The public telephone system is an example of a __________ system.

Simplex communication is one-way only, such as broadcast radio, and cannot describe a telephone system where both parties communicate.

Half-duplex allows two-way communication but only in one direction at a time, like a walkie-talkie - the PSTN does not impose this restriction.

Full-duplex communication enables simultaneous two-way data transmission, meaning both endpoints can send and receive at the same time over the same connection. The PSTN is designed so both callers can talk and hear concurrently without one side needing to stop transmitting for the other to speak, which is the textbook definition of full-duplex operation.

While legacy PSTN originated as analog, modern telephone infrastructure uses digital switching and transmission internally, so 'purely analog' is technically and historically inaccurate.

The PSTN is not purely digital because the local loop (last-mile connection to subscriber premises) has historically used analog signaling, making 'purely digital' incorrect.