User's Manual 17. Media
Version 6.8 183 Mediant 500L MSBR
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).
17.2.2.1.2 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 T.38
Relay (FaxTransportMode = 1).
3. 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)
17.2.2.1.3 Fax over IP using T.38 Transmission over RTP
The device supports Fax-over-IP (FoIP) transmission using T.38 over RTP, whereby the
T.38 payload is encapsulated in the RTP packet, instead of being sent in dedicated T.38
packets (out-of-band). To configure this support, set the coder type to T.38 Over RTP.
To indicate T.38 over RTP, the SDP body uses "udptl" (Facsimile UDP Transport Layer) in
the 'a=ftmp' line. The device supports T.38 over RTP according to this standard as well as
according to AudioCodes proprietary method:
Call Parties belong to AudioCodes Devices: AudioCodes proprietary T.38-over-
RTP method is used, whereby the device encapsulates the entire T.38 packet
(payload with all its headers) in the sent RTP. For T.38 over RTP, AudioCodes
devices use the proprietary identifier "AcUdptl" in the 'a=ftmp' line of the SDP. For
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