Section: 5.3.17 Service Gap Control
Content: Service gap control (SGC) only applies to 3GPP access. The network may control the frequency with which UEs can transition from 5GMM-IDLE mode to 5GMM-CONNECTED mode via the SGC as specified in 3GPP TS 23.501[ System architecture for the 5G System (5GS) ]  [8] and 3GPP TS 23.502[ Procedures for the 5G System (5GS) ]  [9]. If the network supports SGC and the service gap time value i.e. T3447 value is available in the 5GMM context of the UE, the AMF shall consider SGC as active for the UE. The UE and the network negotiate usage of SGC during the registration procedure for initial registration and registration procedure for mobility and periodic registration update: - the UE supporting SGC indicates its support in the REGISTRATION REQUEST message. If the UE supports SGC and the SGC is active for the UE, the AMF includes T3447 value IE in the REGISTRATION ACCEPT message (see subclause 5.5.1.2 and subclause 5.5.1.3). The UE stores the T3447 value; and - for UEs that do not support SGC when the network rejects mobility management signalling requests because SGC is active in the network, the mechanism for general NAS level mobility management congestion control as specified in subclause 5.3.9 applies. The network can provide a new T3447 value to the UE to be used next time it is started or stop timer T3447 in the UE if running with the Generic UE configuration update procedure as specified in subclause 5.4.4. The UE shall start timer T3447 when the N1 NAS signalling connection is released and if: - the UE supports SGC, and the T3447 value is available in the UE and does not indicate zero; and - the N1 NAS signalling connection released was not established for: - paging; - registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; - registration procedure for mobility and periodic registration update with Follow-on request indicator set to "No follow-on request pending" and without Uplink data status IE included; - requests for emergency services; or - requests for exception data reporting. If the SGC is active in the network, after the UE transitions from 5GMM-CONNECTED mode to 5GMM-IDLE mode except when the UE was in 5GMM-CONNECTED mode due to: - paging; - registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; - registration procedure for mobility and periodic registration update with Follow-on request indicator set to "No follow-on request pending" and without Uplink data status IE included, - requests for emergency services; or - requests for exception data reporting, the network shall start timer T3447 if not already running: - with the T3447 value available in the 5GMM context minus 4 minutes, if the UE supports SGC and the T3447 value has been sent to the UE with a non-zero value; or - with the T3447 value available in the 5GMM context if the UE does not support SGC. When timer T3447 is running, the network allows: - requests for emergency service; - requests for emergency services fallback; - requests for high priority access; - requests for exception data reporting; - registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending"; - registration procedure for mobility and periodic registration update without Uplink data status IE included and with Follow-on request indicator set to "No follow-on request pending"; or - service request procedure or registration procedure for mobility and periodic registration update triggered by paging and subsequent MO signalling or MO data, if any, until the UE enters 5GMM-IDLE mode. The UE or the network with a running T3447 timer keeps the timer running when the UE transits from 5GMM-IDLE mode to 5GMM-CONNECTED mode. NOTE: If the UE transitions from 5GMM-IDLE mode to 5GMM-CONNECTED mode due to registration procedure for initial registration with Follow-on request indicator set to "No follow-on request pending" or mobility and periodic registration update request without Uplink data status IE and with Follow-on request indicator set to "No follow-on request pending", the UE initiates no further MO signalling except for mobility and periodic registration update requests without Uplink data status and with Follow-on request indicator set to "No follow-on request pending" until the UE receives mobile terminated signalling (e.g. DL NAS TRANSPORT message for MT SMS) or MT data over user plane, or after the UE has moved to 5GMM-IDLE state and the service gap timer is not running. If timer T3447 is running when the UE changes PLMN or enters state 5GMM-DEREGISTERED, the UE remains switched on, and the USIM in the UE remains the same, then timer T3447 is kept running until it expires. If the AMF determines that the UE operating in single-registration mode has performed an inter-system change from N1 mode to S1 mode and the timer T3447 is running in the AMF, the AMF stops the T3447. Upon inter-system change from S1 mode to N1 mode, if the UE supports service gap control, T3447 is running in the UE, and the T3447 value IE is included in the REGISTRATION ACCEPT message received from the AMF (see subclause 5.5.1.2 and subclause 5.5.1.3), the UE shall keep T3447 running. Additionally, the UE shall store and replace the currently stored service gap time value with the received T3447 value. Upon expiry of the running T3447 timer, the UE shall use the new value when starting T3447 again. If the UE is switched off when the timer T3447 is running, the UE shall behave as follows when the UE is switched on and the USIM in the UE remains the same: - let t1 be the time remaining for timer T3447 timeout at switch off and let t be the time elapsed between switch off and switch on. If t1 is greater than t, then the timer shall be restarted with the value t1 – t. If t1 is equal to or less than t, then the timer need not be restarted. If the UE is not capable of determining t, then the UE shall restart the timer with the value t1.