APPLICATION

MESSAGE

APPLICATION

MESSAGE

Figure 8-2 BSAP message structure

BSMAP message

The message discrimination flag of a BSMAP message contains only the message discrimination parameter that is coded with one byte. If the message discrimination parameter is set to 0, the message is a BSMAP message.

The length indicator is of one byte, indicating the length of subsequent data.

DTAP message

The message discrimination flag of a DTAP message is of two bytes: message discrimination parameter and the data link connection identifier (DLCI) . If the message discrimination parameter is set to 1, the message is a DTAP message. The DLCI indicates the type and treatment of messages transmitted between the BSC and MSC. The DLCI is set to 0 for A1 interface.

The length indicator is of one byte, indicating the length of subsequent data.

8.2.2 Encoding of BSAP Messages

Each A interface message consists of a series of IES. In the following description, the nature of IEs are indicated as follows:

M: mandatory IEs O: optional IEs

Among optional IEs, R denotes required IEs and C denotes conditional IEs.

In the following description, it is conventional to adopt the following sequence to denote the bits and bytes. The bits in one byte are denoted with 0-7. Bit 0 is the least significant bit (LSB) and is transmitted first. Bit 7 is the most significant bit (MSB).

Bytes (or octets) are identified with numbers. Byte 1 is sent first, followed by byte 2, byte3, and so on. A variable-length IE contains one length indicator byte, indicating the length of subsequent IEs.




8-4

The IE includes two types: fixed-length IE and variable-length IE. The bytes contained in a fixed-length IE are predefined, and the length varies with the IE. For variable-length IEs, a length indicator follows the IE indicator. If the IE indicator is omitted (for mandatory IEs it can be omitted), the length byte is the first byte of the variable-length IE.

Four IE types are defined:

IEs with 1/2 octets of content (Type 1) IEs with 0 octets of content (Type 2) IEs with fixed length and at least one octet of content (Type 3) IEs with variable length (Type 4)

I. Type 1

Type 1 IE is a fixed-length IE with one byte. Table 8-1 shows the structure of a type 1 IE.

Table 8-1 Type 1 IE structure

7 6 5 4 3 2 1 0 Octet

1 IEI CIE 1

Bits 0, 1, 2, and 3 (that is, ½ byte) are content of information element (CIE).

Bits 4, 5, and 6 are the information element identification (IEI), except that 010 is used for type 2 IE.

Bit 7 is coded 1.

Type 1 IE can be the optional or a mandatory IE in a BSMAP or DTAP message.

II. Type 2

Type 2 IE is a fixed-length IE. It is of one byte without the CIE. Table 8-2 shows the structure of a type 2 IE.

Table 8-2 Type 2 IE structure

7 6 5 4 3 2 1 0 Octet

1 0 1 0 IEI 1

Bits 0, 1, 2, and 3 are the IEI.

Bits 4, 5, and 6 are coded 010, indicating that the IE is of type 2.

Bit 7 is coded 1.

Type 2 IE cannot be the mandatory IE in a DTAP message.




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III. Type 3

Type 3 IE contains the content of fixed length, followed by the CIE. It can be used as the mandatory IE in a DTAP message.

Optional IE

When the type 3 IE is optional, the IE contains the IEI. Table 8-3 shows the IE structure.

Table 8-3 Type 3 IE structure of type 3 (example 1)

7 6 5 4 3 2 1 0 Octet

0 IEI 1

LI 2

CIE 3

... ...

CIE n

Bits 0–6 in the first byte are the IEI.

Bit 7 in the first byte is 0.

Bytes 2–n denote are the CIE.

Mandatory IE

When the type 3 IE is mandatory, the IEI is omitted. Table 8-4 shows the IE structure.

All bytes denote the CIE.

Type 3 mandatory IE is used in a DTAP message.

Table 8-4 Type 3 IE structure (example 2)

7 6 5 4 3 2 1 0 Octet

CIE 1

CIE 2

... ...

CIE n

IV. Type 4

Type 4 IE contains the variable-length CIE.

Optional IE

When type 4 IE is optional, the IEI is contained. Table 8-5 shows the IE structure.




8-6

Table 8-5 Type 4 IE structure (example 1)

7 6 5 4 3 2 1 0 Octet

0 IEI 1

2

CIE 3

... ...

CIE n

Bits 0–6 in the first byte denotes the IEI.

Bit 7 in the first byte is coded 0.

The second byte is the length indicator, indicating the length of CIE (byte number).

Bytes 3–n denote the CIE.

Mandatory IE

When type 4 IE is mandatory, the IEI is omitted. Table 8-6 shows the IE structure.

Table 8-6 Type 4 IE structure o (example 2)

7 6 5 4 3 2 1 0 Octet

LI 1

CIE 2

... ...

CIE n

The first byte is the length indicator, indicating the length of CIE (byte number).

Bytes 2–n denote the CIE.

Type 4 mandatory IE is used in DTAP messages.

The A interface messages are constructed by the four types of IE described above according to a certain sequence. Generally, mandatory IEs are placed before optional IEs, and different IEs are aligned according to predefined sequence in different messages.

8.2.3 Example of BSAP Messages

The following gives examples of various types BSAP messages.




8-7

I. Complete L3 Information

Complete layer 3 information is sent from the BSC to the MSC upon receipt of the first message from a MS. This message contains a CM Service Request, a Paging Response, or a Location Updating Request message.

A complete layer 3 message contains message category, cell identity, and layer 3 message, as described in Table 8-7.

Table 8-7 Complete layer 3 message

IE Direction Type

Message category BS -> MSC M

Cell identity BS -> MSC M

Layer 3 information BS -> MSC M

II. CM Service Request

CM service request is sent from the BSC to the MSC to request for connection-oriented services (such as voice calls). This message contains such information as radio channel information, subscriber ID, requested service type, MS location, called number, and authentication parameters.

Table 8-8 lists the IEs in the message

Table 8-8 CM service request message

IE Element direction Type

Protocol discriminator BS -> MSC M

Reserved (octet) BS -> MSC M

Message type BS -> MSC M

CM service type BS -> MSC M

Classmark information type 2 BS -> MSC M

Mobile identity (IMSI) BS -> MSC M

Called party BCD number BS -> MSC O

Mobile identity (ESN) BS -> MSC O

Slot cycle index BS -> MSC O

Authentication response parameter (AUTHR) BS -> MSC O

Authentication confirmation parameter (RandC) BS -> MSC O

Authentication parameter COUNT BS -> MSC O

Authentication challenge parameter (RAND) MSC -> BS O




8-8


Service option BS -> MSC O

Voice privacy request (unnecessary in phase 1) BS -> MSC O

Called party ASCII number BS -> MSC O

Authentication event (the RAND and RANDC are mismatched at the BTS side) BS -> MSC O

Authentication data BS -> MSC O

Figure 8-3 gives a CM service request traced on a SS7 link.

Figure 8-3 Example of CM service request

The following describes the traced message. The following IEs are aligned in indent format, and not all IEs are contained. From the top down, they are:

1) MTP message header 2) SCCP message header 3) BSAP message header 4) BSAP message content

For the definition of bits in each IE, refer to 3GPP2 specifications.

83

10------ Network indicator: national network (2)

--00---- Spare1: (0)

----0011 Service indicator: SCCP (3)

00 00 00 Destination point code: (0)

00 00 00 Originating point code: (0)

00

0000---- Signaling link code: (0)

----0000 Spare2: (0)

01 SCCP connection request




8-9

00 00 00 Source local references: (0)

02

0000---- Spare: (0)

----0010 Protocol class: Class 2 (2)

02 Offset 1 : 2

07 Offset 2 : 7

05 Called party address

43

0------- National/International Indicator: no (0)

-1------ Routing Indicator: yes (1)

--0000-- Global Title Indicator: no Global Title included (0)

------1- Subsystem Number Indicator: yes (1)

-------1 Point Code Indicator: yes (1)

00 00 00 Signaling Point Code: (0)

00 Subsystem Number: Not known (0)

0F Data

41 Data length

00 BSMAP

3F BSMAP length

57 Complete L3 Information

05 Cell Identifier

03 Cell Identifier length

02 Cell Identifier discriminator: CI (2)

00 00 Cell: (0)

17 Layer 3 message

37 Layer 3 message length

03

0000---- Reserved4: 0

----0011 Protocol discriminator: call processing call related SS (3)

00 Reserved8: (0)

24 CM service request

91

1001---- CM service type

----0001 Mobile originating call establishment (1)




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0C Classmark information type 2 (length)

00

000----- mobile p rev: (0)

---0---- reserved1a: (0)

----0--- see list of entries: (0)

-----000 RF power capability: class 1 vehicle and portable (0)

00 Reserved8a: 0

40

0------- nar a cap: (0)

-1------ is 95: (1)

--0----- slotted: (0)

---00--- reserved2: (0)

-----0-- dtx: (0)

------0- mobile term1: (0)

-------0 reserved1b: (0)

00 Reserved8b: (0)

00

000000-- reserved6: (0)

------0- mobile term2: (0)

-------0 psi: (0)

00 SCM length: (0)

00 Station class mark: (0)

01 Count of band class entries: (1)

03 Band class entry length: (3)

00

000----- reserved3a: (0)

---00000 band class n: (0)

00

000----- reserved3b: 0

---00000 band class n air interfaces supported: 0

00 Band class n MS protocol level: 0

01 Mobile identity IMSI

0E




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00001--- identity digit: 0

-----110 type of identity: IMSI (6)

5E Called party BCD number

02 Called party BCD number length

80

1------- Ext:1

-000---- Type of number: unknown (0)

----0000 Numbering plan identification: unknown (0)

F0 Digit: 0F

0D Mobile identity ESN

05 Mobile identity ESN length

05

0000---- identity digit1: 0

----0--- odd even indicator: 0

-----101 type of identity: ESN (5)

00 00 00 00 ESN value: 0

42 Authentication response parameter AUTHR

04 Authentication response parameter AUTHR length

01

0000---- reserved4: 0

----0001 auth signature type: AUTHR (1)

00

000000-- reserved6: 00

------00

00 00 Authentication signature value: 0

28 Authentication confirmation parameter RANDC

00 00

40 Authentication parameter count

00

00------ reserved2:0

--000000 count: 00

41 Authentication challenge parameter

05 Authentication challenge parameter length




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01

0000---- reserved4:0

----0001 random number type: RAND (1)

00 00 00 00 Random value: (0)

03 Service option

80 00 Speech 13k (32768)

1D Radio environment and resources

00

0------- reserved1: 0)

-0------ include priority: 0

--00---- forward: not reported (0)

----00-- reverse: not reported (0)

------0- alloc: resources are not allocated (0)

-------0 avail: resources are not available (0)

4A T

01 Authentication event

01 Parameters not received (1)

00 EOP:00

III. Paging Request

Paging request is sent from the MSC to the BSC. It contains sufficient information to locate the cell serving the MS.

This message contains the location area identity (LAI) , Mobile identity, service type, slot cycle index, and so on. Table 8-9 describes the IEs in a paging request.

Table 8-9 Paging request message


Message type MSC -> BS M

Mobile identity (IMSI/ESN) MSC -> BS M

Tag MSC -> BS O

Cell identifier list MSC -> BS O

Slot cycle index MSC -> BS O

Service option MSC -> BS O




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IV. Paging Response

Paging response is sent from the BSC to the MSC when the BSC receives a page response from the MS.

The message contains the mobile identity, radio channel information, slot cycle index, service type, authentication parameter, and so on. Table 8-10 describes the IEs in a paging response.

Table 8-10 Paging request message


Protocol discriminator BS -> MSC M

Reserved (octet) BS -> MSC M


Classmark information type 2 BS -> MSC M

Mobile identity (IMSI) BS -> MSC M

Tag BS -> MSC O

Mobile identity (ESN) BS -> MSC O

Slot cycle index BS -> MSC O

Authentication response parameter (AUTHR) BS -> MSC O

Authentication confirmation parameter (RANDC) BS -> MSC O

Authentication parameter COUNT BS -> MSC O

Authentication challenge parameter (RAND) MSC -> BS O


Voice privacy request (unnecessary in phase 1) BS -> MSC O

Authentication event (when the RAND and RANDC are mismatched at the BTS side) BS -> MSC O

V. Connect

This Connect message is sent by the BSC to the MSC to indicate that the called mobile subscriber has accepted the call.

Table 8-11 lists the IEs of the message.




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Table 8-11 Connect message


Protocol discriminator BS <-> MSC M

Reserved (octet) BS <-> MSC M

Message type BS <-> MSC M

VI. Assignment Request

Assignment request is sent from the MSC to the BSC to request the later to assign radio resources. The message may include the terrestrial circuit to be used if one is needed for the call.

The message contains the circuit identification code, possible calling number, service option, emergency call identifier, and so on. Table 8-12 lists the IEs of the message.

Table 8-12 Assignment request message


Message type MSC -> BS M

Channel type MSC -> BS M

Circuit identification code MSC -> BS O

Encryption information (unnecessary in phase 1) MSC -> BS O

Service option MSC -> BS O

Signal MSC->BS O

Calling party ASCII number MSC->BS O

VII. Assignment Complete

This BSMAP message is sent from the BSC to the MSC, indicating that the requested assignment is completed.

Table 8-13 lists the contents in the message.

Table 8-13 Assignment complete messages



Channel number BS -> MSC M

Encryption information (unnecessary in phase 1) BS -> MSC O





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8.3 BSAP Procedures

BSAP protocol specifies the message format and procedures to support the wireless service functions between the BSC and MSC. Major A interface procedures include:

Mobile origination Mobile termination Call clearing Circuit management

This section introduces these procedures. For handoff procedure, refer to Technical Manual – System Function.

In the protocol stack, BSAP is above SCCP layer. BSAP uses two types of service provided by SCCP: connection-oriented service and connectionless service. Therefore, BSAP messages are carried by SCCP messages.

Table 8-14 lists the SCCP messages used by BSAP.

Table 8-14 SCCP messages used by BSAP

Service SCCP frame User data field (BSMAP/DTAP)

SCCP Connection Request (CR) Optional

SCCP Connection Confirm (CC) Optional

SCCP Connection Refused (CREF) Optional

SCCP Released (RLSD) Optional

SCCP Release Complete (RLC) Not applicable

Connection Oriented (CO) Protocol Class 2

SCCP Data Transfer 1 (DT1) Mandatory

Connectionless (CL) Protocol Class 0 SCCP Unit Data (UDT) Mandatory

For detailed description of the SCCP messages used by BSAP, refer to 3GPP2 specifications.

8.3.1 Location Update

A MS informs the MSC of current location (or parameter) change through the location update procedure as shown in Figure 8-4.




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Figure 8-4 Location update procedure

The procedure is as follows:

1) On receiving the update message from a MS, the BSC constructs a Location Updating Request, places it in the complete layer 3 information, and sends it to the MSC. The BSC then starts timer T3210.

2) The MSC sends a Location Updating Accept message to the BSC to indicate that the request has been processed. Upon receipt of the message, the BSC stops timer T3210.

8.3.2 Mobile Origination

When a MS sends a mobile origination service request, the BSC initiates the mobile origination procedure shown in Figure 8-5.

Figure 8-5 Mobile origination procedure


1) The BSC constructs a CM Service Request, places it in the Complete Layer 3 Information, and sends the message to the MSC. At the same time it starts timer




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T303. For circuit switched calls, the BSC may request the MSC to allocate a preferred terrestrial circuit.

2) If global challenge is used, the MSC will continue the call setup process while waiting for an authentication confirmation (If an authentication failure indication is received at the MSC, it may clear the call).

The MSC sends an Assignment Request to the BSC to request assignment of radio resources. This message includes information of terrestrial circuit, if a terrestrial circuit is to be used between the MSC and BSC. The MSC then starts timer T10.

If the BSC requests a preferred terrestrial circuit in the CM Service Request and the MSC supports that terrestrial circuit, the MSC will use the same terrestrial circuit. Upon receipt of the Assignment Request message from the MSC, the BSC stops timer T303.

3) After the radio channel and terrestrial circuit are set up, the BSC sends an Assignment Complete message to the MSC. This indicates that the calling party is in conversational state and hears the ringback tone or other prompt tones. The MSC stops timer T10 upon receipt of the Assignment Complete message.

8.3.3 Mobile Termination

Figure 8-6 shows the mobile termination procedure.

Figure 8-6 Mobile termination procedure




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1) The MSC finds that an incoming call terminates to a MS in its serving area and sends a Paging Request to the BSC to initiate a mobile terminated call setup scenario. The MSC then starts timer T3113.

2) The BSC constructs a Paging Response message, places it in the Complete L3 Information message, and sends the message to the MSC. After that, it starts timer T303.

The BSC may request the MSC to allocate a preferred terrestrial circuit in this message.

3) The MSC stops timer T3113 upon receipt of the Paging Response from the BSC.

The MSC sends an Assignment Request to the BSC to request assignment of radio resources. This message also includes the information of a terrestrial circuit. The MSC then starts timer T10.

If the BSC requested a preferred terrestrial circuit in the Service Request, the MSC will use the same terrestrial circuit. The MSC may also assign a different terrestrial circuit.

Upon receipt of the Assignment Request from the MSC, the BSC stops timer T303.

4) After the radio traffic channel and terrestrial circuit are established, the BSC sends an Assignment Complete message to the MSC. The MSC stops timer T10 upon receipt of the message, and starts timer T301 to wait for the response of the MS.

5) The BSC sends a Connect message to the MSC to indicate that the call has been answered by the MS. Now the call is considered established and is in conversational state. The MSC stops timer T301 upon receipt of the Connect message from the BSC.

8.3.4 Call Clearing

When a mobile subscriber stops conversation and hooks on the phone, the BSC initiates a clearing procedure (the procedure may also result from other events).

Figure 8-7 shows the call clearing procedure initiated by BSC.




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Figure 8-7 Call clearing initiated by BSC


1) In case of a radio channel failure (for example, due to MS power failure or hook-on) between the MS and the BSC, the BSC sends a Clear Request to the MSC. The BSC then starts timer T300 and waits for the Clear Command from the MSC.

2) The MSC starts timer T315 and sends a Clear Command to instruct the BSC to release associated dedicated resources (such as the terrestrial circuit). The BSC stops timer T300.

3) The BSC returns a Clear Complete message. The MSC stops timer T315 upon receipt of the message and releases the resources at MSC side.

The call clearing procedure initiated by the MSC contains only Clear Command and Clear Complete messages (without Clear Request message). The flowchart is omitted here.

8.3.5 Circuit Block/Unblock

The A interface circuit between the BSC and MSC are maintained through circuit management messages. Because circuit assignment is controlled by the MSC, the BSC must inform the MSC of the circuit status at BSC side.

I. Circuit Block

The MSC must be informed if any circuit at BSC side is out of service (for example, during circuit reset initiated by the MSC). In this case, the BSC sends a Block message to the MSC to inform the later that the circuit is not available any longer. This procedure can be initiated by BSC only.

Figure 8-8 shows the circuit block procedure initiated by the BSC.




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Figure 8-8 Circuit block procedure


1) The BSC sends a Block message to the MSC, containing the information of the circuit to be blocked. Then it starts timer T1.

2) The MSC returns a Block Acknowledge message, indicating that the involved circuit is blocked. On receipt of the message, BSC stops timer T1.

II. Circuit Unblock

When a blocked circuit becomes available again, the BSC sends an Unblock message to the MSC to inform the status change. This procedure can be initiated by BSC only.

Figure 8-9 shows the circuit unblock procedure initiated by the BSC.

Figure 8-9 Circuit unblock procedure


1) The BSC sends an Unblock message to the MSC, requesting the later to unblock the circuit. Then it starts timer T1.

2) The MSC returns an Unblock Acknowledge message to the BSC, indicating that the circuit is unblocked. On receipt of the message, BSC stops timer T1.




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8.3.6 Circuit Reset

The MSC initiates circuit reset when some circuits are abnormal, such as:

Abnormal release of SCCP connections Inaccessible signaling point becoming available Intermittent trunk line failures Manual maintenance operations

Circuit reset can be initiated either by BSC or by MSC.

I. BSC-Initiated Circuit Reset

Figure 8-10 shows the circuit reset procedure initiated by BSC.

Figure 8-10 BSC-initiated circuit reset


1) Once the BSC detects that one or more circuits become idle, it sends a Reset Circuit message to the MSC. Then it starts timer T12.

2) Upon receipt of the Reset Circuit message, the MSC returns a Reset Circuit Acknowledge message to indicate that the circuits are reset. On receipt of the message, BSC stops timer T12.

II. MSC-Initiated Circuit Reset

Figure 8-11 shows the circuit reset procedure initiated by MSC.




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Figure 8-11 MSC-initiated circuit reset


1) Once the MSC detects that one or more circuits become idle, it sends a Reset Circuit message to the BSC. Then it starts timer T12.

2) Upon receipt of the Reset Circuit message, the BSC returns a Reset Circuit Acknowledge message to indicate that the circuits are reset. On receipt of the message, MSC stops timer T12.

III. MSC-Initiated Circuit Reset Failure

Figure 8-12 shows the procedure when the BSC fails to reset the circuit as requested by MSC.

Figure 8-12 MSC-initiated circuit reset failure


1) Once the MSC detects that one or more circuits become idle, it sends a Reset Circuit message to the BSC. Then it starts timer T12.

2) If the BSC fails to set the circuits to idle, it sends a Block message to the MSC and starts timer T1. The MSC stops Timer T12 on receipt of the Block message.




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3) The MSC returns a Block Acknowledge message, indicating the involved circuits are blocked. The BSC stops timer T1 on receipt of the message.

Documents

01-SS7 Signaling System