1.3.1 Networks and topologies — OCR GCSE Computer Science revision notes

Types of network

Figure: LAN and WAN

	LAN — Local Area Network	WAN — Wide Area Network
Area covered	A single site — a home, school or office building	A large geographical area, often between cities or countries
Infrastructure	Owned by the organisation using it	Uses infrastructure leased from telecommunications companies
Typical speed	Faster	Slower, because data travels much further
Examples	A school network, a home Wi-Fi network	The internet, a bank's network linking its branches

The ownership row is the distinction that survives scrutiny — a LAN's cabling belongs to whoever runs it, a WAN's does not.

Factors that affect the performance of a network:

• Number of devices connected — more devices share the same bandwidth, so each gets less and the network slows. • Bandwidth — the amount of data that can be transmitted at once. Higher bandwidth means better performance. • Type of connection — wired connections are generally faster and more reliable than wireless. • Distance and interference — signal quality falls with distance from a wireless access point, and walls, other devices and physical obstructions all degrade it. • Amount of data being transferred — streaming video demands far more than sending text.

Bandwidth: the amount of data that can be transmitted at once; higher bandwidth means better network performance

Client-server and peer-to-peer

Figure: The two models

	Client-server	Peer-to-peer
Roles	Clients request services; a central server provides them	Every computer is equal, acting as both client and server
Where files are stored	Centrally on the server	On individual machines, shared directly
Backups and security	Managed centrally, so easier and more consistent	Each user is responsible for their own
Cost	Higher — needs a dedicated server and someone to manage it	Lower — no server needed
If one machine fails	If the server fails, the whole network loses its services	Only that machine's files become unavailable
Suits	Schools and businesses, where control matters	Small home networks, or sharing directly between a few machines

Hardware needed to connect computers into a LAN

Figure: The five pieces of hardware and what each does

Hardware	Task performed
Wireless access point	Allows devices to join the network wirelessly, by sending and receiving radio signals
Router	Connects one network to another — typically a LAN to the internet — and directs data towards its destination
Switch	Connects devices within a LAN and sends data only to the device it is addressed to
NIC — Network Interface Controller/Card	The component inside a device that lets it connect to a network at all
Transmission media	The cables or wireless signals the data actually travels over — such as Ethernet cable or fibre optic

The distinction most often tested: a switch connects devices within one network, a router connects different networks together.

The internet, DNS and the cloud

The internet is a worldwide collection of computer networks — a network of networks. It is not the same as the World Wide Web, which is the collection of web pages carried over it.

DNS — the Domain Name Service. People remember names like prepgrid.co.uk, but computers route data using IP addresses. A Domain Name Service is made up of multiple Domain Name Servers, and its role is converting a URL into the IP address it corresponds to.

How it works: the browser sends the URL to a DNS server. If that server holds the matching IP address it returns it; if not, it passes the request to another DNS server until one does. The browser then uses the IP address to request the page from the web server.

Hosting means storing files — usually a website — on a server that is permanently connected to the internet, so that others can access them at any time. Businesses often pay a hosting company rather than running their own server.

Servers and clients. A server provides a service; a client requests and uses it. A web server sends web pages; a file server provides file storage and retrieval; a print server manages printing. The device asking for the page or the file is the client.

The cloud means remote provision of services over the internet — storage, software or processing power — run on someone else's servers rather than your own.

Figure: Advantages and disadvantages of the cloud

Advantages	Disadvantages
Files can be accessed from anywhere with an internet connection	No access at all without an internet connection
Storage can be increased or decreased as needed	Ongoing subscription costs rather than a one-off purchase
The provider handles backups, security and maintenance	Data is held by a third party, raising privacy and security concerns
No need to buy and maintain your own servers	Performance depends on connection speed
Easy to share files and collaborate	If the provider has an outage or closes, access may be lost

The internet: a worldwide collection of computer networks — a network of networks — which is not the same as the World Wide Web, the collection of web pages carried over it
DNS: the Domain Name Service, which translates the domain names people remember into the IP addresses computers use to route data

Star and mesh topologies

The same six devices arranged both ways — and the cabling difference that drives the comparison.

A star topology connects every device to a central switch. A mesh topology connects devices directly to each other, with a full mesh connecting every device to every other one.

Star • Advantages: few cables, so cheap and simple to install; each device has its own connection, so performance is good and does not drop as more are added; a single cable failure affects only one device; easy to add or remove a device. • Disadvantage: the central switch is a single point of failure — if it fails, the whole network stops.

Mesh • Advantages: no single point of failure, because data can be rerouted around a break; very reliable; performance holds up as devices are added, since there is no shared central point. • Disadvantages: a great deal more cabling, so expensive to install and difficult to maintain — and the number of connections grows very quickly as devices are added.

Worked example

A hospital needs a network where a single equipment failure must not cut off any department. Recommend a topology.

Identify the requirement: no single failure may disconnect anyone, which is a reliability requirement above all.
Consider star: cheap and fast, but the central switch is a single point of failure — exactly what the hospital cannot accept.
Consider mesh: multiple routes between devices mean data reroutes around any break.
Acknowledge the cost, since a recommendation that ignores it is incomplete.
Commit.

Answer: A mesh topology. Multiple paths between devices mean there is no single point of failure and data can reroute around a break — which the hospital's requirement demands. The cost is far more cabling and a harder installation, but reliability is the priority here in a way it would not be in an ordinary office.

Star topology: a network layout connecting every device to a central switch
Mesh topology: a network layout connecting devices directly to each other, a full mesh connecting every device to every other one

Before you practise