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Chap04 gs 03_kh

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Farzad Ramin
Farzad Ramin

The document describes Gs interface concepts and combined procedures for GPRS and circuit-switched services. It discusses Gs association establishment and release, combined routing area updates, attach procedures, and detach procedures. The key points are: 1. The Gs interface coordinates location information between SGSN and MSC/VLR for combined circuit-switched and packet-switched services. 2. Combined procedures allow updating location information for both domains with a single request. This includes routing area updates, attach procedures when a subscriber becomes attached to both networks, and detach procedures. 3. The Gs association links the subscriber records in SGSN and MSC/VLR and must be established for

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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
Chapter 4 Gs-Combined Procedures 4.1 Gs interface concepts 1. Protocols for Combined Procedures 2. Gs-Association 3. Gs-Association Establishment and Release
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
Gs-Association Gs-Null Gs-Associated IMSI IMSI VLR-no. SGSN-no. SGSN VLR SGSN Association VLR
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, ...)
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
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
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 )
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
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) 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
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.
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 ()
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 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
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 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
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.
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 )
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 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
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 MSs IMSI and the serving VLRs number. The location area identifier (when the 卒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
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, ... )
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.
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
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 MSs 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 MSs 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 Procedures 4.3 Formatting rules 1. BSSAP+ Message Layout
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.

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Chap04 gs 03_kh

  • 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 MSs IMSI and the serving VLRs number. The location area identifier (when the 卒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 MSs 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 MSs 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.

Editor's Notes

  • #4: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling)
  • #6: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) but extended. See TS 23.060 chap. 6.3 Note: The SGSN - MSC/VLR association is removed at the following occasions: - At IMSI detach. - At GPRS detach. When the MSC/VLR receives an LA update via the A or Iu interface from an MS for which an association exists, the MSC/VLR shall remove the association without notifying the SGSN. When the SGSN receives a (non-combined) RA update from an MS for which an association exists, the SGSN shall remove the association without notifying the MSC/VLR. When the MSC/VLR receives a BSSAP+ MS Unreachable message from the SGSN indicating that PPF is cleared, the state of the association shall not be changed at the MSC/VLR.
  • #8: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) See TS 23.060 chap. 6.3 and TS 29.018 Note: The SGSN - MSC/VLR association is removed at the following occasions: - At IMSI detach. - At GPRS detach. When the MSC/VLR receives an LA update via the A or Iu interface from an MS for which an association exists, the MSC/VLR shall remove the association without notifying the SGSN. When the SGSN receives a (non-combined) RA update from an MS for which an association exists, the SGSN shall remove the association without notifying the MSC/VLR. When the MSC/VLR receives a BSSAP+ MS Unreachable message from the SGSN indicating that PPF is cleared, the state of the association shall not be changed at the MSC/VLR.
  • #9: Checke nochmals die timer
  • #12: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) See TS 23.060 chap. 6.3 and TS 29.018 Note: The SGSN - MSC/VLR association is removed at the following occasions: - At IMSI detach. - At GPRS detach. When the MSC/VLR receives an LA update via the A or Iu interface from an MS for which an association exists, the MSC/VLR shall remove the association without notifying the SGSN. When the SGSN receives a (non-combined) RA update from an MS for which an association exists, the SGSN shall remove the association without notifying the MSC/VLR. When the MSC/VLR receives a BSSAP+ MS Unreachable message from the SGSN indicating that PPF is cleared, the state of the association shall not be changed at the MSC/VLR.
  • #13: Checke nochmals die timer
  • #14: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018 Note: T6-1: It should be higher than 2 times the maximum transmission time in the Gs interface, plus the supervision timer of the Update Location procedure [3GPP TS 29.002] Note: T6-2: It should be higher than 2 times the maximum transmission time in the Gs interface, plus 4 times T3350 [3GPP TS 24.008]
  • #16: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #17: Question: Pr端fe: SGSN-HLR: update GPRS location?
  • #18: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #20: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #22: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #24: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #26: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #28: Source: copied (mostly) one-to-one from the TC 1720-5 (UMTS CN signalling) TS 29.018
  • #31: Remark: gr端enen teil pr端fen/端berarbeiten