No notes for slide• • • • • • • When you receive a parcel from a friend, how would you know that the contents are what your friend has send to you? One way is to call your friend up and confirm with him what he has sent to you. Another way is for your friend to include a packing list detailing what is in the parcel, and you can tally the items in the parcel with the packing list. In the same way when your computer/ printer receives a message, how would your computer/printer know that the received message is correct?

One way is for the computer to double check with the sending computer what he has received, just like you calling up your friend to check the contents of the parcel. Another way is for the sending computer to include some information in the message, so that by examining the information you are able to know whether the received message is correct. (Like the packing list). Parity check(Error Detecting Codes) • 1. Error Detection Codes Parity Check 1 By W.I.D.Perera(Esha) Vavuniya Campus, University Of Jaffna, SL • Content Why we need Error Detection? What are the Errors?

What are the Error Detection Mechanisms?  Parity Check  Single Parity Check  Two Dimensional Parity Check  Drawbacks of Parity 2 • Error Detection • Data can be corrupted during transmission. Even with best prevention methods. • For a reliable communication, errors must be detected and corrected. • Error detection and correction are implemented either at data link layer or the transport layer of the OSI model. 3 • Types of Errors • Single bit error:- - Only one bit in the data unit has changed. 4 0 changed to 1 Received Sent • Types of Errors • Burst error:- - It means that two or more bits in the data unit has changed.

5 Sent Received Bits corrupted by Burst Error • Error Detection Codes • Error detecting code is to include only enough redundancy to allow the receiver to deduce that an error occurred, but not which error, and have it request a re- transmission. • Used in Low Noisy Channels – Fiber Optics • Error detection uses the concept of redundancy, which means adding extra bits for detecting error at the destination • To detect an error, something extra has to be added to the data/signal • This extra is an error detection code • 3 Techniques are there: Parity Check, Check Sum, CRC 6 • Parity Check • The Simplest method Available - it’s a linear, systematic block code • 2 Parity Check Methods are there: • Simple Parity - For Single bit Errors • Two Dimensional - For Burst Errors • How to use Parity Methods? • Parity Generate – Sender’s Side • Parity Detect – Receiver’s Side 7 • Single Parity Check(VRC) Vertical Redundancy Check • In Single parity check, a parity bit is added to every data unit so that the total number of 1s is even or odd. 11010011 1 • Therefore, the total number of bits transmitted would be 9 bits. ( 8 bits of information + 1 bit parity = n data bits into n+1 bits) 8 Information(1 byte – 8 bits) Additional Bit added(PARITY BIT) • Single Parity check(Cont.) Parity Generation • There are two ways to generate a Single parity bit. • One is called Even parity and the other is Odd parity. Where Is The Startup Programs Folder In Windows Xp on this page. • What is even parity?

• the total number of 1’s transmitted must be even. • What is odd parity? • the total number of 1’s transmitted must be odd 9 • Parity Generation-VRC • Parity bit is chosen so that the number of 1 bits in the code-word is even (or odd) • computing the (even) parity bit as the modulo 2 sum or XOR of the data bits. • use XOR and XNOR logic gates in circuits to generate parity bits in practically (in devices). History Killer Pro V4.1.1 there. 10 • How is the even parity bit generated? Total number of ‘1’s should be even. If the byte that we want to transmit is: • Step 1: count the number of 1’s in the byte.