User's Manual 146 Document #: LTRT-65417
MP-11x & MP-124
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 'Fax Relay Redundancy
Depth' parameter (FaxRelayRedundancyDepth) and the 'Fax Relay Enhanced
Redundancy Depth' parameter (FaxRelayEnhancedRedundancyDepth). Although this is a
proprietary redundancy scheme, it should not create problems when working with other
T.38 decoders.
13.2.2.1.1 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 'Fax Transport Mode'
parameter (FaxTransportMode) is ignored.
 To configure T.38 mode using SIP Re-INVITE messages:
1. In the SIP General Parameters page (Configuration tab > VoIP menu > SIP
Definitions > General Parameters), set the 'Fax Signaling Method' parameter to T.38
Relay (IsFaxUsed = 1).
2. In the Fax/Modem/CID Settings page, configure the following optional parameters:
• 'Fax Relay Redundancy Depth' (FaxRelayRedundancyDepth)
• 'Fax Relay Enhanced Redundancy Depth'
(FaxRelayEnhancedRedundancyDepth)
• 'Fax Relay ECM Enable' (FaxRelayECMEnable)
• 'Fax Relay Max Rate' (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).
13.2.2.1.2 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:
1. In the SIP General Parameters page (Configuration tab > VoIP menu > SIP
Definitions > General Parameters), set the 'Fax Signaling Method' parameter to No
Fax (IsFaxUsed = 0).
2. In the Fax/Modem/CID Settings page, set the 'Fax Transport Mode' parameter to
RelayEnable (FaxTransportMode = 1).
3. Configure the following optional parameters:
• 'Fax Relay Redundancy Depth' (FaxRelayRedundancyDepth)
• 'Fax Relay Enhanced Redundancy Depth'
(FaxRelayEnhancedRedundancyDepth)
• 'Fax Relay ECM Enable' (FaxRelayECMEnable)
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