IMS Centralized Services (ICS) provides communication services such that all services, and service control, are only based on IMS mechanisms and enablers. It enables IMS services when using CS access (e.g. TS 24.008, 3GPP2 C.S0001-D [27]) for the media bearer. With ICS, the user services are provided by IMS only. User sessions are controlled in IMS via PS or CS access and can be any of those defined in clause 22.4 of TS 22.101. When using a CS access network, or when using a PS access network that does not support the full duplex speech/video component of an IMS-based service, the CS core network is utilized to establish a circuit bearer for use as media for IMS sessions. The transmission of both bidirectional and unidirectional real-time video media flows shall be supported. If the PS access network does support the full duplex speech component of an IMS service then existing IMS session procedures are used as specified in TS 23.228. ICS provides mechanisms to support the use of CS media bearer for IMS sessions. With ICS, IMS sessions using CS media are treated as standard IMS sessions for the purpose of service control and service continuity. ICS defines signalling mechanisms between the UE and IMS for transport of information required for service continuity when using CS access for media transport.

IMS services as defined in clause 22.4 of TS 22.101 shall be consistently provided when using a CS or a PS access network for the media of the IMS service subject to the capability of the UE and the access network.

Service continuity shall be provided when underlying mobility results in a change of access network capabilities, e.g. support of Gm reference point in conjunction with CS bearer may not be possible after handover from UTRAN to GERAN.

An ICS UE supporting multimedia telephony shall manage the IMS multimedia telephony communication service settings data as specified in TS 24.173. For a UE not supporting multimedia telephony, the MSC Server enhanced for ICS may implement a communication service setting conversion function between CS signalling (e.g., as described in TS 24.010) and communication service setting procedures (e.g. as defined in TS 24.173); see also clause 7.6.2.9. For a UE not supporting multimedia telephony, if the MSC Server enhanced for ICS does not implement a communication service setting conversion function or if the UE is connected to an MSC server not enhanced for ICS, refer to clause 7.6.3.7.1. Mechanisms that allow data to be synchronized between the HSS and the TAS when legacy CS signalling is used towards the HSS for service settings data management are not specified.

It shall be possible to provide ICS for an ICS UE and a non ICS UE in home and in visited networks. Home and visited networks with an MSC Server enhanced for ICS shall support call originations and terminations for ICS UEs and non ICS UEs. The MSC Server enhanced for ICS avoids conflicts with other IMS registrations (which includes that used by an ICS UE) by using a Private User Identity and Public User Identity specifically reserved for use by MSC Server registrations. See clause 4.6.2 for more information. An IMS operator who provides services for its subscribers, using both ICS UE's and MSC Servers enhanced for ICS needs to make the following provisions in order to avoid registration conflicts:

• Configure the ICS specific Private User Identity, as defined in clause 4.6.2 to point to the subscriber's implicit registration set; and
• Add the ICS specific Public User Identity, as defined in clause 4.6.2 to the subscriber's implicit registration set.

When the ICS UE establishes the Service Control Signalling Path over I1 or Gm prior to the establishment of the CS Bearer Control Signalling Path, the ICS UE shall use the routing number provided by the network in response to service control signalling as the B-Party number for establishing the CS Bearer Control Signalling Path.