Data can travel one bit at a time down a single wire, or several bits at once down several wires. It can flow one way, both ways in turn, or both ways at once. Exam questions give you a scenario and ask which method suits it, and why.
Every connection is described in two ways. How many bits at once: serial sends one bit at a time down a single wire; parallel sends several bits at the same time down several wires. Which direction: simplex is one direction only; half-duplex is both directions but only one at a time; full-duplex is both directions at the same time. Parallel is faster over short distances but suffers skew (bits arriving at different times) over long ones, so serial is used for long distances.
Send the same byte both ways in the lab. Then make the cable longer and watch what happens to the parallel bits.
| Serial | Parallel | |
|---|---|---|
| How | One bit at a time down a single wire | Several bits at the same time down several wires |
| Advantages | Bits arrive in order, no skew, so it is reliable over long distances; fewer wires make it cheaper | Faster over short distances, because many bits are sent at once |
| Disadvantages | Slower than parallel over a short distance | Skew: bits may arrive at different times over longer distances, corrupting data; more wires cost more and can interfere with each other (crosstalk) |
| Used for | Long distances, USB, connecting to networks | Short distances, e.g. inside a computer between components |
"Parallel is faster" is only true over short distances. If a question involves a long cable, the answer is serial, and the reason is skew.
Switch the lab between simplex, half-duplex and full-duplex and send data from each end.
| Method | Direction | Example |
|---|---|---|
| Simplex | One direction only | Sending data from a computer to a printer that never replies; a radio or TV broadcast; a sensor sending readings |
| Half-duplex | Both directions, but only one direction at a time | A walkie-talkie: one person talks while the other listens |
| Full-duplex | Both directions at the same time | A phone call; a broadband connection that downloads and uploads at once |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is skew?
Which method sends data in both directions, but only one direction at a time?
In the lab, what happened to the parallel bits as the cable got longer?
How many wires does serial transmission use to send the data bits?
A full answer names two things: serial or parallel, and simplex, half-duplex or full-duplex, each with a reason linked to the scenario.
Do not just name the method. A justification such as "because data only needs to travel in one direction" is what earns the second mark.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 1 standard.
Which is an advantage of serial transmission over parallel transmission?
Which situation is best suited to simplex transmission?
A phone call lets both people speak and hear at the same time. Which method is used?
Why is parallel transmission not used to send data 100 metres along a cable?
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