March 1998

BY WENDELL BISHOP AND MIKE EHLICH

Two years ago, the major issue in the fax industry was whether to print on plain paper or thermal paper. One year ago, the issue was determining if users wanted fax machines to double as PC printers. Today, much more dramatic changes are taking place in the communications industry — and fax is in the midst of these changes.

Circuit-switched networks, such as the traditional Public Switched Telephone Network (PSTN), are con-verging with packet-switched networks, such as the Internet and corporate intranets. This convergence is having a profound impact on all communications. Fax is inherently a digital form of communication that can reap great benefits by taking advantage of IP communications technologies. Costs can be greatly reduced by converting fax signals into packetized data and using corporate intranets or the public Internet as the carrier network instead of the PSTN.

IMPACT OF LEGACY FAX STANDARDS
The fact that there are billions of dollars in fax equipment already in place, with an additional $10 billion in new sales yearly, cannot be ignored. Legacy standards from fax will have a major impact on the new IP environment. Any IP fax solution must provide some path for transferring images between new and legacy equipment.

The vast majority of existing fax machines are designed to the Group 3 (G3) standard. G3 was adopted in 1980 by the CCITT, an international communication standards body which is now a subgroup of the International Telecommunications Union (ITU). G3 machines were priced aggressively and quickly made previous standards Groups 1 and 2 obsolete. Group 4 was adopted in 1984 to support digital ISDN connections, but never became widely used. The higher level of compression defined in Group 4, however, was later incorporated into G3 as the T.6 format. G3 defines modem protocols for data transport over audio channels, a protocol (T.30) for two devices to use in exchanging data, and formats (T.4, T.6) for interpreting the data as an encoded bit map image. Outside of the G3 standard, but common in workstation and fax server environments, is the TIFF-F (Tagged Image File Format, the "F" designates the class used for exchanging fax images). This format, while accepted as an industry standard, is only now being considered by standards bodies for official certification.

The T.30 protocol, which was defined at a time when memory was expensive, avoids image storage by sacrificing PSTN network bandwidth. As a result, CTI sending and receiving G3 machines must run in a "lock-step" fashion. This is difficult to maintain over certain existing IP links due to latency. Latency is the time delay between the start of a signal from an originating fax machine and the reception of that signal at the destination fax machine. Not only can an IP network have delays of several seconds, but there are also delays though the gateways. Errors also add to latency in an IP network since faulty packets must be retransmitted in order to guarantee error-free communication.

NEW STANDARDS FOR IP FAX
A working group of the Internet Engineering Task Force (IETF) is close to proposing a standard for sending fax over IP. The focus of this standard will be to support a new set of fax devices with IP only (no telephone) connectivity. This new standard will also be supported in gateways that will serve the installed base of G3 fax machines, allowing them to communicate across an IP digital net-work to either another G3 machine or a new IP fax device. The main components of the new IETF standard are:

• Using T.4/T.6 in TIFF-F for the image format.
• Adopting e-mail protocols for the transport.
• Defining how phone numbers are to be embedded in e-mail addresses to handle routing issues.
• Using return receipt notification to inform the originator of successful (or unsuccessful) delivery.
• Defining a session mode and a store-and- forward mode which indicate whether or not the originator is waiting for receipt notification.

Some IP fax devices have been deployed prior to the adoption of the IETF standard. These devices are based on the WIDE protocol which was developed by several Japanese fax terminal manufacturers. Under the WIDE protocol, fax images are moved over IP networks by store-and-forward servers and gateways. The IETF standard will be adopted early enough and with enough industry participation, however, that it will likely replace WIDE and other early work.

FAX OVER IP GATEWAYS
Gateways for fax over IP serve traditional G3 fax machines at one or both ends of the communication and fall into two main categories: G3-to-IP for real-time and G3-to-IP for store-and-forward. The distinction between real-time and store-and-forward is important. There are no universally-accepted definitions of these terms, so the following descriptions may not apply equally to all vendors.

Real-time Gateways
Users faxing over an IP network want the same experience as faxing over the PSTN, including the knowledge that when a fax session is completed the fax image has been received at the destination. Real-time gateways between G3 machines need to preserve the essential element of a PSTN fax session, namely, an indication of delivery success prior to the completion of the session. In the case of a G3 machine, the interaction is con-strained by the T.30 protocol under which an originating G3 machine will generally retain its grasp on the last physical page of a session until the destination G3 machine has finished receiving. In the case of a successful transfer, the last sent page is simply released; in the case of an unsuccessful transfer, an error report is generally printed or displayed by the originator.

G3-to-IP gateways for real-time permit G3 fax machines to communicate with other G3 fax machines over IP using the interlocking T.30 protocol. The fax communication is received by the gateway and demodulated into a digital image. The image is packetized and transferred over the IP network to a remote server near the destination. The image is then demodulated and transmitted to the destination machine. This is called "demod/remod."

While the delay or latency over the PSTN is negligible, the latency over an IP network can affect the ability to maintain T.30 protocol connections. If the latency exceeds the interlocked timing required, some gateways will terminate the fax sessions and report an error. To better deal with unpredictable IP latency, some real-time fax gateway implementations interact directly with the fax machine at the local end. This inserted protocol is called "spoofing." Spoofing slows down a fax machine to permit the other end to catch up and resynchronize the communication. While implementations that support spoofing are more complex than those that do not, there is no down side to spoofing. It is only introduced into the session when IP net-work latency would otherwise result in the failure of the fax session. A user might choose to set a limit to the amount of spoofing they allow, however, as the duration of the fax session will be extended by the spoofing.

The IETF-defined standard will not support real-time connections between two G3 machines over IP. The session mode defined in the IETF standard does not keep the two G3 machines in lock-step to the extent required by T.30. The TR 29.1 working group of the Telecommunications Industry Association (TIA) is working toward a standard to fill this gap. The work leverages some existing standards (X.5, X.38, X.39) for passing fax over Frame Relay networks. The work of this group will be submitted to the ITU for standards consideration in 1998.

Store-And-Forward Gateways
In a store-and-forward gateway, a complete transaction consists of two independent sessions — transmission to storage in the gateway and transmission to the destination device. The two are separate fax sessions and generally do not overlap in time. In cases where the two sessions do overlap, this is still a store-and-for-ward gateway, not a real-time gateway.

In a G3-to-IP gateway for store-and- forward faxing, the originating fax machine sends its document across the PSTN to the gateway as a normal G3 fax session. The image is typically stored as a file in TIFF-F image format by the gateway. The image is then transferred as a file or e-mail attachment though an IP network to a remote gateway near the destination. The remote gateway can initiate the delivery of the document to a destination G3 fax machine as a normal G3 fax session.

The advantage of store-and-forward is that latency is generally not an issue. Store-and-forward also provides confirmation of the remote delivery via an e-mail or fax return receipt from the destination gateway. In addition, store-and- forward will work with the new IETF standards, which should be integrated directly into future fax machines. These IETF-compatible machines will be capable of communicating directly with IP gateways, or other IETF-compatible machines on the network.

GATEWAY BUYERS BEWARE
Some real-time gateways incorporate a store-and-forward system as a fallback mechanism. G3 machines, however, have no procedure for indicating when a session has fallen back from real-time to store-and-forward. For example, a busy destination device may cause a call to revert to store-and-forward from the start. Or, a long network delay may cause a fallback to store-and-forward, even if both originator and destination sessions are allowed to complete normally. The failure of the destination session may also result in the image being sent to storage near the originator and delivered on a subsequent call. All of these actions would generally be transparent to the originator.

Without accepted standards, it is important to be very careful regarding product features and descriptions. Products may be called real-time but have no support for the interlocking T.30 protocol. They may only provide real-time sessions over private intranets with very low latency, typically less than 1.5 seconds. Some vendors may claim products are real-time under certain conditions, with provisions for fallback to store-and-forward. Often these conditions are not achievable and fallback occurs on every transmission. That this fallback may be transparent to the user means that under-standing the varying definitions of "real-time" on a vendor-to-vendor basis is important. When choosing a gateway, be sure that the features implied in the name are actually supported.

Even though the standards for IP fax are still evolving, some intranet and Internet fax products and services are available today. The economic drive is so compelling that many ISPs, developers, and users are not waiting for standards to be adopted.

Is it too early to jump on the fax over IP bandwagon? Not if a solution is built on open platforms with the flexibility to adapt to emerging standards. The time to enter this market is now. The risks are minimal and those developing IP fax products or services today will reap the benefits as this market expands.

Wendell Bishop is a co-founder of Natural MicroSystems and currently serves as chief scientist, with responsibilities including research for future products and project team management. Michael Ehrlich is a senior product manager with responsibility for fax, speech recognition, and text-to-speech products. Ehrlich also serves as treasurer on the board of directors of the Enterprise Computer Telephony Forum (ECTF). Natural MicroSystems Corp-oration is a leading provider of hard-ware and software technologies for developers of high-value telecommunications solutions. For more information, contact the company at 508-620-9300 or visit their Web site at www.nmss.com.