Month: October 2018

Ethernet CSMA/CD

Ethernet Collision Detection

The original Etheret specification used CSMA/CD to managed collision (on a share Ethernet segment).

CSMA/CD = Carrier Sense Multiple Access/Collision Detection

This is how it works:

  • A device with a frame to send, waits for the ethernet bus to be free (i.e. when no signal can be detected on the bus.
  • When the ethernet bus is free, the device begins sending the frame
  • The device listens to make sure no collision occurs
  • If a collision occurs, all devices that sent a frame send a jamming signal to notify all devices connected.
  • The devices that sent the frame then wait for a random period of time and try again. Devices that didn’t send a frame don’t have to wait

A Collision domain is something like a ethernet hub, where collisions can occur across the hub. A switch however has a collision domain for each port.

Collisions are detected during the first 64 bytes of data being transmitted, and late collisions are detected after the first 64 bytes.