# Comparison of CSMA series: p-pcsma; CSMA / Ca; CSMA / CD

2020-12-07 21:46:28

# CSMA Series summary

CSMA, Also known as carrier sense multiple access protocol . In the computer network course , There are four basic protocols and two practical applications （ Namely 802.11 and 802.3）

busy Free Transmission conflict application
1-p CSMA Keep listening , Waiting for leisure Grab it now （ The transmission probability is 1） （ Conflict detected after transmission ） Wait at random for a while and try again -
Non-p CSMA No more continuous monitoring , It's a random wait and try again Grab it now （ Conflict detected after transmission ） Wait at random for a while and try again -
p-p CSMA【 Slotting 】 Waiting for the next slot Yes p Probability transmission ,1-p The probability is delayed to the next slot （ Conflict detected after transmission ） Wait at random for a while and try again -
CSMA/CD（detect） - - （ Collision detected in transit ） Stop the transmission immediately ; Then randomly wait for a period of time and try again -
Binary exponential backward 1-p CSMA/CD Keep listening Grab it now （ Collision detected in transit ） Stop the transmission immediately ; Then wait [0,2^i-1] Try again after random slots Classic Ethernet -802.3
CSMA/CA（avoid）【 Slotting 】 Pause the timer when a frame is being sent Start timing with a random backward slot Have not received ACK, Then double the number of time slots selected back WLAN -802.11

among ,CSMA/CD What is emphasized is that it is in Signal transmission Using collision detection to speed up transmission , If we must investigate its performance when the channel is busy and when the channel is idle , Consideration should be given 1-pCSMA. And the binary index goes back CSMA/CD And CSMA/CD The difference between the models is that the time algorithm of backoff is clear .

Binary exponential backward 1-p CSMA/CD Every time 2 A random number within the exponential order of begins to recede . It's not hard to see that doing so will lead to more and more time to retreat , So on the 10 After the conflict , This number will no longer increase , And will be in the 16 Give up sending after failed attempts , Return an error report .

and CSMA/CA It is quite different from the previous models . The main The reason is that the radio channel is half duplex type , Can't listen for noise while sending data , So a more cautious strategy was adopted to avoid conflict （ So it's called conflict avoidance ）. The following figure shows the throughput difference between several protocols .

For more detailed introduction, you can follow my blog About computer networks -MAC Content of sublayer ~~

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