validating CRC calculation

0
votes

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...

1
networkingcrc
You have to continue to the end of the message before concluding if there is an error or not. - guga
Hi I edited it with what I think is right. Is it correct? - sam
Not yet. You have to align the 1 from the generator with the 1 in the message (1st operation) - note that the first 2 operations you perform cancel each other. - guga

1 Answers

3
votes
01001001011
 1001||||||
 ----||||||
 0000001011
       1001
       ----
       0010 <- Error!