Lecture9

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1 Announcements Please download “Error Detection and Correction” from class web site (“For Students” -> Fourth Edition -> Reading Supplements). Test 1 will be on October 12 Monday in class (75 minutes). Lab assignment2 is online now (due by midnight of 30 October 2009).

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2 Error Detection And Correction The small size of the transistors, combined with cosmic ray effects causes occasional errors in stored information in large, dense RAM chips. These errors can be detected and corrected by employing error-detecting and –correcting codes in RAMs.
3 Parity Bit To detect errors in data communication and processing, an additional bit is sometimes added to a binary code word to define its parity. A parity bit is the extra bit included to make the total number of 1’s in the resulting code word either even or odd.

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4 Parity Bit Example 1000001 0 1000001 1 1000001 1010100 1 1010100 0 1010100 With Even Parity With Odd Parity 7 bits 8 bits
5 Characteristics of Parity Even parity is more common Parity may be used with binary numbers as well as with codes including ASCII The parity bit may be placed in any fixed position in the code

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6 Parity Bit Generating and Checking An even parity bit is generated at the sending end for all 7-bit ASCII characters The 8-bit characters including the parity bits are transmitted to their destination The parity of each character is then checked at the receiving end If the parity of the received character is not even (assuming even parity is used), at least one bit has changed its value during the transmission
7 Pros & Cons If the number of bits changed is even, the error will not be detected. It can only detects one, three, or any odd number of errors in each character. Parity does not indicate which bit contained the error, even when it can detect it. The data must be discarded entirely, and re- transmitted from scratch. It uses only a single bit, resulting in the least overhead.

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8 Hamming Codes The most common types of error-correcting codes used in RAM are based on the codes devised by R.W. Hamming.
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## This note was uploaded on 11/03/2009 for the course CS 20910 taught by Professor Taoxie during the Spring '09 term at San Diego State.

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