P25 Question

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N8ACR

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I just bought my first scanner in many, many years (i.e. since the days of each agency having their own frequencies) at Dayton Hamvention and have a question about how the Ohio MARCS P25 system works. I have the scanner programmed for my county and have been listening to that fine. With the tornados to my north in the Dayton area last night, I had a thought. Are all talkgroups broadcast throughout the state? So if I program in the talkgroups for another county, would I be able to listen to that county from mine outside of normal radio range? Or do they only broadcast the local county plus a few mutual aid and such?

I'm a ham, so I have some experience in ham DMR talkgroups such as Ohio Statewide which is available all across the state. I just didn't know how the Ohio system is set up.
 

slicerwizard

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Trunking systems do not have the channel capacity to broadcast every talkgroup on every site. If a system user isn't using site x to monitor talkgroup y, then it's highly unlikely site x will be carrying talkgroup y.
 

N8ACR

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Trunking systems do not have the channel capacity to broadcast every talkgroup on every site. If a system user isn't using site x to monitor talkgroup y, then it's highly unlikely site x will be carrying talkgroup y.
Thanks! I suspected that was the case, as that's the same situation in ham DMR systems, but wanted to confirm.
 

ofd8001

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This is a complicated question when it comes to statewide trunking systems. It all depends on the design of the system.

The majority of talkgroups are allowed to affiliate (or "connect to") only on sites in and near their service areas. So Cleveland talkgroups won't be connecting to Cincinnati sites.

There may be some regional talkgroups for mutual aid purposes that are allowed to affiliate to sites in the general region, such as the northeast, southwest, etc.

Then there are statewide talkgroups for much broader use in mutual aid situations, where these talkgroups are allowed to affiliate with all sites on the system.

Whether you hear these regional/statewide talkgroups is a different story. It takes a radio to affiliate with a site in order for that site to carry the transmissions for that talkgroup. For example User A is in Cleveland and turns his radio to the statewide common. His radio is then affiliates with the Cleveland site. Then User B in Cincinnati turns his radio to the statewide common. User B's radio then affiliates with the Cincinnati site. The system controller's then "link" the Cleveland and Cincinnati sites so A can talk to B. An interesting twist would be for User C in Akron to turn his radio to that statewide common, just to monitor the conversation. So that means an additional site, Akron, is linked in.

My advice is to always program interoperability talkgroups into a scanner, because you never know when a conversation will pop up. You may go months without hearing something and then a big, huge, large incident happens involving lots of resources.

Also, if it suits your listening fancy, programming statewide agency talkgroups could be interesting.
 

Jay911

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What even further complicates things is the ability system admins have to restrict or "un-restrict" use of certain TGs in certain areas. To be honest, depending on the whims of a system designer, whether or not you can hear a TG in your area could be a total crap shoot.

In a system I use, the majority of the talkgroups are configured so that their traffic will only be carried on a tower if there is an actual system radio affiliated to that tower and tuned to that talkgroup. No sense in sending audio for FIRE-3 to tower xyz if nobody is there listening to it - you're just wasting a frequency/talk path that could be used for comms that are actually relevant to the local area's users. However I am aware that some systems do allow for certain talkgroups to be forced to be broadcast on all sites (or a specific subset of sites, it's completely programmable) by default as well.

In addition to that, these systems also have the ability to limit where a talkgroup is valid for within the system. My agency's talkgroups are valid for a number of sites surrounding my agency's service area. Once I leave that region, they just won't work any more (I will get a deny/refuse error if I try to tune in). This prevents me from congesting the network by using up talk paths on a site 500 kilometers away from home to listen to what the crews are doing while I'm on vacation/a course/something else far away from the service area. If I need to communicate when outside my coverage area, there are indeed regional channels I can hop on and talk with either locals or dispatch centers.

In this system, there are some talkgroups which are designed for provincewide use, and are only activated when needed. Several of them are in use right now on a major fire in the northern part of my province. I'm not hearing any of that traffic where I am, because I'm 1000km+ away, and even though I have the ability to tune in with my agency radio and "bring" that traffic to my local sites, the system admins frown upon it for the congestion issues mentioned above. It would not be good for the system if half the users decided to eavesdrop on a disaster situation and tie up trunk sites all over the province with traffic just so they could hear what's going on.
 

MTS2000des

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I'm a ham, so I have some experience in ham DMR talkgroups such as Ohio Statewide which is available all across the state. I just didn't know how the Ohio system is set up.
Astro 25 (The Motorola flavor of multi-zone trunking in use in your state) in use allows fleet maps to be designed in geographic areas, or zones, as we call them. Intra and Inter zone communication is usually setup by system managers around stakeholder requirements, balancing load versus available RF resources.

While similar to DMR, there are also many differences. It isn't practical nor needed for local talkgroups in one zone to be simulcast. No one 300 miles away needs to participate in a local talk group call and tie up RF resources in their local zone just listening. During the Minneapolis Bridge collapse of 2007, the state of Minnestoa DOT who manages the statewide P25 system learned quite a bit about how this works and why talk groups needs to cross zone boundaries only when necessary.

As system managers the goal is to make sure the system is highly available to those actually involved in a response. Mindful planning of talk groups and radio IDs are essential to ensuring resources aren't wasted, and of course, use of network management software such as GenWatch ensures things stay stable when their needed most.
 

kayn1n32008

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I think the link below is to a report on how communications worked during the Minneapolis bridge collapse. It is also a good primer on the subject of large statewide systems.

MInnesota ARMER i-35 bridge collapse report - Google Search
It shows that a properly designed system, with proper resources, does work. Depsite the Radio Reference specialnaysayer, armchair, ‘in my day we used lo band simplex and it worked just fine’ quarterbacks.

The one major issue raised, and corrected, in the report, was ‘talkgroup dragging’ to low RF channel count rural sites that caused them to overload due to people not involved listening in, causing the site to overload.


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