Error Detection in Computer Networks | Parity Check

Error Detection in Computer Networks-

 

When sender transmits data to the receiver, the data might get scrambled by noise or data might get corrupted during the transmission.

 

Error detection is a technique that is used to check if any error occurred in the data during the transmission.

 

Error Detection Methods-

 

Some popular error detection methods are-

 

 

1. Single Parity Check
2. Cyclic Redundancy Check (CRC)
3. Checksum

 

In this article, we will discuss about Single Parity Check.

 

Single Parity Check-

 

In this technique,

• One extra bit called as parity bit is sent along with the original data bits.
• Parity bit helps to check if any error occurred in the data during the transmission.

 

Steps Involved-

 

Error detection using single parity check involves the following steps-

 

Step-01:

 

At sender side,

• Total number of 1’s in the data unit to be transmitted is counted.
• The total number of 1’s in the data unit is made even in case of even parity.
• The total number of 1’s in the data unit is made odd in case of odd parity.
• This is done by adding an extra bit called as parity bit.

 

Step-02:

 

• The newly formed code word (Original data + parity bit) is transmitted to the receiver.

 

Step-03:

 

At receiver side,

• Receiver receives the transmitted code word.
• The total number of 1’s in the received code word is counted.

 

Then, following cases are possible-

• If total number of 1’s is even and even parity is used, then receiver assumes that no error occurred.
• If total number of 1’s is even and odd parity is used, then receiver assumes that error occurred.
• If total number of 1’s is odd and odd parity is used, then receiver assumes that no error occurred.
• If total number of 1’s is odd and even parity is used, then receiver assumes that error occurred.

 

Parity Check Example-

 

Consider the data unit to be transmitted is 1001001 and even parity is used.

Then,

 

At Sender Side-

 

• Total number of 1’s in the data unit is counted.
• Total number of 1’s in the data unit = 3.
• Clearly, even parity is used and total number of 1’s is odd.
• So, parity bit = 1 is added to the data unit to make total number of 1’s even.
• Then, the code word 10010011 is transmitted to the receiver.

 

 

At Receiver Side-

 

• After receiving the code word, total number of 1’s in the code word is counted.
• Consider receiver receives the correct code word = 10010011.
• Even parity is used and total number of 1’s is even.
• So, receiver assumes that no error occurred in the data during the transmission.

 

Advantage-

 

• This technique is guaranteed to detect an odd number of bit errors (one, three, five and so on).
• If odd number of bits flip during transmission, then receiver can detect by counting the number of 1’s.

 

Also Read- Checksum

 

Limitation-

 

• This technique can not detect an even number of bit errors (two, four, six and so on).
• If even number of bits flip during transmission, then receiver can not catch the error.

 

EXAMPLE   Consider the data unit to be transmitted is 10010001 and even parity is used. Then, code word transmitted to the receiver = 100100011 Consider during transmission, code word modifies as 101100111. (2 bits flip) On receiving the modified code word, receiver finds the number of 1’s is even and even parity is used. So, receiver assumes that no error occurred in the data during transmission though the data is corrupted.

 

To gain better understanding about single parity check,

Watch this Video Lecture

 

Next Article- Cyclic Redundancy Check

 

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