Comparison and Contrast of the Various VoIP Signaling Alternatives
The various signaling alternatives each offer advantages and disadvantages for system designers. A few highlights are presented here.
First, regarding MGCP and H.323, the scope of the protocols is different. MGCP is a simple device-control protocol, while H.323 is a full-featured multimedia conferencing protocol. H.323 is currently approved up to version 3, while MGCP has not been and may never be fully ratified; it is merely a de facto standard adopted by some manufacturers. As such, MGCP interoperability has been demonstrated, but not industry-wide. Likewise, the complexity of H.323 has inhibited interoperability as well.
MGCP can set up a call in as few as two round-trips, while H.323 typically requires seven or eight round-trips. (Note: H.323v2 provides for a fast start process to set up some calls in only two round-trips, but this is not widely implemented.) Call control is little more than device control for MGCP, while H.323 derives call flow from Q.931 ISDN signaling as a media control protocol. This control information is transmitted over UDP for MGCP, and over TCP for H.323.
SIP and H.323 are more direct competitors. They are both peer-to-peer, full-featured multimedia protocols. SIP is an IETF RFC, while H.323v3 has been approved by the ITU. Interoperability of both protocols has been demonstrated. SIP is more efficient than H.323, allowing some call setups in as little as a single round-trip. In addition, SIP uses existing Internet-type protocols, while H.323 continues to evolve new elements to fit into the Q.931 ISDN model.
Comparison of SIP to MGCP is similar to the comparison of H.323 to MGCP, in that SIP (like H.323) is a media-control protocol and MGCP is a device-control protocol. The same differences emerge as before between client/server and peer-to-peer. The fundamental difference is that peer-to-peer protocols such as H.323 and SIP tend to scale more gracefully, but client/server protocols such as MGCP are easier to design and maintain.
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