Voice over IP:

i. The idea is to use the Internet as a telephone network with some additional capabilities. Instead of communicating over a circuit-switched network, this

ii. Application allows communication between two parties over the packet-switched Internet.

iii. Two protocols have been designed to handle this type of communication: SIP and H.323.

H.323:

H.323 is a standard designed by ITU to allow telephones on the public telephone network to talk to computers (called terminals in H.323) connected to the Internet.

Figure.1 shows the general architecture of H.323.

Fig1: H.323 architecture

A gateway connects the Internet to the telephone network. In general, a gateway is a five-layer device that can translate a message from one protocol stack to another. The gateway here does exactly the same thing. It transforms a telephone network message into an Internet message.

The gatekeeper server on the local area network plays the role of the registrar server.

• Protocols :

  H.323 uses a number of protocols to establish and maintain voice (or video) communication. Figure7 shows these protocols.

H.323 uses G.71 or G.723.1 for compression. It uses a protocol named H.245 which allows the parties to negotiate the compression method.

Protocol Q.931 is used for establishing and terminating connections. Another protocol called H.225, or RAS (Registration/Administration/Status), is used for registration with the gatekeeper.

• Operation :

  Operation of a telephone communication using H.323 with a example is as follows. Figure8 shows the steps used by a terminal to communicate with a telephone.

  1. The terminal sends a broadcast message to the gatekeeper. The gatekeeper responds with its IP address.
  2. The terminal and gatekeeper communicate, using H.225 to negotiate bandwidth.
  3. The terminal, the gatekeeper, gateway, and the telephone communicate using Q.931 to set up a connection.
  4. The terminal, the gatekeeper, gateway, and the telephone communicate using H.245 to negotiate the compression method.
  5. The terminal, gateway, and the telephone exchange audio using RTP under the management of RTCP.
  6. The terminal, the gatekeeper, gateway, and the telephone communicate using Q.931 to terminate the communication.