Ethernet Half-Duplex Collisions

On the original half-duplex Ethernet system the Ethernet medium access control (MAC) system based on CSMA/CD is used to arbitrate access to the Ethernet channel among a set of machines so that each machine gets a fair chance to use the network. One of the components of the MAC system for Ethernet is collision detection and retransmission.

On modern full-duplex Ethernet systems there is no need to share the medium, since it is dedicated to a single computer. Therefore, the CSMA/CD system is no longer used and collisions do not occur on a correctly configured full-duplex link.

Collisions are a normal part of the functioning of a half-duplex Ethernet system, and are the mechanism with which the Ethernet system resolves simultaneous attempts to access the shared channel. On a busy Ethernet with high powered machines, you can expect to see a lot of collisions.

The set of Usenet postings linked to this page describe some of the issues involved in answering the question "What level of collisions is appropriate for my network?"