G.711 Fax and Modem Transport Mode

In this mode, when the terminating device detects fax or modem signals (CED or AnsAM), it sends a re-INVITE message to the originating device, requesting it to re-open the channel in G.711 VBD with the following adaptations:

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Echo Canceller = off

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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

After a few seconds upon detection of fax V.21 preamble or super G3 fax signals, the device sends a second re-INVITE enabling the echo canceller (the echo canceller is disabled only on modem transmission).

A ‘gpmd’ attribute is added to the SDP according to the following format:

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For G.711 A-law:

a=gpmd:8 vbd=yes;ecan=on (or off for modems)

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For G.711 µ-law:

a=gpmd:0 vbd=yes;ecan=on (or off for modems)

The following parameters are ignored and automatically set to Events Only:

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'Fax Transport Mode' [FaxTransportMode]

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'Vxx ModemTransportType' [VxxModemTransportType]