Section: 5.3.17 Service Gap Control
Content: The network may control the frequency UEs can transit from EMM-IDLE mode to EMM-CONNECTED mode via the service gap control (SGC) as specified in 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [10]. If the network supports service gap control (SGC) feature and the service gap time value is available in the EMM context of the UE, the MME shall consider SGC as active for the UE. The UE and the network negotiate usage of the service gap control (SGC) feature during the attach and tracking area updating procedures: - the UE supporting service gap control indicates its support for service gap control in the ATTACH REQUEST and TRACKING AREA UPDATE REQUEST message. If the UE supports service gap control and the SGC is active for the UE, the MME shall include service gap timer T3447 value in the ATTACH ACCEPT message and TRACKING AREA UPDATE ACCEPT message (see clause 5.5.1.2 and clause 5.5.3.2). The UE shall store the service gap time value; and - for UEs that do not support the optional SGC feature when the network rejects mobility management signalling requests due to service gap control is active in the network, the mechanism for general NAS level mobility management congestion control as specified in clause 5.3.9 applies. The UE shall start the SGC timer T3447 when the NAS signalling connection is released and if: - the UE supports SGC feature, and the service gap timer value is available in the UE and does not indicate zero; and - the NAS signalling connection released was not established for: - paging; - attach requests without PDN connection request; or - tracking area update requests without "active" or "signalling active" flag set. If the SGC is active in the network, after the UE transitions from EMM-CONNECTED mode to EMM-IDLE mode except when the UE was in EMM-CONNECTED mode due to: - paging; - attach requests without PDN connection request; or - tracking area update requests without "active" or "signalling active" flag set, the network shall start the SGC timer T3447: - with the service gap time value available in the EMM context minus 4 minutes, if the UE supports SGC feature and the service gap time value has been sent to the UE with a non-zero value; or - with the service gap time value available in the EMM context if the UE does not support SGC feature. When the SGC timer T3447 is running, the network allows: - requests for emergency bearer services; - requests for exception data reporting; - attach requests without PDN connection request; - tracking area update requests without "active" or "signalling active" flag set; - requests from UEs that were received via NAS signalling connections established with RRC establishment cause "High priority access AC 11 – 15"; and - mobile terminated service requests triggered by paging and subsequent MO signalling or MO data, if any, until the UE enters EMM-IDLE mode. If the MME determines that the UE operating in single-registration mode has performed an inter-system change from S1 mode to N1 mode and the timer T3447 is running in the MME, the MME stops the T3447. Upon inter-system change from N1 mode to S1 mode, if the UE supports service gap control, T3447 is running in the UE, and the T3447 value is included in the ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message received from the MME (see clause 5.5.1.2 and clause 5.5.3.2), the UE shall keep T3447 running. Additionally, the UE shall store and replace the currently stored T3447 value with the received T3447 value. Upon expiry of the running T3447 timer, the UE shall use the new value when starting T3447 again. The UE or the network with a running service gap timer shall keep the timer running when the UE transits from EMM-IDLE mode to EMM-CONNECTED mode. NOTE: If the UE transitions from EMM-IDLE mode to EMM-CONNECTED mode due to attach request without PDN connection request or tracking area update request without "active" or "signalling active" flag set, the UE initiates no further MO signalling except for tracking area update request without "active" or "signalling active" flag set until the UE receives network-initiated signalling (e.g. DOWNLINK NAS TRANSPORT message for MT SMS) or MT data over user plane, or after the UE has moved to EMM-IDLE state and the service gap timer is not running. If the timer T3447 is running when the UE enters state EMM-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 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 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.