AudioCodes Mediant 4000 SBC User Manual page 192

CHAPTER 15 Media
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.
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To configure T.38 mode using SIP Re-INVITE messages:
1. On 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)
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).
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.
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To configure automatic T.38 mode:
1. On the Fax/Modem/CID Settings page, configure the 'Fax Transport Mode' parameter to T.38
Relay [FaxTransportMode = 1].
2. 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]
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
the AudioCodes proprietary method:
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Mediant 4000 SBC | User's Manual