Electro/Telecom Industry Trends
February 2005
A Tale of Two Protocols
| Issue Table of Contents |
|
|
 |
After being ratified by the International Telecommunications Union (ITU-T) in 1996, the family of recommended standards for enabling multimedia communications over packet-based networks known as H.323 seemed set to become the defacto call initiation protocol for Voice over IP (VoIP) applications. But by 1999, another standard had been introduced — the Session Initiation Protocol (SIP) — that was soon rapidly gaining acceptance among members of the VoIP community. And with more and more vendors and organizations adopting the standard, it looks like SIP is on the path to becoming the pre-eminent protocol for VoIP communications.
Developed based on another ITU-T standard, H.320, which describes the protocol for wireline videoconferencing, H.323 was originally designed to enable multimedia IP-based communications over a local area network. As it was extended to the Internet and wide area networks, the protocol was quickly adopted by several high-profile vendors, including Microsoft and Netscape. At the time, H.323 was embraced for a number of reasons: its support of a variety of encodings, its interoperability, and its independence from platforms and applications.
However, even as H.323 became more prevalent, its drawbacks were also becoming apparent. Unlike pure Internet protocols, such as HTTP (HyperText Transfer Protocol) and SMTP (Simple Mail Transfer Protocol), which are simple and fit in neatly with the packet-based nature of the Internet, H.323 is complex. The protocol in fact is an umbrella for a number of other protocols and signaling standards that must be followed in order to establish a simple call. This often means that the time it takes to set up a call is noticeably longer than what users experience when using typical wireline phones.
SIP, in comparison, is simple. While H.323 is all encompassing — it supports call signaling, transmission control, multimedia management, and bandwidth control — SIP only has one job: to initiate call sessions. It finds the recipient of an IP-based call using either a phone number or a Web address that is similar to URL conventions, such as SIP:yourname@hostname.com. And like HTTP packets, SIP messages are formatted as text, so that they use up less bandwidth as the system attempts to establish an IP-based call. Also, while H.323 has been designed for applications beyond voice, such as video, SIP can be extended to other, more typically IP-based applications, such as instant messaging and gaming.
Before 1999 and the advent of SIP, the VoIP community didn't have much of a choice when it came to its call initiation protocol. But now, even though there are still proponents for H.323 among VoIP vendors, more and more, they are turning to SIP. They are doing so for a number of reasons, such as the protocol's versatility, its ability to integrate easily with other Internet protocols, and its scalability.
In addition, SIP is also being adopted by vendors developing third-generation wireless technology (3G). In 2001, the Third Generation Partnership Project (3GPP), a group of telecommunications partners dedicated to working collaboratively to develop 3G specifications, chose SIP over H.323 as its signaling protocol. Another related group, the Third Generation Partnership Project 2 (3GPP2) soon followed suit, and adopted the protocol as well. Both moves indicate that SIP's importance to the telecommunications community will only continue to grow.