OxfordAQA A-Level Computer Science: Networking and cyber security (9645) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Use the set to practise section 3.14 (Networking and cyber security) of the OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards. Each question is International A-level only content, examined in the written Unit 4 paper.

Revise first with the study guide and revision notes. Course links: hub, checklist, diagnostics.

Questions 4 to 12 use one context: Pemberwick Dental Practice, a clinic with fourteen staff, two treatment buildings and an online appointment site.

Questions

1. Define bit rate and latency. [2]

2. A link operates at 3,200 baud and uses 64 distinct signal levels.

(a) Calculate the bit rate. Show your working. [2] (b) Calculate how long a 48,000-byte file takes to send at this bit rate, ignoring any overheads. [2] (c) State the effect on the maximum bit rate if the bandwidth of the link is halved. [1]

3. A sensor sends 1,200 characters by asynchronous transmission at 2,400 bit/s. Each character is sent as 1 start bit, 7 data bits and 2 stop bits.

(a) Explain the purpose of the start bit. [1] (b) Calculate the time taken to send all the characters. [2] (c) Calculate the percentage of transmitted bits that are data. [1] (d) State one difference between synchronous and asynchronous transmission. [1]

4. Pemberwick Dental Practice stores patient records centrally.

(a) Explain why a client-server network suits the practice better than a peer-to-peer network. [2] (b) The practice wants thin-client terminals at reception. Describe the hardware and networking requirements of this choice compared with thick clients. [4]

5. The two buildings are to be linked by cable. The link will carry large X-ray image files, and the cable runs past X-ray equipment. Recommend copper or fibre-optic cable, justifying your choice. [3]

6. Staff laptops connect by Wi-Fi.

(a) Explain why wireless networks use CSMA/CA rather than CSMA/CD. [1] (b) Describe how CSMA/CA works when RTS/CTS is used. [4]

7. Name the two hardware components needed to add wireless networking to the practice, and describe three ways the wireless network can be secured. [5]

8. A packet leaves a server with a time to live of 5. Its route crosses seven routers.

(a) State the time to live after the fourth router, and which router discards the packet. [2] (b) Explain why packets carry a sequence number. [2]

9. Patients book at https://appointments.pemberwick-dental.org/slots/today.html.

(a) Identify the FQDN and the domain name in this URL. [2] (b) Describe how DNS finds the IP address for the FQDN when no server has it cached. [4] (c) Explain why Internet registries are needed. [2]

10. Two computers in the practice have addresses 172.19.83.14 and 172.19.95.200.

(a) Using subnet mask 255.255.240.0, show that they are on the same subnet. [3] (b) Determine whether they would be on the same subnet with mask 255.255.248.0. [2] (c) Explain what is meant by a non-routable IP address. [1] (d) Describe how a visiting dentist’s laptop receives an IP address by DHCP, and give one advantage over manual configuration. [5]

11. The practice’s IT contractor works remotely.

(a) Explain why networking protocols such as TCP/IP are organised in layers. [2] (b) Explain what a socket is. [2] (c) Explain why the contractor uses SSH, and why SFTP is preferred to FTP for uploading files to the appointment site. [2] (d) A receptionist emails a patient who reads mail on a phone and a laptop. Describe how SMTP and IMAP are used. [3]

12. The appointment site must be protected.

(a) Explain how stateful inspection differs from packet filtering. [2] (b) Explain how asymmetric encryption solves the key exchange problem of symmetric encryption. [3] (c) Describe how the practice obtains a digital certificate and how a browser uses it. [3] (d) Discuss how a worm and a trojan could each infect the practice’s network, and the vulnerabilities each exploits. [4]

Answers

1. Bit rate: the number of bits transmitted per second [1]. Latency: the time delay between data being sent and being received [1]. Examiner insight: “How fast data goes” fits neither term well; precise definitions earn credit where loose ones do not.

2. (a) log₂(64) = 6 bits per signal change [1]; 3,200 × 6 = 19,200 bit/s [1]. (b) 48,000 × 8 = 384,000 bits [1]; 384,000 ÷ 19,200 = 20 s [1]. (c) It halves, to 9,600 bit/s, because bit rate is directly proportional to bandwidth [1]. Examiner insight: Convert bytes to bits before dividing; forgetting the ×8 is the usual slip.

3. (a) It brings the receiver’s clock into phase with the sender’s clock [1]. (b) 1 + 7 + 2 = 10 bits per character, so 12,000 bits [1]; 12,000 ÷ 2,400 = 5 s [1]. (c) 7 ÷ 10 = 70% [1]. (d) Synchronous sends a continuous stream kept in step by a shared timing signal; asynchronous sends characters individually with start and stop bits [1]. Examiner insight: “Start bit tells the receiver data is coming” is weaker than the clock-phase idea the specification uses.

4. (a) A server holds the records centrally, so everyone works from one up-to-date copy [1]; access rights, security and backups can be managed in one place [1]. (b) Thin-client terminals need little processing power or storage, so they are cheap [1]; the server must be powerful enough to run applications for every terminal [1]; the network must be fast and reliable because all processing traffic crosses it [1]; thick clients need capable local hardware but put less load on the network [1]. Examiner insight: Tie each point to the practice; generic lists of advantages earn less than points applied to the scenario.

5. Fibre-optic [1]: its higher speed and capacity suit large image files [1]; it is immune to electrical interference from the X-ray equipment [1]. Examiner insight: A recommendation without justification linked to the stated situation gains little.

6. (a) A wireless station cannot detect a collision while it is transmitting [1]. (b) The station listens and waits until the channel is idle [1]; it sends a Request to Send to the access point [1]; the access point replies Clear to Send, which other stations hear, so they stay silent [1]; the data is sent and the receiver returns an acknowledgement; no acknowledgement means retry after a random back-off [1]. Examiner insight: Keep the steps in order, and name who sends RTS and who sends CTS.

7. Wireless network adapter in each device [1]; wireless access point [1]. WPA2 encryption, so intercepted data cannot be read [1]; SSID broadcast disabled, so the network is not advertised [1]; MAC address allow list, so only approved devices can join [1]. Examiner insight: For “describe”, say how each measure helps, not just its name.

8. (a) 1 after the fourth router [1]; discarded by the fifth router [1]. (b) Packets may take different routes and arrive out of order [1]; the receiver uses sequence numbers to reassemble them correctly and spot missing packets [1]. Examiner insight: Count carefully: the router that reduces the value to 0 is the one that discards the packet.

9. (a) FQDN: appointments.pemberwick-dental.org [1]; domain name: pemberwick-dental.org [1]. (b) The computer asks its DNS resolver [1]; the resolver asks a root name server, which refers it to the .org name servers [1]; a .org server refers it to the authoritative name server for pemberwick-dental.org [1]; that server returns the IP address, which the resolver caches and returns [1]. (c) They allocate IP addresses and record domain names [1] so that each public address and name is unique and no two organisations clash [1]. Examiner insight: Including “https://” or the path in the FQDN loses the mark.

10. (a) Third octets: 83 = 01010011, 95 = 01011111 [1]; AND with 240 (11110000) gives 01010000 = 80 for both [1]; both network identifiers are 172.19.80.0, so same subnet [1]. (b) AND with 248 (11111000): 83 → 80, 95 → 88 [1]; networks 172.19.80.0 and 172.19.88.0 differ, so not on the same subnet [1]. (c) An address from a private range, used inside local networks and not forwarded across the Internet [1]. (d) The laptop broadcasts a DHCP Discover [1]; a DHCP server replies with an Offer of an address and settings [1]; the laptop sends a Request for that address [1]; the server sends an Acknowledge and the address is leased for a set time [1]. Advantage: no duplicate addresses or typing errors [1]. Examiner insight: Show the binary AND; a bare answer gives nothing to credit if it is wrong.

11. (a) Each layer handles one part of the task and can be changed without altering the others [1]; standard interfaces let hardware and software from different makers work together [1]. (b) An IP address combined with a port number [1], identifying one endpoint of a connection so a host can run several connections at once [1]. (c) SSH gives encrypted remote login to run commands on the server [1]; SFTP encrypts transfers, while FTP sends data and passwords unencrypted [1]. (d) The receptionist’s client sends the email by SMTP to the practice’s mail server [1]; it is relayed by SMTP to the patient’s mail server and stored [1]; the patient’s devices use IMAP, so mail stays on the server and both devices stay in step [1]. Examiner insight: Name the right protocol at each stage; SMTP for retrieval is a frequent error.

12. (a) Packet filtering judges each packet alone against rules on addresses, ports and protocol [1]; stateful inspection tracks connections and admits only packets belonging to an established one [1]. (b) Symmetric encryption needs the shared key delivered securely [1]; the sender encrypts a new symmetric key with the recipient’s public key, which only the recipient’s private key can decrypt [1]; both then use the faster symmetric key [1]. (c) The practice sends its public key and identity details to a Certificate Authority, which verifies them [1]; the CA issues a certificate containing the public key, signed with the CA’s private key [1]; the browser checks the CA’s signature before trusting the site’s public key [1]. (d) A worm spreads by itself across the network [1], exploiting unpatched operating system or service flaws [1]. A trojan arrives disguised as useful software, such as a fake update [1], exploiting users who install it and accounts with excess privileges [1]. Examiner insight: In “discuss”, pair each malware type with its own vulnerability rather than one shared list.

Where marks are usually lost

• Dividing bytes by bit rate without multiplying by 8.
• Using the number of signal levels as bits per change instead of log₂ of it.
• Giving a subnet answer with no binary working.
• Describing CSMA/CD when asked about wireless.
• Swapping the roles of the station and access point in RTS/CTS.
• Writing a whole URL when asked for the FQDN.
• Using the receiver’s private key to sign.
• Listing malware names without the vulnerabilities they exploit.

Next steps

• Revision notes
• Study guide
• Course hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA International AS and A-level Computer Science (9645) specification, Version 1.1, for International AS exams May/June 2025 onwards and International A-level exams May/June 2026 onwards, published by OxfordAQA. Section 3.14 Networking and cyber security.