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Networks - LAN topologies 5.3.4



Revision points: (Full course)
Candidates should be able to:
  • describe different network topologies, identifying briefly the relative advantages of each such as star, ring, bus;

LAN Network topologies:

Network topology is the name given to the way in which the devices (called nodes) are physically connected in a network.

There are three common network topologies, called ring, line (bus) and star.

You will be expected to briefly describe the features of each one, know their advantages and draw simple line diagrams to represent then.

Ring topology:

Network topologies - a RING networkIn a ring topology, the nodes are connected in a ring and data travels in one direction using a control signal called a 'token'.

Advantages:

  • Not greatly affected by adding further nodes or heavy network traffic as only the node with the 'token' can transmit data so there are no data collisions.
  • Relatively cheap to install and expand.

Disadvantages:

  • Slower than a star topology under normal load.
  • If the cable fails anywhere in the ring then the whole network will fail.
  • If any node fails then the token cannot be passed around the ring any longer so the whole network fails..
  • The hardest topology to troubleshoot because it can be hard to track down where in the ring the failure has occurred.
  • Harder to modify or expand because to add or remove a node you must shut down the network temporarily.
  • In order for the nodes to communicate with each other they must all be switched on.

Bus (line) topology:

Network topologies - a BUS (LINE) networkNodes are connected to a main (bus) cable. If data is being sent sent between nodes then other nodes cannot transmit.  If too many nodes are connected then the transfer of data slows dramatically as the nodes have to wait longer for the bus to be clear.

Advantages:

  • The simplest and cheapest to install and extend.
  • Well suited for temporary networks with not many nodes.
  • Very flexible as nodes can be attached or detached without disturbing the rest of the network.
  • Failure of one node does not affect the rest of the bus network.
  • Simpler than a ring topology to troubleshoot if there is a cable failure because sections can be isolated and tested independently.

Disadvantages:

  • If the bus cable fails then the whole network will fail.
  • Performance of the network slows down rapidly with more nodes or heavy network traffic.
  • The bus cable has a limited length and must be terminated properly at both ends to prevent reflected signals.
  • Slower than a ring network as data cannot be transmitted while the bus is in use by other nodes.

Star topology:

Network topologies - a STAR networkIn this type of network, a central computer (server) usually forms the main node and the subsidiary nodes are connected to it and to each other through a switch or hub.

Advantages:

  • The most reliable because the failure of a node or a node cable does not affect other nodes.
  • Simple to troubleshoot because only one node is affected by a cable break between the switch and the node.
  • Adding further nodes does not greatly affect performance because the data does not pass through unnecessary nodes.
  • Easily upgraded from a hub to a switch or with with a higher performance switch.
  • Easy to install and to expand with extra nodes.

Disadvantages:

  • Uses the most cable which makes it more expensive to install than the other two topologies.
  • The extra hardware required such as hubs or switches further increases the cost.
  • As the central computer controls the whole system, the whole system will be affected if it breaks down or if the cable link between it and the switch fails.
  • If the switch, the link to the server or the server itself fails then the whole network fails.

Network topology summary:

Topology Performance with few nodes or low network traffic Change in performance with many nodes or high network traffic Ease of troubleshooting (compared to other topologies) Cost of installation (compared to other topologies) Ease of setting up and modifying (compared to other topologies) Problems to the network caused by cable or node failure
LINE (BUS) Medium Most affected Fairly easy Low Easy to set up and modify Failure of the bus cable causes total failure. Failure of a node has no affect.
RING Slow Least affected Hard High Easy to set up but harder to modify Cable or node failure causes total network failure.
STAR Fast Some affect but the switch/hub can be upgraded easily Easy High Easy to set up and modify Cable or node failure only affects that node.  Failure of the hub/switch or the server causes total network failure.

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