Waiting for the first SIP

Voice over IP was once a hot prospect, conjuring visions of practically free telephone calls via the Internet to anywhere in the world, but the buzz eventually died down. Now, Internet telephony is bidding for a comeback, though this time it's being touted for much more than just cheap phone calls.

The hook, vendors say, is a new standard called Session Initiation Protocol (SIP).

Unlike other standards used for voice over IP, SIP is similar to familiar

Internet-specific protocols such as http and can be used to write Web-based programs just as easily as http is used now. In effect, it makes voice just another Web application that can be easily integrated into any Web service and managed more efficiently by information technology staff with a general set of skills.

Call someone using a SIP phone from your desktop, for example, and the general-purpose IP data network can redirect that call to another phone just as easily as users are directed to other Web pages through a browser. It also makes it much easier to mix and match voice, video and data for specialized applications.

"Unlike with the old central office concept of telephony, SIP means we'll be able to have distributed voice-based applications on the network," said Joe Aibinder, a voice-over-IP director for AT&T. "With IP-delivered services,

we can put them anywhere we want, and it also allows [customers] to design their own services that are much more closely matched to their needs."

In hindsight, experts say, using voice over IP to simply bypass long-distance tolls and therefore save money was a limiting use of the technology, particularly as competition in the traditional circuit-switched phone market drove down tariffs anyway. Using IP packet switching to carry voice traffic was suddenly not that cost-effective anymore, which in turn made the poor sound quality of early Internet telephony even less tolerable than when it was a bargain.

The real use for voice over IP now lies in defining new services, according to Doug Montgomery, manager of the Internetworking Technologies Group at the National Institute of Standards and Technology (NIST).

"And that's where SIP comes in," he said. "When I place a SIP call to someone who is already on the line, I don't get a busy signal, but instead I'm transferred to a Web page that I can navigate to find someone else to talk to and where I can find information about possible alternatives."

SIP also allows programmable call processing, so people can set preferences for how they want their calls to be answered. "We're doing research on those kinds of things now at NIST," Montgomery said.

SIP's origins can be traced as far back as 1996 to the Mbone, or multicast backbone, that was layered on top of the Internet to act as a multimedia delivery network. Mbone included a way for users to listen in on multimedia sessions on the Internet. That mechanism existed more or less unnoticed until the end of 1997, when voice over IP first came to prominent attention and the need for something that could initiate and terminate real-time sessions on the Internet was recognized.

SIP enables users to be found no matter where they are on the Internet at any particular moment, whether at desktop PCs in their offices, at home or on IP phones. SIP then delivers a description of the session that someone wants that user to participate in, conveying the information through a Multipurpose Internet Mail Extension, or MIME, message. The recipient then replies "yes" or "no" to the session invitation, and that answer is relayed to the originator. If necessary, SIP can then be used to end the session. It can also be used to modify the session by sending a new description.

Industry leaders have steadily adopted SIP since the technology received the blessing of the Internet Engineering Task Force early in 1999, though so far that hasn't translated into much SIP-based traffic passing through networks. AT&T's Aibinder believes that less than 1 percent of all of the current voice traffic is represented by voice over IP, "and less than 1 percent of that is SIP."

Gathering Steam

Still, most people believe it's only a matter of time until SIP makes an impact. For one thing, SIP is already in use by the wireless industry to enable the interaction between wireless devices and the people sending data, pointed out Chris Archey, product manager for F5 Networks Inc., a provider of Internet traffic and content management solutions.

SIP will be a vital component of next-generation — so-called 3G and 4G — wireless networks, which will use the IP infrastructure to send voice, data and video to devices.

"With IP applications, it's critically important that you can [regularly] go back to the server you started the connection from, because if you don't, the connection will fail," Archey said. "SIP will allow you to do that."

One reason why SIP is more important than other protocols that could be used for voice over IP is that both carriers and enterprise customers have adopted it, said Joan Vandermate, a vice president for Siemens Enterprise Networks. That's the first time both sides of the industry have supported the same protocol to do the same job.

The other critical move — something that Vandermate says makes SIP "the first [communications] protocol that is truly multimedia" — is its adoption by Microsoft Corp.

Microsoft added support for SIP to both its Windows XP and Windows.Net operating systems, something that enables users of the company's products to set up multimedia sessions over IP networks using Microsoft's Real-Time Communications family of services.

With more than 17 million copies of XP installed in homes and offices since its release last fall, there is already a large base of SIP clients, one that will only grow. Windows XP also includes a SIP application program interface that developers can use to write applications.

However, despite all of this and the speed at which SIP seems to be catching on for IP telephony, it's too early to say if it will become the only protocol used. Other standards, such as the International Telecommunication Union's H.323, have been around for much longer than SIP, and networking products based on them already have a large user base.

For that reason, said Tim McCracken, product manager for Cisco Systems Inc.'s voice technology group, SIP will have to coexist with these standards for a considerable amount of time. In fact, he said, Cisco is betting its business on the fact that hybrid networks, in which SIP and other protocols may actually have to interoperate, will be the rule for the foreseeable future.

"We are working on developing these hybrid networks, so that customers can use either one protocol or the other, or can use them at the same time, without