Nortel Networks Mediant TP-1610 SIP - Manual (Page 203)

Mediant 2000 SIP User’s Manual D. Fax and Modem Transport Modes
Version 4.4 203 July 2005
Appendix D Fax and Modem Transport Modes
D.1 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.
D.1.1 Configuring Fax Relay Mode
When FaxTransportType= 1 (relay mode), then 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
“EnhancedFaxRelayRedundancyDepth” parameters. Although this is a proprietary redundancy
scheme, it should not create problems when working with other T.38 decoders.
Note: T.38 mode currently supports only the T.38 UDP syntax.
D.1.2 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, 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.
When fax/modem transmission ends, the reverse is carried out, and bypass coder is switched to
regular voice coder.