radio channels are required. From another perspective, with a given number of radio channels, a much greater number of user groups can be accommodated. In the example of the police department, this additional capacity could then be used to assign individual talkgroups to specialized investigative, traffic control, or special-events groups which might otherwise not have the benefit of individual private communications. To the user, a trunking radio looks just like an 'ordinary' radio: there is a 'channel select switch' for the user to select the 'channel' that they want to use. In reality though, the 'channel switch' is not switching frequencies as in a conventional radio but when changed, it refers to an internal software program which causes a new talkgroup affiliation to be transmitted on the control channel. This identifies the specific radio to the system controller as a member of a specific talkgroup, and that radio will then be included in any conversations involving that talkgroup. This also allows great flexibility in radio usage – the same radio model can be used for many different types of system users (i.e. Police, Fire, Public Works, Animal Control, etc.) simply by changing the software programming in the radio itself. Since the talkgroups are constantly transmitting on different frequencies, trunked radio systems makes it more difficult for a scanner listener without a programmed trunk tracking scanner to keep up with the conversation. In 1997, radio scanners compatible with trunked systems appeared on the market. One of the first companies to bring these devices to market, Uniden, trademarked the term 'trunk tracking' on December 5, 1997. Types of trunked radio systems This is not designed to be a list of manufacturers' equipment types, it is intended as a list of air protocol types unless significant vendor specific modifications have been made which violate the published standard. Trunked radio technologies today have generally diverged into three distinct types or 'tiers'. These are not 'official', but are clearly defined within protocol types:- Entry-level These systems are relatively simple in their operation and only meet the minimum requirements to be defined as a 'trunked' radio system. They generally do not have enhanced features such as data communications or registration awareness. They will provide simple trunking facilities for voice calls only. SmarTrunk Ericsson GE EDACS Provoice EDACS GE Mark V Logic Trunked Radio LTR Standard LTR Passport LTR Standard and Passport LTR MultiNet LTR-Net Motorola Type