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Feature Article
February 2004


Addressing The Challenges Of Real-Time Fax Over IP

BY JAMES P. RAFFERTY

During the past twenty years, businesses of all sizes have come to rely on fax communications as a sure-fire way to connect with people and businesses throughout the world. There is a myth that fax machines and fax servers are in a steep decline, but this simply does not square with the facts that have resulted in the installation of some 100 million fax machines worldwide, plus a healthy population of fax servers that have been very popular within enterprises. In practice, fax technology and standards have continued to evolve to address expanding business needs for document communications of all kinds. For example, in recent years, many enterprises have migrated to use V.34 facsimile, which offers page transfer rates that are often twice as fast as those supported by the older V.17 fax modems.

One of the hottest trends for today�s enterprises is the move toward IP telephony. IP capable PBX sales have reached the tipping point in 2003 and now exceed the sales of PBX models that do not support IP networks. However, with new technology, there are still some familiar challenges. Today�s business still typically has numerous fax machines and fax servers that were designed to operate over analog or digital phone lines. As a business migrates its voice support to run over an in-house IP network, it is important for IT managers to devise a game plan to provide the same level and quality of fax service over IP as was previously provided over the traditional PSTN.

In the late 1990�s, the fax experts in the ITU and e-mail experts in the IETF saw that communications over IP was an important emerging trend and the two groups agreed to work together to devise standards for Fax over IP. As a result, in 1998, two approaches for Fax over IP were standardized. The IETF focused on extending e-mail in order to be able to support fax effectively and produced a series of related RFCs 2301-2306, which collectively defined a method sending fax images via e-mail. The ITU endorsed this effort and produced a related standard called T.37, which directly referenced the IETF standards. In parallel, the ITU-T developed a standard approach for real-time facsimile communication over IP networks, which resulted in the T.38 standard. As the migration to VoIP has become widespread, T.38 has emerged as the key standard for ensuring that VoIP capable networks can continue to support user requirements for real-time fax communications.

Let�s consider the challenges that T.38 fax over IP has been designed to address. Virtually every fax device in the world supports the T.30 standard for Group 3 facsimile, which was devised by the ITU in 1980 and has continued to be extended in the years since. In 1980, the telephone network was dominated by analog technology, so that T.30 was designed to work effectively over even very poor quality links. Fax negotiations in traditional T.30 operate at only 300 bits per second and have built-in integrity and retry features to help the exchange of fax signals complete successfully. Once the two devices have agreed on a common set of parameters for the fax modem links and the fax page, they then �train� on higher-speed modems such as V.17 and the fax page transfer will happen after that at data rates up to 14.4 kilobits per second. This approach has proven to be very robust over a variety of phone network conditions and means that two T.30 compliant fax devices can connect and communicate over phone links that may be analog, digital, fiber, or even satellite.
 
However, one of the challenges we have seen is that many of the installed fax machines only loosely follow the T.30 standard. In particular, some of the older fax devices may have numerous bugs in the way that they respond to T.30 fax signals, especially during negotiations and training. As a result, the most experienced makers of fax equipment such as fax boards have fine-tuned their T.30 implementations to be somewhat tolerant of errors that may occur during the negotiations and training phases of T.30. These more forgiving implementations of T.30 are thus able to connect with a much higher percentage of the worldwide fax installed base than devices that are less tolerant of protocol or network problems. Over time, fax implementers have developed a collection of techniques for �keeping alive� fax sessions in the face of such problems. These techniques are often referred to as Fax Spoofing.

When developing the T.38 standard, the expert group�s focus was being able to reproduce the real-time facsimile experience for end users while running over packet networks rather than the PSTN. In particular, facsimile users like to be able to watch their documents transfer in real time and then get a confirmation of a successful transmission immediately at the end of the session. After considerable discussion, the group decided that the new real-time protocol for Fax over IP should be able to transparently support all of the same document communication capabilities as T.30 and that there should be minimal elongation of the call as compared to traditional fax. As a result, the new T.38 protocol utilizes the same approach as Group 3 fax in its representation of document characteristics, but is designed to run over standard Internet Protocol transport layers such as UDP/IP and TCP/IP. Since the T.38 protocol encapsulates T.30 messages, this enables gateway developers to quickly transform T.30 signals and data into a functionally equivalent packetized form for transport over the IP network. The T.38 packet sizes for negotiation signals are very small and can be protected using a redundancy method in order to guard against packet loss.

Operation of fax over packet networks offers some different challenges than traditional fax. As a general rule of thumb, if a packet network is sufficiently robust to be able to support VoIP effectively, a T.38 implementation should also work reasonably well. For example, reasonable quality for VoIP typically requires a total round-trip time of about 250-300 milliseconds or less for each packet. The T.30 fax protocol has timers that are measured in seconds rather than milliseconds. For example, fax negotiation signals will be re-sent if no response is received within three seconds. Therefore, even if there are occasional network delays of up to one second, this still gives T.30 and T.38 devices sufficient time to communicate without violating timeouts and causing the session to go down. Consecutive packet loss is more likely to cause issues than delay, especially during the fax negotiations phase, since a T.30 fax session will disconnect if three consecutive signals are missing or unreadable.

The deployment cycle of T.38 has been somewhat delayed by a few factors. First, there was a need to complete a related set of standards that would control the session over the IP network. Initially this was done using H.323 and later on, support was added by the ITU for SIP and Megaco. H.323 deployment for T.38 is common, but support for the use of SIP with T.38 is still very new. In addition, the need for T.38 has been driven by the deployment cycle of VoIP in the enterprise, which slowed greatly during the telecom downturn, but has recently begun to pick up. Third, the initial implementations of T.38 were designed as gateway implementations and many of the companies were relatively inexperienced in communicating with Group 3 fax devices. Fourth, since T.38 is a new technology, there is a need for the suppliers of T.38 gateway technology to work together to ensure that their products interoperate. Finally, we are just beginning to see the first uses of native T.38 fax devices (also called Internet Aware Fax devices) that can connect directly to an IP network without need for a gateway.

As IT managers consider making the move to adopt T.38 fax, the following factors should be taken into account when evaluating such a migration.

� Does the supplier of T.38 technology have experience in connecting successfully to a high percentage of the existing Group 3 fax installed base?

� Does the T.38 implementation interoperate properly with other T.38 implementations?

� Will you be able to minimize changes to existing software infrastructure and applications when moving from traditional to IP fax servers?

� Will the solution provide ways to eventually eliminate the need for all current analog or digital fax lines in the targeted offices?

� Does the T.38 implementation work effectively for the use of fax in your enterprise environment, including ability to connect with remote offices, as well as outside firms such as customers, vendors, law firms, and other outside services?

� Will T.38 be supportable on the existing internal LAN or will upgrades be required to provide sufficient bandwidth for co-existence of voice, fax, and data applications on the same network?

� Will the IP fax solution support T.38 in a native manner or require the use of T.38 compatible gateways?

T.38 shows great promise as an effective solution for bringing facsimile fully into the age of converged networks. Like Group 3 fax itself, a successful T.38 implementation will both follow the standards and be able to operate under less than perfect conditions. As noted, a network whose packet loss and delay characteristics can properly support VoIP should also prove sufficient for support of T.38 fax communications. As today�s fax solution providers add IP fax support and the quality of the underlying packet network also improves, the transition from traditional fax to Fax over IP should become much smoother. As the move to IP PBXs continues to accelerate, a wide variety of options for bringing T.38 IP fax into your organization without disrupting existing business processes should soon be available.

James Rafferty is senior product manager for Brooktrout Technology. Brooktrout Technology is a leading supplier of media processing, network interface, call control and signal processing products that enable the development of applications, systems and services for both the New Network (packet-based) and the traditional telephone (TDM) network. For more information, please visit www.brooktrout.com.

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