Fax Pass-Through
Fax pass-through occurs when incoming T.30 fax data is not demodulated or compressed for its transit through the packet network, as shown in Figure 2-47. The two fax machines communicate directly with each other over a transparent IP connection.
Figure 2-47. Fax Pass-Through
When a gateway detects a fax tone, it switches the call to a high-bandwidth CODEC. The fax traffic, still in PCM form, travels in band over VoIP using G.711 with no VAD. This method of transporting fax traffic takes a constant 64-kbps (payload) stream end to end for the duration of the call. It is very sensitive to packet loss, jitter, and latency in the IP network, although packet redundancy can be used to mitigate the effects of packet loss.
Fax pass-through is applicable when connecting to a third-party voice gateway that does not support T.38 fax relay. Fax pass-through treats the fax call as a simple G.711 voice call with no special handling for fax. When connecting a Cisco fax-enabled router to a third-party voice-enabled router that does not support fax, you should use fax pass-through. The originating Cisco router will treat the call as a G.711 voice call and will not compress, thus preserving the analog properties of the waveshape for the receiving fax machine.
Fax pass-through is supported under the following call control protocols:
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H.323
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Session initiation protocol (SIP)
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Media Gateway Control Protocol (MGCP)
Note
Echo cancellation is enabled and preferred for pass-through using Cisco IOS Release 12.0(3) T and later. Earlier versions of Cisco IOS software required that you disable echo cancellation.