Once a connection is negotiated, SIP gets out of the way, which makes it scalable. Servers can be used to locate and connect all the endpoints involved in a session, including those that support IM. SIP-enabled voicemail and IVR systems can be added to the VoIP PBX function easily, as long as there's IP access.

SIP is also ideally suited for presence applications and can integrate with Web applications, easing CTI (computer-telephony integration) and opening up more options for features like "click-to-talk."

Development and support are simplified by having one protocol and architecture. Any application that requires instantaneous communication is probably already supported by SIP or will be. The beauty of this is that one simple infrastructure, consisting of a SIP Proxy Server and a SIP Registrar Server, can direct requests and track location information. Additional applications can be added with little disruption.

Sure, it's possible to accomplish all this using proprietary systems. However, that route is not necessarily easy and may lock you into a single vendor. Not surprisingly, some vendors with big investments in proprietary alternatives argue that SIP is still a work in progress. But after testing VoIP phones, we say SIP brings to the table solid functionality and interoperability. (For the nuts and bolts of how SIP works, see "It's Time To Take a Look at SIP").

All Aboard

Lots of companies are cooking up products with SIP as a main ingredient. These include such big names as Alcatel, IBM, Microsoft, Novell, Polycom, Siemens, Sun and 3Com, and smaller players like BroadSoft, ipDialog, Mitel Networks, Snom Technology and Zultys Technologies. SIP-enabled VoIP is in the forefront, but messaging and presence, while not quite as far along as VoIP, appear on a lot of vendors' near-term road maps.

For example, IBM has made a strategic decision to incorporate SIP functionality into all future versions of its corporate messaging product, IBM Lotus Instant Messaging and Web Conferencing (formerly known as Sametime and an Editor's Choice award winner; see "IM Grows Up"). The product's messaging gateway now supports the SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions) protocol. This was an obvious choice for a gateway that can link messaging systems from different vendors until they all start incorporating SIP functionality into their client and server products. SIMPLE could do for messaging what SMTP does for disparate e-mail systems.

IBM further plans to incorporate SIP functionality--including presence-based features that make it easy to send availability information with documents--in all its client and server software. Presence, which IBM implements in a nonstandard environment, makes it possible to communicate the availability of an individual via office phone, mobile phone or IM and the preferred method of contact at any given time. An obvious place to implement and manage presence is with groupware, where it can be integrated with schedules. This is why IBM plans to incorporate it into Lotus Notes.

For its part, Microsoft has for a number of years incorporated SIP support in its MSN messenger clients, including its most recent version, MSN Messenger 6.0. Microsoft is also using SIP for its XP Messenger client, which comes installed on every XP desktop. Clearly, like IBM, Microsoft has made a strategic decision to base future voice and video communications on SIP, turning its back on the H.323 standard that powered the NetMeeting product and provided similar functionality.

Indeed, while H.323 has been around longer, you will be hard-pressed to find new development for H.323 products. Most vendors interested in standards-based communications have shifted their efforts toward SIP.

Why? SIP is more extensible, simpler to implement and easier to troubleshoot thanks to its simplicity and text-based messages. The SIP protocol requires fewer packets to be exchanged to set up a call, which means less network traffic, less work for the servers involved and thus greater scalability. SIP is also more flexible than H.323. There's a whole industry developing based upon SIP products. Take a look at www.sipcenter.com for a sampling of some of the equipment and application vendors. A standard like this provides a much larger market for developers than proprietary products can provide, which in turn increases competition and innovation.

Many feel that H.323 is still superior for managing video services. But this hasn't stopped companies like Wave Three, VCON Visual Communications and others from providing SIP-based video products. And some IP vendors with legitimate interests in standards-based connectivity adapted H.323 for their phones before SIP was a viable standard.

There's further evidence of SIP's knockout power on the server side: Microsoft's new RTC (Real Time Communications) server will use SIP for all of its media-related communications. The first application that Microsoft will introduce on this platform will be a corporate messaging product. The company also plans to provide RTC as a development platform. Siemens, for example, will incorporate RTC in its future products to enable it to standardize its messaging and presence applications.

The bottom line, though, is that the industry has declared SIP the winner.