by January 31, 2004 0 comments

VoIP (Voice Over Internet Protocol) is a fast evolving technology and is here to stay. In this article, we will compare the two most widely used signaling protocols of VoIP–H.323 and SIP–and will answer questions such as which is better, which will be the de facto standard, and what are the interoperability issues?

These protocols were developed with the objective of converging voice and multimedia with IP. The difference crept in because H.323 was developed by organizations subscribing to telecommunication and SIP to Internet technology. Before getting into a protocol war let’s have a quick look at the definitions of the two.

H.323 was developed by ITU (International Telecommunications Union) in 1996 for carrying voice over IP service. It describes terminals and other entities for providing multimedia communication services over packet based networks, which do not guarantee quality of service and may provide real time audio/video and/or data communication.

Components of Session Initiation Protocol

SIP (Session Initiation Protocol) was developed by IETF (Internet Engineering Task Force) in 1999 for enabling VoIP connections. It is a protocol that works on the application layer and defines initiation, modification and termination of interactive multimedia sessions between users. SIP is similar to HTTP and SMTP in its functioning. The advantage of SIP is that it is not limited to Internet telephony but can also be used to initiate and manage other sessions such as video, games and text chat.

H.323 Vs SIP
H.323 employs complex ISDN style signals to setup calls across a much simpler IP network. What makes it more complex is the fact that it specifies everything from the codec for the media to how to carry packets in RTP. It uses Binary ASN.1 encoding which is again difficult to understand. All these issues make it complex to handle Internet telephony. The main components of H.323 network are gateways, terminals, gatekeepers and multiple point control units. SIP consists of user agents (that can either function as client or server depending upon if they are making or receiving call) and proxy server to route calls to other entities and a registrar. 



body and Origins
based on technologies of internet, syntax of messages is very much near to
very much near to technologies of telecom and borrows signaling from
Encoding Encoded
in ASCII based text 
coding is difficult to understand
Use in
3G networks
being employed in 3rd generation mobile networks
Not used
Complexity Simple
as HTTP only consist of 37 headers.
complex consist of number of standards and headers
Architecture Simple,
modular SIP easily fits into different other IP Protocols. Clients in SIP
are Intelligent user agents while core servers are proxy, redirect,
location and redirection servers.
consist of services provided by H.323 components Clients in H.323are
intelligent H.323 terminals. Servers are H.323 Gatekeepers.
Scalability Highly
Scalable for small LANs
Scaling as it was basically developed
Highly fits
to internet
Internet fit
Extensibility SIP
is open to new protocols and is easy
to extend because it is  encoded in text format
is difficult to extend and debug because of its binary encoding
very widely deployed, no major project to its credit
widely deployed with many major projects in its kitty.

SIP came out as an alternative to H.323. It has been designed with Internet in mind and works on a client-server model. Strictly speaking SIP is not a telephony protocol. Instead, it’s a protocol used to set up calls, handle errors and handle inter process signals. It is associated only with establishing, modifying and terminating sessions, be it a session of game, voice or multimedia. SIP doesn’t differentiate between them. H.323, on the other hand, is used for transferring only

All this makes SIP very simple, like playing with Lego blocks as SIP defines different building blocks that fit into different other Internet protocols to achieve the task of Internet telephony. 

To have an idea of complexity of H.323 consider the fact that it consists of hundreds of headers while SIP consists of only 37. SIP is text-based, which implies that it is human readable and is easier to extend, debug and process by text-processing tools. SIP messages are like normal Web page source code. Addresses in SIP are not in number format but in URL format such as
which is translated into IP address using DNS. In SIP, the user will be able to make a call by simply clicking the URLs. 

It’s clear by now that SIP has been designed to keep the things simple. The experience of developing the technology and working with it, is as close as it can be to the Internet. Though SIP started late yet it is scaling fast. Big vendors are rolling out their products compatible with SIP. Earlier, Microsoft’s NetMeeting was based on H.323, while now Win XP messenger is based on SIP. 

In today’s scenario H.323 rules the roost as it has an advantage of being deployed earlier and has a major vendor support. Many protocols, groups and organizations dealing with H.323 are working to address the issues such as URL-based IDs and scalability. Problem with SIP is that it is yet to deploy large-scale projects, and it also lacks call state in network, which is necessary for billing and security.

A protocol war is going on to prove which is the best, but the need of the hour is to lay more stress on interoperability issues between SIP and H.323. Many vendors now support both the protocols. No body cares if its SIP or H.323, for both save money by converging voice over data networks. Both protocols provide comparable functionality and similar quality of services by using different mechanisms. Both protocols have their pros and cons. While SIP is very flexible in implementation, H.323 offers better network management and interpretability. VoIP is here for a long time now, yet not a single wonderful application has been developed to make VoIP a hit among the masses. Whosoever brings it first will be the winner.

Siddharth Sharma

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