Error Checking

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Error Checking: Parity Bits

Parity Bits:

Parity Bits are an extra bit added to a string of bits to ensure that a number is correct

There are two types of Parity:

Odd Parity:

In this you will set parity to a 1 when you have an odd number of 1’s

Evan parity

This is basically the same, but you set the parity to 0 when you have an even number of 1’

Examples:

1011010 = 1011010010 (6 1’s)

1110111 = 11101110 (6 1’s)

1001001 = 10010011 (4 1’s)

0010000 = 00100001 (2 1’s)

1010101 = 10101011 (4 1’s)

Parity Problems:

The problems with Parity checking include:

Situations where an even number of errors have have been occurred

When the parity gets changed in transmission

It only tells you that an error has occurred, but it doesn’t tell you which type of error, nor where it is

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Error Checking: Majority Voting

Majority Vote:

Majority voting works by splitting the transmission in to three bits and transmitting each one three times

It accepts the options for each three bit string that appears most often

Problems with Majority Voting:

The majority is not always right

2 errors out of three will correct to the wrong value

Slows down transmission

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Error Checking: Check Digits

Check Digit:

A Check digit is a digit added to a piece of data to check that it is correct

i.e. 241443

It would add up all of there numbers (18) and then add those two numbers together (as 18 does not contain only one digit), so 9 would be the check digit

You then add that to the end of the byte

You can also use Modulo 11:

This is another type of error checking that involves using weighting

To do modulo 11, you:

Multiply the digits in the number by the position they are in the number (starting at 2)

This is weighting

You then remainder - divide the total by 11

Meaning you are after the remainder from the devision

You hen take the remainder away from 11

This is the check digit

If you get a remainder of 10, the check digit is X So as not to confuse the computer