T38, G711 (audio) and CAPI Fax Modes

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Fax Voip T.38 Console supports receiving and sending faxes according to T.38 standard, as well as in audio mode (over G711-ulaw or G711-Alaw codecs). When sending or receiving faxes via ISDN CAPI 2.0 connection, you also can use CAPI fax based on CAPI 2.0 specification (B3 Protocol 4: T.30 for Group 3 fax or B3 Protocol 5: T.30 for Group 3 fax extended).

Both T.38 and audio fax (fax over G.711 codecs) have their advantages and disadvantages when sending/receiving faxes over VoIP.

T.38 - the most reliable standard used to transmit faxes over VoIP in real time. Isolates from the delays, timing jitter, and packet loss experienced in VoIP networks. T.38 support should be provided by your SIP or H.323 VoIP Service provider. Note that T.38 standard is fully compatible with using any of audio codecs (G.711 codecs are not required).

Sending a fax over the audio channel (G711 fax) is not so reliable. Nevertheless, this method can be used with almost any VoIP provider that supports G.711-ulaw or G.711-Alaw codecs. In this case, possibility and quality of faxing depends on many factors, such as the quality of VoIP termination or on the stability of the Internet channel on the client side. Despite the obvious disadvantages, G711 fax often operates very well and stable with many SIP and H.323 providers.

One of the common problems faced by customers is the incomplete support for T.38 on the side of VoIP Service provider. In other words, the provider announces T.38 support, but can not provide T.38 for some call destinations. Or does not have a stable termination to your desired destination. Fax Voip T.38 Console is able to automatically detect the presence or absence of T.38 support at the time of the current call.

In the case of sending or receiving faxes via ISDN CAPI 2.0 line using G711 audio fax mode, you should not expect problems as there are no delays typical for VoIP. On the other hand, we recommend to use CAPI fax mode instead of G711 audio fax mode when sending or receiving faxes via ISDN CAPI 2.0 connection.

Possible fax modes are different in the case of VoIP connection and in the case of ISDN CAPI 2.0 connection and should be considered separately: