Clarify the Portland Public Safety system for me.

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scowl

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I've studied multiple Wikis on how P25 trunking systems work but I don't quite get how everything relates. In the Portland Public Safety system, there are eleven sites. Does that mean that some talk groups use certain sites exclusively?

And there's the "Simulcast Backbone" site. Does that mean that everything on the other sites is also broadcast on this site?

Tri-met is especially confusing. They're on this system yet they also have a separate 100% digital system that they also use. Are they for different purposes?
 
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In general, what talk groups are broadcast on a site depends on what radios are affiliated with the site; a site may only broadcast a specific talk group if there's a need to because a radio has picked that site to communicate with. It's also possible that talk groups can be manually forced to broadcast on a site.

The "Simulcast Backbone" (if I'm correct) actually consists of multiple sites and is designed to cover most of the metro area, and most traffic should be heard on it. The other sites are fill-in sites that cover areas where the backbone doesn't reach good and I think only carry the talk groups that radios using the sites are tuned to. In addition, I've read that there's some talk groups that are only heard on the airport site.

As for TriMet, the digital system is new and currently being transitioned to. Buses have been moved from an old dispatch system using wide band 450 MHz radios, and now supervisors, trains, etc should be getting digital radios so the Portland system lease can be ended. For now, talk groups are patched and can be heard on both systems as needed to reach radio users.
 

scowl

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In general, what talk groups are broadcast on a site depends on what radios are affiliated with the site; a site may only broadcast a specific talk group if there's a need to because a radio has picked that site to communicate with. It's also possible that talk groups can be manually forced to broadcast on a site.
It sounds like from a scanner perspective that we can't really know what talk groups are associated with sites but since the talk group ID is known and doesn't change, we don't need to care which site it comes from. This makes it hard to define the individual systems within the larger system if they exist.

The "Simulcast Backbone" (if I'm correct) actually consists of multiple sites and is designed to cover most of the metro area, and most traffic should be heard on it.
But does it really "simulcast" other sites? It does seem like a "backbone" since most traffic does end up on it.

The other sites are fill-in sites that cover areas where the backbone doesn't reach good and I think only carry the talk groups that radios using the sites are tuned to.
That would make sense for locations like PCC where transceivers will be in buildings and basements and would benefit from closer coverage, although I've seen PCC traffic show up on the backbone too and never from their site (maybe the backbone is a simulcast).

In addition, I've read that there's some talk groups that are only heard on the airport site.
In theory transceivers could monitor the strength of the control channels for each site and select the strongest one. I don't know if that's what they actually do.

As for TriMet, the digital system is new and currently being transitioned to.
That makes sense. While the new system has that obvious digital sound, it has none of the hiss and crunching of FM and sounds pretty good.
 

OregonScanner

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But does it really "simulcast" other sites? It does seem like a "backbone" since most traffic does end up on it.
Yes it is a simulcast. There are several simulcast transmitters that cover almost all of Multnomah County. There are so many user radios affiliated with the simulcast that most if not all talkgroups (with the exception of some airport ones) will almost always be heard on the simulcast even if they are also coming over a fill-in site. So you only need to listen to the simulcast unless you can't get a good enough signal. Then you would need to listen to the appropriate fill-in (non-simulcast) site.

In theory transceivers could monitor the strength of the control channels for each site and select the strongest one. I don't know if that's what they actually do.
Yes that is how they work, they select the strongest one.
 
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That makes sense. While the new system has that obvious digital sound, it has none of the hiss and crunching of FM and sounds pretty good.
I'm pretty sure most of the traffic on the regular talk groups is originating from the analog system, so it isn't pure digital. Only transmissions with a real radio ID start as digital.
 

scowl

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I was talking about the traffic on Tri-met's separate digital system, not their traffic on the backbone. Even if the audio comes from an analog source, my scanner is receiving it much clearer without the pops and crunches I'm getting from the FM backbone. That may be because I'm fairly close to Council Crest.
 
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I was talking about the traffic on Tri-met's separate digital system, not their traffic on the backbone. Even if the audio comes from an analog source, my scanner is receiving it much clearer without the pops and crunches I'm getting from the FM backbone. That may be because I'm fairly close to Council Crest.
I was too. The thought was that digital voice that's been converted from analog would be different then voice that that is digital all the way through.
 

scowl

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I'm not talking about the quality of the voice. I'm talking about the constant pops crunches and hiss I get from the FM system which are completely absent when I'm listening to Tri-met's digital system.
 

PMJ2kx

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As for TriMet, the digital system is new and currently being transitioned to
I was under the impression that the system was live 100% and that it was simulcast to Portland 800 for interop. I know their UHF licenses were canceled, but I didn't think they relied on 800Mhz anymore.

I haven't spoken to my source in months, so maybe I misunderstood what he said.

I'm not talking about the quality of the voice. I'm talking about the constant pops crunches and hiss I get from the 800MHz system which are completely absent when I'm listening to Tri-met's digital system.
And instead of hisses and pops, you're getting dropped packets. :)
 

scowl

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And instead of hisses and pops, you're getting dropped packets. :)
Incredibly I haven't had any reception problems at all with their system. It probably helps that I'm in a fixed location.

Perhaps someday they'll get this "digital radio" working with cell phones. ;)
 
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I was under the impression that the system was live 100% and that it was simulcast to Portland 800 for interop. I know their UHF licenses were canceled, but I didn't think they relied on 800Mhz anymore.

I haven't spoken to my source in months, so maybe I misunderstood what he said.
The sites are operational, but many (maybe most) users excluding the buses do not have digital radios currently. Also, I know that TriMet wants to end the lease payments for using the Portland system, so I'm not sure the talk groups will be regularly broadcast on it.

If you listen to the 800MHz side, you can definitely tell what's been converted from digital.
 

PMJ2kx

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The sites are operational, but many (maybe most) users excluding the buses do not have digital radios currently. Also, I know that TriMet wants to end the lease payments for using the Portland system, so I'm not sure the talk groups will be regularly broadcast on it.

If you listen to the 800MHz side, you can definitely tell what's been converted from digital.
Where I'm at, I can't get 800 real clear, but I can get TriMet's 700 just fine, so I haven't had a chance to compare notes. Anyway, good to know - thanks for the update. :)
 
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