Fax & Modem Transport Modes; Fax/Modem Settings - 3Com V7122 User Manual

Fax & Modem Transport Modes

Fax/Modem Settings

Users can choose to use for fax, and for each modem type (V.22/V.23/Bell/V.32/V.34), one
of the following transport methods:
Fax relay mode (demodulation / remodulation, not applicable to Modem),
Bypass (using a high bit rate coder to pass the signal), or
Transparent (passing the signal in the current voice coder).
When any of the relay modes are enabled, distinction between fax and modem is not
immediately possible at the beginning of a session. The channel is therefore in 'Answer
Tone' mode until a decision is made. The packets sent to the network at this stage are T.38-
complaint fax relay packets.
Configuring Fax Relay Mode
When FaxTransportMode = 1 (relay mode), and when fax is detected, the channel
automatically switches from the current voice coder to answer tone mode, and then to T.38-
complaint fax relay mode.
When fax transmission ends, the reverse is carried out, and fax relay switches to voice. This
mode switch occurs automatically, both at 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 either V32/V22/Bell/V34/Fax),
then when fax/modem is detected, 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 to be generated and
received during the bypass period are regular voice RTP packets (per the selected bypass
coder) but with a different RTP Payload type.
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