Collisions, Collision Domains, and Congestion
Another issue with Ethernet is again caused by
CSMA/CD: the more devices you put on a segment, the more likely you’ll experience collisions, especially along with devices that constantly need access to the wire. For example, when two devices are on an Ethernet wire, you would not expect to see that many
collisions since it would be unlikely that both devices would need to transmit data simultaneously. However, if 100 devices were on the wire, quite a few might need to send traffic simultaneously. When these devices since the wire and simultaneously send their traffic, a collision occurs. And the way
CSMA/CD works, they need to create a JAM signal and back off a random time interval before sending again. Of course, the more devices present, the more likely that the “
random” time interval they choose is the same as another device, thereby creating even more collisions, greatly slowing down a device’s access when trying to transmit data. On top of this, high-performance
PCs, network-based applications, and high-bandwidth applications such as video can create even more contention for access to the wire.
A collision domain basically includes all the devices that share a media type at layer 1 of the
OSI Reference Model—such as all the devices on a single strand of
10Base2 cabling or all the devices connected to a hub or hubs. In a collision domain, each device on the segment will experience the effects of the collisions. The more devices on the segment, the more likely it is that collisions will create bandwidth problems for these devices. This is not to say that collisions are bad—it’s just that collisions are part of how Ethernet functions.