DMR radio is a fairly new addition to the Radio Amateur stable and is a much modified version of the commercial systems currently in use, taking the concept and use far beyond its early beginnings.
So, what is DMR? It stands for Digital Mobile Radio, an open standard for digital radios. It operates using the principles of TDMA (Time Division Multiple Access) as its mode of transmission and is fully digital throughout the transmission path, including network switches, repeaters and radio handsets.
From an amateur point of view, using digital technology means that a single analogue repeater channel of 12.5Khz bandwidth can now effectively be doubled in capacity by carrying a digital signal comprising two 6.25Khz channels. This is achieved by very fast switching (typically 30msec) between the 2 digital timeslots that the mode provides (TS-1 & TS-2). Each repeater is also allocated a digital signature known as a Colour Code (CC) which is similar in function to CTCSS on analogue repeaters. The Colour Code is in the form of a number (0-15) and only radios programmed with the correct colour code for a given repeater will be able to gain access.
Another advantage of DMR, is that the two timeslots available are totally independent of each other, thus giving the impression that you get 2 repeaters for the price of one! Whilst not strictly true, having 2 independent channels means that a repeater keeper or network administrator can allocate each timeslot for a different purpose. Here in the UK on the Phoenix network for example, TS-1 is allocated for international and national use, whilst TS-2 is reserved for regional/inter-regional contacts.
DMR also has the ability of being able to further sub-divide each timeslot into various groups to aid the routing of audio data for specific purposes, so that when programmed appropriately, a radio will only receive traffic directed to that group. It is equally possible for the radio to be programmed to receive more than one group on a given timeslot.
When using the Amateur repeater network, this sub-division into various Talkgroups (TG’s) is handled by a programmable network switch. This device is also responsible for distributing the relevant data to all the connected repeaters that make up the network. In the early days, this was referred to as the ‘C-Bridge’, which was a propriety device and would only work with Motorola repeaters. It also was limited in its capacity as to how many repeaters could be linked to it.
With the advent of repeaters from other manufactures as well as ‘home-brew’ devices, several ways were developed to handle these devices, including the introduction of the BrandMeister DMR network. Here in the UK, the DMR-Marc network became OpenDMR – better known as Phoenix which still links with the Marc network to provide world-wide connectivity. It’s worth noting however, that OpenDMR (Phoenix) and BrandMeister are two separate and independent networks.
For the OpenDMR (Phoenix) network, the back-room boffins came up with IPSC-2. a network control system that would interface with most repeaters and was scalable enough to cope with the growing demand of repeater applications. It handles all the TG routing information as well as providing administrative functions and useful user data – such as Live Monitors and network status information etc.
All network connected repeaters are linked to IPSC-2 via internet connections which are either hard wired LAN, local Wi-Fi links or via 3G/4G mobile network routers, depending on the repeaters physical location.
The connectivity and services provided by IPSC-2 along with the considered use of Talkgroups, ensures that as far as possible network resources are optimised so that only the repeaters that need to be active to form part of the communication chain are used. This approach permits users on non-active repeaters to establish their own conversations with others should they wish to do so.
Using the Phoenix network as an example again, Talkgroup 235 will access ALL the connected repeaters in the UK. Also, because it’s allocated to TS-1, it will effectively make all the other TG’s on TS-1 show as ‘busy’. This is why it is requested that conversations on TG235 are kept fairly brief.
The ideal situation therefore, if you want to have a longer chat, is to make use of another facility provided by IPSC-2 – The User Activated TG. As the name suggests, it is user activated and will only open the repeaters that you and who ever you are in contact with are using. So rather than occupying some 60+ repeaters, your conversation only occupies the least number needed to talk to each other, thus freeing up resources for others to use.
DMR is a very technical mode, not only in terms of transmission, but also extends to the programming and use of radios etc. Out of necessity, this has only been a very brief outline, but hopefully reading through the rest of this document will offer you plenty of information and guidance as to how to overcome some of the issues you may encounter on your journey into the world of Digital Mobile Radio.