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.
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:
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.
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