4.15 IMS in transit network scenarios

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

4.15.1 General concepts

IMS generally provides services to end user customers of a network operator by directly supporting multimedia communications services to or from that operator’s customers. However IMS may also be used in a number of other configurations where the capabilities of IMS are used to support CS domain customers of an IMS operator or in various other kinds of business arrangements where the capabilities may be used to support interconnection of other networks.

Clause 4.15.2 describes several types of configurations in which IMS might be used to support such network interconnection. These are not intended to represent all possible applications of IMS, but rather provide some basis for the mechanisms by which IMS provides these transit functionalities. Further description of IMS transit network procedures are found in Clauses 5.4a.2 and 5.19.

Clause 4.15.3 describes the cases in which IMS application services can be provided in relation to the IMS transit traffic.

4.15.2 IMS transit network configurations

There are at least three general cases in which IMS may be used for transit network support. These could be classified as in the following:

a) IMS operator providing transit functionality for its own, non-IMS (CS domain), customers:
In this case the operator is serving its own customers, some of which have been migrated to IMS while others are still CS Domain subscribers. In this case SIP traffic arrives at a configured entry point and PSTN traffic arrives at the operator’s MGCF. This is similar to the normal Mobile Terminating cases for IMS, but in this case the CS domain subscribers do not have an IMS subscription. For the case where the destination user is not an IMS subscriber, the operator needs to route the session to the CS domain.

b) IMS operator providing transit functionality to enterprise networks:
In this case the operator is serving as a transit network for an enterprise IP network and provides connectivity to both PSTN and IP endpoints. Traffic from the enterprise network arrives at a provisioned routing entity and needs to be routed to either an IP network or to the PSTN depending on the terminating endpoint.

c) IMS operator providing transit functionality to other network operators:
In this case the operator is serving as an IMS session based routing backbone for a PSTN operator or another IP network and provides connectivity to both PSTN and IP endpoints (PSTN <‑> PSTN, IP <‑> IP, PSTN <‑> IP). Traffic from the PSTN operator arrives at configured MGCFs for translation to SIP. IMS traffic arrives at a configured entry point. In either case the operator needs to route the traffic to either an IP network or to the PSTN depending on the terminating endpoint.

An IMS operator can provide transit functionality as above in addition to (originating or terminating) IMS services. In these situations analysis of an incoming SIP request is required before it can be determined whether transit or terminating services need to be provided for this request.

4.15.3 Providing IMS application services in transit network scenarios

When IMS provides transit functionality to other network operators or enterprise networks, the IMS may also provide IMS applications services to the network operator or enterprise network.

Figure 4.15.3-1: IMS application services reference point for transit network scenarios

The Transit service invocation shall be performed based on local configured Transit invocation criteria that are provided for the specific transit scenario.

The Transit invocation criteria for invocation shall have the possibility to take into account,

– (served) preceding network,

– (served) succeeding network, and

– other additional session information.

NOTE: The Transit invocation criteria are intended to be per served interconnected network basis and not per subscriber basis.

Similar to the initial filter criteria for a user profile, the Transit invocation criteria may have service point triggers based on different information in the request, such as source/destination, SIP method, session case, SIP header, and SIP body. The service invocation procedure shall support suppression/avoidance of conflicting services, multiple invocations of the same service and loopback scenarios.

The IMS application services provided can be classified as:

a) Originating IMS application services:

In this case the IMS operator provides IMS application services for SIP traffic being received from the served network operator or enterprise network. The application services, appropriate for the received SIP traffic, will be invoked when received from the served network, after which the traffic is routed towards the destination endpoint.

b) Terminating IMS application services:

In this case the IMS operator provides IMS application services for SIP traffic destined to a served network operator or enterprise network. The application services, appropriate for the received SIP traffic, will be invoked when the served network has been identified as the next network, or when the traffic has been identified as destined to the served network.