Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is a mechanism to send information in a 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 sense the media before putting its own frame on the wire. 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 and MMF), the device checks the light frequencies on the wire.
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 doesn’t sense a frame on the wire, it transmits its own frame. 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 it will 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 frame on the wire at the same time, 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 level will be higher than the level when only one device puts its voltage. If the devices see a collision for their transmitted frames, they have to resend their frames.
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 a small random time period 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
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 one percent of the total traffic.
