Chap 2.2 / 3 - error checking, encryption

Question 1

types of error checking

Answer 1

-Cyclic Redundancy Checks (CRC)
-Check digit (ISBN-13 / Modulo 11)
-Checksum
-Echo check
-Parity Checks
-Automatic Repeat Request (ARQs)

Question 2

which error checking check following transmission

Answer 2

-parity checks
-echo checks
-checksum
-ARQs

Question 3

define CRC

Answer 3

-type of error checking where the no. of 1’s are counted and put as a Hex value in the trailer to be sent

Question 4

how does CRC work

Answer 4

-count no. 1’s
-put value as Hex in the trailer
-receiving comp counts no. 1’s & CRC
-if match = no error; no match = error & packet resent

Question 5

why do you need to check for errors

Answer 5

-risk that data may be tampered with during transmission

Question 6

Errors can occur due to:

Answer 6

-interference = all cables can suffer from electrical interference
-problems during packet switching
-skewing of data = in parallel transmission, bits arrive out of sync

Question 7

define parity checking

Answer 7

-type of error checking where a parity bit is added based on the chosen parity system

Question 8

define parity bit

Answer 8

-extra bit added based on the chosen parity system

Question 9

how does Parity Check work

Answer 9

-choose even or odd
even:
-count no. 1’s
-if even, parity bit = 0; if odd, parity bit = 1
odd:
-count no. 1’s
-if odd, parity bit = 0; if even, parity bit = 1

-receiving comp checks if no. 1’s matched chosen parity system

Question 10

how to find faulty bit in parity check

Answer 10

-Criss cross method
-count no. 1’s in each column & row (parity bit & byte), find one that doesn’t match chosen parity system
-find intersection point
-can determine which digital stream was changed

Question 11

define checksum

Answer 11

-type of error checking where a checksum is calculated form the data using and agreed algorithm & transmitted with the data

Question 12

how does checksum work

Answer 12

-calculate checksum from agreed algorithm
-checksum is sent with data
-receiving comp recalculates checksum & compares them
-if match = no error; no match = error & resend

Question 13

define Echo check

Answer 13

-type of error checking where sent data is sent back to sender to check if any error occurred during transmission

Question 14

how does echo check work

Answer 14

(don’t know if error happened at 1 / 2 transmission)
-data is sent
-copy of data is sent back to sender
-returned & og data are compared
-no diff = no error, yes diff = yes error

Question 15

define check digit

Answer 15

-final digit in code calculated from all other digits in the code

Question 16

types of check digit

Answer 16

-ISBN-13
-Modulo 11

Question 17

what can check digits identify

Answer 17

-incorrect digit entered
-transposition error occurred
-omitted or extra digits
-phonetic errors

Question 18

how is check digit used

Answer 18

-check digit is calculated by human & added to code
-human types numerical code
-comp. calculate check digit based on numerical code & compare with human check digit
-no match = human made an error
-asked to re-enter code

Question 19

ISBN 13 algorithm

Answer 19

1)from left to right, first digit is odd
2)-add all no.s in odd index sequence
2)-add all no.s in even index sequence x 3
3)- 1) + 2) / 10
4)- if remainder = 0, use
5)- else 10- remainder
6) do same for recalculation, 0 = corret transmission

Question 20

Modulo-11 algorithm (generation)

Answer 20

1)-each digit has weighting starting from 2 from right
2)-each digit x their weighting and add all
3)-total / 11
4)-if remainder = 10, check digit = X
5)-else 11- remainder

Question 21

Modulo-11 algorithm (recalculation)

Answer 21

1)-each digit has weighting starting from 1 from right to end
2)-each digit x their weighting and add all
3)-total / 11
4)-if remainder = 0, no. is correct

Question 22

diff btw Checksum & check digit

Answer 22

checksum:
-calculated from block of data
-sent with block of data
-recalculated at receiving
-if diff = error occurred
-used to check errors flowing data transmission
check digit:
-final digit in code
-calculated form other digits in code
-used to check errors during data entry
-detects, incorrect digit entered, transposition error occurred, omitted or extra digits, phonetic errors

Question 23

define ARQs

Answer 23

-type of error checking where an ( - )/( + ) acknowledgment is sent within the timeout before the data is resent

Question 24

how does ARQs work

Answer 24

-data is sent to receiver with an error detection code
-no error = ( + ) acknowledgment, no resend
-yes error = ( - ) acknowledgment, yes resend
-no acknowledgment within timeout = yes resend

Question 25

define acknowledgement in ARQS

Answer 25

-message sent to sender indicated if data had errors during transmission

Question 26

define timeouts in ARQS

Answer 26

time interval allowed to elapse before an acknowledgment is received

Question 27

define plain text

Answer 27

-og messge

Question 28

define cipher text

Answer 28

-texts following and encryption algorithm

Question 29

why is encryption used

Answer 29

-to make data transmission safe
-no interception guaranteed but data will make no sense

Question 30

types of encryption

Answer 30

-symmetric
-asymmetric

Question 31

define pub. key

Answer 31

-key known to all users

Question 32

define pvt. key

Answer 32

-key used to decrypt for asymmetric encryption

Question 33

define shared secret

Answer 33

-key used to encrypt & decrypt for symmetric encryption

Question 34

how does symmetric encryption work

Answer 34

-public key is used to encrypt & decrypt plain & cipher texts btw Alice & bob
-2 pub keys

Question 35

how does asymmetric encryption work

Answer 35

-Alice gives out pub. key to Bob to encrypt plain text
-cipher text is decrypted using Alice’s pvt. key
-only Alice has pvt. key
-pub, & pvt, keys are mathematically linked tgt
-pub & pvt key

Question 36

how does end-to-end encryption work

Answer 36

-Alice & Bod each have pub. & pvt. keys
-they send pub. keys
cipher text send to each other can be decrypted by their own pvt. key

Question 37

describe a situation where parity check wouldn’t identify which bit has been transmitted incorrectly

Answer 37

-even/ odd no. errors so if (parity type) retained
eg, 2 bits have changed

Question 38

how does encryption improve security

Answer 38

-encrypted text is meaningless
-need a key to decrypt

Question 39

how to make encryption more secure

Answer 39

-add more bits to the key so less chance of decryption by brute force

Question 40

3 differences between symmetric & asymmetric encryption

Answer 40

-sym uses the same keys for encryption and decryption but asym uses 2 different keys for encryption and decryption
-asym has a pvt key but sym does not
-asym is used for secure key exchange but sym is used for bulk data transmission

Question 41

how does a data logger calculate the parity bit

Answer 41

-count the number of 1’s in the first 7 bits
-add a 0 or 1 depending on the parity type

Question 42

role of parity byte in error detection

Answer 42

It provides a means to check for errors by ensuring that the total number of 1s in the byte is even or odd

Question 43

disadvantage of symmetric encryption

Answer 43

security problem when transmitting encryption key

Question 44

define symmetric encryption key

Answer 44

-sender & receiver use same key to encrypt & decrypt message