Carrier Sense Multiplex Access/Collision Detect

 

Hi Folks,

Ethernet as said is the most revolutionary technology for last few decades, back to old days when network was evolving and speed  hitting gigabyte was a dream at distant. Networks were running at 10 Mbps and organisation computers competing with each other for bandwidth, we have this CSMA/CD technology for transmitting data and sensing collision for better and reliable communication.

As per definition a collision domain is a network segment where shared access media is permitted. Devices on shared media access  must compete for access when transmitting data.

CSMA/CD technology works with half duplex data transmission. Refer to flowchart above following are the steps a PC go through while transmitting data in half- duplex environment.

Step-1 :  when a PC is ready to send data, it starts sensing the channel by monitoring the data pattern on the channel for a Inter frame Gap (IFG) period. IFG is 9.6 micro second for 10 Mbps and 0.96 micro seconds for 100 mbps. when the same pattern is observed for IFG duration, indicates channel “Free”

Step-2 : PC starts transmitting the data on Tx wire and loops the same data on its Rx wire. As a part of collision detect it keeps comparing data pattern on its Tx and Rx. If some other PC tries to transmit anything in this time, it listens to that on Rx and eventually there is a mismatch in Tx & Rx data pattern resulting in collision detected.

Step -3 : When collision is detected PC starts sending JAM signal by intentionally sending corrupt CRC value in its ethernet frames . Other PCs in the network also listen to this JAM signal  and start  waiting for a random timer based on Binary Exponential Backoff (BEB) algorithm and then try again starting from step 1. After 16 consecutive collision detect it give up and discard the frame.

In Full-duplex network, PCs connect to switch port in point to point manner and  can Transmit and Receive simultaneouly.CSMA/CD is disabled in this case.

Folks your feedback will certainly help me write better.  Thanks and keep simplifying.