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From: dee on 14 Jan 2010 15:19
Subject:
From: deepthi <gdd0014(a)londonmet.ac.uk>

Dear Sir,

I am trying to calculate the FCS Feild of a MAC Frame (IEEE
802.11-2007 p.70).

bits='0000010000110000100000000000011100000011110001101100101011100101100110000000000001100111100010101101111011101101100110001000100010001000100010001000100010001000100010000000000100000011000010110001001100011011001000110010101100110011001110110100001101001011010100110101101101100011011010110111001101111011100000111000101110010011100110111010001110101011101100111011101111000011110010111101000110001001100100011001100110100001101010011011000000001000010001000000110000010100001011000101110010010100110001010010010110110'

I am using a slightly modified code d/loaded from
http://www.cs.washington.edu/homes/dhalperi/useful.html#LATEX

--------------------------------------

function ret = crc32(bits) % receives string of characters (class
char) from main program
poly = [1 de2bi(hex2dec('EDB88320'), 32)]' % generates the poly from
CRC32
bits = bits(:);
bits=str2num(bits) % converts characters to numbers for type
compatibility- i added this line

% Flip first 32 bits
bits(1:32) = 1 - bits(1:32);
% Add 32 zeros at the back
bits = [bits; zeros(32,1)];

% Initialize remainder to 0
rem = zeros(32,1);
% Compute the CRC32

for i = 1:length(bits)
rem = [rem; bits(i)]; %#ok<AGROW>
if rem(1) == 1
rem = mod(rem + poly, 2);
end
rem = rem(2:33);
end

% Flip the remainder before returning it
ret = 1 - rem;
ret=num2str(ret); % converts back to chars before passing back to main
program - i added this line
end

----------------------------------------
I cannot fully understand how this works. But, I feel the output is
not correct.

When I calculated the poly for edb88320 manually it gives 1110 1101
1011 1000 1000 0011 1110 0000 which is different to the output for
poly in the program.

I have spent 2 weeks trying to understand how crc-32 works and how
this works, but it seems that more i look at it, more trouble I am
getting into now.

Can someone pls explain your code or check where I am wrong?

Thank you very much

Deepthi
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