Transmission Modes
🎬 This chapter has a 7:57 video: Data Communication →The five components said who sends and who receives. They did not say whether the receiver is ever allowed to answer. That is the transmission mode, and there are three of them.
1What a transmission mode is
One link, three rules about direction. Watch whether anything ever travels both ways at the same moment.
Data flows in one direction only. One device always sends, the other always receives — it can never reply.
Examples: Keyboard to computer, computer to monitor, radio and TV broadcast
Think of: A loudspeaker announcement at a railway station.
2Simplex
A keyboard only sends characters to the computer. A monitor only accepts what the computer draws on it. A radio or television broadcast goes out from the station to every set, and no set replies. The whole capacity of the link is used in that one direction.
3Half duplex
A walkie-talkie, and why you say ‘over’
An old hub-based Ethernet network worked this way: only one computer could send at a time, and if two started together their signals collided and both had to try again.
4Full duplex
A telephone call is the everyday example: both people can speak at once, and often do. Modern Ethernet over a switch is full duplex as well — a twisted pair cable carries separate wires for each direction, so there is no collision to wait out.
5All three side by side
| Simplex | Half duplex | Full duplex | |
|---|---|---|---|
| Direction | One way only | Both ways | Both ways |
| At the same time? | — | No, one at a time | Yes |
| Can the receiver reply? | Never | Yes, when the line is free | Yes, immediately |
| Performance | Lowest — the link does one job | Better than simplex | Best |
| Examples | Keyboard, monitor, TV and radio broadcast | Walkie-talkie, hub-based Ethernet | Telephone call, switched Ethernet |
Full duplex is not two links. It is one connection that carries both directions at once — that is the whole point of the word duplex.
6Practice
▶Show the answer
Simplex: data flows in one direction only; the receiver can never send back. Example: a keyboard sending characters to a computer.
Half duplex: data flows in both directions but only one at a time. Example: a walkie-talkie.
Full duplex: data flows in both directions simultaneously. Example: a telephone call.
▶Show the answer
Half duplex. Both people can speak, but only one at a time — each must release the button before the other can be heard.
7Check yourself
A television broadcast reaches every set in the city, and no set sends anything back. Which mode is this?
Which statement about half duplex is TRUE?
A phone call where both people can speak at once is an example of: