THE AUSTRALIAN AMATEUR radio dmr repeater network
Digital Mobile Radio (DMR) is a land-mobile radio network standard developed by the European Telecommunications Standards Institute (ETSI). DMR is used by thousands of professional land-mobile radio networks worldwide.
A wide variety of DMR mobile, base and portable equipment is available from numerous manufacturers.
DMR uses Time Division Multiple Access (TDMA) to provide two 30 ms "slots" per 12.5 kHz radio channel.
Each slot can carry independent voice conversations - this means that each DMR repeater provides two simultaneous voice channels - illustrated in the diagrams below:
Analogue vs. TDMA (courtesy Vertex Standard)
Two channel analogue/FDMA vs. TDMA (courtesy dmruk.net)
Each slot can also have an almost unlimited number of "talk groups", which effectively function as discrete channels within that slot. Users on one talk group will not hear those on another.
DMR provides better noise rejection and weak signal performance than analogue systems by using advanced Forward Error Correction (FEC) and codecs to remove noise and reconstruct signals that would otherwise be lost - depicted in the following diagram:
DMR vs. analogue weak signal performance (courtesy Tait Communications)
One of the great attractions of DMR is that repeaters can be linked together directly via IP. This allows interconnection of repeaters in the next suburb, or on the other side of the world...
A number of DMR system designers who were also amateurs realised the potential offered by DMR for wide area connection of amateur repeaters. They formed the DMR-MARC network. This is a world-wide network of more than five hundred interconnected DMR repeaters, across 44 countries, including Australia and New Zealand.
Australian DMR amateur repeaters are located at Cairns, Brisbane, Ipswich, Toowoomba, Sydney (3), Canberra, Melbourne (3), Mt. Buller, Hobart, Launceston (soon), Adelaide and Perth. All operate in the 438/9 MHz (70cm) UHF band. More are planned.
A Google map showing Australian (and all worldwide) DMR repeaters may be found here.