We explore the philosophy of DMR, The construction of GB7HT, the functionality of the DMR Repeater, and simplify access to it and the networks involved.
DMR or Digital Mobile Radio
This is a mild misnomer, as the mode is not the only digital mode, as D-Star, Yaesu Fusion/C4FM/Wires-X, P25, M17 and Tetra(Terrestrial Trunked Radio) are all digital systems, but it’s one of the most technical, Tetra being the most formidable.
DMR as an amateur mode came about, as older equipment in use became available from the world’s most prolific user Motorola. The first networks in the amateur field began with this equipment, but then other manufacturers saw the opportunity. I’m not going to go into the legal implications, but the type of emissions known as quadratic phase shift keying – 4fsk, became very available and cheap.
Naturally the pioneers of amateur DMR using Motorola equipment D-MARC wanted to jealously guard and protect their network from these new and emerging technologies, but this didn’t prevent the arrival of the far East radio name brands now familiar, such as Hytera, Tytera and Retevis to name just a few.
As I arrived on this technology, I was living in France and operating as F5VMR. My initial attempts were accomplished with an early Hotspot known as a DV4Mini, and a Retevis RT3 hand held radio until I built the first DMR Repeater F5ZLR for Limoges.
This was based upon two Motorola GM-350’s, an Arduino Dué with a Ki6ZUM interface board, and a Raspberry Pi 2. (Sadly F5ZLR has recently been removed from operation as the building on which it was situated has been scheduled for demolition.) I then built F5ZLW about which I will relate shortly. At the time, there was no such interface fro F5ZLR that we now know as Pi-Star, so everything has to be compiled and then driven by Bash commands to the terminal, and there was no dashboard to view activity. When Pi-Star arrived, the repeater became instantly viewable by all.
Because of my origins I chose to connect to the UK networks where possible, which at the time were based on reflectors. That has been permanently changed to Talk Group based protocols.
There was also a split in the philosophy, between a network known as Phoenix who saw a regimented format of connections, and a new emerging and worldwide group that we know now as BrandMeister, that showed a more open approach but still with some rules, but far less rigid.
BrandMeister has an entirely different agenda, forming a worldwide network currently of 48 linked servers that fast forward all the connection data, enabling an amateur anywhere on the network to connect to another. Naturally the rules governing connections to this network were firm, in that connections from private resources in the form of servers were frowned upon, to protect the fast forwarding between the authorised servers. This got up the nose of some individuals who wanted to be kings of their own castle, so other networks sprung up, like TGIF and FreeDMR, that splintered the existing users. This means that instead of one calling facility, there are several, which is rather crass. Some of these networks have sought backdoor connections to the IPCS2 (in the UK formerly known as Phoenix) network, as they found they had fewer subscribers.
In the UK the two major networks are BrandMeister and IPSC2, with about equal subscribed connections, but neither are particularly busy, as it appears most repeaters of any sort appear less active these days, with the advent of smartphones and other communication apps and apparatus.
How it works.
This is a little difficult to explain in simplicity, however I will attempt to do so.
A DMR Channel has the capability of carrying two conversations at the same time, and because the information is digital, it is encoded as a single transmission, but each transmission is time sliced into two parts, known as time slots. So one conversation is encoded onto time slot 1, and the other on time slot 2. In addition to this, there are two further pieces of data that are encoded onto each transmission slot that ensures the conditions are maintained, that of the user of the channel, and the path the transmission should ultimately take. This sounds complicated until I explain further.
To enable DMR and some other digital modes to work, A system of registration of each user and connected equipment is required, for the network to be successful. This system had been accepted world wide in the mobile phone industry already, so it has been adapted for amateur radio used as well.
There is an internationally agreed code or codes for each country, known as the Mobile Country Code or MCC. For the UK these are 234 and 235, and as another example, 208 for France, or 240 for Sweden.
So everything related to DMR and a couple of the other modes has evolved from this system. Licenced Amateur Radio Users may register with radioid.net, and obtain a user number of 7 digits for personal use and 6 digits for a recognised repeater. Only one registration for a personal user is required as the user can add the number to all the equipment. It follows that server numbers of 4 digits are similarly allocated, at least on BrandMeister, and transmission paths known as TalkGroups are similarly constructed. However the system was not followed exactly in some areas as you will see momentarily.
Let’s start with the IPSC2 network in the UK, where there are presently 4 main servers and the FreeDMR server. These four servers are interlinked so that transmissions on one server are forwarded onto the linked servers, except in the case of the FreeDMR server that has other functionality not forwarded on to the others. You can find this information on FreeDMR elsewhere.
The main path or calling channel for IPSC2 is fixed on each repeater as Talk Group 235, and for the most part is set on time slot 1, except for simplex hotspots that have only 1 time slot, normally fixed as time slot 2.
Protocol strongly recommends that on making a contact on the calling group, that you take your conversation to another path, that on IPSC2 will be one of several other dynamic paths, talk groups 80 to 84 also on time slot 1, or to other regional groups that are available that are normally on time slot 2.
Naturally on a repeater this removes time slot 1 from use locally, but leaves the rest of the networks repeaters free to have 235 available. A very good practice in my view.
There are other paths that may be available, such as the worldwide path, talk group 1, and possibly others but this is generally at the discretion of the system operator for each repeater.
On Brandmeister, The protocol is similar but activity is less controlled. In the UK, the calling path is 2350 on time slot 2, with free talk paths generally 2351, 2352, and 2353. However talk groups 1 to 90 on repeaters, except in specially adapted circumstances, do not pass over the worldwide network. So a repeater has ample capability to have two local repeater channels on the same frequency. For example on time slot 1 a conversation can take place on talk group 9 for example, with another conversation simultaneously on time slot 2 on talk group 9 or for that matter any other locally available talk group, but the conversations will not interact. In special circumstances talk group 75 had been networked to a specific group of repeaters such as the Salop Cluster in the West Midlands, but still accessible on Internationally available talk group 23575.
Anyone on the BrandMeister network can connect into another country using that countries talk groups. So if I am in the USA on a BrandMeister repeater, I can use any talk group as a dynamic talk group on that repeater to access a fixed group active in the UK. Don KB9BUG in Arizona does this on talk group 2350 on an almost daily basis. Unfortunately some administrators on the US servers have not made this a reciprocal arrangement for their servers and blocked non-US registered users from accessing some state talk groups, from outside the US.
So for ex-pats to access the UK, BrandMeister is the easiest option, otherwise they have to make a direct transmission with a hotspot connected on the IPSC2 network, if that is the network of choice. Incidentally IPSC2 France has significantly fewer connections than BrandMeister France.
Fixed talk groups are those that are always on, and available for traffic without any sort of activation. Dynamic talk groups are those that are not active until called into being by the users making them available, simply by calling on those groups. Normally there is a time after which they self-discontinue after a period of disuse
How you can do this in practical terms.
Firstly, research your locality for DMR repeaters if you have no hotspot. Secondly look at the BrandMeister wiki for the available talk groups, both nationally and internationally. For IPSC2 in the UK, I have already given sufficient information. BrandMeister with its 48 servers is significantly better equipped internationally. There are links below to look at both systems in depth.
Add the title suggests this is also about this specific repeater.
Having begun it’s life in France as F5ZLW, I brought it with me on my return. It is a Tait Series 800 Mark one repeater. The interface is a KI6ZUM MMDVM Board mounted directly on a Rapsberry Pi 2B+. Output is a mean 5 Watts RF to a Comet bi-band antenna around 5 metres above the ground.
Because I view the role of repeater keeper as a service provider, I have programmed the MultiMode Digital Voice Modem on DMR specifically, but have provided the choice of network for the prospective user, to the two main networks, BrandMeister UK and IPSC2.
This means almost unrestricted use subject to the network’s own restrictions, and one small block, that I do not permit connection to talk group 91 on BrandMeister. The activity on 91 is at least difficult and at most chaotic, and difficult from which to disconnect. For that reason only the repeater reports “Not Linked”.
To use GB7HT requires a bit of self training to the programming of a code plug of a radio. The radio has to contain many copies of an identically programmed channel of transmit 439.7375 and receive 430.7375 MHz, the opposite of the repeater. In each of the channels the only thing that changes is the time slot and the talk group.
I was once challenged on why I should have 2350 on time slot 2 on the repeater full time, but it became clear that the correspondent had no clue on repeater philosophy so I ignored the challenger. He wasn’t in the district in any case.
Add it stands currently, the IPSC2 network talk groups are available on time slot 1, but in the code plug need to be installed summed with 8000, for them to be correctly translated by the software in the repeater. So 8235 time slot 1, with talk groups 8080-8084, will all be translated to 235, 80, 81, 82, 83 and 85.
For the BrandMeister network, in general terms, all 4 digit talk groups will be on time slot 2, all 2, 3, 5 digit talk groups will be preferable on time slot 1.
Some talk groups will be dynamic, that is to say will be activated by a user transmitting to the repeater. If the action is permitted, the user will get an acknowledgment tone on the radio. The repeater software makes a path to the connected server, and validates the path with the other servers, and fast forwards the transmission to any other connected user on the same path, thus if you have made a contact in Australia for example, then those path will be confirmed across the whole network. This cannot happen on IPSC2 as the IPSC2 servers in the respective countries are not interlinked. So immediately the advantages of a BrandMeister link are apparent.
The repeater dashboard is publicly viewable here so you can see the interaction. Naturally all the other functions are blocked administratively.
Even without a DMR radio you can listen to the transmissions of BrandMeister, and other talk groups too. You can monitor the dashboards of all worldwide activity on BrandMeister on the LastHeard feature. But clicking on the ‘+’ sign on the right hand end of the search bar, you can search through large number of parameters. Users and Repeaters in the UK are generally connected to either the UK Server or the Ireland server, being the closest, to reduce internet latency. However this does not preclude any system to be connected to any of the servers, as they are for the most part fully up-to-date with all the others.
For IPSC2 networks, it is not so cut and dried, as each server has a different function. All the Hotspots connected can be found here the F servers. All the motorola and Hytera repeaters are here the K server. All the MMDVM repeaters are here the A server. FreeDMR repeaters are fewer in number, are found here. The E server provides the interlinks between the other servers.
On a personal note, I see no reason to have multiple networks that achieve the same ends, it just makes for confusion for the users ultimately. Whilst there is experimentation continuing on mixed modes, GB7HT does not currently support other modes at this time, although from time to time I have attempted cross-mode tests with DMR>C4FM, but the result are not of high quality. I have no equipment for other digital modes.
When using DMR, always listen carefully as the audio lacks quality. When using the calling channels, be circumspect of your conversation length. If it is likely to exceed more than a couple of brief 60 second transmissions, consider an immediate change to a lesser used talk group, leaving the calling group free for further use. Not everyone appreciates a blether, especially if it lacks interest, or is of technical merit to other listeners. The ‘network police’ are not to be tolerated, as they are probably the least frequent of the users, and would see you adhere strictly to the rules. Just be prepared to be embarrased when a timeout occurs, wiping you out.