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Carrier Sense Multiple Access/ Collision Detection (CSMA/CD)

Updated on September 13, 2011

Carrier Sense Multiple Access/ Collision Detection (CSMA/CD)

In networking collision is a phenomena for when two or more devices send data or start transmitting on a single segment simultaneously, if this is the case collision will occur on that segment.

In Ethernet world several collisions may occur on a segment in a day depending on the number of station on that segment and there is mechanism to handle such collision, the mechanism is called carrier sense multiple access/ collision detection (CSMA/CD).

 

 

 

 

Figure 1

To understand the carrier sense multiple access / collision detection assume a small network of four PCs, A, B, C and D (Figure 1). if Station A wants to start sending some data or communicating with B, it will first sense the carrier or segment for any going on transmission, if there is any communication or activity occurring on the segment the station will know that the carrier or segment is busy then it defers its transmission and keep sensing until it found the carrier or segment free. After sensing the segment or carrier free, it will start its transmission, on the other hand when An already starts its communication, The station D also try to initiate the transmission but found the medium busy and defer its transmission.

This whole process of carrier sensing on an Ethernet segment is called carrier sense multiple access CSMA.

Figure 2

Now there is a probability that when A senses the carrier or segment free, at the same time station C which also wants to communicate or transmit data to B sensed the carrier free. If this will be the case then A and C both sense the carrier / segment free at the same time and start sending or transmitting the data simultaneously, this simultaneous activity will cause a collision between A and C on the segment. In CSMA/CD there is mechanism to handle such situation and it handles by Collision detection CD part of CSMA/CD.

The collision detection mechanism states that if any station on the segment listen back their transmission in scramble or distort form it will know that the collision occurs on the segment. After detecting the collision both stations will stop their transmission for the random amount of time, and transmit a 32 bit jam signal on the segment to let other stations know that a collision is occurred at the segment so any other station intended to transmit on the segment will also wait for a random amount of time, the random amount of time known as “Backoff delay” which uses the exponential back off algorithm.

The reason for this random amount of time is as the collision occurs between the two or more stations who wants to transmit and fortunately senses the carrier free at the same time, so there is a probability that in the absence of this back off delay the both station or any other station could again sense the medium free right after the collision and transmit simultaneously, which will again let the collision occur. That’s why this back off delay or random amount wait time reduces the probability of collision on the segment right after the collision.

In our example when A and C both sense the carrier free simultaneously and transmit a collision occur on the segment between their transmission, which returns their transmissions in scramble and distort form and both stations know about the collision, A 32 bit jam signal will be passed all the stations on the segment to let them know about the collision, which invokes exponential back off algorithm on the Station A, B, and C. All three stations will randomly wait before retransmission, and then start transmitting.

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      Suziana 17 months ago

      very nice and simple explanation, i never had a chance to read such a simple but detailed article on this topic

      thank you

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      saya 5 years ago

      good job.. thank you :)

    • profile image

      amit 5 years ago

      can you also exponential back off algorithm

    • profile image

      Nagakrishna 5 years ago

      Thanks a lot..

    • profile image

      waqas 6 years ago

      thanks

    • profile image

      ben 6 years ago

      very helpful with coursework

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      Subash 6 years ago

      very interesting and many thanks for highlighting on such topic which is often confused or misunderstand by peoples, there was some confusions which cleared after reading the Q&A.

      so in the modern world with duplex technologies and high end switches CSMA/CD is really not used, and what about aloha, you miss it in the article.

    • haidry profile image
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      haidry 6 years ago

      each port in layer 2 switch has its own collision domain, that's why the devices connected via a switch doesnot have a chance to have a collision, ya but it this is a hub, then the collision will occur

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      Michel 6 years ago

      very good explanation for the concept, but what abt the CSMA concept on the switch. do this concept applicapable on a layer two switch. or this is only meant to be for BUS topology

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      Waqas 6 years ago

      Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium,

      "Carrier Sense" describes the fact that a transmitter uses feedback from a receiver that detects a carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission.

      In telecommunications, a carrier wave, or carrier is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information.

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      rajesh 6 years ago

      i hav small doubt...in the carrier sense mechanism station will sense the channel that it s free or not...i want to how it ll sense the packet.??does it sends sum packet??if yes,den if packet s lost??please reply me as soon as possible

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      rajesh 6 years ago

      it s really helpful for me..thx

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      surya 6 years ago

      wonderful job....good explanation....

    • haidry profile image
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      haidry 7 years ago

      thanks nam

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      nam 7 years ago

      good job useful

    • haidry profile image
      Author

      haidry 7 years ago

      very nice, the reason for writing this article was just to explain this concept in an easy way. and your comments appreciate my efforts and the intentions, thanks for the feedback

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      N.R 7 years ago

      Great simplified explanation

    • haidry profile image
      Author

      haidry 7 years ago

      thanks mate

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      idreesfarooq 7 years ago from fiverr dot com/idreesfarooq

      Nice Sharing of Thoughts, Welcome to Hubpages!