In an H.323 VoIP network, all call processing intelligence is contained within the PBX and H.323 telephones are little more than stimulus devices. An H.323 telephone informs the PBX when the user pressed a button. The PBX tells the telephone when to light a lamp and what to write to its display. The PBX tells the telephone to turn on and off the voice path. Other than mute which is a local function, an H.323 telephone is a basically a slave to the PBX.
This is very different from how a SIP telephone operates. A SIP telephone is an intelligent endpoint that does far more than simply watch for button presses or requests to write to the screen. In fact, for many of the telephony features we use on a day-to-day basis, a SIP telephone can function just fine without the assistance of a PBX. It’s possible to have two SIP telephones make, hold, conference, transfer, and release calls without anything but an IP network between them.
Of course, a PBX still adds a great deal of value to an enterprise’s communications environment and it will be a long time before we build a network of just SIP endpoints. For instance, until the SIP protocol defines call flows for the over 700 features that an Avaya Communication Manager provides, a network of standalone SIP phones is little more than a lab experiment. So, rather than look for reasons to eliminate the PBX, let’s examine how intelligent SIP endpoints can add richness to your existing communications platform.
Let’s begin with the fact that SIP allows you to create communications sessions of any media type. This means that not only can your SIP “telephone” make voice calls, it can also make video calls, send instant messages, update your presence status, send and receive files, share screen content, etc. In fact, I’ve seen a SIP “telephone” that can play a game of chess. Try and do that with your H.323 telephone.
The developers of SIP were aware that anything you create today will probably be obsolete six months from now. That’s why they insisted that SIP be extensible and not as strictly defined as other VoIP protocols. This openness is accomplished by requiring all SIP entities to ignore request and response headers that those entities don’t understand. As an example, the developers of a new SIP telephone could create a SIP message header that contains the GPS coordinates of the user. When one of these telephones makes a call, the physical location of the user is sent within the SIP INVITE request. That INVITE might pass through a series of SIP proxies and Back-to-Back-User-Agents (B2BUA’s) that weren’t written to understand or process this new header, but they are required to pass it along nonetheless. Now, if that INVITE reaches a SIP telephone that does understand the new GPS header, it can act upon it appropriately. Imagine that the recipient is an emergency responder’s console and the GPS information was sent for an E911 call. Wouldn’t it be useful if the responder knew exactly where the emergency was occurring? That’s the power of openness.
The last thing I want to discuss is computer telephony integration (CTI). For most people CTI means screen-pops, but it can do so much more than that. CTI is the glue between your business applications and your communications system and it can affect anything from call routing to customer relationship management. In the past, CTI was data and processing that ran parallel to a telephone call and external synchronization was required to pass that data to users, applications, and endpoints. However, with SIP that data can be contained within the SIP messages and live within the call flow itself. As opposed to the data being on a separate path that needs to be merged with the call at each decision point, all data is readily available wherever the call might land.
The smart phones we carry around with us today are proof that we demand more from our communications devices. While our older digital and H.323 devices have served us well in the past, there’s a new sheriff in town. SIP services, telephones, and multifunctional endpoints are the building blocks that will create the communications system that meets the needs of the modern enterprise and the ultra-wired employee.