by Philip Carden

H.323 vs. SIP

Given that two standards currently compete for the dominance of IP telephony signaling, how do you decide which is more appropriate? The good news is that the two protocol suites appear to be converging — picking up good ideas from one another. In particular, the third and latest incarnation of H.323 (H.323 v3) has addressed some key performance issues (call setup delay and stateless processing to support UDP), which were initially key SIP advantages.

Most importantly, each suite supports (pretty much equally well) the majority of required end-user functions (including call setup and tear-down, call holding, call transfer, call forwarding, call waiting, conferencing and click-for-dial). The only functional differences are message waiting indication (which only H.323 supports), third-party control (e.g., a secretary placing a call on behalf of a manager, which only SIP supports, and certain conferencing functions. While the range of functions supported is similar, the H.323 v3 suite (by means of H.245) provides a somewhat more robust mechanism for "capabilities exchange" than does SIP, which relies on the less descriptive Session Description Protocol (SDP). "Capabilities Exchange" is the process by which it is determined whether a particular feature is supported by both participating entities.

However, functionality is by no means the only consideration in the H.323 vs. SIP debate. Equally important are Quality of Service (QOS), Scalability/Flexibility and Interoperability. Indeed, whereas the suites are relatively similar in terms of functionality, they differ quite substantially in these areas, as seen in the following table. Because SIP is a significantly less complex protocol, it is argued that it should scale better. This is an important consideration given that the Internet may well come to support 500 million IP telephony devices. However, it is my opinion that this potential advantage doesn’t sufficiently compensate for the protocol’s current weaknesses in terms of QOS and interoperability. Most importantly, SIP doesn’t provide for redundancy (making it unsuitable for carrier applications), doesn’t support the emerging Differentiated Services/Policy Management approach to QOS and has a limited interoperability testing track record (largely because the protocol is new, having only been ratified in February 1999).

Does the H.323 vs. SIP debate even matter?

From a practical perspective, if you go out and research the IP telephony products that are available today you’ll find that many are still vendor specific, several support H.323 version 1, some support H.323 v2 and very few support either SIP or H.323 v3. This is particularly true of products targeted at enterprise solutions as opposed to carrier solutions (where H.323 support is more widespread).

In practice, what is likely to happen is that major vendors will support both approaches until it becomes clear either that one standard is going to die, or that the two are going to merge. That said, it is certainly worthwhile clarifying vendors’ intended strategies with respect to signaling. If you are particularly concerned with high availability and interoperability, then a SIP-oriented solution might be too bleeding edge right now. Otherwise, both approaches pretty much deliver the same in terms of functionality. The only area where there is a noticeable difference is in the implementation of conferencing capabilities. Because SIP can be used to invite multiple parties to join a call, simple conference calls can be initiated without the requirement for a conferencing server (whereas H.323 does require one). In practice however, whether or not this is a constraint depends on the full vendor solution and approach rather than just the protocols that happen to be used.