CAP7001 · Unit 1: Network basics
Transmission modes: simplex, half-duplex and full-duplex
10 practice questions · in the MTE and ETE syllabus
Key points
- Simplex = one direction only
- Half-duplex = both directions, alternately
- Full-duplex = both directions simultaneously
- Half-duplex uses the full capacity for the active sender
Three transmission modes describe the direction of flow on a link.
| Mode | Direction | Example |
|---|---|---|
| Simplex | One way only | Keyboard to computer, broadcast radio |
| Half-duplex | Both ways, one at a time | Walkie-talkie |
| Full-duplex | Both ways at once | Telephone call |
In simplex the sender can only send and the receiver can only receive. In half-duplex each station can send and receive, but not at the same time, so the entire capacity goes to whichever side is transmitting. In full-duplex both stations send simultaneously: either the link has two separate paths, or its capacity is divided between the two directions.
Worked example: two devices each send 4000 bits to the other over a 2000 bps link used half-duplex: 8000 / 2000 = 4 s in total. A full-duplex link of 2000 bps per direction needs only 2 s.
Exam pitfall: full-duplex does not always need two cables, and half-duplex is not a half-speed link. It uses the full capacity, in one direction at a time.
Try two questions
Which example is correctly matched with its transmission mode?
- Keyboard to computer: half-duplex
- Walkie-talkie: full-duplex
- Radio broadcast: half-duplex
- Mobile phone call: full-duplex
Show the answer
D. On a phone call both parties can talk and hear at the same moment, which is full-duplex. A walkie-talkie is the classic half-duplex case.
Why not A: A keyboard only sends to the computer, so it is simplex.
Why not B: A walkie-talkie needs push-to-talk turns, so it is half-duplex.
Why not C: A broadcast flows one way to receivers, so it is simplex.
A half-duplex 8 Mbps link must carry a 24 Mb file from A to B and then a 16 Mb file from B to A. Each reversal of direction costs 0.5 s, and A transmits first. What is the total time?
- 6.0 s
- 5.5 s
- 5.0 s
- 3.0 s
Show the answer
B. A needs 3 s, one 0.5 s turnaround is needed, and B then needs 2 s, so the total is 5.5 s. Only one reversal is required because A starts, and the 3.0 s figure would need full-duplex.
Why not A: That charges a turnaround before A starts as well as after, giving two reversals.
Why not C: That leaves out the turnaround time completely.
Why not D: That is the time for a full-duplex link where both transfers overlap.