Chapter 4

Gs-Combined ProceduresContents:4.1 Gs interface concepts

1. Protocols for Combined Procedures2. Gs-Association3. Gs-Association Establishment and Release

4.2 Combined procedures1. Gs-Association Establishment and Release GMM: Combined Routing Area Update 2. BSSAP+: Routing Area Update 3. Example: Combined LA and RA Update (Part I) 4. Combined Attach Procedure GMM: 5. Combined Detach Procedure 6. Paging of Non-GPRS Services7. BSSAP+: Alerting Procedure 8. BSSAP+: Other Procedures

4.3 Formatting rules1. BSSAP+ Message Layout

Chapter 4

Gs-Combined Procedures4.1 Gs interface concepts

1. Protocols for Combined Procedures2. Gs-Association3. Gs-Association Establishment and Release

Protocols for Combined Procedures

GsMTP level 1

BSSAP+

MTP level 2

MTP level 3

SCCP

MTP level 1SGSN

BSSAP+

MTP level 2

MTP level 3

SCCP

Class A or B MS

• cs and ps service location information co-ordination

VLR

Gs-Association

SGSN VLR

SGSN-no.

VLR-no.

IMSIIMSI

VLR – SGSN Association

Gs-Null

Gs-Associated

Gs-Association Establishment and Release

SGSN

BSSGP+-Location Area Update Request( IMSI, SGSN-no, ...)

BSSGP+-Location Area Update Accept( IMSI, ...)

Start T6-1

Gs-AssociationEstablishment

explicitGs-AssociationRelease

BSSGP+-GPRS Detach Indication orBSSGP+-IMSI Detach Indication

( IMSI, ...)

( IMSI, ...)

BSSGP+-GPRS Detach Indication Acknowledge orBSSGP+-IMSI Detach Indication Acknowledge

Stop T6-1

Start T6-1

Stop T6-1

VLR

Chapter 4

Gs-Combined Procedures4.2 Combined procedures

1. Gs-Association Establishment and Release GMM: Combined Routing Area Update 2. BSSAP+: Routing Area Update 3. Example: Combined LA and RA Update (Part I) 4. Combined Attach Procedure GMM: 5. Combined Detach Procedure 6. Paging of Non-GPRS Services7. BSSAP+: Alerting Procedure 8. BSSAP+: Other Procedures

GMM: Routing Area Update Procedure

Routing Area Update Request( Update type, old RAI, P-TMSI, optional: TMSI status, ... )

Routing Area Update Accept( Update result, optional: allocated P-TMSI, MS Identity (= TMSI), ... )

Routing Area Update Complete

SGSN

MS

Start T3330(= 15 s)

Stop T3330 Start T3350 (= 6 s)only when • P-TMSI and/or• Receive N-PDU

numberswere allocated

Stop T3350

• no valid TMSI available• valid TMSI is available

• RA Update• combined RA/LA Update• combined RA/LA Update with IMSI

Attach• periodic Update

• RA updated• RA/LA updated

optional:BSSAP+

messages

VLR

BSSAP+: Routing Area Update Procedure

BSSAP+-Location-Area-Update-Request( IMSI, SGSN-no, update type, new CGI,

MS classmark 1, TMSI status, ... )

BSSAP+-Location-Area-Update-Accept( IMSI, LAI, optional: new TMSI or IMSI )

BSSAP+-TMSI-Reallocation-Complete

SGSN

Start T6-1

Stop T6-1 Start T6-2 (default 40 s)only when newTMSI was allocated

Stop T6-2( IMSI, CGI, optional: SAI )

HLR

MS

optional

RA Update Accept,RA Update Complete

VLR

Example: Combined LA and RA Update (Part I)

HLR

MS

SGSN GGSNSGSN VLRVLR

newold newold

Routing Area Update Request( old RAI, P-TMSI, TMSI status, ... )

SGSN ContextRequest

SGSN ContextResponse

Authentication

Update PDP Context Request

Update PDP Context Response

Update GPRS Location

Cancel Location

Cancel Location Ack

Insert Subscriber Data

Update GPRS Location Ack

Insert Subscriber Data Ack

Example: Combined LA and RA Update (Part II)

The SGSN can now begin to trigger the location area update:6. The new SGSN sends to the VLR of the location area the BSSAP+ message Location Area Update Request.

IMSI, LAI, new CGI, update type, etc. are provided.7. The VLR can immediately start to update the HLR. In other words there is no second authentication and the VLR

does not have to retrieve the IMSI from the old VLR. The update VLR-HLR is done this way:a. The VLR sends the MAP operation Update Location, which contains the IMSI and the VLR-

ISDN number.b. The HLR can now delete the old subscriber record in the old VLR using the MAP message

Cancel Location. The old VLR will acknowledge this. c. Concurrently to step 7c the HLR will fill the subscriber record in the new VLR with the circuit

switched subscription record using the MAP operation Insert Subscriber Data. Again this message can be repeated several times. Each Insert Subscriber Data needs its own

acknowledgement.d. When the VLR record is filled, then the HLR will complete the procedure with the

acknowledgement for the Update Location operation.8. The new VLR has now to forward the acknowledgement from the HLR to the new SGSN with BSSAP+ Location

Update Accept. In this message the VLR can assign a new TMSI to the MS.9. The new SGSN reports the success of the procedures as GMM message Routing Area Update Accept. In this

message the new TMSI and a new PTMSI can be provided.10. If the last message contained a new temporary identifier, then the MS has to acknowledge it with the GMM

message Routing Area Update Complete. The SGSN shall forward this acknowledgement with BSSAP+ TMSI Reallocation Complete to the VLR if there was a new TMSI assigned to the MS.

Example: Combined LA and RA Update (Part II)

HLR

MS

SGSN GGSNSGSN VLRVLR

newold newold

BSSAP+-Location-Update-Request Update Location

Cancel Location

Cancel Location Ack

Insert Subscriber Data

Insert Subscriber Data Ack

Update LocationAck

BSSAP+-Location-Update-Accept

BSSAP+-TMSI-Reallocation-Complete

Routing Area Update Accept( Update Result, ... )

Routing Area Update Complete

Combined Attach ProcedureThe second procedure that is available as a combined procedure is the GPRS Attach. The possible combinations

are:• GPRS attach only,• GPRS attach with IMSI attach, and• GPRS attach while IMSI attached.The last option is used when a MS is already attached to the VLR and becomes now GPRS attached, too. Then

the consequence of the combined attach is that the Gs-association is established. The second option has the same result, but also clears the IMSI Detach Flag in the VLR.

As acknowledgement for the Routing Area Update Request message the SGSN delivers the GMM message Routing Area Update Accept to the MS. In this message the MS gets an indication whether the MS is attached to GPRS only (or Gs is not present) or whether it is attached to both core network domains in which case also the Gs-association exists.

On the Gs-interface the SGSN uses BSSAP+ for a combined attach. The behavior is for GPRS attach with IMSI attach and for GPRS attach while IMSI attached the same. 

The SGSN sends the BSSAP+ message Location Update Request to the MSC/VLR. Inside the message the SGSN must at least include the IMSI and its own SGSN-ISDN number. The parameter update type can be used to indicate an IMSI attach.

The VLR shall process the message and create a new subscriber record if needed. Then it will answer with BSSAP+ message Location Update Accept. This message possibly includes a new TMSI. In this case the SGSN must later acknowledge the TMSI with BSSAP+ message TMSI Reallocation Complete.

Note that the combined attach has usually only the task to set up the Gs-association.

GMM: Combined Attach

Attach Request

Attach Accept

Attach Complete

SGSN

MS

Stop T3350

• GPRS attach• GPRS attach while IMSI attached• combined GPRS/IMSI attach

• GPRS only attached• combined GPRS/IMSI attached

optional:BSSAP+

messages

VLR

Start T3310(= 15 s)

Stop T3310

( Access type, P-TMSI or IMSI, old RAI, ...)

Start T3350 (= 6 s)(only if (P-)TMSIallocated)

( Attach result, new RAI, optional: allocated P-TMSI, ... )

( )

Example: Combined Attach Procedure

As example let us consider the GPRS attach while already IMSI attached. The procedure is done in the following way:

1. The MS sends the GMM message Attach Request to the SGSN. Inside it will indicate the combined attach type and its old PTMSI with old RAI. 

2. The new SGSN will now check the old RAI and old PTMSI. When the old routing area belongs a different SGSN (old one) the new SGSN will retrieve the IMSI from the old SGSN. This is done with the GTP-C messages Identification Request and Identification Response.

3. Now the new SGSN can update the HLR with the MAP operation Update GPRS Location. The HLR then deletes the old subscriber record and fills the new subscriber record with the packet switched subscription from the HLR.

4. When the HLR update for the packet switched side is done the new SGSN will trigger the set up of the Gs-association with the BSSAP+ message Location Update Request. The VLR will check whether it knows this subscriber and whether a real location update is needed. Then the VLR will acknowledge the Gs association with the BSSAP+ message Location Update Accept.

5. The MS can now be informed that the attach was successful and the Gs-association exists. This is done via the GMM message Routing Area Update Accept where an indicator indicates the successful Gs-association establishment. The message can contain a new P-TMSI, too.

6. In case there was a new P-TMSI the MS has to acknowledge it with the GMM message Routing Area Update Complete.

Example: Combined Attach Procedure

HLR

MS

SGSN SGSN VLR

newold

Attach Request

Routing Area Update Complete

( Access type, P-TMSI or IMSI, old RAI, ...)

here: GPRS attach while IMSI attached

Identification Request

Identification Response Update GPRS Location

Cancel Location

Cancel Location Ack

Insert Subscriber Data

Update GPRS Location Ack

Insert Subscriber Data Ack

BSSAP+-Location-Update-Request

BSSAP+-Location-Update-AcceptRouting Area Update Accept

GMM: Combined Detach Procedure

The MS initiated detach procedure of GPRS and for IMSI deactivation can be combined in three different ways:

• GPRS detach only,• IMSI detach only, and• IMSI and GPRS detach. Additionally the MS has to indicate whether this detach is ahead of a power off. This indication is done in the GMM message Detach Request. In case the MS is not switched off after the detach it

will receive an acknowledgement from the network in form of the GMM message Detach Accept. From the point of view of the Gs-association it is the combined detach procedure that releases the Gs-association.

There exist also three causes for a network initiated detach:• re-attach required,• re-attach not required, and• IMSI detach (after VLR failure)

GMM: Combined Detach

Detach Request( Detach type, P-TMSI, optional: P-TMSI signature )

Detach Accept( )

SGSN

MS

• normal detach• power switched off

• GPRS detach• IMSI detach• combined GPRS/IMSI detach

optional:BSSAP+

messages

VLR

Start T3321(= 15 s)

Stop T3321

Detach Request Start T3322(= 6 s)

Stop T3322Detach Accept

( Detach type )

• re-attach required,• re-attach not required• IMSI detach (after VLR failure)

BSSAP+: Combined Detach Procedure

For the combined detach procedure on the Gs-interface there are two cases:

• combined detach with IMSI detach or• combined detach without IMSI detach. In the first case the SGSN will use the BSSAP+ messages IMSI Detach Indication and the VLR returns IMSI

Detach Indication Acknowledgement. In the message from the SGSN it will be indicated whether the detach is a IMSI and GPRS detach or whether it is a pure IMSI detach triggered by the MS. The third possibility is that the detach is network initiated. The acknowledgement must arrive within T9 time (range: 1 – 30 seconds, default value: 4 seconds). After sending the BSSAP+ message IMSI Detach Indication, the SGSN can already send a GMM message Detach to the MS.

 In the second case the SGSN shall send the BSSAP+ message GPRS Detach Indication. In it the VLR gets an

indication whether the MS or the network triggered detach or whether the detach is because of a restriction of GPRS services. The VLR must acknowledge with GPRS Detach Indication Acknowledge. The acknowledgement must arrive within T8 time (range: 1 – 30 seconds, default value: 4 seconds). After sending the BSSAP+ message GPRS Detach Indication, the SGSN can already send a GMM message Detach to the MS.

GPRS detachand

Gs-associationexists

combined orIMSI detach

andGs-association

exists

BSSAP+: Combined Detach

BSSAP+-GPRS-Detach-Indication( IMSI, SGSN-no, IMSI detach from GPRS service type, CGI, SAI )

BSSAP+-GPRS-Detach-Ack( IMSI )

SGSN

Start T8(default:

4s)

VLR

Stop T8

• MS initiated IMSI detach • combined MS initiated IMSI/GPRS detach • GPRS services not allowed (implicit detach)

BSSAP+-IMSI-Detach-Indication( IMSI, SGSN-no, detach type, CGI, Location info age, SAI )

BSSAP+-IMSI-Detach-Ack( IMSI )

Start T9(default:

4s)

Stop T9

• Network initiated GPRS detach• MS initiated GPRS detach• GPRS services not allowed

Example: MS Initiated Detach Procedure

As example consider a MS initiated detach: 1. The MS sends the Detach Request message with indication of the detach type. 2. If the detach includes a GPRS detach, then the SGSN shall now delete the activated PDP contexts. This is done

using the GTP-C message Delete PDP Context Request. This message is sent to the GGSN which returns Delete PDP Context Response. This message pair must be repeated for all active PDP contexts.

 3. Thereafter the SGSN shall release the Gs-association either with IMSI Detach Indication or GPRS Detach

Indication.

4. If the MS was not switched off, the SGSN returns Detach Accept to the MS.

Example: MS Initiated Detach Procedure

MS

SGSN GGSNVLR

Detach Request

Delete PDP Context Request

Delete PDP Context Response

BSSAP+-GPRS-Detach-Indication

BSSAP+-GPRS-Detach-Ack

BSSAP+-IMSI-Detach-Indication

BSSAP+-IMSI-Detach-Ack

GPRS detach

IMSI or combineddetach

OR

Detach Accept

Decision process in MSC

Paging of Non-GPRS Services

If there is a Gs-association for a MS (class A or B), then the MS is paged for cs- and ps-services only by the SGSN. An example of SGSN assisted cs paging can be seen on the right hand side.

1. A request to establish a mobile terminated call arrives at the VLR. The VLR inspects whether there is a circuit service already activated for the Ms. If not, and if the MS is Gs-associated, then the VLR sends the BSSAP+ message Paging Request to the SGSN. This request must at least include the MS‘s IMSI and the serving VLR‘s number. The location area identifier (when the ´confirmed by radio contact` restoration indication is set true), TMSI, and global CN identifier can be included. The VLR expects a response from the SGSN within T5 (range: 2 to 20 seconds; default value: none).

2. The SGSN has to inform the BSS about the cs paging request. This is done by sending a BSSGP PDU of the type PAGING CS to all BSS, which manage cells of the location area, where the MS has been registered last.If the SGSN cannot serve the paging request of the VLR because the MS is e.g. GPRS detached, then it returns the BSSAP+ message Paging Reject. If the MS is not reachable, then the SGSN returns the BSSAP+ message MS Unreachable.

3. When the MS receives a paging request for circuit services, it requests signalling resources from the BSS to return a response to the VLR. The response is transmitted via the A-interface, which is the BSSAP message Paging Response.

CS servicerequest

SCCPconnection

exists no paging

Gs-association

does not exist

GSM paging

GPRS paging

Example: Paging of Non-GPRS Services

MS

SGSN VLRBSS

IAM

BSSAP+-Paging-Request( IMSI, VLR-no, TMSI, LAI, ... ) BSSGP: PAGING CS(Packet)

Paging Request( P-TMSI or IMSI, paging cause )

Paging ResponseBSSAP: PAGING RESPONSE

BSSAP+-Paging-Request

no paging done,e.g. MS marked non-reachable

BSSAP+-MS-Unreachable( IMSI, Gs cause, ... )

Start T5

Start T5

BSSAP+: Alerting Procedure

In some situations it is necessary for the MSC/VLR to be notified when an inactive subscriber becomes reachable. This is for instance necessary when a short message has failed to be sent to the MS. In this case when the user is reachable again the VLR must alert the HLR and the HLR alerts the service centre, such that the message can be sent again.

 When the Gs-association exists, it is possible that the VLR gets an activity notification from the SGSN. This has to

be activated explicitly by the VLR using the BSSAP+ message Alert Request. The SGSN now knows that the VLR has to be informed when there is subscriber activity. The SGSN will acknowledge the Alert Request with Alert Acknowledge.The VLR waits T7 for an acknowledgement (range: 1 to 30 seconds; default value: 4 seconds).

 When there is now subscriber activity then the SGSN sends a BSSAP+ message MS Activity Notification to the

VLR. Note that now the notification mechanism is deactivated.

reset NGAF

set NGAF

Example: Paging of Non-GPRS Services

MS

SGSN VLR

BSSAP+-Alert-Request( IMSI )

BSSAP+-Alert-Ack

any activity (data or signalling)

BSSAP+-MS-Activity Indication

Start T7(default = 4s)

Stop T7( IMSI, Gs cause)

( IMSI, CGI, SAI )

BSSAP+: Other Procedures

If the HLR, SGSN or VLR have faced an internal failure, the Gs-association for the MSs are no longer reliable. In case of an SGSN or VLR failure, each network element has to inform its peer-entities about the failure by sending the BSSAP+ message Reset Indication. In case of a HLR failure, the HLR informs all associated SGSNs about the recovery. The SGSN sets the NGAF flag for all MS affected by the HLR failure. If there is some activity between the SGSN and the VLR, the SGSN sends a BSSAP+ message MS Activity Indication to the VLR, so that it can update the MS‘s subscription entry from the HLR.

If a VLR requires specific parameters of the MS, the VLR sends the BSSGP+ message MS Information Request to the SGSN. Information requested by the VLR can be the MS‘s IMEI, P-TMSI, Mobile Location Information, etc. The SGSN returns the message MS Information Response.

The VLR can also send MM Information via the SGSN to the MS. The VLR sends the BSSGP+ message MM Information to the SGSN. The SGSN forms the GMM message GPRS Mobility Management message to forward the VLR data.

Chapter 4

Gs-Combined Procedures4.3 Formatting rules1. BSSAP+ Message Layout

BSSAP+ Message Layout

8 7 456 23 1

Message Type

Message Parameters

2

n

1

00000001 BSSAP+-Paging-Request00000010 BSSAP+-Paging-Reject00001001 BSSAP+-Location-Update-Request00001010 BSSAP+- Location-Update-Accept00001011 BSSAP+- Location-Update-Reject00001100 BSSAP+-TMSI-Reallocation-Complete00001101 BSSAP+-Alert-Request

The format of BSSAP+ messages follows the basic standard of layer 3 messages. This means the formatting rules are the same as for GMM, MM, CC and SM protocol.

 The message always begins with a message type indicator which is one octet in length. Then come the message

specific parameters, which can have the formats TLV, LV, V, TV and T.