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July 24, 2009

The Challenge of SIP Interoperability in IP Fax

By Patrick Barnard, Group Managing Editor, TMCnet

What is the importance of session initiation protocol, or SIP, in IP fax systems?

It is essential in order for IP fax systems to work properly. SIP is a signaling protocol (well, actually it is a set of protocols) used for setting up and tearing down multimedia sessions on an IP communications network. Basically, SIP is what enables different IP fax systems from different manufacturers to establish a connection and “talk” to each other. The other important protocol used in IP fax is T.38. This protocol is what facilitates the actual transmission of the data across the network in real time.

But just because two IP fax systems utilize SIP – which, by the way, is a well-established IETF standard – doesn’t necessarily mean they can interoperate. That’s because, going back to what I said earlier, SIP is actually a set, or “stack” or protocols that work together to facilitate the set-up and tear-down of IP sessions. And when the protocol stack is developed for a specific SIP-based solution, the developers are not necessarily all on board with each other, in terms of striving for interoperability between systems.

According to Alan D. Percy of AudioCodes (News - Alert), one factor that makes SIP interoperability difficult to achieve is the way the IETF Requests for Comments (RFCs) are developed. As opposed to the traditional ITU specifications that have been around for decades, “IETF RFCs and Drafts are developed in an open and communal environment, using committees and consensus to craft the specification,” Percy writes in his blog. “This has very many positive benefits, but also a few predictable negative side effects. The problem is that the RFC that defines SIP has become ‘everything to everyone’ and bloated in both size and in flexibility.”

As Percy explains, because the language in the latest version of the specification RFC 3261 from the IETF Network Working Group is intentionally filled with “weak” terms, such as “may,” “can,” and “option,” the standard itself comes under a wide array of interpretations from developers. This results in a “very loose specification that allows the developers of SIP-based systems to make plenty of decisions on features of functions. The byproduct of this is that two systems can be completely RFC 3261 compliant and completely incompatible.”

For example, he points out that, under the current standard, “there are no fewer than five ‘correct’ ways to transport DTMF tones from one end point to another.”

As Percy points out, however, this is “only the tip of the iceberg.” In additional to the technical challenges of SIP interoperability in IP fax there is also the “politics” of which developers, and which vendors, get to establish the “true” standard that is used.

“It appears to me that soon after the authors of RFC 3261 finished their work, the fun really started,” Percy writes on his blog. “As the development teams of the various product and application companies started to build their solutions based on RFC 3261, the looseness of the specification allowed them to make wildly different choices all ‘within specification.’ The result was that you had developers that had invested untold hours of hard work into developing a protocol stack that worked fine in their own lab and with their own products, but had serious interoperability issues with other vendors. To each of the developers, it appeared that ‘everybody else screwed up.’”

Obviously, the developers that poured tons of hard work into their products didn’t want to go back and rework their SIP stacks. So the situation has led to what Percy calls “a classic stand-off, with each of the vendors saying ‘we followed the spec, you should change.’”

Fortunately, multiple vendors, including IP fax solutions provider Sagem (News - Alert)-Interstar, are leading the charge to develop true interoperability between all IP fax systems.

Sagem-Interstar recently announced that it is now a participating member of the SIP Forum FoIP Working Task Group. Currently the group is working on fine-tuning the T.38 specification so that true “out-of the box” interoperability among IP fax solutions can be achieved.

The SIP Forum (News - Alert) is an industry association with members from leading IP companies. Its mission is to advance the adoption of SIP, which work in tandem with the T.38 protocol to enable IP fax transmission. As with SIP, different interpretations of the T.38 specification have led to various interoperability challenges among vendors.
Currently, Sagem-Interstar (News - Alert) is actively contributing in defining the group’s Problem Statement document, which outlines specific interoperability challenges. In addition, the company has committed to participating in future focus groups to find resolutions to previously defined problems.

Patrick Barnard is a contributing writer for TMCnet. To read more of Patrick’s articles, please visit his columnist page.

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