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S6 ICT Practice Paper: Networks, Data Transmission and Security (12 Questions, 100 Marks, 75 Minutes, with Answers)
HKDSE ICT: Networks, Data Transmission and Security
12 questions, 100 marks, 75 minutes: bandwidth and transmission time, subnetting, encryption, firewalls and DMZ, CDN latency, Wi-Fi risks, TCP and UDP, cloud, phishing and backups.
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Paper details Level: S6 | Subject: ICT | Time: 75 minutes | Marks: 100 | Questions: 12 | Answers included
Section A: Data Transmission and Network Addressing (32 marks)
Answer all questions in the spaces provided.
| # | Question | Answer and marking notes |
|---|---|---|
| 1 | Data: a 24 MB file is sent over a link with a bandwidth of 8 Mbps. Assume no other traffic. (a) Calculate the ideal transmission time in seconds. (2 marks) (b) Explain two reasons why the actual time is usually longer than the ideal value. (4 marks) | (a) 24 MB = 24 x 8 = 192 Mbit; 192 / 8 = 24 seconds. (2 marks: working 1, answer with unit 1) (b) Bandwidth is shared with other users so the usable rate is below the nominal value; packet headers and retransmissions add overhead; processing and queuing delays at routers and switches; round-trip latency for each connection. (4 marks, 2 each) |
| 2 | Compare optical fibre with twisted pair cable. (a) State one advantage of fibre in bandwidth and one in resistance to electromagnetic interference, with a brief explanation for each. (4 marks) (b) State one limitation of fibre and explain why it is chosen for a campus backbone. (4 marks) | (a) Bandwidth: fibre carries light signals and supports far higher rates than twisted pair, over 10 Gbps; immunity: light is unaffected by electromagnetic interference and does not radiate, which suits electrically noisy sites. (4 marks, 2 each) (b) Limitation: higher cost, since termination and splicing need special equipment and skilled staff. Reason: a campus backbone carries a large volume of data over long distances, so low attenuation and high bandwidth remove the bottleneck and lower long-term maintenance cost. (4 marks) |
| 3 | A network uses 192.168.10.0/26 as its subnet address. (a) Calculate the number of addresses available for hosts in each subnet. (4 marks) (b) Write the network address and the broadcast address of the first subnet. (4 marks) | (a) Host bits = 32 – 26 = 6, giving 64 addresses; subtracting the network and broadcast addresses leaves 62 usable hosts. (4 marks: working 2, answer 2) (b) Network address 192.168.10.0; broadcast address 192.168.10.63. (4 marks, 2 each) |
| 4 | (a) Explain the difference between symmetric and asymmetric encryption in the use of keys. (3 marks) (b) Explain why an HTTPS connection uses both types of encryption. (4 marks) (c) State the role of a digital certificate in an HTTPS connection. (3 marks) | (a) Symmetric encryption uses one key for both encryption and decryption, so the key must be shared in advance; asymmetric encryption uses a key pair, with the public key encrypting and the private key decrypting. (3 marks) (b) Asymmetric encryption is used in the handshake to exchange a temporary symmetric key securely; symmetric encryption is much faster, so the session content is then encrypted with the symmetric key, giving both security and efficiency. (4 marks) (c) A digital certificate is issued by a trusted certificate authority and binds the server public key to a domain name; the browser uses it to verify the server identity and prevent impersonation by a man in the middle. (3 marks) |
Section B: Security Technology and Wireless Networks (36 marks)
Show your working in calculations and state the units.
| # | Question | Answer and marking notes |
|---|---|---|
| 5 | A system uses a 4-digit numeric personal identification number (PIN). (a) Calculate the total number of possible PINs. (3 marks) (b) If 8 characters are used, each being an upper-case letter, a lower-case letter or a digit, calculate the number of combinations and compare it with (a). Explain why it is harder to break by brute force. (6 marks) | (a) 10^4 = 10,000 PINs. (3 marks) (b) Each position has 26 + 26 + 10 = 62 choices, giving 62^8 = 218,340,105,584,896 combinations, about 2.18 x 10^10 times that of (a); the much larger space makes exhaustive trial infeasible. (6 marks: expression 2, value 2, explanation 2) |
| 6 | A company places a firewall between the Internet and its internal network, and a DMZ for its public web server. (a) Explain how a firewall filters packets according to rules. (4 marks) (b) State one difference between an intrusion detection system (IDS) and an intrusion prevention system (IPS). (2 marks) (c) Explain the benefit of placing the web server in the DMZ instead of the internal network. (3 marks) | (a) A firewall compares each packet with rules on source and destination address, port and protocol; packets matching an allow rule pass and the rest are dropped, and connections can be logged. (4 marks) (b) An IDS only detects and alerts, while an IPS can block or terminate the connection on detecting an attack. (2 marks) (c) If the web server is compromised, the attacker is confined to the DMZ and cannot reach the internal database or staff machines directly, limiting the scope of the intrusion. (3 marks) |
| 7 | Data: a web page needs 4 resources of 1.5 MB each; the link bandwidth is 12 Mbps; each request has a round-trip latency of 80 ms. (a) Calculate the pure transmission time in seconds. (4 marks) (b) Calculate the total loading time including latency, and explain how a content delivery network (CDN) reduces it. (5 marks) | (a) 4 x 1.5 = 6 MB = 48 Mbit; 48 / 12 = 4 seconds. (4 marks: working 2, answer with unit 2) (b) Total time = 4 + 4 x 0.08 = 4.32 seconds. (2 marks) A CDN caches resources on servers near the user, shortening the round trip and latency of each request and spreading the load so that queuing at the origin server is reduced. (3 marks) |
| 8 | A user connects to a free wireless network in a cafe and logs in to online banking. (a) State two risks of data leakage in this situation. (4 marks) (b) Suggest three measures to reduce the risks, each with a brief explanation. (5 marks) | (a) The free network may be run by an attacker who can act as a man in the middle; other users on the same network may sniff unencrypted packets; a rogue hotspot may lure devices to connect automatically. (4 marks, 2 each) (b) Use a VPN to encrypt all traffic; check that the site uses HTTPS and inspect the certificate; disable auto-join and file sharing and use only a trusted network for banking; enable two-factor authentication. (5 marks, about 1 to 2 each) |
Section C: Comparison and Cost Analysis (32 marks)
Answer comparison questions in point form.
| # | Question | Answer and marking notes |
|---|---|---|
| 9 | Compare TCP and UDP, and give one suitable application for each. (8 marks) | TCP is connection-oriented and provides reliable, ordered delivery with acknowledgements and retransmission, at the cost of more overhead and delay; it suits web browsing, file download and email. UDP is connectionless and does not guarantee delivery, with low overhead and delay; it suits video streaming, voice calls and online games. (8 marks, 4 per protocol) |
| 10 | A small firm considers moving customer data from a local server to a cloud service. (a) State two benefits. (4 marks) (b) State two risks and one measure for each. (4 marks) | (a) No upfront hardware investment and pay-as-you-go pricing; the provider handles maintenance, scaling and backup, so capacity can grow quickly; staff can access data from different locations. (4 marks, 2 each) (b) Risks: data held by a third party raises privacy and compliance issues; an outage of the network or provider interrupts business. Measures: encrypt data at rest and sign a data processing agreement; use multiple backups and a contract with a defined service level. (4 marks) |
| 11 | Data: a company receives 40 phishing emails per month, of which 5% lead an employee to click and cause an average loss of 120,000 dollars each time. After security training, the success rate falls to 1%, and the training costs 30,000 dollars per month. Calculate the expected monthly loss before and after training, the amount saved and the net benefit, and judge whether the training is worthwhile. (8 marks) | Before: 40 x 5% x 120,000 = 240,000 dollars. (2 marks) After: 40 x 1% x 120,000 = 48,000 dollars. (2 marks) Saving = 240,000 – 48,000 = 192,000 dollars; net benefit after the 30,000 dollar training cost is 162,000 dollars. (2 marks) The net benefit is positive and substantial, so the training is worthwhile, though technical filtering and regular drills are still needed. (2 marks) |
| 12 | (a) State two common security weaknesses of Internet of Things (IoT) devices. (4 marks) (b) Explain the 3-2-1 backup strategy and its benefit. (4 marks) | (a) Default or weak factory passwords that users rarely change; firmware that is not updated and carries known vulnerabilities; limited processing power that makes strong encryption hard to run. (4 marks, 2 each) (b) Keep three copies of the data, on two different types of storage media, with one copy off-site; this allows recovery from a single media failure, ransomware encryption or a site incident. (4 marks) |
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