Computer Science

1. 1.

   Distinguish a switch's typical role within a local network from a router's role between IP networks.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Compare the forwarding decision at each layer. Devices may combine functions, but the conceptual distinction is frame switching within a link versus packet routing across network boundaries.

   Marking points

   • A switch forwards frames using link-layer addresses within the LAN.
   • A router forwards packets using network-layer destination addresses/routes.
   • The router connects different IP networks rather than merely extending one broadcast domain.

   Examiner tip: Do not infer device function solely from the number of ports.

2. 2.

   Explain what DNS contributes when a browser connects to a named website, and why resolving a name does not itself download the web page.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. A name is convenient for people but routing needs addresses. Resolution identifies where to contact a service; subsequent transport and application exchanges carry the content.

   Marking points

   • DNS resolves a domain name to address information.
   • The browser uses that information to establish a connection to a service.
   • HTTP exchange retrieves the page; DNS is a separate lookup.

   Examiner tip: A DNS record is not the website's HTML.

3. 3.

   A 12 MB file crosses a 24 Mb s^-1 link. Use decimal units and ignore overhead. Calculate minimum transfer time and explain why real time may be longer.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Convert bytes to bits before dividing size by rate: 12 x 8/24. The link's nominal bit rate is not a guarantee that every transmitted bit belongs to the file.

   Marking points

   • File size = 96 Mb.
   • Minimum time = 4 s.
   • Protocol overhead, contention or retransmissions can reduce useful throughput.

   Examiner tip: Capital B denotes bytes; lowercase b denotes bits.

4. 4.

   A file has 5000 payload bytes; each packet can carry at most 1400 payload bytes and adds a 40-byte header. Calculate packet count and total transmitted bytes, with no padding or retransmission.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Three full payloads leave 800 bytes for a fourth packet. The last packet is not padded, but it still needs a header, so add four headers to the original payload size.

   Marking points

   • Packet count = ceil(5000/1400) = 4.
   • Header bytes = 160.
   • Total transmitted = 5160 bytes.

   Examiner tip: Round packet count up; rounding down loses the final partial packet.

5. 5.

   Packets arrive out of order and one is missing. Explain how a reliable ordered transport can deliver the correct byte stream, and why IP routing alone does not guarantee this.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Different paths and losses affect packet arrival, so reliability requires state above basic routing. Sequence positions let the receiver distinguish a late packet from a new stream segment and withhold incomplete ordered delivery.

   Marking points

   • Sequence information identifies byte positions and gaps.
   • Acknowledgements/timeouts can trigger retransmission of missing data.
   • The receiver buffers/reorders data and removes duplicates before delivery.
   • IP is best-effort forwarding, not an end-to-end ordered-delivery guarantee.

   Examiner tip: Reliability belongs to the specified transport service, not every packet network automatically.

6. 6.

   For live voice, evaluate using a low-overhead datagram transport instead of reliable ordered transport, considering late packets as well as lost packets.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Correctness for a file means every byte eventually arrives; usefulness for live audio also requires arrival before playback. Limited buffering and concealment can trade fidelity for latency rather than waiting indefinitely for perfect reconstruction.

   Marking points

   • Low delay matters because audio has playback deadlines.
   • Retransmitted data may arrive too late to be useful.
   • A datagram service avoids mandatory ordered waiting but leaves loss/jitter handling to the application.
   • Choose with loss tolerance, concealment/jitter buffering and network conditions in mind; it is not always superior.

   Examiner tip: Distinguish latency from bandwidth; a high-bandwidth link can still have high delay.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.