2.2 - error detection methods 🔍

Question 1

what can cause data transmission errors to occur?

Answer 1

electrical interference, data packet switching problems, and skewed bits of data

Question 2

how do parity checks work?

Answer 2

they check for the number of bits in bytes of data based on either even (an even number of 1s in the byte) or odd (an odd number of 1s in the byte) parity with one of the bits reserved as the parity bit

Question 3

what should the parity bit be for a byte with even parity?

Answer 3

0 as there’s already an even number of 1s in the byte

Question 4

what should the parity bit be for a byte with odd parity?

Answer 4

1 as it results in an even number of 1s in the byte

Question 5

when are bytes flagged for parity errors at the receiving end?

Answer 5

if they either gain or lose additional 1s on the way (eg. a byte using odd parity in the form of 11001010 at the sending end would be flagged if it’d become corrupted into something like 11000010 by the receiving end)

Question 6

how can parity errors be detected in a certain set of data?

Answer 6

by checking the data that arrives at the receiving end with a parity block where there’s a parity byte appended to the other bytes of data (that is, first check horizontally then vertically to find out where there’s been a parity error)

Question 7

how do checksums work?

Answer 7

they’re calculated from data blocks using a certain algorithm to then be appended to them and transmitted together - they’re then recalculated upon arrival at the receiving end

Question 8

when do data blocks with checksums need to be sent again?

Answer 8

if the checksums at the sending end aren’t the same as those at the receiving end

Question 9

how are check digits calculated and what are some examples of their usage in real life?

Answer 9

they’re calculated from all the other digits in a block of data - ISBN 13, Modulo-11, and UPC all use check digits IRL

Question 10

what data errors can be detected with check digit calculations?

Answer 10

incorrect digit entries, phonetic errors, incorrect transpositions, digit omissions, and extra digit additions

Question 11

how do echo checks work?

Answer 11

they send copies of received data back to the sending side, compare that data against what was initially sent, and determine if there are enough differences between the two to count as being sent w/ error (not really reliable btw (￣m￣）)

Question 12

how do ARQs (often used to keep mobile networks reliable) work?

Answer 12

they use acknowledgements (negative for incorrect data receipts and positive for the polar opposite) and timeouts (how long before acknowledgments must be received) to request the sending side to resend the data until the receipt of a positive acknowledgment or a predetermined number of retransmissions (whichever comes first)

Question 13

(C1) find the check digit of this ISBN-13 code: 393976894689_

Answer 13

(3+9+3+9+7+9+9)+(6+8+4+6+8)=(12+12+16+9)+(18+14)=(24+25)+(18+14)=49+(32*3)=49+96=145/10=14R5 so check digit=10-5=5 and full ISBN-13 code=3939768946895

Question 14

(C2) recalculate the check digit of the full ISBN-13 code from question C1 (3939768946895)

Answer 14

(49+5)+96=54+96=150/10=15R0 so the check digit from question C1 was correct