The migration of telephone infrastructure towards VoIP also increases 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 for compression, as other codecs are inferior at converting fax tones, which are 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, the IFP (Internet Facsimile Protocol), that embeds the signals in UDP/TCP packets.
To enable fax transmissions with T.38, either the fax machines themselves have to support the T.38 standard, or they must be interconnected over the Internet via fax gateways. LANCOM VoIP Routers and LANCOM Routers with the LANCOM VoIP Advanced Option or LANCOM VoIP Basic Option 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:
- Demodulation of incoming T.30 fax signals
- Conversion of T.30 fax signals into T.38 IFP packets
- Transmission of IFP packets between transmitting and receiving gateways
- Conversion of T.38 IFP packets into T.30 fax signals
- Modulation of T.30 fax signals and transmission to the fax machine
With the device type "fax" or "telephone/fax" is selected in the analog or ISDN user settings the LANCOM 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.