Why do collision may occur if there is duplex mismatch?

Question

In a question on Serverfault It is mentioned that "it is possible for a single host connected to one port to experience collisions if there's a duplex mismatch between the host NIC and the switch port to which it is connected". Is it right. If it's right can someone explain how it is happening?

Answer 1

I respect you for thinking critically and trying to learn, but honestly, you should work on your research skills. A simple Google would have taken you to Wikipedia which has this excellent explanation of why it's correct, and how it happens.

Source: http://en.wikipedia.org/wiki/Duplex_mismatch

Communication is possible over a connection in spite of a duplex mismatch. Single packets are sent and acknowledged without problems. As a result, a simple ping command fails to detect a duplex mismatch because single packets and their resulting acknowledgments at 1-second intervals do not cause any problem on the network. A terminal session which sends data slowly (in very short bursts) can also communicate successfully. However, as soon as either end of the connection attempts to send any significant amount of data, the network suddenly slows to very low speed. Since the network is otherwise working, the cause is not so readily apparent.

A duplex mismatch causes problems when both ends of the connection attempt to transfer data at the same time. This happens even if the channel is used (from a high-level or user's perspective) in one direction only, in case of large data transfers. Indeed, when a large data transfer is sent over a TCP, data is sent in multiple packets, some of which will trigger an acknowledgment packet back to the sender. This results in packets being sent in both directions at the same time.

In such conditions, the full-duplex end of the connection sends its packets while receiving other packets; this is exactly the point of a full-duplex connection. Meanwhile, the half-duplex end cannot accept the incoming data while it is sending—it will sense it as a collision. The half-duplex device ceases its current transmission and then retries later as per CSMA/CD. As a result, when both devices are attempting to transmit at the same time, packets sent by the full-duplex end will be lost and packets sent by the half duplex device will be delayed or lost.