3.3: Network Topology

Exam Board:

Eduqas / WJEC

Specification:

2016 + 

What is a network topology?

Network topology refers to layout of computer systems on a network.

 

Devices in a network topology diagram are often called 'nodes'.

What are the different types of network topology?

Bus Topology

The nodes are connected to a bus (a central cable along which all data is transferred across the network).

How it works: 

  • Data packets are sent along the main cable (sometimes known as the 'backbone') from the source computer to each other system in turn.

  • Each system checks the destination address of the data packets.

  • If the addresses match then the data is accepted otherwise it is passed on to the next system.

  • Terminators are required at both ends of the bus to mark the end of the cable.

Advantages: 

  • Because of the simple layout, it is easy to attach another system to the main cable without disrupting the whole network.
     

  • A bus topology is quick to set up once the main cable has been established making it optimal for temporary networks.
     

  • A bus topology is cost-effective because it usually contains less cabling than other topologies and requires no additional hardware (like a hub or switch).

Disadvantages:

  • Poor security as data packets are passed on to each system on the network.
     

  • Data collisions are likely - this is when two systems attempt to transfer data on the same line at the exact same time. Resending the data wastes time and slows down the network.
     

  • The main cable will only have a limited length which can become crowded and slows the  network speed as more systems are attached. The main cable must also be terminated properly.

Ring Topology

Computer systems are connected together in a single loop.

 

How it works:

  • Packets are transferred around the ring in one direction, passing from one computer system to the next in a loop.

  • As the packets arrive at each computer system, the computer checks the destination address contained in the data packet to see if it matches its own address.

  • If the addresses match the computer accepts and processes the data packet, otherwise it passes it on to the next system.

Advantages:

  • Data collisions are avoided as data packets are transmitted in one direction around the ring.
     

  • Attaching more systems to a ring topology won't affect the transfer speed (bandwidth) as much as other layouts like a bus topology because the data is transferred at a consistent speed.

Disadvantages:

  • If any system on the network fails then the whole network fails as the loop is broken and data can't be transferred to all systems.
     

  • To add a new system to a ring topology the network must be temporarily shut down.

Star Topology

Each computer system is connected to a central device, usually a hub or switch.

 

How it works:

  • Each computer system is connected to the central hub or switch and transfers its data packets there.

  • The hub or switch looks at the destination address and transfers the packets directly to the intended computer.

Advantages:

  • A star topology has improved security because data packets are sent directly to and from the hub / switch in the centre and not necessarily all devices like in a bus or ring topology.
     

  • New systems can be attached directly to the central system so the network doesn't need to be shut down. System failures of attached computers won't usually cause complete network failure.
     

  • Transfer speeds are generally fast in a star topology as there are minimal network collisions.

Disadvantages:

  • Extra hardware (the hub or switch) is required to be purchased, installed and maintained.
     

  • If the central system (the hub or switch) fails then the whole network will be unusable until the error is fixed.

Mesh Topology

In a full mesh network, each computer system is connected to every other computer system.

 

There is also a partial mesh network where only some nodes (e.g. a printer) are connected to every other node.

How it works:

  • Data packets are transferred to the destination address along the quickest path, travelling from node to node.

  • If a pathway is broken, there are many alternative paths that the packets can take.

Advantages:

  • If one cable or system fails then data packets can take an alternative route and still reach the destination address.
     

  • Because of the large possible number of systems and connections, a mesh topology can usually withstand large amounts of data traffic.
     

  • New systems can be added to the network without disrupting the entire topology.

Disadvantages:

  • Because of the possibly large amount of cables required (especially in a full mesh topology) this network layout can be expensive to install and maintain.
     

  • Redundant cabling should be avoided - this is when cables are connected between systems that won't ever need to communicate.

Questo's Questions

3.3 - Network Topology:

1. Draw and label diagrams of all four topologies. [12]

2a. A school currently uses a bus topology but is considering changing to a ring topology. Describe two advantages and two disadvantages of both topologies. [8]

2b. An office currently uses a star topology but is considering changing to a mesh topology. Describe two advantages and two disadvantages of both topologies. [8]

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