Section: 5.6.2.4 Paging for SMS
Content: The network shall initiate the paging procedure when it receives an incoming mobile terminating SMS to the UE if the UE is: 1) IMSI attached for non-EPS services or for "SMS only"; or 2) attached for EPS services with CIoT EPS optimization and the UE has requested "SMS only" and the UE is in NB-S1 mode, - no NAS signalling connection exists. - there is no paging restriction applied in the network. For the UE using eDRX, the network initiates the paging procedure when an incoming mobile terminating SMS is received within the paging time window. If an incoming mobile terminating SMS is received outside the paging time window and the eDRX value that the network provides to the UE in the Extended DRX parameters IE during the last attach procedure or the last tracking area updating procedure is not all zeros (i.e. the E-UTRAN eDRX cycle length duration is higher than 5.12 seconds), the network initiates the paging procedure at T time ahead of the beginning of the next paging time window. NOTE: T time is a short time period based on implementation. The operator can take possible imperfections in the synchronization between the CN and the UE into account when choosing T time. To initiate the procedure for SMS when no NAS signalling connection exists, the EMM entity in the network requests the lower layer to start paging (see 3GPP TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [23]). The paging message shall include a CN domain indicator set to "PS". If the paging message includes a UE Paging Identity set to the UE's S-TMSI, the paging procedure is performed according to clause 5.6.2.2.1. If the paging message includes a UE Paging Identity set to the UE's IMSI, the paging procedure is performed according to clause 5.6.2.2.2. The MME shall not start timers T3413 and T3415 for this procedure. If the negotiated UE paging probability information is available in the EMM context of the UE, the EMM entity shall provide the lower layer with the negotiated UE paging probability information (see 3GPP TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [20], 3GPP TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [23]).