5 General requirements

22.2793GPPCombined Circuit Switched (CS) and IP Multimedia Subsystem (IMS) sessionsStage 1TS

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.