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H.323 versus SIP: A Comparison |
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Written by Packetizer
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Friday, 28 December 2007 |
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Page 2 of 4 |
| Media Transport | RTP/RTCP, SRTP | RTP/RTCP, SRTP |
Extensibility -
Vendor Specific | H.323 is extended with non-standard features in such a way as to avoid conflicts between vendors. Globally unique identifiers prevent feature and data element collision. | SIP is extended by adding new header lines or message bodies that may be used by different vendors to serve different purposes, thus risking interoperability problems. The risk is admittedly small, but this problem has already been seen in the real world with similar extension schemes. |
Extensibility -
Standard | H.323 is extended by the standards community to add new features to H.323 in such a way as to not impact existing features. However, new revisions of H.323 are published periodically, which introduce new functionality that is mandatory, yet done in such a way as to preserve backward compatibility. | SIP is extended by the standards community to add new features to SIP in such a way as to not impact existing features. However, new revisions of SIP are potentially not backward compatible (e.g., RFC 3261 was not entirely compatible with RFC 2543). In addition, several extensions are "mandatory" in some implementations, which cause interoperability problems. |
Scalability -
Load Balancing | H.323 has the ability to load balance endpoints across a number of alternate gatekeepers in order to scale a local point of presence. In addition, endpoints report their available and total capacity so that calls going to a set of gateways, for example, may be best distributed across those gateways. | SIP has no notion of load balancing, except "trial and error" across pre-provisioned devices or devices learned from DNS SRV records. There is no means of detecting the load on a particular gateway or to know whether a device has failed, meaning that proxies simply have to try a PSTN gateway, wait for the call to timeout, and then try another. |
Scalability -
Call Signaling | When an H.323 gatekeeper is used, it may simply provide address resolution through one RAS message exchange, or it may route all call signaling traffic. In large networks, the direct call model may be used so that endpoints connect directly to one another. | When using a SIP proxy to perform address resolution for the SIP device, the proxy is required to handle at least 3 full message exchanges for every call. In large networks, such as IMS networks, the number of messages on the wire may be excessive. A basic call between two users may require as many as 30 messages on the wire! |
Scalability -
Statelessness | An H.323 gatekeeper can be stateless using the direct call model. | A SIP proxy can be stateless if it does not fork, use TCP, or use multicast. |
Scalability -
Address Resolution | H.323 defines an interface between the endpoint and gatekeeper for address resolution using ARQ or LRQ. The H.323 gatekeeper may use any number of protocols to discover the destination address of the callee, including LRQs to other gatekeepers, Annex G/H.225.0, TRIP, ENUM, and/or DNS. The endpoint does not have to be concerned with the mechanics of this process, and the processing requirements for address resolution placed on the gatekeeper by H.323 are for just a single message exchange. Although out of scope of H.323, an H.323 endpoint may perform its own address resolution using ENUM and/or DNS and then place a direct call to the resolved address or provide the resolved address to the gatekeeper as an "alias".
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Last Updated ( Tuesday, 12 February 2008 )
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