In order to transmit faxes over the internet and IP-based telephone networks, various ITU-T (the Telecommunications Standardization Sector) recommendations exist. The two main recommendations, T.38 and T.37, form the basis for the successful sending and receiving of faxes over the internet. While T.37 transmits the fax as an attachment to an email in Store-and-Forward mode, T.38 allows the fax transmission to take place in real-time. In contrast to analog faxing in a normal telephone network, in which the faxes exchange information by means of tones, T.38 uses the IP protocols TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) for data transmission.
There are several problems which can arise when you use a fax machine that relies on a network such as the internet. Analog fax messages need to be divided into data packets when they are sent over an IP network. Fax machines depend on a continuous connection with a low delay time if all of the data packets are to be sent successfully. Voice-over-IP connections often cannot guarantee this. In the case of runtime fluctuations or packet losses, the fax machines can lose their synchronisation or the fax connection may not occur at all. Further problems are caused by methods for adapting the frequency response or by the noise suppression and the speech pause detection. The ITU-T recommendation, T.38, avoids these problems by sending the faxes directly via UDP and TCP.
T.38 is a real-time system that sends faxes without delay via UDP and TCP. It is not compatible with conventional G3 faxing. In order to transfer a T.38 fax to a G3 fax, gateways are therefore required between the two incompatible methods. Partially, VoIP (Voice over Internet Protocol) providers provide these gateways. The connection of a normal fax machine to a T.38-compatible VoIP connection is via an ATA (analog telephone adapter) or a special connection on the router, which must also be T.38-capable. T.38 defines how to deal with faulty, lost or repeated data packets.
Providers such as eFax® enable the sending and receiving of faxes from any terminal via the public internet. In order to use the internet fax service, users only need an internet connection. eFax® can be used with existing email mailboxes and does not require the purchase or installation of any additional software or hardware. Another advantage of internet faxing is the management of the faxes takes place in a fully digital, paperless environment. Standard compliance standards are fulfilled thanks to protocol trails (also known as audit trails) and other functions needed for secure transmission.