Half-duplex data transmission

Data can travel in both directions, but only one direction at a time, so devices take turns, like a walkie-talkie.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 29, WA Computer Science ATAR Year 11 — Network Communications — Module 21

Understanding Half-duplex data transmission in IGCSE Computer Science Paper 1

What it does: The arrows show data going in both directions, but in separate turns, which is how half-duplex works. While the other device is sending, the channel is busy, so the device waits. This turn-taking is the key feature of half-duplex. Saying 'Over' signals the end of a turn so the other person can transmit.

Common mistakes: Half-duplex data can travel in both directions, so directions should be 2. A value of 1 describes simplex. In half-duplex the two directions are never used at the same time. Devices take turns, so simultaneous should be FALSE.

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This module's key terms are also in a printable crossword (Modules 131–141) and word search (Modules 121–141).

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Learning

Duo

Which statement describes half-duplex transmission?

Data travels in both directions at the same timeData travels in both directions, but only one direction at a time

Matches

Half-duplex: A <-> B at the same time

↓

This describes full-duplex, not half-duplex. In half-duplex, data goes both ways but only one direction at a time.

Applying

Meanings

Term

half-duplex

Meaning

Data can travel in both directions, but only one direction at a time, so devices take turns, like a walkie-talkie.

Practice

IF channel = "FREE" THEN

Testing

Gaps

mode ← ""

Check Answer

Spots

1mode ← "half-duplex"

2directions ← 1

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Quick Questions

1. Which statement describes half-duplex transmission?

A. Data travels in both directions at the same time

B. Data travels in both directions, but only one direction at a time ✓

Half-duplex allows two-way communication, but the devices must take turns. Sending in both directions at once is full-duplex.

2. Which device is an example of half-duplex transmission?

A. A telephone call

B. A walkie-talkie ✓

With a walkie-talkie, only one person can speak at a time, so the users take turns. A phone call is full-duplex because both people can talk at once.

3. Two devices use half-duplex. Device A is sending data. What must Device B do?

A. Wait until Device A has finished sending ✓

B. Send its own data at the same time

In half-duplex the devices take turns. Device B can only send once the channel is free.

4. True or false: half-duplex data can travel in only ONE direction ever.

A. False: it can travel both ways, but not at the same time ✓

B. True: it only ever travels one way

One-way-only transmission is simplex. Half-duplex supports both directions, one at a time.

5. What is the key idea behind half-duplex transmission?

A. Devices take turns to send and receive ✓

B. Devices send and receive at the same moment

Taking turns is what makes the transmission half-duplex.

Match the Pairs

half-duplex → Data can travel in both directions, but only one direction at a time, so devices take turns

walkie-talkie → A half-duplex example: one person speaks while the other listens, then they swap

Half-duplex lets two devices send data both ways, but never at the same moment → A -> B, then B -> A (one at a time)

In half-duplex, a device must wait for its turn before it can send → if channel_busy: wait()

Half-duplex: A <-> B at the same time → This describes full-duplex, not half-duplex. In half-duplex, data goes both ways but only one direction at a time.

Half-duplex: A -> B only, never B -> A → This describes simplex, not half-duplex. Half-duplex can send in both directions, just not at once.

Key Term

half-duplex

Data can travel in both directions, but only one direction at a time, so devices take turns, like a walkie-talkie.

Fill in the Blanks

1. mode ← "half-duplex"

Hint: Type the name of the transmission mode where devices take turns sending in both directions.

2. maxSenders ← 1

Hint: Type how many devices can send at the same moment in this mode.

3. IF channelBusy THEN WAIT ENDIF

Hint: Type the action a device takes when the other device is currently sending.

4. IF mode = "half-duplex" THEN takeTurns() ENDIF

Hint: Type the comparison operator that checks whether the mode equals a value.

5. IF channelFree THEN SEND(data) ELSE WAIT ENDIF

Hint: Type the keyword that starts the other branch for when the channel is not free.

Spot the Error

Question 1

A mode ← "half-duplex"

B directions ← 1

C sendAtOnce ← FALSE

Line B — Half-duplex data can travel in both directions, so directions should be 2. A value of 1 describes simplex.

Question 2

A type ← "half-duplex"

B bothDirections ← TRUE

C simultaneous ← TRUE

Line C — In half-duplex the two directions are never used at the same time. Devices take turns, so simultaneous should be FALSE.

Question 3

A IF talking = "A" THEN

B A sends data to B

C ELSE

D A sends data to B

E ENDIF

Line D — When it is not A's turn (the ELSE branch), B should send data to A. Having A send in both branches means the devices never swap turns.

Question 4

A direction1 ← "A to B"

B direction2 ← "B to A"

C usedAtOnce ← 2

D // one direction at a time

Line C — Half-duplex has two directions available, but only one can be used at any moment. usedAtOnce should be 1, not 2.

Question 5

A transmission ← "full-duplex"

B devices take turns to send

C walkie-talkie is an example

Line A — The program describes devices taking turns, which is half-duplex. Full-duplex lets both devices send at the same time.

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