The signaling functions are based
on H.225 recommendation, that specifies the use and support
of Q.931/Q932 signaling messages. The calls are sent
by TCP at port 1720. On this port the Q.931 control call messages
are sent for call setup, maintenance and disconnection.
The most common Q.931/Q.932 messages are:
- Setup. It is sent to initiate a H.323 call
or to establish a connection with a H.323 terminal. This information
contains the IP address, port and alias of the calling user
or the IP address and port of the called user.
- Call Proceeding. Sent by the Gatekeeper
to a terminal warning on the attempt to establish a call once
the called number has been analyzed.
- Alerting. Indicates the beginning of tone
generation phase.
- Connect. Indicates the beginning of the
connection.
- Release Complete. Sent by the terminal
to initiate the disconnection.
- Facility. It is a message of the Q.932
standard used like a request of an additional service.
H.245 Control Function
The H.245 Control Function it is a set of
ASN.1 messages used for the call establishment and control
. The main messages are:
• MasterSlaveDetermination (MSD). This
message is used to prevent conflicts between two terminals
which want to start the process. It takes the decision about
which one will be the master and the slave.
• 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.
• CloseLogicalChannel (CLC). Message
to close the information logical channel.
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