What is a defining characteristic of an ITU duplex channel frequency?

The defining characteristic of an ITU duplex channel frequency is that transmit and receive frequencies must be different. In duplex communication, one channel is designated for transmitting while a separate, distinct channel is used for receiving. This separation is crucial to avoid interference between the two signals, allowing for simultaneous communication in both directions.

For instance, in maritime communication, the use of different frequencies ensures that a ship can send and receive messages without the signals overlapping or causing disruption. This characteristic applies to various forms of duplex communication, not just in maritime but across different communication systems regulated by the International Telecommunication Union (ITU).

Other options suggest conditions that do not conform to the principles of duplex communication. Identical frequencies or variations based on communication type would not adequately support simultaneous two-way communication, thus rejecting the fundamental requirement of frequency distinction in duplex channels.