CSMA - Carrier Sense Multiple Access

CSMA/CD - Carrier Sense Multiple Access / Collision Detection

CSMA/CA - Carrier Sense Multiple Access / Collision Avoidance

Carrier Sense Multiple Access protocol is used in situations where multiple users require to access a single channel for data transmission at the same time. If two or more users transmit the data at the same time, it will result in a collision. To detect or avoid such a collision - CSMA, CSMA/CD and CSMA/CA protocols are developed.

CSMA protocol senses the transmission line before transmitting the data. If the line is found to be busy, it waits for a stipulated amount of time and then senses the line again. This time, if the line is found to be free, data is transmitted.

This (CSMA) protocol works fine in some situation, but has some major drawbacks. If two or more transmitting stations senses the line at the same time, and found the line to be free. They will send the data at the same time, and this will result in a collision.

To overcome this problem, an advanced version of the CSMA protocol "CSMA/CD" is introduced. CSMA/CD senses the line just before transmitting the data, and then "again" after the data is transmitted. A high voltage in a line indicates that the transmitted data might have resulted in a collision.

As opposite to CSMA/CD which is a collision detection system, CSMA/CA is a collision avoidance system. It acts to prevent a collision before it occur.