When using a non-deterministic contention-based method, a network device can attempt to access the medium whenever it has data to send. To prevent complete chaos on the media, these methods use a Carrier Sense Multiple Access (CSMA) process to first detect if the media is carrying a signal.

If a carrier signal on the media from another node is detected, it means that another device is transmitting. When the device attempting to transmit sees that the media is busy, it will wait and try again after a short time period. If no carrier signal is detected, the device transmits its data. Ethernet and wireless networks use contention-based media access control.

It is possible that the CSMA process will fail and two devices will transmit at the same time creating a data collision. If this occurs, the data sent by both devices will be corrupted and will need to be resent.

Contention-based media access control methods do not have the overhead of controlled access methods. A mechanism for tracking whose turn it is to access the media is not required. However, the contention-based systems do not scale well under heavy media use. As use and the number of nodes increases, the probability of successful media access without a collision decreases. Additionally, the recovery mechanisms required to correct errors due to these collisions further diminishes the throughput.

CSMA is usually implemented in conjunction with a method for resolving the media contention. The two commonly used methods are:

The figure illustrates the following: