H.323 and SIP are two signalling protocols both of which are used to identify the state of connection between VOIP terminals. But the question is, which one of the two is better?
H.323 has been developed and recommended by the International Telecommunications Union (ITU) and is based on the Q.931 protocol. It is considered to be rooted on traditional circuit switched telephony.
The Session Initiation Protocol (SIP) was developed by the Internet Engineering Task Force (IETF). SIP is primarily based on the Internet and its application includes Internet telephony.
This is a comparison between the two signalling protocols that play an essential role in our lives and industries in the present age. We will be comparing the two based on several parameters and judging various aspects of their functionality and applicability.
Stateful implementations make H.323 a certain complexity. However, H.323 clearly defines the functionality associated with their devices which makes it simplistic. SIP, on the other hand, employs a number of capabilities including video messaging, application sharing etc which makes it complex and gives rise to interoperability issues.
Both H.323 and SIP support a large number of domains and are capable accommodating a number of different technologies and are capable of functioning in either of stateful or stateless manners.
Since the endpoint in H.323 reports to the gatekeeper through the RAS protocol, calls can be successfully billed. The only way to bill calls in SIP is to remain present in the signalling path during the entire duration of the call so the endpoint can be detected upon completion of the call. This increases chances of error in resulting statistics as delays may have taken place during signalling.
When it comes to the services supported by the two protocols, both stand on more or less equal footing. The difference lies in the fact that H.323 standardizes services in the H.450 series of specifications while these are not as strictly defined in SIP. H.323 can be considered to be more capable of supporting new services. However, the general trend remains that H.323 implementations are stateful while those of SIP are stateless.
H.323 employs several fail proofs to deal with potential failures including alternate gatekeepers and alternate endpoints. SIP, on the other hand, does not define any such safety features that may deem it reliable. The failure of a proxy may be detected by the user agent through time expiration.
Hence, to sum it up, while H.323 itself is a good protocol, but SIP does a much better job at adapting to contemporary changes and technological developments. SIP does well to incorporate elements of the Internet as it is an integral part of the modern day communications network while H.323 does not do much in this regard. On the other hand, SIP is faced with potential interoperability issues while H.323 does well to cover this aspect. Albeit overall, most experts are likely to recommend SIP over H.323 due to the long term benefits it promises to bring.