Network Topologies: Star, Mesh, and Bus

This lesson covers the three network topologies you need to know for OCR J277 Section 1.3.1: star, mesh, and bus. A topology describes the physical or logical arrangement of devices in a network.

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What Is a Network Topology?

A network topology is the layout or structure describing how devices (nodes) in a network are arranged and connected. The topology affects the network's performance, reliability, cost, and ease of maintenance.

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Star Topology

Description

In a star topology, all devices are connected to a central device (usually a switch or hub). Devices do not connect directly to each other — all communication passes through the central device.

graph TD
  SWITCH["SWITCH"]
  PC1["PC1"] --- SWITCH
  PC2["PC2"] --- SWITCH
  SWITCH --- PC3["PC3"]
  SWITCH --- PC4["PC4"]

How It Works

1. Device A wants to send data to Device B.
2. Device A sends the data to the central switch.
3. The switch reads the destination address and forwards the data only to Device B.

Advantages of Star Topology

Advantage	Explanation
Easy to add/remove devices	New devices can be connected without disrupting the network
Fault isolation	If one cable or device fails, only that device is affected; the rest of the network continues working
Good performance	The switch sends data only to the intended recipient, reducing unnecessary traffic
Easy to troubleshoot	Faults can be identified and fixed quickly

Disadvantages of Star Topology

Disadvantage	Explanation
Central point of failure	If the switch/hub fails, the entire network goes down
More cabling	Each device needs its own cable to the central device, increasing cost
Cost	The central switch/hub adds to the cost

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Mesh Topology

Description

In a mesh topology, devices are connected to multiple other devices. There are two types:

Type	Description
Full mesh	Every device is connected directly to every other device
Partial mesh	Some devices are connected to all others, but some are only connected to a few

graph TD
  PC1["PC1"] --- PC2["PC2"]
  PC1 --- PC3["PC3"]
  PC1 --- PC4["PC4"]
  PC2 --- PC3
  PC2 --- PC4
  PC3 --- PC4

How It Works

• Data can travel from one device to another via multiple paths.
• If one connection fails, data can be rerouted through an alternative path.
• This makes mesh networks highly resilient.

Advantages of Mesh Topology

Advantage	Explanation
No single point of failure	If one connection fails, alternative paths exist
High reliability	Multiple routes mean the network can tolerate faults
Data can be transmitted simultaneously	Multiple paths allow parallel data transmission
Good for critical systems	Used where downtime is unacceptable

Disadvantages of Mesh Topology

Disadvantage	Explanation
Expensive	Requires many cables and network interfaces
Complex	Difficult to set up and manage
Impractical for large networks	The number of connections grows rapidly as devices are added
Maintenance	More connections mean more to maintain and troubleshoot

Number of connections in a full mesh: For n devices, the number of connections = n(n-1)/2.

• 4 devices: 4(3)/2 = 6 connections
• 10 devices: 10(9)/2 = 45 connections
• 100 devices: 100(99)/2 = 4,950 connections (clearly impractical!)

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Bus Topology

Description

In a bus topology, all devices are connected to a single central cable called the backbone (or bus). Data is sent along the backbone and received by all devices, but only the intended recipient processes it.

graph TD
  T1["Terminator"] --- BACKBONE["Backbone cable"]
  PC1["PC1"] --- BACKBONE
  PC2["PC2"] --- BACKBONE
  PC3["PC3"] --- BACKBONE
  PC4["PC4"] --- BACKBONE
  BACKBONE --- T2["Terminator"]

How It Works

1. A device sends data along the backbone.
2. The data travels in both directions along the cable.
3. All devices on the network receive the data.
4. Only the device with the matching destination address processes the data; others ignore it.
5. Terminators at each end of the backbone absorb the signal to prevent it bouncing back.

Advantages of Bus Topology

Advantage	Explanation
Low cost	Requires less cabling than star or mesh
Simple to set up	Easy to install for small networks
Easy to extend	New devices can be connected to the backbone

Disadvantages of Bus Topology

Disadvantage	Explanation
Single point of failure	If the backbone cable fails, the entire network goes down
Collisions	If two devices transmit at the same time, a collision occurs and data must be resent
Performance	As more devices are added, the network slows down due to increased collisions
Security	All devices receive all data, making it less secure
Difficult to troubleshoot	Faults in the backbone are hard to locate

OCR Exam Tip: Star topology is by far the most common in real-world LANs. Bus topology is largely obsolete but is still tested in the exam. Mesh is used for critical systems (e.g. the internet backbone).

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Topology Diagrams

The following diagrams show the structure of each topology:

graph TD
    subgraph Star Topology
        SW["Switch"] --- S1["PC1"]
        SW --- S2["PC2"]
        SW --- S3["PC3"]
        SW --- S4["PC4"]
    end

graph TD
    subgraph Bus Topology
        B1["PC1"] --- BACK["Backbone Cable"]
        B2["PC2"] --- BACK
        B3["PC3"] --- BACK
        B4["PC4"] --- BACK
        T1["Terminator"] --- BACK
        BACK --- T2["Terminator"]
    end

graph TD
    subgraph Mesh Topology
        M1["PC1"] --- M2["PC2"]
        M1 --- M3["PC3"]
        M1 --- M4["PC4"]
        M2 --- M3
        M2 --- M4
        M3 --- M4
    end

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Comparing Topologies

Feature	Star	Mesh	Bus
Central device	Yes (switch)	No	No (backbone cable)
Reliability	Good (unless switch fails)	Excellent (multiple paths)	Poor (backbone failure = total failure)
Cost	Moderate	High	Low
Performance	Good	Good	Degrades with more devices
Scalability	Easy to expand	Difficult	Moderate
Modern use	Most common for LANs	Internet backbone, critical systems	Rarely used now

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Summary