A collision happens when two stations simultaneously “talk” on the wire. In order to best understand what happens during a collision, think about a conference call where two people start talking at the same time on the phone as a third person tries to listen to the conversation. The third person likely hears a mixture of voices and doesn’t really understand what is being said.
A similar situation happens during a network collision when three different computers communicate on one LAN segment. While one computer is sending information, the other must listen and wait for the transmission to end before it starts transmitting. A collision occurs most commonly after a period of silence, when two computers decide to start talking at the same time and their signals collide on the network. However, collisions are actually a normal part of life for an Ethernet network, and under most circumstances are not considered a problem.
There are two categories of collisions:
1. Early Collisions
Early collisions happen before 512 bits of the frame have been put onto the wire and occur regularly in a normally operating Ethernet network. There is no hardware malfunction or misbehaving station — it’s just that two network interface controllers (NICs) start to talk at the same time. Generally, after the talkers implement the backoff algorithm, which is specially designed to not have both NICs attempt to talk at the same time again, both talkers will successfully put their frame onto the wire. It typically takes no longer than 2-3 milliseconds for a station to recover from a collision and successfully re-transmit its frame.
2. Late Collisions
Late collisions are not normal, and are usually the result of out of spec cabling or a malfunctioning adapter. A late collision happens after 512 bits of the frame have been transmitted. The reason that late collisions are a problem is that once the NIC misses the fact that a collision has occurred, recovery and retransmission are left to the upper layers, and recovery time goes up drastically. While a NIC will typically recover and retransmit a frame in 2-3 milliseconds, it typically takes anywhere from 10 to 100 times that long for the upper layers to recover.
It is important to note that another major cause of late collisions is a malfunctioning NIC. If a NIC malfunctions in such a manner that it is unable to detect that another station is talking, late (and early) collisions will occur. Monitoring for collisions, including why and where they are occurring, is yet another way to make sure your network remains healthy and business remains productive.