2.2. Error detection methods

Question 1

Describe how parity checks work

Answer 1

Detects errors in data transmission.

Parity (odd/even) agreed by sender and receiver.

Data split into 7-bit bytes + 1 parity bit.

Sender adds parity bit to make total 1s even/odd.

Receiver checks parity; a mismatch shows an error.

In block check, the error located at row-column intersection.

Question 2

Question: Describe a situation in which a parity check cannot detect corruption of a byte OR

Describe a situation in which an error during a parity check goes undetected.

Answer 2

• Error will not be detected if multiple errors in the same byte still produce the same parity bit.
• It will not be detected if an even/odd number of digits are changed (depending upon even/odd parity used).
• It will not be detected if a transposition error has occurred.

Question 3

How does a checksum work?

Answer 3

A calculation is done on the data before transmission to produce a checksum value.

The checksum is sent along with the data.

The receiver recalculates the checksum using the same algorithm.

If both checksum values match, the data is correct.

If the values differ, an error is detected and retransmission is requested.

Question 4

How does Echo check work?

Answer 4

When data is sent, a copy is returned to the sender.

The sender compares the returned data with the original.

If both are the same, transmission was successful.

If they differ, an error occurred during transmission.

Not very reliable, as the error could occur either when sending or when returning the data.

Question 5

What’s the process of the check digit?

Answer 5

It is an additional digit that is calculated from the data.
It is then added to the data.
The digit is recalculated when data is entered.
Both digits are compared to check for errors.
If the digits are different, an error is detected.
If digits match, no error is detected.

Question 6

There are a number of different methods used to generate a check digit. Two common methods will be considered here:

Answer 6

1. ISBN 13
2. Modulo-11

Question 7

How does ARQ work?

Answer 7

Uses acknowledgment and timeout.

Sender starts a timer when data is sent.

Receiver checks data using an error detection method.

If no error, a positive acknowledgment is sent.

If error found, a negative acknowledgment is sent, and data is resent.

If no acknowledgment is received in time, sender resends data.

Process repeats until data is received correctly or retry limit is reached.