T.38

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T.38 is an ITU standard which deals with sending fax messages over IP networks. It is used together with Session Initiation Protocol and Session Description Protocol.

The need for reliable faxing over IP networks (the Internet) has increased as the popularity of Voice over IP has increased. Faxes operate by scanning a document, converting the document into data, and sending that data as sounds over a telephone line to a receiving fax which decodes it. This does not operate correctly on many VoIP systems, designed to convert voice sounds to data over the internet, which lose tones required for faxing. The T.38 standard, when implemented on a VoIP ATA and on the VoIP gateway it connects to, is designed to convert fax sounds to data and enable reliable faxing.