A full duplex (often written full-duplex) is a communication system or medium that physically permits simultaneous transmission and reception between endpoints. To understand that the ability to have physical two-way communications but not a usable two-way flow, consider the example of two people, talking face-to-face through the medium of air. While they can both talk at the same time, the conversation will not be intelligible.
Two-way simultaneous (TWS) systems allow simultaneous transmission and reception among the endpoints. It requires a full duplex medium. This is a plausible logical means of communications for such things as computers keeping their files synchronized, with updates flowing in both directions.
Two-way alternating (TWA) systems require either a full duplex medium, a half duplex medium, or a pair of unidirectional communications. This term refers to the logical level of communications, such as the need to talk, and then to listen, to have an intelligible face-to-face conversation.