CRC

AIM 
To write a program for implementing on a data set characters the three CRC polynomials – CRC 12, CRC16 and CRC CCIP

Description:

Calculation of Polynomial Code (CRC) Checksum
1. For degree of generating polynomial G(x) = r , append r zero bits to low-order of frame. The frame now has m+r bits.
2. Divide the bit string corresponding to G(X) into the bit string xrM(x) mod(2)
3. Subtract the remainder R(x) from the bit string xrM(x) mod(2)
  Frame: 1 1 0 1 0 1 1 0 1 1
Generator: 1 0 0 1 1
G(X) = X4 + X + 1
Message after appending four 0’s:
1 1 0 1 0 1 1 0 1 1 0 0 0 0
Remainder: 1110
Transmitted Frame:
1 1 0 1 0 1 1 0 1 1 1 1 1 0

Common CRC Generator Polynomials

CRC-32: 

 x32 + x 26 + x 23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
Used in FDDI, Ethernet.

CRC-CCITT:

 x16 + X12 + x5 + 1
Used in HDLC.

CRC-8:

 x8 + x2 + x + 1
Used in ATM.

PROCEDURES 

Step-1: Read the frame
Step-2: Read the generator polynomial
Step-3: find out the degree of the generator polynomial
Step-4: Append the number of the zero’s to the frame that number is equal to the degree of the polynomial
Step-5: Find out number of digits in the generator polynomial
Step-6: Repeat the following until the number of digits are exhausted
Step-7: If the frame is starting with 1 , then exclusive-or the frame with generator
Step-8: Check whether the result obtained in step 7 is starting with 1, If so exclusive-or the remainder with the generator
Step-9: If the result obtained in step7 is starting with 0, then exclusive –or the remainder(result) with zeros. The number of zeroes must be equal to the length of the generator.

PROGRAM FOR CYCLIC REDUNDENCY CHECK 

#include<stdio.h>
#include<conio.h>
int gen[4],genl,frl,rem[4];
void main()
{
int i,j,fr[8],dupfr[11],recfr[11],tlen,flag;
clrscr();
frl=8; genl=4;
printf("enter frame:");
for(i=0;i<frl;i++)
{
scanf("%d",&fr[i]);
dupfr[i]=fr[i];
}
printf("enter generator:");
for(i=0;i<genl;i++)
scanf("%d",&gen[i]);
tlen=frl+genl-1;
for(i=frl;i<tlen;i++)
{
dupfr[i]=0;
}
remainder(dupfr);
for(i=0;i<frl;i++)
{
recfr[i]=fr[i];
}
for(i=frl,j=1;j<genl;i++,j++)
{
recfr[i]=rem[j];
}
remainder(recfr);
flag=0;
for(i=0;i<4;i++)
{
if(rem[i]!=0)
flag++;
}
if(flag==0)
{
printf("frame received correctly");
}
else
{
printf("the received frame is wrong");
}
getch();
}
remainder(int fr[])
{
int k,k1,i,j;
for(k=0;k<frl;k++)
{
if(fr[k]==1)
{
k1=k;
for(i=0,j=k;i<genl;i++,j++)
{
rem[i]=fr[j]^gen[i];
}
for(i=0;i<genl;i++)
{
fr[k1]=rem[i];
k1++;
}
}
}
}

INPUT:
enter frame :
1 1 1 1 1 1 1 1
enter generator :
1 1 0 1

OUTPUT:
frame received correctly