S5 ICT Practice Paper: Data Communication, Subnetting and Information Security (100 marks)

S5 ICT · Data communication and security

S5 ICT Practice Paper: Data Communication, Subnetting and Information Security (100 marks): 6 questions, 26 parts, 100 marks, 75 minutes.

Section A — Data communication and transmission

Show your working and state the unit in each answer.

#	Question	Answer and marking notes
1	A 4.5 MB file is sent over a 24 Mbps link (1 MB = 8 Mbit). How many seconds does the transfer take?	4.5 × 8 = 36 Mbit; 36 ÷ 24 = 1.5 s
2	If the total delay between sender and receiver is 20 ms, what is the actual time taken in seconds (3 decimal places)?	1.5 + 0.02 = 1.520 s
3	A digital recording uses a 48 kHz sampling rate, 16-bit resolution and two channels. What is the bit rate in kbps?	48 000 × 16 × 2 = 1 536 000 bps = 1 536 kbps
4	The recording lasts one minute. What is the file size in MB (1 MB = 8 Mbit)?	1 536 kbps × 60 = 92 160 kbit = 92.16 Mbit; 92.16 ÷ 8 = 11.52 MB

Answer the following questions about network connections.

#	Question	Answer and marking notes
5	Eight computers are connected in a full mesh topology. How many cables are needed?	8 × 7 ÷ 2 = 28 cables
6	The same computers are reconnected in a star topology with one central switch. How many cables are needed?	8 cables, one from each computer to the switch
7	Each cable costs HK$45. How much more does the mesh topology cost than the star topology?	(28 − 8) × 45 = HK$900
8	How many bytes does 1 GiB exceed 1 GB by (1 GiB = 1 073 741 824 bytes, 1 GB = 1 000 000 000 bytes)?	1 073 741 824 − 1 000 000 000 = 73 741 824 bytes

A network is given the address 192.168.10.0/26. Answer the following.

#	Question	Answer and marking notes
9	State the subnet mask for this prefix in dotted decimal notation.	255.255.255.192
10	How many usable host addresses does each subnet provide?	2 to the power 6 minus 2 = 62
11	How many subnets does the /26 prefix create from 192.168.10.0/24?	2 to the power 2 = 4
12	What is the broadcast address of the first subnet?	192.168.10.63
13	Which subnet contains the host 192.168.10.70? Give the network address.	192.168.10.64/26

Answer briefly.

#	Question	Answer and marking notes
14	A 32 GB backup is uploaded with 100 Mbps bandwidth (1 GB = 8 000 Mbit). How many minutes does it take (1 decimal place)?	32 × 8 000 = 256 000 Mbit; 256 000 ÷ 100 = 2 560 s ≈ 42.7 minutes
15	If the effective bandwidth is only 64 Mbps, how many minutes does it take (1 decimal place)?	256 000 ÷ 64 = 4 000 s ≈ 66.7 minutes
16	What are the default port numbers of HTTPS and HTTP?	HTTPS 443, HTTP 80
17	What is the main function of DNS?	To resolve a domain name such as www.example.com into its IP address

Answer the following questions on account and data protection.

#	Question	Answer and marking notes
18	A password has 8 characters, each being one of 26 lowercase letters or 10 digits. How many combinations are there (3 significant figures)?	36 to the power 8 ≈ 2.82 × 10^12
19	If an attacker tries 10 to the power 9 guesses per second, how many hours are needed to exhaust all combinations (1 decimal place)?	2.82 × 10^12 ÷ 10^9 = 2 821 s ≈ 0.8 hours
20	What is the purpose of adding a salt when storing hashed passwords?	It makes identical passwords hash differently, defeating rainbow tables and bulk comparison
21	Give one example each of symmetric and asymmetric encryption, and state their roles in HTTPS.	AES (symmetric) encrypts the bulk data; RSA or ECDHE (asymmetric) exchanges keys and signs certificates
22	State two features that help identify a phishing email.	Sender domain differs from the official one; links lead to a fake site or demand an immediate password entry

Answer the following and include the unit.

#	Question	Answer and marking notes
23	An uncompressed 1920 × 1080 image uses 24-bit colour. What is its size in MiB (1 MiB = 1 048 576 bytes, 2 decimal places)?	1 920 × 1 080 × 3 = 6 220 800 bytes; 6 220 800 ÷ 1 048 576 ≈ 5.93 MiB
24	An uncompressed video plays at 25 frames per second for 30 seconds. What is its size in GiB (2 decimal places)?	6 220 800 × 25 × 30 = 4 665 600 000 bytes ≈ 4.34 GiB
25	Give one example each of lossless and lossy compression.	Lossless: ZIP or PNG; lossy: JPEG or H.264 video
26	Why is lossy compression normally used for video?	The eye is insensitive to some detail, so discarding it shrinks the file greatly with little visible change

A TutorZone practice paper; original questions and passages, not publisher material.