It is highly advantageous if the set of services that can be supported between two endpoints is known to the endpoints when (or shortly after) communication is established. This information can be used to provide an indication to the user of the services that are available for a particular user-to-user communication session. This can encourage use of available services and avoid invocation of unavailable services, thereby avoiding customer dissatisfaction and unnecessary resource and bearer establishment attempts.
Two types of capability information are described: information about the current radio environment, and UE capability information.
During the radio capability exchange procedure, if the UE find that the remote UE and its current radio environment supports simultaneous CS and PS services and the IM Status indicates that an IMS communication is likely to be successful, then the UE should attempt an IMS registration (in case IMS registration had not previously been performed) based on preconfigured user's preference.
The purpose of the information about the current radio environment is to use it as input to the UE's and/or the user's decision whether to initiate further procedures (e.g. whether to start UE capability exchange, or an IMS session, or "in call MMS", etc.).
This radio environment information exchange occurs over the CS domain during CS call set-up.
This radio environment information is only valid during the lifetime of the CS call. At the end of the CS call, the UE should not store the radio environment information. This information can be used while the CS call is on going to help decide how to present service options to the user and/or whether to initiate a UE capability information exchange.
The following information is exchanged:
The terminal is capable of simultaneous CS and PS services and initiated/received the CS call in a radio environment that currently supports simultaneous CS and PS services.
The IM Status.
UE capability version, which is used for identifying current capabilities of a terminal (to notify capability update).
Personal ME Identifier (as defined in clause 7.4).
The information flows for exchanging this information are shown in clause 8.1.
For the CSI termination, the MGCF may interwork the UUS1 element between the IMS and the CS domain.
The UE capability information provides input to determine the set of services that can be successfully invoked between two users.
It shall be possible to exchange the UE capabilities described below in this clause. Note that the exchange of these capabilities is subject to the availability of the information and privacy control.
IMS Media types which can be supported as IMS media streams (i.e. media component definitions of IMS sessions).
Media format parameters for supported IMS media types (Codecs, media file formats etc.).
MSISDN and preferred SIP URI for the UE sending the UE capability information.
Personal ME Identifier to identify which of the user's MEs the UE capability information is related to.
UE capability version.
Additionally, it shall be possible for the UE to use IMS to exchange capability information about:
CS video telephony capability;
CS voice capability;
Support for other IMS based capabilities or services e.g. PoC.
The UE capability information is exchanged between the calling party and the called party.
The information flows for exchanging UE capabilities are shown in clause 8.2.
The IM Status provided by the sending UE as part of the current radio environment information can be used by the receiving UE as an input for IMS registration, session initiation and any subsequent attempt to perform UE capability exchange via OPTIONS.
The IM Status provided by a UE during CS all setup is valid for the duration of that CS call.
During IMS registration, a UE may register its capability information using SIP User Agent capability registration mechanism specified in RFC 3840  and endorsed by TS 23.228. To facilitate the operation of CSI, it shall be possible for the UE to register at least the following UE capabilities:
CS video telephony capability;
CS voice capability.
Registration of these UE capabilities could help the core IMS network in routing SIP messages to appropriate UE when the caller indicates preference for these capabilities in the a SIP message using mechanism specified in RFC 3841  and endorsed by TS 23.228.
The UE may update registered capabilities as specified in TS 23.228.
For network efficiency, the capability detection functionality requires the terminal to store information about the other terminals' capabilities.
In order to cater for remote parties who use more than one ME (e.g. with the same MSISDN or the same public user ID), CSI needs a mechanism that allows for identifying a particular user's ME. This mechanism shall be capable of identifying a ME upon UE capability exchange, CS call setup and IMS session initiation. This enables the remote party to retrieve the correct stored ME capabilities.
In order to limit network signalling (e.g. use of SIP OPTIONS to trigger other party's request) and avoid an inconsistency between the actual and stored capabilities, it is necessary to exchange the UE capability version.
Procedures are needed to avoid MEs of one user having the same Personal ME Identifier.
The Personal ME Identifier and UE capability version shall fulfil the following requirements:
Minimal impact on SIP signalling and no impact on the IM CN subsystem;
The identifier and the capability version should fit into UUS-1 signalling, also allowing for other services to be run over UUS-1;
The Personal ME Identifier uniquely identifies an ME of the user.
The capability version is updated when UE changes its capability (e.g. UE performing online upgrade or configuration change). The capability version is unique for a given set of capabilities of a UE.