AudioCodes Mediant 800 - Fax Fallback; G.711 Fax; Modem Transport Mode

CHAPTER15 Media
Mediant 800 Gateway & E-SBC | User's Manual
G.711 Fax / Modem Transport Mode
In this mode, when the terminating device detects fax or modem signals (CED or AnsAM), it sends
a Re-INVITE message to the originating device, requesting it to re-open the channel in G.711 VBD
with the following adaptations:
■ Echo Canceller = off
■ Silence Compression = off
■ Echo Canceller Non-Linear Processor Mode = off
■ Dynamic Jitter Buffer Minimum Delay = 40
■ Dynamic Jitter Buffer Optimization Factor = 13
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)
➢ To configure fax / modem transparent mode:
1. Open the Gateway General Settings page (Setup menu > Signaling & Media tab > Gateway
folder > Gateway General Settings), and then from the 'Fax Signaling Method' drop-down list
(IsFaxUsed), select G.711 Transport:
2. Click Apply.
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:
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