Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 4% of your exam marks.
stable
Rare
Stable4%
Serial vs parallel and simplex/half-duplex/full-duplex appear as definition or comparison questions.
The second wired-transmission choice is whether data can flow in one direction or both, and whether both directions can happen at once.
Simplex
Simplex transmission allows data to flow in one direction only.
The receiving end can never send anything back through this connection.
Examples:
- Radio and television broadcasts (the transmitter sends; the receiver listens but cannot reply)
- A doorbell (button → bell)
- A computer to a monitor (computer sends pixels; the monitor cannot send back)
- A keyboard or mouse to a computer (input device → computer)
Half-duplex
Half-duplex transmission permits data to travel in either direction, but just one way at any given moment.
While one end is sending, the other must wait. The roles switch back and forth.
Examples:
- A walkie-talkie: press the button to talk, release it to listen. Both people cannot talk at the same time.
- CB radio
- Some older network connections
- A printer cable that can also send a "low ink" message back to the computer between print jobs
Full-duplex
Full-duplex transmission allows data to flow in both directions at the same time.
Both ends can send and receive simultaneously, as if there were two separate one-way channels.
Examples:
- A telephone call (both people can talk at the same time)
- Broadband internet (you can upload while downloading)
- A video call (each participant streams video and audio in both directions)
- Modern local-area-network (LAN) cables
Side-by-side
| Direction | One-way only | Both ways, not at once | Both ways, simultaneously |
|---|---|---|---|
| Mode | Simplex | Half-duplex | Full-duplex |
| Example | TV broadcast | Walkie-talkie | Telephone call |
| Cost | Cheapest (no return channel needed) | Cheaper than full-duplex (one shared channel) | Most expensive (needs two channels or clever sharing) |
| Speed of two-way exchange | Cannot do two-way | Slower (must take turns) | Fastest |
Combining serial/parallel with simplex/half/full
These two choices are independent, so any combination is possible:
- Serial + simplex: one bit at a time, one direction only (a TV remote control signal)
- Serial + full-duplex: one bit at a time, both directions at once on two wires (a modern Ethernet link)
- Parallel + half-duplex: many bits at once, taking turns between sender and receiver (older system buses)
