1. Chapter 4
Gs-Combined Procedures
Contents:
4.1 Gs interface concepts
1. Protocols for Combined Procedures
2. Gs-Association
3. Gs-Association Establishment and Release
4.2 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 Services
7. BSSAP+: Alerting Procedure
8. BSSAP+: Other Procedures
4.3 Formatting rules
1. BSSAP+ Message Layout

2. Chapter 4
Gs-Combined Procedures
4.1 Gs interface concepts
1. Protocols for Combined Procedures
2. Gs-Association
3. Gs-Association Establishment and Release

3. Protocols for Combined Procedures
• cs and ps service location
information co-ordination
BSSAP+ BSSAP+
SCCP SCCP
MTP level 3 MTP level 3
MTP level 2 MTP level 2
MTP level 1 Gs MTP level 1
SGSN VLR
Class A or B MS

4. Gs-Association
Gs-Null
Gs-Associated
IMSI IMSI
VLR-no.
SGSN-no.
SGSN VLR – SGSN Association VLR

5. Gs-Association Establishment and Release
SGSN VLR
Gs-Association
Establishment BSSGP+-Location Area Update Request
Start T6-1
( IMSI, SGSN-no, ...)
Stop T6-1 BSSGP+-Location Area Update Accept
( IMSI, ...)
explicit BSSGP+-GPRS Detach Indication or
Gs-Association BSSGP+-IMSI Detach Indication
Release Start T6-1
( IMSI, ...)
BSSGP+-GPRS Detach Indication Acknowledge or
Stop T6-1 BSSGP+-IMSI Detach Indication Acknowledge
( IMSI, ...)

6. Chapter 4
Gs-Combined Procedures
4.2 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 Services
7. BSSAP+: Alerting Procedure
8. BSSAP+: Other Procedures

7. GMM: Routing Area Update Procedure
MS
SGSN
Start T3330 Routing Area Update Request
(= 15 s) ( Update type, old RAI, P-TMSI, optional: TMSI status, ... )
• RA Update VLR
• combined RA/LA Update
• combined RA/LA Update with IMSI • no valid TMSI available optional:
Attach • valid TMSI is available BSSAP+
• periodic Update messages
Routing Area Update Accept Start T3350 (= 6 s)
Stop T3330
( Update result, optional: allocated P-TMSI, MS Identity (= TMSI), ... ) only when
• P-TMSI and/or
• RA updated • Receive N-PDU
• RA/LA updated numbers
were allocated
Routing Area Update Complete Stop T3350

8. BSSAP+: Routing Area Update Procedure
SGSN VLR
Start T6-1 BSSAP+-Location-Area-Update-Request
( IMSI, SGSN-no, update type, new CGI, HLR
MS classmark 1, TMSI status, ... )
optional
BSSAP+-Location-Area-Update-Accept
Stop T6-1 Start T6-2
( IMSI, LAI, optional: new TMSI or IMSI )
(default 40 s)
MS only when new
TMSI was allocated
RA Update Accept,
RA Update Complete
BSSAP+-TMSI-Reallocation-Complete Stop T6-2
( IMSI, CGI, optional: SAI )

9. Example: Combined LA and RA Update (Part I)
MS old new old new
SGSN SGSN VLR VLR GGSN HLR
Routing Area Update Request
( old RAI, P-TMSI, TMSI status, ... )
SGSN Context
Request
SGSN Context
Response
Authentication
Update PDP Context Request
Update PDP Context Response
Update GPRS Location
Cancel Location
Cancel Location Ack
Insert Subscriber Data
Insert Subscriber Data Ack
Update GPRS Location Ack

10. 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.

11. Example: Combined LA and RA Update (Part II)
MS old new old new
SGSN SGSN VLR VLR GGSN HLR
BSSAP+-Location-
Update-Request Update Location
Cancel Location
Cancel Location Ack
Insert Subscriber
Data
Insert Subscriber
Data Ack
BSSAP+-Location- Update Location
Routing Area Update Accept Update-Accept Ack
( Update Result, ... )
Routing Area Update Complete
BSSAP+-TMSI-
Reallocation-Complete

12. Combined Attach Procedure
The 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.

13. GMM: Combined Attach
MS
SGSN
Attach Request
Start T3310
( Access type, P-TMSI or IMSI, old RAI, ...)
(= 15 s)
VLR
• GPRS attach
• GPRS attach while IMSI attached optional:
• combined GPRS/IMSI attach BSSAP+
messages
Stop T3310 Attach Accept
Start T3350 (= 6 s)
( Attach result, new RAI, optional: allocated P-TMSI, ... )
(only if (P-)TMSI
allocated)
• GPRS only attached
• combined GPRS/IMSI attached
Attach Complete Stop T3350
()

14. 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.

15. Example: Combined Attach Procedure
MS old new
SGSN SGSN VLR HLR
Attach Request
( 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
Insert Subscriber Data Ack
Update GPRS Location Ack
BSSAP+-Location-
Update-Request
BSSAP+-Location-
Routing Area Update Accept Update-Accept
Routing Area Update Complete

16. 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)

17. GMM: Combined Detach
MS
SGSN
Start T3321 Detach Request
(= 15 s) ( Detach type, P-TMSI, optional: P-TMSI signature )
• GPRS detach • normal detach
• IMSI detach • power switched off
• combined GPRS/IMSI detach VLR
Detach Accept optional:
Stop T3321 BSSAP+
( ) messages
Detach Request Start T3322
( Detach type )
(= 6 s)
• re-attach required,
• re-attach not required
• IMSI detach (after VLR failure)
Stop T3322
Detach Accept

18. 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.

19. BSSAP+: Combined Detach
SGSN VLR
BSSAP+-GPRS-Detach-Indication
Start T8
( IMSI, SGSN-no, IMSI detach from GPRS service type, CGI, SAI )
(default:
4s)
GPRS detach • MS initiated IMSI detach
and • combined MS initiated IMSI/GPRS detach
Gs-association • GPRS services not allowed (implicit detach)
exists
BSSAP+-GPRS-Detach-Ack
Stop T8
( IMSI )
BSSAP+-IMSI-Detach-Indication
Start T9
( IMSI, SGSN-no, detach type, CGI, Location info age, SAI )
(default:
combined or 4s)
IMSI detach • Network initiated GPRS detach
• MS initiated GPRS detach
and
• GPRS services not allowed
Gs-association
exists BSSAP+-IMSI-Detach-Ack
Stop T9 ( IMSI )

20. 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.

21. Example: MS Initiated Detach Procedure
MS
SGSN VLR GGSN
Detach Request
Delete PDP Context Request
Delete PDP Context Response
BSSAP+-GPRS-Detach-Indication
GPRS detach
BSSAP+-GPRS-Detach-Ack
OR
BSSAP+-IMSI-Detach-Indication
IMSI or combined
detach
BSSAP+-IMSI-Detach-Ack
Detach Accept

22. 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 service
request
SCCP exists no paging
connection
Gs- does not GSM paging
association exist
Decision process in MSC GPRS paging

23. Example: Paging of Non-GPRS Services
MS
BSS SGSN VLR
IAM
BSSAP+-Paging-Request
(Packet) BSSGP: PAGING CS ( IMSI, VLR-no, TMSI, LAI, ... )
Paging Request
( P-TMSI or IMSI, paging cause )
Paging Response
BSSAP: PAGING RESPONSE
BSSAP+-Paging-Request Start T5
no paging done,
e.g. MS marked
non-reachable
BSSAP+-MS-Unreachable Start T5
( IMSI, Gs cause, ... )

24. 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.

25. Example: Paging of Non-GPRS Services
MS
SGSN VLR
BSSAP+-Alert-Request Start T7
( IMSI ) (default = 4s)
set NGAF
BSSAP+-Alert-Ack Stop T7
( IMSI, Gs cause)
any activity (data or signalling)
BSSAP+-MS-Activity Indication
( IMSI, CGI, SAI )
reset NGAF

26. 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.

27. Chapter 4
Gs-Combined Procedures
4.3 Formatting rules
1. BSSAP+ Message Layout

28. BSSAP+ Message Layout
8 7 6 5 4 3 2 1
1 Message Type
2
≈ Message Parameters ≈
n 00000001 BSSAP+-Paging-Request
00000010 BSSAP+-Paging-Reject
00001001 BSSAP+-Location-Update-Request
00001010 BSSAP+- Location-Update-Accept
00001011 BSSAP+- Location-Update-Reject
00001100 BSSAP+-TMSI-Reallocation-Complete
00001101 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.