Answer:
Assuming you're talking about radio communications - There are three main methods of transmission...
Simplex - where a conversation takes place on a single frequency (eg. CB radio) each party has to end their conversation to hear the other person.
Duplex - where the base station transmits on one frequency, and the handset or mobile transmits on a different frequency (eg. taxis and emergency services) again - each party has to end their transmission in order to hear the other.
Full duplex - works like a telephone - base station & mobiles transmit on different frequencies - but both parties can talk at the same time