|Trunking is an idea that has been around for a while. Phone companies have been using switching systems for years, which is similar to trunking. All stations and mobile units in a trunked radio system usually share a group of five or more channels. Some large agencies may have a trunking system all to itself. In some of the smaller to medium size communities, the system may be shared by police, fire and other public services. Non-emergency services may use most of the air time during the time that the police department isn't busy. One system may be shared with several communities. Trunking assures full and equal usage of each channel and prevents one department from "stepping on" another while it is transmitting.