Two Questions About Every Connection
Part 1 showed how data is packaged and routed. This guide zooms in on a single connection and asks how the bits are actually sent along it.
Students often mix up the five methods in this topic because they try to learn them as one list. They are really the answers to two separate questions:
| Question | Possible answers |
|---|---|
| How many bits are sent at the same time? | Serial (one at a time) or parallel (several at once) |
| In which direction can data travel? | Simplex, half-duplex or full-duplex |
A connection has an answer to both questions. For example, a connection can be serial and full-duplex. Keep the two questions separate and the topic becomes much easier.
Where This Fits in the Syllabus
Cambridge IGCSE Computer Science (0478, 2026–2028) section 2.1 asks you to:
- describe how data is transmitted using serial, parallel, simplex, half-duplex and full-duplex, including the advantages and disadvantages of each
- explain the suitability of each method for a given scenario
- understand the USB interface, how it is used to transmit data, and its benefits and drawbacks
Notice "for a given scenario". Many marks come from choosing a method and justifying it using the details in the question.
Check Your Understanding 1
A student says, "Full-duplex is a type of parallel transmission." Why is this a muddle?
Check the answer
The terms answer different questions. Full-duplex describes the direction of data (both ways at once). Parallel describes how many bits are sent at the same time. A connection could be full-duplex and serial.
Serial Transmission — One Bit at a Time Syllabus
In serial transmission, data is sent one bit at a time along a single channel. The bits travel one after another, like cars in single file on a one-lane road. Because only one data channel is needed, a serial cable generally needs fewer conductors (wires) than an equivalent parallel cable.
▼
1 → 0 → 1 → 1
→ 0 → 0 → 1 → 0
▼
Receiver
One channel: the first 1 is sent first, then each bit follows the one before it
| Advantages | Disadvantages |
|---|---|
| The bits travel one after another along a single channel, so they cannot arrive out of step with each other (no skew, explained below). This makes serial reliable over long distances | In the traditional comparison, sending one bit at a time can be slower than a parallel link over a short distance (but see the Extension box below) |
| Fewer wires, so the cable is cheaper and there is less interference between wires | Data may need converting between serial and parallel form at each end, which adds processing |
Typical uses: USB connections, and network connections that carry data over long distances.
Parallel Transmission — Several Bits at Once Syllabus
In parallel transmission, several bits are sent at the same time, each along its own wire. With eight wires, a whole byte can be sent in one go — like eight cars side by side on an eight-lane road.
Wire 1 ── 1 ──▶
Wire 2 ── 0 ──▶
Wire 3 ── 1 ──▶
Wire 4 ── 1 ──▶
Wire 5 ── 0 ──▶
Wire 6 ── 0 ──▶
Wire 7 ── 1 ──▶
Wire 8 ── 0 ──▶
▼
Receiver gets all 8 bits at the same time
(if there is no skew)
The problem: skew
The wires are never perfectly identical, so the bits sent at the same moment can arrive at slightly different times. This is called skew. Over a short distance the difference is tiny. Over a long distance it grows, until bits from one group get mixed up with bits from the next, and the data is corrupted.
Running many wires side by side also causes interference between them (sometimes called crosstalk), which can also corrupt data.
| Advantages | Disadvantages |
|---|---|
| Can be faster over short distances, because several bits are sent at once | Skew means bits can arrive out of step, so it is unreliable over long distances |
| Several bits (for example, a whole byte) are transferred at the same time, which suits components that are designed to send and receive data in parallel, such as those on a computer's internal parallel bus | More wires make cables more expensive and cause more interference |
Typical uses: very short connections, such as a computer's internal data bus, or older printer cables. (Even inside computers, many modern links are now serial — see the Extension box below.)
Serial vs parallel at a glance
| Feature | Serial | Parallel |
|---|---|---|
| Bits sent at once | One | Several (e.g. 8) |
| Wires or channels | One | Several |
| Skew? | No | Yes — worse with distance |
| Best for | Long distances | Short distances |
It can be faster over short distances. Over long distances, skew corrupts the data, so serial is the better choice. And modern high-speed serial links often outperform older parallel ones even over short distances.
Check Your Understanding 2
What is skew, and why does it make parallel transmission unsuitable for long distances?
Reveal the answer
Skew is when bits sent at the same time along different wires arrive at slightly different times. The longer the cable, the bigger the difference, until bits get out of step and the data is corrupted.
Serial and parallel describe how the bits travel. Now the second question: can data travel one way, or both?
Simplex, Half-Duplex and Full-Duplex Syllabus
A ──────▶ B
one way only; B can never send back
HALF-DUPLEX
A ──────▶ B
then, taking turns
A ◀────── B
both ways, but only one direction at a time
FULL-DUPLEX
A ──────▶ B
and at the same time
A ◀────── B
both directions at once
Simplex — one direction only
Data can only travel one way, from the sender to the receiver. The receiver can never send anything back. A radio or TV broadcast is an example: the station transmits, and your radio only receives.
- Advantage: simple; all of the connection's capacity is used for one direction, and data from the two ends can never collide.
- Disadvantage: the receiver cannot reply, so it cannot confirm that data arrived or ask for it to be sent again.
Half-duplex — both directions, but one at a time
Data can travel in both directions, but not at the same time. Each side must take turns. A walkie-talkie is the classic example: you press the button to talk, then release it to listen — you cannot do both at once.
- Advantage: two-way communication is possible using a single channel, which keeps it simpler and cheaper than full-duplex.
- Disadvantage: each side has to wait for the other to finish, so communication is slower.
Full-duplex — both directions at the same time
Data can travel in both directions simultaneously. A phone call is the example most people know: both people can talk and hear each other at the same time. A home broadband connection works this way too, uploading and downloading at once.
- Advantage: the fastest form of two-way communication, because neither side has to wait.
- Disadvantage: more complex and usually more expensive, because both directions must be carried at once without interfering with each other.
| Method | Directions | At the same time? | Example |
|---|---|---|---|
| Simplex | One | — | Radio or TV broadcast |
| Half-duplex | Both | No — taking turns | Walkie-talkie |
| Full-duplex | Both | Yes | Phone call, broadband |
That is simplex. Half-duplex can send in both directions — just not at the same time.
Check Your Understanding 3
Name the method used in each situation.
- Two friends chat on a video call, talking over each other.
- A display board at a station only receives train times from a central computer.
- Two builders use radios where only one can transmit at a time.
Check your answers
- Full-duplex — both directions at the same time.
- Simplex — data travels in one direction only.
- Half-duplex — both directions, but one at a time.
USB — The Universal Serial Bus Syllabus
USB stands for Universal Serial Bus. It is a standard way of connecting devices such as keyboards, mice, printers, phones and flash drives to a computer. As the name tells you, it uses serial transmission.
What happens when you plug in a USB device?
- The computer detects that a device has been connected.
- The device tells the computer what type of device it is.
- The computer loads the right driver — the software that lets it communicate with the device. Many common devices use standard drivers already built into the operating system, so this happens automatically. If no suitable driver is installed, the operating system may download one, or the user may need to install it.
- Data can now be transmitted between the device and the computer. The device can also draw power through the cable, up to a limit that depends on the port and the USB version.
| Benefits of USB | Drawbacks of USB |
|---|---|
| A universal standard supported by a huge range of devices, from keyboards to phones and printers | Limited cable length — a standard cable can only be a few metres long before the signal becomes unreliable (the exact limit depends on the USB version); longer distances need a hub or an active cable |
| Devices are detected automatically and the driver is usually loaded automatically | Older USB versions are much slower than newer ones. A connection usually runs at the speed of the slowest part, so a fast device on an old port or cable only transfers data at the older speed |
| Can supply power as well as data, for example to charge a phone or run a keyboard | The power supplied is limited and varies by port and version, so devices that need more than the port can give need their own power supply. (USB-C Power Delivery can supply much more, enough to charge many laptops, but only when both the charger and the device support it) |
| Newer versions are usually backward compatible with older ones (they still work, but at the older speed). Traditional connectors such as USB-A and micro-USB are keyed so they only fit one way round and cannot be plugged in incorrectly; the newer USB-C connector is reversible and fits either way up | There are several different connector types (such as USB-A, micro-USB and USB-C), so an adapter or a different cable may be needed |
Check Your Understanding 4
Give one benefit and one drawback of using USB to connect a device.
Reveal the answer
Benefit, any one of: a universal standard used by many devices; devices are detected automatically and drivers loaded; can supply power as well as data; backward compatible; connectors are shaped so they cannot be plugged in incorrectly (keyed connectors fit only one way round, and USB-C is reversible). Drawback, any one of: limited cable length; older versions are slower; limited power supply; several connector types.
Choosing a Method for a Scenario
Exam questions give you a situation and ask which method is most suitable. Pick out the clues — distance, direction, whether replies are needed — then justify your choice with them.
Scenario A — Across the school site
Data must be sent between two buildings, 80 metres apart. Serial or parallel?
Reveal the analysis
Serial. Over 80 metres, parallel transmission would suffer from skew, so bits would arrive out of step and the data could be corrupted. Serial sends bits one after another along one channel, so it stays reliable over long distances, and the cable is cheaper.
Scenario B — Inside the computer
Data must be moved very quickly between two components a few centimetres apart on a circuit board. Serial or parallel?
Reveal the analysis
Parallel is a suitable answer for this simplified scenario. Over such a short distance, skew is not a problem, so sending several bits at once can make transfer fast. (In real modern computers, many fast internal links, such as PCI Express, are actually high-speed serial — so justify your choice using the clues in the question, such as the very short distance.)
Scenario C — The baby monitor
A basic baby monitor sends sound from the baby's room to a parent's unit. The parent's unit never sends anything back.
Reveal the analysis
Simplex. Data only needs to travel in one direction. (If the monitor had a "talk back" button where parent and baby unit took turns, it would be half-duplex.)
Scenario D — Online gaming
A player's console sends their moves to a game server while constantly receiving the positions of other players.
Reveal the analysis
Full-duplex. Data needs to flow in both directions at the same time. Making each side wait for the other (half-duplex) would cause noticeable delays in the game.
Exam Focus — Choose and Justify
These are original FutureLogic practice questions in the Cambridge style. Try each one before revealing the model answer. The marks shown are only a guide: a model answer shows the kind of points examiners look for, but it cannot guarantee marks. Real mark schemes decide, and they accept a range of correct points.
Question 1 (3 marks)
A company needs to send data to an office 200 metres away. Explain why serial transmission is more suitable than parallel transmission.
Reveal the model answer
Syllabus link: 2.1.2(b) — explain the suitability of each method for a given scenario.
Parallel transmission sends several bits at once along different wires. Over 200 metres the bits would suffer from skew and arrive at different times, so the data could be corrupted. Serial transmission sends one bit at a time along a single channel, so the bits arrive in order and the data stays reliable over a long distance. Serial also needs fewer wires, so the cable is cheaper and there is less interference.
Why this is a strong answer: it explains the problem with parallel (skew) and why serial avoids it, and links both to the 200-metre distance in the question.
Question 2 (2 marks)
Describe the difference between half-duplex and full-duplex data transmission.
Reveal the model answer
Syllabus link: 2.1.2(a) — methods of data transmission.
In half-duplex, data can travel in both directions, but only one direction at a time. In full-duplex, data can travel in both directions at the same time.
Why this is a strong answer: it makes the key difference — "one at a time" versus "at the same time" — explicit for both methods.
Question 3 (4 marks)
A student connects a printer to a computer using USB. Describe two benefits and two drawbacks of using USB.
Reveal the model answer
Syllabus link: 2.1.3 — the benefits and drawbacks of the USB interface.
Benefits: the printer is detected automatically when it is plugged in, and the correct driver is usually loaded; USB is a universal standard, so the same port can be used for many other devices.
Drawbacks: the cable length is limited, so the printer must be close to the computer; if the computer has an older USB port, data will transfer at the slower speed of that older version.
Why this is a strong answer: each point is specific and linked to the printer, rather than general words such as "fast" or "easy".
Common Cambridge Exam Mistakes
Serial and parallel are about how many bits at once. Simplex and duplex are about direction.
It can be faster over short distances, but not always — modern high-speed serial links often outperform older parallel ones. Over long distances, skew corrupts parallel data.
That is simplex. Half-duplex is both directions, one at a time.
It is Universal Serial Bus — a reminder that USB uses serial transmission.
In a scenario question, use the details given — distance, direction, need for replies — to explain your choice.
Quick Quiz
- What is the difference between serial and parallel transmission?
- What is skew?
- Which is more suitable for long distances, and why?
- Describe simplex transmission and give an example.
- How does half-duplex differ from full-duplex?
- Give an example of full-duplex communication.
- What does USB stand for, and which transmission method does it use?
- Give two benefits of USB.
Quick Quiz Answers
Reveal all answers
- Serial sends one bit at a time along one channel; parallel sends several bits at the same time along several channels.
- Skew is when bits sent at the same time along parallel wires arrive at slightly different times.
- Serial, because it does not suffer from skew, so data is not corrupted over distance.
- Data travels in one direction only, e.g. a radio or TV broadcast.
- Half-duplex sends in both directions but one at a time; full-duplex sends in both directions at the same time.
- A phone call, or a broadband internet connection.
- Universal Serial Bus; serial transmission.
- Any two of: universal standard; automatic detection and driver loading; supplies power; backward compatible; connectors cannot be plugged in incorrectly (keyed, or reversible USB-C).
Final Summary
Serial
One bit at a time, one channel. Reliable over long distances.
Parallel
Several bits at once. Can be fast over short distances; skew over long ones.
Simplex
One direction only.
Half-Duplex
Both directions, one at a time.
Full-Duplex
Both directions at the same time.
USB
Universal Serial Bus: a standard serial interface that carries data and power.