Hello Sam,

only half-duplex interfaces can see collisions.

Collision detection is performed in hardware by comparing transmitted signal with signal sensed on wire: if the two differ a collision has happened.

collisions: actually only the first 64 bytes of the frame are checked if they pass the frame is considered valid.

after a collision is detected a random counter is started and the frame is transmitted again in the hope of a successful transmission.

Up to 16 attempts with random counters using exponential backoff timers (increasing at each attempt).

This lead to the definition of multiple collisions.

Excessive collisions count how many times even with 16 attempts the frame was not transmitted successfully.

Finally, if collisions happen after first 64 bytes of frame they are called late collisions.

see

http://www.cisco.com/en/US/products/hw/modules/ps2033/products_tech_note09186a008009446d.shtml

and more general

http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html

Hope to help

Giuseppe