In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode. This means when a device wants to send data, it must first check to see whether the networking media is busy. The device must check if there are any signals on the networking media. After the device determines the networking media is not busy, the device will begin to transmit its data. While transmitting its data in the form of signals, the device also listens. It does this to ensure no other stations are transmitting data to the networking media at the same time. After it completes transmitting its data, the device will return to listening mode.
Networking devices are able to tell when a collision, a situation that can occur when two hosts begin transmitting at the same time on the same network, has occurred because the amplitude of the signal on the networking media will increase. When a collision occurs, each device that is transmitting will continue to transmit data for a short time. This ensures that all devices see the collision. Once all devices on the network have seen that a collision has occurred, each device invokes a waiting algorithm. After all devices on the network have backed off for a certain period of time (different for each device), any device can attempt to gain access to the networking media once again. When data transmission resumes on the network, the devices that were involved in the collision do not have priority to transmit data.
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