Microsoft® Office Communications Server 2007 Resource Kit

Bridging VoIP to the PSTN Network by Using a Media Gateway

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.

Table 3-2. Protocol Translation Performed by Media Gateway

Technology	PSTN Network	IP Network
Signaling protocol	SS7 and others	SIP
Media codecs	G.711	RTAudio
Transport Protocol	T-Carrier/E-Carrier	RTP (Real-Time Protocol)/SRTP (Secure RTP)
Network Type	Circuit switched	Packet-based

Such a media gateway is provided by Microsoft partners. (See the Additional Resources section at the end of this chapter for the link.) ...