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:
1. SETUP
- 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.
3. AUDIO
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.
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