SIP User's Manual 250 Document #: LTRT-65415
MediaPack Series
8.2.6.2.2 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.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
automatically set to the mode called ‘transparent with events’.
To configure fax / modem transparent mode, set IsFaxUsed to 2.
8.2.6.2.3 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 parameter FaxTransportMode is ignored and automatically set to
‘transparent’.
To configure fax fallback mode, set IsFaxUsed to 3.
8.2.6.2.4 Fax/Modem Bypass Mode
In this proprietary mode, when fax or modem signals are detected, the channel
automatically switches from the current voice coder to a high bit-rate coder (according to
the parameter FaxModemBypassCoderType). In addition, the channel is automatically
reconfigured with the following fax / modem adaptations:
Disables silence suppression
Enables echo cancellation for fax
Disables echo cancellation for modem
Performs certain jitter buffering optimizations
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