That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once. A walkie-talkie typically has one channel, and a CB radio has 40 channels. A walkie-talkie can transmit about one mile using a 0.25 watt transmitter. A CB radio, because it has much higher power, can transmit about five miles using a 5 watt transmitter.
Cell phones operate within cells , and they can switch cells as they move around.
Cells give cell phones incredible range. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach. In simplex radio, both transmitters use the same frequency. Only one party can talk at a time. In duplex radio, the two transmitters use different frequencies, so both parties can talk at the same time.