I am going over the following past paper question:
Consider the 4-bit generator, G = 1001, and suppose that the data portion of a bit stream to be transmitted prior to Cyclic Redundancy Check (CRC) calculation is 11001001. Show the actual bit stream transmitted. Suppose the leftmost bit in the transmitted bit stream is inverted due to noise on the transmission link. Show that this error is detected at the receiver’s end.
I have calculated the CRC to be appended to the end of the transmission using XOR as follows:
11001001000
1001|||||||
----|||||||
0101|||||||
1001||||||
-----||||||
001000||||
1001||||
----||||
000001100|
1001|
----|
01010
1001
----
0011
So R = 011 is appended to the transmission and is what is sent.
For the second part of the question I do the same thing except due to the error the leftmost but is now 0 so:
01001001011
1001|||||||
----|||||||
1101|||||||
1001|||||||
----|||||||
01001||||||
1001||||||
----||||||
0000001011
1001
----
0010 therefore there is an error
Where do I go from here? if it is all zeroes do I stop? But this would mean there is no error...
01001001011
1001||||||
----||||||
0000001011
1001
----
0010 <- Error!