System supports working with faxes.


There are several modes to transmit a fax in telephony:

  • inband.
    This is a default mode for E1/T1.
    This mode can be used in VoIP when using G.711 audio codec.
    When using other audio codecs with compression, reliable fax transmission is not possible, because compressions algorithms are designed to compress voice, not fax signal.
    However even with G.711 fax transmission in VoIP will be not reliable. The reason is that during stream transmission through IP network losses and delays are possible.
    For E1/T1 there is no such issue, because these technologies use time-division multiplexing (timeslots) with reliable delivery.
  • T.38
    This protocol has been designed to provide reliable fax transmission in VoIP.
    For transmission UDPTL protocol over UDP is used.

Error correction for T.38

Following error correction modes are supported:

  • FEC
  • redundancy
  • auto - use mode, which is suggested by a remote side

In case if during signalling session different modes for bridged peers have been chosen, system supports real-time translation between modes.

Translation between different fax transmission modes.

System supports translation between different fax transmission modes.
This is sometimes referred as Fax gateway.
This can be useful in next cases:

  • You'd like to use more reliable T.38 for fax transmission between your partners in a scheme, when a system is a transit node, however T.38 is supported by only one of partners.
    You can use T.38 on behalf of partner which supports it, and use inband mode with partner which doesn't support T.38.
  • You want to receive fax from E1/T1 stream and transmit it to VoIP partner and vice versa.

To activate fax gateway mode you should enable T.38 gateway in SIP parameters of a peer which doesn't support T.38.
Enabling this parameter will activate audio stream sniffing on this peer.
In case if fax tones will be detected - they will be translated into T.38 messages and sent to bridged peer.
And vice versa, T.38 messages from bridged peer will be translated on fly into fax tones and sent inband.

However, be careful, as this mode added CPU overhead.
Also it disables usage of Media proxy.
Therefore, use it only for appropriate peers.

For reason of minimizing CPU overhead you can configure T.38 gateway timeout option.
It defines the period of time which the system will sniff the audio stream to detect fax.
Its usage is based on assumption that fax is usually transmitted at the beginning of call.
And if there is no fax detected - the call is assumed to be a regular voice call.
And therefore a sniffing thread could be detached.

System as a fax termination endpoint.

System can act as a termination point for sending and receiving of faxes.
This produces VAS, that rely on this function.
For example:

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