Cabling · Networking
Duplex / Simplex
Simplex transmits in one direction only. Half-duplex can go both ways but not simultaneously. Full-duplex transmits and receives at the same time — the standard for modern Ethernet.
These terms describe the directionality of a communication channel:
Simplex — one direction only. A broadcast antenna is simplex. In fiber cabling, a single-strand fiber is sometimes called simplex (one fiber, one direction) vs duplex (two fibers, bidirectional).
Half-duplex — both directions, but not simultaneously. Early Ethernet hubs operated in half-duplex: only one device on a segment could transmit at a time, and all others had to wait. Walkie-talkies are half-duplex.
Full-duplex — simultaneous bidirectional transmission. All modern switched Ethernet is full-duplex: the switch creates a dedicated collision domain for each port, so devices can transmit and receive at the same time without collisions. A 1Gbps full-duplex link is 1Gbps in each direction simultaneously.
In fiber, duplex connectors pair two fibers (transmit and receive) in a single connector body — an LC duplex or SC duplex connector handles both directions in one plug, as opposed to two separate simplex fibers.
The distinction matters when configuring network equipment: if both ends of a link aren't set to the same duplex mode, you'll see a duplex mismatch — excessive errors, poor performance, and link instability that can be hard to diagnose.