Fundamentals of computer networks: free AQA GCSE Computer Science practice questions

The mistakes that cost the most marks

Each of these confuses one half of a pair with the other.

• Saying a MAC address and an IP address do the same job. The MAC address is fixed and identifies the device locally. The IP address can change and is used to route between networks.
• Saying HTTPS stops hackers. It encrypts data in transit so intercepted traffic cannot be read. It does nothing about a weak password or malware on your machine.
• Confusing a switch with a router. A switch works within one network. A router connects networks together.
• Saying wireless is always slower without the reasons. The exam wants interference, distance, obstacles and sharing the channel.
• Describing layering as making the network faster. The benefits are that layers can be changed independently and that different vendors can build different layers.
• Saying a bus topology is better because it is cheaper, without saying that one break in the backbone takes down the whole network.

What a network is and what it costs

A network is two or more computers connected so they can share data and resources. The benefits are sharing files, sharing hardware such as printers, central backup and central management of software and users. The costs are the equipment and cabling, the need for someone to manage it, and the fact that one attack or one failure can reach many machines at once. A LAN covers a small geographical area, such as one site, and the organisation usually owns the cabling. A WAN covers a large area and uses infrastructure the organisation does not own, such as leased lines. The internet is the best known example of a WAN.

Wired against wireless, star against bus

Wired connections, usually twisted pair copper or fibre optic, are faster, more reliable and more secure, because you have to physically connect to them. Wireless is more convenient and cheaper to install, but it is slower, more affected by distance and obstacles, and easier to intercept. In a star topology every device has its own connection to a central switch. One cable failing affects only that device and performance stays good under load, but it needs more cable and the central device is a single point of failure. In a bus topology all devices share one backbone cable with terminators at each end. It uses less cable and is cheap, but a break in the backbone brings the whole network down, and performance falls as traffic rises because devices compete for the one cable.

Protocols and layering

A protocol is an agreed set of rules for how devices communicate. Without shared protocols, devices from different manufacturers could not talk to each other. Networks are built in layers, where each layer does one job and provides a service to the layer above. Layering means a change in one layer does not force a rewrite of the others, and different companies can build different layers. TCP splits data into packets, checks they all arrive and reassembles them in the right order. IP handles addressing and routing packets between networks.

The protocols you have to name

HTTP transfers web pages. HTTPS is HTTP with encryption, so data sent to and from the site cannot be read if it is intercepted. FTP transfers files between computers. SMTP sends email. IMAP retrieves email from a server while leaving it there, so the same mailbox looks the same on every device. Encryption on a network scrambles data so that anyone intercepting it cannot read it without the key. On wireless networks that is essential, because the signal is available to anyone in range.

Addresses and hardware

A MAC address is a physical address burned into a network interface card by the manufacturer and does not change. It identifies a device within a local network. An IP address identifies a device on a network and can change, for example when you connect to a different network. It is used to route data between networks. The hardware a network needs is the network interface card in each device, a switch to connect devices within a LAN and send traffic only where it is needed, a router to connect different networks and route traffic between them, and the transmission media that carry the signal. A switch sends a frame only to the port the destination device is on, which is why a switched star network handles heavy traffic much better than a shared bus.