Section: 5.31.16 Service Gap Control
Content: Service Gap Control is an optional feature intended for CIoT UEs to control the frequency at which these UEs can access the network. That is, to ensure a minimum time gap between consecutive Mobile Originated data communications initiated by the UE. This helps reducing peak load situations when there are a large number of these UEs in an operator network. Service Gap Control is intended to be used for "small data allowance plans" for MTC/CIoT UEs where the applications are tolerant to service latency. NOTE 1: Time critical applications, such as regulatory prioritized services like Emergency services can suffer from the latency caused by the Service Gap Control feature. Therefore Service Gap Control feature is not recommended for subscriptions with such applications and services. Service Gap Time is a subscription parameter used to set the Service Gap timer and is enforced in the UE and in the AMF on a per UE level (i.e. the same Service Gap Timer applies for all PDU Sessions that the UE has). The UE indicates its capability of support for Service Gap Control in the Registration Request message to the AMF. The AMF passes the Service Gap Time to the UE in the Registration Accept message for a UE that has indicated its support of the Service Gap Control. The Service Gap Control shall be applied in a UE when a Service Gap Time is stored in the UE context and applied in the AMF when the Service Gap Time is stored in the UE Context in the AMF. Service Gap Control requires the UE to stay in CM-IDLE mode for at least the whole duration of the Service Gap timer before triggering Mobile Originated user data transmission, except for procedures that are exempted (see TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [47]). The Service Gap timer shall be started each time a UE moves from CM-CONNECTED to CM-IDLE, unless the connection request was initiated by the paging of a Mobile Terminated event, or after a Mobility or Periodic Registration procedure without Follow-on Request indication and without Uplink data status, which shall not trigger a new or extended Service Gap interval. When a Service Gap timer expires, the UE is allowed to send a connection request again. If the UE does so, the Service Gap timer will be restarted at the next CM-CONNECTED to CM-IDLE transition. The Service Gap control is applied in CM-IDLE state only and does not impact UE Mobile Originated user data transmission or Mobile Originated signalling in CM-CONNECTED state. The Service Gap timer is not stopped upon CM-IDLE state to CM-CONNECTED state transition. The UE shall not initiate connection requests for MO user plane data, MO control plane data, or MO SMS when a Service Gap timer is running. The UE shall not initiate PDU Session Establishment Requests when a Service Gap timer is running, unless it is for Emergency services which are allowed. CM-CONNECTED with RRC_INACTIVE is not used for UEs that have a Service Gap Time configured. NOTE 2: As a consequence of allowing Initial Registration Request procedure, the UE with a running Service Gap timer does not initiate further MO signalling, except for Mobility Registration procedure, until the UE receives MT signalling or after the UE has moved to CM-IDLE state and the Service Gap Timer is not running. NOTE 3: Implementations need to make sure that latest and up-to-date data are always sent when a Service Gap timer expires. The AMF may enforce the Service Gap timer by rejecting connection requests for MO user plane data, MO control plane data, or MO SMS when a Service Gap timer is running. The AMF may enforce the Service Gap timer by not allowing MO signalling after Initial Registration requests when a Service Gap timer is running except for Mobility Registration procedure, Periodic Registration procedure or access to the network for regulatory prioritized services like Emergency services, which are allowed. When rejecting the connection requests and the SM signalling after Initial Registration Requests while the Service Gap timer is running, the AMF may include a Mobility Management back-off timer corresponding to the time left of the current Service Gap timer. For UEs that do not support Service Gap Control (e.g. pre-release-16 UEs), Service Gap Control may be enforced using "General NAS level congestion control" as defined in clause 5.19.7.2. NOTE 4: After MT signalling in CM-CONNECTED state the AMF does not further restrict MO signalling when a Service Gap timer is running as this case is considered equal to a connectivity request in response to paging. When the AMF starts the Service Gap timer, the AMF should invoke the Service Gap timer with a value that is slightly shorter than the Service Gap Time value provided to the UE based on the subscription information received from the UDM. NOTE 5: This ensures that the AMF does not reject any UE requests just before the Service Gap timer expires e.g. because of slightly unsynchronized timers between UE and AMF. A UE which transitions from a MICO mode or eDRX power saving state shall apply Service Gap Control when it wakes up if the Service Gap timer is still running. Additional aspects of Service Gap Control: - Service Gap Control applies in all PLMNs. - When the Service Gap timer is running and the UE receives paging, the UE shall respond as normal. - Service Gap Control does not apply to exception reporting for NB-IoT. - Access to the network for regulatory prioritized services like Emergency services are allowed when a Service Gap timer is running. - Service Gap Control shall be effective also for UEs performing de-registration and re-registration unless access to the network for regulatory prioritized services like Emergency services is required. - If the Service Gap timer is running, the Service Gap is applied at PLMN selection as follows: a) Re-registration to the registered PLMN: The remaining Service Gap timer value survives. b) Registration to a different PLMN: The remaining Service Gap timer value survives. c) USIM swap: The Service Gap timer is no longer running and the Service Gap feature does not apply, unless re-instantiated by the serving PLMN. - Multiple uplink packets and downlink packets are allowed during one RRC connection for UE operating within its Rate Control limits. The following procedures are impacted by Service Gap Control: - Registration Procedure, see clause 4.2.2.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]; - UE Triggered Service Request, see clause 4.2.3.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]; NOTE 6: Since UE triggered Service Request is prevented by Service Gap timer, this implicitly prevents the UE from initiating UPF anchored Mobile Originated Data Transport in Control Plane CIoT 5GS Optimisation (see clause 4.24.1 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]), NEF Anchored Mobile Originated Data Transport (see clause 4.25.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]) and MO SMS over NAS in CM-IDLE (see clause 4.13.3.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]).