Fax Fallback

In this mode, when the terminating device detects a fax signal, it sends a re-INVITE message to the originating device with T.38. If the remote device doesn’t support T.38 (replies with SIP response 415 "Media Not Supported"), the device sends a new re-INVITE with G.711 VBD with the following adaptations:

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Echo Canceller = on

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Silence Compression = off

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Echo Canceller Non-Linear Processor Mode = off

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Dynamic Jitter Buffer Minimum Delay = 40

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Dynamic Jitter Buffer Optimization Factor = 13

This section is applicable only to the Gateway application.

When the device initiates a fax session using G.711, a ‘gpmd’ attribute is added to the SDP according to the following format:

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For G.711A-law:

a=gpmd:8 vbd=yes;ecan=on

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For G.711 µ-law:

a=gpmd:0 vbd=yes;ecan=on

In this mode, the 'Fax Transport Mode' [FaxTransportMode] parameter is ignored and automatically set to Disable (transparent mode).

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To configure fax fallback mode:

Open the Gateway General Settings page (Setup menu > Signaling & Media tab > Gateway folder > Gateway General Settings), and then from the 'Fax Signaling Method' drop-down list (IsFaxUsed), select Fax Fallback:

Click Apply.