Computer Networking

Transport Layer

CSMA With Collision Detection (CSMA/CD)

In CSMA/CD, if any station wants to transmit data then it must follow the following steps

Step 1: Continuous sense if the transmission medium is idle?

By sending some dummy data on shared medium, status of medium (idle or busy) can be found. If there is no collision with dummy data then it means medium is free for that node.

Step 2: Transmit the data & check for collisions.

  • If the medium is idle. Then, sender transmits its data on the medium. CSMA/CD use Collision signals instead of ‘acknowledgement’ Because ACK increase collision rate because of additional functionality.
  • During transmission, if collision signal is received by the sender, then transmission is stopped by sending a Jam/stop signal.
  • After stop transmission, Sender waits for random period of time to resend frame by repeating above process.

Step 3: If collision signals are detected in propagation, the sender completes its frame transmission.

Transmission Time in CSMA/CD

If a station receive collision signal after its completion of transmission time then sender cannot recognize the collision and can’t be able to resend frames. So in CSMA/CD, transmission time is always two times of propagation delay as in the following formula

Transmission time ≥ 2× Propagation Delay

We can further expand the above equation as.

Note: Transmission time = length of message/Bandwidth of medium

Why Transmission Time is 2 Times Greater Than Propagation Delay?

Let explain with example, Suppose Station A and B where the propagation time is 1Hour and transmission time is 2 Hour for each, if both sending data to each other at a time. Then collision will occur just after one 30-mint.

Suppose another case,  when collision can happen when the frame of Station A reach just before the Station B after time 59.59 minutes.  At the mean while Station B senses the medium and found the medium is idle. So, station B also transmits the data toward Station A. The result will be in collision of data of both stations.


  • Station B will receive the collision signals just after 2 second (one second for transmission propagation and 1 second for collision signal back propagation to Sender B).
  • Station A will receive the collision signals after 1 hour and 59.58 minute (59.59 minutes for transmission propagation and 59.59 minutes for collision signal back propagation to Sender A).

So, in the above case Transmission time for each station should be 2 Hours. 1 hour will define the propagation time and other hour will use if collision occurs.

Efficiency Formula of CSMA/CD 



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