Concept of Network and Data Communication

Introduction

When two people talk, information passes from one mind to another through speech, air and hearing. Computers and other digital devices do the same thing, but they exchange bits instead of words, and the study of how those bits move accurately from one machine to another is called data communication. When many such machines are joined so that they can share this data and the resources attached to them, we have a computer network. This chapter builds up from a single communication system to full networks: the elements involved, the media that carry signals, the problems those media introduce, the devices and layouts that connect machines, and finally the OSI model and IP addressing that make worldwide communication possible.

The communication system

A communication system is the complete set up that carries a message from a source to a destination. Its basic elements are:

• Source (sender): the device that produces the message, for example a computer.
• Transmitter: converts the message into a signal suitable for the medium (a modem, for instance).
• Medium (channel): the physical path along which the signal travels, such as a cable or the air.
• Receiver: converts the received signal back into a usable message.
• Destination: the device for which the message is intended.
• Noise: unwanted disturbance that the medium adds along the way.

A simple model of communication therefore runs: source produces a message, the transmitter encodes it into a signal, the signal travels through the medium (picking up some noise), the receiver decodes it, and the destination gets the message.

Data communication and its elements

Data communication is the exchange of data between two devices through a transmission medium such as a cable. For it to be effective it must deliver the right data, to the right destination, accurately and in time. Its five elements are the message (the information to be communicated), the sender, the receiver, the transmission medium and the protocol (the agreed set of rules that governs the exchange; without a shared protocol two devices cannot understand each other).

Modes of data transmission

The direction in which data can flow gives three transmission modes:

• Simplex: data flow in one direction only, like a keyboard to a computer or a radio broadcast.
• Half duplex: data flow in both directions but only one direction at a time, like a walkie talkie where each person must wait for the other to finish.
• Full duplex: data flow in both directions at the same time, like a telephone call where both people can speak at once.

LAN and WAN

Networks are commonly classified by the area they cover.

A Local Area Network (LAN) connects computers within a small area such as a room, a building or a campus. It is privately owned, offers high data rates, has a low error rate and is cheap to set up. A school computer lab is a LAN.

A Wide Area Network (WAN) connects computers spread over a large geographical area, across cities or countries, usually by leasing communication links from telecom operators. The Internet is the largest WAN. Compared with a LAN it covers a huge area but generally has lower data rates, a higher error rate and higher cost.