AudioCodes E-SBC - Fax Fallback; G.711 Fax; Modem Transport Mode

CHAPTER15 Media
Mediant 1000 Gateway & E-SBC | User's Manual
■ Device initiates a call: The device always sends the SDP offer with the proprietary token
"AcUdpTl" in the 'fmtp' attribute. If the SDP answer includes the same token, the device
employs the proprietary T.38-over-RTP mode; otherwise, the standard mode is used.
■ Device answers a call: If the SDP offer from the remote party contains the 'fmtp' attribute
with "AcUdpTl", the device answers with the same attribute and employs the proprietary T.38-
over-RTP mode; otherwise, the standard mode is used.
If both T.38 (regular) and T.38 Over RTP coders are negotiated between the call parties,
the device uses T.38 Over RTP.
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:
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