Section: 4.3.17.9 Service Gap Control
Content: Service Gap Control is an optional feature intended for MTC/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 emergency services and regulatory prioritised 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 MME on a per UE level (i.e. the same Service Gap Timer applies for all PDN connections that the UE has). The UE indicates its capability of support for Service Gap Control in the Attach Request message and TAU Request message to the MME. The MME passes the Service Gap Time to the UE in the Attach Accept message and/or Tracking Area Update Accept message for UE that has indicated its supports 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 MME when the Service Gap Time is stored in the MM context. Service Gap Control requires the UE to stay in ECM-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 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]). The Service Gap timer shall be started each time a UE moves from ECM-CONNECTED to ECM-IDLE, unless the connection request was initiated by the paging of a Mobile Terminated event, or after a TAU procedure without active flag or signalling active flag, 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 ECM-CONNECTED to ECM-IDLE transition. The Service Gap control is applied in ECM-IDLE state only and does not impact UE Mobile Originated user data transmission or Mobile Originated signalling in ECM-CONNECTED state. The Service Gap timer is not stopped upon ECM-IDLE state to ECM-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 also not initiate Attach Requests when a Service Gap timer is running, unless it is Attach Request without PDN connectivity or Emergency Attach which are allowed. NOTE 2: As a consequence of allowing Attach without PDN connectivity procedure, the UE with a running Service Gap timer does not initiate further MO signalling, except for tracking area updating procedure, until the UE receives MT signalling or after the UE has moved to ECM-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 MME may enforce the Service Gap timer by rejecting connection request for MO user plane data, MO control plane data, or MO SMS when a Service Gap timer is running. The MME may enforce the Service Gap timer by not allowing Attach Requests when a Service Gap timer is running, unless it is Attach Request without PDN connectivity or Emergency Attach which are allowed. When rejecting the connection requests and the Attach Requests while the Service Gap timer is running, the MME may include a Mobility Management back-off timer corresponding to the time left of the current Service Gap timer. For the UEs that does not support Service Gap Control (e.g. pre-release-15 UEs), Service Gap Control may be enforced using "General NAS level Mobility Management control" as defined in clause 4.3.7.4.2.1. When the MME starts the Service Gap timer, the MME should invoke the Service Gap timer with a value that is slightly shorter than the Service Gap Time value provided to the UE in the subscription information received from the HSS. NOTE 4: This ensures that the MME doesn't reject any UE requests just before the Service Gap timer expires e.g. because of slightly unsynchronized timers between UE and MME. A UE which transitions from a PSM 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 applies to low priority (delayTolerant) and normal traffic. - Service Gap Control does not apply to exception reporting for NB-IoT. - Emergency Attach and Attach without PDN Connectivity are allowed when a Service Gap timer is running. - Service Gap Control shall be effective also for UEs performing detach and reattach unless it is Attach Request without PDN connectivity or Emergency Attach. - Tracking Area Update with active flag or signalling active flag is not allowed when a Service Gap timer is running except for emergency bearer services or if the UE is accessing with high priority access class in the range AC 11-15. - If the Service Gap timer is running, the Service Gap is applied at PLMN selection as follows: a) Re-attach to the registered PLMN: The remaining Service Gap timer value survives and controls the re-attach. b) Attach or Tracking Area Update to a different PLMN: The remaining Service Gap timer value survives and controls the Attach/Tracking Area Update to the new PLMN. c) USIM swap: The Service Gap timer is no longer running and the Service Gap feature does not apply, unless re-instatiated by the serving PLMN. - Multiple uplink packets and downlink packets are allowed during one RRC connection for UE operating within its APN Rate Control limits. The following procedures are impacted by Service Gap Control: - E-UTRAN Initial Attach, see clause 5.3.2.1; - Tracking Area Update procedures, see clause 5.3.3; - UE Triggered Service Request, see clause 5.3.4.1; - Connection Resume Request, see clause 5.3.5A. NOTE 5: Since UE triggered Service Request and Connection Resume Request are prevented by Service Gap timer, this implicitly prevents the UE from initiating MO data in Control Plane EPS Optimisations (see clause 5.3.4B.2), MO NIDD procedure (see TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [74]) and MO SMS (see TS 23.272[ Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2 ]  [58]).