Fax Fallback; Fax/Modem Bypass Mode - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 15 Media
After a few seconds upon detection of fax V.21 preamble or super G3 fax signals, the device sends
a second Re-INVITE enabling the echo canceller (the echo canceller is disabled only on modem
transmission).
A 'gpmd' attribute is added to the SDP according to the following format:
■ For G.711 A-law:
a=gpmd:0 vbd=yes;ecan=on (or off for modems)
■ For G.711 µ-law:
a=gpmd:8 vbd=yes;ecan=on (or off for modems)
The following parameters are ignored and automatically set to Events Only:
■ 'Fax Transport Mode' (FaxTransportMode)
■ 'Vxx ModemTransportType' (VxxModemTransportType)

Fax Fallback

In this mode, when the terminating device detects a fax signal, it sends a re-INVITE message to
the originating device with T.38. If the remote device doesn't support T.38 (replies with SIP
response 415 "Media Not Supported"), the device sends a new re-INVITE with G.711 VBD with the
following adaptations:
■ Echo Canceller = on
■ Silence Compression = off
■ Echo Canceller Non-Linear Processor Mode = off
■ Dynamic Jitter Buffer Minimum Delay = 40
■ Dynamic Jitter Buffer Optimization Factor = 13
When the device initiates a fax session using G.711, a 'gpmd' attribute is added to the SDP
according to the following format:
■ For G.711A-law:
a=gpmd:0 vbd=yes;ecan=on
■ For G.711 µ-law:
a=gpmd:8 vbd=yes;ecan=on
In this mode, the 'Fax Transport Mode' (FaxTransportMode) parameter is ignored and automatically
set to Disable (transparent mode).

Fax/Modem Bypass Mode

In this proprietary mode, when fax or modem signals are detected, the channel automatically
switches from the current voice coder to a high bit-rate coder, according to the 'Fax/Modem Bypass
Coder Type' parameter (FaxModemBypassCoderType). The channel is also automatically
reconfigured with the following fax / modem adaptations:
■ Disables silence suppression
■ Enables echo cancellation for fax
■ Disables echo cancellation for modem
■ Performs certain jitter buffering optimizations
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Mediant 4000 SBC | User's Manual