O-LevelComputer ScienceData Transmission2210/12 · May/June 2015 · Q59 Marks

Parity checks are often used to check for errors that may occur during data transmission.

(a)A system uses even parity. Tick (✔) to show whether the following three bytes have been transmitted correctly or incorrectly. Received byte 11001000 01111100 01101001[3]
(b)(i)One of the bits has been transmitted incorrectly. Write the byte number and column number of this bit: Byte number Column number[2]
(b)(ii)Explain how you arrived at your answer for part (b)(i).[2]
(c)Give the denary (base 10) value of the byte: 10111110[1]
(d)A parity check may not identify that a bit has been transmitted incorrectly. Describe one situation in which this could occur.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2015). The mark or mark type is shown in brackets.

[1] (a) 1 mark per correctly placed tick Received byte Byte transmitted correctly Byte transmitted incorrectly 11001000 ✔ 01111100 ✔ 01101001 ✔
[2] (b)(i) byte number: 7 column number: 6
[2] (b)(ii) Any two from: – letter "A"(byte 7) transmitted as odd parity (three 1s) – column 6 has odd parity (seven 1s) – intersection of byte 7 and column 6 indicates incorrect bit value
[1] (c) 190
[1] (d) Any one from: – 2 bits interchanged (e.g. 1 → 0 and 0 → 1) that won't change parity value – even number of bits/digits are transposed – If there are multiple errors in the same byte/column, that still produce the same parity bit, the error will not be detected

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