Digital Mobile Radio (DMR) is an open digital radio standard for Professional Mobile Radio (PMR) users specified in the European Telecommunications Standards Institute (ETSI) Standards . Its primary goal is to specify a digital system with low complexity, low cost and interoperability across brands, acting as a direct replacement for analogue PMR. DMR provides voice, data and other supplementary services. DMR is a two-slot, Time Division Multiple Access (TDMA) system offering voice, data and a range of other features and applications.
Available for both licensed and unlicensed spectrum use, PMR applications extend from low-cost walkie-talkies aimed at the consumer market through to public safety and mission-critical systems. The PMR/DMR markets can be roughly divided into three broad categories. DMR has the capability to serve them all:
- Consumer and short-range industrial
- Professional / Business-Critical applications
- Public Safety / Mission-Critical applications.
DMR is a scaleable system that can be used in unlicensed mode (in a 446.1 to 446.2 MHz band), and in licensed mode, subject to national frequency planning. It is developed in three 'tiers':
- Tier 1 is the low-cost, licence-exempt 'digital PMR446'
- Tier 2 is for the professional market offering peer-to-peer mode and repeater mode (licensed)
- Tier 3 is for trunked operation (licensed).
Features supported include fast call set-up, calls to groups and individuals, short data and packet data calls. The communications modes include individual calls, group calls, broadcast calls and, of course, a direct communication mode among the mobiles. Other important DMR functions such as emergency calls, priority calls, full duplex communications, short data messages and IP-packet data transmissions are supported.
TETRA offers the automatic operation and frequency efficiency of trunking combined with the terminal autonomy of a conventional PMR. TETRA has a multi-mode capability by combining these two modes, trunking and direct (conventional), into a single terminal equipment, and it also provides the standardised way of inter-working between these two modes. TETRA is designed to offer bandwidth-on-demand, a facility to have a variable amount of bandwidth allocated for the call duration, depending on the application.
TETRA is also designed for emergency situations, when almost instantaneous communication is required, both between individuals and within a group of an unlimited size. Priority calls can be made, backed by call pre-emption if required, and on occasions an all-informed communication.BENEFITS OF TETRA
TETRA offers fast call set-up time, group communication support, direct mode operation between radios, packet data and circuit data transfer services, frequency economy and security features. TETRA uses time division multiple access (TDMA) technology with four user channels on one radio carrier and 25kHz spacing between carriers. This makes it inherently efficient in the way that it uses the frequency spectrum. For emergency systems in Europe the frequency bands 380-383MHz and 390-393MHz have been allocated by a single harmonized digital land mobile system. Additionally, whole or appropriate parts of the bands 383-395MHz and 393-395MHz can be utilized should the bandwidth be required.
For civil systems in Europe, the frequency bands 385-389.9MHz and 395-397.9MHz, 410-420MHz and 420-430MHz, 450-460MHz and 460-470MHz, 870-876MHz and 915-921MHz have been allocated for TETRA.
A TETRA trunking facility provides a pooling of all radio channels, which are then allocated on demand to individual users, in both voice and data modes. By the provision of national networks, countrywide roaming can be supported, the user being in constant seamless communications with his colleagues. TETRA supports point-to-point and point-to-multipoint communications both through the TETRA infrastructure and by use of direct mode without infrastructure.