The radio is normally in receive mode so the user can hear all other transmissions on the channel. When the user wants to talk, they press a ?push-to-talk? button, which turns off the receiver and turns on the transmitter; when the button is released, the receiver is activated again. Multiple channels are provided so separate user groups can communicate in the same area without interfering with each other and some radios are designed to scan the channels in order to find a valid transmission. Other two-way radio systems operate in full-duplex mode, in which both parties can talk simultaneously.