Section: 4.3.5.2 Reachability Management for UE in ECM-IDLE state
Content: The location of a UE in ECM-IDLE state is known by the network on a Tracking Area List granularity. All cells of the Tracking Areas in which a UE in ECM-IDLE is currently registered needs to be taken into account for paging. The UE may be registered in multiple Tracking Areas. All the tracking areas in a Tracking Area List to which a UE is registered are served by the same serving MME. An EMM-REGISTERED UE performs periodic Tracking Area Updates with the network after the expiry of the periodic TAU timer. The MME may allocate long periodic TAU timer value to the UE according to clause 4.3.17.3. If the UE is out of E-UTRAN coverage (including the cases when the UE is camped on GERAN/UTRAN cells) when its periodic TAU timer expires, the UE shall: - if ISR is activated, start the E-UTRAN Deactivate ISR timer. After the E-UTRAN Deactivate ISR timer expires the UE shall deactivate ISR by setting its TIN to "P-TMSI". - if ISR is activated and the UE is camping on a GERAN/UTRAN cell (or returns to coverage in GERAN/UTRAN) and the UE is EPS/IMSI attached, perform a LAU procedure in NMO II or a combined RA/LA update procedure in NMO I. - when EMM-REGISTERED, perform a Tracking Area Update when it next returns to E-UTRAN coverage. For UE using a RAN that provides discontinuous coverage (e.g. for satellite access with discontinuous coverage), if the UE knows how the E-UTRAN coverage varies with time based on information defined in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] (e.g. from the ephemeris data of a satellite access system that the UE is using) then the UE may deactivate its Access Stratum functions in order to optimise power consumption until coverage returns. Details are specified in TS 36.304[ Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode ]  [34] and TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]. If the UE is camped on an E-UTRAN cell or is in ECM-CONNECTED state when the UE's periodic RAU timer expires, the UE shall: - if ISR is activated, start the GERAN/UTRAN Deactivate ISR timer. After the GERAN/UTRAN Deactivate ISR timer expires the UE shall deactivate ISR by setting its TIN to "GUTI". - perform a Routing Area Update when it next returns to GERAN/UTRAN coverage. If the UE is EPS attached only and either camps on an E UTRAN cell or is in ECM CONNECTED state when the UE's periodic LAU timer expires, the UE shall perform a Location Area Update procedure in NMO II or combined RA/LA update in NMO I when it next returns to GERAN/UTRAN coverage. The E-UTRAN Deactivate ISR timer is stopped when the UE performs a successful Tracking Area Update or combined TA/LA Update; and the GERAN/UTRAN Deactivate ISR timer is stopped when the UE performs a successful Routing Area Update or combined RA/LA Update. Expiry of the periodic TAU timer, or, the periodic RAU timer, or, the periodic LAU timer shall not cause the UE to change RAT. The UE's periodic TAU timer is restarted from its initial value whenever the UE enters ECM-IDLE mode and when the UE leaves the E-UTRAN connection due to handover to GERAN/UTRAN. UTRAN RRC state transitions and GERAN GPRS STANDBY/READY state transitions shall have no other impact on the periodic TAU timer. E-UTRAN RRC state transitions shall have no impact on the periodic RAU timer or periodic LAU timer except that handover from GERAN/UTRAN to E-UTRAN shall cause the periodic RAU timer to be started from its initial value. Handover from E-UTRAN to UTRAN/GERAN shall cause the periodic TAU timer to be started from its initial value. Typically, the MME runs a mobile reachable timer. Whenever the UE enters ECM IDLE mode the timer is started with a value similar to the UE's periodic TAU timer. If this timer expires in the MME, the MME can deduce that the UE is not reachable. However, the MME does not know for how long the UE is not reachable, so, the MME shall not immediately delete the UE's bearers. Instead the MME should clear the PPF flag in the MME and start an Implicit Detach timer, with a relatively large value and if ISR is activated, at least slightly larger than the UE's E-UTRAN Deactivate ISR timer. Tracking Area or RAT specific MME configuration can be used to support UEs using a RAN that provides discontinuous coverage (e.g. for satellite access with discontinuous coverage). NOTE 1: For example, if a satellite system only provides coverage to a UE for 20 minutes when a satellite passes, and the maximum time before a satellite passes any point on the earth is 10 hours, the MME could configure the periodic TAU timer and mobile reachable timer to be just greater than 20 minutes and the Implicit Detach timer to be greater than 10 hours to avoid unintended implicit detach due to coverage gap. Such configuration does not require MME to be aware of detailed coverage times for each UE or for different locations. Further enhancements to handle discontinuous coverage for satellite access is provided in clause 4.13.8.2. If MME has allocated an Active Time to the UE, then the MME starts the Active timer with the value of Active Time whenever the UE enters ECM IDLE mode. If this timer expires in the MME, the MME can deduce that the UE is not reachable and should clear the PPF flag in the MME. With the PPF clear, the MME does not page the UE in E-UTRAN coverage and shall send a Downlink Data Notification Reject message to the Serving GW when receiving a Downlink Data Notification message from the Serving GW. If the Implicit Detach timer expires before the UE contacts the network, then the MME can deduce that the UE has been 'out of coverage' for a long period of time and implicitly detach the UE as described in clause 5.3.8.3 "MME-initiated Detach procedure". If the MME is requested to monitor Reachability for Data and the UE enters ECM-CONNECTED, the MME sends a Monitoring Report message to the address that was indicated in the related Monitoring Request as described in TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [74]. When the MME applies General NAS level Mobility Management Congestion Control to a UE, the MME may need to adjust the mobile reachable timer and/or Implicit Detach timer (as clause 4.3.7.4.2.4). NOTE 2: The SGSN has similar functionality as the MME. NOTE 3: Alternative MME implementations are permitted, however, the externally visible MME behaviour should conform to the above description.