On a number of occasions I’ve mentioned how I prefer SIP over H.323. Some of you, rightfully so, are probably wondering why? Just because SIP is newer doesn’t make it better. Well, here are a number of reasons why I’ll take SIP over H.323 any day of the week.
First, let’s look at who controls each protocol. H.323 was developed by the International Telecommunications Union (ITU), an organization that dates back to before Alexander Graham Bell invented the telephone. The ITU has been instrumental in building the Public Switched Telephone Network (PSTN) as it existed for the first 100 years or so. In fact, H.323 is basically an IP wrapper around Q.931 which is the call setup protocol of ISDN. In other words, H.323 is clearly rooted in traditional circuit switched telephony.
SIP is controlled by the Internet Engineering Taskforce (IETF) which is responsible for the protocols that make the Internet work. The same folks who make enhancements to Hypertext Transfer Protocol (HTTP) and the Domain Name System (DNS) also make enhancements to SIP. While H.323 takes its cues from the TDM world, SIP looks towards the Internet and the web for its guiding light.
In H.323, endpoints are dumb. They are what I like to refer to as stimulus devices. The phone tells the call server that a button has been pushed. The call server tells the phone to write something to its display. Without an intelligent network element telling an H.323 telephone exactly what to do, it’s little more than an expensive paperweight.
SIP endpoints are smart. A lot of processing occurs in a SIP phone and it’s possible to create and manage all sorts of communication flows without the assistance of any network components.
H.323 is a byte-based, hexadecimal protocol and as such is hard to troubleshoot and debug. SIP is text based and anyone familiar with the protocol can easily look at a SIP trace to understand what is happening.
Despite being a standard, H.323 has evolved into being a very proprietary protocol. You cannot take an Avaya H.323 phone and expect it to work on a non Avaya system. However, despite some of the liberties that vendors have taken with SIP, there is a high degree of probability that you can run anyone’s SIP phone on any SIP compliant system. You may not get a particular vendor’s proprietary call control features, but you can expect to make calls, hold calls, transfer calls, create conferences, and most of the other common telephony functions.
SIP allows you to embed information about the call within the SIP messages. For instance, you can add a customer’s account number to a SIP message and have the number stay with the call no matter how many times it is transferred around a contact center. H.323 does not have the mechanism to easily attach data to a call which leads to a fully parallel Computer Telephone Integration (CTI) network.
H.323 works well for voice and video, but it hasn’t been extended to support other media types. On the other hand, SIP is media agnostic and can be used for instant message, presence, file transfer, etc. In fact, when I was working for Nortel developing the MCS 5200 soft phone, we used SIP to play chess across the Internet.
In summary, H.323 is not a bad protocol. It does what it does and it does it well. However, as communications have evolved, H.323 has pretty much stayed where it was 10 years ago. SIP not only addresses the media types that H.323 ignores, it has absorbed the properties that have made the Internet as pervasive as it is today. SIP is clearly the future and its relevance will proportionally rise as H.323’s relevance falls.