Errors Where do errors occur n Anywhere data

Where do errors occur? n Anywhere data is transferred, processed, stored, etc. ¡ Input

How do we detect errors? n verification ¡ ¡ n “making sure that the

Verification n visual verification ¡ n proofreading double entry ¡ force re-entry of data

Validation n Range checks ¡ n Type checks ¡ n Is this birthday acceptable

Which of these numbers has an error? 1235 n 9053 4213 How do you

Check Digits n 1. 2. 3. Append additional data for comparison Apply some algorithm

Parity Bit n Even Parity ¡ n Check digit (1 or 0) is added

Parity Bit Examples n Even parity w/ NO ERROR ¡ n Even parity w/

Parity Bit Creation Assume you have the following bit string: 10111101 with even parity,

Unweighted Check Digit 1. 2. 3. Add up each of the digits to get

Other Errors n Hardware errors ¡ n Software errors ¡ n n n defective

How do we fix errors? n Re-input ¡ n Re-transmission ¡ ¡ n Prompt

Slides: 13

Download presentation

Errors!

Where do errors occur? n Anywhere data is transferred, processed, stored, etc. ¡ Input n ¡ Storage n n ¡ user error magnetic hard drive errors physical tampering (hax 0 ring) Communication n n electrical interference / noise physical tampering (hax 0 ring)

How do we detect errors? n verification ¡ ¡ n “making sure that the data on the source documents is exactly the same as the input to the computer system” ve ~equal data = input validation ¡ ¡ “making sure that the data input into the system makes sense” va ~ acceptable data makes sense

Verification n visual verification ¡ n proofreading double entry ¡ force re-entry of data (e. g. passwords or e -mails)

Validation n Range checks ¡ n Type checks ¡ n Is this birthday acceptable (M/D/YY)? 1/62/07 How old are you? 8 a years old Format checks ¡ Please enter your birthday (M/D/YY): 1/16/2007

Which of these numbers has an error? 1235 n 9053 4213 How do you detect errors if you had nothing to compare your data against?

Check Digits n 1. 2. 3. Append additional data for comparison Apply some algorithm on the data to generate a check digit Attach the check digit to the data To verify the data in the future, apply the same algorithm to the data to generate the check digit again – compare against old check digit to verify

Parity Bit n Even Parity ¡ n Check digit (1 or 0) is added to make sure there an even number of 1 s Odd Parity ¡ Check digit (1 or 0) is added to make sure there an odd number of 1 s

Parity Bit Examples n Even parity w/ NO ERROR ¡ n Even parity w/ ERROR ¡ n 101010 Odd parity w/ NO ERROR ¡ n 101110 101111 Odd parity w/ ERROR ¡ 000101

Parity Bit Creation Assume you have the following bit string: 10111101 with even parity, add the parity bit to the right end to maintain even parity

Unweighted Check Digit 1. 2. 3. Add up each of the digits to get a number If number is multidigit, then add up digits of that number Repeat until you have a single digit Example: 23455 2345 2 + 3 + 4 + 5 = 14 14 1 + 4 = 5 Check digit for 2345 is 5 Attach at the end, so we store

Other Errors n Hardware errors ¡ n Software errors ¡ n n n defective hard drive e. g. , decimal in the wrong place Logic errors Runtime errors Syntax errors

How do we fix errors? n Re-input ¡ n Re-transmission ¡ ¡ n Prompt the user to re-enter data If there was electrical interference/noise over a cable, we can always resend the data Transmission Control Protocol (TCP) Backup your data! ¡ Verify data against backups