Digital Mobile Radio (DMR) is a digital radio standard for voice and data transmission over non-public radio networks. It was created by the European Telecommunications Standards Institute (ETSI) and is designed to be low-cost and easy to use. DMR, along with P25 Phase II and NXDN are the main competing technologies to achieve 6.25 kHz equivalent bandwidth using the proprietary AMBE+2 voice codec. DMR and P25 II both use two-slot TDMA on a 12.5 kHz channel , while NXDN uses discrete 6.25 kHz channels using frequency division and TETRA uses four-slot TDMA on a 25 kHz channel. DMR was designed with three levels. DMR levels I and II (conventional) were first published in 2005 and DMR III (Trunked version) was published in 2012, with manufacturers producing products within a few years of each publication. The primary objective of the standard is to define a digital system with low complexity, low cost and inter-brand interoperability, so that radio buyers are not locked into a proprietary solution. In practice, given the current limited scope of the DMR standard, many vendors have introduced proprietary features that make their product offerings non-interoperable with other brands. SPECIFICATIONS The DMR interface is defined by following ETSI standards: TS 102 361-1: Air interface protocol TS 102 361-2: Voice and General services and facilities TS 102 361-3: Data protocol TS 102 361-4: Trunk protocol The DMR standard works within the existing channel spacing 12.5 kHz used in terrestrial mobile frequency bands worldwide, but achieves two channels of voice through two-slot TDMA technology built around a 30 ms structure. The modulation is 4-state FSK, which creates four possible symbols on the air at a rate of 4,800 symbols/s, which corresponds to 9,600 bits/s. After overhead, forward error correction, and splitting into two channels, 2,450 bits/s remain for a single voice channel using DMR, compared to 4,400 bits/ s using P25 and 64,000 bit/s with traditional telephone circuits. The standards are still (as of late 2015) under development with revisions made regularly as more systems are developed and improvements are discovered. It is very likely that further improvements will be made to the standard, something which will require future firmware upgrades in terminals and infrastructure to take advantage of these new improvements, with potential incompatibility issues if not done. DMR covers the RF range from 30 MHz to 1 GHz. There are DMR implementations, (from early of 2016), operating up to 66 MHz (within the European Union, in "Lo-Band VHF" 66–88 MHz.) The DMR Association and manufacturers often claim that DMR has superior coverage performance to analog FM. Forward error correction can achieve higher voice quality when the receive signal is still relatively high. In practice, however, digital modulation protocols are much more prone to multipath interference and fail to provide service in areas where analog FM would otherwise provide degraded but audible voice service. At higher voice quality, DMR outperforms analog FM by about 11 dB. But at lower voice quality, analog FM outperforms DMR by about 5 dB. Where digital signal processing has been used to improve quality of analog FM audio, then analog FM generally outperforms DMR in all cases, with a typical improvement of 2–3 dB for "high quality" voice and about 5 dB improvement for "lower quality" voice. Where digital signal processing is used to improving analog FM audio, the overall "delivered sound quality" is also significantly better than DMR. However, the DSP processing of analog FM audio does not remove the 12.5 kHz requirement, so DMR is still more efficient spectral. DMRDMR Tier I SERIES DMR Tier I products are intended for unlicensed use in the European PMR446 band. Tier I products are specified for non-infrastructure use only (ie without the use of repeaters). This part of the standard provides for consumer applications and low power commercial applications, using a maximum RF power of 0.5 watts. Note that there is no license free assignment on this frequency outside of Europe, which means that PMR446 radios, including DMR Tier I wireless radios, can be legally used in other countries only if the appropriate radio license is obtained by the operator. Some DMR radios sold by Chinese manufacturers (mainly Baofeng) have been mislabeled as DMR Tier I. A DMR Tier I radio will only use the unlicensed PMR446 frequencies and have maximum transmit power of 0.5 W as required by law for all PMR446 radios. Although the DMR standard allows DMR Tier I radios to use continuous transmission mode, all known Tier I radios currently use TDMA, the same with Tier II. This is likely due to the battery savings that come with transmitting only half the time instead of continuously. DMR Tier II DMR Tier II covers licensed conventional radio systems, mobile and handheld devices operating in PMR frequency bands from 66 –960 MHz. The ETSI DMR Tier II standard is aimed at those users who need spectral efficiency, advanced voice functions and integrated IP data services in licensed bands for high power communications. Some manufacturers market DMR Tier II compliant products. ETSI DMR specifies two TDMA slots on 12.5 kHz channels for Tier II and III. DMR Tier III A portable radio compliant with the DMR Tier III digital radio standard. DMR Tier III covers trunking in frequency bands 66–960 MHz. Tier III supports TETRA-like voice and short message handling with embedded 128-character status messages and short messages with up to 288 bits of data in various formats.It also supports packet data service in various formats, including support for IPv4 and IPv6. Tier III compliant products were released in 2012. In April 2013, Hytera participated in the completion of the DMR Tier III Interoperability Test (IOP). DMR ASSOCIATION In 2005, a memorandum of understanding (MOU) was established with potential DMR suppliers, including Tait Communications, Fylde Micro, Selex, Motorola, Hytera, Sanchar Communication, Vertex Standard, Kenwood and Icom to establish common standards and interoperability. While the DMR standard does not specify the vocoder, members of the MoU agreed to use the vocoder half-rate DVSI Advanced Multi-Band Excitation (AMBE) to ensure interoperability. In 2009, MS members established the DMR Association to work on interoperability between vendors' equipment and provide information on the DMR standard. Official testing interoperability tests have been conducted since 2010. The results are published on the DMR Association website. There are approximately 40 members of the DMR Association. The standard allows DMR manufacturers to implement additional features beyond the standard, which has led to practical issues of non-interoperability between commercial signals, in violation of the DMR MS. HAMMER USE DMR is used in the VHF and UHF amateur radio bands, which was initiated by DMR-MARC around 2010. The FCC officially approved the use of DMR by amateurs in 2014. At amateur sites, coordinated DMR identification numbers are assigned and managed by RadioID Inc. The coordinated database can be uploaded to DMR radios to display the name, call sign and location of other operators. Internet-connected systems such as DMR-MARC, Free-Star ( an experimental approach to implementing a vendor neutral and open source, digital communication network for amateur radio), BrandMeister network, TGIF, FreeDMR and many others (including many previously closed clusters now connected to larger networks in wide area accessibility), allow users to communicate with other users around the world through connected DMR repeaters or 'hotspots' which are often based on a Raspberry Pi board computer. There are currently more than 5,500 repeaters and 16,000 'hotspots' connected to the BrandMeister system worldwide. The low cost and the increasing availability of Internet-connected systems have led to an increase in the use of DMR in the amateur radio bands. The development of Raspberry Pi-based hotspots, often those using Pi-Star software, has allowed users to connect their radios to one or more Internet-connected systems simultaneously. DMR hotspots are often based on the open source Multimode Digital Voice Modem or MMDVM, hardware with firmware developed by Jonathan Naylor.