6.5 Interworking 3GPP Release 5 with Other Networks

Connection to circuit-switched networks, such as the PSTN, GSM and 3GPP Release 1999 networks, requires interworking at both the speech path level and the signalling level. Media gateways (MGWs) are included in the IM subsystem to terminate the IP transported speech paths and convert them to circuit-switched TDM, transcoding if necessary. The circuit-switched networks generally use the ITU-T SS7 integrated services user part (ISUP) to control calls. Signalling gateways (SGW) in the IM subsystem map between the message transfer part levels of SS7 and the SIP transport protocol (e.g. TCP/IP) used in the IM subsystem.

It is not possible to simply map ISUP signalling messages into SIP messages, since the service context of the messages must be known. A media gateway control function (MGCF) is used to perform the mapping of the IM subsystem voice service (and SIP signalling) to the voice service of the other network (e.g. PSTN voice service and ISUP signalling). The MGCF communicates with the S-CSCF or I-CSCF using SIP. The MGCF also controls the MGW, for example using the H.248/ Megaco protocol [5].

Interworking with other VoIP networks that are not compatible with 3GPP Release 5, such as those based on ITU-T Recommendation H.323, also requires signalling and media gateways in order to map any differences in lower layer protocols and police the IM subsystem. An MGCF is also needed to ensure appropriate mapping of the voice service between the networks. Figure 6.12 shows the functional entities...