P25 Trunked scanning question

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cwachs

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I have a BCD 396T scanner. I live in Colorado where the fire dept I work for uses the P25 statewide system. I am having trouble getting my head around the correct programming method for this system and my scanner.

I have the correct channel trunk ID's and have them programmed.
As I understand it, all I need to do is enter the CONTROL frequencies into my frequency list, correct? Since there are well over 100 repeater sites in the state on this system and since I am in my car much of the time when listening, I've entered the control only frequencies for about 40 repeater sites near me into the scanner.

First question, is this the correct approach? Will the scanner lock onto the strongest control frequency and then hop top the next as I travel, giving me seemless coverage like our department portables do?

To test the scanner, I entered all the frequencies for the closest repeater site (control and all) and then entered the truck ID's. Everything worked perfectly until I traveled out of the reception area for that repeater.

After doing some reading on how P25 works, I then removed all the frequencies and entered only control frequencies for the 40 closest repeater sites.

If I do an ID Search, I hear traffic on various Trunk ID's, but none of the ones I want to listen to. If I do an ID Scan for only my Truck ID's, no traffic comes through on many of the control frequencies.

Is this the best approach for handling a mobile scanner on a P25 system?
Am I running into a situation where various trunk ID's are blocked on some repeater sites and I need to simply remove control frequencies of blocked sites?
 

captclint

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AFAIK, the 396 requires a separate system for each site. So you would need to create 40 systems, enter the CC and Alt CC for each, and then duplicate the TGID's in each. The 996 handles this much better. I don't have a 396, so I am sure someone will correct me if I am wrong, and hopefully provide an easier way.
 

W6KRU

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AFAIK, the 396 requires a separate system for each site. So you would need to create 40 systems, enter the CC and Alt CC for each, and then duplicate the TGID's in each. The 996 handles this much better. I don't have a 396, so I am sure someone will correct me if I am wrong, and hopefully provide an easier way.
That's the way I do it. You can get strange behavior if you put multiple sites in the same system. The 396 will stop on the first control channel it can hear, pause, and then if there is no activity, move to the next system.
 

n4yek

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It's most likely your Fire department isn't broadcasting over the entire state, that would be pointless. They would only have a few repeaters they would use, I doubt 40 of them. (I have been wrong before though)
If you are wanting to ONLY monitor your departments comms, you would need to find out which repeaters they are utilizing and only program those systems into your scanner.
Then give each system you create the same Quick Key number so 1 push of the button turns them all on.
 
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cwachs

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We have had chief officers take portable radios to all parts of the state and they work perfectly. It does appear that our truck IDs are active on all repeaters if our portables (or mobile units in the trucks) are any indicator.

I like the suggestion of building a number of different systems and use the same QuickKey. I'll give that a try. Only annoyance is when I want to add/remove truck ID's, I have to do it to perhaps 40 systems...
 

n4yek

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We have had chief officers take portable radios to all parts of the state and they work perfectly. It does appear that our truck IDs are active on all repeaters if our portables (or mobile units in the trucks) are any indicator.
Your radios are programmed with 'home' repeaters in them, so if you were across the state and keyed up
your radio, the system would know where your radio was wanting to go and link it up to that repeater. But to constantly transmit your local fire department across the state would be a waste of spectrum. As long as a radio wasn't somewhere else in the state, I doubt it is linking to other repeaters all the time.
So that being the case, your departments radios will receive no matter where you are in the state, but your scanner will only receive what it can hear because it is only receiving and has no way to tell the system you want to monitor a particular talkgroup ID.

If I am wrong about this way of thinking, then I apologize to all.
 
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UPMan

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It is kind of like "if a tree falls in a forest..."

In this case, the system is smart enough to know that if there is no one there to hear it (i.e. no one on that group has affiliated with the tower) not to start chopping (well, the analogy totally fell apart..."not to rebroadcast your comms from that tower").
 

cwachs

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Hopefully my last dumb question... Colorado P25 sites are broken up into 3 "zones" according to the database on this site. They are geographically distinct (the 3 zones). Would it be safe to assume that sites within those 3 zones broadcast everything in that zone - meaning each site re-broadcasts all the traffic of all the other sites in that same zone? Or am I probably reading too much into the zones?
 

jerk

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Hopefully my last dumb question... Colorado P25 sites are broken up into 3 "zones" according to the database on this site. They are geographically distinct (the 3 zones). Would it be safe to assume that sites within those 3 zones broadcast everything in that zone - meaning each site re-broadcasts all the traffic of all the other sites in that same zone? Or am I probably reading too much into the zones?
No they do not, *unless* they have a group specifically setup for it to do that through the site controller... this is usually Mutual Aid groups or a "City-wide" group. The only way it is done is if a radio is affiliated, with that particular TG and has "permission" is affiliated (connected) the computer in the radio says here I am and the site says ok if someone on fire1 talks, I'll let you know. But if someone talks on fire2, and the radio had not "phoned home" or told that site I’m listening on fire2 instead of fire1, then no transmission is sent out to the radio and nothing is received on the scanner.
Only if the site and the radio are talking to each other will you hear something on a particular talkgroup, provided the scanner has it programmed or is in "search" mode.
 

cwachs

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No they do not, *unless* they have a group specifically setup for it to do that through the site controller... this is usually Mutual Aid groups or a "City-wide" group. The only way it is done is if a radio is affiliated, with that particular TG and has "permission" is affiliated (connected) the computer in the radio says here I am and the site says ok if someone on fire1 talks, I'll let you know. But if someone talks on fire2, and the radio had not "phoned home" or told that site I’m listening on fire2 instead of fire1, then no transmission is sent out to the radio and nothing is received on the scanner.
Only if the site and the radio are talking to each other will you hear something on a particular talkgroup, provided the scanner has it programmed or is in "search" mode.
So to continue, or finish this educational lesson for me, I think this can be given the analogy of a computer network??

For this example, let's say the "system" (state of Colorado P25) has 200 repeater sites. These are all connected together like a bunch of computers on a LAN via data lines (fiber, microwave, whatever). This LAN has a central router that routes packets intelligently to the different repeaters. This "router" is the P25 computer, or site controller.

One fire department has 10 portable radios. They are on and connected to 2 repeater sites. They are authorized to be anywhere on the network. When dispatch talks on their talk group, the system computer acts like a network switch and routes that traffic (which is only digital data anyway) to the 2 repeater sites that have radios on that talk group listening. The other 198 repeater sites are quiet on that talk group since no authorized radio has phoned home on that talk group. The system acts like a switch, not a hub, and only sends traffic to the sites that have listeners, not broadcasting to all sites hoping someone is listening, like a hub does.

If you have a scanner and want to hear a particular talk group, you need to connect to a repeater site that has that talk group active on it.

I understand this is a simple analogy but is it accurate? Sure, there are talk groups that probably broadcast to every site like mutual aid groups. There are also probably repeater sites that always transmit on certain talk groups whether or not any radios are "logged in" on those sites - but is the basic analogy accurate? In this way, if you take an authorized system radio to any place in the state, as soon as it boots up and phones home to the site controller with a list of talk groups it is listening on, those talk groups will begin to be transmitted to that repeater site for that radio. Some repeater sites are really busy with lots of talk groups in the metro areas and some are quiet with perhaps just a couple talk groups. You can see this in the number of actual frequencies assigned to the various repeaters.

As I travel with my scanner, I need to hit repeater sites that would normally have radios in my talk groups attached to them or I likely won't hear any traffic on those groups.

I think I have my head around this - though clearly there will be exceptions to this analogy.
 

slicerwizard

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After doing some reading on how P25 works, I then removed all the frequencies and entered only control frequencies for the 40 closest repeater sites.

Is this the best approach for handling a mobile scanner on a P25 system?
It's a very good approach. The scanner will lock on to the first control channel it detects. If it's not the one you want to monitor, just press Hold to hold on the CC and then press L/O to force the scanner to try your other CC frequencies. ID Search will then pick up all active comms on whatever zone you're monitoring.

To unlock locked CCs, press Hold and then long press L/O.
 

UPMan

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That is one of the limitations of scanning statewide (or regionally) for talk groups that are only normally active locally. Since a scanner cannot affiliate with a site, it cannot trigger a site controller to include traffic of interest on that site.
 

slicerwizard

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if you take an authorized system radio to any place in the state, as soon as it boots up and phones home to the site controller with a list of talk groups it is listening on, those talk groups will begin to be transmitted to that repeater site for that radio.
I doubt that a radio can drag a bunch of talkgroups over to a site (a zone, actually), since it only affiliates to one talkgroup. If it could, you'd quickly overload sites with traffic.
 

FlashSWT

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cwachs, I think that was a great analogy and you've got it pretty well figured out now.

slicerwizard, what if that radio was in "scan" mode listening across a whole bank of channels? Wouldn't that "drag them over" or does that not create the needed affiliation?

.
 
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