Category Archives: Digital Mobile Radio (DMR)

Updated Maryland DMR Codeplugs

I just updated my Mid-Atlantic (now called Maryland) codeplugs and have made them available on my Maryland DMR project page.  Right now the codeplugs only contain repeaters in Maryland but I will be working on them over the next few days to include Washington DC and Virginia repeaters that are nearby.

Codeplugs are available for the CS-700, CS-750, and CS-800.  Please let me know if you’re using them and if there are any errors I need to fix (I know of one already).

An interesting way to deploy a DMR c-Bridge

I made an inquiry to Rayfield regarding their c-Bridge product that can be used to join several (up to 50) Motorola DMR (MOTOTRBO) repeaters together.  Here’s what I found out:

There is a software option that is $600 less than the hardware solution.  The software runs on CentOS 5 or 6 (I’m assuming RHEL would also work here).  I was able to quickly stand up an un-licensed version (doesn’t actually do anything) to test with in a VM on my laptop and I didn’t see any resource problems.

I suspect one could easily run this on a 1GB Linode for $10/mo and have really good availability.  The best part of using a hosted virtual machine for this type of system is that you can duplicate it (to have a hot backup elsewhere), geographically move it around (to counter physical disasters) and be afforded a great deal of availability if needed (of which you’ll want since this is a radio system you’re dealing with).  Plus it’s easy to increase the resources afforded to the VM if needed (as the system expands to allow more repeater connections).

I may be incorrect on my assumptions, and if anyone has tried it I’d really like to know, but I think this sounds like a good and cheaper way to deploy a c-bridge.

Dipping my toe into the DMR pool

A few weeks ago I ventured down to Cary and to RARSfest to see some friends.  It was a good event and the weekend was nice and relaxing.  While at RARSfest I noticed many of my friends carrying DMR portables.  They told me to go talk to the NC PRN guys who had a booth there.  Well, fast forward to today and I now have a DMR radio sitting on my desk.

What I know so far…

Well…  I did a lot of reading to try to understand how the system is setup.  As I used to have to deal with trunking systems [in a previous life] I understand talkgroups and the problems with having too many available without the channels to support them all when you have simultaneous use.  DMR is no different.  Throughout the world I see many different thoughts on how to allocate the talkgroups to make many options for the users without completely killing the system.  Some groups do it well, others… well, I’m not sure how their systems can even work.

They systems in the Mid-Atlantic seem to do well.  I’d probably make some adjustments but overall they seem to run smoothly.  The repeaters seem to have “local” on one timeslot and everything else on another.  That keeps local open all the time for conversations.  This is in contrast to the NC PRN network which lumps local onto a timeslot with a bunch of “on-demand” talkgroups.  Their system-wide talkgroup, PRN, is kept on its own timeslot without interference.  Again, it’s all about priorities.

The system’s efficiency is at risk when you put too many users on too many talkgroups, especially when those talkgroups occupy timeslots on all the repeaters all the time.  Of course someone thought of this and created TAC-310; an on-demand talkgroup that most everyone (globally) has access to but doesn’t occupy a timeslot on a repeater unless it’s been activated there.  Makes sense, correct?  You can get off the calling channel and get over on a discrete channel that doesn’t utilize that many resources.  Unfortunately there is only one of these talkgroups.  Even on a system the size of NC PRN their get-off-of-PRN talkgroup is named ‘Southeast’ where the talkgroup has many more potential off-network users.  Still it’s all about priorities and fulfilling a certain mission.

CalDMR seems to have a good way of separating communications onto their timeslots.  I guess the hope is that you won’t have too many conversations at each level happening simultaneously.

One system I’ve been quite impressed with the Connecticut’s DMR ARES system.  Designed to facilitate intra-state communications, they have added ten tactical on-demand tactical channels to help spread out the load away from system-wide channels while supporting many simultaneous communications.  Still, I wonder about the load and whether or not the repeater congestion will be a problem.

Most of these networks are young and I suspect some shifting around will be required on all networks as well as an understanding of the priorities of the network by its users.

So where are you hanging out?

As for me, I’m listening to the North America calling channel as well as local on the Upper Marlboro and Charlotte Hall repeaters.  Feel free to give me a shout!