IGCSE Computer Science: Data Transmission — Revision Notes

Condensed for the final weeks. For the full explanation, use the Data Transmission study guide.

Packets and packet switching — narrate it as a sequence

1. A message is broken into packets.
2. Each packet gets a header (destination address, packet number, originator’s address), a payload, and a trailer.
3. Routers control the route each packet takes — different packets from the same transmission can travel by different routes.
4. Packets can therefore arrive out of order.
5. The receiving device uses the packet numbers to reorder them once the last one arrives.

Exam tip: “describe the process” questions want this as a sequence, not a list of isolated facts.

Transmission methods — match scenario to method fast

Method	Best for
Serial	Long distance, cheaper cabling
Parallel	Short runs, high speed
Simplex	One direction only (e.g. keyboard to computer)
Half-duplex	Alternating two-way (e.g. walkie-talkie)
Full-duplex	Simultaneous two-way (e.g. phone call)

The exam typically gives a scenario (printer cable, long-distance network link, keyboard) and asks which method suits it — build this table into automatic recall rather than re-deriving trade-offs under time pressure.

USB: a standardised, widely supported connector that also supplies power (benefit); has transmission speed limits compared with some dedicated interfaces, and a maximum practical cable length (drawback). Past questions ask for one of each — prepare both.

Error detection — keep the three methods distinct

• Parity check (odd/even, including parity byte and parity block): counts the number of 1-bits and compares against an expected odd or even total.
• Checksum: a calculated value sent alongside the data, recalculated on arrival for comparison.
• Echo check: the received data is simply sent back to the sender to compare with the original.

Check digit (e.g. ISBN, barcodes) is a related but separate idea — it validates one piece of entered data, not an entire transmitted packet. Don’t conflate it with the three methods above.

ARQ (automatic repeat query): uses positive and negative acknowledgements plus a timeout to confirm data was received without error.

Encryption — the one distinction that carries the marks

• Symmetric encryption: one shared key for both encrypting and decrypting.
• Asymmetric encryption: a mathematically linked public/private key pair — data encrypted with the public key can only be decrypted with the matching private key.

State the difference precisely — just naming both terms without the mechanism usually only earns partial credit.

Connect 2.1 and 2.2 — why error detection exists at all

A network cannot guarantee a clean, uninterrupted path from sender to receiver — that is exactly why packets are switched and routed independently (2.1). That same unreliability — interference, congestion, equipment faults along the packet-switched journey — is exactly why error detection (2.2) exists at all. A strong “why do we need error detection” answer draws this link explicitly rather than treating the two sub-topics as unrelated.

Exam traps

• Confusing a movement/change in one error-detection method for another because all three serve the same general purpose — keep them distinct by what each one actually checks.
• Treating check digits as a fourth error-detection method rather than a separate, single-value validation.
• Naming symmetric/asymmetric encryption without explaining the public/private key mechanism.
• Treating USB as too minor to examine — past papers have asked for both a benefit and a drawback.

Where this topic sits in the wider syllabus

Data transmission is the second of six topics grouped under “Computer systems,” between Data representation and Hardware. Everything here assumes candidates already accept that data is ultimately binary (Topic 1); the packet structure and network concepts introduced here are picked up again in Topic 3.4 (Network hardware) and Topic 5 (The internet and its uses), both of which assume candidates already understand how data physically moves across a network before discussing the hardware and protocols built on top of it. Revising this topic thoroughly therefore pays off twice later in the syllabus, not just on its own questions.

Choosing between transmission methods — worked scenario

Consider a printer connected to a computer by a short cable versus a network link connecting two offices several kilometres apart. The printer connection favours parallel transmission because the distance is short and speed benefits outweigh cost; the office-to-office link favours serial transmission because parallel cabling becomes unreliable and expensive over long distances due to signal degradation between the multiple wires. Practising this kind of side-by-side comparison, rather than memorising the definitions in isolation, is what lets you answer a “justify your choice” question rather than only a “name the method” question.

Distinguishing the 0478 and 0984 sibling syllabuses

Cambridge IGCSE Computer Science 0478 is the plain A*-G qualification and is not tiered — every candidate sits the same paper regardless of ability. It is distinct from the numeric-grade Cambridge IGCSE (9-1) Computer Science 0984 sibling syllabus, which uses a different grading scale. Confirm which syllabus code your school has entered you for before assuming a past paper or mark scheme applies, since content overlaps closely but grading and paper structure differ between the two.

Self-test

1. What does a packet header contain?
2. Why might packets from the same transmission arrive out of order?
3. Name the three error-detection methods that check different things.
4. What does a check digit validate that the other three methods do not?
5. What is the key mechanism that distinguishes asymmetric from symmetric encryption?

Answers: 1. Destination address, packet number, originator’s address. 2. Routers can send different packets by different routes. 3. Parity check, checksum, echo check. 4. A single piece of entered data (e.g. an ISBN), not an entire transmitted packet. 5. A mathematically linked public/private key pair, where the public key encrypts and only the matching private key can decrypt.

Official syllabus

Cambridge IGCSE Computer Science 0478 syllabus for 2026, 2027 and 2028 (Version 5, December 2025) — cambridgeinternational.org.