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Unit 4: Ethernet and CablingLesson 4.9 (9 of 9 in this unit)24 of 84 in the Network Fundamentals course

Collision domains

Why hubs shared one collision domain, how switches split them, and why full duplex ended collisions.

Beginner · 6 min read

Collision domain is a network segment in which frames from two devices can collide if both transmit at the same time, as on a shared hub or coaxial cable running half duplex. Each switch port is a separate collision domain, and a full-duplex link has no collisions at all.

In simple terms: On old, shared Ethernet, only one device could talk at a time, and if two talked at once, both messages were garbled. A collision domain is the group of devices that share that one channel.

Shared Ethernet and collisions

Early Ethernet was a shared medium: one coaxial cable, or later a hub, which simply repeats every bit it receives out of every other port. Only one device can transmit at a time. If two devices transmit at once, their signals collide and both frames are destroyed. A collision domain is the set of devices that can collide with each other.

Hubrepeats everythingPC APC BPC C
  1. 1. Two PCs transmit at once. PC A and PC C both start sending.
  2. 2. Collision. The hub repeats the mixed signal out of every port. Both frames are destroyed.
  3. 3. CSMA/CD recovers. Each sender detects the collision, sends a jam signal, waits a random time and tries again.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) was the set of rules: listen before sending (carrier sense), detect collisions while sending, then back off for a random time. It works, but the more devices share the domain, the more collisions there are and the lower the real throughput.

Switches split collision domains

A switch does not repeat signals like a hub. It reads each frame, forwards it only where needed, and buffers frames when an output port is busy. So every switch port is its own collision domain.

SwitchPC APC BPC C
  1. 1. Two PCs send at once… …both to PC B. The switch buffers one frame briefly and sends the two frames one after the other. There is no collision.
  2. 2. Full duplex: PC A and PC C can even send to each other at the same time, because each link carries traffic in both directions at once.

Full duplex ends collisions

On a switch port running full duplex, the device and the switch can send and receive at the same time. There is no shared medium, so collisions can't happen and CSMA/CD is switched off. Collisions on a modern switched network are a symptom of a problem, usually a duplex mismatch.

Learn more: Ethernet FundamentalsSpeed and Duplex

Counting collision domains

DeviceEffect on collision domains
Hub / repeaterExtends one collision domain to all its ports
Switch / bridgeEach port is a separate collision domain
RouterEach interface is a separate collision domain (and broadcast domain)

Check yourself

Predict · scenario 1

A 24-port switch has 20 PCs connected. How many collision domains are there on the switch?

Predict · scenario 2

Four PCs are connected to a hub, and the hub is connected to one switch port. How many collision domains do the hub and its PCs form?