Duplex 5 : Two-frequency radio operation in which one frequency is used for a mobile station to talk to a base station, and the second frequency is used for a case station to talk to a mobile station
Ralph W. Burklin & Robert G. Purington, Fire Terms: A Guide to Their Meaning and Use, National Protection Association, Boston, MA 1980 USA. (Guide: Meaning and Use, Nat Prot Association Fr Srvc - Amer) {457}