For voice calls from Office Communications Server 2007 to reach the Public Switched Telephone Network (PSTN), a third-party media gateway is required to bridge the IP network and the PSTN network by translating the signaling and media to the protocols used by each network, as shown in Table 3-2.

Such a media gateway is provided by Microsoft partners. (See the Additional Resources section at the end of this chapter for the link.) The two types of media gateways that Office Communications Server 2007 supports are:

The third-party basic media gateways cannot translate media from real time (RT) Audio codecs to G.711 codecs or the other way around. Therefore, a Mediation Server role provided by Office Communications Server 2007 is required to translate RT Audio to G.711, and it must be understood by the basic media gateway. If the media gateway does not support MTLS, the Mediation Server translates SIP from MTLS to Transmission Control Protocol (TCP). There must be a one-to-one mapping of Mediation Server to media gateway. You cannot configure a Mediation Server to service multiple media gateways.

The third-party advanced media gateways do not require the assistance of a Mediation Server and are capable of directly translating RT Audio to and from G.711. Essentially, the advanced media gateway incorporates the functionality of the Mediation Server in the media gateway.

Alternatively, if the media gateway runs on Windows Server 2003, it might be possible to co-locate the Mediation Server and the basic media gateway on the same server, if supported by the media gateway vendor. Although not an advanced media gateway, this configuration improves overall return on investment (ROI), as it reduces the number of servers required to bridge to the PSTN network.