4-BSSPAR- BSS Protocols and Signalling Capacity

6-90155v 1.0

© Nokia Oyj 1 (23)

BSSPAR

BSS Protocols & Signalling CapacityTraining Document

BSSPAR CTXX 04 S10.5 12-2002

6-90155v 1.0

© Nokia Oyj 2 (23)

The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia Networks' customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia Networks. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Networks welcomes customer comments as part of the process of continuous development and improvement of the documentation.

The information or statements given in this document concerning the suitability, capacity, or performance of the mentioned hardware or software products cannot be considered binding but shall be defined in the agreement made between Nokia Networks and the customer. However, Nokia Networks has made all reasonable efforts to ensure that the instructions contained in the document are adequate and free of material errors and omissions. Nokia Networks will, if necessary, explain issues which may not be covered by the document.

Nokia Networks' liability for any errors in the document is limited to the documentary correction of errors. Nokia Networks WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of this document or the information in it.

This document and the product it describes are considered protected by copyright according to the applicable laws.

NOKIA logo is a registered trademark of Nokia Corporation.

Other product names mentioned in this document may be trademarks of their respective companies, and they are mentioned for identification purposes only.

Copyright © Nokia Oyj 2003. All rights reserved.

BSS Protocols & Signalling Capacity

6-90155v 1.0

© Nokia Oyj 3 (23)

Contents

1 Module Objectives.................................................................... 4

2 Introduction............................................................................... 5

3 Procedures of the Basic Call Function ................................... 6

4 Call Phases ............................................................................... 8

5 Call Assignment ..................................................................... 105.1 Mobile Originating Call (MOC) .................................................. 105.2 Mobile Terminating Call (MTC) ................................................. 12

6 Location Update ..................................................................... 13

7 Disconnect .............................................................................. 147.1 Network Initiated....................................................................... 147.2 Mobile Station Initiated ............................................................. 15

8 Handover................................................................................. 168.1 Synchronised Handover ........................................................... 178.2 Non-Synchronised Handover.................................................... 188.3 Handover Failure ...................................................................... 18

9 Signalling Channel Capacity.................................................. 19

10 High Capacity Signalling Links.............................................. 20

11 Key Learning Points ............................................................... 21

12 Review Questions................................................................... 22

BSSPAR CTXX 04 S10.5 12-2002

6-90155v 1.0

© Nokia Oyj 4 (23)

1 Module ObjectivesAt the end of the module, the participant will be able to:

• State the procedures and the phases of a basic call

• List five examples of signalling messages that are sent between BSS and MS

• Describe the signalling phases in MOC and MTC

• Discuss the signalling exchange that takes place during a Location Update

• Explain the differences between Network-initiated and MS-initiated Disconnect

• Describe the Handover protocols used between BSS and MS

• Show how to determine the required signalling capacity of each type of link in the BSS

• Discuss the benefits of implementing High Capacity Signalling Links in the A-interface


6-90155v 1.0

© Nokia Oyj 5 (23)

2 Introduction The capacity and performance of each cell and its channels, is largely determined by Radio Network parameter settings.

In this module, we see the various signalling transactions which occur during the various phases of a call, and which create a load on the signalling channels of a cell.

Signalling protocols in Radio Resource Management in the open GSM specifications contain descriptions of the signalling between the Mobile Station and the Base Station in various situations. Following are the protocols of the most usual situations.

BSSPAR CTXX 04 S10.5 12-2002

6-90155v 1.0

© Nokia Oyj 6 (23)

3 Procedures of the Basic Call FunctionThere are 11 signalling procedures used during the phases of a basic call to provide CS speech or CS data services to a subscriber:

1. The paging procedure offers the network the means to start the establishment of a mobile terminating call with a given mobile station (MS).

2. The channel required procedure offers the MS the means to start the establishment of a mobile originating call or to answer the PAGING REQUEST message.

3. Channel reservation procedure: The MS uses the CHANNEL REQUEST message for requesting a dedicated signalling channel SDCCH. After receiving the CHANNEL REQUIRED message from the BTS or the ASSIGNMENT REQUEST message from the MSC, the BSC starts searching for a relevant channel. The BSC can allocate SDCCH or TCH channels as follows:

• SDCCH, TCH/F or TCH/H channel due to the CHANNEL REQUIRED message

• TCH/F or TCH/H channel due to the ASSIGNMENT REQUEST message

• SDCCH, TCH/F or TCH/H channel in case of a handover

• Up to four TCH/F channels in case of a high-speed circuit switched data call.

The number of the TCH and SDCCH channels depends on the radio configuration.

4. Channel activation procedure: After a successful reservation of a new SDCCH or TCH channel, the BSC activates it by sending the CHANNEL ACTIVATION message to the BTS. The GSM timer T9103 is used for supervising the channel activation procedure. The BSC can activate TCH or SDCCH channels. If the BTS refuses to activate the new channel, it sends the CHANNEL ACTIVATION NACK message to the BSC with the reason for the failure.

5. The IMMEDIATE ASSIGN message facilitates assignment of the MS onto an SDCCH channel; the IMMEDIATE ASSIGN REJECT message facilitates refusal of the access sent by the MS. The GSM timer T3101 is used to supervise the immediate assignment procedure. If the SDCCH channel reservation or activation has failed, the BSC sends the IMMEDIATE ASSIGN REJECT message to the MS.

6. The BSC starts the SCCP connection establishment procedure for the MS after receiving the ESTABLISH INDICATION message for the SDCCH establishment from the BTS. The GSM timer T9105 is used to supervise this procedure. If the SCCP link establishment fails, the BSC releases all resources related to this transaction.

7. Transmission of Transparent L3 messages is used in communication between the MS and the MSC. The BTS and the BSC forward the transparent messages.


6-90155v 1.0

© Nokia Oyj 7 (23)

8. Ciphering procedure is one of the security procedures designed to protect the subscriber identity and data. It is an optional procedure in GSM. When ciphering is active, all information exchanged between the mobile and the network on the dedicated radio channels is encrypted. The key previously set between the network and the MS is used to encipher and to decipher the encrypted information. The Authentication procedure checks the MS identity in order to prevent unauthorised use. During the procedure the ciphering key Kc is set between the network and the MS. Ciphering is initiated after the ciphering key is set on the dedicated signalling channel (SDCCH or FACCH).

9. The assignment procedure is used in assigning the MS to the correct TCH channel. The selection of the TCH channel is based on the information received from the MSC.

10. The channel release procedure is used for releasing a dedicated radio channel. Either the BSC or the MSC starts the channel release procedure. If the MSC starts the channel release procedure, it sends the CLEAR COMMAND message to the BSC. If the BSC starts the channel release procedure, it sends the CLEAR REQUEST message to the MSC, which then sends the CLEAR COMMAND message to the BSC.

11. System information broadcasting procedure: The network periodically broadcasts messages on the BCCH. The MS uses this information to gain access to the system via the current cell. The network sends similar information in messages on the SACCH after a handover. The information broadcast can be grouped in the following classes:

• Information giving unique identification of the current network, Location Area and cell

• Information used for cell selection and candidate cell measurements for handover procedures

• Information describing the current control channel structure

• Information controlling the random access channel utilisation

• Information defining different options supported within the cell

• Information about length of the part of the message belonging to phase 1 protocol.

Cell reselection criterion C1 enables the MS to select a cell based on power level. The C2 reselection criterion may be defined by the operator. This enables avoiding undesirable reselection of a micro cell in an environment where there is coverage by both a micro cell and a macro cell.

BSSPAR CTXX 04 S10.5 12-2002

6-90155v 1.0

© Nokia Oyj 8 (23)

4 Call PhasesA typical Mobile Terminated Call (MTC) goes through about 6 to 8 phases in its duration. There are 15 possible phases for all types of call and transactions such as Location Update.

get

service

get

SDCCH

establish

SDCCH

connection

get

TCH

establish

TCH

connection

call

phaserelease

phase

Call Completion RateCall SetupSuccess Rate

SDCCH

Blocking

(system)

and

SDCCH

call

Blocking

TCH

Blocking

(system)

and

TCH Call

Blocking

SDCCH

Success Rate

Overall Call Success Rate

Figure 1. Typical Call Phases


6-90155 v 1.0

© Nokia Oyj

9 (23)

PHASE PHASE_NAME1 Paging2 MM signalling3 Basic assignment4 Release5 FACCH assignment6 SMS establishment (TCH)7 SMS establishment (SDCCH)8 Ciphering9 External handover (source)10 Internal handover intra (source)11 Internal handover inter (source)12 External handover (target)13 Internal handover Intra (target)14 Internal handover inter (target)15 Conversation (TCH)

Figure 2. List of Various Possible Call Phases

Figure 3. MT Call Phases Sequence

MS BTS BSC

AUTHENTICATION (SDCCH)Phase 2 : MM signalling

CIPHERING MODE (SDCCH)Phase 8 : Ciphering

TMSI REALLOCATION (SDCCH)

SETUP (SDCCH)Phase 2 : MM signalling

CH. REQUEST (RACH)

IMMEDIATE ASSIGN(AGCH)

SERVICE REQUEST (SDCCH)

Phase 1 : Paging, initial MS

CH.RELEASEPhase 4 : Release

ALERTING & CONNECT (FACCH)Phase 2 : MM signalling

CONN. ACK. and MEASUREMENTPhase 15 : Conversation

DISCONNECT & RELEASE (FACCH) Phase 4 : Release

ASSIGNMENT (SDCCH-FACCH)Phase 3 : Basic assignment

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

10 (23)

5 Call Assignment Call assignment takes place when a Mobile Station makes a call (Mobile Originating Call) or receives a call (Mobile Terminating Call).

5.1 Mobile Originating Call (MOC)

Authentication

Ciphering Mode Setting

Service Request

Immediate assignmentCHAN REQ (RACH)IMM ASSIGN (AGCH)

CM SERV REQ (SDCCH)

AUTH REQ (SDCCH)AUTH RES (SDCCH)

CIPH MOD CMD (SDCCH)CIPH MOD COM (SDCCH)

MS NETWORK

Call Confirmation

Call Accepted

Assignment of Traffic Channel

Call InitiationSETUP (FACCH)

CALL PROC (SDCCH)

ASSIGN CMD (SDCCH)

ALERT (FACCH)

CONNECT (FACCH)CONNECT ACK (FACCH)

MS NETWORK

ASSIGN COM (FACCH)

Figure 4. Mobile Originating Call

As seen above, the main phases can easily be separated: Immediate Assignment, Service request, Authentication, Ciphering Mode, Call Initiation, Assignment of Traffic Channel, Call Confirmation and Call Acceptation. The same phases can actually be found in the Mobile Terminating Call, which is described below.


6-90155 v 1.0

© Nokia Oyj

11 (23)

Figure 5. Mobile Originated Call Trace

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

12 (23)

5.2 Mobile Terminating Call (MTC)

Authentication


Service Request

Immediate assignment

PAG REQ (PCH)CHAN REQ (RACH)

PAG RES (SDCCH)



MS NETWORK

IMM ASSIGN (AGCH)

Call Confirmation

Call Accepted

Assignment of Traffic Channel

Call InitiationSETUP (FACCH)

CALL CONF (SDCCH)

ASSIGN CMD (SDCCH)

ALERT (FACCH)

CONNECT (FACCH)CONNECT ACK (FACCH)

MS NETWORK

ASSIGN COM (FACCH)

Figure 6. Mobile Terminating Call


6-90155 v 1.0

© Nokia Oyj

13 (23)

6 Location Update The MSC needs to know under which Location Area the Mobile Station can be reached. So the MS is required to make Location Updates, and the information is used by the BSC for paging of MS's from BTS's in the LA.

Authentication


Service Request

Immediate assignmentCHAN REQ (RACH)IMM ASSIGN (AGCH)

LOC UPD REQ (SDCCH)



LOC UPD ACC (SDCCH)TMSI REAL COM (SDCCH)

CHAN REL (SDCCH)Channel Release

MS NETWORK

Figure 7. Location Update

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

14 (23)

7 Disconnect The disconnect protocol is needed when the Mobile Station or the Network want to finish a call for any.

7.1 Network Initiated

Disconnect, Network Initiated

Release

Call Clearing

DISCONNECTREL

CHAN REL

MS NETWORK

REL COM

Figure 8. Disconnect, Network Initiated


6-90155 v 1.0

© Nokia Oyj

15 (23)

7.2 Mobile Station Initiated

Disconnect, MS Initiated

Release

Call Clearing

DISCONNECTREL

CHAN REL

MS NETWORK

REL COM

Figure 9. Disconnect, Mobile Initiated

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

16 (23)

8 Handover Maintaining the traffic connection with a moving subscriber is made possible with the help of the handover function. When the subscriber moves from the coverage area of one cell to another, a new connection with the target cell has to be set up and the connection with the old cell has to be released.

There are two reasons for performing a handover:

1. Handover due to measurements occurs when the quality or the strength of the radio signal falls below certain parameters specified in the BSC. The deterioration of the signal is detected by the constant signal measurements carried out by both the mobile station and the BTS. As a consequence, the connection is handed over to a cell with a stronger signal.

2. Handover due to traffic reasons occurs when the traffic capacity of a cell has reached its maximum or is approaching it. In such a case, the mobile stations near the edges of the cell may be handed over to neighbouring cells with less traffic load.

The decision to perform a handover is always made by the BSC that is currently serving the subscriber, except for the handover for traffic reasons. In the latter case the MSC makes the decision. There are four different types of handover:

1. Intra cell - Intra BSC handover

This is where the subscriber is handed over to another traffic channel (generally in another frequency) within the same cell. In this case the BSC controlling the cell makes the decision to perform the handover.

2. Inter cell - Intra BSC handover

The subscriber moves from the serving cell to the target cell. In this case the handover process is controlled by BSC. The traffic connection with the serving cell is released when the connection with target cell is set up successfully and the handover is complete.

3. Inter cell - Inter BSC handover

The subscriber moves from one cell to another, served by another BSC. The handover process is carried out by the MSC, but the decision to make the handover is still done by the first BSC. The connection with the first BSC (and BTS) is released when the connection with the new BSC (and BTS) is set up successfully.

4. Inter MSC handover

The subscriber moves from a cell controlled by one MSC/VLR, to a cell in the domain of another MSC/VLR.

The MSC/VLR currently serving the subscriber (also known as the anchor MSC), contacts the target MSC/VLR and the traffic connection is transferred to the target MSC/VLR. As both MSCs are part of the same network, the connection is established smoothly.


6-90155 v 1.0

© Nokia Oyj

17 (23)

Air A

BTS

Old Cell / BTS

New Cell / BTS

BTS

BSC TC

BSC TCVLRMSC

VLRMSC

Figure 10. Inter Cell - Inter MSC Handover

In synchronised handover, no timing advance information is needed, so the protocols are slightly different. This reduces signalling because no physical information needs to be sent. Both handover cases - synchronised and non-synchronised - are presented separately. The Handover failure procedure has been presented as well.

8.1 Synchronised Handover

Handover, Synchronized

New Channel, New Cell

HANDO CMDHANDO ACC

ACTIVE CALL

MS NETWORK

HANDO ACCHANDO ACC

HANDO ACCHANDO COM

ACTIVE CALL

Old Channel, Old Cell

Figure 11. Synchronized Handover

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

18 (23)

8.2 Non-Synchronised Handover


MS NETWORK

ACTIVE CALL

HANDO CMD

HANDO ACC …….

HANDO ACC

PHYS INFO

PHYS INFO

HANDO COM

ACTIVE CALL


T 3124

Ny1

ParametermaxNumberOfRepetition ( 5 … 35 )

Figure 12. Non-Synchronized Handover

8.3 Handover Failure

Handover Failure


HANDOVER CMD

ACTIVE CALL

MS NETWORK

HANDOVER FAIL

ACTIVE CALL



Timer T3124 expiry or Radio Link Failure

Figure 13. Handover Failure


6-90155 v 1.0

© Nokia Oyj

19 (23)

9 Signalling Channel Capacity Common channels, dedicated channels, LAPD links, and CCS7 links, each have capacity limitations determined by parameter settings and hardware limitations

For a comprehensive study of this topic we will use the document entitled:

BSS (BSC) Traffic Handling Capacity, Overload Protection and Network Planning dn9812388 Issue 5-0 en (46 pages), from S10.5 ETSI documentation set.

Our Objective is to: Show how to determine the required signalling capacity of each type of link in the BSS.

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

20 (23)

10 High Capacity Signalling Links The GSM network has been originally specified (in GSM 08.08) and designed to use 64 kbit/s CCS7 links in the A-interface. For various reasons, these CCS7 links can become congested. This situation can now be improved with the use of higher capacity CCS7 links.

In BSS10, higher capacity CCS7 links (128 or 256 kbit/s) are supported between the BSC and MSC. They require the use of the Nokia BSC2i with plug-in-units AS7-V, and Nokia MSCi with plug-in-units AS7-A.

Normally, there are 4 timeslots reserved for 64 kbit/s CCS7 links. However, with the high capacity CCS7 link feature, there are always 16 timeslots reserved for CCS7 usage, regardless of how many timeslots are actually needed. So implementation of high capacity CCS7 links may have an impact on the maximum LAPD capacity of the BSC, depending on the configuration.

Table: Link Capacity per BCSU - Normal and High capacity CCS7 links

High Capacity BSC2i: 64 kbit/s CCS7 link

128 kbit/s CCS7 link

256 kbit/s CCS7 link

Maximum Number of CCS7 links

4 4 4

Maximum Number of BCFSIG LapD links

32 32 32

Maximum Number of TRXSIG LapD links

64 64 64

Maximum number of LapD links (BCFSIG+TRXSIG+ISDN +ET-LapD)

124 112 112


6-90155 v 1.0

© Nokia Oyj

21 (23)

11 Key Learning Points • John

• Examples of signalling messages that are sent between BSS and MS include call assignment (mobile originate and mobile terminated calls), location update, disconnect (network initiated and MS initiated), handover (synchronised and non-synchronised)

• Call Assignment is used for mobile originated and mobile terminated calls. The phases of call assignment are Immediate Assignment, Service Request, Authentication, Ciphering, Call initiation, Traffic Channel Assignment, Call Confirmation, and Call Acceptance

• Location Area updates are sent by the MS when it changes LA. This information is used for paging purposes. The phases involved are immediate assignment, service request, authentication, ciphering, and channel release.

• Disconnect messages are sent by either the MS or Network to break the connection between an MS and the Network. The sequence of events in a disconnect message is: Disconnect, Release, Release Complete, and Channel Release.

• Handover takes place when an MS is required to get continued CS service from another Base Station. There are two types of handover: synchronised and non-synchronised. No timing advance information is needed in synchronised handover.

• Common channels, dedicated channels, LAPD links, and CCS7 links, each have capacity limitations determined by parameter settings and hardware limitations

• The GSM network has been originally specified and designed to use 64 kbit/s CCS7 links in the A-interface. For various reasons, these CCS7 links can become congested. This situation can now be improved with the use of higher capacity CCS7 links (128 and 256 kbit/s).

BSSPAR CTXX 04 S10.5 12-2002

6-90155 v 1.0

© Nokia Oyj

22 (23)

12 Review Questions 1. Which of the following is not phase in a call?

A. Paging

B. MM signalling

C. Basic assignment

D. Release

E. Mobile Terminated Call

2. Which of the following types of signalling messages are sent between BSS and MS?

F. Call Assignment

G. Location Update

H. Disconnect

I. Handover

J. All of the above

3. What is the first phase in call assignment message between BSS and MS?

A. Service Request

B. Authentication

C. Ciphering Mode Setting

D. Channel Release

E. Immediate Assignment

4. Which of the following phase takes place last during location update?

A. Service Request

B. Authentication

C. Ciphering Mode Setting

D. Channel Release

E. Immediate Assignment


6-90155 v 1.0

© Nokia Oyj

23 (23)

5. What is the sequence of events in an MS initiated disconnect message?

A. Disconnect, Release Complete, Release, Channel Release

B. Disconnect, Release, Release Complete, Channel Release

C. Disconnect, Release, Channel Release, Release Complete

D. Disconnect, Channel Release, Release, Release Complete

E. None of the above

6. The major difference between a synchronised handover and a non synchronised handover is

A. It is initiated by MS

B. It is initiated by Network

C. Timing advance information is needed

D. Timing advance information is not needed

E. It is used only in the case of handover failure

7. Indicators of congested signalling links are

A. Buffer overflow

B. Lost messages

C. Quality alarms

D. Message retransmissions

E. All of the above.

8. What capacities can be used on the CCS7 links of the A-interface?

A. 64 kbit/s

B. 16, 32, 64, 128 kbit/s

C. 16, 32, 64, 256 kbit/s

D. 64, 128, 256 kbit/s

E. 128, 256 kbit/s

Documents

4-BSSPAR- BSS Protocols and Signalling Capacity