Networks and network models

Computer Science AS · Communication. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A network links computers so that they can share files, printers, software and an internet connection. A LAN covers a small area such as one building and is usually owned by the organisation that uses it. A WAN covers a large area and joins LANs together, often over links rented from other companies.

In the client-server model a server holds the files and services, and clients send requests to it. In the peer-to-peer model every computer has equal status and can both share and use resources.

You must also know how devices are arranged (topology), what cloud computing offers, and when to choose cables or wireless links.

Rules to remember

Client-server: central storage, security and backup; needs a server and someone to manage it. Peer-to-peer: cheap and simple for a few computers; each user manages their own files and security
Thin client: relies on the server for processing and storage. Thick client: does most of its own processing and can work without the server
Bus: all devices share one backbone cable; a break in the backbone stops the network, and the shared cable allows collisions
Star: each device has its own cable to a central switch, which sends a packet only to the destination; one cable failing affects one device
Mesh: devices have many links to each other, so a packet can take another route if a link fails. Hybrid: a mixture of topologies
Public cloud: services from a provider, shared by many customers over the internet. Private cloud: used by one organisation only
Fibre-optic cable: higher bandwidth, less signal loss over distance and no electrical interference compared with copper; wireless gives mobility but is slower and easier to intercept

Common mistake

Students say that a star network fails if any one cable breaks. Only the device on that cable is cut off; the whole network fails only if the central switch fails.

Worked example

A school has 30 computers in one room and wants a network where one faulty cable does not stop the lesson. Should it choose a bus or a star topology?

In a bus, all 30 computers share one backbone cable, so a break in it stops every computer.
In a star, each computer has its own cable to the switch, so a faulty cable affects only one computer.
A star also avoids the collisions of a shared cable, because the switch sends each packet only to its destination.

Answer: A star topology, with every computer connected to a central switch.

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