Private Mobile Radio
Private Mobile Radio (PMR) - sometimes called Professional Mobile Radio - is a mobile system that serves a closed user group and that is normally owned and operated by the same organization as its users. PMR was developed for business users who need to keep in contact over relatively short distances with a central base station, also called a dispatcher. A typical example is a taxi company. PMR is also widely used by emergency services. PMR networks consist of one or more base stations and a number of mobile terminals.
From their early designs, PMR systems have developed into 'trunked' systems, the most notable of which is TETRA, Terrestrial Trunked Radio. Trunking is a technique where the resources of the communications network are shared. Typically, a communication channel is allocated for the duration of a call and then automatically released to allow it to be used by another user. The technique also enables multiple base stations to be connected and to provide coverage across a wider area than with a single base station.
PMR systems generally provide facilities for closed user groups, group call and push-to-talk, and have call set-up times which are generally short compared with cellular systems. Many PMR systems allow Direct Mode Operation (DMO) in which terminals can communicate with one another directly when they are out of the coverage area of a network.