Error detection

Question 1

State 3 methods for checking for errors in the transmission of data

Answer 1

Parity Check
Checksum
Echo Check

Question 2

1 method for checking for errors in data input

Answer 2

Check Digit

Question 3

Even Parity

Answer 3

The sender and receiver decide on the type of parity

The sender

counts the number of ones in the bit pattern
It adds a 0 if there are an even number of 1’s
It adds a 1 if there are an odd number of 1’s

The receiver

counts the number of ones in the bit pattern
If there are an even number of ones it accepts the message
If there is an odd number of 1’s it identifies there is an error and requests that the message is resent

Question 4

Checksum

Answer 4

The sender

Calculates the checksum for the block of bytes
Transmits the checksum with the block of data
The receiver

Checksum is recalculated from the block of data
Compares the calculated checksum to the one transmitted
If same, data transmitted has been received without errors
If different a request is sent for date to be retransmitted

Question 5

ARQ

Answer 5

Automatic Repeat Request

Message is sent by sender and it waits for a set amount of time
If message received correctly a message (acknowledgement) is sent back by the receiver indicating that data has been received correctly
If message not received correctly a message (ACKNOWLEDGEMENT) is not sent back.
TIMEOUT occurs and the message is automatically resent.

Question 6

Check Digit

Answer 6

A check digit is calculated from other digits and added to the end of a long code

Used to identify errors in data entry caused by mistyping or misscanning a barcode.

Question 7

How is a check digit used?

Answer 7

Full code is input (manual or scanned) e.g. 0 12345 67890 5
Check digit is recalculated from inputted digits
Compares the calculated check digit to the one entered
If same, check digit entered without errors
If different error detected and code is not accepted

Question 8

2 types of errors detected by check digits

Answer 8

2 digits transposed

Incorrect digit entered

Question 9

Echo check

Answer 9

The receiving computer sends a copy of the data immediately back to the sending computer
The sending computer compares the received data to the sent data
If they are the same the data is deemed to have arrived without error