Section: 5.3.3.0 Triggers for tracking area update
Content: A stand-alone tracking area update (with or without S-GW change, described in clauses 5.3.3.1 and 5.3.3.2 respectively) occurs when a GPRS-attached or E-UTRAN-attached UE experiences any of the following conditions: - UE detects it has entered a new TA that is not in the list of TAIs that the UE registered with the network (except for the case of a UE configured to perform Attach with IMSI when entering a TA in a new non-equivalent PLMN in RRC-IDLE mode); - the periodic TA update timer has expired; - UE was in UTRAN PMM_Connected state (e.g. URA_PCH) when it reselects to E-UTRAN; - UE was in GPRS READY state when it reselects to E-UTRAN; - the TIN indicates "P-TMSI" when the UE reselects to E-UTRAN (e.g. due to bearer configuration modifications performed on GERAN/UTRAN); - the RRC connection was released with release cause "load re-balancing TAU required"; - the RRC layer in the UE informs the UE's NAS layer that an RRC connection failure (in either E-UTRAN or UTRAN) has occurred; - a change of the UE Network Capability and/or MS Network Capability and/or UE Specific DRX Parameters and/or TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  [47] MS Radio Access capability (e.g. due to GERAN radio capability change, E-UTRAN, NG-RAN capability change or cdma2000 Radio Access Technology Capability change) information of the UE. - for UE supporting RACS in ECM-IDLE as defined in clause 5.11.3a, a change in UE Radio Access capability (e.g. due to GERAN radio capability change, E-UTRAN, NG-RAN capability change or cdma2000 Radio Access Technology Capability change) corresponding to signalling a different UE Radio Capability ID. - at every change between a cell that does not broadcast SystemInformationBlockType31(-NB) and an E-UTRA cell that broadcasts SystemInformationBlockType31(-NB). - a change in conditions in the UE require a change in the extended idle mode DRX parameters previously provided by the MME. - for a UE supporting CS fallback, or configured to support IMS voice, or both, a change of the UE's usage setting or voice domain preference for E-UTRAN. - for a SR-VCC capable UE, a change of MS Classmark 2 and/or MS Classmark 3 and/or Supported Codecs. - UE manually selects a CSG cell whose CSG ID and associated PLMN is absent from both the UE's Allowed CSG list and the UE's Operator CSG list. - UE receives a paging request from the MME while the Mobility Management back off timer is running and the UE's TIN indicates "P-TMSI". - a change in any of the values of information included in Preferred Network Behaviour as defined in clause 4.3.5.10 that would create incompatibility with the Supported Network Behaviour provided by the serving MME. - with satellite access for Cellular IoT upon changing to a suitable cell indicating one or more TACs for the RPLMN all of which are outside the UE's Tracking Area List in both ECM-CONNECTED and ECM-IDLE. - a UE that is using a RAN that provides discontinuous coverage (e.g. for satellite access with discontinuous coverage) is about to leave the satellite network coverage as described in clause 4.13.8.2. - when the UE has informed the network that it is unreachable and now returns to coverage using either satellite or terrestrial access as described in clause 4.13.8.2. NOTE 1: The complete list of TAU triggers is specified in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]. NOTE 2: With satellite access for Cellular IoT, more than one TAC can be indicated to a UE for each PLMN in any cell, see clause 4.13.6. The procedure is initiated by an UE in either ECM-IDLE state or ECM-CONNECTED state. The decision to perform S-GW change during the tracking area update procedure is made by the MME independently from the triggers above. If SIPTO is allowed for the APN associated with a PDN connection, the MME should re-evaluate whether the PDN GW location is still acceptable. If the MME determines that PDN GW re-location is needed, the MME may initiate PDN deactivation with reactivation requested according to clause 5.10.3 at the end of the tracking area/routing area update procedure. NOTE 3: It depends on the operator's configuration in the MME whether to use the deactivation with reactivation request or allow the continued usage of the already connected GW. If SIPTO at the local network is allowed for the APN associated with a PDN connection the MME handles the SIPTO at the Local Network PDN connection as follows. For a L-GW collocated with (H)eNB: - For intra-MME mobility, upon completion of the TAU procedure the MME shall deactivate the SIPTO at the local Network PDN connection with the "reactivation requested" cause value according to clause 5.10.3. If the UE has no other PDN connection, the MME initiates "explicit detach with reattach required" procedure according to clause 5.3.8.3. - For Inter-MME/SGSN mobility, as part of the Tracking Area Update procedure, the source MME shall remove the bearer(s) corresponding to the SIPTO at Local Network PDN connection and shall release the core network resources associated to the SIPTO at the Local Network PDN connection by performing the MME-initiated PDN Connection Deactivation before sending the Context Response message. For a stand-alone GW: - For intra-MME mobility, upon completion of the TAU procedure the MME checks that the Local Home Network ID has changed and decides whether to deactivate the SIPTO at the local Network PDN connection with the "reactivation requested" cause value according to clause 5.10.3. If the UE has no other PDN connection, the MME initiates "explicit detach with reattach required" procedure according to clause 5.3.8.3. - For Inter-MME/SGSN mobility, upon completion of the TAU/RAU procedure the new MME/SGSN checks that the Local Home Network ID has changed and decides whether to deactivate the SIPTO at the Local Network PDN connection with the "reactivation requested" cause value according to clause 5.10.3. If the UE has no other PDN connection, the MME initiates "explicit detach with reattach required" procedure according to clause 5.3.8.3. If LIPA is active for a PDN connection of the UE, the source MME (or S4-SGSN) shall not include LIPA bearer(s) in the EPS bearer Context during Tracking Area Update procedure and shall release the core network resources of this LIPA PDN connection by performing the MME requested PDN disconnection procedure according to steps 2 to 6 of clause 5.10.3 before it responds with the Context Response message in the case of inter-MME/SGSN mobility or after it receives Tracking Area Update Request in the case of intra-MME mobility. NOTE 4: The source MME may not be able to release the LIPA PDN connection after the Context Response is sent as when there is no S-GW relocation, the S-GW will assign the S11 control tunnel of the UE to the new MME after the new MME updates the context information. During the Tracking Area Update procedure, if the MME supports SRVCC and if the UE SRVCC capability has changed, the MME informs the HSS with the UE SRVCC capability e.g. for further IMS registration. The cell selection for UTRAN is described in TS 25.304[ None ]  [12] and TS 25.331[ None ]  [33]. If during the Tracking Area Update procedure the MME detects that the Serving GW or/and the MME needs be relocated, the old MME may reject any PDN GW initiated EPS bearer(s) request received since the Tracking Area Update procedure started and if rejected, the old MME shall include an indication that the request has been temporarily rejected due to mobility procedure in progress. The rejection is forwarded by the Serving GW to the PDN GW, with the indication that the request has been temporarily rejected. In the case of satellite access for Cellular IoT, upon receiving the TAU Request message, the MME may verify the UE location and determine whether the PLMN is allowed to operate at the UE location, as described in clause 4.13.4. If the UE receives a TAU Reject message with cause value indicating that the selected PLMN is not allowed to operate at the present UE location, the UE shall attempt to select a PLMN as specified in TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [10]. Upon reception of a rejection for an EPS bearer(s) PDN GW initiated procedure with an indication that the request has been temporarily rejected due to mobility procedure in progress, the PDN GW start a locally configured guard timer. The PDN GW shall re-attempt, up to a pre-configured number of times, when either it detects that the Tracking Area Update procedure is completed or has failed using message reception or at expiry of the guard timer. A Multi-USIM UE may trigger a TAU to send a Requested IMSI Offset in order to derive an alternative IMSI as defined in clause 4.3.33. NOTE 5: As an exception, during a TAU procedure due to mobility to a new Tracking Area outside the Tracking Area List and detecting paging collision at the same time, a Multi-USIM UE implementation can decide to indicate to the MME a Requested IMSI Offset even if it does not know whether the MME serving the new Tracking Area supports it.