Integrating VoIP and Fax
Fax and the Internet
With all the attention being paid to VoIP and talking over the Internet, many forget about an important service which has also traditionally relied on phone lines, namely fax. Though the importance of fax has gone down recently due to the proliferation of scanners and email, there are still millions of fax users worldwide and very often the only way a person can receive a copy of a document is via fax and a telephone connection. So those ditching the PSTN system in favor of VoIP need to understand how fax works with it.
Unfortunately, fax doesn't work very well with a default VoIP set up. There's nothing preventing someone from hooking up a fax machine to the VoIP system since it behaves just like a regular phone system. But the results won't be pretty. The primary reason for this is that VoIP digitizes and encodes the analog signals of a regular phone line using whichever codec is most appropriate and these manipulations are targeted towards voice data alone. When we try and do the same thing to fax data, it doesn't work out so well. Because of this, we need to tweak the VoIP set up in order to make it work with fax systems.
Making Fax work with VoIP
In order to ensure that faxes aren't broken up by VoIP systems, the international body that's responsible for these things has created the T.38 standard. This is a special protocol which specifies all the "fax specific" things which need to be done to make sure that outgoing and incoming faxes are not garbled.
This protocol is implemented in the ATA routers to which you connect your regular telephone lines and are responsible for the conversion of digital into analog and vice versa. Of course, you'll also need a VoIP gateway which supports the same protocol to complete the set up. Conversely, you can do away with a fax machine entirely and set up your server with fax software which makes it behave just like a fax machine to send and receive images.
Finally, we have separate fax machines which are specifically designed to work with "Fax Over IP" or FoIP as it is called. Using these specialized machines, fax over the Internet is a snap. Just hook it up like a regular telephone system and you should be ready to go. One of the measures taken to maintain the integrity of FoIP is that each packet contains some data from the previous packet. This means that more than one packet has to be lost in order to lose the entirety of the contents. It's just one of the ways the T.38 protocol uses to ensure that the fax arrives in decent shape at the other end.