H. 323. Chapter 4. Definition. The H.323 standard provides a foundation for audio, video, and data communications across IP-based networks, including the Internet.
transmission formats and between communications procedures
1. The endpoint that initiates the call knows the called number (122) but it does not know the IP address associated with that number. At the same time, since it is registered with the gatekeeper, it must ask the gatekeeper for a permission to place the call. It does so by sending the Admission Request message to the gatekeeper. The Admission Request (ARQ) will contain the called number (122), indicating to the gatekeeper that the endpoint needs to have the number resolved to an IP address.
2. The gatekeeper will check it's database of registered endpoints whether it contains the number 122. If so, the gatekeeper will check if 121 is allowed to call 122 and if it is possible to place the call — for example, if there is enough bandwidth (a bandwidth limit could be configured on the gatekeeper for a subnet that's connected via some WAN link). After that, the gatekeeper will form an answer — the message Admission Confirm (ACF) with an IP address and send the ACF to the calling endpoint.
3. The endpoint 121 will now open a call signalling channel to the address provided by the gatekeeper in the ACF message. The call signalling messages are sent over TCP and the protocol is H.225.0, embedded in Q.931 (we will denote this as Q.931/H.225.0). With the gatekeeper-routed call model, the endpoint 121 will open a TCP channel to the gatekeeper and send the Q.931/H.225.0 message Setup. The gatekeeper will in turn open a second TCP channel to the endpoint 122 and forward the Setup message.
4. The endpoint 122 will first respond with the Q.931/H.225.0 message Call Proceeding to indicate it has started working on setting up the call and the gatekeeper will forward the message to the calling endpoint. After that, 122 will ask the gatekeeper for a call permission (Admission Request, ARQ) and the gatekeeper will respond with Admission Confirm (ACF).
5. The called telephone (122) starts ringing and this is signalled to the other party with the Q.931/H.225.0 message Alerting.
6. The called party picks up the handset and the endpoint can signal the call has been accepted. This is done by sending the Q.931/H.225.0 message Connect. At this point, the parties will need to negotiate parameters for audio (and optionally video) channels. The protocol H.245 is used for this negotiation.
7. The calling endpoint opens a TCP channel to the H.245 address it has received in the Connect message, and the gatekeeper will establish the second "half" of the H.245 signalling channel. The endpoints can start exchanging H.245 messages. The H.245 negotiation has three parts:
Deciding which endpoint is the "master" and which is the "slave".
Exchanging information about the capability set of each party.
Deciding what codec’s will be used
8. Finally, the two endpoints can start sending the RTP streams and the two people will hear one another. Note that each of the two directions can be encoded with a different codec.