collision detection (CD)
In the case of collision-affected access methods, the focus is on safer collision detection( CD) on the one hand, and on measures to eliminate collisions on the other. With collision methods such as CSMA/CD, collisions must be reliably detected regardless of the signal propagation times in the lines.
In the CSMA/CD access method, all stations willing to transmit listen to the transmission medium to determine whether it is unoccupied. If it is free, the corresponding station (A) transmits. Since the signal has a certain propagation time, the signal from station A is already on its way, while station B, which has also listened to the transmission channel, still recognizes it as free and also starts transmitting the signal. This results in a collision of the signals from both stations, which is reflected in an increased level on the transmission medium. During its own transmission process, each station monitors the transmission medium until the complete data packet is on the transmission medium.
There is a parameter, the collision parameter (k), whose magnitude is crucial for collision detection and which is calculated from the quotient of the maximum signal propagation time and the transmission time for the data packet. The collision coefficient (k) should be "1. If this quotient is >1, the entire data packet is on the transmission medium without a bit having reached the destination station. The transmission process for the sending station A is thus completed and station A can no longer detect the collision.
As countermeasures, the collision domain can be made smaller or the packet length can be extended. In this case, the distance between the stations furthest apart should correspond to twice the signal propagation time. The extension of the packet length is implemented in Gigabit Ethernet by the additional Carrier Extensiondata field. In addition, other wait and retry strategies can be used with immediate, delayed or randomly triggered retransmission as in Binary Exponential Backoff( BEB).