Physical and Logical Topologies
Unit 1: Networking Foundations. Lesson 2 of 49 in NetPlus Academy.
A topology is the shape of a network. The physical topology is how cables actually run. The logical topology is how data flows. They can differ: a modern office is a physical star wired to a switch, and it behaves logically like a star too, but old Token Ring was a physical star with a logical ring.
The shapes you must recognize
- Star (also called hub and spoke): every device connects to one central device. Easy to add or remove hosts and one cable fault affects one host, but the center is a single point of failure. This is the standard LAN design.
- Mesh: devices interconnect with each other. Full mesh links every pair, which gives maximum redundancy but the link count explodes: n(n-1)/2 links for n devices, so 6 devices need 15 links. Partial mesh links only the important pairs. WAN cores and wireless mesh systems use partial mesh.
- Point-to-point: one direct link between two endpoints, such as a leased line between two offices or a wireless bridge between buildings.
- Point-to-multipoint: one central endpoint talks to many, such as a wireless access point serving clients.
- Ring: each device connects to two neighbors. A single break can stop the ring unless a second counter-rotating ring exists.
- Bus: one shared cable with terminators at each end. A single break kills everyone. Legacy only.
- Hybrid: any combination, which describes almost every real network.
Physical and Logical Topologies, explained slowly
Two questions about any network
A topology answers "what shape is it?" But there are two shapes to ask about. The physical topology is where the cables actually run. The logical topology is the path the data takes. Usually they match. Sometimes they do not: old Token Ring networks were wired as a star (every cable went to a central box) but the data went around in a circle from one PC to the next, so the logical topology was a ring.
The shapes, one at a time
Star (also called hub and spoke). Every device gets its own cable to one central device, which today is a switch.
[PC]
|
[PC] --[Switch]-- [PC]
|
[PC]Add a PC: plug in one cable. One cable breaks: one PC is down, everyone else keeps working. That is why every office LAN is a star. The weakness is the middle. If the switch dies, everyone is down. That is the single point of failure.
Mesh. Devices connect to each other, not through a middle. Full mesh links every pair. Maximum redundancy: any link can fail and there is still a path. The cost is the number of links. Each of n devices needs a link to the other n minus 1 devices. That counts every link twice (A to B and B to A), so divide by two.
Devices Links needed (n x (n-1) / 2) 3 3 4 6 5 10 6 15 8 28 10 45
Nobody builds a full mesh of 50 offices; 1,225 links. So real WANs use partial mesh: the important sites get redundant links, the small ones get one.
Point-to-point. One link, two ends. A wireless bridge from one roof to another. A leased line between two offices.
Point-to-multipoint. One center talks to many, but those many do not talk to each other directly. A Wi-Fi access point and its clients.
Ring. Each device connects to its two neighbors and data circles around. One break stops the ring unless there is a second ring going the other way.
Bus. One long cable everyone taps into, with a terminator at each end that soaks up the signal. Remove a terminator or cut the cable anywhere and the whole segment dies. Legacy.
Hybrid. Any combination. A star in each building with a partial mesh between buildings is a hybrid, and that describes almost every real network.
The part that trips people up
Questions usually describe a property and want the shape.
- "Most fault tolerant" or "no single link failure can isolate a site": full mesh.
- "Easiest to add devices and troubleshoot": star.
- "One cable break brings down the whole segment": bus (or a single ring).
- "A single point of failure at the center": star. The star is easy to manage and also has a single point of failure. Both are true.
- "Two buildings connected by a directional wireless link": point-to-point.
What to memorize
- Star: one center, one cable per device, the center is the weak point.
- Full mesh links: n(n-1)/2.
- Bus and single ring: one break, everyone down.
- Physical is the wiring, logical is the data path.