An AM or FM two-way radio system uses one frequency to communicate. While in the beginning of radio communication one radio antenna was enough to cover a large part of Los Angeles, soon there was no space and time enough to transmit all messages for all districts and all emergency services. A growing number of frequencies were needed to service all districts and all emergency services. However, during quiet times all those frequencies were occupied without any use.
Early two-way radio systems communicated only on one channel. It required for every unit to listen in on one channel to hear reports. A unit or a group of units switched to another channel if a talkgroup (a group of users that belong for example to the same district and the same emergency service, other kinds of talkgroups are possible) conversation was needed. The switching was manual and the radio signal was analogue.
Trunked Radio System is a complex computer controlled radio system. The computer does the switches the communication for the various talkgroups. The system allows to use a limited number of frequencies for a maximum number of talkgroups. To further reduce the number of used radio frequencies, a digital signal is used now.