This forced me to think about DMR from an "outsider's" perspective in order to explain the differences between standard analogue repeater operation and DMR network-based operation. Up until now, our local amateur community has educated each other as we go, as we're all relatively new to the mode. This means that you have to get a radio and get on the air to behold the mysteries. A better approach might be to have that information available beforehand.
There's already information about programming various radios, the different networks, and the technical aspects of the mode itself. I'm not going to duplicate that here. A lot of the 'introductory' material I've found assumes that the prospective operator understands a good deal more than they actually do. Nobody likes to feel awkward and "out of the loop", so I'm going to explain some of the basic concepts that will give a prospective DMR operator a head start.
Time Slots
There's no analogy to analogue repeaters here. All technological discussion aside, every DMR repeater operates as if it is, essentially, two repeaters. Two completely different and independent QSOs can take place simultaneously, if they are on different time slots. The time slot that you use is programmed in to your radio, and is determined by the talk group that you choose. I'll get to talk groups shortly, so bear with me here. Two people can't use the same time slot at the same time, just as two people can't simultaneously talk on an analogue repeater. On a DMR repeater, however, you can't "double" -- the repeater simply won't allow you in if that slot is currently in use. Of course, two (or more) operators can alternate on a time slot, just as on an analogue repeater.
Most of the time, you don't need to worry about time slots. It's not as if you need to adjust your radio and choose your slot. It's built in to your choice of talk group. Each talk group has a time slot assigned to it, and you can't change that. On our local network, for example, the talk group (TG) "Local 2" uses time slot (TS) 2. If you are transmitting on "Local 2", then time slot 2 (TS2) is "tied up". No one can transmit on any of the other talk groups assigned to TS2 while you are transmitting on "Local 2" (TG2). For example, "Midwest Regional" (TG 3169) is assigned that same time slot. Anyone wanting to transmit on "Midwest Regional" while you are transmitting on "Local 2" will either have to use a different repeater or wait until you un-key.
However, if you are talking on "Local 2" (TG2, TS2), another person can simultaneously be on "North America" (TG3, TS1), because that talk group uses the other time slot.
The vast majority of talk groups on our local network use TS2. You can find the list of available talk groups, along with their assigned number (TG) and time slot (TS) here under the "Talkgroups" heading.
I mention time slots here because you might find yourself unable to get in to the repeater at certain moments because the time slot you are trying to use is "tied up". It doesn't mean that anything is wrong with your code-plug or your radio.
Talk groups in general
Talk groups are like channels on an analogue radio -- though I hasten to clarify that the term "channel" on DMR has a specific and different meaning. Without talk groups, every DMR operator anywhere in the world would be sharing the same space -- kind of like all HF operators sharing the same frequency. It would be messy and unusable, mixing people trying to talk across town with people trying to talk to other continents.
To avoid this, DMR uses a variety of talk groups (TGs) aimed at a specific user group. Every TG is assigned a number and a time slot.
Unless you are programming a radio, you normally don't need to concern yourself with TG numbers or TS numbers. The exception would be when another operator asks you to "QSY" to another talk group. You aren't really changing frequencies, so "QSY" is an inaccurate term, but it's what people understand. Someone may ask you to "move to 310", for example. The important thing here is that you don't need to memorise TG numbers and punch them into a keypad in order to use talk groups. All of that is already programmed in to your radio when the code-plug is loaded.
Mostly, talk groups are based on locations. Examples would be "Local 2" (local area of the repeater), "Midwest Regional", "North America", "SE Regional", "World Wide", etc. There are exceptions, with some talk groups being based on language, such as "World Wide German" or "World Wide English".
So, the basic procedure is that you choose one of the talk groups (TGs) that's programmed into your radio and start talking on it. Assuming that the time slot is open, it's just that easy.
PTT talk groups
This is a somewhat confusing topic for the uninitiated. There are a lot of PTT TGs. "PTT" stands for "push to talk", of course. These TGs are also referred to as "push up" talk groups.
In a sense, these talk groups don't actually exist until you use them. Well, they exist, but the repeater isn't connected to them, so it's as if they don't exist from the standpoint of the end user.
An example on our local network would be "World Wide English" -- TG13, TS1. If you set your radio on this talk group to listen to all of the fascinating global QSOs, you'll probably be disappointed. Unless someone else on the same repeater that you are using has actually transmitted on "World Wide English" in the past five minutes or so, that talk group isn't even connected to the repeater. You won't hear anything.
The solution is to key up (PTT) on that talk group. This connects the repeater to TG13 ("World Wide English"), and you can either talk on it or listen.
There are two important things to keep in mind about using (including monitoring) any PTT TG.
The first is that opening that PTT talk group sets a timer off, and the repeater will disconnect if you don't transmit within about five minutes. There's no notification given when a disconnect happens, so you have to "ker-chunk" the talk group regularly even if you are just listening.
The second thing to bear in mind is that using that PTT TG ties up the time slot assigned to that talk group. In the case of "World Wide English" (TG13, TS1), TS1 would be tied up for the entire time that you are talking on that PTT TG. Anyone who wanted to use, say, "North America" (TG3, TS1) would have to either wait until you were done or use another repeater.
As I mentioned before, there are a lot of these PTT TGs. Practically all of the "regional" talk groups are PTT, for example. I'll explain "TAC" (tactical) talk groups shortly, but all TAC TGs are PTT. Knowing how PTT talk groups work is completely opaque to those new to DMR, as there's no equivalent on analogue repeaters. Unfamiliarity with the PTT TG concept causes friction and confusion.
PTT TGs are used to conserve bandwidth and reduce repeater transmit time. You need to understand that any DMR repeater is constantly transmitting the content of every non-PTT talk group. Making a talk group PTT is a compromise that makes the content of talk group available on demand. There's nothing whatsoever wrong with using these PTT TGs, but you need to be aware of the behind-the-scenes process.
TAC talk groups
TAC TGs (short for "tactical" for some obscure reason) are nifty and useful, but they are another aspect of DMR operation that has no parallel in analogue repeater operation. All TAC talk groups are PTT -- they are not connected to the repeater until you actually key up on them. Pretty much all of them operate on TS2 (at least on our local network), so using them means that no local communication is possible on that particular repeater while a TAC TG is open. The only exception is "COMM 1" (TG 3777215, TS1).
TAC TGs are often called "repeater-to-repeater" channels, but that's not exactly true. It's more of a an indication of how TAC talk groups are actually used. There's a considerable amount of traffic on "TAC 310" (TG 310, TS2), for example. Anyone on any DMR repeater can listen to it or join in, provided the talk group has been opened on the particular repeater (since it's PTT, remember). So it's not really any more of a "repeater-to-repeater" talk group than any regional PTT talk group would be.
There's an idea that TAC TGs are somehow "private", but they aren't. Two operators "QSY" to a TAC TG and create a connection between their two different repeaters, but anyone else can open that TG on their repeater as well. It's kind of like moving to simplex from a repeater.
In virtually all cases of TAC TG use, two operators on a crowded (or "calling") talk group decide to move to a TAC talk group to continue their conversation. Not all TAC TGs are carried on all repeaters, or on all networks, so there may be some "negotiation" involved. TAC 310 is pretty universal, but European operators seem to prefer "TAC 1" (TG 8951, TS2).
There are TAC TGs that only exist on a particular network, and there are politics involved as to which get carried on which networks. Our local network (K4USD) has eight TAC TGs exclusive to the network, for example: TAC 312 - TAC 319. There's no point in suggesting that someone on a different network "QSY" to TAC 315, in other words.
Any time you are on a "calling channel" (which I'll explain shortly), it's considered polite to move to a TAC talk group for an extended conversation. Again, it's similar to moving off of 146.52 (on FM simplex) to another channel to avoid monopolising the "calling channel". This concept, however, probably isn't too familiar to new hams.
There's absolutely no point in moving from a local talk group to a TAC talk group. You'll simply tie up the slot and prevent anyone from using the local TG by opening the PTT TAC TG. The only exception would be moving to (the previously-mentioned) "COMM 1" TAC TG, since that's on time slot 1 (TS1).
You can, of course, monitor TAC talk groups. The issue of timed disconnect still applies, however.
It's a good idea to familiarise yourself with your radio ahead of time so that you know which zone your TAC talk groups are in, and how they are named. You don't want to figure all of that out when someone asks you to move to one of them.
There is a net on TAC 310 on Sunday evenings.
"Calling" channels
This is a topic that new operators are often unaware of, and create friction with other users unknowingly. There are certain talk groups that are designated for "calling", and there are certain operators that are very anal about enforcing this. On our network, most of these are PTT TGs. The exception is "North America" (TG3, TS1), which is "full time".
On "North America", the consensus seems to be that ten minutes is a reasonable limit. If your conversation has gone on for close to that, you really should start negotiating a TAC TG to move to, or break off the QSO.
In the case of "World Wide" (TG1, TS1) -- which is PTT on our network -- the limit is two minutes. This basically means that you need to start setting up another option as soon as contact is made. I should point out that "World Wide" is the top-level talk group carried by all repeaters, and is different from "World Wide English" (TG13, TS1) -- which is also a "calling" channel.
The other calling channels on our local repeaters would be "World Wide German" (TG10, TS1) and "World Wide Nordic" (TG17, TS1). Obviously, these are non-English talk groups. Please don't be the guy that goes on these TGs speaking English.
On the language-based calling channels (WW English, WW German, WW Nordic), the time limit is a bit softer: around five minutes.
In practical terms, you don't have to move to a TAC TG from a calling channel. You can move to "Midwest Regional" (TG 3169, TS2) from "North America", for example. Just be aware that someone in, say, California might not have that Midwest talk group available on their network. The odds of an operator outside of the U.S. having such a talk group available to them locally is extremely thin. Using a TAC talk group is probably your only option in these cases.
Again, your familiarity with TAC talk groups really pays off when using calling channels. At a minimum, know where you can find TAC 310, TAC 311, TAC E (a/k/a "UA English 1" or TAC Eng 1"), and TAC 1 on your radio. Consider modifying the zones with calling channels on them to include these four common TAC TGs on the same zone.
Regional, Statewide, Local, and TAC talk groups are not considered to be calling channels and have no time limits.
Local 2 vs. Local 9
On our local network, all of the repeaters carry these two talk groups "full time". They are both on TS2, which means that they both can't be used simultaneously on the same repeater.
"Local 9" (TG9, TS2) is the local channel for the repeater. This was a meaningless distinction when we only had one working DMR repeater in town. Now that we will soon have three (or maybe four), it kind of matters. Think of "Local 9" as working like your standard analogue repeater. Talking on Local 9 puts you in touch with other people on the same repeater (provided they are listening to "Local 9", of course).
For example, if you are on the Dayton 442.850+ machine using Local 9, nobody on the Oakdale 443.425+ machine will hear you. Here's a nifty thing to keep in mind, though: you won't tie up the time slot on the other repeater by using Local 9 on the Dayton machine.
"Local 2" (TG2, TS2) is the linked local channel for all of the metro repeaters. Talking on Local 2 means that you are carried on all three (or four) local DMR repeaters. Think of it as getting on the HandiHam linked network.
Of course, using Local 2 ties up TS2 on all of the local repeaters, too. Or looking at from another way, if someone is using TAC 310 on the Oakdale repeater (using time slot 2, remember), you won't be able to reach Local 2 on the Oakdale repeater from the Dayton machine.
Summary
Activity is so low right now that "tying up a slot" is hardly ever a concern. Still, we are using a shared resource, and understanding how your use of that resource impacts others is the key to considerate operation.
On an analogue repeater, everyone understands the concepts of "doubling" and monopolising a repeater. It's transparent. On a DMR repeater, however, none of this is obvious from the outside. Things that you expect will work, simply do not, for reasons that are mysterious and opaque.
Likewise, people can use analogue repeaters extensively and be unfamiliar with "calling channels" and "moving to simplex". Certainly, the idea of PTT talk groups is alien to those who do already understand those two concepts.
This all may seem complicated, but it becomes second nature very quickly once you comprehend how the entire network of repeaters interacts. I haven't covered everything, but I hope that I've imparted enough information that a new DMR operator can hit the ground running and avoid the most common pitfalls, conflicts, and baffling situations. It's certainly more than I understood when I first waded in to the environment.
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