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CSMA with Collision Detection (CSMA/CD)

If two or more stations sense the channel idle at about the same time, and begin transmitting the same time, the collision is sure.In CSMA/CD protocols, the stations abort their transmission as soon as they detect the collision.

A station, after abortion waits a random period of time known as back-off delay and tries the same thing again.Abortion of collisions saves the bandwidth as in csma/cd.

A jam signal is sent due to which all transmitters stops transmissions by random intervals, reducing the probability of a collision after the first retry. CSMA/CD can be in one of three states: -transmission, -contention, or -idle. Normally the csma/cd protocol follows the mechanism shown in the diagram below.

If two stations starts transmitting at the same time, a collision will occur which is detected by the power of the received pulse and then comparing it with the transmitted signal power. After the collision is detected,the station stops transmitting and waits random amount of time and then again starts transmitting assuming that no other station is transmitting in this time.This time slot can be regarded as contention slot. When no station is transmitting, the channel is idle.


The minimum time for detecting the collision is the time it takes the signal to propagate from one station to the other. How long does it take to detect collision, after its occurrences, by the involved stations?

When a transmission starts, a station can detect within the round trip propagation period, after which every other station can detect the collision. A round trip time is defined as the time taken by data(frame,packet) to reach destination,gets processed in proper time and then reaches back to sender in the form of acknowledgment. Remember that CSMA/CD is a layer 2 access method, not a protocol of the OSI model.The CSMA/CD method is internationally standardized in IEEE 802.3 CSMA/CD was used in bus topology Ethernet variants and in early versions of twisted-pair Ethernet.

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