Modem Pass-Through

Modem Pass-Through

Modem pass-through, as illustrated in Figure 2-48, is similar to fax pass-through, except that there is a computer modem at each end of the connection. The two modems communicate directly with each other over a transparent IP connection.

Figure 2-48. Modem Pass-Through

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When a gateway detects a modem tone, it switches the call to a high-bandwidth CODEC. The modem traffic, still in PCM form, travels in band over VoIP using G.711 with no VAD. This method of transporting modem traffic takes a constant 64-kbps (payload) stream end to end for the duration of the call. It is highly sensitive to packet loss, jitter, and latency in the IP network, although packet redundancy can be used to mitigate the effects of packet loss. Packet redundancy is defined in RFC 2198 and describes a way in which RTP carries the modem audio essentially twice. The redundant packets are sent in case there is packet loss. This scheme produces significant overhead, therefore, and may not be acceptable in all applications. You can enable or disable packet redundancy when configuring modem pass-through.

When using modem relay, which is illustrated in Figure 2-49, computer modem signals are demodulated at one gateway, converted to digital form, and carried in Simple Packet Relay Transport (SPRT) protocol packets to the other gateway. When it reaches the other gateway, the modem signal is recreated and remodulated and then passed to the receiving computer modem. SPRT is a protocol running over User Datagram Protocol (UDP). At the end of the modem session, the voice ports revert to the previous configuration and the DSPs switch back to the original voice CODEC. This method uses less bandwidth (Real-Time Transport Protocol [RTP] is not required) and is much less sensitive to jitter and clocking mismatches than modem pass-through.

Figure 2-49. Modem Relay

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The following call control protocols support modem relay:

• H.323

• SIP

• MGCP