A duplex network system is a point-to-point system whereby multiple end systems are connected to one another. A duplex system allows two-way communication between two end systems but, depending on implementation, may restrict when an end system can send and receive.
Duplex communications are descriptive of common transport layer protocols like transmission control protocol (TCP) and User Datagram Protocol (UDP). Duplex systems are found in one of two forms:
Full Duplex: End systems participate in simultaneous two-way communication such that an end system can receive and transmit data at the same time.
Half Duplex: End systems participate in two-way communications but cannot receive while sending or vice versa.
A full-duplex system is like speaking over the telephone: one party can interrupt or speak over the other party. Voice data is being transmitted in both directions simultaneously. A half-duplex system is more akin to walkie-talkies where one party much hold a button to transmit during which time no incoming transmissions can be received.