Section: 4.3 UE mode of operation 4.3.1 General
Content: A UE attached for EPS services shall operate in one of the following operation modes: - PS mode 1 of operation: the UE registers only to EPS services, and UE's usage setting is "voice centric"; - PS mode 2 of operation: the UE registers only to EPS services, and UE's usage setting is "data centric"; - CS/PS mode 1 of operation: the UE registers to both EPS and non-EPS services, and UE's usage setting is "voice centric"; and - CS/PS mode 2 of operation: the UE registers to both EPS and non-EPS services, and UE's usage setting is "data centric". A UE configured to use CS fallback, shall operate in CS/PS mode 1 or CS/PS mode 2. Such UE may also be configured to use IMS, in which case the voice domain preference for E-UTRAN as defined in 3GPP TS 24.167[ 3GPP IMS Management Object (MO); Stage 3 ]  [13B] shall be used for the selection of the domain for originating voice communication services. NOTE 1: The domain selected for originating voice communication services can be ignored by attempting a CS emergency call. Upon request from upper layers to establish a CS emergency call: - if the UE needs to initiate a CS fallback emergency call but it is unable to perform CS fallback, the UE shall attempt to select GERAN or UTRAN radio access technology, and a UE with "IMS voice not available" should disable the E-UTRA capability (see clause 4.5) to allow a potential callback, and then progress the CS emergency call establishment; - if the UE needs to initiate a 1xCS fallback emergency call but it is unable to perform 1xCS fallback, the UE shall attempt to select cdma2000® 1x radio access technology to establish the call. NOTE 2: Unable to perform CS fallback or 1xCS fallback means that either the UE was not allowed to attempt CS fallback or 1xCS fallback, or CS fallback or 1xCS fallback attempt failed. A UE configured to use SMS over SGs shall operate in CS/PS mode 1 or CS/PS mode 2. The behaviour of the UE in CS/PS mode 1 of operation, upon failure to access the CS domain or upon reception of a "CS fallback not preferred" or "SMS only" indication, will depend on the availability of voice over IMS. In the present document, "IMS voice not available" refers to one of the following conditions: a) the UE is not configured to use IMS; b) the UE is not configured to use IMS voice, i.e. when the voice domain preference for E-UTRAN, as defined in 3GPP TS 24.167[ 3GPP IMS Management Object (MO); Stage 3 ]  [13B], indicates that voice communication services are allowed to be invoked only over the CS domain; c) the UE is configured to use IMS voice, but the network indicates in the ATTACH ACCEPT message or the TRACKING AREA UPDATE ACCEPT message that IMS voice over PS sessions are not supported; or d) the UE is configured to use IMS voice, the network indicates in the ATTACH ACCEPT message or the TRACKING AREA UPDATE ACCEPT message that IMS voice over PS sessions are supported, but the upper layers: - provide no indication that the UE is available for voice call in the IMS within a manufacturer determined period of time; or - indicate that the UE is not available for voice calls in the IMS. NOTE 3: If conditions a, b and c evaluate to false, the upper layers need time to attempt IMS registration. In the event an indication from the upper layers that the UE is available for voice calls in the IMS takes longer than the manufacturer determined period of time (e.g. due to delay when attempting IMS registration or due to delay obtaining an EPS bearer context for SIP signalling), the NAS layer assumes the UE is not available for voice calls in the IMS. Other conditions may exist but these are implementation specific. In the present document, "IMS voice available" refers to the conditions a, b, c and d, and other implementation specific conditions for "IMS voice not available" evaluate to false.