The MS or the RAN decides to perform an intersystem change which makes the MS switch to a new cell where Iu mode has to be used, and stops transmission to the network.
The MS initiates an RRC connection establishment and sends a Routeing Area Update Request (P-TMSI, Old RA, Old P-TMSI Signature, Update Type, CM, Voice domain preference and UE's usage setting) message to the combined 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 attach requested and also if the MS has a follow on request, i.e. if there is pending uplink traffic (signalling or data). The SGSN may use, as an implementation option, the follow-on request indication to release or keep the Iu connection after the completion of the RA update procedure. The SRNS shall add an identifier of the area where the message was received before passing the message to the 2G+3G SGSN. The 2G+3G SGSN stops transmission of N PDUs to the MS. The UE sets the voice domain preference and UE's usage setting according to its configuration, as described in clause 5.3.15
Security functions may be executed.
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, 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 SGSN Number. In networks that support network sharing, the Location Update Request includes the identity of the selected core network operator if the SGSN has received this information from the RNS, as described in TS 23.251
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:
The new VLR sends an Update Location (new VLR) to the HLR.
The HLR cancels the data in the old VLR by sending Cancel Location (IMSI) to the old VLR.
The old VLR acknowledges with Cancel Location Ack (IMSI).
The HLR sends Insert Subscriber Data (IMSI, subscriber data) to the new VLR.
The new VLR acknowledges with Insert Subscriber Data Ack (IMSI).
The HLR responds with Update Location Ack (IMSI) to the new VLR.
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.
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 the network supports the MOCN configuration for network sharing, the SGSN may, if the MS is not a 'Network Sharing Supporting MS', in this case decide to initiate redirection by sending a Reroute Command to the RNS, as described in TS 23.251
instead of rejecting the routeing area update. 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 Routeing Area Update Accept (P-TMSI, P-TMSI Signature, IMS voice over PS Session Supported Indication, Emergency Service Support) message is returned to the MS. The 2G+3G-SGSN derives for this intersystem change the corresponding PDCP sequence numbers from the N PDU sequence numbers stored in the SGSN PDP contexts by adding eight most significant bits "1". These PDCP sequence numbers are stored in the SGSN PDP contexts. The IMS voice over PS Session Supported Indication is set as described in clause 5.3.8
The Emergency Service Support indicator shall be included when going to UTRAN to inform the MS that Emergency PDP contexts are supported, i.e. the MS is allowed to request activation of emergency PDP context when needed.
The MS acknowledges the new P-TMSI by returning a Routeing Area Update Complete message to the SGSN.
The 2G+3G SGSN sends a TMSI Reallocation Complete message to the VLR if the MS confirms the VLR TMSI.
If the MS has pending uplink data or signalling, it shall send a Service Request (P-TMSI, RAI, CKSN, Service Type) message to the SGSN. Service Type specifies the requested service. Service Type shall indicate one of the following: Data or Signalling.
The 2G+3G SGSN requests the SRNS to establish a radio access bearer by sending a RAB Assignment Request (RAB ID(s), QoS Profile(s), GTP SNDs, GTP SNUs, PDCP SNUs, UE-AMBR, MSISDN, APN, Charging characteristics) message to the SRNS. If Direct Tunnel is established the SGSN provides to the RNC the GGSN's Address for User Plane and TEID for uplink data. The PDCP sequence numbers are derived from the N PDU sequence numbers and stored in the PDP contexts in step 7). The SRNS sends a Radio Bearer Setup Request (PDCP SNUs) message to the MS. The MS responds with a Radio Bearer Setup Complete (PDCP SNDs) message. The SRNS responds with a RAB Assignment Response message. MSISDN, APN and Charging characteristics are optional parameters and only transferred if SGSN supports SIPTO at Iu-ps.
If the SGSN established Direct Tunnel it shall send Update PDP Context Request to the GGSN(s) concerned and include the RNC's Address for User Plane, downlink TEID for data and DTI to instruct the GGSN(s) to apply Direct Tunnel specific error handling as described in clause 13.8
. The GGSN(s) update the Address for User Plane and TEID for downlink 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.
Traffic flow is resumed between the 2G+3G SGSN and the SRNS. N-PDUs that were already sent to the MS in acknowledged mode SNDCP and that are not yet acknowledged by the MS are tunnelled by the 2G+3G SGSN to the SRNS together with their related N-PDU number (SNDCP sequence number). No PDCP sequence numbers shall be indicated for these N-PDUs. The SRNS shall discard all N PDUs with N PDU sequence numbers older than the eight least significant bits of PDCP-SND received from the MS. Other N PDUs shall be transmitted to the MS. The MS shall discard all N PDUs with sequence numbers older than the eight least significant bits of the PDCP SNU received from the SRNS. All other N PDUs shall be transmitted to the SRNS. The SRNS negotiates with the MS for each radio bearer the use of lossless PDCP or not regardless whether the old 2G-SGSN used acknowledged or unacknowledged SNDCP for the related NSAPI or not.
The traffic flow is resumed between the SRNS and the MS.
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.