Question

What is Access collision Detection (CSMA/CD) in detail ? How it works detail information ? What is the purpose ? Why we need to use it?  please draw an explanation diagram about CSMA/CD.

Answer 1

CSMA/CD:

CSMA/CD stands for Carrier Sense Multiple Access/Collision Detection which is a MAC method
The meaning of the individual terms in CSMA/CD are defined below :
Carrier sense (CS): The carrier state detection makes sure that all network participants check whether the medium is currently free – only then does the protocol initiate data transmission
Multiple access (MA): Several participants (computers connected to the network) share a transmission medium
Collision detection (CD): The collision detection is an extension of the original protocol and regulates how to proceed in case data packets happen to collide

Hence CSMA/CD is a modification of pure carrier-sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

Working of CSMA/CD :
Step 1: Check if the sender is ready for transmitting data packets.
Step 2: Check if the transmission link is idle?
Sender has to keep on checking if the transmission link/medium is idle. For this it continously senses transmissions from other nodes. Sender sends dummy data on the link.If it does not receive any collision signal, this means the link is idle at the moment.If it senses that the carrier is free and there are no collisions, it sends the data. Otherwise it refrains from sending data.
Step 3: Transmit the data & check for collisions.
Sender transmits its data on the link. CSMA/CD does not use ‘acknowledgement’ system. It checks for the successful and unsuccessful transmissions through collision signals. During transmission, if collision signal is received by the node, transmission is stopped. The station then transmits a jam signal onto the link and waits for random time interval before it resends the frame. After some random time, it again attempts to transfer the data and repeats above process.
Step 4: If no collision was detected in propagation, the sender completes its frame transmission and resets the counters.

Find the flow chart for above explanation:



Purpose and Usage :
In wired communication, like Etherent or CAN protocol, CSMA/CD is used. This is because in wired communication, you can also sense what you have transmitted, which eventually means you can receive what you had transmitted, which is not the case in wireless.
CSMA CD only minimizes the recovery time after collision which will occur frequently in wired network
Hence the CSMA/CD is used in wired communication