Network Engineer Exam note--------CRC cyclic redundancy check calculation

CRC Detailed verification code calculation

(Network engineer Exam required)

Example Description: 2008 second half of the morning question (18).

The CRC is used to test the error, the generation of the polynomial is G (X) =x4+x+1, the information code word is 10110, the computed CRC check code is:

A. 0000 B. 0100 C. 0010 d.1111

Analysis

The symbolic representation assumes that the coefficients of the polynomial and the polynomial are represented by the same symbol, as

G (X) = X4+x+1

G (X) =10011

1. The known conditions are as follows:

The original code word to do m (x), namely: M (x) = 10110

Generate Polynomial notation g (x), i.e.: g (x) = 10011

The highest order of G (X) does r, here R = 4

2. Calculation Steps

(1) Calculation xrm (X)

That is, add the tail of M (X) to R 0.

XrM (X) = 10110 0000

(2) Calculate XRM (x) long except g (x), the remainder is recorded as Y (x)

The "Long divide" method here is calculated as follows:

XrM (X) 10110 0000

--G (X) 10011 (note bit correspondence mode, corresponding bit is different or operation can be)

00101 0000

--G (X) 100 11 (Same as calculation method)

001 1100

--G (X) 100 11

01111 (this number is already less than g (x), the calculation ends here, i.e. y (x))

Note that Y (x) is the number of bits R (here is 4), so y (x) = 1111

Y (X) is the CRC check code.

(3) Calculate transfer code word t (x) = XrM (x)-Y (x)

Calculation method: Connect y (x) at the end of M (x) to

i.e.: T (X) = 10110 1111

Answer

This problem only calculates the checksum y (X). The correct answer is: D