SIP emerges as lightweight IP call setup vehicle

Emerging IP-based network operators who want to offer converged services are starting to demand something more of their voice-gateway vendors than simply an emulation of traditional telephone network features.

They want that emulation to be simple and not add a lot of overhead to IP voice packets.

That's why a group of vendors is beginning to introduce products to the carrier and to the enterprise markets based on a new, lightweight packet-voice call setup protocol called Session Initiation Protocol (SIP).

The protocol, for which the Internet Engineering Task Force has established a working group, is largely a challenge to the International Telecommunication Union's H.323 standard, which has developed for the past several years as a way for two gateways or endpoints to negotiate a multimedia packet session including voice.

The problem, say SIP proponents, is that H.323 was designed at a time when desktop videoconferencing and whiteboarding applications, rather than simply voice over packet, was considered the possible killer application.

As a result, H.323, which is a derivative of ISDN, requires many messages to travel between two different endpoints before a communications link is established, often resulting in a several-second call setup delay and sometimes ongoing latency. By contrast, SIP, built on the HTTP Web-server protocol, requires the exchange of half a dozen or fewer messages before the communications link is established, dramatically reducing call set-up time.

"It's lightweight and easy to implement," says Scott Hoffpauir, chief technology officer at BroadSoft, a provider of service-creation software for next-generation carriers. By contrast, H.323 "really tries to reuse a lot of existing protocols by adding in multiparty videoconferencing," Hoffpauir says. "It ended up being a very heavy, complex protocol."

As a result: Vendors and carriers have had a difficult time getting their H.323-based products to interoperate, Hoffpauir says. That's created a situation where users potentially have to hunt among ISPs and regional carriers to find ones that happen to use the same IP telephony gateways.

A major target of SIP products: the nascent IP Centrex market. Traditional circuit-switched Centrex from Bell companies and other incumbent local carriers takes the burden of managing voice customer premises equipment off the user. But in return it forces them to learn a series of nonintuitive interactions with service providers on their telephone keypads to do such routine tasks as moves, adds and changes - if not requiring actually multiple phone calls just to make such routine changes.

As conceived by vendors such as BroadSoft with its BroadWorks suite, IP Centrex is designed to let network professionals at user organizations enter such commands through Web-browser interfaces.

But SIP is also starting to be deployed in enterprise products, such as IP-based PBX replacements. For example, the No. 3 U.S. PBX vendor, Siemens, is implementing SIP in several components of its HiPath 5000 IP telephony platform. Siemens officials say their tests have shown SIP cuts down call set-up time to imperceptible.

More information about SIP is available at www.sipforum.org.