Section: 5.3.26 Support for unavailability period
Content: If the UE and network support unavailability period and an event is triggered in the UE making the UE unavailable for a certain period of time, the UE may store its 5GMM and 5GSM context in USIM or non-volatile memory in the ME to be able to reuse it after the unavailability period. NOTE 1: How and where the UE stores its contexts depends upon the UE implementation. The UE can store some or all of its contexts in the ME or USIM using existing ME or USIM functionality. To activate the unavailability period, the UE provides unavailability information, including a type of unavailability, an unavailability period duration if known, and the start of the unavailability period if known, during the registration procedure or during the de-registration procedure (see 3GPP TS 23.501[ System architecture for the 5G System (5GS) ]  [8] and 3GPP TS 23.502[ Procedures for the 5G System (5GS) ]  [9]). The support for the unavailability period is negotiated in the registration procedure. If the UE is registered to a PLMN via a satellite NG-RAN cell, the AMF may provide the unavailability period duration of the UE during the registration procedure. The AMF may consider the unavailability period duration provided by the UE if available and determine the unavailability period duration of the UE as described in subclause 5.5.1.3.4. If the UE provided unavailability information in the last registration procedure or de-registration procedure, the AMF considers the UE unreachable until the UE re-register for a normal service without providing unavailability information. If the UE did not include a start of the unavailability period, the AMF shall consider the start of the unavailability period to be the time at which AMF received the REGISTRATION REQUEST message or the DEREGISTRATION REQUEST message from the UE. During the registration procedure, the AMF may determine the values of the negotiated extended DRX parameters, the timer T3324, and the periodic registration update timer (T3512) to be provided to the UE based on the unavailability period duration and the start of the unavailability period based on their availability. The AMF should set the value of the mobile reachable timer and implicit de-registration timer based on the unavailability period duration and the start of the unavailability period. The AMF releases the N1 signalling connection after the completion of the registration procedure in which the UE provided unavailability information without providing the start of the unavailability period. NOTE 2: If the UE supports MUSIM and the UE and the networks the UE is registered with support the unavailability period, then the UE can indicate a different unavailability period to each registered network. If for discontinuous coverage the UE has stored a discontinuous coverage maximum time offset as described in subclause 5.4.4.3, 5.5.1.2.4, and 5.5.1.3.4, the UE shall set the discontinuous coverage maximum time offset value to a random value up to and including the stored discontinuous coverage maximum time offset for this PLMN and satellite NG-RAN RAT Type, determine a time point equal to the time when the UE is about to lose satellite coverage minus the discontinuous coverage maximum time offset value, and send the REGISTRATION REQUEST message for mobility registration update to the AMF indicating out of coverage at the determined time point. If for discontinuous coverage the UE has stored a maximum time offset as described in subclause 5.4.4.3, 5.5.1.2.4, and 5.5.1.3.4, upon returning in coverage of a TA in the current registration after being out of coverage due to discontinuous coverage, and in the last REGISTRATION ACCEPT message the end of unavailability period report was set to "UE needs to report end of unavailability period", the UE shall start the discontinuous coverage maximum time offset timer with a random value up to and including the stored maximum time offset value, if available, for this PLMN and NR satellite access, and starts the discontinuous coverage maximum time offset timer. The UE shall enter 5GMM-REGISTERED.ATTEMPTING-REGISTRATION-UPDATE state and shall not initiate any NAS signalling on that NR satellite access and PLMN while the maximum time offset timer is running. The UE shall stop the maximum time offset timer and initiate NAS signalling if the UE receives paging message, has pending emergency services, is establishing an emergency PDU session or is performing emergency services fallback procedure or when the UE enters a TAI outside the registration area. At expiry of the discontinuous coverage maximum time offset timer the UE shall perform a registration procedure for mobility registration update. If the end of unavailability period report indicates "UE does not need to report end of unavailability period" then upon returning in coverage of a TAI in the current registration area the UE shall choose an appropriate substate of 5GMM-REGISTERED and does not need to perform a registration procedure for mobility registration update due to end of the unavailability period. If for discontinuous coverage the AMF sets the End of unavailability report bit in Unavailability configuration IE value to “UE does not need to report end of unavailability”, the UE is not required to trigger the registration procedure for mobility registration update when the unavailability period duration has ended. If the AMF does not provide the Unavailability configuration IE or the AMF sets the End of unavailability report bit in Unavailability configuration IE value to “UE needs to report end of unavailability”, the UE should trigger registration procedure for mobility registration update when the unavailability period duration has ended. When the unavailability period is activated, all NAS timers are stopped and associated procedures aborted except for timers T3512, T3346, T3447, T3448, T3396, T3584, T3585, any back-off timers, T3245, T3247, the timer T controlling the periodic search for HPLMN or EHPLMN or higher prioritized PLMNs, and the timer TSENSE controlling the periodic search for PLMNs satisfying the operator controlled signal level threshold (see 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [5]). When the UE activates the unavailability period using registration procedure without providing the start of the unavailability period, then after successful completion of the procedure the UE shall enter the state 5GMM-REGISTERED.NO-CELL-AVAILABLE and may deactivate AS layer. Otherwise, if the UE provided the start of unavailability period in the registration procedure, the UE shall enter the state 5GMM-REGISTERED.NO-CELL-AVAILABLE and may deactivate AS layer only after the UE activates the unavailability period. When the UE activates the unavailability period using the de-registration procedure, then after successful completion of the procedure the UE shall enter the state 5GMM-DEREGISTERED.NO-CELL-AVAILABLE and deactivate the AS layer. When the UE comes out of the unavailability period the UE shall activate the AS layer if deactivated and perform registration procedure as described in subclause 5.5.1.2.2 and 5.5.1.3.2.