Error Detection and Correction-Cont

ƒ     Performance of VRC o    VRC can detect all single bit errors o    Can also detect Burst errors as long as the total number of bits changed is ODD (1,3,5 etc) Examples Š    We  have  an  Even  Parity  data  unit  where  the  total  number  of  1's  including  the parity bit is '6' : 1000111011 Š    If  3  bits  change  value  resulting  parity  will  be  odd  and  an  error  will  be  detected: 1111111011: 1's = 9 Š    If  2  bits  change  value  resulting  parity  will  still  be  even  and  error  will  not  be detected: 1110111011: 1's = 8 o    VRC cannot detect errors when the total number of bits changed are even o    VRC checker will return a result of 1 and the data unit will be rejected o    The same hold true for any odd number of errors o    In  the  second  case,  VRC  checker  will  check  parity  and  will  return  an  even number although the data unit contains two errors o    VRC cannot detect error when the number of bits changed is even o    If any two bits change in TX, the changes cancel each other and the data unit will pass a parity check even though the data unit is damaged o    Same holds true for any even number of errors