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Commonly confused · Networking Fundamentals

Half Duplex vs Full Duplex: What's the Difference?

Half duplex lets a device either send or receive at a given moment, but not both, so collisions are possible and CSMA/CD is used. Full duplex sends and receives simultaneously on separate paths, which eliminates collisions and effectively doubles usable bandwidth.

Half Duplex

Full definition
vs

Full Duplex

Full definition
Half DuplexFull Duplex
DirectionOne direction at a timeBoth directions at once
CollisionsPossibleNone
CSMA/CDRequiredDisabled
Typical deviceHub, legacy Ethernet, Wi-Fi radiosSwitch port to host, modern Ethernet
AnalogyWalkie-talkieTelephone call
Mismatch symptomShows late collisions when the other end is full duplexShows CRC errors and runts when the other end is half duplex
Remember it

Half duplex is a walkie-talkie; full duplex is a phone call.

Exam tip

Duplex mismatch is a favorite troubleshooting scenario: slow performance with late collisions on the half-duplex side and CRC or runt errors on the full-duplex side. Also know that a hub forces half duplex while switches support full duplex.

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