Confused about Talk Groups

[rockfordpi]

I am confused about control freq and talk groups for LARGE systems. My county, Shelby County Alabama is a part of the Alabama First Responders Network. Shelby County Has 5 different control frequencies, see https://www.radioreference.com/apps/db/?sid=7258&siteSort=county#sites .

Within the Shelby County designation, there are 40-50 talk groups. I obviously do not need to tune to ALL of these TG’s. Is each TG associated with a specific control channel or ALL control channels? My question is, how do I know which talk group is linked to which control channel, or does it matter?

BTW, I have been able to connect to three of the control channels and they all use the same NAC, I know that tells me something, just not sure exactly what, LOL

Sorry, I am very new here, only had my dongle about 2 weeks. I have it working on trunked p2, but I am still trying to learn and understand what I am doing. My only previous radio experience was CB back in the 70's, so please excuse what I am sure are dumb/simple questions

 

[n0nhp]

It is up to the system manager to assign which talk group can associate with which site. The control channel just tells the radios associated with each site what the neighbor sites are (control channel freqs) , whether that radio Talk Group can associate with it and if it can which channel on it's site is active for that talk group.
Control channel scanning or monitoring will tell your radio (scanner) the talk group number and the channel number in use by each talk group. If you are monitoring all talk groups, the first to light up will grab your receiver. If you have particular talk groups set up to be monitored the radio software or firmware will ignore any you don't want to hear and move to channel and open squelch on any you do want to hear.
You will only hear talk groups associated with that tower. If there are lots of units in the neighborhood chances are real good that you will hear most all of the traffic on the talk group you are interested in. If it is a low count, there may not be any units associated and you may miss traffic if you are locked to one control channel.


Much more can go in here. Look at the wiki on wide area trunking for full details

Bruce

 

[slicerwizard]

Is each TG associated with a specific control channel or ALL control channels? My question is, how do I know which talk group is linked to which control channel, or does it matter?

Generally, talkgroups show up on the sites where the users are. If no talkgroup x user is hanging out on site y, site y is unlikely to carry talkgroup x traffic.

BTW, I have been able to connect to three of the control channels and they all use the same NAC, I know that tells me something, just not sure exactly what, LOL

It tells you that the system manager set it up that way and not much else.

 

[WA0CBW]

The control channel listens for subscriber channel access requests for a talk group and assigns a voice channel for all subscribers listening to that particular talk group. Depending on the system programming the controller may assign a different voice channel for every PTT. A site uses only one control channel at a time. It may use several (or all voice channels) as alternate control channels. In a simulcast system ALL the sites have the same channels and talk groups. Stand alone sites may be linked to other stand alone sites or simulcast sites or a combination of this. Talk groups show up on site(s) depending on subscriber site affiliation or the system programming.
It goes like this: subscriber selects talk group and PTT's on the control channel and then listens for a channel grant -- controller hears the request and assigns a voice channel and sends a channel grant on the control channel -- all subscribers hear the grant and if a subscriber is listening to that talk group their radio changes to the voice grant channel frequency and the conversation takes place.
More than you wanted to know about the control channel.
BB

 

[rockfordpi]

Thanks for all of the feedback and references. I think I am slowly beginning to understand.

 

[poltergeisty]

You have a control channel (frequency) that dictates what system channel (frequency) a talk group will use. So you could have a control channel and say five system channels. At any given time a talk group could talk on any one system channel depending on who else is talking at the same time on another system channel. So you could have the state police on their talk group and at the same time another agency like the fire department could be on their talk group. Each talk group is using their own system channel that the control channel just so happens to hand out at that time.

You listen to the control channel, and the system channel is thus assigned for your talk group. Next time you key your radio you may be assigned another system channel depending on what the control channel give you.

It's like standing in line at a bank waiting for your turn. You never know which teller will take you based on when they are done with the previous patron.

 

[rockfordpi]

You have a control channel (frequency) that dictates what system channel (frequency) a talk group will use. So you could have a control channel and say five system channels. At any given time a talk group could talk on any one system channel depending on who else is talking at the same time on another system channel. So you could have the state police on their talk group and at the same time another agency like the fire department could be on their talk group. Each talk group is using their own system channel that the control channel just so happens to hand out at that time.

You listen to the control channel, and the system channel is thus assigned for your talk group. Next time you key your radio you may be assigned another system channel depending on what the control channel give you.

It's like standing in line at a bank waiting for your turn. You never know which teller will take you based on when they are done with the previous patron.

I got that part, my main confusion was how do I know which control channel (freq) in the county to listen to to get a given TG. If I am understanding the setup here, I need to just pick the strongest signal in the county I can get because the TG's are configured for all of them and as long as there is one member of a given TG connected to the same control channel as me, then I can/will get all that TG traffic if I want it.

 

[poltergeisty]

That's a hard one, really. Ideally, the strongest control channels you can hear may carry the talk groups in your area that you're after.

 

[amcferrin90]

This kind of question should really be asked in the Alabama state thread hopefully in a topic that may exist discussing that specific system. I say that because it really depends on how the system is set up. Once you know that, it will simplify things for monitoring.

For example I'm in Columbus OH and in Columbus are several systems all tied into the statewide MARCS system. Certain nodes I'll call them will only let certain users on them. The node for the city of Columbus is primarily for public safety agencies in the city and county. There is a county node also that the sheriff uses and some suburban police agencies. There is the state node which hosts all state agencies, universities, hospitals and a few suburban public safety agencies in the county. Some talkgroups will air on all of these nodes simultaneously such as hospitals, some will air on certain nodes such as the sheriff and then depending on the user tuned to that talkgroup (say a city PD unit has a radio tuned to the sheriff because his precinct neighbors the sheriff he can be aware of situations nearby, so the sheriff talkgroup would air both on the county node and city node because the city PD user is steered to the city node). So there is an exception where I can county on hearing the sheriff on the county node, maybe on the city node so if I'm having a problem with interference or signal on one, I can change nodes and hopefully listen on the other.

I hope that makes sense.

 

[dave3825]

my main confusion was how do I know which control channel (freq) in the county to listen to to get a given TG.

The county of Shelby has 7 sites. Each site has its own freqs. If your doing this from home, find out which of the 7 sites is closest to you. Then only work with those freqs. Out of the freqs on whatever site you choose, only 1 freq will be an active control channel. That tells each radio on the system where it may transmit and receive.

You said you have one dongle. How are you using it and what are you using to decode the digital? You can't just listen to 1 freq to hear a certain tg in a trunked system.

46B    4 (4)    011 (B)    Hoover (Greystone)    Shelby    769.58125c    771.40625    772.10625    773.18125               
46B    4 (4)    015 (F)    Hoover (Spain Park)    Shelby    769.85625c    771.68125    772.35625    773.88125               
46B    4 (4)    017 (11)    Pelham    Shelby    770.50625c    770.80625a    771.15625    772.61875    773.74375    774.39375
     
46B    4 (4)    018 (12)    Montevallo    Shelby    769.09375c    771.09375    772.21875    772.94375    773.99375    774.64375 
    
46B    4 (4)    023 (17)    Shelby    Shelby    771.04375c    771.46875    771.81875    772.31875    772.89375    773.24375 
46B    4 (4)    028 (1C)    Columbiana    Shelby    851.750c    852.2375a    852.575    853.2875    853.500    853.900     
46B    4 (4)    029 (1D)    Sterrett    Shelby    771.06875c    771.54375a    771.79375    772.24375    772.54375    772.91875

 

[Ensnared]

Consider yourself very lucky. Shelby does not have a ton of encrypted talk groups like other agencies. Some have gone nuts.

If you click on the RR locations at the bottom all the listed frequencies, enlarge the map, you should see the sites that are within your county. If you want to listen at home, you might want to program those within your county with corresponding talk groups. Make sure you don't program the talk groups marked "DE" because this indicates full time encryption.