The ITU-T H.323 protocol supports a distributed VOIP Network, in which VOIP terminals are managed by gatekeepers. Peer gatekeepers communicate with each other to enable end-to-end voice communications between VOP terminals. In the case of requiring a VOIP terminal to communicate with the traditional switched network such as PSTN, a gateway is required to facilitate the communications.
There are two types of gatekeeper call signaling methods:
- Direct Endpoint Signaling - This method directs call setup messages to the terminating gateway or endpoint.
- Gatekeeper-Routed Call Signaling (GKRCS) - This method directs the call setup messages through the gatekeeper.
A major functionality of gatekeepers is to keep track of other H.323 zones and forward calls appropriately. When many H.323 zones are present, gatekeeper configurations can become administratively intensive. In such large VoIP installations it is possible to configure a centralized directory gatekeeper that contains a registry of all the different zones and coordinates LRQ-forwarding processes. No full mesh is needed between inter-zone gatekeepers with directory gatekeepers.
In a typical H.323 call setup scenario, after RAS messages are exchanged, H.225 setup messages are sent over a control channel. For example, in the following figure, both gateways are registered to the same gatekeeper, and the gatekeeper has chosen direct call signaling.
- Gateway 1 (the calling gateway) initiates the admission request (ARQ) (1)/admission confirmation (ACF) (2) exchange with that gatekeeper.
- The gatekeeper returns the call signaling channel address of Gateway 2 (the called gateway) in the ACF.
- Gateway 1 then sends the setup (3) message to Gateway 2 using that transport address.
- The setup is complete and the call is proceeding (4).
- If Gateway 2 wishes to accept the call, it initiates an ARQ (5)/ACF (6) exchange with the gatekeeper.
- The gatekeeper responses with ACF/ARJ (6).
- Gateway 2 sends an alerting (7) message to Gateway 1. (If Gateway 2 receives an admission reject [ARJ] (6) message instead of an ACF message, it sends a release complete message to Gateway 1 instead of the alerting message.)
- Gateway 2 responds with the connect (8) message to Gateway 1
Example of H.323 Call Flow
Related Terms: H.323, Gatekeeper, Gateway, H.323 call flow, H.323 call setup
Reference Links: http://www.javvin.com/protocolVOIP.html: VOIP Protocols