Section: 4.3.5.10 Preferred and Supported Network Behaviour
Content: A UE includes in a Preferred Network Behaviour indication the Network Behaviour the UE can support and what it would prefer to use. The Preferred Network Behaviour includes this information: - Whether Control Plane CIoT EPS Optimisation is supported. - Whether User Plane CIoT EPS Optimisation is supported. - Whether Control Plane CIoT EPS Optimisation is preferred or whether User Plane Plane CIoT EPS Optimisation is preferred. - Whether S1-U data transfer is supported. - Whether SMS transfer without Combined Attach is requested. - Whether Attach without PDN Connectivity is supported. - Whether header compression for Control Plane CIoT EPS Optimisation is supported. If SMS transfer without Combined EPS Attach is requested by the UE, a supporting MME provides SMS transfer without the UE performing the combined EPS attach specified in TS 23.272[ Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2 ]  [58]. An MME connected to NB-IoT should support SMS transfer without the UE being required to perform a Combined Attach.This feature is only available to UEs that only support NB-IoT. If S1-U data transfer is supported is indicated by the UE, the UE supports data transfer that is not subject to CIoT EPS Optimisations. If the UE indicates support of User Plane CIoT EPS Optimisation then it shall also indicate support of S1-U data transfer. If Attach without PDN connection is supported, the UE need not establish a PDN connection as part of the Attach procedure and the UE and MME may at any time release all the PDN connections and remain EPS attached. The MME indicates the network behaviour the network accepts in the Supported Network Behaviour information. This indication is per TAI List. The MME may indicate one or more of the following: - Whether Control Plane CIoT EPS Optimisation is supported. - Whether User Plane CIoT EPS Optimisation is supported. - Whether S1-U data transfer is supported. - Whether SMS transfer without Combined Attach is accepted. - Whether Attach without PDN Connectivity is supported. - Whether header compression for Control Plane CIoT EPS Optimisation is supported. If the MME indicates support of User Plane CIoT EPS Optimisation then it shall also indicate support of S1-U data transfer. If the UE and MME indicate support for User Plane CIoT EPS Optimisation, MME sets the UE User Plane CIoT Support Indicator to "supported" in S1-AP messages as defined in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]. For NB-IoT UEs that only support Control Plane CIoT EPS Optimisation, the MME shall include support for Control Plane CIoT EPS Optimisation in NAS accept messages. A UE that supports the NB-IoT shall always indicate support for Control Plane CIoT EPS Optimisation. In a network that supports Dedicated Core Networks (see clause 5.19), the Preferred Network Behaviour indication from the UE may be used to influence policy decisions that can cause rerouting of the Attach or TAU from an MME to another MME. Other CIoT EPS Optimisations include "Attach without PDN connection establishment"; "PDN type = non-IP"; and "UE connection to SCEF". These features are requested by implicit and explicit signalling described within the relevant clauses of this TS.