DMR

DMR is a digital communication standard developed by ETSI (European Telecommunications Standards Institute), used both in professional (PMR) and amateur radio fields. It stands out for its spectral efficiency, digital audio quality, and data transmission capabilities.

It uses a bandwidth of 12.5 kHz. It employs 4FSK modulation. It supports voice and data (text, GPS, telemetry). It allows two simultaneous communications on the same physical channel thanks to TDMA (Time Division Multiple Access), a technique that divides time into "slots", assigning each transmission a portion of the total time:

• Each 12.5 kHz channel is divided into 2 time slots, called Time Slot 1 and Time Slot 2.
• Each slot has a duration of 30 ms and alternates (30 ms for slot 1, then 30 ms for slot 2, and so on).
• In effect, it is like having two logical channels on a single physical one.

Example:

If two operators are talking on TS1 and TS2 simultaneously, their transmissions alternate in time, but neither overlaps: they are synchronized in time.

Transmitting at regular 30ms intervals produces, if listened to in analog, the classic "machine gun" effect. Since with DMR you only transmit for 50% of the time unit, the battery life of mobile devices also benefits significantly.

DMR, being TDMA-based, requires all involved devices to be well synchronized in time. If a transceiver is too far away, a propagation delay large enough to make the signal "slip" into the wrong time slot can occur, causing collisions and loss of synchronization.

What is the limit?

The maximum distance at which two DMR Tier 2 devices can transmit without GPS synchronization before the timeslots overlap is theoretically about 150 km.

A brief explanation:

DMR and TDMA: DMR (Digital Mobile Radio) uses 2-slot TDMA technology. This means that on a single channel, communication is split into two alternating "timeslots", allowing two separate conversations to happen simultaneously. Guard Band: To prevent transmissions from overlapping, the DMR system includes a "guard band" between timeslots. This guard band is a small empty time interval that compensates for signal propagation delays and is defined by the standard to be 1 millisecond. Propagation delay: The speed of light (and therefore of radio waves) is finite. When a signal travels from one device to another, it takes a certain amount of time. The greater the distance, the greater the delay. ETSI Specification: The ETSI standard for DMR (TS 102 361-1) defines a propagation allowance of 1 ms within the guard band. Calculation: Considering that radio waves travel at about 300,000 km/s, a delay of 1 ms corresponds to a distance of 300 km (300,000 km/s * 0.001 s = 300 km). Since the signal must travel round-trip (or at least the maximum distance is half the total path allowed by the delay), the maximum distance without overlap problems is about 150 km.

DMR Networks

There are multiple networks accessible via access nodes (where a node means either a repeater or a hotspot):

• BrandMeister
• TGIF (Thanks God It's Friday)
• FreeDMR
• ...

The most organized (and least free) worldwide is BrandMeister.

The BrandMeister Network

BrandMeister is an IP-based, non-commercial DMR Tier II network that interconnects:

• DMR repeaters (connected via the internet)
• Personal hotspots (such as Pi-Star, OpenSpot, WPSD, MMDVM, etc.)
• Regional and national servers All these components allow a ham radio operator to speak with other colleagues anywhere in the world, as long as they are connected to the same network, using a DMR ID and a talkgroup.

What is a DMR ID and where do you get it?

The DMR ID (or Radio ID) is a unique numeric identifier assigned to every ham radio operator (and their devices) to access and transmit on digital DMR networks like BrandMeister, TGIF, FreeDMR, ...

It is the digital equivalent of a ham radio callsign. It is used to:

• Identify you uniquely on the network
• Show your name, callsign, location, and country when you transmit
• Manage your hotspots, radios, and repeaters
• Access extra features (APRS, SMS, dashboard...)

It is mandatory for using amateur radio DMR networks.

A DMR ID is a 7-digit number (optionally expandable with a suffix of 2 additional arbitrary numbers to be used when setting up a hotspot):

• 2221123 (Italy has the prefix 222; the subsequent numbers identify the user)
• 2220001, 2220002...

What do you need to register?

• Your valid ham radio callsign
• A document proving its validity (e.g., a scan of your license)
• First name, last name, email address
• (Optional) Data for your first device (brand, model)

What is a TalkGroup (TG)?

A TalkGroup (TG) is like a "virtual channel", or a "room" by analogy with rooms in other digital systems, on the BrandMeister network:

• TG 91 = Worldwide
• TG 222 = Italy
• TG 2222 = National Italy on TS2
• TG 22201 = Lombardy
• TG 222112 = Tuscany (example)

As for BrandMeister, the list of Italian talkgroups (and starting from that page, you can also find the others) can be found at the Italy - BrandMeister URL. Keep in mind that just about any number with 7 or fewer digits can be used to talk on a talkgroup: if two operators are on the same TG, they can talk to each other. The list presented is only because, by convention, "named" TGs have been created (i.e., assigned a name) to give ham radio operators a point of reference (for example: TG 22251 is the thematic talkgroup for Tuscany).

Okay, this is pure theory. But now I have a DMR radio in my hand and I want to make some contacts. In short, what do I need to do?!

Communication between two radio operators

Let's recap everything and see how to operate in DMR, both via a repeater and in direct mode, and how timeslots are used.

Communication via Repeater (Duplex)

This is the most common mode and serves to extend the range of communications.

How it works:

1. Frequencies: The DMR repeater uses two different frequencies: a receive frequency (input) and a transmit frequency (output). The user's radio transmits on the repeater's input frequency and receives on the repeater's output frequency. Nothing new compared to analog here.
2. Synchronization: The repeater acts as a master clock, providing a precise synchronization signal to all connected radios. This is crucial for TDMA, so each radio knows exactly when to transmit in its assigned timeslot.
3. Two Logical Channels: Within the repeater, the two frequencies (input and output) are used to create two independent logical channels (Timeslot 1 and Timeslot 2).
4. Use of Timeslots:

• Timeslot 1 (TS1): Often used for "global" or "national" communications via the internet. For example, the Italy Talk Group (TG 222) on the BrandMeister network is typically associated with TS1. If you speak on this TG, your voice is transmitted to the repeater on TS1, and the repeater sends it to the network, which redistributes it to other repeaters worldwide that have that TG active. Keep in mind that CONVENTIONALLY timeslot 1 is used for global TGs, but nothing prevents an operator from asking their OWN repeater/hotspot to connect timeslot 2 to TG 222. Therefore, the fact that you transmit on timeslot 1 guarantees that anyone else connected to TG 222 will receive you and can speak with you (regardless of the timeslot their repeater is hooked onto).
• Timeslot 2 (TS2): Generally reserved for "local" or "regional" communications, which remain confined to the specific repeater or the network of locally interconnected repeaters. For example, Talk Group "22251" or another specific regional TG is often associated with TS2. This allows a group of users to talk to each other without occupying national or international bandwidth. Conventionally, TG 9 is the local TG: it is used to allow operators under the coverage of the same repeater to talk locally without routing traffic to the master server (i.e. to the Internet). Therefore, TG 9 is a "special" talkgroup: traffic remains confined to the repeater. Another special TG is "4000": it is used to tell the repeater to "unlink" from all previously connected TGs.

To talk via repeater:

Your radio must be programmed with:

• The transmit frequency (repeater input).
• The receive frequency (repeater output).
• The repeater's Color Code (conventionally, for hams this is always 1).
• The desired Timeslot (TS1 or TS2).
• The desired Talk Group for that timeslot.

When you press the PTT (Push-To-Talk), your radio sends your ID and the digitized voice signal on the selected timeslot and talkgroup. The repeater receives the signal, retransmits it (if it's TG 9), or routes it through the network (if it's a network TG) to reach the radios that are also connected to the same Talkgroup.

Direct Communication (Simplex)

This mode allows radios to communicate directly with each other, without the aid of a repeater. It is the digital equivalent of the analog "walkie-talkie".

How it works:

1. Simplex Frequency: Radios transmit and receive on the same frequency.
2. No External Synchronization: Unlike repeater mode, there is no central infrastructure providing precise time synchronization. The radios rely on their own internal oscillators and "guard bands" (like the 1ms band mentioned earlier) to avoid collisions.
3. Single Timeslot: In DMR Tier 2 direct mode, despite the two-slot TDMA technology, it is common practice to use only one timeslot (typically Slot 1) for communication. This is because, without a repeater coordinating timing, attempting to use both slots simultaneously from two different radios would significantly increase the risk of overlaps and collisions, making communication unreliable. Only one radio can transmit at a time to ensure the transmission doesn't overlap.

To talk in simplex:

Your radio must be programmed with:

• The same frequency for transmit and receive.
• A common Color Code (often 1 by convention among ham radio operators).
• The Timeslot set to 1 (or sometimes 2, the important thing is that both radios use the same one).
• The desired Talk Group (here too, it must be the same for all radios wishing to communicate).

When you press the PTT, your radio transmits directly to the other radio within range, on the specified timeslot and talkgroup.

Okay, clear. No, actually not so much. Can you explain it to me in other terms?

You know how an analog repeater works? Okay, when you tune in and key the PTT, everyone on the same repeater hears you. Perfect.

Now let's assume there are two groups of people who want to talk to each other, without what group A says interfering with what group B says. How is it done? Simple, you double the repeaters: if we have two repeaters on different frequencies, this is possible.

But what if we don't have two repeaters with two frequency pairs (TX, RX), but only one repeater with a single TX, RX frequency pair? You could take turns. You check the clock and decide that for the first five minutes group A talks, the next 5 minutes group B talks, and so on, to cover the 60 minutes of the hour. That sucks, but it's possible, right? Everyone's clocks just need to be synchronized, so everyone knows if it's their group's turn to speak or not. What time is it? 5:12 PM → so we are at the twelfth minute of the hour, and knowing that:

• from 5:00 to 5:05 the first group speaks
• from 5:05 to 5:10 the second group speaks
• from 5:10 to 5:15 the first group speaks again
• we understand that at 5:12 it is the first group's turn.

We agree this is abominable, but we have introduced the concept of a "Time Slot", which in our case is equal to 5 minutes.

Now let's pretend that the repeater keeps the "tempo". When group A keys up, it only receives what group A says when group A is actually authorized to speak; and the same goes for B. So what happens if group A makes a 12-minute speech? Basically, we will hear what they say only from minute 0 to minute 5, and from minute 10 to minute 12, losing the central 5 minutes.

So if both group A and group B speak at the same time (pretending that the transmissions don't step on each other and that it took 12 minutes to say these two short sentences):

• A says: "Hi, how are you? I saw you yesterday downtown doing some shopping."
• B says: "Last night I went horseback riding and had a lot of fun."

The repeater receives and retransmits: "Hi, how are you? horseback riding and had a lot of doing some shopping." Of course the result is still 12 minutes, but the central 5-minute part was taken by group B.

Let's shorten the times: let's make it so the timeslots aren't 5 minutes long but 30 milliseconds. It is still true that the sentences intersect and are therefore incomprehensible, but there isn't a 5-minute "hole" in the middle where you can't understand anything: the holes are many more, but they are much smaller. It's like taking a two-hour movie, cutting out an hour of plot in one chunk, and splicing in an hour of a cartoon, or replacing one frame out of two with those of the cartoon.

The real reason you don't understand anything is that the resulting movie is the fusion of two movies: if you could tell your video player "show me only the odd frames" you could see one movie, and telling it "show me only the even frames" you could see the cartoon.

With DMR it works like this: your radio, which is synchronized with the repeater, understands which "frames" to discard. Are you transmitting on TS1? Then discard the even ones. Are you transmitting on TS2? Then discard the odd ones.

By doing this, two groups of people can talk on the same frequency pair. So the timeslot is the choice you make to speak (and hear) what happens during the "even" or "odd" frames.

And for now we have untangled the timeslot knot. Let's move on to Talkgroups. Pretend you want to talk to the people of Tuscany. You are in Florence, you tune to your repeater in Florence, normal stuff, you talk to your fellow citizens. But what about the guy in Grosseto? Or Siena? Or Livorno? And your aunt, a well-known ham radio operator, who is on vacation in the US? How do they do it? Obviously they can't talk by connecting to YOUR repeater. This is where "thematic rooms" or talkgroups come in. You tell your repeater "Hey REPEATER, I'm contacting you on timeslot TS2: know that I want to speak in the Tuscan talkgroup! (TG: 22251)". Whereupon the repeater takes your voice arriving on timeslot TS2 and shoots it via the Internet to the BrandMeister master server. The ham operator in Grosseto, the one in Siena, Livorno, and your aunt do the same: they call their repeater and explain they want to join TG 22251. Each will choose their own timeslot: remember that the TS serves for communication between radio and repeater; once the message is on the Internet, there is no longer a distinction. The content of my TS1 can easily be played back on your TS2. The BrandMeister master server will say: "Oh look at all these people who want to talk on TG 22251: I'll put them in communication with each other." And here the magic happens. Done.

Now, to consolidate these ideas, I'll explain a couple more concepts. Talkgroups can be static, dynamic, or autostatic.

A talkgroup is defined as static on a repeater's Timeslot when the repeater administrator requires that specific talkgroup to be always active. If TG 222 is static on TS1 of repeater IR5UDN, it means that if you use that repeater and ask it to connect to TG 716 on TS1, it will connect, HOWEVER it will also remain connected to 222. After a preset time (10 minutes) during which no traffic belonging to TG 716 is detected, the latter will be dropped by the repeater, and if someone speaks on it at minute 11, you won't hear them. You will have to key up again, which will extend the repeater's stay in that TG for another 10 minutes. When two (or more, up to a maximum of 5) talkgroups are active on the same timeslot of a repeater, you will hear ALL conversations belonging to those TGs. In case of collision, the first one to arrive takes over (having multiple TGs on the same timeslot is detrimental, especially if they are TGs with decent traffic). The fact that TG 716 disconnects after 10 minutes makes that TG "dynamic". That is, previously your repeater was not connected to it, then it is, and then it goes away after a while. If you don't want to wait for the 10-minute timeout to disconnect from a TG, you can key up a couple of times on TG 4000: that's the TG used to tell the repeater to "unlink from all non-static TGs". Autostatic TGs, on the other hand, are those TGs that once linked by the repeater are not released until you ask the repeater to connect to another TG. In reality, they are not used in repeaters or dual-hat hotspots, but only in simplex simplex-frequency ones:

1. The hotspot is not connected to anything.
2. You key up on TG 222 → TG 222 becomes autostatic, meaning it won't disconnect automatically after 10 minutes.
3. You key up on another TG, for example 716 → TG 716 becomes the new autostatic one. TG 222 becomes dynamic and if no traffic occurs for 10 minutes, it will drop.
4. To disconnect from autostatic TGs: key up twice on TG 4000.

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