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Unit VIII data communication systems

I ntroductory text

A multi-user system is made up of a number

of terminals sharing a mainframe computer. Each

terminal consists of at least one input device and output device and allows the user to communicate with the computer.

There are a number of different types of terminals. For example, a teletype terminal is made up of a keyboard and a printer which gives a hardcopy output.

A VDU terminal has a keyboard and a screen. It usually has its own internal processor which deals with the input/output and editing of data and is, therefore, referred to as an intelligent terminal.

A graphics terminal is similar to a VDU terminal but is specially designed to display graphics such as pictures, maps, graphs, diagrams etc. and has a lightpen attached to allow the user to ‘draw’ on the screen. It is normally used for Computer Aided Design.

Often a mainframe computer and its terminals are situated a long distance apart. They may be in different rooms, buildings, cities or even countries. These remote access terminals are connected to the mainframe computer by telecommunication links (e.g. telephone lines). Computers and terminals connected together using telecommunications are known as a data communications (or datacomm) system.

Datacomm systems may also be formed from a number of computers and peripherals connected together in a network. This allows software and hardware to be shared by a large number of users over long distances.

A network contained in a relatively small area, such as an office building, factory site or university campus, is known as a Local Area Network or LAN. The two main types of LAN are single cable networks and loop networks.

If the network is connected over a large area it is called a Wide Area Network or WAN. This may be linked using telephone lines. Dedicated lines are permanently connected to a system but dialed lines are only connected when needed. However, existing telephone lines are designed to carry analogue signals and not the digital signals used by computers. A special device is therefore needed at each end of the telephone line to convert the signals to the correct form. This device is known as a modulator/demodulator or modem. A special kind of modem known as an acoustic coupler has rubber cups into which an ordinary telephone can be plugged. Although this avoids having to make a special connection into the telephone system, it is more likely to suffer from corruption of data due to interference.

Modern telecommunications systems are being developed using fibre-optic cables. These allow data to be transmitted as pulses of light along thin strands of glass (about the thickness of a human hair). In this way, data can be communicated at very high speed, measured in bauds i.e. bits per second. It also results in very low level of interference.

Data can be communicated over long distances and over water by using microwave radio links. These transmit and receive high frequency microwave signals using small dish aerials. Since there must be a clear path between these aerials, they are usually situated at the top of high towers.

Data communications over long distances can make use of communications satellites, which are placed in fixed orbits above the Earth. The satellites receive signals from ground stations on the Earth, amplify the signals then retransmit them to other ground stations, which may be in another country or another continent.

The development of efficient data communications systems is an important aspect of the growth in information technology.