Errors!

Where do errors occur? Anywhere data is transferred, processed, stored,

etc. Input

user error Storage

magnetic hard drive errors physical tampering (hax0ring)

Communication electrical interference / noise physical tampering (hax0ring)

How do we detect errors? verification

“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 visual verification

proofreading

double entry force re-entry of data (e.g. passwords or e-mails)

Validation Range checks

Is this birthday acceptable (M/D/YY)?

1/62/07

Type checks How old are you?

8a years old

Format checks Please enter your birthday (M/D/YY):

1/16/2007

Which of these numbers has an error?

1235 9053 4213

How do you detect errors if you had nothing to compare your data against?

Check Digits Append additional data for comparison1. Apply some algorithm on the data to

generate a check digit2. Attach the check digit to the data3. 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 Even Parity

Check digit (1 or 0) is added to make sure there are an even number of 1s

Odd Parity Check digit (1 or 0) is added to make sure there

are an odd number of 1s

Parity Bit Examples Even parity w/ NO ERROR

101110 Even parity w/ ERROR

101010 Odd parity w/ NO ERROR

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 Digit1. Add up each of the digits to get a number2. If number is multidigit, then add up digits of that

number3. Repeat until you have a single digit

Example:2345 2 + 3 + 4 + 5 = 1414 1 + 4 = 5Check digit for 2345 is 5Attach at the end, so we store 23455

Other Errors Hardware errors

defective hard drive

Software errors e.g., decimal in the wrong place

Logic errors Runtime errors Syntax errors

How do we fix errors? Re-input

Prompt the user to re-enter data

Re-transmission 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