If I have CRCs for 2 blocks of data, is it practical to compute the CRC

for the combined block.

Yes, CRC's are just the block modulo the CRC polynomial.Quote:>If I have CRCs for 2 blocks of data, is it practical to compute the CRC

>for the combined block.

So you can combine two of them in the obvious way.

>> If I have CRCs for 2 blocks of data, is it practical to compute the CRC

>> for the combined block.

> Yes, CRC's are just the block modulo the CRC polynomial.

> So you can combine two of them in the obvious way.

the final output, but before such complications are added,

CRC(a XOR b) = CRC(a) XOR CRC(b).

To deal with the CRC of blocks that are appended to each other,

you have to realize that a CRC considers a message as a

polynomial in x, where the bits are the coefficients.

I.e. you're sending m(x). The CRC has a ganerator polynomial

p(x) of degree n, and CRC(m) = m(x)*x^n % p(x), where %

is the modulo remainder.

If you have a second message m'(x) which you want to prepend to the first,

then you need to find the length k of the first message and

compute CRC(m'm) = (m'(x)*x^(n+k) + m(x)*x^n) % p(x).

This can be converted to

CRC(m'm) = (CRC(m') * x^k + CRC(m)) % p(x).

Remembering that adding CRCs is actually done by XORing them, all you

need to do is to "shift" the CRC of m' by k positions, then you

can XOR it with the CRC of m.

The details aren't that hard to work out, although you won't find

polynomail multiplcation in the usual example CRC code.

--

-Colin

1. Help CRC-16, CRC-CCITT, BCH

Hi, everyone.

As you know

CRC-16 = x^16 + x^15 + x^2 + 1.

CRC-CCITT = x^16 + x^12 + x^5 +1.

BCH code => Various generator polynominals

My questions are as follows..

1. Could you show me some cases of these kinds of situation?

2. How exact each method can detect these kinds of erranous situation

in the 'normal'communication environment?

(Statistical data is much favored..)

For example:

For CRC-16-> single bit error : ?(%)

double bit error : ?(%)

Odd number bit error : ?(%)

(Various kind of)burst error : ?(%)

Please....

I'll be very pleased if I can get to know any 'little' answers.

TIA...

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