Configuring Fax Relay Mode; Configuring Fax/Modem Bypass Mode; Supporting V.34 Faxes - 3Com VCX v7111 User Manual

Configuring Fax Relay Mode

When FaxTransportMode = 1 (relay mode), then on detection of fax the channel
automatically switches from the current voice coder to answer tone mode, and then to T.38-
compliant fax relay mode.
When fax transmission has ended, the reverse switching from fax relay to voice is
performed. This mode switching automatically occurs at both the local and remote endpoints.
Users can limit the fax rate using the FaxRelayMaxRate parameter and can enable/disable
ECM fax mode using the FaxRelayECMEnable parameter.
When using T.38 mode, the user can define a redundancy feature to improve fax
transmission over congested IP network. This feature is activated by
FaxRelayRedundancyDepth and FaxRelayEnhancedRedundancyDepth parameters.
Although this is a proprietary redundancy scheme, it should not create problems when
working with other T.38 decoders.
T.38 mode currently supports only the T.38 UDP syntax.

Configuring Fax/Modem Bypass Mode

When VxxTransportType= 2 (FaxModemBypass, Vxx can be one of the following:
V32/V22/Bell/V34/Fax), then on detection of fax/modem, the channel automatically switches
from the current voice coder to a high bit-rate coder (G.711 or G.726), as defined by the
user, with the FaxModemBypassCoderType configuration parameter.
During the bypass period, the coder uses the packing factor (by which a number of basic
coder frames are combined together in the outgoing WAN packet) set by the user in the
FaxModemBypassM configuration parameter. The network packets generated and received
during the bypass period are regular voice RTP packets (per the selected bypass coder) but
with a different RTP Payload type.
When fax/modem transmission ends, the reverse switching, from bypass coder to regular
voice coder, is carried out.

Supporting V.34 Faxes

V.34 faxes do not comply with the T.38 relay standard. We therefore provide the optional
modes described under Using Bypass Mechanism for V.34 Fax Transmission and Using
Relay mode for both T.30 and V.34 faxes:
Note that the CNG detector is disabled (CNGDetectorMode=0) in all the following examples.
Using Bypass Mechanism for V.34 Fax Transmission
In this proprietary scenario, the media gateway uses a high bit-rate coder to transmit V.34
faxes, enabling the full utilization of its speed.
See the following configurations:
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3Com VCX V7111 VoIP Gateway User Guide