This TS defines a call control protocol for use in the IP Multimedia (IM) Core Network (CN) subsystem
based on the Session Initiation Protocol (SIP), and the associated Session Description Protocol (SDP).
Where possible the present document specifies the requirements for this protocol by reference to specifications
produced by the IETF within the scope of SIP and SDP. Where this is not possible, extensions to SIP and SDP are
defined within the present document. The document has therefore been structured in order to allow both forms of
As the IM CN subsystem is designed to interwork with different IP-Connectivity Access Networks (IP-CANs), the IP-CAN independent aspects of the IM CN subsystem are described in the main body and annex A of this specification.
Aspects for connecting a UE to the IM CN subsystem through specific types of IP-CANs are documented separately in
the annexes or in separate documents.