Computer Networks.

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Element 4.3 Computer Networks

2. Types of Networks

A LAN (local area network) is a collection of computers connected together over a small area, usually a single building or site (e.g. a school).  If a school was to have a LAN it would own all the equipment and be able to run the network as it sees fit.  If changes need to be made to the network, such as adding new terminals, it will be relatively easy and cheap.

A WAN (wide area network) consists of computers connected over a large area using telecommunications equipment, such as telephone or satellite communications links.  A school could have a LAN which is connected to other schools around the country (or world) to form a WAN.

WANs are more sophisticated than LANs as the operator has to deal with telecommunications companies and doesn’t own all the equipment.  Wires between one LAN and another are owned supplied by a telecommunications service provider, each of which has it’s own regulations and service charges.

Two schools with a LAN could be connected using two modems and appropriate software on each machine together with a public telephone line.  This method is fairly cheap to set up with no need for much extra equipment, but data transfer is slow and the phone bills can be high.  As it is a public telephone line it is not very secure and so vulnerable to hackers.  Renting a dedicated leased line from a telecommunications service provider, of which you are sole user, is much more secure but more expensive.

Examples of Wide Area Networks:

Broadcast services

These are services that use radio links.  The commonest example is teletext where images from computers are sent as part of the television signal.  Broadcast services also include networks with terminals or workstations linked by radio or infrared signals.  This could be useful in many situations such as in supermarkets where checking stock can be done using a wireless hand-held terminal that transfers the data directly from the shelf to the main system.

Public switched data network

More commonly known as Switchstream or X-stream, this is one of four digital networks owned by British Telecom.  The Switchstream is used for international communications because it uses a recognised standard regulated by CCITT.  This network is a packet switching type, which means it splits each message into packages.  Each package contains the message, the destination address, correct sequence the package should be assembled into, information identifying the source and finally the ability to detect and correct errors.  The user of this type of network uses a packet terminal in which a packet is created for transmission along the dedicated dataline.  If the user is not using a packet terminal a dial up connection is used, this needs a modem and the message must go through a PAD (packet assembler/dissembler).  This device converts the message into the required protocol to be transmitted along the network and then reconverts it to be received by the destination computer.

Public switched telephone network

It is called ‘public switched’ because the user chooses the number dialled.  Dialling can be done automatically by the modem in a communications system, as the telephone numbers are stored in the computer.

There are two types of telephone line: the standard telephone line and the dedicated or leased line.  We all use the standard line for using telephones but it can also be used for transporting data.  Large organisations (e.g. banks, police, etc) tend to use dedicated lines because they are faster, less prone to signal problems and more secure, but the expense of leasing them means smaller organisations could not afford them.

Integrated services digital network (ISDN)

To transmit data along telephone lines the digital material has to be changed to analogue form and then converted back to digital at the other end.  As files become larger and it is not just textual data the needs to be transferred the current telephone system is finding it harder to transfer data.

The UK has seen a gradual development of a completely digital telephone system; known as integrated services digital network.  The main advantage of such a system is that it is much faster.

Private wide area network

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This is a WAN that uses a privately owned or leased line, which only subscribers have access to.  The Technology Colleges Trust Intranet is an example of a private WAN, which has many technology colleges subscribed to it.  Colleges can contribute and access data on this WAN.  Joint Academic Network (JANET) is another example of a private WAN, set up by the universities.  Internet service providers such as CompuServe and AOL are types of private WAN.

3. Components


A workstation is either a ...

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