leedesmond’s blog

The H.225 and H.245 are protocols from the H.323 family that deal specifically with VoIP calls. In a H.323 gateway-to-gateway call, several network packet exchanges must occur. H.225 ’s job is to take care of setting up the call (or “call setup”). This is followed by H.245 negotiating gateway capabilities (such as supported CODEC) and performing call control of the end-points. Once the connection is established, the Real Time Protocol (RTP)* is responsible for streaming the audio payload directly between the gateways.

When one or more gatekeepers (GK) are present, a H.323 end-point or gateway must first request (ARQ message) and be granted** permission (ACF message) from the former. Only then can it initiate the call setup process as previously described to actually place the call. Using a gatekeeper topology, H.323 calls can easily scale- out and -up to improve both availability and redundancy. To communicate with one another (GW to GK, GK to GK), gatekeepers and gateways make use of the Registration, Admission and Status (RAS) channel setup as part of a call by the H.225 protocol.

* RTP Control Protocol = companion media control/monitoring protocol over UDP for QoS e.g. delay, jitter, packet lost
** or denied permission (ARJ = Admission Reject)

ARQ = Admission Request
ACF = Admission Confirm

Technorati tags: VoIP, Unified Communications

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