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

📌Highlights: Hello Friends ! in this post we discuss about Carrier Sense Multiple Access (CSMA) / Collision Detection (CD), CSMA/CD protocol.In this post we covered about Non persistent CSMA protocol, about One Persistent CSMA protocol, about P - Persistence CSMA protocol, how channel  can detect collision.

⧭ Let we know about CSMA

➧ It has wire oriented channel

➧ It is used to communicate in small areas.

➧ Here single channel can share large number of users.

➧ Carriers means to carry same information.

Here collision will also occur but less than ALOHA.

⧭ CSMA PROTOCOLS

1. Non persistent CSMA : Before transmission each and every station  suppose to sense the channel and do the following;

➧ Step 1: If the channel is free than transmit.

➧ Step 2: If channel is busy than wait for random amount of time and go to step 1

Here random means generate random number.

➧ Step 3: If collision is found ( because of lack of acknowledgement ) than wait for random time and go to step 1.

2. One Persistent CSMA : Before transmission each and every station suppose to sense the channel and do the following.

Step 1: If  the channel is free than transmit.

Step 2: If the channel is busy than continuously sense the channel until it become free and than transmit.

Step 3: If collision is found ( because of lock of acknowledgement ) than wait for random time and go to step 1.

⧭ Note : It is Greedy approach because it continuously sense the channel.

➧ In light load delay will less.

➧ But collision will be more.

➧ It performance is not good.

➧ It is worst case.

➧ It is rarely used.

3. P - Persistence CSMA : Before transmission each and every station suppose to sense the channel and do the following.

If channel is free than transmit with probability (P) (Probability of success) and differ the operation by one slot with probability (1-P) (probability of delay).

If channel is busy than continuously sense the channel until it become free and than go to step 1.

If collision is found ( because lack of acknowledgement) than wait for random time and goto step 1.

Explain with example:

For example , If there are two channel than the probability are.

So we can say that if there are two channel and want to transmit than their probability of success is 50%. or 0.5

If B & C sense the channel and see if the channel is empty than both generate the random number ( through number generator ) between (0-1) ( 0.0, 0.1, 0.2, 0.3, -----, 0.9, 1 )

If suppose B = 0.4 & C = 0.5

Through protocol , If the random number is less than 0.5 than transmit at that time , But if more than 0.5 than differ for next slot.

If 4 station are there than probability or variable = 1/4 i.e 0.25 and if 4 station sense the channel and one station will get less than 0.25 and other will get more than 0,25 than 0.25 than collision will reduced.

Note : P = 1/n   

where : n is number of ready station ( those will sense channel simultaneously ) 

Note: 

1. If the value of P is underestimated than delay will increase i.e ( wait necessary).

Example : If our estimated value is 0.5 for two channel. But if it is used by 10 channel then the value will be (0.1) . But here the value is still (0.5) than most of the channel are is wait.

It is the case for underestimation 

2. If the value of  P will be overestimated than collision will increase.

Example : If our estimated value is 0.1 for 10 channel . But if it is used by 2 channel than the value will be 0.5, but still the value is 0.1 than most of channel are ready for transmission. So collision will increase.

Carrier Sense Multiple Access (CSMA) / Collision Detection (CD) [CSMA / CD ]:

⧭ Each station has a special electronic circuit that can detect the collision because of fluctuation in voltage (single level) during collision. The circuit is active only for those station which are currently transmitting.

After detection the collision, station suppose to do following

➧ Immediately stop their transmission ( because transmission are unnecessary & useless ) so stop transmission will improve for channel efficiency.

➧ Send high frequency positive analog signal called jammin signal into channel so that other station should not inter mean during collision period.