3Com® VCX V7111 VoIP Gateway User Guide 253Configuring Fax Relay ModeWhen FaxTransportMode = 1 (relay mode), then on detection of fax the channelautomatically 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 isperformed. 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/disableECM fax mode using the FaxRelayECMEnable parameter.When using T.38 mode, the user can define a redundancy feature to improve faxtransmission over congested IP network. This feature is activated byFaxRelayRedundancyDepth and FaxRelayEnhancedRedundancyDepth parameters.Although this is a proprietary redundancy scheme, it should not create problems whenworking with other T.38 decoders.T.38 mode currently supports only the T.38 UDP syntax.Configuring Fax/Modem Bypass ModeWhen VxxTransportType= 2 (FaxModemBypass, Vxx can be one of the following:V32/V22/Bell/V34/Fax), then on detection of fax/modem, the channel automatically switchesfrom the current voice coder to a high bit-rate coder (G.711 or G.726), as defined by theuser, with the FaxModemBypassCoderType configuration parameter.During the bypass period, the coder uses the packing factor (by which a number of basiccoder frames are combined together in the outgoing WAN packet) set by the user in theFaxModemBypassM configuration parameter. The network packets generated and receivedduring the bypass period are regular voice RTP packets (per the selected bypass coder) butwith a different RTP Payload type.When fax/modem transmission ends, the reverse switching, from bypass coder to regularvoice coder, is carried out.Supporting V.34 FaxesV.34 faxes do not comply with the T.38 relay standard. We therefore provide the optionalmodes described under Using Bypass Mechanism for V.34 Fax Transmission and UsingRelay 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 TransmissionIn this proprietary scenario, the media gateway uses a high bit-rate coder to transmit V.34faxes, enabling the full utilization of its speed.See the following configurations:
