Error detection and correction
- 1. ERROR DETECTION AND CORRECTION By, Mrs.Alamelu.G Assistant professor of BCA E.M.G Yadava Women’s College Madurai.
- 2. CYCLIC CODES Cyclic Code Analysis: ♥ Analyze a cyclic code to find its capabilities by using polynomials. ♥ where f(x) is a polynomial with binary coefficients. Dataword: d(x) Codeword: c(x) Generator: g(x) Syndrome: s(x) Error: e(x) ♥ If s(x) is not zero, then one or more bits is corrupted. ♥ However, if s(x) is zero, either no bit is corrupted or the decoder failed to detect any errors.
- 3. IN A CYCLIC CODE 1. If s(x):0 one or more bits is corrupted. 2. If s(x)=0 either a. No bit is corrupted , or b. Some bits are corrupted , but the decoder failed to detect them. ♥ In our analysis we want to find the criteria that must be imposed on the generator , g(x) to detect the type of error we especially want to be detected. ♥ Let us first the relationship among the sent codeword , error, received codeword, and the generator. Received Codeword=c(x) + e(x)
- 4. • In other words , the received codeword is the sum of the sent codeword and the error. • The receiver divides the received codeword by g(x) to get the syndrome. Received codeword / g(x) = c(x) + e(x) / g(x) ♥ The first term at the right hand side of the equality has a remainder of zero . ♥ So the syndrome is actually the remainder of the second term on the right hand side. ♥ If this term does not have a remainder either e(x) is 0 or is divided by g(x). ♥ We do not have to worry about the first case there is no error , the second case is very important. ♥ Those errors that are divisible by g(x) are not caught.
- 5. SINGLE BIT • What should the structure of g(x) be to guarantee the detection of a single bit error? • A single bit error is e(x)=X^I ,where I is the position of the bit. • If a single bit error is caught , then X^I is not divisible by g(x). • If g(x) has at least two terms and the coefficient of x^0 is not a zero , then e(x) cannot be divided by g(x). Example: Book
- 6. TWO ISOLATED SINGLE BIT ERRORS • Now imagine there are two single bit isolated errors. • We can show this type of errors as e(x) =X^j+x^i. • The value of I and j define the position of the errors , and the differences j-i defines the distance between the two errors. Representation of two isolated single bit errors using polynomial. difference:j-i <------------------> 0 1 0 1 1 1 0 1 0 1 0 0 0 0 1 1
- 7. STANDARD POLYNOMIAL • Some standard polynomial used by popular protocols for CRC generation along with the corresponding bit pattern. Name Polynomial Used in CRC-8 100000111 ATM header CRC-10 11000110101 ATM AAL CRC-16 10001000000100001 HDLC CRC-32 10000010011000001110110110111 LANs
- 8. ADVANTAGES OF CYCLIC CODES • That cyclic codes have a very good performance in detecting single bit errors , double error, burst errors. • They can easily be implemented in hardware and software. • They are especially fast when implemented in hardware. • This has made Cyclic Code a good candidate for many network.
- 9. OTHER CYCLIC CODES • The cyclic codes we have discussed in this section are very simple. • There are however more powerful polynomial that are based on abstract algebra involving Galois fields. • These are beyond the scope of this book. • One of the most interesting of these codes is the Reed Solomon Code used for both detection and correction.
- 10. HARDWARE IMPLEMENTATION • One of the advantages of a cyclic code is that the encoder and decoder can easily and cheaply be implemented in hardware by using a handful of electronic devices. • Also, a hardware implementation increases the rate of check bit and syndrome bit calculation. • We try to show step by step the process .The section however is optional and does not affect the understanding of the rest.
- 11. • Divisor • Augmented Dataword • Remainder • Odd number of errors • Burst errors • General Design
- 12. DIVISOR 1. The divisor is repeatedly XORed with part of the divided. 2. The divisor has n-k-1 bits which either are predefined or are all 0s. ♥ In our previous example, the divisor bits were either 1011 or 0000. ♥ The choice was based on the leftmost bit of the part of the augmented data bits that are active in the XOR operation. 3. A close look shows that only n-k bits of the divisor are needed in the XOR operation. ♥ The leftmost bit is not needed because the result of the operation is always 0, no matter what the value of this bit. ♥ The reason is that the inputs to this XOR opration are either both 0s or both 1s.
- 13. AUGMENTED DATAWORD • In our paper and pencil division process the augmented data word as fixed in position with the divisor bits shifting to the right , 1 bit in each step. • The divisor bits are aligned with the appropriate part of the augmented data word. • Now that our divisor we needed to modify our divisor is fixed ,we needed instead to shift the bits of the augmented data word to the left to align the divisor bits with the appropriate part. • There is no need to store the augmented data word bits.
- 14. REMAINDER 1. We assume that the remainder is originally all 0s. 2. At each time click , we repeat the following two action: a. We use the leftmost bit to make a decision about the divisor. b. The other 2 bits of the remainder and the next bit form the augmented data word are XOR with the 3 bit divisor to create the next remainder.
- 15. ODD NUMBERS OF ERRORS • A generator with a factor of x+1 can catch all odd numbers of errors. • This means that we need to make x+1 a factor of any generator. • We are not saying that the generator itself should be x+1. • We are saying that it should have a factor of x+1. • If it is only x+1 , it cannot catch the two adjacent isolated errors .
- 16. BURST ERRORS • Lets us extend our analysis to the burst error , which is the most important of all. • A burst error is of the form e(x)=(xj+……….+xi). • Note the difference between a burst error and two isolated single bit errors. • The first can have two terms or more, the second can only have two terms. • We can factor out xi and write the error as xi(xj-1+…….+).
- 17. GENERAL DESIGN OF ENCODER AND DECODER OF A CRC CODE • A general design for the encoder and decoder. • Note that we have n-k1 bit shift register in both the encoder and decoder. • We have up to n-k XOR devices , but the divisors normally have several 0s in their pattern. • In the decoder however the entire codeword must be fed to the decoder before the syndrome is ready. Example: Book
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