SIP User's Manual 402 Document #: LTRT-65412
MP-11x & MP-124
9.5.2.7 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:
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 device 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.
9.5.3 V.34 Fax Support
Unlike T.30 fax machines, V.34 fax machines have no relay standard to transmit data over
IP to the remote side. Therefore, the device provides the following operation modes for
transporting V.34 fax data over the IP:
îš„ Using bypass mechanism for V.34 fax transmission (refer to ''Using Bypass
Mechanism for V.34 Fax Transmission'' on page 402)
îš„ Usi
ng relay mode, i.e., fallback to T.38 (refer to ''Using Relay mode for both T.30 and
V.34 faxes'' on page 403)
Usi
ng the ini file parameter V34FaxTransportType, you can determine whether to pass V.34
Fax-over-T.38 fallback to T.30, or use Bypass over the High Bit Rate coder (e.g. PCM A-
Law).
Note: The CNG detector is disabled (CNGDetectorMode = 0) in all the subsequent
examples.
9.5.3.1 Using Bypass Mechanism for V.34 Fax Transmission
In this proprietary scenario, the device uses bypass (or NSE) mode to transmit V.34 faxes,
enabling the full utilization of its speed.
Configure the following parameters to use bypass mode for both T.30 and V.34 faxes:
îš„ FaxTransportMode = 2 (Bypass)
îš„ V34ModemTransportType = 2 (Modem bypass)
îš„ V32ModemTransportType = 2
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