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DMR Networks and Talk Groups

For the purposes of this post, I’ll define a DMR Network as a uniquely named group of repeaters linked together in some way such that any two or more radios, properly set up to intercommunicate through the network can do so by connecting each one independently to any repeater in the group.

The above definition, to keep it simple, is for a network meeting basic DMR requirements. Radio access to the network via hotspots (see below) is an optional extra. Typically, a repeater will also allow local RF-only communication, on one of its slots, without passing this traffic to other repeaters in the group. In this mode of operation, that repeater slot is acting stand-alone, while its other slot may simultaneously be networking.

The repeaters, of course, give over-the-air access to radios within their range. DMR has a protocol for that. Repeaters may link to others by any means but by far the most common is by passing on the DMR voice data packets (and other kinds) using the Internet Protocol (IP). DMR has a protocol for that, too! For the benefit of those who are unable to connect via a repeater, some networks allow access via a personal hotspot; others do not. A hotspot is a gateway that passes traffic between a radio and the network using DMR’s over-ther-air protocol on one side and IP on the other.

Two well-known DMR networks in use in the UK are BrandMeister and the South West Cluster. There are others. To find out which ones you can access through your accessible repeaters, check ukrepeater.net. I chose these two networks to illustrate their different approaches to DMR networking:-

• BrandMeister[the network] is synonymous with BrandMeister[the system] that implements the network. BrandMeister is a popular global network, partly because it encourages the use of hotspots as well as providing a large number of repeaters but also, undoubtedly, because it provides an extensive range of useful features for its users.
• The South West Cluster (SWC) is a network of repeaters covering SW England. Two of its original aims (AFIK) were to provide DMR communication across this region only and to make the network easy to use. Today’s SWC remains true to those aims by default but it is also now possible to talk to people further away with only a slight amount of sleight of hand on the part of the user. Because of its relative simplicity and because I actually use it, I have chosen the SWC as my example of a DMR network in what follows below.

The SWC, at the time of writing, comprises 19 linked repeaters, 18 of which are currently operational. They are scattered across the South West of England from Cornwall to Hampshire. The repeaters use non-conflicting frequencies and colour codes, of course, but every one of them is associated with the same two Talk Groups (see below), namely TG 9 on slot 1 and TG 950 on slot 2. I normally access the cluster through GB7HD and have two channels for it (one per slot) set up in my radio. I have named them GB7HD1 and GB7HD2. The SWC does not support the use of hotspots.

So, moving on…

• What is a Talk Group? A talk group is simply a group of people who can talk to each other, over a given network, by setting up the same numeric talk group ID in their radios. Their radios must, of course, be connected to the network but not necessarily through the same repeater. A talk group usually embraces a geographic region but not necessarily. For example, Raynet members, wherever they are, have the use of TG 23531 on BrandMeister. Each talk group is identified by a numeric ID.
• Who organises and allocates talk group IDs and decides what they represent? The people who run the network do!There is no global organisation in charge of this; each DMR network has and manages its own set of talk groups. The SWC. for example, has two default talk groups: TG 9 is for the local area covered by the single repeater you are connected to (no networking involved); TG 950 is for the area covered by the entire cluster (all SWC networked repeaters).
• How are talk group IDs used within the network? The DMR protocols use these numbers as destination IDs to ensure that the relevant digital voice packets can be routed properly through the network.
• What did I actually have to do to be able to join the appropriate the appropriate SWC talk group in order to communicate within it? To be part of a talk group I must have the correct talk group ID set up in my radio. Although the channel and the talk group are different, independent things, in the DMR worldview, I had to provide a talk group ID as a default “contact” for each DMR channel in my radio setup. I specified TG 9 for GB7HD1 and TG 950 for GB7HD2. I will never be tempted to program any more than those two channels for that repeater because the SWC repeaters accept and repeat voice traffic using only these two talk group IDs. That’s not so restricting as it sounds. All the major DMR networks, including seemingly isolated clusters such as the SWC, offer me ways to fire up talk groups beyond their usual remit and to use them temporarily, overriding the default set up in my radio. More on this in the next section.

Inter-networking and Talk Groups Galore!

The previous section highlighted, among other things, that in the UK and around the world there are quite a few DMR networks, developed by different people, with each defining its own talk groups.That’s fine as long as we do not want to stray outside the network our radio is connected to but sometimes we do, of course.

Linking disparate networks together is called Inter-networking and the Internet, having been developed over many decades for that very purpose is, de facto, the tool for the job. Practically all DMR networks have now adopted the Internet to set up links to other DMR networks as well as, in most cases, internally, to link their own repeaters together. Working through these links and into the talk groups of the networks they lead to can greatly extend our range and the number of our potential contacts. This can, though, present organisational problems with our radios. Here is a summary of how I deal with that…

• My DMR radios, in common with most, allow me to create huge lists of channels and talk groups. My general strategy is to keep these lists as short as possible, for two reasons. Firstly, it minimises the time I spend editing and reloading the code plugs. Secondly, it minimises the amount of scrolling and knob-twiddling I need to do during operation. These are my tactics:-
• Normally, I allow myself only two programmed channels (one per slot) for each DMR repeater, choosing my two preferred default talk groups from those available. Finding out which talk groups are provided by the network (by searching the Web) is relatively easy. Finding out which of those can be be accessed through each slot on a particular repeater can sometimes be more tricky but perseverance usually pays off.
• My AT878UV insists that any TG I intend to set as a channel default must previously have been stored in the radio’s Contact List. Apart from this I seldom use that list.
• I use the Manual Dial facility (a standard DMR radio feature) to set up, temporarily, any non-default TG I might need, then just press the PTT switch to activate it. The exact procedure for activating a temporary TG may vary between networks but it is always possible to use Manual Dial to do the job. I intend to deal with such practical matters in more detail in future posts.

Radio IDs and the Worldwide Amateur Contact Database

You must set up a valid radio ID, which is just a number, in your radio before you attempt to transmit with it in DMR mode. This identifies the radio and is sent out over the air whenever you transmit. Any DMR radio, upon receiving a transmission, will display the caller’s radio ID on its screen throughout the transmission.

A radio amateur’s transmission can potentially be received by any other radio amateur in the world. To avoid any “clashing” IDs, a worldwide database and allocation scheme for radio IDs was needed. Amazingly, in the diverse and anarchic world of amateur radio, some people got together and set one up! It is now administered by RadioID Inc.

To get a Radio ID, go to radioid.net and “sign up” to get an account, then follow instructions. This service is free; do not be lured into “subscribing” (aka “paying”) for extra, optional services, at this stage, unless you’re just feeling magnanimous. On the assumption that you will not be simultaneously transmitting on more than one radio, you need only one ID, which will be associated with your call sign in the database. Suddenly your radio ID has become a personal ID, instead!

Obtaining your radio ID may take a little time because your call sign has to be verified. Once you have it, it needs to installed in each of your DMR radios. As well as a field for the number, there will be another field for some text. The first thing to go in there should be your call sign. You may want to follow this with some other information, such as your name. This text is your so-called “Air Alias”. With luck (the feature is newish and is not universally supported), the text will be displayed on on the screens of receiving radios, alongside your ID.

There is also another older and more reliable way to ensure, when you are receiving, that more than just the caller’s ID is shown. You can get the entire, worldwide contact database from radioid.net, in CSV format (another free service, last time I used it), and upload it into your radio as part of your code plug! Then, your radio will use any incoming caller ID to look up the additional data (supplied originally to RadioID Inc. by that caller) and display it on the screen. If the whole database won’t fit (radios, are playing catchup, here, as more amateurs sign up for DMR), you can edit the CSV file with a spreadsheet program or you can pay RadioID Inc to do the job for you by specifying what you want on the radioid.net website. To make the best use of the database in your radio, you’ll need to update it from time to time. This can be a bit tedious, especially if you need to edit the file each time.

Private Calls

Most of our talking on DMR is done, aptly enough, within talk groups. In DMR parlance, a transmission addressed to a talk group ID is known as a Group Call.

DMR also supports Private Calls. A private call is addressed to an individual radio ID. Private voice calls are not necessary in amateur communication, which is supposed to be done openly. There are ways to eavesdrop on them, anyway, and we can do all the talking we need to do using group calls.

Private data calls, though, have their uses. For example, some networks, notably those running FreeDMR software (e.g. SWC) use brief private data calls from users for the setting up and closing down of temporary talk groups; some others, notably BrandMeister, allow users to send text messages.

Private data calls are also the basis of other DMR facilities, such as checking the availability of another radio or, if both radios are GPS-equipped, displaying its range and bearing. Experimentation along these lines is best done over simplex channels; repeater keepers tend to disallow such things.

The End

I expected this series to run to three shortish posts but it became two longish ones, instead. Thank you for your patience.

Future posts about DMR will cover more practical matters and will be more narrowly focused.