5.3.26 Support for unavailability period

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 and the unavailability is not due to discontinuous coverage, 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, either 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 the UE provides unavailability information, including the type of unavailability and the unavailability period duration if known, during the de-registration procedure (see 3GPP TS 23.501 [8] and 3GPP TS 23.502 [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 for discontinuous coverage provide the unavailability period duration of the UE, the start of the unavailability period or both to 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 clause 5.5.1.2.4 and 5.5.1.3.4. The AMF may also consider the start of the unavailability period provided by the UE, if available, and determine the start of the unavailability period of the UE as described in clause 5.5.1.3.4.

If the UE provided unavailability information in the last registration procedure for mobility registration or the de-registration procedure, the AMF considers the UE unreachable until the UE registers for a normal service. 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 discontinuous coverage maximum time offset, 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 is registered with the support of unavailability period, then the UE can indicate a different unavailability period to each registered network.

NOTE 3: The AMF sets the value of the mobile reachable timer longer than the unavailability period duration.

If for discontinuous coverage the UE has stored a discontinuous coverage maximum time offset as described in clause 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 the stored discontinuous coverage maximum time offset for this PLMN and satellite NG-RAN access technology, 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 and periodic registration update to the AMF indicating out of satellite coverage at the determined time point.

NOTE 4: How the UE determines that it is about to lose satellite coverage is up to UE implementation.

If for discontinuous coverage the UE has stored a discontinuous coverage maximum time offset as described in clause 5.4.4.3, 5.5.1.2.4, and 5.5.1.3.4, upon returning in satellite coverage of a TA in the current registration area after being out of satellite coverage due to discontinuous coverage the UE starts the discontinuous coverage maximum time offset timer to a random value up to the stored discontinuous coverage maximum time offset value, if available, for this PLMN and satellite NG-RAN access technology, 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 satellite NG-RAN access technology and PLMN while the discontinuous coverage maximum time offset timer is running. The UE shall stop the discontinuous coverage maximum time offset timer and initiate NAS signalling if the UE:

a0) receives a paging message;

• receives a NOTIFICATION message over non-3GPP access as described in case b) clause 5.6.3.1;
• has pending emergency services;
• is establishing an emergency PDU session;
• is performing emergency services fallback procedure;
• enters a TAI outside the registration area;
• is a MUSIM UE and needs to request a new 5G-GUTI assignment as specified in clause 5.5.1.3.2;
• is allowed to use exception data reporting (see the ExceptionDataReportingAllowed leaf of the NAS configuration MO in 3GPP TS 24.368 [17] or the USIM file EFNASCONFIG in 3GPP TS 31.102 [22]) and the UE has to transmit user data related to an exceptional event; or
• is a UE configured for high priority access in selected PLMN.

At expiry of the discontinuous coverage maximum time offset timer, if the EUPR bit of the Unavailability configuration IE in the last REGISTRATION ACCEPT message was set to "UE needs to report end of unavailability period", the UE shall perform a registration procedure for mobility registration update.

If the AMF sets the EUPR bit value in the Unavailability configuration IE to "UE does not need to report end of unavailability period", then upon returning to a TA in the current registration area the UE shall choose an appropriate substate of 5GMM-REGISTERED and 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 EUPR bit value in the Unavailability configuration IE to "UE needs to report end of unavailability period", when the unavailability period duration has ended, the UE returns in satellite coverage of a TA in the current registration area after being out of satellite coverage due to discontinuous coverage and the discontinuous coverage maximum time offset timer has expired, if any, the UE shall trigger registration procedure for mobility registration update.

When the unavailability period is activated, all NAS timers are stopped and associated procedures aborted except for timers T3512, T3324, T3346, T3444, T3445, T3447, T3448, T3396, T3526, T3584, T3585, T3587, any back-off timers, T3245, T3247, the timer T controlling the periodic search for HPLMN or EHPLMN or higher prioritized PLMNs, the timer TSENSE controlling the periodic search for PLMNs satisfying the operator controlled signal level threshold, the timer TF, the timer TG, the timer TS (see 3GPP TS 23.122 [5]), the timer TNSU, the timer instance associated with the entry in the list of "PLMNs not allowed to operate at the present UE location", slice deregistration inactivity timer when the UE activates the unavailability period using registration procedure and the timer instance associated with the entry in the list of "LSP PLMNs not allowed to operate at the present UE location", the UE shall reset the registration attempt counter and service request attempt counter.

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 is in the unavailability period and the UE needs to perform a de-registration procedure, the UE may perform a local de-registration and enter the state 5GMM-DEREGISTERED NO-CELL-AVAILABLE.

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 clause 5.5.1.2.2 and 5.5.1.3.2.