Trunked Systems with Multiple Sites... Help me understand

[joelincoln]

I'd like to better understand how a large trunked system with multiple sites operates:

Let's assume you have a system with two sites - sites 1 and 2 in two separate cities. And there is a mobile unit 1 normally operating in city 1 on talkgroup 1.

When unit 1 keys its mic, site 1 will broadcast this on one of its freqs as talkgroup 1. Will site 2 also broadcast this? If so, how does site 2 get the transmission - through the air at a distance, or directly from site 1 via some dedicated link?

If unit 1 drives into city 2 and keys its mic, what will happen on sites 1 and 2?

Can anyone help?
Thanks.

 

[IAmSixNine]

Its not quite that simple.
Some systems can simulcast talk groups on all towers. You usually see these in larger cities where they have 1 system with a wide area of coverage. So they share the same frequencies on towers across the city.
Then you have some systems that are for larger coverage. Think a state wide system. Less frequenices per tower. Each tower has its own designation. So RFSS 1 Site 1, RFSS 1 Site 2, RFSS 1 Site 3 and so on. A system like that will use different frequencies so they can get farther distance on each tower and only re-use frequenices when towers are not going to interfere with each other. BUT traffic on one tower may not be hard on all of them. A radio has to log in to a tower to make that Talk Group pass audio. Even then some systems might restrict what TG use what tower.

https://en.wikipedia.org/wiki/Trunked_radio_system

https://www.google.com/search?q=p25...KHbRdC-MQ9QEwEHoECAMQBA#imgrc=BWtRLAGwm87POM:

 

[Spitfire8520]

I find that our own local Trunking Basics article does a good job with explaining how trunked radio systems work. The affiliation concept in the article is what drives networked sites and determines where talkgroups show up.

One of the basic requirements for a radio system is that it be networked together in some way. Many times it can be a dedicated data line or a microwave link. This allows sites to pass transmissions back to a central controller that then distributes the transmission as needed to different sites that have a radio affiliated with it.

 

[joelincoln]

Thanks for the response. Please verify my conclusions...

So, to paraphrase: Separate sites can act independently or in unison depending on how they're setup.

This means that in a system with geographically distinct functions/sites/freqs, only certain TGs are assigned to certain sites and cannot 'use' other sites. So, in my example, if unit 1 drove into city 2 and wasn't allowed on site 2, nothing would happen if its mic was keyed.

Is there anyway to tell, based on the data in RadioReference, which sites might support which talkgroups?

 

[KB7MIB]

The database may give you general information, based on what listeners have discovered. For example, the Phoenix Regional Wireless system notes that Phoenix PD and city services are on Simulcast A, while Phoenix FD and city services are on Simulcast B.

Where the site is located can give you a general idea as well. To continue with the Phoenix RWC, Simulcast C is located in the City of Chandler, so the primary TG's will be for Chandler PD and city services, as well as the Phoenix FD Regional Dispatch Center. Simulcast F is in the City of Tempe (PD, city services, Phoenix FD RDC). Simulcast G has multiple sites throughout the West Valley, covering multiple cities. Simulcast H is in the City of Scottsdale. Simulcast J is in the City of Buckeye. Each of these simulcasts also allows a few TG's from neighboring agencies to affiliate. Phoenix PD has a couple of TG's on virtually every simulcast, corresponding to the adjacent precinct(s) to that simulcast.

A list of which specific talkgroups are allowed on which specific sites may be found in a wiki article, if you or someone else in your local area makes one. Or, you may find a thread in your state forum that lists them.

Or you can use the wildcard or ID search feature of your scanner and make up your own lists of which TG's are active on which sites. I've done just that on my GRE PSR-500, setting up a V-Scanner which one simulcast or intellirepeater per scan list and setting a wildcard to catch the TG's, which I then add to the scanlist by pressing the F3 button. There are a couple of software packages that can do the same thing, logging the TG's on your computer.

John
Peoria, AZ

 

[joelincoln]

A list of which specific talkgroups are allowed on which specific sites may be found in a wiki article, if you or someone else in your local area makes one. Or, you may find a thread in your state forum that lists them.

Or you can use the wildcard or ID search feature of your scanner and make up your own lists of which TG's are active on which sites. I've done just that on my GRE PSR-500, setting up a V-Scanner which one simulcast or intellirepeater per scan list and setting a wildcard to catch the TG's, which I then add to the scanlist by pressing the F3 button. There are a couple of software packages that can do the same thing, logging the TG's on your computer.

John
Peoria, AZ

Thanks, John.
I guess that's what I'll have to do... oh for the good ol' days

 

[joelincoln]

I find that our own local Trunking Basics article does a good job with explaining how trunked radio systems work. The affiliation concept in the article is what drives networked sites and determines where talkgroups show up.

Spitfire... I read that section of the wiki and it does explain the situation very well. I never realized that these affiliations were dynamically and automatically assigned.

I'm copying the relevant paragraphs here:

Say you grew up in Smalltown and would like to hear it to keep up with what is going on, but you moved to Big City for a job. Unless patrolman Bob (or someone else) comes to Big City and keeps his radio tuned to Smalltown Dispatch, you will never hear it because no one in Big City has a radio affiliated with the Smalltown Dispatch talkgroup on the Big City site, and therefore, the system controller doesn't send the Smalltown Dispatch audio to your tower.
Say you are listening to a couple of state policeman who are doing some surveillance many miles away and they are communicating on the regular dispatch channel for the local state police station. They can be heard, even though they are not affiliated on the local site, because the local state police station's radio is affiliated with the dispatch channel. If the two state policemen (who are affiliated with some other site you can't normally hear) decide to switch channels to a TAC (talk around channel) that no one in your area has tuned, then you will lose the conversation because it will not be carried on the local site.

 

[nd5y]

, how does site 2 get the transmission - through the air at a distance, or directly from site 1 via some dedicated link?

A trunked system has one or more controllers for the whole system or zones that route the audio from user radios between sites. The type of link used could be T1 lines, microwave radio or other network depending on the system and where the sites are located.

 

[jim202]

Every trunking system is unique to itself. I don't think you will find any 2 trunking systems in the country programmed or set up the same.

Another way of looking at it is to think that every user has their own privileges. That is to say the main switch controls what each user can and can not do or use which tower sites that user can use.

But if the system is a simulcast system, then al what I have said is mute. In this type of system like others have mentioned, you come in on one tower site and go out on all the other tower sites on the same voice channel. Your not restricted to just one or a couple of the towers. You come out on every tower in the system.

The simulcast system has both it's good and bad sides. The bad side is it ties up channels over the entire system when any user talks. This can allow others on the same talk group to hear the transmission no matter where they are in the system. But it can limit the system to a much lower user count at any one time.

The other down side to a simulcast system is it needs to be set up very carefully as to the time delays between each tower site in the entire system. If not, then you will get distortion and can even have areas where you can't hear anything intelligible. This is why scanner users have a bad time trying to listen to this type of a system if they happen to live in a coverage overlap region of multiple towers.

There is much more to understanding trunking systems, but just thought I would bring up some of the simulcast issues.

Jim

 

[joelincoln]

Great input folks, Thanks.

 

[mmckenna]

When unit 1 keys its mic, site 1 will broadcast this on one of its freqs as talkgroup 1. Will site 2 also broadcast this? If so, how does site 2 get the transmission - through the air at a distance, or directly from site 1 via some dedicated link?

Kind of off topic, but related…

In the above example, it would depend on how the system is set up. It's usually efficient to set up the system to only route the traffic on Talk Group 1 from Site 1 to Site 2 if there is a reason for it to do that. Often this is handled by radios "affiliating" with their local trunked radio site.
If there was a radio affiliated to Site 2 and on Talk Group 1, the system would transmit talk group 1 from both sites 1 and 2. If there was no radios affiliated with Site 2 on Talk group 1, then the system may not transmit that traffic from site 2. That way that trunk can be used by someone else on another talk group. In other words, no need to tie up infrastructure transmitting traffic out to no one.

That is the reason you will often see people tell others not to set up transmit capable trunked radios on a system they don't have authorization to access. If a hobbyists trunked radio accidentally affiliates, traffic can be routed to a trunked site where it doesn't really need to be. That can tie up resources and hamper communications for authorized users.

As others said, it all depends on how the system was set up.

Site linking on many older system is T-1, either from the phone company, private network, or microwave.
Most modern systems are using IP networking to do this. Cheaper, more flexible, easier, etc.

 

[joelincoln]

Great details. Thanks.

 

[ofd8001]

And to add to your confusion, just in case you didn't know...

Scanning trunked systems is different than scanning conventional systems. With conventional, the scanner "listens" to each unlocked/unavoided channel for activity. If activity is detected, then the scanner remains on that channel "for the duration".

On trunked systems, the scanner "listens" to control channels. When it detects a "channel grant" the scanner notes information. The scanner then looks at the list of programmed talkgroups. If the "channel grant" is for an unlocked/unavoided talkgroup, the scanner will "do internal things" that end up with you hearing the conversation.

With multiple sites, it goes something like this: Listen to Site 1 control channel, then listen to Site 2 control channel, then listen to Site 3 control channel until each site is "monitored" and then the process is repeated (assuming you are not scanning other "stuff" like conventional systems).

So with that in mind:

1. It doesn't matter how many talkgroups you have programmed in your scanner. The scanner does not cycle through each and every programmed talkgroup, rather it listens to the control channel for activity on any other programmed talkgroups. Thus you are kind of actually "scanning" a trunked system a little faster. (By that I mean the scanner checks for activity on 1,000 talkgroups much faster than it would check 1,000 conventional frequencies for activity).
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2. There is no reason to program sites that are too far to receive because all it is doing is taking up time trying to hear something it cannot.
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3. Because each system has its own "twists and turns" if there is one system of interest to you, going to the applicable page for the affected state and asking about the system probably will give you the best specifics on the system.
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4. It does take some time to wrap your head around big systems and most of us have struggled in the beginning.
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[joelincoln]

Thanks for the basics but I've been listening to trunked systems for years. I just wanted to understand multi-site systems better.

 

[pb_lonny]

With our EDACS system they are grouped by area but at times talk groups for other areas will appear on sites in other parts of the state, in most cases this is due to a radio being set to the talk group from that area.