Unit 2.2 - Error Detection

Question 1

parity check

Answer 1

uses and even or odd parity (adding a parity bit to make sure the protocol (odd or even) is followed

Question 2

parity bit

Answer 2

bit added to a byte of data to ensure that the byte follows the correct even/odd parity protocol

Question 3

parity block

Answer 3

horizontal and vertical parity check on a block of data being transmitted

Question 4

parity byte

Answer 4

byte of data sent at end of parity block (made of parity bits generated from vertical parity check of data block)

Question 5

check sum

Answer 5

1) an algorithm is agreed on
2) the check sum is calculated from the block of data being sent
3) checksum value is sent with data block
4) receiver recalculates checksum
5) If it matches, no errors have occured, if error has occured, ask for re-send

Question 6

ARQ (automatic repeat request

Answer 6

1) receiver also gets an error detection code with data
2) If no error, positive acknowledgement is sent, if error, negative acknowledgement is sent, requesting a re-send
3) if no acknowledgement has been received after a certain period of time, it is a time out so the data is re-sent until a positive acknowledgement is received

Question 7

echo check

Answer 7

data is sent to a receiver and then immediately sent back to the sender. sender then checks if the received data matches the sent data
–> No difference = no error
–> difference = error while sending or receiving

Question 8

check digit

Answer 8

additional digit added to a number to check if the entered number is error free

Question 9

why do errors occur?

Answer 9

-interference (data loss/gain)
-packet switching problems (data loss/gain)
-skewing of data

Question 10

2 examples of check digits

Answer 10

-ISBN
-Barcodes

Question 11

what type of errors occur due to mis-typing

Answer 11

-incorrect digit entered
-transposition errors (2 numbers changed orders)
-omitted or extra digits (forgot or added more)
-phonetic errors (13, not 30)

Question 12

CRC (cyclic redundancy checks)

Answer 12

1) sender adds up all the 1 bits in payload and stores it as a hex value in trailer before it’s sent
2) receiver recalculates number of 1 bits and checks the value against the one in the trailer
3) if two values match, no errors. if they don’t match, packet needs to be resent