We know the operation of modern communication networks is a very complex process that involves the interaction of many systems. A communication network , is a set of equipment and facilities that provides a service, the transfer of information between users located at various geographical points. The ability of communication networks to transfer communication at extremely high speeds, allows users to gather information in large volumes almost instantly with the help of computers for long distances.

Radio and television broadcasting are probably the most common communication services. Various stations transmit an ensemble of signals simultaneously over radio or cable distribution networks.

Telephone service is the most common real-time service provided by a network. Two people are able to communicate by transmitting their voices across the network. The service is considered as connection-oriented in the sense that users must first interact with the network to set up a connection

Telephone networks provide a broad class of call management services that use the originating number or the destination number to determine the handling of a call. For example , in call return the last originating number is retained to allow it to be automatically called by the destination user at a later point in time. Caller ID allows the originating number and sometimes name, of the originating call to be displayed to the destination user when the receiving device is display capable. Voice mail allows a destination user to have calls forwarded to a message receiving device when the destination user is not available.

Cellular telephone service extends the normal telephone service to mobile users who are free to move within a regional area covered by an interconnected array of smaller geographical areas called cells. Each cell has a radio transmission system that allows it to communicate with users in its area. Cellular system should handle “handoff” when user moves from one area to another area.

Electronic mail (e-mail) is another popular network service. The user provides a text message and a name and/or address to a mail application. The application interacts with a local mail server, which in turn transmits the message to a destination server across a computer network. The destination user retrieves the message by using a mail application. E-mail is not a real-time service , where large delays can be tolerated. Here security and privacy is matter of concern.

Many applications that involve an interaction between processes running in two computers may be characterized by client/server interaction. Internet is an example of distributed system. The World Wide Web (WWW) is accessed through a browser program that displays the document and allows the user to access other documents by clicking the links. Each link provides the browser with a uniform resource locator (URL) that specifies the name of the machine where the document is located as well as the name of the file that contains the requested document.

Video on demand characterization another type of interactive service. The objective of the service is to provide access to a video library. The user initiates the service by accessing a menu from which a selection is made. Video which requires lot of bandwidth to pass from source computer to the destination.

Streamed audiovisual services over the internet provide an example of a service with some of the feature of video on demand. An application like “real player “ can be used to access a channel that provides an audio visual stream to the client machine.

Another class of service is audio-visual conferencing. Here both audio and video are transmitted over a distance. This service requires real-time response requirement. This service is being used by many companies for training, client interaction from a site, group discussion etc.