CSMA or CD
 


Carrier sense multiple access with collision detection (CSMA/CD) is a Media Access Control method in which:[1]

a carrier sensing scheme is used.
a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

Main procedure

  1. Is my frame ready for transmission? If yes, it goes on to the next point.
  2. Is medium idle? If not, wait until it becomes ready
  3. Start transmitting.
  4. Did a collision occur? If so, go to collision detected procedure.
  5. Reset retransmission counters and end frame transmission.

Collision detected procedure

  1. Continue transmission until minimum packet time is reached to ensure that all receivers detect the collision.
  2. Increment retransmission counter.
  3. Was the maximum number of transmission attempts reached? If so, abort transmission.
  4. Calculate and wait random backoff period based on number of collisions.
  5. Re-enter main procedure at stage 1.


This can be likened to what happens at a dinner party, where all the guests talk to each other through a common medium (the air). Before speaking, each guest politely waits for the current speaker to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time (in Ethernet, this time is measured in microseconds). The hope is that by each choosing a random period of time, both guests will not choose the same time to try to speak again, thus avoiding another collision.

Methods for collision detection are media dependent, but on an electrical bus such as 10BASE-5 or 10BASE-2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus.

 

 Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD to physically monitor the traffic on the line at participating stations. If no transmission is taking place at the time, the particular station can transmit. If two stations attempt to transmit simultaneously, this causes a collision, which is detected by all participating stations. After a random time interval, the stations that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off.

CSMA/CD is a type of contention protocol.  Networks using the CSMA/CD procedure are simple to implement but do not have deterministic transmission characteristics. The CSMA/CD method is internationally standardized in IEEE 802.3 and ISO 8802.3.