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CAIE 9618 · Paper 1 · Topic 1.2.2

Network
Topologies

Bus · Star · Ring · Mesh · Physical vs Logical Topology

CSZone Cambridge International AS & A Level Computer Science 9618
Learning Objectives

By the end of this video you will be able to...

Draw and describe bus, star, ring, and mesh topologies
Compare advantages and disadvantages of each topology
Distinguish between physical and logical topology
Explain how topology affects network performance and fault tolerance
Bus Topology

All Devices on a Single Cable

ADVANTAGES
Simple and cheap to install
Requires less cable than other topologies
Easy to add new devices
DISADVANTAGES
Cable failure = entire network fails
Collisions occur when multiple devices transmit
Performance degrades as more devices added
Limited cable length and number of devices
🔌 A single backbone cable (bus) runs through all devices. Each end has a terminator to prevent signal reflection.
Star Topology

All Devices Connect to a Central Switch

ADVANTAGES
One device failure does NOT affect others
Easy to add new devices without disruption
Easy to diagnose faults (centralised)
Better performance — no collisions (full duplex switch)
DISADVANTAGES
Central switch failure = entire network fails
Requires more cable than bus
Switch is expensive
Most common modern LAN topology — used in virtually all Ethernet networks today
Mesh Topology

Every Device Connected to Every Other

FULL MESH
Every device directly connected to every other
n devices → n(n-1)/2 connections
Maximum redundancy and fault tolerance
PARTIAL MESH
Only some devices have multiple connections
Compromise between cost and redundancy
Used in internet backbone/WAN routing
✓ Pros: No single point of failure, multiple paths for data
✗ Cons: Very expensive, complex to configure and maintain
Physical vs Logical Topology

How It Looks vs How It Works

PHYSICAL TOPOLOGY
The actual physical layout of cables and devices — how they are physically connected in the real world.
LOGICAL TOPOLOGY
How data actually flows through the network — the path data takes regardless of physical layout.
KEY EXAMPLE
A network may be physically wired as a star (cables run to a central switch) but logically operate as a bus (data broadcast to all devices simultaneously) — e.g. an older hub-based network.
Exam Practice

Cambridge-style questions

Question 1
A school is setting up a new computer network. Discuss the suitability of a star topology for this purpose. Include advantages and one disadvantage.
4 marks
✓ MARK SCHEME
1 mark
Fault isolation: failure of one device/cable does not affect the rest of the network
1 mark
Easy management: faults easy to locate and fix from the central switch
1 mark
Good performance: switch prevents collisions, each device has dedicated bandwidth
1 mark
Disadvantage: if the central switch fails, the entire network fails / switch is a costly single point of failure
Exam Practice
Question 2
Explain the difference between physical and logical topology. Give an example where they differ.
3 marks
✓ MARK SCHEME
1 mark
Physical topology: the actual arrangement of cables/devices in the real world
1 mark
Logical topology: the path/flow of data through the network
1 mark
Example: a network physically wired as a star (to a hub) but logically operating as a bus (hub broadcasts to all)
Common Mistakes

Don't lose easy marks

1
Saying "star topology has no single point of failure" — the central switch IS a single point of failure. If it fails, the entire network fails. This is the key disadvantage of star over mesh.
2
Confusing bus with a "line" topology — in bus topology, each end needs a terminator. Data travels along the bus and every device receives it, not just the target. Without terminators, signals reflect and corrupt data.
3
Not distinguishing physical from logical — in exam questions about topology, always specify whether you mean physical (how cables are laid) or logical (how data flows). Mixing these up loses marks.
Topic Summary — 1.2.2

What You Need to Know

TopologyKey FeatureMain AdvantageMain Disadvantage
BusSingle backbone cableCheap, simpleCable failure kills network
StarCentral switchFault isolation, fastSwitch = single point of failure
MeshMultiple pathsHighly fault tolerantVery expensive, complex
Physical vs Logical: physical = cable layout; logical = data flow path. They can differ (star physical, bus logical).
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1.2.3
MAC Addresses, IP Addresses & Wireless
Addressing · SSID · Encryption · CSMA/CA
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