The migration of telephone infrastructure towards VoIP has also increased the demand for fax devices to communicate over VoIP. Even in the age of e-mail, fax transmissions continue to be highly important in legal respects as legally binding documents such as contracts and invoices can be far more easily handled by fax than with the alternative of e-mails with digital signature. The integration of fax devices into VoIP infrastructure can be implemented in two ways:
- Fax messages are transmitted via landline just like a conventional fax.
- The transmission takes place over an Internet connection. Options for this are as follows:
- The fax signals are transmitted like voice data over a VoIP connection, referred to as "fax over VoIP". Fax transmission should only make use of the G.711 codec, as other codecs are inferior at converting the fax tones designed for analog networks into digital VoIP data. Due to the highly sensitive nature of fax connections, this method can only be used with high-quality connections, whereby the transmission speed is sub-optimal.
- For example, with the "store-and-forward" principle (ITU-T.37), fax messages are passed from the fax machine to a gateway that stores and converts the fax document. In a second step the fax is transmitted to the destination for conversion back into a fax format. Alternatively fax messages can be sent by e-mail (fax-to-mail and mail-to-fax). Solutions of this type may not meet the legal requirements mentioned above, due to the fact that there is no direct connection between transmitter and receiver.
- With "real-time routing" of fax messages, on the other hand, a direct connection is established between the two fax machines and all data is transferred in real time. The fax machines are connected virtually over the Internet. Communication between the two fax machines follows the ITU-T.38 standard for converting standard fax signals. This variant is also known as Fax over IP (FoIP). The fax messages are not transferred as acoustic signals via VoIP, but rather in a special protocol, that embeds the signals in UDP/TCP packets.
To enable fax transmissions with T.38, the fax machines themselves either have to support the T.38 standard or they must be interconnected over the Internet via fax gateways. LANCOM VoIP routers support the T.38 standard and are thus suitable for operation as fax gateways in VoIP infrastructure.
The fax machines are connected to the LANCOM VoIP routers by means of a suitable interface. The fax gateway in the LANCOM VoIP router handles the conversion of the signals for transmission and reception of fax messages:
- Conversion of T.38 fax data to G.711/T.30
- Conversion of G.711/T.30 fax data to T.38
- Passthrough of G.711/T.30 fax data
- Passthrough of T.38 fax data
If the device type "fax" or "telephone/fax" is selected in the user settings of the ISDN or analog user, the LANCOM Business VoIP router automatically recognizes a fax for transmission and it attempts to transmit via F.38/FoIP. If the remote site does not support this method, the LANCOM VoIP router automatically uses the fax over VoIP-version using G.711 compression.
