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.