Full Duplex vs Half Duplex Explained
The difference is whether both directions can transmit at once: full duplex sends and receives simultaneously (no collisions, standard today), while half duplex can only do one direction at a time (older, collision-prone, now rare).
Side by side
| Factor | Half Duplex | Full Duplex |
|---|---|---|
| Transmission | One direction at a time | Both directions simultaneously |
| Collisions | Possible (needs CSMA/CD) | None |
| Typical use today | Legacy hubs, some Wi-Fi | Standard switched Ethernet |
| Effective throughput | Lower (contention) | Full rated bandwidth |
The details that matter
Old hub-based Ethernet was half duplex — devices had to take turns, using CSMA/CD to detect and recover from collisions when two spoke at once. Modern switches give every port a dedicated full-duplex link — collisions become physically impossible. A duplex mismatch (one side full, one half) is a classic real-world fault: it doesn't stop connectivity but causes climbing CRC errors on one side and late collisions on the other, with mysteriously slow performance. Best practice: let both ends auto-negotiate, or set both manually to match. See show interfaces for spotting this in the error counters.
Frequently asked questions
What is the difference between full duplex and half duplex?
Full duplex transmits and receives simultaneously with no collisions; half duplex can only do one direction at a time and needs collision detection.
What causes a duplex mismatch?
One side of a link is set to full duplex and the other to half (often from manual misconfiguration) — causing errors and poor performance rather than a hard failure.
Is half duplex still used?
Rarely in wired Ethernet today; virtually everything is full duplex. Some wireless and legacy shared-media systems still operate in a half-duplex-like manner.
Related articles
Want hands-on training?
Learn this on real Cisco lab devices with placement support at Attila Technologies, Ahmedabad.