AQA 7517 · A-Level Computer Science · ~16 min read
What is a Network?
A network is two or more computers connected together to share resources and data. Networks allow file sharing, communication, shared peripherals, centralised storage, and internet access.
LAN vs WAN
Feature
LAN (Local Area Network)
WAN (Wide Area Network)
Geographic scope
Single site (room, building, campus)
Multiple sites, countries, global
Ownership
Owned by one organisation
Leased from telecoms providers
Speed
High (up to 10 Gbps on modern Ethernet)
Slower, variable
Example
School network, home network
The internet, bank branches connected nationally
Cable type
Ethernet (twisted pair / fibre)
Fibre, satellite, leased lines
Network Hardware
Router: connects different networks; forwards data packets between networks using IP addresses; connects LAN to WAN (internet)
Switch: connects devices within a LAN; uses MAC addresses to send frames only to intended device; more efficient than hub
Hub: sends all incoming data to all connected devices (broadcasts) — inefficient, replaced by switches
Wireless Access Point (WAP): extends LAN wirelessly using IEEE 802.11 (Wi-Fi)
NIC (Network Interface Card): hardware in each device with a unique MAC address; connects device to network
Modem: modulates/demodulates analogue signals to digital — needed for DSL broadband over telephone lines
Firewall: monitors and filters incoming/outgoing traffic based on rules; can be hardware or software
Network Topologies
Topology
Structure
Advantages
Disadvantages
Bus
All nodes share single backbone cable; terminators at ends
Simple, cheap to set up
Collision prone; single cable failure = full network down
Star
All nodes connect to central switch/hub
Device failure doesn't affect others; easy to add devices
Switch failure = whole network down; more cable needed
Ring
Each node connects to two neighbours forming a ring
Equal access, no collisions (with token passing)
Single node failure can break ring; difficult to add devices
Mesh
Every node connected to every other (full mesh) or some (partial)
Highly fault tolerant; multiple data paths
Very expensive; lots of cable
Transmission Media
Copper twisted pair: RJ45 Ethernet cables; cheaper; susceptible to electromagnetic interference; up to 100m per segment
Fibre optic: light pulses in glass/plastic; immune to EMI; very high speed (100+ Gbps); expensive; harder to install
Wireless (Wi-Fi): radio waves; no cable needed; subject to interference, walls reduce signal; security risk (eavesdropping)
Bandwidth and Latency
Bandwidth: maximum data transfer rate of a network connection (Mbps, Gbps)
Latency: time delay for data to travel from source to destination (milliseconds); affected by distance and routing
Exam tip: AQA 7517 may ask you to "compare LAN and WAN" or "describe the role of a router vs a switch". Key distinction: routers work at IP (Network) layer and connect different networks; switches work at MAC (Data Link) layer within one network. Star topology is most common in modern LANs — know its advantages and what happens when the central switch fails.
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Worksheet — 4.9.1 Networks Introduction
8 questions · instantly marked · AQA 7517 standard
Q1State two differences between a LAN and a WAN.[2]
✅ Mark scheme
Mark scheme
Any two differences (1 mark each point + 1 mark for elaboration): Geographic scope — LAN covers a single site (e.g. school) [1], WAN spans multiple sites/countries (e.g. internet) [1]; Ownership — LAN owned by one organisation [1], WAN uses leased lines from telecoms providers [1]; Speed — LAN typically faster (up to 10 Gbps) [1], WAN slower and variable [1].
Q2Explain the difference between a router and a switch.[2]
✅ Mark scheme
Mark scheme
Router: connects different networks together [1]; uses IP addresses to forward data packets between networks (e.g. connects LAN to the internet) [1]. Switch: connects devices within a single LAN [1]; uses MAC addresses to send data frames only to the intended device on that network [1].
Q3Describe one advantage and one disadvantage of a star network topology.[3]
✅ Mark scheme
Mark scheme
Advantage: if one node (device) fails, the rest of the network is unaffected [1] — other devices can still communicate through the central switch [1]. Disadvantage: if the central switch fails, the entire network goes down [1]; also requires more cable than a bus topology [1].
Q4Compare fibre optic and copper twisted pair cables for use in a network.[3]
✅ Mark scheme
Mark scheme
Fibre: uses light pulses in glass/plastic [1]; immune to electromagnetic interference [1]; much faster (100+ Gbps) and suitable for longer distances [1]; but more expensive and harder to install [1]. Copper: cheaper and easier to install [1]; susceptible to EMI [1]; maximum ~100m per segment, lower bandwidth [1].
Q5Explain the role of a Network Interface Card (NIC) and state what makes each NIC unique.[2]
✅ Mark scheme
Mark scheme
NIC: hardware component in each device that connects it to the network [1]; handles sending and receiving data frames [1]. Each NIC has a unique MAC (Media Access Control) address assigned by the manufacturer [1].
Q6State one advantage and one disadvantage of using wireless (Wi-Fi) instead of a wired connection in a LAN.[2]
✅ Mark scheme
Mark scheme
Advantage: no cables needed — devices can connect from anywhere within range [1]. Disadvantage: slower than wired; susceptible to interference from walls/other devices; less secure (data can be intercepted) [1].
Q7Explain the difference between bandwidth and latency.[3]
✅ Mark scheme
Mark scheme
Bandwidth: the maximum amount of data that can be transmitted over a network connection per unit time [1]; measured in Mbps or Gbps [1]. Latency: the time delay for data to travel from the sender to the receiver [1]; measured in milliseconds (ms) — affected by physical distance and routing [1]. High bandwidth does not mean low latency — they are independent properties.
Q8A school is designing a new network. Justify whether they should use a star or bus topology.[3]
✅ Mark scheme
Mark scheme
Star recommended [1]: fault tolerance — one device failing does not affect others [1]; easier to add devices [1]; central switch directs traffic more efficiently [1]. Bus is cheaper to install but single cable failure = entire network down, and collisions reduce performance — not suitable for school with many users [1]. Award marks for justified recommendation with comparison points.
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Mini Test — Networks Introduction
10 questions · 10 minutes
⏱ 10:00
Section A — Multiple Choice [5 marks]
Q1A router's primary function is to:
Q2In a star topology, if the central switch fails:
Q3Which transmission medium is immune to electromagnetic interference?
Q4A WAN differs from a LAN because:
Q5Latency in a network refers to:
Section B — Short Answer [5 marks]
Q6State the difference between a hub and a switch.
Mark schemeHub: broadcasts data to all connected devices [1]. Switch: sends data only to the intended device using MAC addresses [1] — more efficient.
Q7What is a MAC address? Where is it stored?
Mark schemeA unique hardware identifier for a NIC (network interface card) [1]; assigned by the manufacturer and stored on the NIC [1].
Q8Give one advantage of a mesh topology over a star topology.
Mark schemeMesh provides multiple paths between nodes — if one link fails, data can be rerouted [1], making it highly fault-tolerant [1].
Q9State the role of a modem in a network.
Mark schemeModulates/demodulates signals — converts digital data to analogue for transmission over telephone lines (DSL), and converts analogue back to digital on receipt [1].
Q10State one security risk of using Wi-Fi and one way to mitigate it.
Mark schemeRisk: data transmitted wirelessly can be intercepted by unauthorised users [1]. Mitigation: encrypt the network using WPA2/WPA3 so intercepted data is unreadable [1].