Syllabus & Goals 3 min
Cambridge 2.1 · Types and methods of data transmission Paper 1 · Computer Systems
By the end of this lesson you can:
- Describe serial and parallel data transmission and compare them.
- Describe simplex, half-duplex and full-duplex transmission.
- State what the Universal Serial Bus (USB) is and give its benefits.
Recap / Warm-Up 5 min
Packets travel along wires. Now we look at how the bits move along those wires — one at a time, or many at once, and in which direction.
Quick starter
A walkie-talkie lets two people talk, but only one at a time. A phone call lets both talk at once. Which feels faster, and why?
Reveal the answer
The phone call — both directions work at the same time. That is the difference between half-duplex (walkie-talkie) and full-duplex (phone call).
Key Concept 14 min
1 · Serial vs parallel
2 · Direction: simplex, half-duplex, full-duplex
| Mode | Direction | Example |
|---|---|---|
| Simplex | One direction only | Computer → printer |
| Half-duplex | Both directions, but not at the same time | Walkie-talkie |
| Full-duplex | Both directions at the same time | Broadband / phone call |
3 · The Universal Serial Bus (USB)
USB is a form of serial transmission and the standard way to connect devices to a computer. It supports half-duplex and full-duplex.
Worked Example 12 min
Question: name the transmission type being described in each case.
| Description | Answer |
|---|---|
| One bit at a time, in one direction only | Serial, simplex |
| 8 bits at a time, in one direction only | Parallel, simplex |
| 16 bits at a time, both directions at once | Parallel, full-duplex |
| One bit at a time, both directions at once | Serial, full-duplex |
USB — benefits and drawbacks
| Benefits | Drawbacks |
|---|---|
| Devices auto-detected; drivers auto-loaded | Max cable length ~5 m (needs a hub beyond that) |
| Connector only fits one way (older types) | Slower than, e.g., Ethernet for very large transfers |
| Industry standard; backward compatible | Very early (V1) devices may not be supported |
Try It Yourself 12 min
Goal: State whether sending data from a computer to a printer is simplex, half-duplex or full-duplex.
Goal: Give two reasons why USB (serial) is preferred over parallel for connecting an external device.
Goal: Explain why a computer's internal circuits use parallel transmission, even though long parallel wires can skew.
📝 Exam Practice 10 min
Define serial data transmission.
Mark scheme
- Bits are sent one at a time down a single wire / channel (1).
Compare half-duplex and full-duplex transmission.
Mark scheme
- Both send data in both directions (1).
- Half-duplex cannot do both at the same time, whereas full-duplex can (1).
Give two benefits of using USB to connect a device to a computer.
Mark scheme
- Any two of: device auto-detected / driver auto-loaded · connector fits only one way · industry standard · backward compatible · supplies power (1 each, max 2).
Recap & Key Terms 3 min
Serial sends one bit per wire (reliable, long distance); parallel sends a byte across many wires (fast, short distance, can skew). Direction is simplex, half-duplex or full-duplex. USB is serial and auto-detects devices.
- Serial transmission
- One bit at a time down a single wire.
- Parallel transmission
- Several bits at once down several wires.
- Full-duplex
- Data sent in both directions at the same time.
- USB
- Universal Serial Bus — the standard serial connection that auto-detects devices.
Homework 1 min
Task (≤ 15 min): For each device, name the most suitable transmission direction (simplex / half-duplex / full-duplex): (a) a wireless doorbell sensor, (b) a walkie-talkie, (c) a video call.
Model answer
- (a) Simplex — the sensor only sends.
- (b) Half-duplex — both can send, but one at a time.
- (c) Full-duplex — both directions at once.