Fax Negotiation and Transcoding

The device can allow fax transmissions to traverse transparently without transcoding or it can handle the fax as follows:

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Allow interoperability between different fax machines, supporting fax transcoding if required.

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Restrict usage of specific fax coders to save bandwidth, enhance performance, or comply with supported coders. These coders include G.711 (A-Law or Mu-Law), VBD (G.711 A-Law or G.711 Mu-Law), and T38.

Fax configuration is done in the Coders Groups table and IP Profiles table. The Coders Groups table defines the supported coders, which is assigned to the IP Profile associated with the SIP entity. The IP Profiles table also defines the negotiation method used between the incoming and outgoing fax legs, using the following fax-related parameters:

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'Fax Mode': Defines the offer negotiation method - pass fax transparently, negotiate fax according to fax settings in IP Profile, or enforce remote UA to first establish a voice channel before fax negotiation.

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'Fax Coders Group': Defines the supported fax coders (from the Coders Groups table).

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'Fax Offer Mode': Defines the fax coders sent in the outgoing SDP offer.

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'Fax Answer Mode': Defines the fax coders sent in the outgoing SDP answer.

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'Remote Renegotiate on Fax Detection': You can also configure the device to detect for faxes (CNG tone) immediately after the establishment of a voice channel (i.e., after 200 OK) and within a user-defined interval. If detected, it can then handle the subsequent fax renegotiation by sending re-INVITE messages to both SIP entities (originating and terminating faxes). For more information, see the parameter in Configuring IP Profiles.