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Dar es Salaam institute of Technology (DIT)
ETU 07420
Switching Systems
Ally, J
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Signaling Network (SS7)
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Common Channel Signaling System No.7 An international standard for exchange of information on call setup,
call routing and control in the PSTN
Standardized by the ITU-T North American variant standardized by ANSI and Bellcore
The SS7 network enables enhanced services such as: Call setup, management and teardown CallerID, call forwarding, 3-way calling, ... Toll-free (800/888) and toll (900) services Wireless roaming Wireless subscriber authentication
SS7 information travels over a separate network Message-based packet network SS7 signaling links are dedicated channels, separate from voice
channels
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SS7 Signaling Between Exchange The SS7 signalling system is described in the Q.700
series of ITU-T recommendations. A common channel signaling system, optimized for
digital networks, it allows direct transfer of call information transfer between exchange processors.
Comprising a number of layered and modular parts, each with a different function, it is a powerful general-purpose signaling system capable of supporting a range of applications and administrative functions, including ISDN (integrated services digital network) Intelligent networks (INS) Mobile services (e.g. cellular radio) Network administration, operation and management
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SS7 Architecture SS7 signaling is out-of-band, meaning that a signaling link is not in
a voice channel Out-of-band signaling enables:
Faster call setup than would be possible with in-band signaling using multifrequency tones
Support for intelligent network features such as database systems Types of SS7 signaling points:
Service Switching Points (SSP’s) Signal Transfer Points (STP’s) Service Control Points (SCP’s)
Addressing and routing Signaling points are addressed by unique point codes Message routing uses the source and destination point codes
and routing tables at each signaling point
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An Example of Signaling Network
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SS7 Signaling Points Service Switching Points (SSP’s)
SSP’s are PSTN switches that originate or terminate calls, or route calls to other switches (tandem switches)
SSP’s exchange SS7 messages to set up, manage and release voice circuits
Service Control Points (SCP’s) SCP’s are database servers that respond to requests from SSP’s for
call routing information
Signal Transfer Points (STP’s) STP’s are packet switches that serve as routers in the SS7 network
Incoming SS7 messages are switched to outgoing links based on routing
information contained in the messages (not just based on the destination numbers)
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Signaling Point (SP) A Signaling Point (SP) is a switching or, processing node in a
signaling network, with the functions of SS7 implemented.
All Signaling Points in a SS7 Signaling Network are identified by a unique code (14 bits 0r 24 bits) known as a Signaling Point Code.
A signaling point, at which a signaling message is generated, is called the Originating Point.
A signaling point, to which a signaling message is destined, is called a Destination Point.
A signaling point, at which a message is received on one signaling link and then transferred to another link, without processing the contents of the message, is called a Signaling Transfer Point (STP).
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SS7 Signaling Links
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Types of SS7 Signaling Links A (access) link Connects end nodes to STP’s
B (bridge) link Interconnects primary STP’s from different networks
C (cross) link Connects STP’s performing identical functions into a mated pair
D (diagonal) link Interconnects secondary STP’s
E (extended) link Connects an SSP to a secondary STP; alternate access link
F (fully associated) link Interconnects two end nodes
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Evolution of SS7 Common Channel Signaling System No. 7 (i.e.,
SS7 or C7), which was specified in 1979/80, is intended primarily for digital networks, both national and international, where the high transmission rate (64 kb/s) can be exploited. It may also be used on analog lines.
SS7 signaling has not only been designed to control the setting up and supervision of telephone calls but of non -voice services also.
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Advantages of Using SS7SS7 has several advantages compared with traditional signalingsystems. Some obvious advantages are the following:
FAST - the time for call set up is reduced to less than one second in most cases.
HIGH CAPACITY - each signaling link can handle the signaling for several thousand simultaneous calls.
ECONOMICAL - much less signaling equipment is required, compared to traditional signaling systems.
RELIABLE - by using alternate signaling routes, the signaling network can be made very secure.
FLEXIBLE - the system can contain many more signals, for example, and can be used for other purposes than telephony.
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Fundamentals of SS7 Signaling System 7 (SS7) is an architecture for
performing out-of-band signaling in support of the call-establishment, billing, routing and information-exchange functions of the public switched telephone network (PSTN) or public land mobile network (PLMN).
It identifies functions to be performed by a signaling-system network and a protocol to enable their performance.
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An Introduction to Signaling In a telephony context, signaling means the passing of information
and instructions from one point to another relevant to the setting up and supervision of a telephone call.
By tradition, Signaling has been divided into two types: Subscriber Signaling i.e. signaling between a subscriber terminal
(telephone) and the local exchange Trunk Signaling i.e. signaling between exchanges
L o c a lE x c h a n g e
L o c a lE x c h a n g e
T r u n kS i g n a l l i n g
S u b s c r i b e rS i g n a l l i n g
S u b s c r i b e rS i g n a l l i n g
ccs
cas
ccs
cas
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Subscriber Signaling
Calling Party Called PartyA_Number B_Number
H O O K O F F
D I A L T O N E
N U M B E R
R I N G I N G T O N E R I N G I N G S I G N A L
B A N S W E R
C O N V E R S A T I O N
H O O K O N H O O K O N
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Trunk SignalingThe Trunk Signaling has further been divided into:
Channel Associated Signaling (CAS) i.e. signaling in the speech channel (in-band) or in a channel closely associated with the speech channel (out-of-band), and
Common Channel Signaling (CCS) i.e. signaling in a
channel totally separated from the speech channels (out-of-band) and where this Signaling Channel is common for a large number of speech channels.
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Channel Associated Signaling (CAS)Characteristic for CAS is that for each speech channel there is one unambiguously defined signaling path, either On-speech-path, i.e. the signals are transferred in the speech channel (in-band signaling) or Channel-associated, i.e. the signals are transferred in a separate signaling channel, for example the line signals are transferred in time slot 16 in PCM system.
All of these signaling systems have a number of limitations like: Relatively slow, Limited information capacity, etc.
Switch1
Switch2
Voice TrunkSignaling Link
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Common Channel Signaling (CCS)In type of CCS, signaling for numerous circuits can be handled by a few fast signaling data links. The signaling is performed in both directions, with one signaling channel in each direction.
The signaling information that will be transferred is grouped into signal units (data packets). Besides the signaling information itself, there is also need for speech circuit identification and address information (label) and information for error control.
SS7 is a kind of CCS signaling system.
SP SP
STP STP
Voice Trunk
Signaling Link
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Signaling Link & Signaling Link Set The common channel signaling system uses
Signaling Links (SLs) to convey the signaling messages between two signaling points.
Physically, a Signaling Link consists of a Signaling Terminal at each end of the link and some kind of transmission media (normally a time slot in a PCM -link) interconnecting the two Signaling Terminals.
A number of parallel signaling links that directly interconnect two signaling points constitute a Signaling Link Set.
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Signaling ModesThe term Signaling Mode refers to the association between the path taken by a signaling message and the speech path (or data path) to which the message refers.
In the Associated mode of signaling, the messages related to a call follow the same path as the speech between two adjacent signaling points.
In the Quasi-associated mode of signaling, the signaling messages are following another path other than the speech.
SP SPAssociated SP SP
STP STP
Quasi-Associated
Signaling Relation
Signaling Link Set
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Signaling Route & Signaling Route Set The pre-determined path, that a message takes
through the signaling network between the origination point and the destination point is called a Signaling Route. It may consist of a succession of SP/STPs and the interconnecting SLs.
All the Signaling Routes that may be used between an origination point and a destination point by a message traversing the signaling network is the Signaling Route Set for that signaling relation.
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SS7 Protocol Stack
The OSI Reference Model and the SS7 Protocol Stack
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Message Transfer Part (MTP)The Message Transfer Part (MTP) is divided into three levels.
MTP Level 1 Physical and electrical interfaces of SS7 digital signaling links E-1 (2048kb/s), DS-1 (1544 kb/s), DS-0 (64 kb/s), V.35 (64 kb/s),
DS-0A (56 kb/s)
MTP Level 2 Handles message transmission over a physical link Includes flow control, packet sequencing, error detection, retransmission
MTP Level 3 Handles message routing between SSP’s Provides congestion control
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SS7 Transport and Higher Layers Telephone User Part (TUP) Analog call circuit setup/teardown
ISDN User Part (ISUP) Setup, management & release of trunk circuits
Signaling Connection Control Part (SCCP) Transport layer for TCAP-based services such as 800/888 numbers, wireless
roaming, etc. Provides subsystem numbers (like port numbers in TCP/UDP), which
enable addressing to specific applications at destination signaling points
Transaction Capabilities Application Part (TCAP) Used for SCP-SSP communications concerning routing of 800/888/900 calls, to encapsulate Mobile Application Part (MAP) messages containing customer profile information for roving mobile subscribers,
and for calling card calls
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Telephone User Part (TUP)
Overview of Telephone User Part (TUP)
The Telephone User Part defines the necessary telephone signaling functions in SS7 for international as well as national telephone traffic. It provides the same features for telephone signaling as other ITU-T signaling systems.
The telephone signals are transferred in the signaling network as the form of signaling messages, which are the contents in the SIF field in the Message Signal Units (MSUs).
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ISDN User Part (ISUP)Overview of ISUP
The ISDN User Part (ISUP) defines the protocol and procedures used to set-up, manage, and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN) or ISDN network. ISUP is capable of processing ISDN specific information which is more complex than telephony signaling.
ISUP is used for both ISDN and non-ISDN calls. Calls that originate and terminate at the same switch do not use ISUP signaling.
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Commonly Used ISUP Signals
Initial Address Message (IAM) An IAM is sent in the "forward" direction by each switch needed to
complete the circuit between the calling party and called party until the circuit connects to the destination switch. An IAM contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.
Address Complete Message (ACM) An ACM is sent in the "backward" direction to indicate that the
remote end of a trunk circuit has been reserved. The originating switch responds to an ACM message by connecting the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The originating switch also sends a ringing tone to the calling party's line.
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Commonly Used ISUP Signals (2) Answer Message (ANM) When the called party answers, the destination switch terminates
the ringing tone and sends an ANM to the originating switch. The originating switch initiates billing after verifying that the calling party's line is connected to the reserved trunk.
Release Message (REL) A REL is sent in either direction indicating that the circuit is being
released due to the cause indicator specified. An REL is sent when either the calling or called party "hangs up" the call (cause = 16). An REL is also sent in the backward direction if the called party line is busy (cause = 17).
Release Complete Message (RLC) A RLC is sent in the opposite direction of the REL to
acknowledge the release of the remote end of a trunk circuit and end the billing cycle as appropriate.
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Basic ISUP Call Control
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Signaling Connection Control Part (SCCP)Where is SCCP?
INAP: IN Application Protocol MAP: Mobile Application PartCAP: CAMEL Application Part TCAP: Transaction Capability Application PartTUP: Telephone User Part ISUP: ISDN User PartMTP: Message Transfer Part SCCP: Signaling Connection
Control Part
TUP ISUPTCAP
INAP CAP MAP
MTP
SCCP
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What is SCCP? In SS7 signaling system, SCCP and MTP layer three
together are responsible for signaling network layer function. SCCP expands the MTP functions in the following points:
Enable to convey various non-circuit-related signaling message.
Provide enhanced addressing and routing function, and enable to achieve the direct global transmission between different SS7 networks by using GT (Global Title) addressing.
Expand the user part of MTP. SCCP supports up to 256 kinds of sub systems in stead of 16 in MTP.
Enable to provide connectionless service and connection-oriented service.
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Service Functions of SCCP NetworkAccording to the various service requirements, SCCP provides four classes of service: two connectionless services and two connection-oriented services. The four classes are: 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.
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Transaction Capabilities Application Part (TCAP)
The main purpose of TCAP is to provide support for interactive applications in a distributed environment.
TCAP is a general protocol which makes it easy to introduce new features in telecommunication networks. It reduces the need for development of new protocols whenever new features are introduced.
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Applications of TCAPApplications of TCAP: Data Exchange between switching systems Switching systems access network database center Network databases establish remote operation
dialogue
Examples of TCAP applications: Mobile service applications Free phone service (800-service) Credit Card calling Operation & Maintenance applications
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Intelligent Network Application Part (INAP)
The functions of INAP
INAP defines operation criterion among the IN functional entities SSF, SCF, SRF and SDF.
INAP is transmitted by SCCP UDT data, using connectionless services.
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