In the following example we could see a H.323 call. The different colors show different protocolos.
A H.323 call has 4 different processes:
- Terminal 1 register itself with the gatekeeper using the RAS protocol (Register, admisaion, status) sending an ARQ message and receiving an ACF message.
- Using H.225 protocol (used for setup and release of the call) terminal T1 sends a SETUP message to T2 requesting a connection. This message contains the IP address, port and alias of the calling user or the IP address and port of the called user.
- T2 sends a CALL PROCEEDING message warning on the attempt to establish a call
- Now, T2 terminal must register itself in the gatekeeper as T1 previously do.
- Alerting message indicates the beginning of tone generation phase.
- And finally, CONNECT message shows the beginning of the connection.
2. CONTROL SIGNALLING
In this phase a negotiation using H.245 protocol is opened (conference control), the interchange of the messages (request and answer) between both terminals establishes who will be the master and who the slave, the capacities of the participants and the audio and video codecs to be used. When the negotiation finishes the communication channel is opened (IP addresses, port).
The main H.245 messages used in this step are:
- TerminalCapabilitySet (TCS). Message capabilities supported by the terminals that take part in a call
- OpenLogicalChannel (OLC). Message to open the logical channel which contains information to allow the reception and codification of the data. It contains information of the data type that will be sent.
Terminals start the communication using the RTP/RTCP protocol.
4. CALL RELEASE
- The calling or the called terminal can initiate the ending process using the CloseLogicalChannel and EndSessionComand messages to finish the call using again H.245.
- Then using H.225 the connection is closed with the RELEASE COMPLETE message.
- And finally the registration of the terminals in the gatekeeper are cleared using RAS protocol.