Data Transmission - Error Detection, Check Digits, Encryption

Question 1

when can data transmission errors occur and what is the consequence

Answer 1

• due to ** electrical interference**: data loss, data gain and data
  change
• alters pattern of the 0s and 1s –> affecting the integrity / correctness of the data

Question 2

what are the most common reasons for data corruption

Answer 2

• interference - data changed
• packet switching - data lost or gained

Question 3

describe the stages of a parity check

Answer 3

1. computers can either use odd or even parity
2. when sending data, the most significant ne bit is used as the parity bit
3. parity bit = 1 OR 0 at the start of transmission –> makes the total number of 1s odd or even, depending on machine
4. 1s are counted at the receiving end –> if wrong number of bits are ‘on’ or 1 –> error detected + data is resent

Question 4

when is a parity block check used

Answer 4

• used when sending multiple bytes in one transmission block
• can locate the position of the error

Question 5

when it is an even parity, what is the parity bit set to

Answer 5

0

Question 6

describe the stages of error detecting using a parity block check

Answer 6

1. each byte has its own parity bit
2. a final parity bit is used for each column
3. a horizontal parity check is performed on each byte –> identifies error in one of the bytes
4. Vertical parity check performed on whole block
5. error located + corrected without having to ask for data to be re-sent

Question 7

when is a checksum used

Answer 7

to verify of data has become corrupt after a transmission

Question 8

describe the staged of a checksum

Answer 8

1. data sent in blocks with a checksum added at the end
2. before data is transmitted, a checksum is calculated from the block of data based on a pre-agreed algorithm
3. Data + checksum transmitted
4. receiving device uses the same pre-agreed algorithm to recalculate the checksum
5. recalculated checksum is compared to original checksum –> if two checksums are the same = no errors.
6. Otherwise, request to re-send the original data is sent

Question 9

describe how an echo check is performed

Answer 9

• data transmitted to destination device
• recipient device returns an exact copy of data back to the originator
• returned data is compared* with original data –> if *different = ** error occurred** + data resent –> same = data arrived without error

Question 10

what does ARQ stand for

Answer 10

Automatic Repeat Query

Question 11

what does it ensure

Answer 11

to verify that transmitted data has arrived unchanged, and error free

Question 12

what is a positive acknowledgement in ARQs

Answer 12

message sent to the **sender **indicating that the data has been received correctly

Question 13

what is a negative acknowledgement in ARQs

Answer 13

message sent to the sender indicating the data has not been received correctly + wants data resent

Question 14

what is timeout

Answer 14

• feature used by the sending device
• when no acknowledgement has returned within a certain time period - timeout is registered and sending device automatically resends data

Question 15

describe the stages of the ARQ

Answer 15

1. destination device receives the data transmission along with a error detection code
2. code used to determine if data contains any transmission error
3. if no errors –> positive acknowledgement sent back
4. if error detected –> negative acknowledgement: re-transmission request sent back to sending device
5. if no acknowledgement is returned after a certain time period, a timeout is registered and the sending computer resends the packet
6. process repeats until packet received is correct OR until ARQ limit is reached

Question 16

what do check digits do

Answer 16

form of validating data entry rather than detecting errors

Question 17

what is a check digit and how does it detect errors

Answer 17

• is an last additional digit at the end of a string of numbers calculated from the other digits in the code
• is recalculated on data entry
• if recalculated check digit matches the one at the end –> numbers are deemed correct and in the right order

Question 18

how do you calculate the check digit

Answer 18

Modulus 11 system:
- Weight: start with weight = 2 on the RHS E.g. for code 45305, 5=2, 0=3, 3=4 …
- Multiplication: code digit x weight no.
- Addition: add all numbers
- Remainder: remainder when sum divided by 11
- Subtraction: subtract remainder from 11

Question 19

how do you check the check digit

Answer 19

Same as calculating check digit BUT:
- weight start with 1 on the rhs
- when total divided by 11, if remainder = 0 –> number is valid

Question 20

id the result of subtracting the remainder from 11 is 10, what is the check digit

Answer 20

X = Roman 10

Question 21

where are check digits commonly used:

Answer 21

• barcodes
• ISBN numbers (international standard book numbers)
• Vehicle Identification Numbers (VIN)

Question 22

what is plaintext

Answer 22

the original message or data file to be encryted

Question 23

Ciphertext =

Answer 23

the encrypted message that is the result of putting a plaintext through an encryption algorithm

Question 24

encryption algorithm =

Answer 24

the complex piece of software that transform plaintext into ciphertext

Question 25

describe the steps of symmetric encryption

Answer 25

• private key encryption
• uses the same pre-shared key to encode + decode data
• key must also be transferred separately to the recipient to decrypt message

Question 26

what is the problem with symmetric encryption

Answer 26

can be intercepted –> anyone that obtains the key can encrypt or decrypt the message