SIP or H.323: Which Protocol for Multimedia Communication?

Wednesday, June 07, 2006

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Voice over IP and IP Telephony are two hot topics today, with just about every organization being in the process of seriously deploying them nowadays. One of the names that crops up nowadays in relation to these two terms is SIP or Session Initiation Protocol. It's a protocol meant to address the problem of multimedia communication over IP. Another name also crops up here, called H.323, which is the older and more accepted protocol for the job. It has a wider deployment base than SIP, and is also far more complex. Since SIP has been around since 1999, there's been a constant debate on whether it would prove better than H.323. Let's take a look.

Starting with H.323, the protocol is quite complex, and being an older standard, it was primarily used on ISDN lines for video conferencing. But that's not the only thing that this protocol does. It's currently the main protocol that allows voice, video and data communication, and not to mention Voice over IP as well. It's actually a part of a larger set of recommendations by the ITU, called H.3x, which contains many protocols. These define all aspects of the conference network, such as what all equipment would be used, such as multi-control units, terminals, gatekeepers, etc. It also defines what protocol would be used for call initiation, termination, registration, forwarding, etc. While all this definitely means that H.323 protocol has a proven and well-defined track record, it also means that it carries a heavy baggage of multiple protocols with it. The protocol has predefined methods for everything. So for instance, it specifies the protocol Q.931 has to be used for call setup, H.245 has to be used for exchanging terminal capabilities, and so on and so forth. SIP, on the other hand, is a far simpler protocol than H.323, and has a broader scope as well. Unlike H.323, which defines all the protocols that must be used, SIP doesn't mandate any such thing. It's only a session initiation protocol and is very modular in nature. It therefore leaves it up to the implementer to decide what can be added to it. While that's an advantage, it can end up being a disadvantage too, as it could lead to interoperability issues. If different vendors develop their own proprietary ways of using the protocol, it could mean a lot of trouble for the customer. For instance, there is no single way of implementing SIP. H.323 on the other hand has a predefined procedure of implementation, which is followed everywhere.

Anil Chopra, Associate Editor

One benefit of SIP is that it's a lightweight protocol, and uses only text messages for all its communication. H.323, on the other hand, uses binary encoding during communication which makes it fairly difficult to troubleshoot. Another key advantage of SIP is that it also has inherent support for communication over a wide area network, whereas H.323 was designed keeping LANs in mind. This makes SIP far more scalable.

Another factor in favor of SIP is its addressing capabilities, wherein it can use URLs, email ids, or even H.323 conventions to identify specific hosts. H.323, on the other hand, supports limited addressing. No wonder then that SIP is being embraced by all key players in the market.  For instance, 3GPP has adopted the protocol, and there are various types of SIP based products available from key vendors, like Avaya, Microsoft, Bea, Cisco, Linksys, Hughes Software Systems, and many others. Avaya has a converged communications server, which does SIP based telephony. Bea has an integrated J2EE-SIP application server. Hughes has a SIP application server that supports proxying, registrar, redirecting of calls, conferencing, and much more. Another company called Active Telecom has developed an IP class Centrex, which provides features like virtual PBX, unified messaging, CTI and many others.

As can be seen, the key benefit of SIP is its extensibility, which allows vendors to take it beyond basic voice and video conferencing, and also includes instant messaging and e-mail. A vendor could come up with a software SIP server supporting just voice, and later provide additional modules for video, and other communications channels. Possibly, this would be the key enabler in making unified messaging a reality.

Unified messaging has been a much talked about buzzword for quite some time now, but still has a long way to go. Possibly, SIP might bring some order to the chaos and make it a reality very soon.

So which protocol standard will prevail in the future? Currently, the ball seems to be in favor of SIP, because of its broader scope, which going far beyond what H.323 offers. But given the fact that H.323 has been more widely deployed, and is fairly robust, it won't go away so soon. SIP's

popularity will increase as vendors start using its broad scope to bring out better applications for communication. Already, as per various market research organizations, it's expected to become the biggest VoIP carrier in the near future.

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