Content for  TS 23.228  Word version:  19.0.0

The MGCF in the IM CN subsystem is a SIP endpoint that initiates and receives requests on behalf of the PSTN and Media Gateway (MGW).Other nodes consider the signalling as if it came from a S-CSCF. The MGCF incorporates the network security functionality of the S-CSCF. PSTN termination may be done in the same operator's network as the S-CSCF of the session originator. Therefore, the location of the MGCF/MGW are given only as "Terminating Network" rather than "Home Network" or "Visited Network". Further, agreements between network operators may allow PSTN termination in a network other than the originator's visited network or home network. This may be done, for example, to avoid long distance or international tariffs.

The PSTN termination procedure is as follows:

This sub clause describes the IMS session setup procedures towards external SIP clients that don't support the required IMS SIP extensions. In this scenario, the UE originates an IMS session without requiring the support for precondition capabilities, towards an external SIP entity that does not support those capabilities. Since required resources are not yet available at the UE, all the media components are set to inactive. In this example the external SIP client does not support the Precondition extension of SIP so the related precondition information within SIP/SDP is ignored. When both parties have agreed to the session and media parameters and the UE has established resources for the media, the UE initiates session modification setting the status of the media components to active and is thus enabling the media transfer to start. Figure 5.19b and Figure 5.19c together illustrate session flows for one possible originating session establishment towards a non-IMS client in an external network with QoS authorization and Policy and Charging Control support. For illustration purposes these session flows show the case of a non-roaming origination. This flow is a variant of MO#2 defined in sub clause 5.6.2. The same principles apply in roaming cases, i.e. analogous variants of MO#1 defined in sub clause 5.6.1 are also supported for interworking with SIP clients that do not support the required IMS procedures.

The UE initiates an INVITE message, which indicates the support of the precondition capability Since required resources are not yet available, the UE sets all media components to inactive state, as shown in Figure 5.19b.

Once the session parameters have been agreed and the UE has successfully reserved resources for the media components, the session set-up continues by setting the media components to active, as shown in session flow 5.19c. At this point in time, the session is established between the two parties.