Bits on the Wire

Learning objectiveCambridge IGCSE 0478 / 0984 · §2.1

Describe how data is transmitted between devices using serial, parallel, simplex, half-duplex and full-duplex methods, and explain which method is suitable for a given scenario.

By the end of the activity you should be able to: state how many bits travel per clock tick under each form; state how many data lines each combination requires; explain why a shared half-duplex line must be turned around; explain why increasing the length or the clock rate of a parallel link eventually corrupts data; and justify a choice of method for a described situation.

Mapped to Cambridge IGCSE Computer Science 0478 / 0984 (2026–2028), §2.1 Types and methods of data transmission, including the universal serial bus.

Form — Direction — Data lines — Tick 0

Computer A

Transmitter

Message to send

Sent 0 / 0 bits0 B

Received from B

Computer AcableComputer B

Computer B

Receiver

Message to send

Sent 0 / 0 bits0 B

Received from A

Form Direction

Clock 6.0/s

The six combinationsSerial / parallel × simplex / half / full

Form and direction are independent choices, which is why every pairing exists. Select one to load it onto the bench — the same switches sit on the control bar above.

Key terms · Ngā kupu matuaEnglish · te reo Māori · 中文 · 한국어

English	Te reo Māori	中文（简体）	한국어	What it means on this page

On the te reo Māori column. Terms marked as gaps have no attested equivalent in the sources checked — Paekupu (Hangarau Matihiko), Te Aka and the CS Unplugged kuputaka. Following the practice used elsewhere in this project, no coinage is printed as though it were established; where a class needs one, commission it from Te Taura Whiri i te Reo Māori and credit the translator. Te reo Māori is not italicised and takes no plural “s”.

Teacher notesMisconceptions · prompts · answers

How to drive it in class

Shorten the message first — a single word, or even one letter — then open on serial simplex with the clock at 1/s and step through the first byte by hand, reading the bits aloud. Students should see that the first bit on the wire is the most significant bit of the first byte. Restore the full default message and the point makes itself: 34 characters is 272 bits, and at one bit per tick that is a long wait. Switch to parallel × 8 and the same 272 bits clear in 34 ticks, because a whole byte leaves at once — not because anything travels faster. Save half-duplex for last; the Force a collision button is the moment worth building to.

The macron demonstration is worth two minutes: type Tēnā koe into a message box and watch the byte count exceed the character count. UTF-8 spends two bytes on ē, which is a concrete reason character encoding sits on the specification alongside transmission.

Common misconceptions

“Parallel is faster, so modern computers use it.” The opposite happened. Push either cable slider past halfway and watch characters corrupt. Both faults get worse as the cable lengthens and the clock speeds up, which is why USB, SATA, PCIe and Ethernet are all serial links that comfortably beat the parallel buses they replaced. Parallel survives over short, matched distances — inside a chip, across a motherboard.

“Skew and crosstalk are the same thing.” They are different failures with different fixes, and the bench separates them. Skew is a timing fault: the lines are not identical in length or impedance, so bits sent together arrive apart, and the receiver samples some lines before their bit has settled — it latches the previous column's value. You see it as a ragged column drifting out of line. Crosstalk is an interference fault: a line that switches from 0 to 1 couples capacitively and inductively into its neighbours and induces a false pulse in a line that should have stayed quiet. You see it as a bit visibly flipping halfway down the cable, ringed in red. Skew is cured by matching line lengths or slowing the clock; crosstalk is cured by spacing the conductors, twisting or shielding pairs, or putting a grounded guard line between them.

“A quiet line is a safe line.” The reverse, for crosstalk. The victim of crosstalk is precisely the line that is not switching — a line already changing state is far less disturbed by its neighbours. Set the crosstalk slider high and check which bits flip: they are the ones whose value was the same as the previous column, sitting beside a line that changed.

“Full-duplex is twice as fast as half-duplex.” Full-duplex means both directions at once, not double the rate in one direction. A full-duplex link sending in one direction only runs at exactly the same rate as a half-duplex one. What it saves is the turnaround delay, which the bench shows explicitly.

“Half-duplex is just simplex with a switch.” No — simplex hardware has no transmitter at the far end at all. A half-duplex device is a transceiver; it holds its data and waits its turn. Watch B's queue sit full while A transmits.

“Serial means one wire.” One data line. A real link still needs a return path, and usually either a separate clock line or a coding scheme that lets the receiver recover the clock from the data. The counts on this page are data lines only.

“In parallel the whole byte arrives at once, so bit order doesn't matter.” Order across the lines matters exactly as much as order in time does. Line 0 must be wired to line 0.

“You pick either serial or duplex.” They answer different questions — how many bits at once, and which way traffic flows. Every one of the six pairings is buildable, which is the point of the grid.

Discussion prompts

A wired doorbell, a car reversing camera and a television remote are all one-way. Which are genuinely simplex, and which merely happen never to send anything back?
Walkie-talkies are half-duplex and telephones are full-duplex. What does that force people to change about the way they speak?
A hard drive ribbon cable was 40-way parallel; its replacement, SATA, is serial and much faster. What changed to make that trade worth it?
Full-duplex parallel needs double the data lines. Where in a real machine would that cost be acceptable, and where would it be absurd?

Exam-style questions and answers

1. Describe the difference between serial and parallel data transmission. [2]

Answer. In serial transmission, bits are sent one at a time, one after another, over a single data line. In parallel transmission, several bits are sent at the same time, each over its own data line. (1 mark each; “one bit at a time” and “multiple bits simultaneously over multiple wires” are the discriminators.)

2. State one advantage and one disadvantage of parallel over serial transmission. [2]

Answer. Advantage: more data is transferred per unit time over a short distance, because several bits travel together. Disadvantage: over longer distances the bits arrive at slightly different times (skew) and the lines interfere with one another (crosstalk), so data becomes corrupted; it also needs more wires, so the cable costs more.

3. A security camera sends video to a monitor in a locked office. It never receives data. Identify the method of transmission and justify your answer. [2]

Answer. Simplex. Data travels in one direction only, from camera to monitor, and the camera has no need — and no hardware — to receive anything back. (Accept: the return path is not required, so the simplest and cheapest arrangement suffices.)

4. Explain why a telephone call must be full-duplex but a walkie-talkie conversation can be half-duplex. [3]

Answer. A telephone call requires both people to speak and hear at the same time, so data must travel in both directions simultaneously — that is full-duplex. A walkie-talkie sends in both directions but only one at a time, so a user must finish and release the button before the other can reply — that is half-duplex. Half-duplex is acceptable because users accept the delay while the line is turned around, and it is achievable over one shared channel rather than two.

5. Universal Serial Bus is a serial interface. State two benefits of USB. [2]

Answer. Any two of: it is a universally adopted standard so devices are interchangeable; connectors only fit one way, so it cannot be inserted incorrectly; it supports several transmission speeds; it can supply power to the device; drivers are usually installed automatically; it detects devices when they are plugged in.

6. A parallel cable is lengthened from 0.5 m to 15 m and errors appear. Explain why. [3]

Answer. Each line has a slightly different length and electrical characteristics, so bits sent together arrive at slightly different times — skew — and the difference grows with cable length. If the spread exceeds the point at which the receiver samples the lines, it latches some bits from the wrong clock cycle, so the received byte differs from the byte sent. Longer parallel lines also run alongside one another for longer, so crosstalk increases: a signal changing state on one line couples into its neighbours and induces a false signal in them.

7. Distinguish between skew and crosstalk in a parallel cable, and give one method of reducing each. [4]

Answer. Skew is a difference in arrival time: bits transmitted simultaneously on different lines reach the receiver at different moments because the lines differ in length or electrical properties, so the receiver may sample a line before its bit has settled. It is reduced by matching the physical length of the lines, shortening the cable, or lowering the clock rate so each bit has longer to settle. (2)

Crosstalk is unwanted interference between lines: a line changing state produces a changing electromagnetic field that couples into an adjacent line and induces a false signal in it, which can flip a bit that should not have changed. It is reduced by increasing the spacing between conductors, twisting or shielding pairs, or running an earthed guard line between signal lines. (2)

Extension activity

Set the form to parallel × 8, the clock to 0.5/s and the message to a single word. Run it three times: skew at 70% with crosstalk at zero, then crosstalk at 70% with skew at zero, then both. Ask students to predict each time, before pressing Play, which bits will fail — both faults are deterministic here, so the answers are genuinely checkable. Skew always hits the same lines; crosstalk hits whichever quiet line happens to sit beside a switching one, so it moves around as the message changes. Then have them propose fixes for each and check against what industry actually did: match the line lengths, slow the clock, space and shield the conductors — or abandon parallel altogether for a serial link with clock recovery.