AudioCodes Mediant 600 - Fax; Modem Transport Mode

Version 6.4 145 March 2012
SIP User's Manual 12. Media
Although this is a proprietary redundancy scheme, it should not create problems when
working with other T.38 decoders.
12.2.2.1.1 Switching to T.38 Mode using SIP Re-INVITE
In the Switching to T.38 Mode using SIP Re-INVITE mode, upon detection of a fax signal
the terminating device negotiates T.38 capabilities using a Re-INVITE message. If the far-
end device doesn't support T.38, the fax fails. In this mode, the parameter
FaxTransportMode is ignored.
To configure T.38 mode using SIP Re-INVITE messages, set IsFaxUsed to 1. Additional
configuration parameters include the following:

FaxRelayEnhancedRedundancyDepth

FaxRelayRedundancyDepth

FaxRelayECMEnable

FaxRelayMaxRate
Note:
The terminating gateway sends T.38 packets immediately after the T.38
capabilities are negotiated in SIP. However, the originating device by default,
sends T.38 (assuming the T.38 capabilities are negotiated in SIP) only after it
receives T.38 packets from the remote device. This default behavior cannot
be used when the originating device is located behind a firewall that blocks
incoming T.38 packets on ports that have not yet received T.38 packets from
the internal network. To resolve this problem, the device should be configured
to send CNG packets in T.38 upon CNG signal detection (CNGDetectorMode
= 1).
12.2.2.1.2 Automatically Switching to T.38 Mode without SIP Re-INVITE
In the Automatically Switching to T.38 Mode without SIP Re-INVITE mode, when a fax
signal is detected, the channel automatically switches from the current voice coder to
answer tone mode, and then to T.38-compliant fax relay mode.
To configure automatic T.38 mode, perform the following configurations:

IsFaxUsed = 0

FaxTransportMode = 1

Additional configuration parameters:
• FaxRelayEnhancedRedundancyDepth
• FaxRelayRedundancyDepth
• FaxRelayECMEnable
• FaxRelayMaxRate
12.2.2.2 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