Fax Relay Mode - AudioCodes Mediant 2000 User Manual

SIP User's Manual
9.5.2.1 T.38 Fax Relay Mode
In Fax Relay mode, fax signals are transferred using the T.38 protocol. T.38 is an ITU
standard for sending fax across IP networks in real-time mode. The device currently
supports only the T.38 UDP syntax.
T.38 can be configured in the following ways:

Switching to T.38 mode using SIP Re-INVITE messages (refer to ''Switching to T.38
Mode using SIP Re-INVITE''

Automatically switching to T.38 mode without using SIP Re-INVITE messages (refer to
''Automatically Switching to T.38 Mode without SIP Re-INVITE''
When fax transmission ends, the reverse switching from fax relay to voice is automatically
performed at both the local and remote endpoints.
You can change the fax rate declared in the SDP, using the parameter FaxRelayMaxRate
(this parameter doesn't affect the actual transmission rate). In addition, you can enable or
disable Error Correction Mode (ECM) fax mode using the FaxRelayECMEnable parameter.
When using T.38 mode, you can define a redundancy feature to improve fax transmission
over congested IP networks. This feature is activated using the FaxRelayRedundancyDepth
and FaxRelayEnhancedRedundancyDepth parameters. Although this is a proprietary
redundancy scheme, it should not create problems when working with other T.38 decoders.
9.5.2.1.1 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:
Version 5.8
on page
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).
411 )
411
9. IP Telephony Capabilities
on page 412 )
October 2009