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Content for  TS 23.060  Word version:  16.0.0

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6.13  Intersystem ChangeWord‑p. 181
An intersystem change takes place when an MS changes between Iu mode and A/Gb mode of operation by the Routeing Area Update procedure or by PS handover. A prerequisite for an intersystem change is that the MS is GPRS-attached. The transition of the mobility management states is as specified for the corresponding mobility management procedures.
There is no transition of the session management states at an intersystem change.

6.13.1  Intra SGSN Intersystem Change

An SGSN that supports both the Gb and Iu PS interfaces may support an intra-SGSN intersystem change if the radio access technology nodes serving the MS before and after the intersystem change are both served by this SGSN.

6.13.1.1  Iu mode to A/Gb mode Intra SGSN Change

6.13.1.1.1  Iu mode to A/Gb mode Intra SGSN Change using Gn/Gp |R8|
The intersystem change from Iu mode to A/Gb mode takes place when an MS changes from UTRAN or GERAN Iu mode to A/Gb mode. Depending on the PMM state before the intersystem change and whether the RA is changed or not, one of the following procedures is initiated by the MS:
  • When an MS in PMM IDLE state changes to the A/Gb mode without changing the RA, the MS shall follow the selective RA update procedures, see clause "Selective RA Update".
  • When an MS in PMM IDLE state changes to the A/Gb mode and the RA changes, the MS shall initiate the GPRS RA update procedure, see clause "Intra SGSN Routeing Area Update".
  • When an MS in PMM CONNECTED state changes to the A/Gb mode, the MS shall initiate the GPRS RA update procedure independent of whether the RA has changed or not. The RA update procedure is either combined RA / LA update or only RA update.
A combined RA / LA update takes place in network operation mode I when the MS enters a new RA or when a GPRS-attached MS performs IMSI attach. The MS sends a Routeing Area Update Request message indicating that an LA update may also need to be performed, in which case the SGSN forwards the LA update to the VLR. This concerns only idle mode (see TS 23.122), as no combined RA / LA updates are performed during a CS connection. In the context of this specification, the terms RNS or RNC refer also to a GERAN BSS or BSC (respectively) when serving an MS in Iu mode.
Reproduction of 3GPP TS 23.060, Figure 52: Iu mode to A/Gb mode Intra SGSN Change
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Step 1.
The MS or RAN decides to perform an intersystem change which makes the MS switch to a new cell where A/Gb mode has to be used, and stops transmission to the network.
Step 2.
The MS sends a Routeing Area Update Request (old RAI, old P-TMSI Signature, Update Type, Voice domain preference and UE's usage setting) message to the 2G+3G SGSN. Update Type shall indicate RA update or combined RA / LA-update or, if the MS wants to perform an IMSI attach, combined RA / LA update with IMSI attached requested. The BSS shall add the Cell Global Identity including the RAC and LAC of the cell where the message was received before passing the message to the 2G+3G SGSN. The UE sets the voice domain preference and UE's usage setting according to its configuration, as described in clause 5.3.15.
If there is an ongoing emergency bearer service and a Routing Area Update Request is received the Routing Area Update shall be rejected with a cause code indicating that access to GERAN is not allowed.
Step 3.
If the MS is PMM CONNECTED state, the 2G+3G SGSN sends an SRNS Context Request (IMSI) message to the SRNS.
Upon reception of the SRNS Context Request message, the SRNS starts buffering and stops sending downlink PDUs to the MS. The SRNS responds with an SRNS Context Response (GTP SNDs, GTP SNUs, PDCP-SNDs, PDCP SNUs) message. The GTP sequence numbers are included for each PDP context indicating the next in-sequence downlink GTP-PDU to be sent to the MS and the next in-sequence GTP PDU to be tunnelled to the GGSN. For each active PDP context, which uses lossless PDCP, the SRNS also includes the uplink PDCP sequence number (PDCP SNU) and the downlink PDCP sequence number (PDCP-SND). PDCP SNU is the PDCP sequence number for the next expected in-sequence uplink packet to be received from the MS. PDCP-SND is the PDCP sequence number for the first downlink packet for which successful transmission has not been confirmed. The 2G+3G SGSN shall strip off the eight most significant bits of the passed PDCP sequence numbers, thus converting them to SNDCP N PDU numbers of the respective 2G GPRS PDP contexts.
Step 5.
Security functions may be executed.
Step 6.
If the MS is PMM CONNECTED, the 2G+3G SGSN sends an SRNS Data Forward Command (RAB ID, Transport Layer Address, Iu Transport Association) message to the SRNS. This informs the SRNS that the 2G+3G SGSN is ready to receive data packets. Upon reception of SRNS Data Forward Command message from the 2G+3G SGSN the SRNS shall start the data-forwarding timer.
Step 6a.
If Direct Tunnel was established in Iu mode the SGSN sends Update PDP Context Request to the GGSN(s) concerned to establish the GTP tunnel between SGSN and GGSN. The GGSN(s) update the address for User Plane and downlink TEID for data and return an Update PDP Context Response. Otherwise, if there were changes of for example the RAT type that e.g. can be used for charging, the SGSN sends Update PDP Context Request (SGSN Address and TEID, QoS Negotiated, RAT type) message to the GGSN.
Step 7.
For each RAB indicated by the SRNS Data Forward Command the SRNS starts duplicating and tunnelling the buffered GTP-PDUs back to the 2G+3G SGSN. For each radio bearer which uses lossless PDCP the GTP-PDUs related to transmitted but not yet acknowledged PDCP PDUs are duplicated and tunnelled back to the 2G+3G SGSN together with their related downlink PDCP sequence numbers. The 2G+3G SGSN converts the PDCP sequence numbers to SNDCP sequence number (by stripping off the eight most significant bits of the PDCP sequence numbers).
Step 8.
The 2G+3G SGSN sends an Iu Release Command message to the SRNS. When the RNC data-forwarding timer has expired, the SRNS responds with an Iu Release Complete message.
Step 9.
If the association has to be established i.e. if Update Type indicates combined RA / LA update with IMSI attach requested, or if the LA changed with the routeing area update, then the 2G+3G SGSN sends a Location Update Request (new LAI, IMSI, SGSN Number, Location Update Type) to the VLR. Location Update Type shall indicate IMSI attach if Update Type in step 1 indicated combined RA / LA update with IMSI attach requested. Otherwise, Location Update Type shall indicate normal location update. When the SGSN does not provide functionality for the Intra Domain Connection of RAN Nodes to Multiple CN Nodes, the VLR number is derived from the RAI. When the SGSN provides functionality for Intra Domain Connection of RAN Nodes to Multiple CN Nodes, the SGSN uses the RAI and a hash value from the IMSI to determine the VLR number. The VLR creates or updates the association with the 2G+3G SGSN by storing the SGSN Number.
Step 10.
If the subscriber data in the VLR is marked as not confirmed by the HLR, the new VLR informs the HLR. The HLR cancels the data in the old VLR and inserts subscriber data in the new VLR:
  1. The new VLR sends an Update Location (new VLR) to the HLR.
  2. The HLR cancels the data in the old VLR by sending Cancel Location (IMSI) to the old VLR.
  3. The old VLR acknowledges with Cancel Location Ack (IMSI).
  4. The HLR sends Insert Subscriber Data (IMSI, subscriber data) to the new VLR.
  5. The new VLR acknowledges with Insert Subscriber Data Ack (IMSI).
  6. The HLR responds with Update Location Ack (IMSI) to the new VLR.
Step 11.
The new VLR allocates a new VLR TMSI and responds with Location Update Accept (VLR TMSI) to the 2G+3G SGSN. VLR TMSI is optional if the VLR has not changed.
Step 12.
The 2G+3G SGSN validates the MS's presence in the new RA. If due to roaming restrictions or access restrictions the MS is not allowed to be attached in the RA, or if subscription checking fails, the 2G+3G SGSN rejects the routeing area update with an appropriate cause. If all checks are successful, the 2G+3G SGSN updates MM and PDP contexts for the MS. A new P-TMSI may be allocated. A logical link is established between the new 2G+3G SGSN and the MS. 2G+3G-SGSN initiates the establishment procedure. A Routeing Area Update Accept (P-TMSI, P-TMSI Signature, Receive N PDU Number (= converted PDCP SNU), IMS voice over PS Session Supported Indication) message is returned to the MS. Receive N PDU Number contains the acknowledgements for each NSAPI which used lossless PDCP before the start of the update procedure, thereby confirming all mobile-originated N PDUs successfully transferred before the start of the update procedure. If Receive N PDU Number confirms the reception of N PDUs, these N PDUs shall be discarded by the MS. The IMS voice over PS Session Supported Indication is set as described in clause 5.3.8.
Step 13.
The MS acknowledges the new P-TMSI by returning a Routeing Area Update Complete (Receive N PDU Number) message to the SGSN. Receive N PDU Number (= converted PDCP SND) contains the acknowledgements for each NSAPI which used lossless PDCP before the start of the update procedure, thereby confirming all mobile-terminated N PDUs successfully transferred before the start of the update procedure. If Receive N PDU Number confirms the reception of N PDUs, these N PDUs shall be discarded by the 2G+3G-SGSN.The MS deducts Receive N PDU Number from PDCP SND by stripping off the eight most significant bits. PDCP SND is the PDCP sequence number for the next expected in-sequence downlink packet to be received in the MS per radio bearer, which used lossless PDCP. The new 2G-SGSN negotiates with the MS for each NSAPI the use of acknowledged or unacknowledged SNDCP regardless whether the SRNS used lossless PDCP or not.
Step 14.
The 2G+3G SGSN sends a TMSI Reallocation Complete message to the VLR if the MS confirms the VLR TMSI.
Step 15.
The 2G+3G SGSN and the BSS may execute the BSS Packet Flow Context procedure.
For some network sharing scenario (e.g. GWCN) if the PLMN-ID of the RAI supplied by the RNC is different from that of the RAI in the UE's context, then the SGSN shall informs the HLR.
The CAMEL procedure calls shall be performed, see referenced procedure in TS 23.078:
C1)
CAMEL_GPRS_Routeing_Area_Update_Session, CAMEL_PS_Notification and CAMEL_GPRS_Routeing_Area_Update_Context.
  • The procedure CAMEL_GPRS_Routeing_Area_Update_Session is called once per session. In Figure 52, the procedure returns as result "Continue".
  • Then the procedure CAMEL_PS_Notification is called once per session. The procedure returns as result "Continue".
  • Then, the procedure CAMEL_GPRS_Routeing_Area_Update_Context is called once per PDP context. In Figure 52, the procedure returns as result "Continue".
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6.13.1.1.2  Iu mode to A/Gb mode Intra SGSN Change using S4 |R8|Word‑p. 184
In this case, clause 6.13.1.1.1 applies except for steps 6a and 7, as well as section specific general statements stated below.
Reproduction of 3GPP TS 23.060, Figure 52-2: step 6a for Iu mode to A/Gb mode Intra SGSN Change using S4
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a)
In this procedure flow the Serving GW is not relocated. If Direct Tunnel was established in Iu mode or if there were changes of for example the RAT type that e.g. can be used for charging, the SGSN sends Modify Bearer Request (SGSN Address and TEID, serving network identity, CN Operator Selection Entity, RAT type) message to the Serving GW.
b)
The Serving GW informs the P-GW(s) about the change of for example the RAT type that e.g. can be used for charging, by sending the message Modify Bearer Request (Serving GW Address and TEID, RAT type) to the concerned P-GW(s). If dynamic PCC is deployed, and RAT type information needs to be conveyed from the P-GW to the PCRF, then the P-GW sends RAT type information to the PCRF as defined in TS 23.203.
c)
Each P-GW updates its context field and returns a Modify Bearer Response (MSISDN, P-GW address and TEID) message to the Serving GW. MSISDN is included if available in the stored UE context.
d)
The Serving GW updates the address for User Plane and downlink TEID for data and return a Modify Bearer Response (Serving GW address and TEID, P-GW address and TEIDs (for GTP based S5/S8) or GRE keys (for PMIP based S5/S8) at the PDN GW(s) for uplink traffic) message.
e)
In case Direct Tunnel in Iu mode was not established, for each RAB indicated by the SRNS Data Forward Command the SRNS starts duplicating and tunnelling the buffered GTP-PDUs back to the 2G+3G SGSN. For each radio bearer which uses lossless PDCP the GTP-PDUs related to transmitted but not yet acknowledged PDCP PDUs are duplicated and tunnelled back to the 2G+3G SGSN together with their related downlink PDCP sequence numbers. The 2G+3G SGSN converts the PDCP sequence numbers to SNDCP sequence number (by stripping off the eight most significant bits of the PDCP sequence numbers).
In case Direct Tunnel in Iu mode was established, the packets are forwarded via the S-GW.
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