By: Mamadou Korka Diallo

Mohammed S. Ibrahim

Bello Ahmed Ibrahim

Under the Guidance of: Mr. Sharif Omar Salem

Network must be able to transfer data from one device to another with acceptable accuracy. For must applications a system must guaranty that the data received are identical to the data transmitted. Anytime data are transmitted from one node to the next, they can become corrupted in passage. Many factors can alter one or more bits of message. Some applications request a mechanism for detecting and correcting errors.

However, some application can tolerate a small level of errors. For example a random error in audio or video transmission may be tolerable, but when we transfer a text, we expect a very high level of accuracy.

INTRODUCTION

Single bit errorA single bit error is an isolated error condition that

alters one bit but does not affect nearby bits.Burst errorA burst error is a contiguous sequence of bits in

which the first and last bits and any number of intermediate bits are received in error.

TYPES OF ERROR

A single bit can occur in the preserve of while noise, when a slight random deterioration of single-to-noise ratio is sufficient to confuse the receiver’s decision of a single bit. On the other hand burst errors are more common and more difficult to deal with. Burst error can be caused by impulse noise.

HOW A SINGLE BIT ERROR DOES DIFFER FROM A

BURST ERROR?

Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination there are three types of redundancy checks are common in data communication: Parity check Cyclic Redundancy check (CRC) Checksum

THREE TYPES OF REDUNDANCY CHECKS USED IN DATA

COMMUNICATION.

A parity bit is a bit that is added to ensure that the number of bits with the value (one) in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code. There are two variants of parity bits: even parity bit and odd parity bit. When using even parity, the parity bit is set to 1 if the number of ones in a given set of bits is odd, making the number of ones in the entire set of bits even. If the number of ones in a given set of bits is already even, it is set to a 0. When using odd parity, the parity bit is set to 1 if the number of ones in a given set of bits is even, keeping the number of ones in the entire set of bits odd. And when the number of set bits is already odd, the odd parity bit is set to 0. In other words, an even parity bit will be set to "1" if the number of 1s + 1 is even, and an odd parity bit will be set to "1" if the number of 1s +1 is odd.

WHAT IS PARITY CHECK ?

In case of redundancy check method we have to append the data unit with some extra bits. These extra bits are called parity.This parity or parity hit can be even or odd.in case of even parity we have to make number of 1’s even, including the parity hit e.g. 1110001 is the data unit where the no. of l’s is already even then we will insert 0 at the next to data unit it’, 1110001. In case of odd parity we have to make no. of l’s odd, including the parity bit. e.g. 1111000 is the data unit, where the no. of 1’s is even then we will insert I at the next to data unit i.e. 11110001.

THE DIFFERENCE BETWEEN EVEN PARITY AND ODD

PARITY

Parity is used in many hardware applications where an operation can be repeated in case of difficulty, or where simply detecting the error is helpful. For example, many microprocessor instruction caches include parity protection. Because the I-cache data is just a copy of main memory, it can be disregarded and re-fetched if it is found to be corrupted.

THE USAGE OF PARITY BIT CHECKER

In this technique, a redundant bit called a parity bit is added to every data unit so that the total number of 1s in the unit becomes even (or odd). Suppose we want to transmit 1100001. Adding the number of 1’s gives us 3, an odd number. Before transmitting, we pass the data unit through a parity generator. The parity generator counts the 1’s and appends the parity bit to the end (al in this case).

HOW CAN THE SIMPLE PARITY BIT DETECT A DAMAGED DATA

UNIT ?

Regardless of the design of the transmission system, there will be errors, resulting in the change of one or more bits in a transmitted frame. When a code word is transmitted one or more number of transmitted bits will be reversed due to transmission impairments. Thus error will be introduced. It is possible to detect these errors if the received code word is not one of the valid code words. To detect the errors at the receiver, the valid code words should be separated by a distance of more than 1.

The concept of including extra information in the transmission of error detection is a good one. But instead of repeating the entire data stream, a shorter group of bits may be appended to the end of each unit. This technique is called redundancy because the extra bits are redundant to the information; they are discarded as soon as the accuracy of the transmission has been determined.(Hamming Code).

ERROR CORRECTION

In the even parity system the parity bit is used to ensure that the total number of 1's when added together equals an even number.

Even parity works by counting the number of 1's and if the total is even then the parity bit is set to zero and if it is odd the parity bit is set to one. Thus, the total number of 1's in the byte is always even.

PARITY BIT GENERATOR

PARITY BIT GENERATOR

Transmission sent using even parity:

A wants to transmit: 1001A computes parity bit value: 1^0^0^1 = 0A adds parity bit and sends: 10010 Date will be send is : 10010

In the odd parity system the parity bit is used to ensure that the total number of 1's when added

together equals an odd number.

Odd parity works by counting the number of 1's and if the total is odd then the parity bit is set to zero

and if it is even the parity bit is set to one. Thus, the total number of 1's in the byte is always odd.

PARITY BIT GENERATOR

Transmission sent using odd parity:

A wants to transmit: 1001 A computes parity bit value: ~(1^0^0^1) = 1A adds parity bit and sends: 10011Date will be sent is : 10011

PARITY BIT GENERATOR

•At the receiving side there will be an algorithm used to detect if there is an error occur in the original data sent

•XOR logic gate is used to check the specific block of data received

•This mechanism enables the detection of single bit errors, because if one bit gets flipped due to line noise, there will

be an incorrect number of ones in the received data

FUNCTIONS OF DETECTING AND CORRECTING

FUNCTIONS OF DETECTING AND CORRECTING

XOR truth table

Transmission sent using even parity:

A wants to transmit: 1001 A computes parity bit value: 1^0^0^1 = 0

A adds parity bit and sends: 10010 *** TRANSMISSION ERROR ***

B receives: 11010 B computes overall parity: 1^1^0^1^0 = 1

B reports incorrect transmission after observing unexpected odd result.

FUNCTIONS OF DETECTING AND CORRECTING

Data can be corrupted during transmission. Some application require that errors be detected and corrected.Parity bit is one of the mechanism applied to detected errors but it does have ability to correct that error its does not determine at which specific bit that error occur

CONCLUSION

Thank You

Questions?