Understanding DMR – 1

By Toshen, KEØFHS – last updated Dec 2017
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Digital Mobile Radio Association logo

Why?

After weeks of intense learning and effort, I feel like I'm just beginning to get a handle on D-STAR, having put together a nicely working solution for my shack as well as for mobile. So why am I adventuring off in a new direction already?

If you read my D-STAR story, you know that I learned a lot from a group of hams in Colorado, the Parker Radio Association (PRA). One of the things I've been enjoying doing occasionally with D-STAR is joining their digital radio net on D-STAR reflector XRF223.

During one of their nets, some of them mentioned the new SharkRF openSPOT personal access point device that they recently received, and they were really enthusiastic about it. So I ordered one to try out for myself. It should arrive any time now.

Then, during another net, they announced they were going to put up a DMR repeater on the BrandMeister Network. At first I thought, well, that's interesting, but I'm going to stay focused on D-STAR for now. However, their enthusiasm is pretty infectious, so after the net was finished, I got thinking about it and realized that this was a golden opportunity to learn something new, and also would give me a great way to try out the openSPOT device.

So I decided to dive right back into another bowl of baffling info-soup and learn how to swim all over again. I ordered a DMR HT, and as soon as it and the openSPOT arrive, I'll share my new adventure here.

1) What is DMR?

Short answer

Another digital voice system that amateur radio enthusiasts can play around with.

The logos of D-STAR, DMR, System Fusion, and FreeDV

Among the other digital voice systems being developed are D-STAR, the original (Icom and Kenwoood), System Fusion (Yaesu), and an open system that's in development, a combination of FreeDV software and an open source speech codec/vocoder, Codec 2.

Longer answer

DMR is a digital radio standard originally designed for commercial use and developed by the European Telecommunications Standards Institute (ETSI) beginning in 2005, which hams now are adapting for amateur radio use.

If something about using DMR for Amateur Radio doesn’t make sense, remember that DMR was created for commercial use, and was never designed nor intended for Amateur Radio use.
Mike's DMR Doctrine, by Mike, K0NGA, of Rocky Mountain Ham Radio

And it's true, initially I did find it quite challenging to get up and running with DMR. It's different enough that I couldn't apply a lot of what I had just learned getting going with D-STAR. I also had the additional challenge of needing to figure out how to get started via a hotspot, while most info available online is from the point of view of having access to a repeater.

Finally and thankfully, some Elmers with hotspot experience helped me put it all together and get on the air.

I also met Mike, K0NGA, in person at the recent 2017 Longmont Amateur Radio Club's LARCFest, and he graciously spent some time answering some questions I had and helping clarify a few things about DMR. He's a great guy. Thanks for your help, Mike!

There's humor in them thar signals!

Laurel and Hardy lookalikes at Shoreham Airshow in West Sussex on August 30, 2014
Laurel & Hardy lookalikes, Shoreham Airshow, 2014
Photo by Philip Bird LRPS CPAGB, Shutterstock

I found it amusing that once I finally did get things working and then spent an evening listening to hams ragchewing on various talkgroups (I'll discuss those more later, but they're sort of like conference calls or chat rooms), a lot of the chatter was hams discussing questions about DMR like, "Why does it work that way?" and "How do I make it do this thing?" So I guess I'm not alone in going through learning challenges with DMR.

In order to use DMR radios, in addition to using frequencies, channels, and repeaters/reflectors like you do with D-STAR, you also need to understand a bunch more puzzle pieces, and fit them together properly, including:

And thar's a bit of frustration, too!

Whenever I found myself growling "Why isn't this working??? ! ! !," it was helpful to keep Mike's DMR Doctrine in mind and take a step back to think about DMR in a commercial setting, like a big hospital, corporate campus, or casino. In an organization like that where there are many different departments using the radios (security, housekeeping, maintenance, catering, etc.), there typically will be a centralized IT team doing all the programming of all the locked-down radios they assign to members of the various departments. DMR makes it relatively easy for the IT team to organize communications so that the members of each team can talk amongst themselves, as well as to control whether or not they have permission to talk with or monitor other teams, all within the closed ecosystem of their organization.

Of course, that's a very different scenario than a lone ham who wants to set up a radio for his or her own use in order to connect to open repeaters put up by amateur radio clubs that allow them to be used to connect to other hams or groups of hams locally, regionally, statewide, nationwide, or worldwide. Adapting a complex system designed for the needs of a big organization within a closed ecosystem so that it can be used by hams in a more open worldwide ecosystem is a bit like trying to fit a simple square peg into a more complex hole in the shape of a 10-pointed star polygon.

And yet hams are figuring out how to make it work!

1a) Time slots

When there are so many puzzle pieces that need to be put together to set up a DMR radio successfully, it's challenging to figure out where to begin. It's a bit of a "Which comes first, the chicken or the egg?" problem, but I think a good place to start is with time slots, since they're fundamental.

If you read my Alphabetsoupese – example 2 note in the D-STAR article, you might recall that one way DMR differs from D-STAR is that it uses Time Division Multiple Access (TDMA) to generate its signal instead of the Frequency Division Multiple Access (FDMA) that D-STAR uses. Specifically, DMR uses 2-slot TDMA (the slots are numbered 1 or TS1 and 2 or TS2).

If, like me, you're more fluent in English than Alphabetsoupese, what this means is that calls on two different channels can share the same frequency simultaneoulsy. Each call is sliced up into chunks of a few milliseconds, and the slices from the two calls are interleaved on the signal. This happens so fast that we hear what we perceive of as a continuous transmission even though it's coming in chunks, looking like this:

Time slots interleaved on the signal

This also means that when you program a DMR channel, you must specify both the frequency and the time slot, so that your radio and the repeater can encode and decode which chunks on the signal belong to the channel you're using.

The convention is that TS2 is used for local talkgroups and TS1 is used for non-local or wide-area talkgroups (for example, statewide, regions, nationwide, and worldwide). I know, I know, here I am talking about talkgroups again before I've explained what they are, but I had to start somewhere!

For more details about time slots, see Amateur Radio Guide to DMR (PDF), 2015, by John Burningham, W2XAB.

1b) Color codes

Another fundamental piece of the puzzle is color codes (CC). I'm not sure why they're called color codes, but they're like the Continuous Tone-Coded Squelch System (CTCSS) tones used for analog radio. When you want to use a DMR repeater, you need to program in the appropriate color code to open it up. There are 16 color codes, 0 - 15. (Strange, right? ! The color codes don't even have color names!)

Just as with analog radio CTCSS tones, you need to get the appropriate color codes from the organization operating the repeater in order to be able to use the repeater. When you program a DMR channel, in addition to the frequency and the time slot, you must specify the color code; otherwise, you won't be able to access the repeater. So the magic formula to get onto a DMR repeater successfully = frequency + color code + time slot.

For example:
Frequency: 446.9875−
Color code: 7
Time slot: 1
will get you onto the wide-area time slot of the Lee Hill repeater located in Boulder, Colorado that is linked into the RMHAM (Rocky Mountain Ham) network.

Frequency: 445.075−
Color code: 1
Time slot: 2
will get you into onto the local time slot of the Pinery repeater located in Parker, Colorado that is linked into to BrandMeister network.

1c) Talkgroups

Finally … let's talk about talkgroups!

Of course, the whole point of getting onto a DMR repeater is to talk to other hams, and you do that by visiting a talkgroup, which enables one-to-many communication, sort of like a conference call or a chat room. The effect of using a talkgroup is similar to linking to a D-STAR reflector; anything transmitted to a talkgroup is transmitted to everyone listening to that talkgroup.

There are worldwide, nationwide, regional, statewide, area, and local talkgroups, as well as language-based talkgroups. For example, on the BrandMeister network, talkgroup 91 is Worldwide, 93 is North America, 3100 is Nationwide, 3108 is Colorado, 31090 is the Midwest Region (the same as callsign assignment region 0), 3171 is the Northern Colorado area, and 2 or 9 is the Local talkgroup.

In addition, there are three TAC channels, 310, 311, 312, that hams can move to for their longer chats in order not to tie up the channels that are more widely shared. For example, some hams may begin a chat on 3100, and then move to 310 to continue it.

Some talkgroups are linked across networks, for example, 3100 also is Nationwide on the DMR-MARC and DMRPlus networks, and TAC channels 310, 311, and 312 are linked across BrandMeister and DMR-MARC.

Static and dynamic talkgroups

Talkgroups are either static (always on) or dynamic (user-activated). When you activate a dynamic talkgroup on a repeater's time slot by keying up, it typically remains activated while there are transmissions on it, then drops from the repeater after some period of inactivity, for example, after 10 or 15 minutes. You don't need to manually unlink from a talkgroup. Again, the convention is to use time slot 1 to key up dynamic wide-area talkgroups, while local talkgroups typically are static on time slot 2.

Similar to how it works when you link a D-STAR repeater to another D-STAR repeater or to a D-STAR reflector, when you activate a dynamic talkgroup on a repeater's time slot, that's the only talkgroup that can be used on that time slot as long as it's active.

On some repeaters, like those on the DMR-MARC network, the talkgroups you can use on a time slot are specified by the repeater administrator, including any dynamic talkgroups, if they are even allowed. On other repeaters, like those on the BrandMeister network, users can specify any dynamic talkgroup they want to use; however, once again the convention is to key up dynamic talkgroups only on time slot 1.

Here's a section from PAPA System's BrandMeister Getting Started Guide (PDF) explaining dynamic talkgroups on the BrandMeister network:

When a user keys up on a dynamic talkgroup, the BrandMeister system creates a subscription for that talkgroup on that repeater. Then, for 15 minutes, the system will send audio for that talkgroup to the repeater. 15 minutes after the last transmission on a given repeater, that talkgroup will be dropped from the repeater.
   Users have the ability to key up on any talkgroup and use it. For us here in California, a user could key up on a Florida state talkgroup and it would work fine. 15 minutes later the connection would drop. Interestingly, the timeslot is not important. The network will route the audio to the repeater and transmit on the same timeslot. It is recommended that timeslot 1 be used for wide area conversations leaving timeslot 2 available to local communications.
   Unlike other linking systems there is no need to drop a connection or restore a link.
   Dynamic talkgroups are time slot agnostic. This means I can key up on talkgroup 123 on timeslot 1 and could be talking with someone on talkgroup 123 on timeslot 2 [ed. note: in this example, for the person on time slot 2, talkgroup 123 likely would be a local talkgroup]. Inside the network there is no time slot, this is only considered when the talkgroup is delivered to a repeater, and it is delivered on the timeslot of the most recent transmission.

By the way: "Talkgroups" is one of those terms that doesn't seem to have a standard form of usage. I've seen TalkGroups, Talk Groups, Talk groups, Talkgroups, and talkgroups used, sometimes interchangeably in the same article. For consistency, I'm sticking with talkgroups (capitalized only at the beginning of a sentence), but am respecting how other authors use it when I reference their articles and presentations. Similarly, while most people use "time slots," some use "timeslots," again at times interchangeably in the same article. I'm sticking with time slots.

There are many sources where you can find lists of talkgroups, including:

You also can find a few lists of various talkgroups I've been exploring at the end of this article in the A few other notes section.

1d) Zones

Zones are an organizational tool, like file folders, for your channels; in other words, a zone is a group of channels.

The channel selector on many DMR radios let's you choose from 16 channels. In order to get past that limitation, you can use zones. For example, I have a DMR radio that can handle 64 zones. If I fill all of them with 16 channels each, that gives me a total capacity of 1,024 channels.

You can organize your zones however you want, for example, you might want one or more "Home" zones for channels that correspond to your favorite talkgroups that you can reach via the repeaters that are within range of your home. You might want a "Commute" zone that corresponds to the talkgroups you most often use via the repeaters that are in range as you drive to and from work. If you use a hotspot, you might want some "Hotspot" zones for the talkgroups you key up via your hotspot. You might want some other zones that correspond to the groups of talkgroups your club uses, or to nets you frequently participate in. Some people set up zones based on repeater locations.

It's pretty easy to use zones: you create a zone, name it, and then add channels to it (up to 16 for a radio that supports 16 channels per zone). Optionally, you can change the order of the channels within the zone.

1e) RX group lists

When you program your channels, you optionally can choose to monitor more than just the talkgroup associated with that current channel.

For example, if you're on a repeater that has multiple talkgroups that share a time slot and your channel is for one of those talkgroups, you might want to monitor all of the talkgroups that share that time slot to hear whether there is ongoing activity on any of the other talkgoups. This can be very helpful to ensure that you don't disrupt a call that is ongoing on a talkgroup that is on the same time slot as the one programmed into your current channel.

Another example is that you might want to create an Rx group list for a group of your favorite talkgroups, so you can monitor whatever activity is going on in them. However, it might be better to use the scan feature for this instead.

Even if you have set up an RX group list with multiple contacts, when you transmit, it goes only to the TX contact set up for the channel you're on. If you hear someone on a talkgroup that is different from the one you have programmed for the channel you're currently tuned to, you can quickly tune to the channel programmed for that other talkgroup in order to join that conversation/

However, in many cases, you might want to monitor only the channel you're on, in order to avoid any confusion.

Note: Some radios have an optional feature that, when enabled, will switch to the channel of the last talkgroup that was received as long as you PTT shortly after that last received transmission ended. This feature is both good and bad. On the good side, it enables you to reply to a received transmission without having to manually change channels. On the bad side, you might find that you've accidentally been transmitting to a talkgroup other than the one you intended to.

1f) Scan groups

You can monitor multiple channels by creating scan groups. For example, you might want to monitor all the channels in a region, all the analog channels in a frequency range, or a group of your friends.

A scan group can combine digital and analog channels, and can include channels from multiple zones, time slots, and color codes. If your radio has limits on the number of channels per zone, that limitation doesn't apply to the number of channels you can have in a scan group.

One thing to keep in mind is that scanning takes a lot of power, so will drain your battery faster than regular use; although in general, DMR radios are more power efficient than other Digital Voice radios because of the way the bandwidth usage is optimized with two simultaneous channels.

1g) DMR networks

This is one area where DMR seems to be quite different from D-STAR. Just as in the D-STAR playground, some DMR repeaters are standalone and used for local communication only. DMR and D-STAR diverge in how they handle it when repeaters are interconnected.

In D-STAR, repeaters can be linked to reflectors, for example, there are a bunch of D-STAR repeaters here in Colorado that are permanently linked to a single statewide reflector, and then whatever is transmitted on one of those linked repeaters is reflected to all the other linked repeaters. It's also possible for a ham to temporarily link an individual repeater to another repeater or reflector for a call.

In DMR, repeaters using static talkgroups are linked together in network configurations that are decided by the system administrations, which can't be changed by individual hams. Some repeaters also allow hams to temporarily subscribe to specified dynamic talkgroups.

Initially, there were two main worldwide umbrella networks for amateur radio, DMR-MARC (the largest), built on MotoTRBO products, and DMRPlus, built on Hytera products. It's my understanding that the two networks didn't interconnect initially, but more recently, the two teams have been collaborating on building interconnectivity.

More recently, a new worldwide network has launched, BrandMeister. In a little over a year (as on Jan 2016), it apparently has grown to be the largest amateur radio DMR network in the world. As per the section I previously quoted from the PAPA System's BrandMeister Getting Started Guide (PDF) BrandMeister users can key up to (a.k.a., temporarily subscribe to) and use any talkgroup.

There also are other, typically regional, networks, for example, Rocky Mountain Ham Radio operates its own RMHAM network that covers Colorado and parts of New Mexico and Wyoming. RMHAM repeaters up and down a large swath of the Rocky Mountain region are linked together making it possible for a large community of hams to communicate on the supported talkgroups.

The BrandMeister network

BrandMeister logo

On the BrandMeister wiki, they say:

If you are an amateur radio operator working in digital voice modes like D-Star, DMR, C4FM, APCO25 or others (not all are supported yet!), you do not need to know much about BrandMeister, and it's very easy to operate on its infrastructure.

However, I'm the kind of person who doesn't like black boxes, so I wanted to learn a little bit more about BrandMeister as part of getting up and running on DMR. Here's what I've learned so far. First, "BrandMeister" is a play (in German) on the words "brand new master server."

It's a decentralized, worldwide, community-driven network being developed by an international team: Artem, R3ABM, Moscow, Russia; Rudy, PD0ZRY, Ultrect, The Nederlands; Yentel, West-Flanders, Belgium; Jonathan, EA1HET, Spain, Wijnand, PD0MZ, The Nederlands; Denis, DL3OCK, Berlin, Germany; and Adam, SQ7LRX, Poland. They are joined by teams in countries worldwide that are bringing master servers online, as well as by other teams putting repeaters online. As of Nov 2016, there were more than 30 BrandMeister DMR Servers deployed, connecting hundreds of repeaters in more than 40 countries.

As of early 2017, BrandMeister is just a bit more than a year old: development work began in 2014, and the first master servers went online in November of 2015. It's spreading as fast as a wildfire, which I guess is appropriate since, in German, "Brandmeister" (little "m") means fire chief.

From their wiki:

BrandMeister is an operating software for master servers participating in a worldwide infrastructure network of amateur radio digital voice systems.… BrandMeister allows you to connect to MOTOROLA DMR-MARC and Hytera DMRplus networks, this means you can operate with other DMR amateur radio operators on both infrastructures the same time.

BrandMeister has a really nice, robust User Dashboard that includes activity meters, a real-time Last Heard page, and a fun feature called the Hose Line, which enables you to listen in on transmissions around the world, including drilling down to specific talkgroups.

BrandMeister User Dashboard

The dashboard also earns my respect for its clean, modern, and fully responsive design, which scales nicely to any size device. It's nice to see an amateur radio-related website that looks like it was coded in this century.

BrandMeister User Dashboard

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