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# Error Detection and Correction

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Error Detection and Correction presentation for an IT Student

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### Error Detection and Correction

1. 1. ERROR DETECTION & CORRECTION Topic Taken From Unit-3
2. 2. IntroductionTo Error Detection And Correction  When a transmission of digital signal takes place between two systems such as computers as shown in given figure.  They get contaminated due to the addition of NOISE to it.  It is necessary to detect and correct errors because these errors can become a serious threat to the accuracy of the digital system. Binary signal Binary signal contaminate d by noise Computer 1 Computer 2
3. 3. TYPES OF ERRORS- There are three types of errors- 1.CONTENT ERRORS 2.FLOW INTEGRITY ERRORS 3.BIT ERRORS 1. CONTENT ERRORS:- The content errors are nothing but errors in the content of a message For e.g. ”0” may Receive as “1“or vice versa. Such errors are introduced due to noise added into the in data signals during its transmission. 2. FLOW INTEGRITY ERRORS:- Data is sent as data packets across a data network. It is not necessary that data packets sent by a source to the destination take the same path. As you see in figure, packet 1 goes via nodes A, B, and D. Packet 2 goes directly from node A to node D. Packet 3 starts its journey from nodes A to D via nodes B and C. Flow of these data packets from the source to the destination may be affected in several ways.
4. 4. 1.SINGLE BIT ERROR 2.BURST ERROR SINGLE BIT ERROR:- • The term single bit error suggests that only one bit in the given data unit such as a byte is in a error. • That means only one bit can be change from 1 to 0 or 0 to 1, as shown in the figure. BURST ERRORS:- • If two or more bits from a data unit such as byte change from 1 to 0 or from 0 to 1 then burst errors are said to have occurred. • The length of the burst is measured from the first corrupted bit to the last corrupted bit. Some of the bits in between may not have been corrupted. • Burst errors are illustrated in the given figure. 0 1 1 1 0 0 1 1 10 01011 0 1 01 1 1 0 0 11 0001110 Medium Medium Transmitted Byte Transmitted Byte Received Byte Received Byte Error Errors Types Of Bit Errors
5. 5. Error Detection When a code word is transmitted, one or more number of transmitted bits will be reserved (0 & 1 or vice versa) due to transmission impairments. Thus the errors will be introduced. It is possible for the receiver to detect these errors if the received codeword (corrupted) is not one of the valid codeword's. When the errors are introduced, the distance between the transmitted & received codeword's will be equal to the number of errors as illustrated in given figure. Hence to detect the errors at the receiver, the valid codeword's should be separated by a distance of more than 1. Otherwise the incorrect received codeword’s will also become some other valid codeword’s & the error detection will be impossible. Transmitted code word 10101100 11101011 00100101 Received code word 11101100 01111011 10110001 Number of errors 1 2 3 Distance 1 2 3 errorerrorerror
6. 6. Error Detection Methods What is ERROR DECTECTION METHODS? Before thinking of correcting the errors introduced in the data bits is necessary to first detect them. For error detection & correction it is necessary to add some check bit to a block of data bits. These check bits are also known as “Redundant bits” because they do not carry any useful information. Some of the most important error detection methods are as follows- 1. Parity Checking. 2. Checksum Error Detection. 3. Cyclic Redundancy Check (CRC). Error Detection Methods Parity Checking Checksum Error Detection Cyclic Redundancy Check (CRC)
7. 7. Error Detection Method 1. PARITY:- The parity of the 8-bit transmitted word can be either even parity or odd parity. Even Parity means the number of 1’s in the given word including the parity bit should be even (2,4,6,8,…..). Odd Parity means the number of 1’s in the given word including the parity bit should be odd (1,3,5,7,…..). 2.CHECKSUM:- A checksum is a count of the number of bits in a transmission unit that is included with the unit so that the receiver can check to see whether the same number of bits arrived. If the counts match. Its assumed that the complete transmission was received. Both TCP and UDP communication layers provide a checksum count and verification as one of their services. 3. Cyclic Redundancy Check (CRC) CRC is based on binary division. A sequence of redundant bits called CRC or CRC remainder is appended at the end of a data unit such as byte. The resulting data unit after adding CRC remainder becomes exactly divisible by another predetermined binary number.
8. 8. Error Correction Technique Codes are generated by adding a group of parity bits or check bits. The source generates the binary data which is accepted by encoder to add the check bits to them to produce the code words. The check bits are used by decoder to detect and correct the errors. The encoder adds check bits to the data bits according to the prescribed rule. Decoder uses parity bits to detect and correct errors after separating data and check bits. The data bits are then applied to the destination. Data source DestinationEncoder Decoder Data bits Check bits Data bits Code words Data bits
9. 9. Block Diagram of the basic ARQ System Encoder Input buffer & controller Forward Transmission channel Return Transmission channel Output buffer & Controller Detector Feedback path Feedback channel Message Input ACK / NAK
10. 10. Operations of ARQ System The encoder produces codeword's for each message signal and its input. Each codeword at the encoder output is stored temporarily and transmitted over the forward transmission channel. At the destination a decoder will decode the code words and look for errors. If decoder doesn’t get a error than it is positive (ACK), and if he gets negative than its negative (NAK). If its NAK than its will return back to “(controller)” and it will correct the word and pass it back to input buffer. A word can be retransmitted twice or more no of times. The output will be displayed as the errors are being cleared.