This post is the first in a series that will describe how the routing of SIP service-related requests is performed in the IMS.
Before going into the details of service profiles, initial filter criteria and the ISC (IMS Service Control) interface, I will start with an attempt at placing IMS service routing in the larger context of end-to-end SIP routing within IMS.
I think this is something missing in most representations of the IMS service architecture, which tend to fragment the overall perception of this architecture, and make it more difficult to comprehend for the novice.
The figure you can see at the top (click to enlarge) shows the end-to-end interaction between two IMS users. However, there exist variants related to which entity initiates the SIP interaction (IMS client or IMS AS), which entity terminates the SIP interaction (IMS client or IMS AS), and whether the end-to-end SIP interaction fully takes place between IMS entities or involves entities in an external network like the circuit-switched network or the Internet. I will address these variants below using specific colors.
Before describing what IMS SIP routing permits to achieve, let's start with one of its apparent limitations compared with how SIP can be used in a non-IMS network.
Signalling path termination: the SIP interaction may terminate at phase 2 if one of the application servers decides to act as an endpoint for the SIP request.
The S-CSCF routes the SIP request according to its request-uri.
IMS entry point: instead of an S-CSCF in the originating network, the request may come from the circuit-switched network (through an MGCF), or from a non-IMS SIP network (e.g. the Internet).
A specific case for having an IMS AS issuing a SIP request to the IMS is when it acts as a gateway towards another protocol and/or domain. This is the symetric case to what was described in purple for phases 2 and 5 above.
There is no direct relationship to this post, but I just added the initial architecture proposed in 3GPP for the SCIM in the first post I wrote about it in May.