Section: 4.3.5.6 Idle mode signalling reduction function
Content: The Idle mode Signalling Reduction (ISR) function provides a mechanism to limit signalling during inter-RAT cell-reselection in idle mode (ECM-IDLE, PMM-IDLE, GPRS STANDBY states). NOTE 1: The Idle mode Signalling Reduction function is mandatory for E-UTRAN UEs that support GERAN and/or UTRAN and optional for core network. The UE's ISR capability in the UE Core Network Capability element is for test purpose. The MME/SGSN activates ISR only if the Serving GW supports the ISR. How MME/SGSN determines a Serving GW supports ISR is implementation dependent. ISR shall be activated by decision of the CN nodes and shall be explicitly signalled to the UE as "ISR activated" in the RAU and TAU Accept messages. The UE may have valid MM parameters both from MME and from SGSN. The "Temporary Identity used in Next update" (TIN) is a parameter of the UE's MM context, which identifies the UE identity that the UE shall indicate in the next RAU Request, TAU Request or Attach Request message. The TIN also identifies the status of ISR activation in the UE. The TIN can take one of the three values, "P-TMSI", "GUTI" or "RAT-related TMSI". The UE shall set the TIN when receiving an Attach Accept, a TAU Accept or RAU Accept message according to the rules in table 4.3.5.6-1. Table 4.3.5.6-1: Setting of the TIN When "ISR Activated" is indicated by the RAU/TAU Accept message but the UE shall not set the TIN to "RAT-related TMSI" is a special situation. Here the UE has deactivated ISR due to special situation handling. By maintaining the old TIN value the UE remembers to use the RAT specific TMSI indicated by the TIN when updating with the CN node of the other RAT. Only if the TIN is set to "RAT-related TMSI" ISR behaviour is enabled for the UE, i.e. the UE can change between all registered areas and RATs without any update signalling and it listens for paging on the RAT it is camped on. If the TIN is set to "RAT-related TMSI", the UE's P-TMSI and RAI as well as its GUTI and TAI(s) shall remain registered with the network and shall remain valid in the UE. Table 4.3.5.6-2: Temporary UE Identity that the UE shall indicate in Attach Request and TAU/RAU Request (as "old GUTI" or as "old P-TMSI/RAI" information element) Table 4.3.5.6-2 shows which temporary identity the UE shall indicate in a Tracking or Routing Area Update Request or in an Attach Request message, when the UE stores these as valid parameters. Situations may occur that cause unsynchronized state information in the UE, MME and SGSN. Such special situations trigger a deactivation of ISR locally in the UE. The UE shall deactivate ISR locally by setting its TIN to the temporary identity of the currently used RAT in following special situations: - Modification of any EPS bearer context or PDP context which was activated before the ISR is activated in the UE; - At the time when the UE moves from E-UTRAN to GERAN/UTRAN or moves from GERAN/UTRAN to E-UTRAN by means other than PSHO, if any EPS bearer context or PDP context activated after the ISR was activated in the UE exists; - At the time when the UE moves from GERAN/UTRAN to E-UTRAN by means other than PSHO and CS to PS SRVCC, if the PDP contexts were suspended in GERAN and not successfully resumed before returning to E-UTRAN; - After updating either MME or SGSN about the change of the UE specific DRX parameters to guarantee that the other CN node is also updated; - After updating either MME or SGSN about the change of the UE Core Network Capabilities to guarantee that the other CN node is also updated; - E-UTRAN selection by a UTRAN-connected UE (e.g. when in URA_PCH to release Iu on UTRAN side); - E-UTRAN selection from GERAN READY state; - GERAN selection by an E-UTRAN-connected UE via Cell Change Order that is not for CS fallback; - After a LAU procedure if the UE has CS fallback and/or SMS over SGs activated. - For a UE that is IMS registered for voice, then after that UE moves from a Tracking Area List that supports IMS voice over PS sessions (see 4.3.5.8 for more information) to one that does not, and vice versa. It shall be possible, e.g. using Device Management or initial provisioning, to configure the UE to apply/not apply this particular exception. NOTE 2: A UE moving between Tracking Area Lists that both support IMS voice over PS sessions, or that both do not support IMS voice over PS sessions, is unaffected by the above. The UE shall deactivate ISR locally by setting its TIN to the temporary identity of the RAT that is still available to the UE in following special situations: - After the RAT-specific Deactivate ISR timer expires, e.g. because the coverage of that RAT is lost or the RAT is no more selected by the UE (this may result also in implicit detach by SGSN or MME). ISR shall be deactivated in the UE by the CN node using normal update signalling, i.e. by omitting the signalling of "ISR Activated", in following special situations: - CN node change resulting in context transfer between the same type of CN nodes (SGSN to SGSN or MME to MME); - Serving GW change; - When the UE only has bearers related to emergency bearer service; - When the UE is registered for RLOS service; - TAU or RAU when UE moves over the border between local and macro network where SIPTO at local network with stand-alone GW and Serving GW relocation without mobility are supported in the core network. - TAU or RAU when the network confirms to use PSM for the UE. If tracking area list or routing area covers both local network and macro network, the ISR shall not be activated if the UE is allowed to use SIPTO at local network and Serving GW relocation without mobility are supported in the core network.