CSMA/CD stands for Carrier Sense Multiple Access/Collision Detection. It is a widely used MAC protocol. Its specifications have been standardized by the IEEE 802.3 standard. When a station wants to transmit a frame in CSMA, it will sense the channel to check if it is free or not. If the channel is free the station transmits the frame, now it might happen that some other station also transmits the frame, and thus, a collision occurs. Now, the station has to wait(round-trip propagation delay) for an acknowledgment after the transmission in case of CSMA. But with CSMA/CD this time can be reduced.
In CSMA/CD the station wishing to transmit first listens to make certain that the link is free, then transmits data and listens again. The station checks the line for extremely high voltages that indicate a collision. After a station detects a collision, it aborts it’s transmission waits for the Back Off period of time and tries again. The transmitting stations also release a jam signal to indicate that a collision has occurred.
Characteristics of CSMA/CD
- CSMA/CD has carrier sense i.e. it senses the channel for transmissions to check if it is busy or not.
- After the collision is detected, the stations have to wait for a random amount of time before retransmitting the frames.
- A jam signal is used to indicate the other stations that collision has taken place and that the stations have to wait.
- A higher transmission priority mechanism can be implemented.
Advantages of CSMA/CD
- Collisions are detected in a shorter span of time.
- Wasteful transmission of frames is avoided thus all available bandwidth is utilized if possible.
- It has low overhead.
Disadvantages of CSMA/CD
- It is not possible over long distances and large networks.
- It does not reduce the possibility of collision and collisions degrade performance.
- The performance is inversely proportional to the number of stations. Thus, more stations degrades the performance exponentially.
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