Friday, March 1, 2019

DMR Defined

This is a introduction to Digital Mobile Radio, DMR.  This post helps support my series on DMR tips and other DMR articles.  This is the best place to start if you don't know what DMR is.
(Click on images for larger)


DMR: DMR starts out as scary and mysterious for existing hams, but all it is is a way to send your voice to another radio in digital form.  It's mostly used on repeaters but you can use it for simplex communications too (and there are advantages to doing this).  




Terms and features of DMR


Zones:  I really like the "Zone" feature, which is nothing more than a grouping of channels.  Zones are great for me since I only want to see some channels when when I'm at my summer home or working with my ARES team.   This is similar to the Yaesu "Bank" but Yaesu banks are hard to use, not labeled on some radios and kind of kludgy.  The way you switch banks is different on my two Yaesu HTs and I’m always forgetting how it works.  

The Digital Part of DMR - Before I go on any further, I need to explain, as briefly as I can, what DMR (Digital Mobile Radio) is and how its different.  DMR is digital which does a few things right off  the bat. 
  1. TDMA (Time Division Multiple Access) allows repeaters to handle more than just one conversation at a time.  In this case, DMR repeaters use TDMA allows two conversations at a time by switching back and forth between two “time slots”.  This is done so quickly you don’t notice (like on the old TDMA cell phones, or aka GSM).
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  2. Because your voice is digitized when it comes out of the radio, it makes it easy to integrate other digital technologies and add additional features.  For example, all of the BrandMeister repeaters worldwide are  linked together via the internet.
  3. Each transmission has some header info about who you are, the on-ramp that you took and what “talkgroup” you want to communicate with.
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  4. Connections are without noise since it’s voice over digital, the voice is there or it isn’t.  There’s bound to be some data loss so the protocol includes some amount of forward error correction that’s being done.  So, in theory, users should get better range .  My testing has shown DMR outperforms FM range when doing side-by-side testing. 

    According to this article and a Wikipedia article, your coverage won't be any better on DMR.  In my testing, I've found DMR to have better range than standard FM.  Because of forward error correction, you get great receive quality right up to the end, then you get ZERO, ZILCH, NADA, you vaporize! (attribution Bob KB6CIO).  With analog FM, you can sometimes turn off squelch and still pick up a very weak signal but when doing side-by side tests, DRM wins every time.  I conducted multiple test from a mountain top and couldn't get anything at all on FM but when switching to DMR, the person came in perfectly.  My tests consistently show DMR has better range and when both are working at long range, DMR quality is always better.

  5. Because it's digital, you can easily build or buy a little box that we refer to as a "hotspot", but it's acts like a tiny personal repeater.  This allows you dedicate frequencies that you want to listen to all the time to your own repeater and experiment without taking up one of the slots on a public repeater.


Talkgroups: The best way to think about talkgroups is to think of your local repeater as a little "talk group".  Your buddies are on xyz repeater so you dial it in and have a chat.  Now you know what a DMR "talk group" is!  DMR takes it to a whole new level.  You may have used linked repeaters in your area that allows operators to communicate with each other over a much larger area.  Examples of a linked repeater system are the Papa System in Southern California, and Rimlink or EARS in Arizona.  These linked systems are considered more useful because of the massive coverage area that they have with just a small hand-held radio.

DMR takes this idea to the next level by linking all DMR repeaters in the network worldwide.  Think of the Brandmeister network as the internet, and a talkgroup as a "chat room" on the internet.  Most Brandmeister connected repeaters allow hams to tap into a local repeater (or you can use a little mini hotspot as your own repeater) and basically jump into any talk group not confined to the repeater.

Because the repeater uses the internet to link all the repeaters together, you could communicate on the San Diego Hangout talk group (310014) that I usually monitor and give me a shout.  If I'm there, I'll answer.  There are over 1300 talk groups out there and most repeaters that I've used allow you to instantly connect to one even though it isn't "static".  Here's a listing of Brandmeister talk groups.

When a repeater or hotspot (I’ll just say repeater from now on) is subscribed to a talk group, any transmission that is destined for that group is heard by all the operators listening (subscribed to the talkgroup).  Just think of it as a massive website of pages where a page is a talkgroup and instead of a web page, it’s voice.  Lastly, if an operator is using a DMR repeater and wants to talk to his pals on the SoCal 31066 talk group but the repeater isn’t currently subscribed to it full-time, all she has to do is dial in the repeater and configure the radio with the SoCal talk group and then transmit.  The radio sends the destination talkgroup with each transmission and the repeater will dynamically add the group to the subscribed list on the repeater if it’s isn’t already.  Now the operator just joined the group and can hear everyone else no matter what their on-ramp is.  Think of it as being able to connect to any repeater in the world from any repeater with just the push of a button. 

For me, DMR using the Brandmeister network really opens up a new world of capabilities.  Sure, users of some analog repeaters can link the repeater to another repeater,  but only if it is supported and they know how.  I’ve heard local hams do this by linking a repeater I’m on to one in New York or elsewhere but it's a bit complex.  DMR changes this because it makes connecting to a talkgroup from anywhere in the world as easy as pressing the transmit button on your HT.  If I was in Russia, I could use a local repeater or hotspot and yak with my pals that monitor the SoCal or other group here in San Diego in seconds.  If you are into radio, this is pretty cool. 

There’s one catch though.  If at any time the repeater becomes disconnected from the internet, it acts like any old-school repeater by only having the capability to communicate with users on that specific repeater.  In a disaster, this is not a problem, just choose the “local” talk group and the repeater knows not to even try to use the Internet. Even if you don't choose "local", people on the same repeater will hear you.

SMS
I'm starting to play with DMR text messaging.  This is a very cool feature, similar to APRS.  It allows you to send a text message to the entire group, or an individual radio.  Unlike cell phones however, the system has no guaranteed delivery mechanism.  If your radio isn't on, or the system can't find your radio, you'll miss the message.


Brandmeister

Brandmeister is a network of repeaters.  Think of it as it's own network with centralized (worldwide cluster) of control servers.  Although Brandmeister is the largest amateur network of connected repaters, there are other amateur radio networks of repeaters, like "Anarchy" as well as private networks.  Most public service agencies, like police and fire, have private networks.  Unless the network owners set it up, users of one network aren't automatically bridged to the other.

Repeaters can be connected to only one network at a time.  So if you are using a Brandmeister connected repeater and your friend is using a repeater connected to Anarchy or some other network, you won't be able to communicate with each other.

DMR Advantages

  • Digital provides the best quality all the way to the max range.
  • The signal is combined with other data, like your call sign and in some cases your GPS position if supported by the radio making it more useful.
  • It supports Voice, text messaging, and other capabilities as part of the protocol.
  • The use of TDMA doubles the capacity of repeaters allowing each repeater to look like two.
  • The use of the internet or other linking capabilities enables massive networks of repeaters worldwide.  Combined with a massive number of talkgroups, DMR enables a lot more capability.
  • DMR give you longer battery life.  Because DMR is really only transmitting half the time, batteries last longer.  
  • DMR has better range.  My testing shows that the forward error correction built into the protocol improves range and other operators tell me that SMS messages can be sent from even longer ranges. 

Comparison to Analog


Range
Testing shows DMR wins when analog doesn’t work at all.  When at the fringe, DMR provides better quality for 100% copy.  Text works at longer ranges.
Quality
DMR has better quality with zero “path noise”.  Can have digital type dropouts
Complexity
FM analog wins but DMR only adds two additional factors, talkgroup & time slot.
Availability of radios
Analog has been around over 100 years so it has a huge head start
Accessibility
DMR talkgroups can be accessed from anywhere in the world via internet connected repeaters or personal hotspots at home or in the car.  Analog is constrained to a single repeater, private network, or IRLP linked repeaters. Other digital radios can use DMR via hotspots that “bridge” to DMR.
Additional features
DMR typically shows a callers call-sign, name, and location.  Other data like location is supported and it also supports group and private text messaging

Other Things To Keep In Mind

  • DMR has some latency while your radio syncs to the channel.  Wait until you hear the permit tone before you start talking.
  • It's hard to tell when there is a double because only one QSO can be heard.
  • Because of the linked nature, others could subscribe to the talkgroup so you may be heard by 1000s of users. 
  • Because of digital encoding delays, leave long pauses for breaking traffic.
  • Kerchunking is normal, as operators dynamically subscribe to the TG.
  • You can use “dual slot monitoring” to hear all traffic on a repeater.

My Observations

At first, I was a little bit frustrated with DMR.  It was hard getting use to the longer pauses between people.  Operators have to wait just an extra second or two for the call to be fully setup between the radio and the repeater and the rest of the network.  Sometimes I'll just loose the conversation entirely, it will vaporize and I have no idea that I've lost touch with the repeater.  It finally dawned on me why I'm frustrated by this while talking to another ham this morning.  With analog, as you or the other operator move to the fringe of coverage, it communicates this by having the signal get a little more noise in it, then small drop outs begin occurring then larger drop outs (can't break squelch) then bang, the station isn't breaking squelch.  In DMR, it's there or it isn't...  Yes, at times, you will hear digital dropouts or  R2D2 sounds before the loss of communication but not always.

The one big plus with DMR is you know you have a problem when using a repeater if your radio gives you an error tone while transmitting.  If if you aren't making it into the repeater (this is important) it lets you know with a tone and a message on the display.  DMR is able to do this because your DMR radio transmissions are chopped up into very small increments and when it's not actually sending data, it receives data back from the repeater during your entire transmission.  If it doesn't get enough of these back, you get an an error tone on your radio so you know it isn't getting into the repeater.  This is a nice feature!  As you approach the edge of reception range, your receive quality can go from excellent quality to zero reception because of the digital nature.  Sure, sometimes there is some digital interference and some R2D2 sounds to let you know you may be approaching the edge, but in my experience, I go from good to nothing.


On the bright side, you know for sure if you are getting into a repeater when you don't get an error.  Also, in my experience, I can use a HT in my car with no external antenna around town with excellent quality.  If I were using FM, the quality wouldn't be nearly as good.  Without an external antenna in the car, he range is definitely affected but we have enough repeaters on the small mountains and hills in and around San Diego for this to be doable.  Adding an antenna really helps in the range department though. 

I'm looking forward to AnyTone coming out with a vehicle mounted DMR unit so that I can replace my Yaesu mobile unit and get down to one radio in the car.

I think that DMR is the future for VHF and UHF communications.



REFERENCE




73,
Chris Claborne, N1CLC
(aka christian claborne)

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