3.4
Networked computer systems

A LAN (usually):

is owned and operated by a single organisation;
is of limited geographical area;
has a high bandwidth;
has a simple architecture;
uses broadcast techniques;

3.4.2

A server is generally a larger machine in terms of RAM, processing power and disc space. It holds users files and the operating system software.

It may use several discs and a large LAN may have several servers.

Cable types are discussed further below .

The bus is the single cable in the middle.

A WAN (usually):

is implemented using equipment owned by different organisations;
is of large geographic area;
has moderate or low bandwidth;
uses switching techniques;

On this page: [ basics | hardware | protocols ]

Basics

In this section: [ LAN topologies | hubs and switches | LAN pros & cons | WAN's ]

A network consists of two or more computers that are connected together such that they may exchange data between them.

If the connection is physical , such as a cable , and if the network extends over a small geographical area , such as a single building or site, it is usually referred to as a Local Area Network or LAN . A LAN is often a broadcast network - every station on the network can potentially receive all transmissions.

If the connection is via some sort of communication system , such as the telephone system or via satellite , it is usually known as a Wide Area Network or WAN and usually extends over large distances . A WAN is a circuit-based network - a specific path is established between sender and receiver for all or part of the data being transmitted.

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LAN Topologies
The significant LAN topologies (shapes) are star and bus (ring topology is not mentioned in the program guide). They may be connected to other networks via gateways and a large network will probably be a hybrid type - ie it will involve both of the standard topologies. A large LAN may thus have subnetworks linked by routers ( bridges ) and gateways . A gateway differs from a bridge in its ability to connect subnetworks using different protocols .

Star
In this configuration the individual computers (or workstations) are connected to the server via a hub (or to the servers via hubs). A hub is a repeater which sends signals to the devices (computers, printers) which are attached.

This configuration is expensive since it uses a lot of cabling ( twisted pair which is more expensive but allows faster transmission than simple coaxial ). It is also vulnerable to a hub failure. However if one cable breaks the other computers still operate.

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Hubs and switches
The hub is often a simple switching device. In the broadcast configuration, the hub sends every signal from the server to every connected workstation . This does not give a great advantage over a bus network (see below). This is known as a shared-medium hub . Only one station can transmit at one time.

However, a switched hub allows a particular data packet to be sent to a single destination. In this system multiple stations can transmit simultaneously , increasing the speed of the LAN.

Printers and print servers (computers or, these days, small control devices) may be connected directly to the hub to handle the print queue (jobs sent by workstations which are waiting to print).

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Bus
The cheapest form of network which uses a common cable (typically coaxial ) to which all devices are connected. The cable must be properly terminated so that signals are absorbed and don't bounce up and down the bus. 

When pc's communicate on a LAN the signal is sent over the wire ( broadcast ) unless another pc is already transmitting. All connected devices sense the signals but only the intended recipient decodes it. If two devices transmit at the same time a collision occurs and after a random time interval they try again. Bus networks can be rather fragile in that a bad connection as well as an actual break can bring down the entire network .

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Some pros and cons of LAN's

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WAN 's
As discussed above a wide area network, as its name suggests, can be spread over a large geographical area - countrywide or even worldwide. The key distinction from a LAN is the use of a communication system of some kind.

At it's simplest the WAN could use the existing telephone system, in which case some form of modem is required (see below). Othe systems might be satellite -based, use microwave transmitters or even use dedicated links (these might be leased from a telephone company for the exclusive use of one client).

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Hardware

Definitions (from the IB Computer Science Guide - adapted and re-printed by permission of Pearson Education from Glossary of Computer Science Terms , The British Computer Society , 2002):

According to William Stallings (see base of page for reference), a router operates on an internet (a set of interconnected networks) and has the capability to decide between different routes by wich data can be sent across the network. Hubs and switches , in contrast, operate only on LAN's.

Cables etc
The medium of transmission may vary between simple cable ( coaxial or twisted pair ), fibre optic cables, radio and microwave links (land-based), satellite links and laser or infra-red links. The table gives some information on different ways of transmitting data around networks of different kinds:

We don't necessarily have the latest data on every device available on the market (you can look in computer journal's or surf the web for this). However, there is already more than you are likely to need for an IB question on the topic. Just remember to make comparisons rather than try to quote absolutes which are misleading:

This.. 

 "Twisted pair is often used in LAN's because of its higher bandwidth than coaxial cable, although twisted pair is more expensive."

..is better than this

 "Twisted pair cable is expensive and fast."

(Compared to what? satellite, tortoise, superman?)

POTS and Modems
The Plain Old Telephone System is often used when connecting WAN's.

A modem ( mo dulator/ dem odulator) converts a computer's digital data (stream of bits) into analogue (continuous wave) data which can then be transmitted over the telephone line.

Speeds are measured in bps ( bits per second ) and the current limit is around 56K . Other forms of connection using the telephone system are ISDN (International Standard Digital Network with transfer rates of up to 128K) and ADSL (Asynchronous Digital Subscriber Line) with incoming rates of about 2.5 Mb and outgoing rates of 512K). None of these rates are achieved in practice. You do not need to know the technical details of these systems for your IB examinations.

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Protocols

A protocol is a set of rules - an agreed way of communicating data between devices. Some elements of a protocol are:

• transmission speed
• structure of data (eg where is the destination address, data part, sequence number)
• parity type used
• block character check frequency
• character code in use
• send/receive signals

Clearly, if one station sends at twice the speed the other station is expecting, the receiver will miss every other data block.

Specific details of parity checks and checksums are to be found on the next page under integrity.

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related: [ Common core home | previous: types of processing | next: networks (more) ]

Further reading:
Data & Computer Communications, William Stallings,
Prentice Hall, 2000, ISBN 0-13-186388-2.

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