Talk digital - an intro to DMR

Back in 1999 when I was first active as OE3DZW voice communication was analogue only. Some repeaters used DTMF to control remote functions. Frauenstaffel, OE3XER was famous for its hidden but extensive functionality controlled by a Commodore C-64 computer.

Intro

Today the majority of repeaters is digital. They are using a protocol, which was originally developed for professional radio by Motorola. ETSI has standardized the radio interface as TS 102 361(PDF)

DMR

In an amateur context, that radio interface is called “DMR” which stands for “digital mobile radio”. But, DMR is just one standard for digital mobile radio, there are others. There are several flavours of TS 102 361, so called “tiers”. Low-end license free “DMR446” devices use “Tier 1”, a low-end version of DMR. Amateurs, especially amateur repeaters (mainly) use “Tier 2” devices. The main difference is the use of TMDA and the use of a different voice codec. TMDA in DMR is very simple. The system only uses two time slots, which would allow for full duplex communication without duplex filters. But, the amateur flavour uses the two different time slot for a different purpose, additional capacity. Thus, an amateur DMR repeater can allow two different QSOs on the same frequency. One is on time slot 1 (TS1), the other on time slot 2 (TS2).

Identities

The endpoints of DMR communication use an ID, the so called TSI (abbreviation for TETRA subscriber identity).Amateurs use a TSI which starts with the mobile country code (MCC) of the callsign, eg.”232” for Austria. IDs can be registered at https: //register.ham-digital.org/, a web page operated by DL5DI. My IDs are 2323190 for OE3DZW and 2321216 for OE1DZW. The page also allows to download a list of all users (callsign, ID, first name, country), a feature which is useful for the configuration of radios (see below).

Repeaters

In good old analogue times, the only information you needed was the transmit frequency of the repeater. The received frequency had a defined offset (600kHz lower in the 2m band, 7.6MHz lower in the 70cm band). The only question was - is there a 1750MHz call tone or not, thus configuration was very simple. This is totally different with DMR. DMR need a set of parameters for each repeater but the user interface of most radios does not allow to entering or modifying most of the parameters (if any). This means that the radio needs to be configured with a computer before the first QSO can start.

Codeplug

This all means that it is rather difficult to configure a radio for DMR. One would expect that vendors of amateur radios would supply relevant information on their products and that amateur radio clubs and operators of repeaters would make such information available. But the situation is different. There are a few enthusiasts who provide so called “codeplugs”. These codeplugs are configuration files which can be uploaded to the radio using a device specific software. When I ordered a Retevis RT3s I thought I could use such a code plug. But I was wrong. The codeplugs I found were for different firmware versions of the same device, I failed to upload these pre-configured configuration files. Thus, I had to learn how DMR radios need to be configured.

Configuration parameters

Here is a list of key configuration parameters

Frequency (transmit and receive frequency)
Own TSI (see above)
Timeslot (TS1 or TS2)
Colour code (0-15, but usually 1 used)
Encryption (not used in Amateur Radio)
Called group (the destination of the transmission)
Received groups (a difficult topic, see below)

Frequency

This is the only easy topic. Frequencies of repeaters can be found at various sources, an up to date source is aprs.fi, another comprehensive, but inaccurate source is RadioID.

Timeslot

Repeaters allow for communication on two different time slots, TS1 and TS2. As communication is possible on both time slots, both can be used and configured.

Colour code

The idea behind colour codes is to differentiate between neighbouring, possibly interfering repeaters on the same frequency. Communication is only possible if sender and receiver use the same colour code. This concept originates from cellular networks where base stations have a rather limited reach and interference is tolerated in order to foster the reuse of the same frequency. Although the situation in an amateur environment is different (interference between repeaters is avoided), the use of different colour codes would reasonable. Anyway, nearly all amateur repeaters use the colour code “1”.

If you get this step wrong, you will not receive anything. I wonder why only a few sites mentions this important setting.

Encryption

DMR originates from professional radio and such radios depend on secure and private communication. But, amateurs have different concept, thus encryption remains deactivated.

Talk group

On an analogue repeater all amateurs within its range can listen and talk to each other. In terms of DMR these amateurs form a common talk group (TG). DMR allows to use different talk groups identified by numbers (E.g. 232). Talk groups do not add capacity, with other words when the repeater is used by a specific talk group it is occupied and can’t be used for a different group at the same time (on the same timeslot). The different group have are used for a number of purposes, but for now let us assume that we know that we want to talk on group 232. Mobiles typically allow to configure a default talk group or to use a group out of a contact list (which is a preconfigured list of groups stored in the radio).

Called group

Communication in DMR is always addressed to a specific group (or private ID, see below). Thus, amateurs who use DMR and want to talk to each other need t agree on a group number. So for example, let us assume amateurs agree to transmit on group ID “232”. Such a transmission would in the original design of DMR only be audible for amateurs, which have included “232” in the list of receive groups. All other amateurs would see that the repeater (on that timeslot) is busy, but could not hear anything as the radio would mute such traffic.

Receive groups

In an amateur environment, one would expect that all QSOs on a repeater can be followed, but DMR is different and the expected configuration is actually an exception, which is only supported by some radios and the support is partly limited. The normal configuration is that the radio is configured to listen only to a predefined list of talk groups. E.g., the radio might be configured to listen to group 9 and 232. All other QSOs, e.g. on group 110 would be ignored (muted). The radio might indicate that the channel is busy (e.g. green light). Thus, if the radio is not smart enough, the only option is to include all “well known” groups in the receive groups list.

Private calls

A private or direct call is a communication between two radios. The called party is identified by its private ID (TSI). The design is optimized for professional use and does not really fit into ham radio although also amateurs might be interested in a quiet radio, which only responds to calls addressed to themselves. In the original design, only the participating parties can listen to a private call. Some - but not all - amateur radios allow listening to private calls. Thus the term “private” is a bit misleading, it only refers to the way the calling party is addressed, but does not imply privacy.

How to configure

All these parameters lead to the question: Where to get all that information? The easiest way is probably to find another amateur who already configured their device and is happy to share the configuration. In Austria, http://ham-dmr.at/ is a good place start the search. Even if the codeplug there are not compatible with the own device, they can be reverse-engineered to find out what frequencies and groups are used for what purpose. Please note: group IDs and private IDs are different, it is important to store and use the right type of IDs when configuring the radio.

DMR networks

The above description only explains how DMR works in principle. However, one interesting feature of DMR is that repeaters are typically not operated in standalone mode, but are connected to each other. This allows for communication between repeaters. The group “232” might for example might be available on all repeaters of some network within a specific area (e.g. IPSC2 in Austria). Some groups might always be active on a repeater (as statically configured by the sysop), some other groups might only be active a repeater if activated. The activation procedure in IPSC2 is very simple: Just start a transmission on a group and the repeater will join that group until a timeout of typically 15min (as defined by the sysop). The current configuration of groups can be found at http://ham-dmr.at/index.php/auflistung-der-weltweiten-dmrplus-reflektoren/ .

Besides these static and dynamic groups, additional “groups” which are called reflectors can be activated and deactivated by a private (!) call to ID of the reflector (e.g. 4191). After such a call the repeaters transmits the communication on that reflector on a predefined group ID. There is a special private ID to query the active reflector (private ID 5000) and to disconnect the active reflector (private ID 4000). Besides IPSC2, the Brandmeister network is popular in a number of countries. Groups might only be active if it is activated. A list of reflectors can be found at http://ham-dmr.at/index.php/auflistung-der-weltweiten-dmrplus-reflektoren/ .

How to check configuration

In the analogue world, one would set the radio to local repeater frequency and check if there is any activity (or scan for activity on repeater channels). Then one would transmit and check for a response to a test transmission on. (e.g. roger-beep). But in DMR there is not such response, so how to know if communication is possible? On way of doing it is to monitor DMR dashboards. E.g., all activity on ISPC2 in Austria can be followed at http://ham-dmr.at/index.php/dashboard/ If you find your ID (and callsign) on this board, you know that your transmission was received at the repeater you selected. Such dashboards also help you to understand which groups are available and which groups are really used. By the way, “transmission” is not accurate. A DMR device in Tier-2 mode is requesting access at the repeater, and if the repeater does not grant that access (because it is busy, the signal is too weak or the frequency is wrong), then the radio does not transmit your audio at all. Radios have an indication on this, e.g. when a LED goes red the radio might indicated transmission while flashing red might indicate a failure.

Why it might not work

There is a number of parameters, and for a beginner it might be hard to find out what source of information is accurate and up to date. Another reason might be that some low-end devices claim to be DMR compatible while they are not (e.g. some Baofeng devices). DMR as used in ham radio uses two different time slots, TS1 and TS2. Low-end devices do not support TMDA - turn on the transmitter on the active time slot and turn it off on the other timeslot. They use simple continuous transmission, like in analogue FM.