collisions on ethernet links

on shared ethernet,errors are often the result of collisions.two stations try to send a frame at the same time and the resulting collision damages the frames,causing CRC errors.

depending on the size of the ethernet network,many of these collisions happen in the 8-byte preamble of the frames and are not registered by troubleshooting tools.

if the collision happens past the preamble and somewhere in the first 64 bytes of the data frame,this is registered as a legal collision and the frame is called a runt frame.

a general goal for ethernet collisions is that less than 0.1% of the frames should be affected by a legal collision(not counting the collisions that happen in the preamble).

a collision should never occur on full-duplex ethernet links.if they do,there's probably a duplex mismatch.

to detect a duplex mismatch,look at the number and type of errors on either end of the link.look for CRC and runt errors on one side and collisions on the other side of the link.

the side that is set for full duplex can send whenever it wants.it doesn't need to sense carrier.

the sied that is set for half duplex senses carrier and will stop transmitting if it detects a simultaneous transmission from the other side.it will back off and retransmit,and report a collision.

the result of the half-duplex station's stopping transmission is usually a runt frame(shorter than 64 bytes) and is always a CRC-errored frame.

the full-duplex side receives runts and CRC-errored frames and reports these errors.the half-duplex side reports collisions.

notice the asymmetry of the errors when there is a duplex mismatch.if you see collisions and CRC errors on both sides of the link,the problem is probably something other than a duplex mismatch,perhaps a wiring problem or bad NIC.