H.323 versus SIP: An (un)objective Comparison
September 18, 2007
I came across an interesting comparison between H.323 and SIP in a Cisco related blog. They make a pretty good technical analysis, but the comparison lacks in its completeness.
Both H.323 and SIP are used today for VoIP, and they are considered interchangeable solutions. The comparison made covers the following issues:
- Philosophy – H.323 does calls, SIP does sessions
- Reliability – H.323 reliable by design, SIP by responsible user agents
- Message Definition – H.323 uses ASN.1, SIP uses ABNF
- Message Encoding – H.323 is binary, SIP is mostly textual
- Media Transport – both use RTP/RTCP and SRTP
- Extensibility – H.323 extensible by design, SIP breaks interoperability with extensibility
- Scalability – H.323 scalable by design, SIP by implementation or by additional IETF standards
- Addressing – H.323 supports multiple addressing schemes, SIP has only URIs
- Billing – H.323 has billing by design, SIP by implementation
And the list goes on to other issues. It seems strange to me that in all, H.323 either excels or does as good as SIP. This being the case, why does every new developer looking for SIP?
I have been working with H.323 and SIP for several years now, and I can say that both have their advantages and both are broken in some places. H.323 is a lot better today in issues of interoperability – a lot of it can be easily attributed to the IMTC’s work in this area. I also have a warm place in my heart for this particular protocol – I have been working and dealing with it for many years. That said, the comparison above lacks two main points:
IMS
The 3GPP’s next generation network, which has been adopted by the Tispan and CableLabs (making it the de-facto network in the world in the future). This happened as the 3GPP added interfaces scenarios and call flows to SIP, giving more advantages to it.
H.323 is not part of IMS and is irrelevant for IMS.
SIP is at the core of IMS.
Market
H.323 is dominant today and has large deployments around the world. It is a lot better where it comes to video conferencing, and can be found a lot more in the enterprise.
SIP is the protocol of choice for most developers today – it is quite strong in the consumer and service provider markets. If you are a company about to develop a communication product, you will probably be selecting SIP. It is not as good for video conferencing, but it is getting there.
Services
There is another parameter that is important, and that is what services are part of the protocol and what new services can be offered easily?
H.323 focuses on multimedia calls in all of their flavors. Voice only, video, data collaboration, conferences and a rich set of telephony services.
SIP doesn’t seem to focus on anything in particular. You can use sessions to make calls with it (voice, video – whatever), you use it for presence and instant messaging, and you can use it for a large array of additional services as well.
That said, these services can be added to H.323 as well – this statement would be true to trying to add new services to SS7 though…
Now, if you opened a company now, which protocol would you decide use? What would be your decision looking only on technical aspects, and what would it be looking only on market aspects?
Technorati Tags: Cisco, IMS, IMTC, H.323, SIP, VoIP, Interoperability, Tsahi Levent-Levi
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October 22, 2007 at 10:52 pm
I think you are right that most things currently possible in SIP could also be done with other protocols, but I also another very key attribute of its nature should be added to your (well thought-out) comparison:
Coupling of session control to nature of media:
H.323 - strong component of protocol
SIP - uncoupled by design (through SDP)
So rather than ask:
“Now, if you opened a company now, which protocol would you decide use? ”
perhaps a more key question is:
If you are a network operator facing declining margins and you are continually being told that your future revenues lie with purported “IP services”, but no-one can define these yet, how important is it to you to implement an access protocol which is truly payload agnostic?
Even if SIP is a ‘one-trick pony’ in some senses, this is a pretty good trick given the size of the financial stakes being bet against a future IP-brokered service world…
Cheers,
Micah
July 21, 2008 at 3:12 am
Micah,
I disagree that this is a “pretty good trick”. Perhaps this statement just exemplifies your point that people do not fully understand IP-based services.
There are many things we can do on the Internet. VoIP is just one of them. But, one should not use H.323 or SIP for applications where they do not fit. Don’t take the SIP hammer and start pounding on screws.
SIP was primarily designed for voice. It has grown since then to include more capabilities, most of which are even less interoperable than simple voice calls.
Now, let’s explore the various IP services one wants. Say, streaming music. SIP? No. Wrong protocol. What about an Internet data storage system? No. Wrong protocol again. Billing system? No. Banking application? Let’s hope not!
SIP is really limited in scope to providing a few basic services, primarily centered around real-time voice, video, etc.
SIP is more than 12 years old now and one should not expect significant new kinds of functionality. Sure, there are and will be enhancements to improve on the application domain for which it was designed, but it would be wrong to use it for more than that. And, I get the impression that many people do want to use it for other things– and they will likely be disappointed.
Both H.323 and SIP were developed in the mid ’90s and they’ve reached a point of maturity. Now, users are now looking to do more and to utilize a number of different devices. After all, if you just replace a user’s phone with a voice phone, what is the appeal?
For these and other reasons, the ITU started working on the next-generation multimedia system that will enable communication across a multiplicity of devices in parallel. It will use XML as the basis of the protocol and, of course, will provide a lot more application functionality than previous systems. The new project will ultimately be H.325.
Paul