VoIP Call Control Protocols

As of this writing, the main VoIP call-control protocols are H.323, Simple Gateway Control Protocol (SGCP), Internet Protocol Device Control (IPDC), MGCP, and SIP. They are defined as follows:

• H.323 is the ITU-T recommendation with the largest installed base, simply because it has been around the longest and no other protocol choices existed before H.323. Chapter 10, "H.323," discusses this protocol in detail.

• SGCP was developed starting in 1998 to reduce the cost of endpoints (gateways) by having the intelligent call-control occur in a centralized platform (or gateway controller). Chapter 12, "Gateway Control Protocols," covers this in more detail.

• IPDC is very similar to SGCP, but it has many other mechanisms for operations, administration, management, and provisioning (OAM&P) than SGCP. OAM&P is crucial to carrier networks because it covers how they are maintained and deployed.

• In late 1998, the IETF put IPDC and SGCP in a room and out popped MGCP. MGCP is basically SGCP with a few additions for OAM&P. MGCP is covered in more detail in Chapter 12.

• SIP is being developed as a media-based protocol that will enable end devices (endpoints or gateways) to be more intelligent, and enable enhanced services down at the call-control layer. Chapter 11, "Session Initiation Protocol," covers SIP in detail.

To briefly explain the various differences between these call-control protocols, let's take a look at how they signal endpoints.

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