Fax Relay Mode - AudioCodes Mediant 3000 User Manual

12.2.2.1 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 (see 'Switching to T.38 Mode
using SIP Re-INVITE' on page 158)

Automatically switching to T.38 mode without using SIP Re-INVITE messages (see
'Automatically Switching to T.38 Mode without SIP Re-INVITE' on page 158)
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
FaxRelayRedundancyDepth
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:
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
SIP User's Manual
IP networks.
and
The terminating gateway sends T.38 packets immediately after the T.38
capabilities are negotiated in SIP. The originating device sends no-op T.38
packets after the T.38 capabilities are negotiated in SIP. For this feature to
function, no-op packets must be enabled (NoOpEnable = 1).
This feature is
FaxRelayEnhancedRedundancyDepth
158
Mediant 3000
activated using the
parameters.
Document #: LTRT-89712