In FoIP, there are two primary transmission methods:
- In the store-and-forward approach, fax information is transferred from fax server (also called a gateway) to fax server as e-mail attachments. This uses a lower-level Internet protocol (like SMTP). If you've been sending faxes from your PC for the last five years, you've probably been using the store-and-forward approach. It's kind of like sending an e-mail who's destination is a fax machine instead of someone's inbox. The drawback here is that the fax machines are not exchanging information in real-time, so it doesn't feel like a traditional fax session. The machines can't discuss their capabilities (if the sender is a color fax machine and the receiver is black and white, somebody is going to be disappointed), and the sender does not receive instant confirmation that each page has been received.
- In real-time IP faxing, fax information is transferred from fax server to fax server as IP data packets using a high-level Internet Protocol such as TCP or UDP. These protocols allow for real-time connections that let the fax machines exchange information along each step of the way (see How OSI Works to learn about protocol layers). A real-time IP fax feels just like a traditional phone-line fax.
The big thing in FoIP right now is the real-time approach. There are a lot of possible configurations for a real-time IP faxing system. A few of the more common ones include:
- G3 (traditional) fax machine to G3 fax machine
- Fax-equipped PC to G3 fax machine
- IP fax machine to G3 fax machine
- IP fax machine to IP fax machine
No matter which method you use, IP addresses are involved at least somewhere along the transmission path. An IP address is the identifying code of any device connected to the Internet. If you're transmitting a fax between two IP fax machines, the phone number you're sending the fax to is just an alias. The phone number is immediately converted to the corresponding IP address for the receiving machine. If you're sending a fax from an IP fax machine to a G3 fax machine, the IP fax machine uses the destination phone number to generate the IP address of the fax server located closest to the receiving machine. Proximity matters -- you want a local fax server so there are no expensive long-distance lines involved in the transmission of the fax from the server to the receiving machine.
Here's a more detailed look at a simplified, real-time FoIP session between an IP fax machine (as the sender) and a G3 fax machine (as the receiver):
On the surface, this is all there is to an IP fax transmission. But behind the scenes, there's a lot going on to make the conversion between phone-line faxing, which is based on a protocol called T.30, and real-time IP faxing, which uses a protocol called T.38. In the next section, we'll take a closer look at the process.