Cyclic redundancy check (CRC) codes
seminar surveyer Active In SP Posts: 3,541 Joined: Sep 2010 
30122010, 02:55 PM
Cyclic redundancy ppt 2010.ppt (Size: 437.5 KB / Downloads: 86) What is CRC? A systematic error detecting code a group of error control bits (which is the remainder a polynomial division of a message polynomial by generator polynomial) is appended to the end of the message block with considerable bursterror detection capability The receiver generally has the ability to send retransmission requests back to the data source through a feedback channel. Steps involved Following are the steps that are involved in sending our message with CRC so that receiver can check for the reliability of the message sent. Step:1 Representing n+1 bits using n degree polynomial. And we call it M(x). CHOOSING CRC CRC bit can be choosen from the following table Which is polynomial of degree ‘k’ and it is based on our application .we call it C(x): Step 3: Making P(x)[which is n+1 bit message bit + k bit crc] exactly divisible by C(x). Basic idea: The polynomial for the received code word P(X) is divided by the generator polynomial C(X). If the remainder is not zero: An indication that an error has occurred in transmission and the received codeword is not a valid code word. POLYNOMIAL DIVISION Rules: 1.On dividing B(x) by C(x),B(x) must be of Higher degree polynomial than C(x). 2.If Degree of both B(x) and C(x) are same then Quotient=1 3.To get remainder subtract C(x) from B(x). 4.For Subtracting use XOR gate. To make P(x) exactly divisible by C(x) follow the following three steps: 1.Add k (Degree of CRC choosen) zeros at the end of message M(x) and call the resulting polynomial T(x). 2.Divide T(x) by C(x) and find the remainder. 3.Subtract the remainder from T(x). Resulting bit is the message that we have to send. 


