Daily Learning: Computer Networks – Access Control Methods – CSMA(Carrier Sensing Multiple Access)/CD(Collision Detection)


  1. There can be multiple stations in a network.
  2. It will sense if the transmission line is busy or not. If it is not busy then it will transmit the data.
  3. It will also sense for any type of collision while sending the data.

Persistence Methods

  1. 1 Persistent – It sense the medium continuously. When the medium is free it sends the packet imediately.
  2. Non-Persistent – It first sense the medium then waits for a random amount of time then again sense.
  3. P Persistent – It fist sense the medium continuously then when the medium is free it generate a random number and check if the number is less than the probability of which host if the number is less than some host’s probability then that host will transmit the data.


Vulnerable Time – It is the total propagation time Tp. If first bit of the packet reaches to the end of the medium then every station will heard of the transmission and no one will transmit.


The Carrier Sense Multiple Access/ Collision Detection protocol is used to detect a collision in the media access control (MAC) layer. Once the collision was detected, the CSMA CD immediately stopped the transmission by sending the signal so that the sender does not waste all the time to send the data packet. Suppose a collision is detected from each station while broadcasting the packets. In that case, the CSMA CD immediately sends a jam signal to stop transmission and waits for a random time context before transmitting another data packet. If the channel is found free, it immediately sends the data and returns it.

It is used in the wired medium and used by Ethernet.

Transmission Time – Tt = 2 * Tp


CSMA stands for Carrier Sense Multiple Access with Collision Avoidance. It means that it is a network protocol that uses to avoid a collision rather than allowing it to occur, and it does not deal with the recovery of packets after a collision. It is similar to the CSMA CD protocol that operates in the media access control layer. In CSMA CA, whenever a station sends a data frame to a channel, it checks whether it is in use. If the shared channel is busy, the station waits until the channel enters idle mode. Hence, we can say that it reduces the chances of collisions and makes better use of the medium to send data packets more efficiently.

It is used in the wireless interface

  • Interframe Space(IFS) = Collision are avoided by deferring transmission even if the channel is found idle. When an idle channel is found, the station does not send immediately. It waits for a period of time called the interframe space.
  • Contention Window – We divide the network into windows and if some signal collide the it has to wait for the next window which is 2^n [n = number of collision]

Minimum amount of data – L ≥ 2 * Tp * B

Efficiency(η) = Tt / (C * 2 * Tp) + Tt + Tp

Back off Algorithm – Wt – K * Tslwt [ k = 0 – 2^n-1 ] [ n = collision number ]


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