O-LevelComputer ScienceData Transmission2210/11 · May/June 2021 · Q811 Marks
Four 7-bit binary values are transmitted from one computer to another. A parity bit is added to each binary value creating 8-bit binary values. All the binary values are transmitted and received correctly.
- (a)Identify whether each 8-bit binary value has been sent using odd or even parity by writing odd or even in the type of parity column. 8-bit binary value Type of parity 01100100 10010001 00000011 10110010[4]
- (b)An error may not be detected when using a parity check. Identify why an error may not be detected.[1]
- (c)(i)Describe how data is sent using parallel half-duplex data transmission.[4]
- (c)(ii)State two drawbacks of using parallel data transmission. Drawback 1 Drawback 2[2]
Mark scheme
Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.
- [4] (a) Odd Odd Even Even
- [1] (b) Any one from: there is a transposition of bits it does not check the order of the bits (just the sum of 1s/0s) even number of bits change incorrect bits still add up to correct parity
- [4] (c)(i) Four from: Multiple bits are sent at the same time Uses multiple wires Data is sent in both directions but only one direction at a time
- [2] (c)(ii) Any two from: Bits may arrive skewed More expensive to setup/manufacture/purchase cable Limited distance More prone to interference/error
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