Introduction to Networking

# 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: Continuously sense if the transmission medium is idle.

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

Step 2: Transmit the data & check for collisions.

• If the medium is idle, then the sender transmits its data on the medium. CSMA/CD uses Collision signals instead of ‘acknowledgment’ Because ACK increases the collision rate because of additional functionality.
• During transmission, if a collision signal is received by the sender, then the transmission is stopped by sending a Jam/stop signal.
• After stopping transmission, the Sender waits for a random period of time to resend the frame by repeating the 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 receives a collision signal after its completion of transmission time, then the sender cannot recognize the collision and can’t be able to resend frames. So, in CSMA/CD, transmission time is always two times the propagation delay as in the following formula.

Transmission time ≥ 2× Propagation Delay

We can further expand the above equation.

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

### Why is Transmission Time 2 Times Greater Than Propagation Delay?

Let’s explain, for example, in Station A and B, where the propagation time is 1 hour and transmission time is 2 Hours for each if both are sending data to each other at a time. Then, the collision will occur just after one 30-minute period.

Suppose another case when a collision can happen when the frame of Station A reaches just before Station B after time 59.59 minutes.  In the meantime, Station B senses the medium and finds the medium idle. So, station B also transmits the data toward Station A. The result will be a collision of data from both stations.

• Station B will receive the collision signals just after 2 seconds (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 minutes (59.59 minutes for transmission propagation and 59.59 minutes for collision signal back-propagation to Sender A).

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