Medium Access Control: Stochastic Methods - CSMA/CD

Carrier Sense Multiple Access with Collision Detection

CSMA improves channel utilization compared to ALOHA. However, the performance can still be improved further, especially in wired networks.

Consider the situation of two terminals that are connected to the same cable. This cable could, for example, be a coaxial cable, or it could also be built with twisted pairs. Before extending CSMA, it’s useful to understand more intuitively, how frames are transmitted in such a network and how collisions can occur.

The slides below illustrate the physical transmission of a frame on such a cable. To transmit its frame, host A must send an electrical signal on the shared medium.

1. The first step is thus to begin the transmission of the electrical signal. This is depicted in the first slide below.

2. This electrical signal will travel along the cable. Although electrical signals travel fast, we know that information cannot travel faster than the speed of light (i.e. $300,000$ kilometers/second). On a coaxial cable, an electrical signal is slightly slower than the speed of light in a vacuum which is at about $200,000$ ...