There are several ITU-T recommendations for the transmission of fax data over the internet and VoIP (Voice over Internet Protocol) based telephone networks. One of these recommendations is T.37. It handles the sending of faxes via email over the Internet. Another recommendation for internet faxing is T.38. T.37 was first set up 1998 and, together with T.38, forms the basis for the transmission of faxes over the Internet.
While faxing via analog telephone networks is usually pretty straightforward, faxing through VoIP tends to be a bit more complicated. The lack of a data packets in an analog telephone call doesn’t cause any major issues. However, with a VoIP connection, a single lost data packet can result in the entire fax connection being terminated.
Faxes rely on continuous and complete data streams with minimal delay. If these are not available, incomplete fax transmissions or unsuccessful fax connections can occur. Runtime variations of packets which are too large can cause fax machines to lose their synchronisation. Also complicating the process of faxing over a VoIP network are measures such as the adjustment of the frequency responses as well as noise reduction and speech pause detection. Under certain circumstances, these measures can cause interference with the fax machine which is receiving the fax signal.
Because these problems, the T.37 and T.38 standards were created to allow ease of faxing over IP networks. While T.38 handles fax transmissions in real time over the IP network, T.37 describes the transmission of faxes.
To put it simply, T.37 converts fax transmissions into email attachments. The actual fax document is first converted into an image file in TIFF format and then sent as an email attachment to the desired recipient fax number. This procedure makes the actual transmission of the data independent of the continuous data streams. Since the faxes sent are usually monochrome documents (i.e. they don’t use an elaborate colour pallette), the simple TIFF-F format is used. Although there are now standards for the transfer of colour faxes, these methods are not commonplace in today’s market, with monochrome faxing dominating in most industries.