Central Idaho Amateur
Radio Club
     
CIARC - Resources - Primer
Resources - DMR PRIMER

IMPORTANT: The low level DMR repeaters described on this page are not owned or operated by the Central Idaho Amateur Radio Club. The repeaters referenced on this page all support the DMR operating practices found on the Idaho ARES web-site, and require all amateur radio operators using these repeaters to do the same. These repeaters include:


CALL-SIGN DMR-MARC-ID FREQUENCY OFFSET COLOR-CODE
W7CIA 311602 146.960 - CC1
N7IBC 311603 444.125 + CC1
W7ELE 311607 146.640 - CC1


The following articles are recommended:

This article extends the information in the first two articles above to cover networking infrastructure and features, primarily focused towards the Brandmeister network.

The first article, TRBO Hits the Amateur Bands, gives a good fundamental overview of the hardware aspects of DMR (note: TRBO is Motorola's shorthand reference to Mototrbo, which is their branding of DMR).

The second article, Amateur Radio Guide to Digital Mobile Radio, is much the same. The first article is a shorter read, and that article and the following text may be enough to give you a good foundation.

The articles talk about TDMA, but the critical information is that there are two time slots, 1 & 2, and that these alternate at approximately 30 millisecond intervals (when operating on a DMR Tier 2 or Tier 3 repeater, where the repeater provides the timing frames for the switching of time slots in the user's radio). When operating on a repeater, your radio will transmit and receive on only 1 time-slot. For transmit mode, this also means that your transmitter is not on at a 100% duty cycle, and you will realize approximately a 40% battery savings for transmit mode when compared to analog FM. An immediate benefit that jumps out of TDMA is spectral efficiency in that a single repeater can carry two distinct independent conversations, using the two time-slots. Time slot 1 users do not hear time slot 2 users and visa versa.

On Color Codes, these should be thought of as a digital mode equivalent to CTCSS or DCS use on analog FM. It is simply a qualifying criteria to gain access to the repeater transmitter.

Contacts, which are comprised of both Talk-Group IDs and Private IDs, were originally meant, in the context of land mobile DMR, to further limit access to a repeater transmitter to a very specific set of users and to support private calls between users (the latter applies to both repeater and simplex operation).

Enter internet support.

Although the ETSI specification for DMR calls out how radios of different manufacturer are to interoperate with each other, the ETSI DMR specification provides no guidance on how to interconnect radios / repeaters via the internet. Each manufacturer has developed their own proprietary method of linking repeaters over the internet. The DMR offerings from the two dominant manufacturers, Motorola and Hytera, do interoperate over the internet, or at least not without some sort of translation service through some server.

Motorola TRBO equipment was initially interconnected through a technology referred to as a ‘C-Bridge', which was responsible for routing traffic between repeaters. Hytera has their own equivalent technology.

The internet connectivity of Motorola and Hytera are very similar, and for the sake of explanation, let's envision a number of geopolitical structured chat rooms. These are organized hierarchically in the following manner:

Talk-Group Visualization

Additional listings of talk groups labels and values can be found here or here.

When you key up a DMR radio, your radio transmits the chat-room reference, in the form a talk-group, which is received by a repeater that is connected to the internet, and then forwarded to a server. That server will then distribute your audio to every other server (or hot spot, like a SharkRF OpenSpot) that is subscribed to that chat-room. If you want to visit a different chat-room, use a different channel that has a talk-group number that references the chat-room that you want to use. The server will accept traffic from any source, determine the destination chat-room (by talk-group number), and then distribute the audio to all repeaters that are subscribed to that chat-room.

The network also allows users of a repeater network to overload the use of a chat-room reference (talk-group number) through the creation of a ‘cluster'. Repeaters that subscribe to a cluster, using the cluster's talk-group number, will not communicate over the internet with repeaters using the same talk-group number that are not subscribed to a cluster. For example, a repeater that uses the Idaho state-wide chat-room, using talk-group 3116, would hear other repeaters that are also subscribed to 3116 without the use of a cluster. A repeater that uses the same 3116 talk-group through a cluster, will only have traffic from other repeaters that are also subscribed to that cluster. This allows for the semi-private linking of repeaters that are affiliated.

The repeaters have several different ways in supporting a chat room. These include:

A dynamic (push-to-talk) talk-group exists when the repeater is not subscribed to a chat room. You, as a user, can create a temporary subscription to a talk-group by selecting a channel that holds the talk group number for the chat room that you want to enter, and the key and unkey your radio. The repeater will establish a temporary subscription (usually for 20-minutes), and any traffic on that chat room will be heard by repeater users on the time-slot from which the subscription was obtained. If the server receives no further traffic from the repeater, the subscription will time out and the subscription is cancelled.

A static (always on) talk-group is always on, and associated with a specific time slot, as configured by the repeater administrator. The repeater will maintain a permanent chat-room subscription. Users need not key up to subscribe.

A timed static (scheduled to be always on) talk-group has a scheduled period for which a talk-group is made static. After the period expires, the subscription is converted to a dynamic subscription, and subject to t ime-out as any other dynamic subscription would. This is used for NET participation, where the NET resides in a chat-room, and where you don't want to have a station key up every 10-minutes to maintain the connection to the NET.

The configurations of these are set by the repeater administrator, and there are conventions associated with these (which are not well publicized, and often not followed). The convention has dynamic / PTT talk groups assigned to slot 1 and static talk groups assigned to slot 2.

Enter inter-brand internet compatibility. A group of programmers (mostly in Europe and Russia) obtained disclosure on the internet protocols that are implemented by Motorola, Hytera, SharkRF OpenSpot, and MMDVM (home-brew DMR), and developed a network where all of these protocols are made to appear compatible. Chat-room subscriptions are honored across these proprietary technologies. This network is referred to as the Brandmeister Network, and allows direct interconnectivity to occur. For Motorola TRBO repeaters that are on a C-Bridge, there is a translation to allow these to access the Brandmeister network, but the majority of hardware has a direct connection to the Brandmeister network.

For repeaters that are directly connected to the Brandmeister network, you can look up a repeater on Brandmeister Network. As an example, visit the W7CIA repeater. When you visit the page, you will see a section titled Slot details, where you'll see:

The use of two clusters is to support two simultaneous conversations between repeaters on our affiliated network. This also has advantages for an EMCOM NET, where traffic can be moved off the NET time-slot, to the opposing time-slot, while assuring that COMMS won't be lost when moving channels (since the same hardware is being used).

DMR also supports private calls, but these should be renamed guaranteed call for two reasons. First, the call cannot be made private (no encryption on ham radio, is available to DMR scanners, and can be listened to on the internet). Second, a call can be nearly guaranteed, provided the called station informs the network of what repeater they are operating on, which is done simply by keying and unkeying every time they change repeaters. You could private call Larry, without knowing what talk-group, time-slot, color-code, or repeater that Larry is operating on. If Larry has informed the Brandmeister network as to how to route a call to him (by keying after changing repeaters), your call will be routed to him. Once the call is established, and if the repeater operator desires to not support sustained private conversations, you may need to coordinate to move to a public talk-group.

Private calls may not be supported by all repeaters, and operators should consult the repeater owner before using this feature as it can be disruptive to repeater operations as it will take over a time-slot. Should private call be suppored, there are two ways to make a private call:

  1. Manual mode, where you use the menus in the radio to either select a private call ID from the contact list that is programmed into the radio.
  2. Channel mode, where a channel is programmed to carry the private call ID.

In manual mode, the radio has a timer that, as long as you transmit prior to the timer timing out, will maintain you in private call after receiving a private call. The maximum time period for this timer in many radios is 7-seconds. Some other radios allow longer times, up to 15-seconds. If you receive a private call, and respond within the timer period, your radio will transmit the private ID, regardless of the talk-group that is programmed into the channel that you are currently using. If your transmission is late, the timer times out and your transmission will then go out with the talk-group that is programmed into the current channel, and the calling station will not hear you.

In channel mode, you simply select a channel that has the private ID and the timers are no longer in play.

So it is most likely that initial contact via private call will have the call recipient operating in the equivalent of manual mode as the timers are in play. After receiving the private call, it is best to switch to channel mode (if you have a channel programmed for other station) so that you don't revert to non-private mode if you're late in responding.


Posted 17 December 2017