Computer Science · May / June 2024 · Paper 11 · Question 9

0478

As printed in the paper

The question as text

A company owner has installed a new network. Data is correct before it is transmitted across the network. The company owner is concerned that data might have errors after transmission.

Read automatically from the paper; the printed page above is exact.

Show the answer and mark scheme

9(a)[3]
Any three from:
Data could be lost
Data could be gained/added
Data could be changed
Bits could be reassembled in the wrong order
Interference could occur
Crosstalk could occur
Data collisions could occur
Data packets could time out/reach their hop count
Network could be infected with malware
9(b)(i)[8]
Any eight from:
The 1s are counted (in each byte)
Each byte has a parity bit
If the number of 1s are odd the parity bit is 0 (otherwise it is 1)
(The first packet of) data is sent and a timer is started
The receiving device counts the number of 1s (in each byte)
If the number of 1s are odd/data meets odd parity an acknowledgement is sent to say the data is error free …
… the sender then sends the next packet of data …
… and the timer is restarted
If the number of 1s is even an acknowledgement is not sent
If no acknowledgement is received within a set timeframe/before timeout …
… the data packet is resent
9(b)(ii)[2]
Any two from:
Echo check
Checksum
Even parity check
Negative ARQ
9(c)(i)[5]
Any five from:
Criteria can be set (for traffic)
… such as a blacklist/whitelist (of IP addresses)
It will examine traffic coming into the network
It will check that the traffic meets the set criteria
… and will reject it if it does not meet criteria
Certain ports used by hackers can be blocked/closed
9(c)(ii)[2]
Any two from:
Example:
Virus
Worm
Trojan horse
Spyware
Adware
Ransomware

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