August 2000

BY JIM MACHI

To answer the first question, let's consider the perspective from a U.S. office. I remember just a few years ago sending and receiving several faxes a day. Today, I still send and receive faxes -- but now it's only a couple of times a week. Why even that many faxes, you ask? Because I still send faxes to submit my forms for speaking engagements at trade shows. Companies like TMC, which sponsors the Internet Telephony Conference and Expo, want my signature to show I'm committed. I also sometimes fax documents to other company offices with my notes scribbled on them, since it's often faster than editing an online document. It's also more convenient to edit a hard copy when I'm on a plane. Plus, I frequently receive faxes from my company's offices and customers in the Pacific Rim.

Though e-mail is my primary means of communication, the fact that I'm still faxing tells me something: Faxing is important overall and is important for a full IP telephony solution. In this context, we're talking about real-time fax service that fulfills users' expectations of walking up to a fax machine, sending a fax, and having a piece of paper simultaneously come out of a receiving machine at the destination phone number. The bottom line? Next-gen service providers and IP centric enterprise communication systems need to ensure that this type of fax capability is part of their infrastructure solution.

FAX STANDARDS
Now the question is how to go about it. One major way is standards. The International Telecommunications Union (ITU) has developed two different standards addressing fax over IP, ratifying them in June 1998. These standards began showing up in commercial systems in the second half of 1999. The first standard, T.37, is used mainly for store-and-forward. It merely defines the format in which fax is to be delivered as an e-mail attachment. The second standard, T.38, defines the protocol for real-time delivery of fax over IP (FoIP).

With T.37, the fax is sent over IP as an e-mail attachment and delivered to the destination over the public switched telephone network (PSTN) by the gateway closest to the destination. While T.37 allows for cost savings through toll arbitrage, from a user's perspective it is a store-and-forward model and is not real time. You can't immediately receive confirmation that the fax was successfully delivered. Although a sender might claim a fax "has been sent," the T.37 model doesn't let you automatically assume a confirmation of delivery has been sent. If an en-route e-mail server happened to be down (for example, because of the recent "I Love You" virus), the confirmation wouldn't go through.

To satisfy the need for real-time FoIP, it's possible to use the G.711 coder to transmit the fax. The problem here is very high bandwidth usage. Unlike voice transmission -- where the penalty is just human discomfort -- jitter, packet loss, and latency can all cause a fax machine to immediately terminate a call. The world is still learning traffic engineering for multi-media transport over IP. In some locations, the bandwidth is extremely limited -- making network problems virtually impossible to avoid.

Any real-time protocol over IP that bridges the call path must meet the requirements of the T.30 protocol (the protocol for standard fax calls over the PSTN). The T.30 protocol has very stringent requirements -- for example, signals are exchanged every 75 ms. To solve this problem, ITU-T Study Group 8 developed the T.38 protocol. Transporting a fax using T.38 takes only a half-duplex channel and 14.4 Kbps plus packet overhead. Transporting a fax using the G.711 channel takes a full duplex 64 Kbps plus packet overhead.

T.38 is modeled as a smart T.30 interpreter. It executes extensive training, signaling, and data exchange with T.30 to determine the line quality on a PSTN network. This is meaningless with a packet network, since the IP packets can take any available route. Gateways at each end execute full T.30 for communication with fax machines. However, all the data is not transferred over IP. While the fax machine sends the entire CNG/CED-type tones for signaling, the gateways using T.38 only exchange octets that indicate whether they've succeeded or failed at detecting tones. Essentially, the two gateways only exchange simple results such as confirming success or failure, and only transmit the pages intended for delivery.

ALIVE AND KICKING
The most recent Study Group 8 meeting was held in Gaithersberg, MD from June 11-15, 2000. The focus was on developing V.34 extensions to the protocol to allow 33.6 Kbps fax transmission over IP. More work was done in alternative call control models such as SIP and H.248, as well as on standardizing techniques for switching from voice to fax and vice versa for both H.323 and H.248.

Is fax still alive and kicking? Yes. And it will continue to be important for IP telephony. As standards evolve the picture will continue to grow clearer. 

Jim Machi is director, product management, CT Server and IPT Products, for Dialogic Corporation (an Intel company). Dialogic is a leading manufacturer of high-performance, standards-based computer telephony components. Dialogic products are used in fax, data, voice recognition, speech synthesis, and call center management CT applications. 

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