AS Computer Science 9618: Comparing Bus, Star and Mesh Topologies

BrightDesk Architects has 12 computers in one open-plan office, all within a single building. The office's IT technician, Priya, is choosing how to cable the computers together to form a local network, and is comparing three cabling arrangements: a bus topology, a star topology and a full mesh topology.

(a) State whether the network Priya is designing is a LAN or a WAN, and give one reason for your answer based on the scenario. [2]

(b) Priya decides to connect all 12 computers to a central switch, so that each computer has its own individual cable running back to the switch (a star topology), rather than connecting all 12 computers to a single shared backbone cable that they all tap into (a bus topology). Each computer connects to its cable using its network interface card (NIC). Explain one advantage and one disadvantage of Priya's star topology choice compared with a bus topology, in the context of this 12-computer office. [4]

(c) A colleague suggests wiring the 12 computers as a full mesh instead, with a dedicated cable directly connecting every computer to every other computer. State one advantage this would give over the star topology, and one reason it is not a practical choice for this office. [2]

Part (a): LAN or WAN?

A LAN (Local Area Network) covers a single site, such as one building or one floor of a building. A WAN (Wide Area Network) connects devices or networks that are spread across multiple, geographically separated sites, typically using external telecommunications links (or the internet) to join them.

All 12 of BrightDesk’s computers are in one open-plan office within a single building, so this is a LAN. [2 marks]: [1] for stating “LAN”, [1] for a valid reason tied to the scenario (a single site/building), rather than a reason based on who owns or uses the network.

Part (b): Star topology vs bus topology

In a bus topology, every computer connects to one shared backbone cable, and any data sent travels along that whole cable until the addressed computer picks it up. In Priya’s star topology, every computer instead has its own individual cable running directly to a central switch, connecting through its NIC.

Advantage of the star topology: if one of the 12 individual cables, or one computer, develops a fault, only that single computer loses its connection, the switch keeps forwarding data normally to and from the other 11. This is a clear improvement on the bus topology, where a single break anywhere along the one shared backbone cable disconnects every computer on the network, not just one.

Disadvantage of the star topology: it needs far more cable overall than a bus topology, since each of the 12 computers requires its own dedicated cable run all the way back to the switch, rather than all 12 simply tapping into one shared cable. This makes installation more expensive. The star topology also introduces a new single point of failure at the switch itself: if the switch fails, all 12 computers lose connectivity to each other and to the rest of the network, whereas in the original bus topology there was no equivalent central device to fail.

[4 marks]: [2] for a correctly explained advantage that compares against the bus topology’s actual behaviour, [2] for a correctly explained disadvantage that compares against the bus topology’s actual behaviour.

Part (c): Full mesh topology

Advantage over the star topology: in a full mesh, every computer is directly wired to every other computer, so there are many independent paths through the network and no single point of failure at all, not even a central switch. If any one cable fails, the two computers it connected can still communicate with everything else, and the rest of the network is completely unaffected.

Reason it is not practical here: the number of cables needed in a full mesh grows very quickly as computers are added. For n computers, a full mesh needs n x (n - 1) / 2 separate cables. For BrightDesk’s 12 computers, that is 12 x 11 / 2 = 66 separate cables (and a free NIC port on every computer for each of those connections). Far too much cabling, cost and complexity for an ordinary 12-computer office, and it would only get worse if more computers were added later.

[2 marks]: [1] for a valid advantage tied to redundancy/no single point of failure, [1] for a valid reason it is impractical (cost or the rapidly growing number of cables/ports needed).

Final answers

(a) LAN. All 12 computers are in one building/site.
(b) Advantage: only the affected computer loses its connection if a cable fails (vs the whole network in a bus). Disadvantage: more cabling is needed, and the switch becomes a single point of failure.
(c) Advantage: no single point of failure due to redundant direct links. Impractical because: 66 separate cables would be needed for 12 computers, which is too costly and complex.