DMR Scanning, Private Calls and Talkgroups

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m6mdr

DMR Scanning, Private Calls and Talkgroups

Post by m6mdr » Fri Jan 15, 2021 11:23 am

Hello.

I’m a curious SWL/Scanner and I’ve found a frequency local to me on 440.125Mhz which contains DMR traffic.

I’m using OpenGD77 and have the radio on that frequency and set to DMR, with no filters i.e Promiscuous mode but the radio isn’t decoding all of the traffic I see on the screen. It will decode some talkgroups and private calls but not all and I’m not sure why.

There’s constantly a variety of TGs and PCs appearing on the display but only about 1/3 of which are decoded in to audio.

As I write this, I’ve just heard both sides of a private call between two IDs (235 and 214) so I can’t figure out why I hear some traffic and not others??

Can anyone shed any light please?

Many thanks,

G4EML
Posts: 919
Joined: Sat Nov 16, 2019 10:01 am

Re: DMR Scanning, Private Calls and Talkgroups

Post by G4EML » Fri Jan 15, 2021 1:48 pm

Many commercial systems use text messaging and data in addition to voice. If the system is used by multiple companies some of the voice traffic might be encrypted. OpenGD77 only decodes un-encrypted voice calls.

VK3KYY
Posts: 7486
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: DMR Scanning, Private Calls and Talkgroups

Post by VK3KYY » Fri Jan 15, 2021 9:08 pm

G4EML wrote:
Fri Jan 15, 2021 1:48 pm
Many commercial systems use text messaging and data in addition to voice. If the system is used by multiple companies some of the voice traffic might be encrypted. OpenGD77 only decodes un-encrypted voice calls.
To add to slightly to this.

Its not possible to decode fully encrypted calls. The official firmware can't do this either.

It is technically possible to find the encryption key for transmissions using "basic" encryption, but its almost certainly illegal to do this in most countries.

m6mdr

Re: DMR Scanning, Private Calls and Talkgroups

Post by m6mdr » Sat Jan 16, 2021 10:10 am

Thank you for the replies everyone. Very much appreciated!

Yes, I forgot about encryption.

I’m not exactly sure what the frequency is but there seems to be all sorts of traffic on it.

I know one of the users are NCP Carparks or similar.
They use TG 11 and their radios have short ID numbers like 220, 235 etc and they communicate between themselves and a base station / main office which I can hear all sides of the conversations. I’ve also heard their private calls too.

I hear them talking about changing shifts, checking CCTV and other parking related stuff.

I also see lots of other Private calls on frequency flashing up on screen from what I assume are other users of the frequency with TX lasting anywhere from 30 secs upwards. On the screen it will say:
Private Call
ID 2345678
=> 1234567

Many of these private calls I see on screen originate from different IDs that are around the same numerical length e.g 123467 and are all directed to the same one or two recipient IDs.
So I assume these are part of another frequency usergroup based on their ID structuring.

Other times it’s long IDs using long TG numbers like:

TG 45671234
ID 23123123

So quite a mix of uses and configurations.

All the traffic mentioned above is just from TS1 CC4 but both timeslots are in use on that frequency along with multiple CC - so there’s a lot going on.

I think it’s either a business license frequency, maybe shared or some sort of Digital PMR.

Being nosey and curious, I’d like to know more hi hi.

KG5UTD
Posts: 1
Joined: Wed Jun 30, 2021 9:33 am

Re: DMR Scanning, Private Calls and Talkgroups

Post by KG5UTD » Thu Jul 15, 2021 10:10 pm

I didn't see if you mentioned where your local area is. But, Google can be useful. This was one result. https://repeaterbook.com/repeaters/deta ... 6&ID=12717

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