📚 Knowledge Library — Topic 2.3B — Data Transmission

Simplex, Half-Duplex and Full-Duplex

Understand the direction data can travel in a connection: one way, both ways one at a time, or both ways at the same time.

Start LearningWorked Example
1. Invitation

Data does not always travel both ways.

When two devices communicate, the direction of data matters.

Some connections only send data one way. Others allow both devices to send data, either one at a time or simultaneously.

💡 Remember: this lesson is about direction, not speed.
Figure 1.1
Direction Matters
Device A
↔
Device B
The arrows show how data can travel.
2. Big Idea

There are three direction patterns.

Simplex sends data in one direction only.

Half-duplex sends data both ways, but not at the same time.

Full-duplex sends data both ways at the same time.

💡 One-way, turn-taking, or simultaneous.
Figure 2.1
Three Patterns
Simplex →

Half-duplex ⇄
one at a time

Full-duplex ⇆
same time
3. Bridge

Think of road systems.

Simplex is like a one-way street.

Half-duplex is like a narrow bridge where traffic can go either way, but only one direction at a time.

Full-duplex is like a dual carriageway where traffic moves both ways together.

💡 Road direction helps students remember data direction.
Figure 3.1
Road Bridge
One-way street
→

Narrow bridge
⇄ one at a time

Dual carriageway
⇆ same time
4. Simplex

Simplex sends data one way only.

In simplex transmission, the sender only sends and the receiver only receives.

There is no reply path for data in the opposite direction.

Example

Microphone → Speakers
💡 Simplex = one direction only.
Figure 4.1
Simplex
Sender
────►
Receiver
Data travels one way.
5. Half-Duplex

Half-duplex allows turn-taking.

In half-duplex transmission, data can travel in both directions.

However, both devices cannot transmit at the same time. They must take turns.

Example

Walkie-talkie conversation
💡 Half-duplex = both directions, not simultaneously.
Figure 5.1
Half-Duplex
Device A
────►
Device B

then

Device A
◄────
Device B
One direction at a time.
6. Full-Duplex

Full-duplex works both ways at once.

In full-duplex transmission, both devices can send and receive data simultaneously.

This is useful when two-way communication needs to happen in real time.

Worked Example

Scenario
Need
Method
Phone call
Both speak and listen
Full-duplex
A phone call uses full-duplex because both people can talk and hear at the same time.
Figure 6.1
Full-Duplex
Device A
◄════►
Device B
Both directions simultaneously.
7. Exam Tip

The timing words earn marks.

In exam answers, do not just write that data can go both ways.

You must say whether it happens at the same time or not at the same time.

🎯 Exam Tip: full-duplex = both directions at the same time. Half-duplex = both directions, but not at the same time.
Figure 7.1
Mark-Winning Words
Half-duplex
not at same time

Full-duplex
at same time
8. Common Mistake

“Both ways” is not enough.

Students often say half-duplex and full-duplex both send data in both directions. That is true, but incomplete.

The difference is whether the two directions can happen simultaneously.

⚠️ Common Mistake: writing “both ways” without saying “at the same time” or “not at the same time”.
Figure 8.1
Do Not Stop Here
Both ways
?
same time
or
turn-taking?
9. Summary

Direction of transmission in one screen.

Simplex: data travels in one direction only.

Half-duplex: data travels in both directions, but not at the same time.

Full-duplex: data travels in both directions simultaneously.

💡 Key idea: direction + timing decides the transmission type.
Figure 9.1
Final Pattern
Simplex →

Half-duplex ⇄
turn-taking

Full-duplex ⇆
same time