Microsoft® Office Communications Server 2007 R2 Resource Kit

Bridging VoIP to the PSTN Network by Using a Media Gateway

For voice calls from Office Communications Server to reach the 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 4-3.

Table 4-3. Protocol Translation That Media Gateway Performs

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

Microsoft partners provide such a media gateway. (See the section titled "Additional Resources" at the end of this chapter for the link.) The three ...