The Basics
DMR 101
Overview
On the surface, Digital Mobile Radio (DMR) appears complicated, and of course I would say that... I use it everyday. Yes, learning a new technology can be rough, especially if you don't use it everyday. So, I'll start with the basics and build on them. During this technical presentation, I will do my best to stay away from manufacturer-proprietary features. We will need to build some terminology, so be ready for a ton of new terms.
Ready?
Preface
In an analog world, channels are allocated in such a way that a transmitted signal uses the entire spectrum available for each channel, currently 25 kHz for the amateur band 12.5 kHz in the commercial band. To convert analog audio (your voice) into either an AM or FM signal, a radio transmitter modulates a carrier (a radio signal) much the same way that grooves are cut into wax records when recording live audio, where subtle changes in the noise captured by a microphone are translated into groves on the wax disc. A radio receiver demodulates the audio signal from the radio channel much like a record player, where the subtle changes of the record’s groove while it spins underneath the needle are translated back to audio. In both cases, the goal is to convey information in the form of audible tones, usually voice. Practically speaking, with today's conventional analog radio transceivers, an audio transmission occupies the entire radio channel bandwidth. Ideally, only one transmitter is ever active on the channel at any one time, and any number of receivers may be able to detect and demodulate that one transmission (as long as they are close enough for the signal to be heard properly).
DMR offers the ability to add a second, concurrent talk path to that same channel spectrum normally used by conventional radios, along with throwing in a bunch of cool features to boot.
What is DMR?
Digital Mobile Radio (DMR) is a commercial grade, globally accepted, non-proprietary, digitally modulated RF mode, that implements manufacturer agnostic standardized features, using Time Division Multiple Access (TDMA) technology, to make efficient use of the RF spectrum, developed by the DMR Association and ratified by the European Telecommunications Standards Institute (ETSI). It converts your analog voice into data and then sends it as a stream of ‘ones-and-zeroes’ via an AMBE II++ vocoder that is sent in the form of packets using TDMA technology (don’t worry… we’ll talk more about TDMA and packets a little bit later).
Those published DMR standards are followed by various manufacturers who build their own brands of digital communications equipment that are able to interoperate with any other standards-compliant equipment. Interestingly, not all of the features are included in the DMR standard. This means that some features available from certain manufacturers are NOT actually ‘...standardized.’ We will talk about some non-standard features in future chapters.
Since DMR is the transmission of digital information over radio, it is able to send any kind of data payload that the ETSI standards will allow. The data rate is fast enough that it supports streaming voice information, along with data based services like caller ID, GPS, text messaging and others.
Timeslot Illustration
DMR relies on a technology called ‘Time Division Multiple Access’ (TDMA). TDMA allows for more than one user to access a given resource based on a division of time of that resource. The DMR TDMA standard was built around ‘narrow’ bandwidths, with RF channels 12.5 kHz wide, with the channel divided into two 30ms timeslots along with the implementation of Forward Error Correction to ensure the data stream is passed correctly.
DMR timeslots are divided into 30ms segments. When DMR is used, the data stream bits are packaged into each 30ms segment. Each segment or packet of digital information contains a header and data payload... and sometimes more but we are going to leave it simple for now. All radios that comply with the DMR standard use their radio transmitter for exactly 30ms at a time to send each packet, and then turn off for another 30ms. A series of packets is sent by continually turning on the transmitter, modulating the data for the segment, and then turning off the transmitter. And yes, they do that very fast (about 33 times a second). What this means to you is an extended battery life since the transmitter of the radio is only on for about 50% of the time. To add to the quick deep dive, most DMR radios have a frequency tolerance that is down to 5 PPM. In other words, they are not allowed to drift at all.
As mentioned above, the DMR standard employs the concept of a timeslot, 2 of them to be specific. Each timeslot allows for an independent conversation, meaning that 2 concurrent conversations can occur on the same frequency without contention or interference. Each timeslot uses the entire 12.5 kHz channel bandwidth. Most radios require that a channel assignment include the timeslot assignment. In other words, and we will go into this deeper in section “103 Programming” later, a channel usually has a talkgroup assigned as a TX Contact, and that is then assigned a timeslot 1 or 2. If you do not set the timeslot correctly, things will not work as desired. Subscriber radios (Mobiles & Portables) typically only transmit in a single timeslot at a time. That means that the radios transmitter is physically on for only the timeslot it is assigned. Subscriber radios that transmit in both timeslots should be avoided at all costs. There were reports that there was a radio that took up both timeslots. Since then, a firmware update has resolved the issue. All repeaters transmit on both timeslots, however, any unused timeslots are left blank and only timing information is sent. It is possible for some DMR subscriber devices to transmit on both timeslots at the same time. Currently, this feature is proprietary and limited to a single radio manufacturer. This feature allows a subscriber to become a real time single frequency or simplex repeater.
The DMR Standard has 3 Tiers, or levels. Tier I is very similar to the Family Radio Service (FRS). Products are built very similarly to FRS with fixed output power and fixed antennas, and a like. They are in what are considered unlicensed bands, usually in the 460 MHz band. We will not be talking about them here. I am going to jump to Tier III, which is the trunking implementation of DMR. T3 systems are typically owned and operated by commercial and government organizations. Since amateurs aren’t deploying trunking systems, again, we aren’t going to talk about them here. That leaves us with Tier II. Tier II is the conventional, if you want to call it that, implementation of DMR. It closely matches analog implementations and is usually the easiest to convert from analog to DMR.
Summary
I know that was a ton of information along with a bunch of links. That’s my version of the quick and dirty. If you want to know more, there are several resources to aid in your research. Let’s move on and get to explaining key features.