A tgid question for my SDR project

[Roveer]

I have a couple of Uniden scanners that I have programmed to our county wide radio system P25P1. Our system has 7 Police, 7 Fire and 5 EMS groups. My town (Mendham Township) if in Group 1 with 3 other towns:

Police 1 - Chester Police (47), Mendham Borough (18), Mendham Township (19), and Washington Township (38)

Whenever I scan and have traffic on my police channel It could be for any one of the 4 towns. I remember asking back when I programmed my scanner if there was a way I could filter the 3 (2 actually, I'd prefer to listen to Mendham Twp and Boro) other towns so they would be excluded. I remember the response being that I would have to have all of the radio id's and I could put them into the scanner and then it would only allow those through. I don't know if I will be able to get all the id's but I have a question now that I'm contemplating a SDR project.

Would it not be possible to use SDR to allow only the two towns (18 and 19) as listed above to be accepted? I'm assuming the 18 and 19 are their TG prefix? Seems like something that could be very simple with SDR?

Actually just trying to justify getting a SDR project together since I completed my "record fire calls and send them to my phone in real time" project.

 

[boatbod]

Sure... you can do anything you want in software!

Op25 for example, allows you to assign whitelists, blacklists and individual tgid priorities. You can limit reception to specific tgids and then have them prioritized so important dispatch calls preempt the tac channel chatter.

 

[slicerwizard]

My town (Mendham Township) if in Group 1 with 3 other towns:

Police 1 - Chester Police (47), Mendham Borough (18), Mendham Township (19), and Washington Township (38)

Whenever I scan and have traffic on my police channel It could be for any one of the 4 towns. I remember asking back when I programmed my scanner if there was a way I could filter the 3 (2 actually, I'd prefer to listen to Mendham Twp and Boro) other towns so they would be excluded. I remember the response being that I would have to have all of the radio id's and I could put them into the scanner and then it would only allow those through. I don't know if I will be able to get all the id's but I have a question now that I'm contemplating a SDR project.

Would it not be possible to use SDR to allow only the two towns (18 and 19) as listed above to be accepted? I'm assuming the 18 and 19 are their TG prefix? Seems like something that could be very simple with SDR?

The traffic you want to monitor, as well as the traffic you want to block, are carried on talkgroup 257. Filtering would have to be done based on the radio IDs used by the organizations that you want to monitor. Probably best if you identified the radios that you don't want to hear and flag them; that way, you'd never miss any comms that you do want to hear and when you hear something that you don't want to, you just go and flag the unwanted radio ID(s) that you missed.

SDR software can easily be written or modified to deal with your use case, but I have my doubts that any of them actually currently do what you want, but a simple request like this to the authors often gets results.

 

[APX8000]

Except that the same console ID from the MCC7500s at Morris County are used on numerous talkgroups, not just that one individual talkgroup. So you will only hear half of the conversation if you lock out the console ID....all your town units and no response from the dispatcher. And if you lock out all the other town IDs instead, you will hear the dispatcher calling all the other agencies on that same talkgroups and no response from the other towns.


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[Roveer]

Except that the same console ID from the MCC7500s at Morris County are used on numerous talkgroups, not just that one individual talkgroup. So you will only hear half of the conversation if you lock out the console ID....all your town units and no response from the dispatcher. And if you lock out all the other town IDs instead, you will hear the dispatcher calling all the other agencies on that same talkgroups and no response from the other towns.


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Wow... Sounds like what I want to do is harder than I thought. I remember learning about console id and that being a problem.

So, how are the actual police car radios doing it? They are only hearing their towns calls, whether car/portable or console generated. I was thinking prefix white listing? 18 for boro, 19 for twp?

When the console key's up for 19 car (twp) isn't there identifiable data that could be used for filter? Same for the other radios?

 

[APX8000]

The console can “private call” the car if configured to do so. I used that option on an old EDACS system we used. But it’s not really private per se, because someone with a scanner can monitor the call...while other units could not. That’s why we later switched to an encrypted tac separate from the dispatch channel.

On Police 1 everyone hears everyone using Police 1.


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[Roveer]

On Police 1 everyone hears everyone using Police 1.

Is that true in the police car? Does an 18 unit (Mendham Borough) hear Chester and Washington Twp Car and Console traffic? I didn't think that was the case?

I'd still like to see if I could set up an SDR that could filter to remove Chester & WT Car and console traffic.

If there is prefix data (town specific), I think it could be done pretty easily.

I was going to set up one of the scanner software that allow me to look at the data stream and see if I can discern patterns that I could then filter by,

 

[APX8000]

Every agency that uses the POLICE 1 talkgroup hears every other agency assigned to and using the same talkgroup. So 18, 19, 47 and 38 all hear each other. Same goes with the agencies assigned to POLICE 2 through 7, EMS RESP 1-5 and FIRE RESP 1-7. Yes, it is 100 % true and not Fake News.


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[Roveer]

Every agency that uses the POLICE 1 talkgroup hears every other agency assigned to and using the same talkgroup. So 18, 19, 47 and 38 all hear each other. Same goes with the agencies assigned to POLICE 2 through 7, EMS RESP 1-5 and FIRE RESP 1-7. Yes, it is 100 % true and not Fake News.


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Since I have no first hand knowledge I will have to take you on your word. I'm trying to improve a little on that and want to filter 38 & 47. Kind of hoping when I look at datastream I will see prefix's that would allow such filtering but not sure what I will find. Guess if they all hear each other than I won't find these prefix's, but thought I read about them in a wiki. Might have just been reading about tg prefix through. I'll share what I find.

 

[Roveer]

From my data collection tonight I think it's impossible to do what I want. Then again, after listening, there's so little traffic it's not even worth worrying about. I was able to capture traffic from all 4 towns.

Dispatch seems to have a UID in the 20's I saw several different id's used, but my data below shows all ID 24

Cars have varying ID's and its possible that each town has a range, but no real way to tell.

Biggest problem I see is what another poster pointed out. Console talking to all 4 towns on a UID in the 20's no way to filter town by town and if I filtered cars then I'd always hear console talking to all 4 towns without responses to those I filtered.

Would have thought there would be more segmentation in the UID and that it would be describable (therefore filterable) town by town. But, if they are all talking in one talk group, then I have to accept that's all the separation I'm going to get.

But with all cars having computers, there's really not a lot to listen to unless there is an active incident. They basically just call dispatch to let them know they are on a vehicle stop and give plate. Pretty boring stuff.

TGID UID Car Town
257 Municipal Police 1 55052 47 Car Chester
257 Municipal Police 1 24 Dispatch

257 Municipal Police 1 55144 18 Car Mendham Borough
257 Municipal Police 1 24 Dispatch

257 Municipal Police 1 55779 19 Car Mendham Twp
257 Municipal Police 1 24 Dispatch

257 Municipal Police 1 56088 38 Car Washington Twp
257 Municipal Police 1 24 Dispatch