Im trying to understand CRC and I'm getting confused as how to calculate the 'divisor'.
In the example on wikipedia the divisor is 11 (1011) for input of 11010011101100
11010011101100 000 <--- input left shifted by 3 bits
1011 <--- divisor (4 bits) = x³+x+1
------------------
01100011101100 000 <--- result
How is the divisor calculated? In this example (x³+x+1) x is 2? Where did the 2 come from?
Answer
From the "Mathematics of CRC" section of that same wikipedia it starts "Mathematical analysis of this division-like process reveals how to pick a divisor that guarantees good error-detection properties." This is the key to it. Some divisors are better than others so you just find a standard one and use that usually.
The bottom of that page describes some of the different CRCs used and the polynomial that defines their divisors.