Scanning a P25 Multi Site Sytem

[gwbaker]

I have searched and searched the net wide to help me figure this out and I have not found a description that lets me wrap my head completely around this (I have been searching for weeks).

System
Cobb County, GA - a Project 21, Phase I system using APCO-25

It is a multi site system with four control channels (two in county, two out of county). There are MANY trunk ID's on this system as the entirety of the Cobb County uses it as well as neighboring counties. I believe the system is a work in progress to get the metro Atlanta agencies to be able to interact using any officer's radio (at least that is what they told me at the Sheriff's Office). When the logistics of the 96 Olympics highlighted the need of inter departmental communication, a Metro wide initiative was launched to remedy the problem. Now 20 years later and many elections and administration changes with various departments, I don't know where it all wound up, but it is definitely not what was originally envisioned.

Now to my questions...

When I am at work in a building I scan my departments channels, the fire department, and all of the police departments in the county (total of 7 departments) I hear nothing from the police departments. This does not surprise me as there is a lot of metal around me to stop the signals. But, this is what I do not get. Here (the jail admin area - less metal) I can hear everything on the Sheriff's Office talk groups. About 60 percent of the time I can also hear all of the fire department traffic. Other days, I hear nothing from the fire department.

Not fully understanding how this system works and why this is happening is bugging me.

The four control sites (two of which SHOULD be heard where I am). The other two listed out of the county, I doubt.

Can someone point me to a description of how that traffic is actually handled through this system. I need to be able to visualize this in my mead as I am a visual learner. The old days were easy. Car to tower, tower to scanner. I do understand the basics as I have read everything that pops up on the first four pages of a google search for this topic. But I am just not there yet.

P.S. See post #4 for a better drill down of my questions.

Thanks!

 

[darunimal]

Double check your squelch is on 2, if you want to try 0 it may help. Also if the System swings wildly while watch the digital decode threshold menu "volume" "function" "volume knob" then watch when it's actually receiving audio change the Decode Threshold to Manual and that number to 7,8 or 9. But do remember each group of towers is a site and traffic may only be on specific sites and not multicast on all sites.

 

[darunimal]

If it is a 800mhz system look to upgrade to a remtronics Bnc 800 antenna.

 

[gwbaker]

If it is a 800mhz system look to upgrade to a remtronics Bnc 800 antenna.

Thanks for the advice, both of which I have already done. I have 4 antennas for the scanner now.

What my whole rant boils down to is that I need a FULL understanding of how this system works. Where the RF comes from to an officer's portable etc (not necessarily my scanner). And, with 4 control channels, which one is a radio listening to (the officer's radio)? Do they listen to them all? When they frequency hop, I assume that changes the actual transmitter site from which RF is sent for the group to hear? Is there a separate tower and transmitter for each frequency?

 

[troymail]

I assume you have all of the frequencies (or at least the primary and alternate control channel frequencies) for each site programmed into your radio (separate sites?)....

Your radio is always trying to find the best site to lock on to. It's possible that in one location you are locking onto site 1 but when you change locations, you are locking onto site 3 (as an example) and all sites may not carry the same activity/traffic.

The system may have a wide range of configurations but in general, multi-site systems work this way:

Each of the 4 sites are intended to provide coverage in a geographic area. Authorized users "affiliate" with the best site available as determined but their radio/signal strength. A user radio could be allowed to affiliate with all 4 sites (any 1 at any given time) or be limited to only certain site(s). It's all up to how the system is configured.

If the system is still being built out, that might explain why you're hearing things sometimes and not other times.

However, understand how the system works is key to understanding that it is very possible that any of the 4 given sites may not carry traffic that the other sites are carrying. And - this will/can be dynamic.

Each of the sites has it's own frequencies and each of the sites are linked together in some way (microwave, fiber, etc.). A given talkgroup can appear on all 4 sites at the same time but more than likely less.

If there is no user "affiliated" with a given site and using a given talkgroup, you may not hear it on that site. There is no need for traffic for a given talkgroup to be carried on any site unless there is a user on that talkgroup in that site footprint. Why transmit it if no one is listening?

For sake of discussion, let's make some assumptions (just for the discussion):

-pretend the 4 sites are all in a straight line across and area like this:

S1 ---- S2 ---- S3 ---- S4

It is very possible that a talkgroup is active with a user in S1 and S4 and none on that talkgroup in S2 and S3. In this case, it is likely(possible) that you will not hear the activity on S2 and S3.

A bit later, a user becomes active on S2 on that same talkgroup so then S2 (along with S1 and S4) now carry the activity. 5 minutes later, the user in S4 switches talkgroups and S4 stops carrying the traffic.

I'm not saying this is what is happening in your case but the fact that you sometimes hear things and other times, you don't could be a result of this kind of situation.

As far as your scanner, you may want to "take control" and decide for yourself which site(s) to monitor at any given time by "duplicating" the system and creating 4 systems - 1 system for each site. Put the duplicated system in 4 different scanlists and only enable one site for each of the 4 copies. This way, you can control through the scanlist selections which site(s) you monitor at any given time. This configuration also ensures that you monitor each of the sites you have enabled at any given time (assuming they are in range).

 

[gwbaker]

Troymail thanks for the response, I think I am getting there.

Now, in your S1 -- S2 --- S3 --- S4 Example.

Are all of the additional frequencies that the radios "hop" around as needing coming from the same location as S1, S2, etc. Or are the other frequenices scattered around the county using different transmitters and receiver antennas like a traditional repeater?

As you mention there are users in S3 that need to hear the same talk group that is being received by S1, is the radio traffic then sent out of S1 and S3 at the same time? I am also assuming that a site knows it needs to send to a user as our radios (here at SO) seem to work like cellular phones, they seem to transmit when changing channels (aka talkgroups).

So, again assuming I and following you and my thoughts are correct. Lets say most of a precinct is near S1. I would then think all of the traffic comes and goes from S1, unless a unit drives to the other side of town (maybe to the jail) near S3. Would S1 and S3 be used for the talkgroup?

Lastly I have a county radio. I cannot hear traffic from a precinct in the southern most area of my county. But when I tune my SO issued radio to their talkgroup, I can communicate with them. Would this then not allow my scanner to hear the same traffic sitting right next to my county radio?

AM I OVER THINKING THIS ALL???

OH YEAH, and I totally understand what you are suggesting below. I am definitely going to take that approach.

As far as your scanner, you may want to "take control" and decide for yourself which site(s) to monitor at any given time by "duplicating" the system and creating 4 systems - 1 system for each site. Put the duplicated system in 4 different scanlists and only enable one site for each of the 4 copies. This way, you can control through the scanlist selections which site(s) you monitor at any given time. This configuration also ensures that you monitor each of the sites you have enabled at any given time (assuming they are in range).

 

[darunimal]

Simulcast - The RadioReference Wiki

You may not want to transmit directly next to your scanner at anytime, it may blow reception out

 

[AggieCon]

That's a great description so there might not be a need for me to rehash it. But I will if you think it would be helpful to hear it explained a bit differently.

Definitely put each site as its own system. There are some recent threads on here where I give step by step on how to accomplish that the easiest way.

I suggest monitoring a new site each day. See what traffic you get from it (assuming you can get decent reception from your location). That way you will get a feel for who is usually on which site.

The county jail sites can be like musical chairs. On some systems, entire departments drop on and off the site as an officer brings someone to the county jail. Then when they head back home, their radio re-affiliates with the home site and that department's traffic is no longer on the "jail site." I can also receive random southeast Texas departments when someone is driving up I-45, for instance.

Perhaps your scanner was on a different site, talk group, or not getting decode at the time of your test. Next time hold it on your local site and on the correct talk group.

Importantly, program the active control channel only for each site. Run it with the squelch open. If the T is not on most of the time, probably not worth trying to nab that site.

 

[AggieCon]

I just did more research on the system.

The Cobb County site is multicast. Your scanner isn't going to do the best with that. All of those tower locations use the same set of frequencies. Consider the whole thing as one "site" with a single control channel.

The Mableton site is a single tower location. Your scanner will like it more. It uses a different set of frequencies.

When someone makes a call carried by multiple sites, all of the sites have to be available and the call is transmitted at the same time.

The computer rotates through the repeater bank for each call. The control channel tells the radio where to listen. If you want, program each system frequency as a conventional P25 channel and you can get a feel for how the system distributes the traffic.

Once you get UniTrunker setup you will be able to visualize all of this and it will make a lot more sense. You will be able to see everything that is happening on each site that you are able to monitor. You can even read the "raw" control channel commands.

 

[gwbaker]

I just did more research on the system.

The Cobb County site is multicast. Your scanner isn't going to do the best with that. All of those tower locations use the same set of frequencies. Consider the whole thing as one "site" with a single control channel.

The Mableton site is a single tower location. Your scanner will like it more. It uses a different set of frequencies.

When someone makes a call carried by multiple sites, all of the sites have to be available and the call is transmitted at the same time.

The computer rotates through the repeater bank for each call. The control channel tells the radio where to listen. If you want, program each system frequency as a conventional P25 channel and you can get a feel for how the system distributes the traffic.

Once you get UniTrunker setup you will be able to visualize all of this and it will make a lot more sense. You will be able to see everything that is happening on each site that you are able to monitor. You can even read the "raw" control channel commands.

Okay, thanks for taking the time to look into my specific situation!

I have my fingers on it now and I see I have more work to do to get to the understanding that I want. But, you guys have given me the knowledge to get started on how to figure it out.

Thanks again!

 

[gwbaker]

I just did more research on the system.

The Cobb County site is multicast. Your scanner isn't going to do the best with that. All of those tower locations use the same set of frequencies. Consider the whole thing as one "site" with a single control channel.

The Mableton site is a single tower location. Your scanner will like it more. It uses a different set of frequencies.

When someone makes a call carried by multiple sites, all of the sites have to be available and the call is transmitted at the same time.

The computer rotates through the repeater bank for each call. The control channel tells the radio where to listen. If you want, program each system frequency as a conventional P25 channel and you can get a feel for how the system distributes the traffic.

Once you get UniTrunker setup you will be able to visualize all of this and it will make a lot more sense. You will be able to see everything that is happening on each site that you are able to monitor. You can even read the "raw" control channel commands.

When you stated "the Cobb County site is multicast," did you mean that it is the LSM Multicast that I have seen everyone moaning about?

 

[Spitfire8520]

Now, in your S1 -- S2 --- S3 --- S4 Example.

Are all of the additional frequencies that the radios "hop" around as needing coming from the same location as S1, S2, etc. Or are the other frequenices scattered around the county using different transmitters and receiver antennas like a traditional repeater?

Each of the sites has a pool of frequencies that it can use. If you take a look at Cobb County P25 Trunking System, you will see that each site has specific frequencies listed. All the frequencies that are not actively being used as the control channel is typically considered free game for voice traffic. These frequencies will stay with the specific sites.

As you mention there are users in S3 that need to hear the same talk group that is being received by S1, is the radio traffic then sent out of S1 and S3 at the same time? I am also assuming that a site knows it needs to send to a user as our radios (here at SO) seem to work like cellular phones, they seem to transmit when changing channels (aka talkgroups).

Yes, the system knows what site specific radios are on and will automatically figure out where the audio needs to go to reach all the radios on the system.

So, again assuming I and following you and my thoughts are correct. Lets say most of a precinct is near S1. I would then think all of the traffic comes and goes from S1, unless a unit drives to the other side of town (maybe to the jail) near S3. Would S1 and S3 be used for the talkgroup?

It is extremely likely that traffic for that specific precinct will be heard on S1 throughout the entire day unless something weird was happening. You are also correct that S3 will carry that precinct's traffic if a unit is in the area. S1 and S3 would both carry that talkgroup until the radio affiliates to a different site, switches to a different talkgroup, or is turned off.

Lastly I have a county radio. I cannot hear traffic from a precinct in the southern most area of my county. But when I tune my SO issued radio to their talkgroup, I can communicate with them. Would this then not allow my scanner to hear the same traffic sitting right next to my county radio?

Your scanner would suddenly start to receive them (and you) if it happens to be locked onto the same site your radio is using. If your radio or the scanner locks onto a different site, then it might not receive anything.

AM I OVER THINKING THIS ALL???

I would say you have a firm grasp of your radio system. You probably understand more than 99% of the other guys with radios. Many of them only care about how to turn the radio on, change channels, and press the button to talk.

When you stated "the Cobb County site is multicast," did you mean that it is the LSM Multicast that I have seen everyone moaning about?

He should have said simulcast, but you are correct as to what he is trying to get at. It appears that the Cobb County site is actually a 5 tower simulcast site using Linear Simulcast Modulation (LSM) that people are complaining about.

Multicast generally refers to the same data on different frequencies. Simulcast is the same data on the same frequency. The whole talkgroup on multiple different site example that you have been given would be an example of multicast where you get traffic on both S1 and S3.

That said, you have noted that you can reliably hear the Sheriff's Office talkgroups most of the time. It certainly sounds like an affiliation deal more so than simulcast issues. Some additional explanations can also be found on the Wiki. The Trunking Basics page would be a good place to start.

 

[gwbaker]

Spitfire,

Thanks so much for breaking it down and answering each question. I now know that I do understand the system and why I am not hearing everything (from inside a building at the jail). I have also noticed that my county radio will not hear some groups either and the signal strength meter shows nothing. That was one of the clues that led me to believe our radios work like cellphones.

Not ideal for me, but a good external antenna, well placed, should allow me to hear all that I want. As I have mentioned in other threads I returned to scanning after about 16 years of not. Boy things sure have changed. It was a whole lot simpler back then.

 

[gwbaker]

Wow, now this is kinda funny.

One of the things that had my perplexed about this system as some days I could hear all of the fire dipatch traffic, and for many days I would not. Then again I could, all 12 hour shift long. Today, nothing. So I tuned my issued Motorola APX 6000 radio to the fire dispatch channel (Talkgroup). Viola, scanner starts singing all of the fire dispatches (while I stayed tuned to this talkgroup). This confirms what you guys have helped with with in relation to a radio being affiliated with the site near me.

Then it hit me why before I figured this out, why some days I could hear fire, others I could not. Fire HQ is right up the street from my building. Someone at fire HQ must leave their portable on in the charger some times and that was when I could hear fire.

That was one of the things that confused the crap out of me when I was trying to learn this system. Now it all is making sense.

GEEZ!

 

[troymail]

Wow, now this is kinda funny.

One of the things that had my perplexed about this system as some days I could hear all of the fire dipatch traffic, and for many days I would not. Then again I could, all 12 hour shift long. Today, nothing. So I tuned my issued Motorola APX 6000 radio to the fire dispatch channel (Talkgroup). Viola, scanner starts singing all of the fire dispatches (while I stayed tuned to this talkgroup). This confirms what you guys have helped with with in relation to a radio being affiliated with the site near me.

Then it hit me why before I figured this out, why some days I could hear fire, others I could not. Fire HQ is right up the street from my building. Someone at fire HQ must leave their portable on in the charger some times and that was when I could hear fire.

That was one of the things that confused the crap out of me when I was trying to learn this system. Now it all is making sense.

GEEZ!

It's nice to have that confirmation. Thanks for the update.

 

[AggieCon]

I meant "simulcast" not "multicast." Thanks Spitfire for catching that and elaborating.

To be technical, "LSM" is Motorola's proprietary simulcast technology. Harris's technique is not called "LSM." I did not check to see who built your system.

Does the FD exclusively use that radio system or is it a patch from their conventional or other "regular" system?

My local FD transitioned to P25 from EDACS. However the EDACS system is still live too (the link I sent you). Sometimes, for days at a time, the FD is dropped from the EDACS system. My guess is that only a handful of people are using old radios on the EDACS system to monitor and when they all turn them off of the talk groups, then those talk groups are dropped from the EDACS system (either that or the patch just gets messed up intermittently). At any rate, it's similar to what you described. Heck, when you are at work and turn to that channel, you are helping all the other local scanner owners

Glad you nailed down that one mystery.

 

[gwbaker]

I meant "simulcast" not "multicast." Thanks Spitfire for catching that and elaborating.

To be technical, "LSM" is Motorola's proprietary simulcast technology. Harris's technique is not called "LSM." I did not check to see who built your system.

Does the FD exclusively use that radio system or is it a patch from their conventional or other "regular" system?

My local FD transitioned to P25 from EDACS. However the EDACS system is still live too (the link I sent you). Sometimes, for days at a time, the FD is dropped from the EDACS system. My guess is that only a handful of people are using old radios on the EDACS system to monitor and when they all turn them off of the talk groups, then those talk groups are dropped from the EDACS system (either that or the patch just gets messed up intermittently). At any rate, it's similar to what you described. Heck, when you are at work and turn to that channel, you are helping all the other local scanner owners

Yes, FD exclusively uses the same radio system. They may have a backup, but I am not sure. There certainly do not patch it to a conventional system. At the Sheriff's Office we still have the old VHF repeaters and radios in all of the cars. It serves as a backup in the mind of our Sheriff and the one before him. But, it was a good idea to keep it as in GA there are still some 2 meter freqs that are used state wide. For example, I was in south GA bringing a prisoner back and a jack knife tractor wreck happened right in front of me. I used one of the statewide 2 meter freqs to reach the local dispatch who then sent locals via their radio system. We do not patch the old to the new either.

 

[jdavida10]

OK, I understand about the multiple sites now. But, with a TRX-1, how do I unable a site, or turn off a site? I live in a medium size county (Orange, Ca.), but if I am in the South part, I don't want to have it scanning through all of the control channels for all of the sites, if I just want to monitor the area I am near. When I downloaded my system by ZIP code, it only put in Countywide. There are 9 other sites. I think I understand how to add them, but not how to control them.

 

[troymail]

OK, I understand about the multiple sites now. But, with a TRX-1, how do I unable a site, or turn off a site? I live in a medium size county (Orange, Ca.), but if I am in the South part, I don't want to have it scanning through all of the control channels for all of the sites, if I just want to monitor the area I am near. When I downloaded my system by ZIP code, it only put in Countywide. There are 9 other sites. I think I understand how to add them, but not how to control them.

When you import a multi-site system on the TRX, the radio will decide which single site will be monitored at any given time based upon signal quality.

The only way to control multiple sites being scanned on the TRX is to import a system with it's multiple sites and then "duplicate" the system - one time for each site. Then, lockout all but one site in each copy of the system and assign all talkgroup objects in each copy to a different scanlist.

As far as the sites imported based on zip code - chances are the other sites are out of range for your location - but maybe not. If you want all sites for a system imported, import the system using the "Preferred" method vs a locational import such as zip code or county.

 

[AggieCon]

Duplicate the system into as many sites as you need. Delete all but one site for each. Name the system with the system + site name.

Add all of the talkgroups for each system-site to a DIFFERENT scanlist. Name the scanlist for the site. That way you can turn on and off each site as you travel.

Join the TRX Facebook group and post this, I'll post a picture or two as an example.