If you're planning to move to IP telephony, remember that voice won't be the only thing running over your phone lines.
Voice over IP may be getting more attention right now, but fax over IP could be a quick, simple way to enter the VoIP network realm and save money in the process.
How extensively does your enterprise use fax among internal points? In some industries, such as health care, this can be a surprising amount of your traffic. I have seen 30 to 40 percent fax traffic among the offices of a multi-centre health system.
When you look at your fax usage, also consider overnight delivery and courier usage and whether any can be replaced by quality fax. Remember that some documents, such as legal forms, must still be handled in physical form.
Analogue fax traffic is more delay-tolerant than analogue voice and can be a perfect candidate to start your VoIP deployment. Fax machines look like telephones to the public switched telephone network (PSTN) and plug into telephone-oriented interfaces on routers or private branch exchanges (PBX).
Look carefully at the applications or fax machines that generate the traffic, and you may soon find that a good many transmissions could be deferred until your links are largely idle. Effective management of non-urgent fax can even reduce the need for costly telephone links.
Stay conventional?For occasional fax use, it may be reasonable to use either traditional fax machines or PCs equipped with individual phone lines that talk directly either to physical fax machines or to modems and fax software. Most commonly, this means you will receive faxes as T.37-defined e-mail attachments that you can print.
And for fax output, you can simply treat the fax capability as a printer in the menu of printers you can select. Receiving does require transmission to your phone number.
Rather than use an analogue PSTN line, you may send the signal over VoIP lines that use a high-bit-rate codec compatible with fax. Current analogue fax machines are Group 3 (G3), and use the V.34bis protocol for transmission. These fax machines can send at 33kbit/s, but can fall back to lower speeds on bad lines. The fallback speeds are still faster than the fallback speeds of earlier generations.
General fax standardsThe ITU T.30 standard involves the negotiation of capabilities between real or virtual fax machines, while ITU T.4 specifies the actual scanning and reproduction of fax images, including things such as paper size, and is agnostic about being carried over analogue or digital packet facilities. These protocols have become substantially more complex since they were first introduced for analogue transmission, and their timing needs are responsible for much of the quality of service (QoS) requirements for FoIP.
While FoIP is less sensitive to message delay than is VoIP, fax protocols do have more sensitivity to network impairments than does, say, browsing Web documents. As with VoIP calls, both the network and processing add delay. Absolute delay, in the absence of keep-alive mechanisms, can cause the fax session to drop. While FoIP is still sensitive to variable delay (jitter), it is less sensitive to it than is VoIP, because packets can be time-stamped and put into the correct sequence. Were VoIP packets to be reordered, the voice call sound like Yoda would.
FoIP is more sensitive to packet errors or loss than is VoIP, and FoIP must contain error correction. Information can be repeated in frames, or an error-correcting but delay-adding protocol such as TCP can be used. If one end of the fax session is traditional, there's no way to get true delivery confirmation unless you can complete the message transfer within the limits of the T.30 timers on the sending machine and get the T.30 protocol confirmation.
True FoIP standardsThe newer methods encapsulate the T.30/T.4 fax in Simple Mail Transfer Protocol (SMTP), as a Tag Image File Format (TIFF) attachment. SMTP fax can be sent in two ways. Neither real-time or store-and-forward is better; each has advantages and disadvantages in specific applications.
Basic concepts• Store and Forward -- This means the faxes are sent whenever an SMTP server delivers them as e-mail messages to the fax-capable router. Real-time means that they are sent one at a time, if and only if there is an active connection between both ends, so that a delivery confirmation can be sent.
T.37 is the ITU store-and-forward standard for sending faxes in e-mail. Store-and-forward fax breaks the fax process into distinct sending and receiving processes and allows fax messages to be stored between those processes. Store-and-forward fax also enables fax transmissions to be received from or delivered to computers rather than fax machines. Store-and-forward can exploit all the flexibility of e-mail, such as sending to distribution lists.
Store-and-forward lets you set up mail servers to try continuously, so the sending user never sees a busy signal. You can use all the features implicit in e-mail, such as distribution lists that send traffic to multiple fax machines. Synchronisation would be very difficult for real-time among multiple destinations. If transmission is delayed until idle time, and the enterprise is already connected to the Internet, long-distance charges for fax disappear.
Almost any Internet connection will be faster than 14.4kbit/s or 34kbit/s fax. The biggest problem is that store-and-forward doesn't implicitly link transmission and reception, so there could be a problem if receipt confirmation is needed in real time.
Receiving fax as e-mail is especially attractive to remote workers. A travelling worker with a virtual private network (VPN) connection also can send through the company's fax servers.
• On-Ramp -- Store-and-forward uses an on-ramp gateway to get onto the Internet, sending TIFF images as e-mail attachments. Obviously, there must be a SMTP server between the PC that creates the TIFF file and the fax gateway router. There are physical fax machines than handle documents in the traditional way, but connect via the T.37 store-and-forward technique rather than analogue. Such machines may also have a backup G3 interface, or you can use a commercial service that accepts TIFF e-mail and sends it to G3 analogue machines.
• Off-Ramp -- Still within T.37, an off-ramp gateway receives TIFF digital fax from the Internet and delivers it to local analogue fax functions in fax machines or PCs with fax software. Again, there are commercial services that perform this function. As with on-ramps, this gateway may also have analogue capability.
• T.38: Real-Time Fax in Packets? -- Conventional fax machines that transmit using the T.30 protocol learn from T.30 messages if the transmission is considered complete by the receiver. Knowing that the message has been received can be important when, for example, you are negotiating a contract with marked-up documents. Note that a T.30 fax should give an error message if it has a paper jam or another non-protocol problem, but this doesn't always happen.
T.38 gets around the delivery-confirmation problem but gives up some of the flexibility of store-and-forward. A typical system takes traditional T.30 fax from the PSTN, routes it over an IP network, connects in real-time to a T.38 gateway and then converts it back to a T.30 fax session at or near the destination fax machine.
T.38 can experience the equivalent of a busy signal if an Internet connection cannot be made. Some T.38 gateways also have backup G3 analogue interfaces.
Applications• Point-to-Point -- Quite a few enterprises have dedicated pairs of fax machines, such as between the sales department and the warehouse. You can use simple FXS interfaces on your routers, riding on existing data links between the locations. Just use Private Line Automatic Ringdown if the fax machines support simple off-hook, or program the appropriate number into the router.
• Reducing the number of Phone Lines -- You can make your VoIP network fax-aware, such that the voice gateways detect analogue fax tone and take appropriate action. This enables you to reduce the cost of analogue phone lines, since the same line can detect if the incoming call voice or fax. Unfortunately, this often runs into multivendor interoperability problems.
• Headquarters and Branches -- Using store-and-forward, a headquarters can send a single e-mail to multiple destinations.