A file server is used as a central data store for a network of computers. Rory sends data from his computer to a file server that is approximately 100 metres away. It is important that the data is transmitted accurately. Rory needs to be able to read data from and write data to the file server at the same time.

Mark scheme

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

1. [1] (a)(i) Serial ✔
2. [1] (a)(i) Duplex ✔
3. [1] (a)(ii) Serial has less/lower interference
4. [1] (a)(ii) Serial is (more) reliable/accurate over distances
5. [1] (a)(ii) In serial the bits won't be skewed
6. [1] (a)(ii) In serial it is easier to collate the bits together again after transmission
7. [1] (a)(ii) Duplex transmits data in both directions at the same time
8. [1] (a)(ii) simplex/half-duplex/remaining methods won't allow read and write at same time
9. [1] (b) Checksum
10. [1] (b) A value is calculated from the data // Description of calculation
11. [1] (b) Value is transmitted with data
12. [1] (b) Value is recalculated after transmission
13. [1] (b) If the values match the data is (more likely to be) accurate
14. [1] (b) Parity check
15. [1] (b) A parity bit is transmitted with each byte of data
16. [1] (b) Odd or even (parity can be used)
17. [1] (b) Counts / checks number of 1's // counts / checks to see if 1's are even // counts / checks to see if 1's are odd
18. [1] (b) (Each byte is) checked after transmission to see if it matches the odd/even parity used
19. [1] (b) Automatic Repeat Request (ARQ)
20. [1] (b) Uses acknowledgement and timeout
21. [1] (b) When a device detects an error in data transmission it asks for the packet to be resent / no error detected, positive acknowledgment sent
22. [1] (b) The sending device resends the packet after the request to resend/ timeout received
23. [1] (b) This process is continuous until the data received is correct/until the ARQ limit is reached
24. [1] (b) Echo (check)
25. [1] (b) Copy of data is sent back to sender
26. [1] (b) Data is compared to see if it matches
27. [1] (b) If it does not match error detected