The present document specifies service requirements for Combining CS and IMS services using a CS speech or CS multimedia call in association with an IMS session. The IMS session may consist of one or more IMS services. Requirements for the following capabilities are included:

• Radio capability exchange.
• Terminal capability exchange.
• E.164 number exchange.
• Adding IMS session to an ongoing CS call.
• Adding a CS call to an ongoing IMS session.
• Supplementary services as they relate to CSICS.

It is intended that the capabilities defined herein for CSICS shall support interoperability between different operator networks, and roaming.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

Combinational services are applicable to both UTRAN and GERAN and enables the unidirectional or bi-directional exchange of PS data within the context of an IMS session A specific subscription for combinational services is not necessary. However, both users A and B shall as a minimum be provisioned with CS telephony (TS11) as well as for accessing the IMS. The existing address context is reused when the combined service is established, which makes the combined service simple to invoke for the user.

In addition to the existing IMS requirements [3], the following general requirements apply for CSICS: It shall be possible to establish a combinational service between two users within the same PLMN or within different PLMNs. It shall be possible to establish a combinational service between two users camped on identical or different RATs. It shall be possible to establish a combinational service when roaming, assuming the visited operator supports GPRS roaming. The user (A or B party) shall only need to know one address in order to establish the combinational service. It shall be possible to add an IMS session to a CS speech call, thereby creating a combinational call. It shall be possible to add a CS speech call to an IMS session, thereby creating a combinational session. It shall be possible to add an IMS session to a CS Multimedia call, thereby creating a combinational call. It shall be possible to add a CS Multimedia call to an IMS session, thereby creating a combinational session. The following two service modes will exist in regards to IMS registration depending on different UE implementations:

1. IMS pre established state: the CSICS capable UE performs the IMS registration at switch on.
2. IMS on demand state: the CSICS capable UE performs the IMS registration:
   • to start the communication, or
   • to add a IMS session to an existing CS call.

Interoperability between UEs that implement such different approaches shall be enabled. During a CS call it shall be possible to request establishment of the IMS session whether the invited UE is IMS registered or not. The invited user shall be able to accept or reject the IMS registration request. A combinational service shall enable both unidirectional and bi-directional exchange of PS data within the context of the IMS session.

When one of the participating users terminates the CS call of a combinational service, the IMS session may continue. When one of the participating users terminates the IMS session of a combinational service, the CS call may continue. When the user A sends media to a user B, the user B can accept or reject the media (confirmation from the receiving party is needed) and vice versa. If media, or parts thereof, accepted by a user cannot be rendered by the UE simultaneously with the CS call, conflicts shall be resolved such that the user is presented with:

• CS speech with preference over IMS speech/audio;
• IMS video and images with preference over CS video.

It shall be possible to initiate a combinational service with user perceived simultaneous setup of IMS session and CS call. The CS call and IMS session can be established sequentially and on the failure of any of the setups the user may be prompted to decide whether to continue. The terminating user shall be able to accept or reject CS call or IMS session independently.

There is no standardised supplementary service defined for IMS session, however mechanisms exist (service capabilities) to emulate the behaviour of some of the most common supplementary services that exist in the circuit switched domain. The intention in this clause is NOT to define "supplementary services" for IMS, just to explain the service behaviour during a combinational session for some specific cases as indicated below:

• IMS session hold: In an ongoing combinational service, the user may decide to suspend the IMS session. When this service is invoked the user should be able to decide whether the CS call of the combinational service should also be put on hold.
• IMS session waiting: In an ongoing combinational service, the user should be able to receive an alert of an incoming IMS session towards his UE. Subject to the capability of the UE, the user should be provided with the option to switch between the ongoing session and the new incoming one, or accept the new one in parallel with the existing one. The CS call of the combinational service should continue during the alerting of the subscriber and the user may decide to put the CS call on hold when switching to the new IMS session.
• IMS session redirect: It should be possible to add CS call to a redirected IMS session, subject to the capability of the recipient UE.
• Identity presentation: Existing Session Originator Identity Presentation rules apply to IMS session.
• Identity restriction: Existing Session Originator Identity Presentation Suppression rules apply to IMS session, even if this results in the called party being unable to establish a combinational session.

The current section lists the supplementary services as specified in TS 22.004.

For combinational services, it must be possible to charge as follows: It shall be possible to provide charging information on the CS call and IMS session for correlation purposes in order to allow off-line charging. The charging information shall continue to be produced for any remaining multimedia components or the CS call when a multimedia component or the CS call drops during the communication between the two parties. The home operator should be able to correlate charged media components and CS call in order to introduce dedicated charging schemes, e.g. discounts. This applies to on-line charging as well as off-line charging.

The combinational service should not place additional provisioning requirement on the operator.