I'm not an "expert" about the MetroKC Trunked Radio System, but I think I can shed a little light on the subject for you, Steve.
There are three major dispatch entities (not sure that all of them are PSAP's - 911 centers - but they are subject to different political subdivisions). There is the King Communications/Valley Communications (sub)system, the ESPSCA (sub)system and the Seattle (sub)system.
All of them are the same 800MHz trunked radio system, hence the reason they all share the same system ID.
Each major dispatch entity (for lack of a better term, and due to my infamiliarity with how it actually works up there) has it's own simulcast system as well as several "intellipeater" sites. Technically speaking, any radio from the system could go anywhere that there was coverage and get the talkgroups from their (sub)system. I seem to recall hearing, though, that the system engineers have turned off roaming between the (sub)systems though, so in reality, it is not possible.
Anyway, with a SmartZone system, which is what is in operation there, each tower location is it's own independent "system" (the exception is simulcast sites, which all share the same frequencies, and are specifically tuned to reduce or eliminate multi-path), and does not necessarily repeat traffic from any other site... but could. Each radio "affiliates" with the system when it is turned on (and tuned to a trunked channel), and when the channel is changed. The guts of the system keeps track of which radios are affiliated at which sites, and routes the audio that those radios want to listen to (based on the talkgroup to which the radio is affiliated) to only those sites (unless the system is programmed to
always route the audio for a certain talkgroup to a site).
To maybe make it a little more clear, pretend that I have a radio for (say) Seattle PD. When I am working in Seattle, and turn my radio on, theoretically it gets the best coverage from one of the City of Seattle sites. If I was to travel to Kirkland, though, and the radio thought it was getting better coverage from an ESPSCA site, it would attempt to affiliate to that site. If the system let the radio affiliate, then you would start to hear the Seattle PD channels on the ESPSCA site. (Again, I don't know that system really well, so this may work anyway, or I could have my geography all screwed up, but I hope you see my point).
For all intents and purposes they are separate systems, but they are all the same system (that is one reason why the talkgroup IDs aren't duplicated). They also all appear in a single system
here in the database at RadioReference. Hmm. I see a second entry for ValleyComm too... guess I'll be doing work on Washington once I get finished with Oregon.....
Hope that helps.
