Network topologies: bus, star, ring and mesh
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A topology is the shape of the links between devices. Join the same six devices as a bus, a star, a ring, a mesh and a tree, send one message across each, then cut a cable and see what still works.
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01On an old thin-coax bus network, someone cuts the cable in the middle. What happens?
A bus needs a terminator at each end. A cut leaves an open end, signals reflect off it and garble everything, so the whole segment fails.
02In a star, one device's cable is cut. Who is affected?
Each device has its own cable to the centre, so only that device drops off. The real weak spot is the switch itself: if it fails, everyone is cut off.
03How many links does a full mesh of 6 devices need?
Each of the 6 devices links to the other 5, and each link joins two devices: 6 × 5 ÷ 2 = 15. Twenty devices would need 190.
04Why do city fibre rings usually run a second ring in the other direction?
Data in a ring travels one way round, so one cut would stop it. With a second ring, traffic turns back at the break and reaches its destination the long way.
05An old Ethernet hub has a cable to every device, but repeats every message out of every port. What is its topology?
The cables form a star (physical topology), but every device hears every message, exactly as on a shared bus (logical topology).
المسرد
- Topology
- The shape of the links in a network: which devices are connected to which.
- Physical topology
- How the cables (or radio links) actually run between devices.
- Logical topology
- The path data really takes, which can differ from the cabling: a hub wired as a star behaves like a bus.
- Bus
- Every device taps into one shared cable. Simple, but one break stops everything and only one device can send at a time.
- Star
- Every device has its own cable to a central switch or access point. Easy to grow; the centre is a single point of failure.
- Ring
- Each device links to two neighbours and data travels round in one direction. A second ring the other way survives a cut.
- Mesh
- Devices link to many others (full mesh: every pair), so data can route around a broken link.
- Single point of failure
- One part whose failure stops the whole network, like the switch at the centre of a star.
الملاحظات والمصادر
- 10BASE2 (IEEE 802.3 clause 10): 50-ohm thin coax, segments up to 185 m, at most 30 stations, a terminator at each end. Stations share the cable using CSMA/CD (IEEE 802.3 clause 4): listen first, and back off after a collision.
- Twisted-pair Ethernet (IEEE 802.3, 10BASE-T onward) runs one cable of up to 100 m from each device to a switch port. Wi-Fi in infrastructure mode (IEEE 802.11) joins every station through one access point: a star made of radio.
- Token Ring is IEEE 802.5 (4 or 16 Mbit/s). FDDI (ISO 9314) used two counter-rotating fibre rings; metro Ethernet rings today use ITU-T G.8032 protection, which aims to switch over in under 50 ms. At 2/3 the speed of light, fibre adds about 5.0 µs per km, so the long way round this 10 km-per-hop ring adds about 100 µs.
- A full mesh of n nodes has n(n−1)/2 links: 15 for 6, 190 for 20, 4,950 for 100. Wi-Fi mesh is IEEE 802.11s; Zigbee runs on IEEE 802.15.4 radios. The internet core is a partial mesh of routers.
- A hub repeats every bit to every other port, so all its devices share one collision domain: physically a star, logically a bus. Switches (IEEE 802.1Q bridges) forward each frame only where it needs to go, and spanning tree turns a switched network with spare loops into a logical tree.
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