SIP User's Manual 344 Document #: LTRT-68806
Mediant 2000 & TP-1610 & TP-260/UNI
îš„ V34ModemTransportType = 2
îš„ BellModemTransportType = 2
7.2.2.4 G.711 Fax / Modem Transport Mode
In this mode, when the terminating gateway detects fax or modem signals (CED or
AnsAM), it sends a Re-INVITE message to the originator gateway asking it to reopen the
channel in G.711 VBD with the following adaptations:
îš„ Echo Canceller = off
îš„ Silence Compression = off
îš„ Echo Canceller Non-Linear Processor Mode = off
îš„ Dynamic Jitter Buffer Minimum Delay = 40
îš„ Dynamic Jitter Buffer Optimization Factor = 13
After a few seconds upon detection of fax V.21 preamble or super G3 fax signals, the
gateway 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:
îš„ For G.711A-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 parameters FaxTransportMode and VxxModemTransportType are ignored and are
automatically set to the mode called ‘transparent with events’.
To configure fax / modem transparent mode, set IsFaxUsed to 2.
7.2.2.5 Fax Fallback
In this mode, when the terminating gateway detects a fax signal, it sends a Re-INVITE
message to the originator gateway with T.38. If the remote gateway doesn’t support T.38
(replies with 415 Media Not Supported), the gateway sends a new Re-INVITE with G.711
VBD with the following adaptations:
îš„ Echo Canceller = on
îš„ Silence Compression = off
îš„ Echo Canceller Non-Linear Processor Mode = off
îš„ Dynamic Jitter Buffer Minimum Delay = 40
îš„ Dynamic Jitter Buffer Optimization Factor = 13
When the gateway initiates a fax session using G.711, a ‘gpmd’ attribute is added to the
SDP according to the following format:
îš„ 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 parameter FaxTransportMode is ignored and automatically set to
‘transparent’.
To configure fax fallback mode, set IsFaxUsed to 3.
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