It's a digital audio mode used by generally smaller agencies and businesses. Also it's getting popular in the ham world. Perhaps the best known brand name for it is MotoTRBO by Motorola. It is a published standard so compatibility between different companies products is generally easy to accomplish (so long as you don't get sucked into the proprietary extensions that the salesmen always push to force you to be stuck with only their products). As far as scanners go, the upgrades should handle most any DMR system in the database. As usual, any encrypted signals will still not be monitorable.
There are two standard tiers:
* Tier 1 is the lowest end and is basically just an upgrade from radio to radio analog to make it digital, although some special features such as GPS positioning can be active. This is used only in the lowest end situations, such as in a school where a teacher in the pick-up line can call for a student that's still in the building so the walkie-talkie equipped teachers can locate the child and get them out to their ride.
* Tier 2 is the more popular and more useful mode. It typically uses a repeater and can provide two separate and independent talk paths on a single repeater pair. The radios send quick pulses of data on their assigned time slot and all radios will only listen to their assigned time slot so there is no interference between the two independent conversations.
* Tier 3 is basically just an extension to tier 2 that adds trunking into the mix. Basically that extends the two "channels" that are fixed use in standard tier 2 to allow different uses for an available "channel". You're still limited to two active "channels" per repeater, but using the trunking feature, a channel can be for the maintenance crew one second and for the security team the next. A system can be further extended by adding additional repeaters to increase this 2 "channel" maximum to be 2 "channels" per repeater in the system. Often the additional repeater functionality is one of those proprietary extensions.
