There are various methods used to detect errors that can occur during data transmission and storage. Describe each of the following error detection methods.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. Two marks for each correct description
2. Parity Check ∞ Checks a byte of data ∞ Check is performed when data is received ∞ A parity bit is added (to the parity byte) ∞ Counts / checks number of 1's // counts / checks to see if 1's are even // counts / checks to see if 1's are odd ∞ Can be even or odd ∞ If parity is incorrect, error is detected
3. Check digit ∞ A digit that is calculated from the data // uses modulo to calculate digit // valid description of modulo ∞ It is appended / added to the data ∞ Digit is recalculated when data is entered ∞ Digits are compared to check for error
4. Checksum ∞ A value is calculated from the data // Valid description of calculation ∞ It is transmitted with the data ∞ Value is recalculated after transmission ∞ Values are compared after transmission to check for error
5. Automatic Repeat reQuest ∞ Uses acknowledgement / request and time-out ∞ Error control protocol ∞ Check performed on receiving data // error is detected by e.g. parity check, check sum ∞ If error detected, request is sent to resend data // negative acknowledgement is used ∞ Resend request is repeated till data is sent correctly / requests time out / limit is reached ∞ Send acknowledgement that data is received // positive acknowledgement is used ∞ If acknowledgement not received in set time data is resent