1.2.2 Network Topologies — Cambridge 9618

What is a Network Topology?

A network topology describes the arrangement/layout of devices (nodes) and connections in a network. There are two aspects: physical topology (how devices are physically connected) and logical topology (how data flows through the network).

Bus Topology

All devices connect to a single shared cable called the backbone. Both ends of the backbone are terminated with a terminator to absorb signals and prevent reflection.

Only one device can transmit at a time — collisions handled by CSMA/CD (Carrier Sense Multiple Access/Collision Detection)
Data travels both directions along the bus; all devices receive the signal but only the addressed device accepts it

Advantages	Disadvantages
Simple and cheap to set up (one cable)	Single point of failure — backbone breaks = whole network fails
Easy to add devices	Performance degrades with more devices (collisions)
Uses less cable than star	Data less secure — all devices receive all transmissions
—	Difficult to identify fault location

Star Topology

Each device connects directly to a central switch or hub. All communication passes through the central device.

Switch — more intelligent; sends data only to the destination device (point-to-point)
Hub — broadcasts data to all devices (older technology)

Advantages	Disadvantages
Failure of one cable/device doesn't affect others	Central switch = single point of failure for whole network
Easy to identify and isolate faults	More cable required (one run per device)
Better performance — no collisions with switch	More expensive (switch hardware)
More secure — switch sends only to destination	Dependent on central device performance

Ring Topology

Devices connect in a closed loop. Data travels in one direction (unidirectional), passing through each device in turn.

Uses token passing — a special control frame (token) circulates; a device can only transmit when it has the token
No collisions (unlike bus topology)
Dual-ring: two rings for redundancy (one goes clockwise, one anti-clockwise)

Advantages	Disadvantages
No collisions — token passing prevents simultaneous transmissions	Single ring — one break can bring down whole network
Equal access for all devices (fair)	Slower than star — data must pass through each node
Performs consistently as load increases	Difficult to add/remove devices (must break ring)

Mesh Topology

Every device connects directly to every other device (full mesh) or to multiple other devices (partial mesh).

Maximum redundancy — multiple paths for data
Common in WANs and the internet backbone
Number of connections in full mesh with n nodes: n(n-1)/2

Advantages	Disadvantages
Highly fault-tolerant — data can be re-routed if link fails	Very expensive — many cables/connections needed
Multiple simultaneous paths — high throughput	Complex to set up and manage
No single point of failure	Network Interface Card (NIC) on each device needs many ports

Comparing Topologies

Feature	Bus	Star	Ring	Mesh
Single point of failure	Backbone	Central switch	Any node (single ring)	None
Cost	Low	Medium	Medium	Very high
Performance	Degrades with load	Good (switch)	Consistent	Very high
Fault finding	Difficult	Easy	Moderate	Complex
Scalability	Limited	Good	Limited	Good but expensive

Exam tip: Cambridge exam questions often give a scenario and ask which topology is most suitable. Star is the standard choice for office/school LANs. Bus is used in simple low-cost networks but is largely obsolete. Mesh is used for WANs and where high reliability is needed. Always explain WHY a topology is suitable using its characteristics — don't just name it.

⚠️ Common Mistakes

Confusing hub and switch — a hub broadcasts to all; a switch sends only to the destination device
Saying bus topology has no single point of failure — the backbone is the single point of failure
Forgetting terminators in bus topology — without them, signals reflect back and cause errors
Confusing number of cables needed: full mesh with n nodes needs n(n-1)/2 connections (not n²)

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Worksheet — 1.2.2 Network Topologies

8 questions · instantly marked · Cambridge 9618 standard

Q1State two advantages of a star topology over a bus topology.[4]

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Any two (2 marks each): Failure of one cable/device does not bring down the whole network — unlike bus where backbone failure kills all connections [2]; Star uses a switch so data is sent only to the intended device — more secure and no collisions, unlike bus where all devices receive all data [2]; Faults are easier to identify in star — a problem can be isolated to one connection [2].

Q2Explain the role of a terminator in a bus topology network.[2]

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Terminators are placed at each end of the backbone/bus cable [1]; they absorb signals reaching the end, preventing them from reflecting back along the cable which would cause interference and corrupt data [1].

Q3Explain how token passing works in a ring topology.[3]

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A token (special control frame) circulates continuously around the ring [1]; a device wishing to transmit must wait until it receives the token — it then takes the token off the ring and sends its data [1]; once transmission is complete, the token is released back onto the ring so another device can use it — this prevents collisions [1].

Q4A school wants to set up a LAN for 30 computers. Discuss whether star or bus topology is more suitable.[4]

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Star is more suitable [1]: In a school with 30 computers, heavy concurrent use means bus would suffer many collisions/slow performance [1]; Star provides dedicated connections via a switch, so each computer gets better bandwidth [1]; If one connection fails in star, only that computer is affected — less disruptive in a school environment [1].

Q5Calculate the number of direct connections (cables) needed for a full mesh network with 6 nodes.[2]

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Formula: n(n-1)/2 [1]; = 6×5/2 = 15 connections [1].

Q6Distinguish between a hub and a switch. State one reason why switches are preferred in modern networks.[3]

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A hub broadcasts incoming data to all connected devices [1]; a switch reads the destination MAC address and sends data only to the intended device [1]; Switches are preferred because they reduce unnecessary traffic and improve performance — no collisions and more secure [1].

Q7Explain what is meant by 'physical topology' and 'logical topology'. Why might they differ?[3]

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Physical topology: the actual physical layout of cables and devices [1]; Logical topology: how data flows through the network, which may differ from the physical layout [1]; Example: a network may be physically wired as a star (each device connected to a central hub) but operate logically as a bus (hub broadcasts to all devices) [1].

Q8Explain why mesh topology is used in WANs and the internet backbone but not typically in small office LANs.[3]

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Mark scheme

Mesh is used in WANs for high fault tolerance — if one link fails, data is rerouted via alternative paths [1]; The internet must remain operational even if individual connections fail — no single point of failure [1]; However, mesh is too expensive and complex for small office LANs — many cables/ports needed and difficult to manage for relatively few devices [1].

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What is a Network Topology?

Bus Topology

Star Topology

Ring Topology

Mesh Topology

Comparing Topologies

Worksheet — 1.2.2 Network Topologies

Mini Test — 1.2.2 Network Topologies