I.1 Scope

24.2293GPPIP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)Release 18Stage 3TS

Additional routeing functionality is necessary for support of:

– transit traffic as operators may use the IM CN subsystem as a transit network to provide transit functionality for their own CS networks, enterprise networks, or other network operators;

– other traffics such as roaming traffic and incoming traffic destined to CSI UEs (Combining Circuit Switched (CS) and IP Multimedia Subsystem (IMS) services) traffics;

– traffic for the roaming architecture for voice over IMS with local breakout; and

– originating traffic if required by local policy as specified in subclause 5.4.3.2.

Depending on the additional routeing functionalities, the required specific functions may reside in a stand-alone entity or may be collocated with an MGCF, a BGCF, an I-CSCF, an S-CSCF, or an IBCF as appropriate for the specific scenario.

When colocated with an I-CSCF, the additional routeing capabilities may be performed in advance of I-CSCF procedures as specified in subclause 5.3, or after I-CSCF procedures have failed to identify an S-CSCF supporting the user identified by the Request-URI.

When colocated with an MGCF, the generated requests can be routed to an I-CSCF or to possible targets of the routeing procedures defined in subclause I.2.

The BGCF procedures specified in subclause 5.6 are a subset of the more general routeing procedures provided in this annex.

NOTE: Depending on the host entity for the additional routeing functions, the functionality can be accessed as:

a) the last set of functions on the host before forwarding a request (e.g., on an MGCF, an S-CSCF or an IBCF);

b) the first set of functions performed by the host entity when receiving a request at the host entity’s entry point (e.g., on a BGCF, I-CSCF or IBCF);

c) a specified point in the logic of the host (e.g., on the I-CSCF at failure to identify an S-CSCF for the Request-URI); or

d) a set of functions associated with a separate entry point (e.g., at a separate entry point associated with a BGCF, I-CSCF, IBCF or stand-alone entity).