Section: 5.3.1.3 Suspend and resume of the NAS signalling connection
Content: Suspend of the NAS signalling connection can be initiated by the network in EMM-CONNECTED mode when user plane CIoT EPS optimization is used. Resume of the suspended NAS signalling connection is initiated by the UE. In the UE, when user plane CIoT EPS optimization is used: - Upon indication from the lower layers that the RRC connection has been suspended, the UE shall enter EMM-IDLE mode with suspend indication, shall not consider the NAS signalling connection released and shall not consider the secure exchange of NAS messages terminated (see clause 4.4.2.3 and 4.4.5). Based on further indications provided by the lower layers, the UE shall update the status of the suspend indication for the EMM-IDLE mode; - Upon trigger of a procedure using an initial NAS message when in EMM-IDLE mode with suspend indication, the UE shall: i) if the initial NAS message is a TRACKING AREA UPDATE REQUEST message which includes a UE radio capability information update needed IE, enter EMM-IDLE mode without suspend indication and proceed with the tracking area updating procedure; and ii) otherwise, request the lower layer to resume the RRC connection. In this request to the lower layer the NAS shall provide to the lower layer the RRC establishment cause and the call type according to annex D of this document; NOTE 1: In NB-S1 mode, in the request to the lower layer the data volume information of the initial NAS message is provided to the lower layers. Interactions between the NAS and the lower layers in order to obtain the data volume information of the initial NAS message (see 3GPP TS 36.321[ Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification ]  [49], 3GPP TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [22]) is left to implementations. - Upon indication from the lower layers that the RRC connection has been resumed when in EMM-IDLE mode with suspend indication, the UE shall enter EMM-CONNECTED mode. If the pending NAS message is: i) a SERVICE REQUEST message; ii) a CONTROL PLANE SERVICE REQUEST message, and the UE did not include any ESM message container, NAS message container, EPS bearer context status information element, or UE request type information element; or iii) an EXTENDED SERVICE REQUEST message, and the Service type information element indicates "packet services via S1" and the UE did not include any EPS bearer context status information element, or UE request type information element; the message shall not be sent. Otherwise the UE shall cipher the message as specified in clause 4.4.5 and send the pending initial NAS message upon entering EMM-CONNECTED mode; NOTE 2: If a NAS message is discarded and not sent to the network, the uplink NAS COUNT value corresponding to that message is reused for the next uplink NAS message to be sent. - Upon fallback indication from the lower layers at RRC connection resume when in EMM-IDLE mode with suspend indication, the UE shall enter EMM-IDLE mode without suspend indication, send any pending initial NAS message and proceed as if RRC connection establishment had been requested; - Upon indication from the lower layers that the RRC connection resume has failed and indication from the lower layers that the RRC connection is suspended, the UE shall enter EMM-IDLE mode with suspend indication and restart the ongoing NAS procedure if required; and - Upon indication from the lower layers that the RRC connection resume has failed and indication from the lower layers that the RRC connection is not suspended, the UE shall enter EMM-IDLE mode without suspend indication and restart the ongoing NAS procedure if required. In the network, when user plane CIoT EPS optimization is used: - Upon indication from the lower layers that the RRC connection has been suspended, the network shall enter EMM-IDLE mode with suspend indication, shall not consider the NAS signalling connection released and shall not consider the secure exchange of NAS messages terminated; and - Upon indication from the lower layers that the RRC connection has been resumed when in EMM-IDLE mode with suspend indication, the network shall enter EMM-CONNECTED mode. For the case that not all suspended bearers are resumed, see clause 6.4.4.6.