Version 6.2 445 February 2011
SIP User's Manual 8. IP Telephony Capabilities
To configure automatic T.38 mode, perform the following configurations:
IsFaxUsed = 0
FaxTransportMode = 1
Additional configuration parameters:
• FaxRelayEnhancedRedundancyDepth
• FaxRelayRedundancyDepth
• FaxRelayECMEnable
• FaxRelayMaxRate
8.3.7.2.2 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.
8.3.7.2.3 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:
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