Section: 5.3.4B Data Transport in Control Plane CIoT EPS Optimisation 5.3.4B.1 General
Content: If the UE and MME use the Control Plane CIoT EPS Optimisation, they can transfer data in NAS PDUs including the EPS Bearer Identity of the PDN connection they relate to, for which there is no S1-U bearers established (i.e. when an S1-U bearer is established the UE shall use S1-U to transfer data PDUs). All PDN types are supported. If the UE and the MME support Control Plane CIoT EPS Optimisation, then for SMS transfer and EPC Mobile Originated Location Request (EPC-MO-LR) or EPC Mobile Terminated Location Request (EPC-MT-LR) the Service Request procedures defined in clause 5.3.4 are not used for MO and MT SMS or for EPC-MO-LR and EPC-MT-LR, but instead UE and MME shall be using the Data Transport in Control Plane CIoT EPS Optimisation. This is accomplished by using the NAS transport capabilities of RRC and S1-AP protocols and the data transport of GTP-u tunnels between MME and S-GW and between S-GW and P-GW, or if a Non-IP connection is provided by via the MME with the SCEF, then data transfer occurs as indicated in TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [74]. For IP data, the UE and MME may perform header compression based on ROHC framework IETF RFC 5795 [77]. For uplink IP data, UE implements ROHC compressor, and MME implements the decompressor. For downlink IP data, MME implements the ROHC compressor, and UE implements the decompressor. The uplink and downlink ROHC channels are bound by UE and MME to support feedback. The configurations for the header compression are established during the PDN connection establishment procedure. To minimise potential conflicts between NAS signalling PDUs and NAS Data PDUs, the MME should complete any security related procedures (e.g. Authentication, Security Mode Command, GUTI reallocation) before alerting the HSS, MSC or SGW of the UE's entry into ECM-CONNECTED state, and before commencing downlink transfer of NAS Data PDUs. The priority handling between the EMM/ESM NAS signalling PDUs and NAS Data PDUs is specified in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46].