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Sunday, July 7, 2013

Differentiate between persistent and non-persistent CSMA?

- CSMA or Carrier Sense Multiple Access makes use of LBT or listen before technique before making any transmission. 
- It senses the channel for its status and if found free or idle, the data frames are transmitted otherwise the transmission is deferred till the channel becomes idle again. 
- In simple words, we can say that CSMA is an analogy to human behavior of not interrupting others when busy. 
- There are number of protocols out which the persistent and the non – persistent are the major ones. 
- CSMA is based on the idea that if the state of the channel can be listened or sensed prior to transmitting a packet, better throughput can be achieved.
- Also, using this methodology a number of collisions can be avoided. 
- However, it is necessary to make the following assumptions in CSMA technology:
  1. The length of the packets is constant.
  2. The errors can only be caused by collisions except which there are no errors.
  3. Capture effect is absent.
  4. The transmissions made by all the other hosts can be sensed by each of the hosts.
  5. The transmission time is always greater than the propagation delay.
About Persistent CSMA
- This protocol first senses the transmission channel and acts accordingly. 
- If the channel is found to be occupied by some other transmission, it keeps listening or sensing the channel and as soon as the channel becomes free or idle, starts its transmission. 
- On the other hand, if the channel is found empty, then it does not wait and starts transmitting immediately. 
- There are possibilities of collisions. 
- If one occurs, the transmitter must wait for random time duration and start again with the transmission. 
- It has a type called 1 – persistent protocol which makes transmission of probability 1 whenever the channel is idle. 
- In persistent CSMA there are possibilities of occurrence of collisions even if the propagation delay is 0. 
- However, collisions can only be avoided if the stations do not act so greedy. 
We can say that this CSMA protocol is aggressive and selfish. 
- There is another type of this protocol called the P – persistent CSMA. 
This is the most optimal strategy. 
- Here the channels are assumed to be slotted where one slot equals the period of contention i.e., 1 RTT delay. 
- The protocol has been named so because it transmits the packet with probability p if the channel is idle otherwise it waits for one slot and then transmits.

About Non–Persistent CSMA
- It is deferential and less aggressive when compared to its persistent counterpart. 
- It senses the channel and if it is busy it just waits and then again after sometime senses the channel unlike persistent CSMA which keeps on sensing the channel continuously. 
- As and when the channel is found free, the data packet is transmitted immediately. 
- If there occurs a collision it waits and starts again.
- In this protocol, even if the two stations become greedy in midst of transmission of some other station they do not collide probably whereas, in persistent CSMA they collide.
- Also, if only one of the stations become greedy in midst of some other transmission in progress, it has no choice but to wait. 
- In persistent CSMA this greedy stations takes over the channel up on completion of the current transmission.
Using non – persistent CSMA can reduce the number of collisions whereas persistent CSMA only increases the risk. 
- But the non – persistent CSMA is less efficient when compared to the persistent CSMA.
- Efficiency lies in the ability of the protocols of detecting the collisions before starting the transmission. 

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