ERROR CHECKING

Computers need methods to check for data transmission errors caused by interference.

These methods include the use of:

1. Parity bits
2. Parity block check 
3. Check digits
4. Checksums and 
5. Automatic Repeat Requests (ARQ) 

Parity block check

This method of error checking is similar to a traditional parity check except the data is arranged in a virtual table with parity bits added to each row and column. A single corrupted bit can be spotted at the intersection of a corrupted row and column. Multiple corrupted bits would also flaw this system.

Parity bits

• Computers use either odd or even parity
• When sending a byte of data, one bit is used as a parity bit 
• This bit is set to a 1 or 0 to make the total number of 1s or 0s in the byte (including the parity bit) odd or even depending on the machine

Your parity bit will always be the most significant bit

Using parity for error detection

• When data is transmitted, the parity bit is set at the transmitting end, and parity is checked at the receiving end
• If the wrong number of bits is ‘on’, an error has occurred
• The receiving computer notifies the transmitting end and the data is resent
• How could a parity check allow an erroneous byte past detection?

Check digits

• A check digit is an additional digit at the end of a string of other numbers designed for mistakes in input or transmission

Modulo 10 function allows u to find the remainder of 10 

• Printed books and other products have a unique barcode with an ISBN (International Standard Book Number) or EAN (European Article Number) 

Checksums

1. A checksum works in a similar way to a check digit

• A total sum of all bytes in a transmission of data is calculated using an algorithm
• The sum is often sent with the data
• The receiving computer recalculates the checksum based on the data it received and compares it with the checksum received or a known, expected value
• If it does not match, the data may have been altered or corrupted during transmission and the data may be resent

2. Checksums are used with Credit Card numbers [=       

Automatic Repeat Requests

• Computers using ARQs will automatically return an acknowledgment that the data is correct
• The simplest Stop-and-wait ARQ will resend the data if an acknowledgment is not received within a specific period; it will assume that there was a flaw in the data or transmission error
• It is sometimes used with mobile phone network data