Fax / Modem Transport Mode; Fax Fallback - AudioCodes Mediant 3000 User Manual

User's Manual
13.2.2.2 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:

In the SIP General Parameters page (Configuration tab > VoIP menu > SIP
Definitions > General Parameters), set the 'Fax Signaling Method' parameter to
G.711 Transport (IsFaxUsed = 2).

13.2.2.3 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).
Version 6.6 171
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Mediant 3000