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What is CSMA/ CD?

The carrier sense multiple access with collision detection or the CSMA/ CD is a method for the media access control which makes use of the following:
1. A carrier sensing scheme
2. A transmitting data station

– The purpose of the transmitting station is detecting the other signals while a frame is already under the process of transmission.
– If another is detected, it stops the transmission of the current and sends out a jam signal.
– It waits for the appropriate time interval for re-sending the frame.
– This is actually a modified form of the CSMA developed in order to improve its performance.
– This it does by stopping transmission up on detection of a collision.
– Thus, it shortens the time between the several attempts.

Procedure for CSMA/CD

The main procedure for CSMA/ CD has been defined as follows:
– If the frame is ready for transmission, then let it go to the next point otherwise not.
– Go ahead with the transmission if the medium is idle otherwise wait for it to become idle.
– Start the transmission.
– If a collision is detected proceed with the collision detected procedure.
– End the transmission of the frame and reset the counters.

The collision detected procedure has been defined as follows:
– The transmission is continued till the minimum packet time is finished in order to make sure that the collision is detected by all receivers.
– Increment the counter.
– If the maximum allowed number of transmissions has been reached, abort the transmission otherwise carry on.
– Wait for some time (backoff period) based up on the number of detected collisions.
– Follow the main procedure from stage 1.

Applications of CSMA/CD

– The methods for detecting the collisions are based on medium that exists between the two sources.
– On electrical buses (for example 10BASE – 2 or 10BASE – 5) the transmitted data can be compared with the received data for detecting the collision.
– Another way is recognition of higher signal amplitude than the normal.
– A signal carrying 32 – bit binary pattern is termed as the jam signal.
– This signal is sent by the data station for informing all the stations not to transmit.

CSMA/ CD has a number of applications such as the following:
– Shared media Ethernet variants that are now obsolete such as 10BASE2, 10BASE5 etc.
– Twisted – pair Ethernet that use repeater hubs.
– Early Ethernet networks. Modern networks use full – duplex connections and switches do not use CSMA/ CD
– A varying concept of CSMA/ CD is used by radio frequency systems. These systems are based automatic packet reporting system.

Performance of CSMA/CD

– The performance calculation of a CSMA/ CD network that involves only one node and one attempt is simple.
– However, where there are more than two nodes, it becomes difficult to predict the performance of network.
– A part of the bandwidth is wasted by both the collisions and back-off delays which results in a fall in a throughput ad utilization of the network.
– An overload condition may occur if a large number of NICs try to share the bandwidth.
– This results in an increasing utilization causing the overload condition.
– But the throughput of the Ethernet reduces by a significant amount.
– CSMA/ CD algorithms wasted too much of its capacity.

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