Error detection and encryption — GCSE Computer Science practice questions

Question 1

A fitness band syncs a week of step data to a phone using a wireless connection.

Which of these could cause errors in the data while it is being transmitted?

Accepted Answer

B: Interference that changes, adds or removes some of the bits. Data can be corrupted during transmission by interference, for example from other wireless devices or electrical equipment. Interference can change bits (0 to 1 or 1 to 0), or make bits go missing or appear. Encryption, parity bits and lossless compression are deliberate, reversible processes, so they do not cause errors.

Question 2

A drone sends photos to a ground station using positive automatic repeat query (ARQ). After sending each block of data, the drone starts a timer.

What happens if the timer reaches the timeout before an acknowledgement arrives?

Accepted Answer

A: The drone sends the same block of data again. With positive ARQ, the ground station sends an acknowledgement only when a block arrives without an error. If the timeout is reached with no acknowledgement, the drone assumes the block was lost or corrupted and sends it again.

Question 3

A weather station on a mountain sends temperature data to a university in blocks: six bytes, then a parity byte. **Even** parity is used along every row and down every column, so each row and each column should contain an even number of 1s. Column P holds each row's parity bit, and columns b7 to b1 hold the data (b7 = bit 7, … b1 = bit 1).

On the way, interference flipped exactly one bit. This is the block the university received:

| Row | P | b7 | b6 | b5 | b4 | b3 | b2 | b1 |
|---|---|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1 |
| 2 | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 0 |
| 3 | 0 | 1 | 1 | 1 | 0 | 0 | 0 | 1 |
| 4 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 1 |
| 5 | 0 | 1 | 0 | 1 | 0 | 1 | 1 | 0 |
| 6 | 1 | 0 | 1 | 0 | 1 | 1 | 0 | 0 |
| Parity byte | 1 | 1 | 1 | 0 | 1 | 0 | 1 | 1 |

Which bit was flipped?

Accepted Answer

C: 4 · b5. Row 4 (00001011) contains three 1s, so it fails even parity; every other row passes. Counting each column, only column b5 has an odd number of 1s (three). The flipped bit is where the failing row and column cross: row 4, column b5 (bit 5).

Row	P	b7	b6	b5	b4	b3	b2	b1
1	0	0	1	1	0	1	0	1
2	0	1	0	0	1	1	1	0
3	0	1	1	1	0	0	0	1
4	0	0	0	0	1	0	1	1
5	0	1	0	1	0	1	1	0
6	1	0	1	0	1	1	0	0
Parity byte	1	1	1	0	1	0	1	1