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FCS (Frame Check Sequence) :

This field occupies 4 bytes in the whole frame. It is the one that is responsible for ensuring the integrity of data received. It relies on mathematical calculations to make sure that the data sent by a certain transmitter is intact when arriving at the receiver. To make it plain for you, consider this rather oversimplified scenario.
Let us assume that we have two computers ; one computer is a sender and the other is a receiver. Surely we want an error-free data transmission. For this goal, before data is transmitted, it is subjected to an operation which takes the whole binary number of data (because in network data is seen as zeroes and ones ), divides it by a predetermined number (also in binary), and the remainder is used as the check value (CRC).  This latter is then put in the trailer of the frame, which is FCS.
The frame, which contains data inside it, is put on wires for transmission. When getting at the receiving machine, the frame is de-encapsulated, in the meanwhile, the FCS field is opened to check the CRC value. Now, the receiver machine will carry out another operation by taking the data received and calculate it relying on the same mathematical method done at the sender. The results will be compared with the CRC value contained in the FCS field. If the CRC value, put by the sender in the frame, matches the data value, then the data would be considered to be intact and error-free; but if the values do not match, the data would be considered to be damaged.

Now let us make this explanation more clear by including decimal numbers ( which is roughly the same conception as the actual one) ; however, computer tasks are done using binary system.

Suppose that  the infromation to be transmitted is the number ‘2000’. Before this number is sent, it is divided by a predetermined number, for example, 6. So, 2000 divided by 6 equals 333 with a remainder of 2. Next, the remainder is taken as the check value, because it is always much smaller than the data itself as well as it is not wasting too much bandwidth, and put in the frame.

When it is received, the receiver takes the data, which is, in this case, ‘2000’, then extracts the CRC value from the frame, which is 2, and divides 2000 by 6. If the outcome is 2 as the one included in the frame, then it assumes that no error has occurred ; but if the outcome is another number than 2, then it assumes that an error has occurred and data is damaged. ( read this lesson further to know more about CRC calculation).

Note : 2000, 6, and 2 numbers are given just  to make things clear for you. There are no such numbers in real calculation, but the idea is the same.

This illustration up to now is just an oversimplification on the grounds that numbers are divided by a polynomial ; nonetheless, the principle is the same. In the actual process, the remainder, in binary system,  is also utilized as a short check value to see if an error has occured.

The next lesson is a sequel to this one
10: Ethernet - Ethernet II (version 2, or DIX) frame - FCS (Frame Check Sequence) field part 13 Reviewed by BOUFTIRA on 10:36:00 AM Rating: 5