In this era of convergence, sending voice over networks-like Internet Protocols, Asynchronous Transfer Mode (ATM is broadband switching and transmission technology) and frame relay-has become the hottest technology facilitating real time communication. And the basic element for all of them is a packet called Voice over Internet Protocol (VoIP).

How it works: VoIP uses the packet switching networks to transfer voice across the network. For this transmission, it routes its service through the traditional circuit-switching network. International Telecommunication Union has defined a standard -H.323 for such packet based multimedia networks.

A circuit switched network or the public switched telephone network creates a dedicated path between the original and the destination call. Although the same two-end points of the call can follow different paths, the circuit is fully available for the duration of the call, making it unavailable to other network users.

In packet switched networks, there is no dedicated path between the users. Here the content is broken up into packets and sent across the network. Each packet carries a header, which determines the destination of the packet. Here too the packets take different paths towards the destination but they share their paths with other packets.

H.323 and the system: H.323 packet is connected to the Integrated Services Digital Network (ISDN), Public Switched Telecommunications Network (PSTN) and local landline or wireless devices. Other components of H.323 are terminals that are end points to the LAN Gateways, Gatekeepers and Multipoint control units.

The H.323 are the LAN based end point terminals. These terminals include the compression/decompression format -codec (They are devices that encode or decode a signal) and accomplish the voice transmission functions. When the signal is determined to be voice, it is usually compressed by a DSP to the Codec format. The voice packet is constructed as an IP packet, to avoid TCP/IP to attempt to correct a corrupted packet by re-transmitting the packet. Any attempt to retransmit the packet would result in unnecessary delays. They are hardware or software components providing digital encoding and decoding of analog signals. They send and receive packetized voice. These terminals also need to support signaling functions that are used for call setup. The terminals are connected to the gatekeepers through their RAS (Registration, Admission, and Status) point.

Gateways are interfaces between the LAN and switched circuit network. The gateway serves as the interface between the two different physical networks and facilitates a connection between the packets switched network and the traditional circuit switched network. It translates the signaling messages between the two sides and compresses and decompresses the voice. Gatekeepers are usually present to perform a set of functions like providing address translation (routing) for devices in their zone.

The Multipoint Control Units can be standalone devices, or be integrated into a terminal, a gateway or a gatekeeper. They have two parts, each performing certain functions. The Multipoint controller handles the signaling and control messages essential to set up and manage conferences. The Multipoint processor accepts messages from endpoints, replicates them and forwards them to the correct participating points. But the most important aspect of the technology is the voice quality. This can be affected primarily due to the type of Codec used and also because of latency, jitter and packet loss.

But the technology enables the transfer of your voice in the form of packets through a traditional circuit-switching network to its right destination.

Source: CIOL