Which method sends bits one at a time down a single wire?

serial. Serial means one after another.

Which method lets data travel in both directions at the same time?

full-duplex. A phone call is full-duplex: both people can speak at once.

A walkie-talkie lets either person talk, but only one at a time. This is:

half-duplex. Both directions, but not together.

Why is serial transmission preferred over long distances?

The bits cannot arrive out of step, and the cable is cheaper. With a single wire there is nothing to get out of step with.

Which is a benefit of USB?

A device is detected automatically when it is plugged in. The computer recognises the device and loads its driver.

Which is an example of simplex transmission?

a keyboard sending key presses to a computer. Data only ever goes one way.

Serial and parallel, simplex and duplex, and USB · Cambridge IGCSE Computer Science

In plain words

There are two separate questions about any link between devices. How many bits travel at once: one after another down a single wire (serial), or several side by side down several wires (parallel)? And which way can the data go: one way only, both ways in turn, or both ways at once?

7 things to know

Serial transmission sends bits one at a time down a single wire. It is reliable over long distances and cheap, because it needs few wires.
Parallel transmission sends several bits at the same time down several wires. It is faster over a short distance, but over a long one the bits can arrive out of step (skewed), and the cable costs more.
Simplex: data travels in one direction only, such as from a keyboard to a computer.
Half-duplex: data travels in both directions, but only one way at a time, such as a walkie-talkie.
Full-duplex: data travels in both directions at the same time, such as a phone call.
USB (universal serial bus) is a serial connection. A device is detected automatically when it is plugged in and the right driver is loaded. The connector fits only one way round, it can supply power, and it is an industry standard that works with older versions.
Drawbacks of USB: the cable length is limited (about 5 metres), and older versions transfer data slowly.

Worked example

A factory computer sends data to a printer 30 metres away. Choose serial or parallel transmission, and justify the choice.

30 metres is a long distance for a cable between two devices.
Over that distance the bits in a parallel cable could arrive out of step, causing errors, and the many-wired cable would be expensive.
Serial is the better choice: one wire, so no skew, and a cheaper cable.

Tips and tricks

Serial or parallel is about how many bits at once. Simplex or duplex is about direction. A link has one of each, such as "serial, full-duplex".
When justifying a choice, use the scenario: long distance points to serial, and a need for data both ways at once points to full-duplex.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Serial and parallel, simplex and duplex, and USB: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

Which method sends bits one at a time down a single wire?
- parallel
- serial (the answer)
- duplex
- simplex
Serial means one after another.
Which method lets data travel in both directions at the same time?
- simplex
- half-duplex
- full-duplex (the answer)
- serial
A phone call is full-duplex: both people can speak at once.
A walkie-talkie lets either person talk, but only one at a time. This is:
- simplex
- half-duplex (the answer)
- full-duplex
- parallel
Both directions, but not together.
Why is serial transmission preferred over long distances?
- It sends more bits at once.
- The bits cannot arrive out of step, and the cable is cheaper. (the answer)
- It needs more wires.
- It only works one way.
With a single wire there is nothing to get out of step with.
Which is a benefit of USB?
- The cable can be any length.
- A device is detected automatically when it is plugged in. (the answer)
- It sends data in parallel.
- It needs no software at all.
The computer recognises the device and loads its driver.
Which is an example of simplex transmission?
- a phone call
- a video call
- a keyboard sending key presses to a computer (the answer)
- two people texting each other
Data only ever goes one way.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 7 marks. Write your answers on paper, then check them.

Serial and parallel, simplex and duplex, and USB

Cambridge IGCSE Computer Science 0478 · 7 marks · papermunch.org

Name ______________________________ Date ______________

Describe the difference between half-duplex and full-duplex transmission.[2]
Show answerHide answer
In half-duplex, data can go in both directions but only one way at a time. In full-duplex, data goes in both directions at the same time.
I got this one
Give three benefits of using a USB connection to attach a device to a computer.[3]
Show answerHide answer
The device is detected automatically and its driver loaded. The connector can only be put in the right way round. It can supply power to the device. (Also: it is a widely used standard.)
I got this one
Explain why parallel transmission is not suitable over long distances.[2]
Show answerHide answer
The bits travel along separate wires and can arrive at slightly different times (skew), which causes errors. The cable is also more expensive.
I got this one

Answers: Serial and parallel, simplex and duplex, and USB

1. In half-duplex, data can go in both directions but only one way at a time. In full-duplex, data goes in both directions at the same time.
2. The device is detected automatically and its driver loaded. The connector can only be put in the right way round. It can supply power to the device. (Also: it is a widely used standard.)
3. The bits travel along separate wires and can arrive at slightly different times (skew), which causes errors. The cable is also more expensive.