Understand LANs and WANs, star and bus topologies, wired and wireless networks, the role of switches, routers and the Wi-Fi standards including CSMA/CA and SSID.

A focused answer to AQA A-Level Computer Science 4.8.2 and 4.8.3, covering LANs and WANs, star and bus topologies, wired versus wireless networks, network hardware, and wireless networking with CSMA/CA and the SSID.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
LANs and WANs
Topologies
Wired versus wireless
Wireless networking: SSID and CSMA/CA

What this dot point is asking

AQA wants you to distinguish LANs and WANs, describe the star and bus topologies, compare wired and wireless networks, and explain wireless networking including CSMA/CA and the SSID.

LANs and WANs

The defining differences are scale and ownership. A LAN is small and owner-controlled, so the organisation manages and secures all of its hardware; a WAN spans large distances and usually leases lines or uses public infrastructure, which the organisation does not own and over which it has less control. This is why data crossing a WAN, such as the internet, generally needs encryption: it travels over equipment the sender does not control and could be intercepted.

Topologies

A switch is the key piece of hardware in a star network: it receives a frame and forwards it only to the port of the intended recipient, rather than broadcasting to everyone, which reduces unnecessary traffic and collisions. This targeted forwarding is a major reason the star topology is fast and dominant in modern wired LANs, whereas the bus topology, where all devices contend for one shared medium, is largely obsolete.

Wired versus wireless

A wired network (typically Ethernet over copper or fibre) is fast, reliable and more secure because access needs a physical connection, but it needs cabling and limits mobility. A wireless network is convenient and supports mobility but is generally slower, less reliable over distance and obstacles, and less secure unless encrypted, because the radio signal can be received by anyone in range. The choice trades convenience and mobility against speed, reliability and inherent security.

Wireless networking: SSID and CSMA/CA

Choose a topology and explain a wireless transmission

Target: recommend a topology for an office and outline how a wireless device sends data.

step 1: Choose the topology

For a modern office, recommend a star topology: each computer has its own cable to a central switch, so a single cable fault isolates only one machine, and the switch forwards traffic efficiently to the intended recipient.

step 2: Note the trade-off

The star's weakness is the central switch as a single point of failure, so a critical office might use a backup switch; the bus topology is rejected because of collisions and whole-network failure if the backbone breaks.

step 3: A device joins the wireless network

A laptop reads the SSID being broadcast and connects to the matching access point, authenticating with the network's encryption (such as WPA).

step 4: The device transmits using CSMA/CA

Before sending, the laptop listens to check the channel is clear (carrier sense); if it is busy it waits a random back-off time, and uses request-to-send and clear-to-send signalling to avoid colliding with other devices, then transmits.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksCompare the star and bus network topologies. In your answer, explain what happens to the rest of the network when a single connection fails in each topology.

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In a star topology every device connects to a central switch by its own cable. If one device's cable fails, only that device is cut off; the rest of the network is unaffected because each has its own link. However, if the central switch fails, the whole network goes down, since all communication passes through it.

In a bus topology all devices share a single backbone cable. If a device's drop connection fails only that device is affected, but if the backbone cable itself fails or a terminator is lost, the whole network goes down, because there is no longer a complete shared medium.

Markers reward the per-device versus whole-network failure analysis for each topology, especially that a star fails wholesale only if the central switch fails, and a bus fails wholesale if the backbone fails.

AQA 20214 marksExplain why a wireless network uses CSMA/CA, describing what the technique does, and state the purpose of the SSID.

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A wireless network uses CSMA/CA (carrier sense multiple access with collision avoidance) because devices share a single radio channel and cannot reliably detect collisions while transmitting (unlike wired Ethernet). CSMA/CA has a device listen first to check the channel is clear (carrier sense), and use techniques such as request-to-send and clear-to-send signals and a random back-off before transmitting, so collisions are avoided rather than merely detected after they happen.

The SSID (service set identifier) is the name of the wireless network, which devices use to identify and connect to the correct access point among the several that may be in range.

Markers reward the shared-channel and cannot-detect-collisions reason, a description of listening and avoiding collisions, and the SSID as the network name used to connect to the right access point.

Related dot points

Sources & how we know this

AQA A-level Computer Science (7517) specification — AQA (2015)