Two error detection methods that Allison's computer uses are check digit and checksum.

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

1. [2] 4(a) Any two from: They both calculate a value from the data; They both append the calculated value to the data; They both recalculate the value; They both report an error if they don't match
2. [4] 4(b) One mark for method, three marks for description: 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 timeout / limit is reached; Send acknowledgement that data is received // positive acknowledgement is used; If acknowledgement not received in set time data is resent OR Parity Check A parity bit is added (to the parity byte); Counts / checks number of 1's; Can be even or odd; If parity is incorrect, error is detected