Help understanding relationship between groups and sites in FreeSCAN/scanner

[RTracey]

While I've been reading a lot about trunked systems in the wiki (very helpful!), I seem to be missing a piece of the puzzle when it comes to fully understanding the relationship between the groups and sites in a trunking system - as programmed in a scanner. For example, looking at the CT Land Mobile Radio Network (CLMRN) when downloaded into FreeSCAN, there are multiple groups [GRP] and what I believe are the sites [MP25]. The [GRP] contain what I understand are the different talk groups and their IDs (TGID). The [MP25] contain the site trunking frequencies.

So my question. The CLMRN system contains multiple [GRP]s and multiple [MP25]s; how does the scanner know which [GRP]s are associated with which [MP25}s? It looks to me in FreeSCAN like the [GRP]s and [MP25]s are just thrown together in CLMRN in no logical manner. Thanks.

 

[doctordialtone]

So my question. The CLMRN system contains multiple [GRP]s and multiple [MP25]s; how does the scanner know which [GRP]s are associated with which [MP25}s? It looks to me in FreeSCAN like the [GRP]s and [MP25]s are just thrown together in CLMRN in no logical manner. Thanks

System is CLMRN is broken down to

Groups - or talk groups - are groups of similarly used radios. Think DOT, CSP, etc.

Sites are radio facilities or transmitter locations.

 

[doctordialtone]

MP25 is just short for P25

APCO Project 25 - The RadioReference Wiki

wiki.radioreference.com

 

[RTracey]

Thanks Doc, I understand those points. But how does the scanner know to associate the correct groups [GRP] with the correct sites/trunk frequencies [MP25]? I has to do this, no? In other words, how does it know to associate one of the Troop"X"Simulcast [MP25] sites with the TGIDs in the State Police [GRP], instead of the TGIDs in the DepofCorrections [GRP]?

Maybe I'm overthinking this or I'm just thrown off by the multiple [MP25] sites in the CLMRN system, vs. for instance, the single [MP25] site in the Waterford system. Are the groups [GRP] just a convenient (but not required) way of grouping the similarly used radios while programming the scanner? So if you wanted - from a programming perspective - could you combine all of the individual [GRP]s/TGIDs in the CLMRN system into a single huge [GRP]? It would be a real mess with hundreds of TGIDs, but would it work?

 

[doctordialtone]

But how does the scanner know to associate the correct groups [GRP] with the correct sites/trunk frequencies

Are the groups [GRP] just a convenient (but not required) way of grouping the similarly used radios while programming the scanner?

1) The scanner knows what to do by what it receives over the Control Channel.
2) When the system is designed, groups are developed containing similarly used radios. (DOT, CSP, EMS, etc.)
3) When the system is designed, the designers know what areas the radios need to cover (Sites)
4) Yes, the groups are a convenient (Not required) way of grouping radios.
5) When a radio in a given group affiliates with a site - if it's allowed - it can use the frequencies at that site.
6) Yes you could put everyone in one huge group but it wouldn't allow people to use it because it would be too busy. This is why the system is divided up into troops for example. Plus a state trooper doesn't care if RT 2 needs sand or not. That's a DOT matter.

 

[jonwienke]

Thanks Doc, I understand those points. But how does the scanner know to associate the correct groups [GRP] with the correct sites/trunk frequencies [MP25]? I has to do this, no?

No. Any talkgroup can be broadcast on any frequency. There is no "association" that the scanner has to worry about. When a transmission is broadcast, the scanner looks up the talkgroup ID to see what to do with it, and goes from there. That's it.

 

[RTracey]

Thanks both, I think I now understand!