Network topologies

The essentials

Topology is the arrangement of computers, cables and devices in a network. The choice of topology affects cost, speed of setting up, reliability and ease of finding faults.

Key points

• Bus: all nodes are connected to a single main cable (backbone), with terminators at both ends. It is cheap and needs little cable, but a break in the backbone stops the whole network and faults are hard to locate. Data travels in both directions.
• Star: every node is connected to a central device (hub or switch). It is easy to install, add nodes and find faults; failure of one cable affects only one node. If the central device fails, the whole network stops. Needs more cable than bus.
• Ring: each node is connected to two neighbours, forming a closed loop; data passes from node to node. A break in the ring can disrupt the network.
• Mesh: each node is connected to many or all other nodes. It is very reliable because there are alternative paths, but it is costly. A fully connected mesh of n nodes needs n(n − 1)/2 links. For 5 nodes: 5 × 4 ÷ 2 = 10 links.
• Tree (hierarchical): a combination of bus and star; groups of star networks are connected to a backbone. It is easy to expand, but failure of the backbone affects the branches.

Exam tips

• Case study: "central server room with buildings connected to it" suggests star; "highly reliable with redundant paths" suggests mesh.
• Practise the mesh formula with small numbers.

Common mistakes

• Saying failure of one node stops a star network. Only failure of the central device does.
• Using n(n − 1) instead of n(n − 1)/2 for the number of links in a full mesh.