The CSMA/CD Mechanism Explained

Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is a mechanism that allows end devices to exchange data in a half-duplex shared environment. It involves three steps: CS (Carrier Sense), MA (Multiple Access), and CD (Collision Detection). Each step describes a particular functionality of this mechanism.

Carrier Sense

In a traditional hub-based wired Ethernet environment, only one device can send a frame at a time. All other devices have to wait until the transmission is over. To ensure that no other device is currently transmitting a frame, a device senses the media before sending its own frame. If the wire is a copper cable (such as UTP and STP), the device checks the voltage levels on the wire. If the wire is a fiber-optic cable (such as SMF or MMF), the device checks the light frequencies on the cable.

Sensing the media before sending frames is the first step of the CSMA/CD mechanism. This step is known as Carrier Sense. All devices that use this mechanism to send frames must go through this step.

If a device doe not detect a frame on the wire, it transmits its own. If it finds a frame on the wire, it will wait till the transmission is over.

Multiple Access

If two or more devices simultaneously sense the wire and see no frame, they place their frames on the wire. This situation is called Multiple Access. The more devices you place on an Ethernet segment, the more likely it is to happen. If you put too many devices in the segment, it will occur more frequently.

Collision Detection

When two or more devices simultaneously put their frames on the wire, a collision occurs. In this situation, the voltage levels on a copper wire or the light frequencies on a fiber wire get messed up.

For example, if two devices put the same voltage on an electrical wire, the voltage will be higher than when only one device is supplying voltage. If devices detect a collision with their transmitted frames, they have to resend them.

When a collision occurs, each device that is transmitting a frame creates a special signal, called a jam signal, on the wire. It then waits for a short, random time and senses the wire again. If it senses no frame on the wire, it retransmits its original frame. The time period that the device waits is measured in microseconds. It can’t be detected by a human. Likewise, the time period the device waits is random to help ensure a collision does not occur again when these devices retransmit their frames.

A collision seriously affects the throughput. Therefore, you should monitor the number of collisions on each network segment. The more collisions you experience, the less throughput you’ll get. Normally, collisions should be less than 1% of total traffic.

Conclusion

CSMA/CD is an essential protocol for managing data transmission in shared Ethernet environments. By ensuring that devices sense the carrier, manage multiple access, and detect collisions, the network can minimize data loss and maintain efficiency. Understanding and monitoring CSMA/CD processes are crucial for optimizing network performance and preventing excessive collisions, which can significantly impact throughput.