P25 Trunking System FAQ
What is P25?
Project 25 (P25 or APCO-25) is a suite of standards for digital mobile radio communications designed for use by public safety organizations in North America. P25 radios are a direct replacement for analog (typically FM) radios, but add the ability to transfer data as well as voice, allowing for more natural implementations of encryption and text messaging.
What is Trunking?
"Trunked" radio systems differ from "conventional" radio systems in that a conventional radio system uses a dedicated channel (frequency) for each individual group of users, while "trunking" radio systems use a pool of channels which are available for a great many different groups of users.
For example, if public safety communications are configured in such a way that twelve conventional channels are required to permit countywide dispatch based upon geographical areas, during periods of slow dispatch activity, much of that channel capacity is idle. In a trunked system, the units in a given geographical area are not assigned a dedicated channel, but instead are members of a talk-group entitled to draw upon the common resources of a smaller pool of channels.
To the user, a trunking radio looks just like an "ordinary" radio: there is a "channel 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 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, EMS, Public Works, Animal Control, etc.) simply by changing the software programming in the radio itself.
What is a Talkgroup?
A talkgroup is an assigned group on a trunked radio system. Unlike a conventional radio which assigns users a certain frequency, a trunk system takes a number of frequencies allocated to the system. Then the control channel coordinates the system so talkgroups can share these frequencies seamlessly. The purpose is to dramatically increase bandwidth. Many radios today treat talkgroups as if they were frequencies, since they behave like such.
What is a Fleetmap?
Groups are assigned in a fleet map. This was implemented to make it easier to assign group ID numbers. For example, EMS would be on a separate 'fleet' than police. In those fleets are subfleets or the actual talkgroups. The Radio ID corresponds to a location or agency. For example, it is common to see a '1' or '2' in front of a police group and another number in front of a fire group.