Data flow (also called the transmission mode) describes the direction in which data can travel between two connected devices, and whether both can send at the same time.
- Simplex: one direction only (keyboard to computer, TV broadcast).
- Half-duplex: both directions, but one at a time (walkie-talkie, old hubs).
- Full-duplex: both directions at once (telephone, modern switched Ethernet).
Each mode explained
Simplex is like a one-way street. One device always sends and the other always receives; the receiver can never reply on the same channel. A keyboard only sends to the computer, a radio station only broadcasts to listeners, and a traditional CCTV camera only sends pictures to the recorder. The full capacity of the channel is used in one direction.
Half-duplex is like a narrow bridge that only one car can cross at a time. Both devices can send, but they must take turns. A walkie-talkie user presses the button to talk, and while talking cannot hear the other person. Old Ethernet networks built with hubs worked this way: if two computers sent at the same moment, their signals collided and both had to send again. Wi-Fi is also half-duplex on each channel: a device and the access point take turns to transmit.
Full-duplex is like a two-lane road with traffic in both directions at once. Both devices send and receive at the same time, so a 1 Gbps full-duplex link can carry 1 Gbps in each direction. A telephone call is full-duplex: both people can speak and hear at once. Modern switched Ethernet is full-duplex, because each cable has separate wire pairs for each direction and the switch gives every device its own link, so there are no collisions.
| Mode | Directions | At the same time? | Example |
|---|---|---|---|
| Simplex | One | Not applicable | Keyboard, TV broadcast |
| Half-duplex | Two | No, they take turns | Walkie-talkie, Wi-Fi, hub-based Ethernet |
| Full-duplex | Two | Yes | Telephone, switched Ethernet |
Duplex mismatch: if one end of a cable is set to full-duplex and the other to half-duplex, the link still works, but very badly. The half-duplex side sees "collisions" whenever both sides send, and performance drops sharply. This is a classic cause of a slow but working connection (see 9.5). Leaving both ends on automatic negotiation usually avoids it.
Remember
- Simplex: one way only. Half-duplex: both ways, taking turns. Full-duplex: both ways at once.
- Modern switched Ethernet is full-duplex and has no collisions.