A message leaves one computer as binary, crosses the cable, and is rebuilt at the other end. Choose how it's sent with the two toggles in the title bar: the form (serial or parallel) and the direction (simplex, half-duplex or full-duplex). Type your own messages into the boxes under each computer, then press Play, or Step a tick at a time.
- Serial: data is sent one bit at a time, down a single wire.
- Parallel: several bits are sent at the same time, each down its own wire (8 wires send a whole byte at once).
- Simplex: data is sent in one direction only.
- Half-duplex: data is sent in both directions, but only one direction at a time.
- Full-duplex: data is sent in both directions at the same time.
The wires run together inside one cable, as they do in a real one. Each bit is a dot on its wire: filled for a 1, hollow for a 0. In parallel, the Skew and Crosstalk sliders show why parallel only works over short distances: skew is bits sent together arriving at different times, and crosstalk is a switching wire inducing a false signal in its neighbour. In half-duplex, Force a collision shows what happens when both ends send onto the shared line at once.
The six combinations
Form and direction are separate choices, so every pairing exists. Choose one to load it.
Teacher notes
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 default message and the point makes itself: Learn It With Me is 16 characters, 128 bits, and at one bit per tick that's a long wait. Switch to parallel × 8 and the same 128 bits clear in 16 ticks, because a whole byte leaves at once, not because anything travels faster. Save half-duplex for last; Force a collision 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 each on ē and ā, so 8 characters take 10 bytes.
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, or across a circuit board.
“Skew and crosstalk are the same thing.” They are different failures with different fixes. Skew is a timing fault: the lines aren't identical in length or electrical properties, so bits sent together arrive apart, and the receiver samples some lines before their bit has arrived: it latches the previous column's value. You see it as a ragged column drifting out of line. Crosstalk is interference: a line that switches couples into its neighbours and induces a false pulse in a line that should have stayed steady. You see it as a bit flipping halfway along the cable, ringed in red. Skew is reduced by matching line lengths, shortening the cable or slowing the clock; crosstalk by spacing the conductors, twisting or shielding pairs, or an earthed guard line between them.
“A quiet line is a safe line.” The reverse, for crosstalk: the victim is the line that isn't switching. Set the crosstalk slider high and check which bits flip: they're the ones whose value was the same as in the previous column, 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. Sending one way only, full-duplex runs at the same rate as half-duplex. What it saves is the turnaround delay, which the bench shows.
“Half-duplex is just simplex with a switch.” Simplex hardware has no transmitter at the far end at all. A half-duplex device can send and receive; 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 a clock line or a coding scheme the receiver can recover the clock from. The counts here are data lines only.
“In parallel the whole byte arrives at once, so bit order doesn't matter.” Order across the lines matters as much as order in time: line 0 must be wired to line 0.
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's 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 twice 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. Serial: data is sent one bit at a time, down a single wire. Parallel: multiple bits are sent at the same time, down multiple wires.
2. State one advantage and one disadvantage of parallel over serial transmission. [2]
Answer. Advantage: faster transmission, as several bits are sent at the same time. Disadvantage: it's only suitable for short distances, as over longer ones the bits can arrive skewed (out of step) and the wires suffer interference (crosstalk), so errors occur; 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 only needs to be sent in one direction, from the camera to the monitor; nothing needs to be sent back.
4. Explain why a telephone call must be full-duplex but a walkie-talkie conversation can be half-duplex. [3]
Answer. On a telephone call both people may speak and hear at the same time, so data must travel in both directions at once: full-duplex. With a walkie-talkie data travels in both directions but only one at a time: one person speaks, releases the button, then the other replies. Half-duplex is acceptable because users accept that turn-taking, and it needs only one shared channel.
5. A universal serial bus (USB) connection is used. State two benefits of USB. [2]
Answer. Any two of: it's a universal (industry) standard; the device is detected and its drivers installed automatically; it can power or charge the device; it supports different transmission speeds; it's backward compatible; the cable can only be plugged in one way; as it's serial, the data is less likely to be skewed.
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 properties, so bits sent together arrive at slightly different times (skew), and the difference grows with the length of the cable. If it's large enough, the receiver reads some bits from the wrong clock cycle, so the received data differs from the data sent. The lines also run alongside each other for longer, so there's more crosstalk: a signal changing on one line induces a false signal in its neighbours. (Parallel cables are rarely made longer than about 5 m.)
7. Distinguish between skew and crosstalk in a parallel cable, and give one way of reducing each. [4]
Answer. Skew is a difference in arrival time: bits sent at the same time on different lines reach the receiver at different moments, so the receiver may read a line before its bit has arrived. It's reduced by matching the lengths of the lines, shortening the cable, or lowering the clock rate. Crosstalk is interference between lines: a line changing state induces a false signal in the line next to it, which can change a bit. It's reduced by spacing the conductors further apart, twisting or shielding pairs, or an earthed guard line between signal lines.
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. Before each, ask students to predict which bits will fail: both faults are deterministic here, so the answers can be checked. Skew always hits the same lines; crosstalk hits whichever steady line sits beside a switching one, so it moves as the message changes. Then have them propose fixes and compare with what industry did: match the line lengths, slow the clock, space and shield the conductors, or replace parallel with a serial link.
Objective: describe how data is transmitted using serial, parallel, simplex, half-duplex and full-duplex methods, with the advantages and disadvantages of each, and explain which is suitable for a given situation (Cambridge IGCSE Computer Science 0478 / 0984, 2.1 Types and methods of data transmission).
Where this fits
- AQA: AQA A Level Computer Science (7517) Goes beyond AQA A Level Computer Science (7517): Simplex, half-duplex and full-duplex aren't in AQA 7517; serial against parallel is.
- Cambridge: Cambridge IGCSE Computer Science (0478)
