Thanks guys. I already knew this was a statewide system, so therefore now suggested by a couple of you guys I hearing other counties. How strange.
Not really. With a multi-site system like the MPSCS you really almost have to throw out the old concepts of one tower and frequency for a specific agency. With the size of the system the concept of only being able to talk back to your dispatch from within a specific radius of a single tower site also goes by the wayside.
Obviously, with the number of agencies using the system, it's not possible or desirable for every site on the system to carry every talkgroup at all times. To handle this situation, radios must let the system know that they are in range of a tower site and that they are using a specific talkgroup. This is known as affiliating with the system/tower site.
Basically, when turned on, the radio will search for control channels that are nearby. When it finds one that is strong enough to use, it will send a message to the site that basically says "I wish to use this tower site and this specific talkgroup". If that talkgroup is allowed on that site, the tower will acknowledge the request and start transmitting that talkgroup when it is active. If the talkgroup is not allowed, the control channel will send back a denial message, and the radio will continue searching for another control channel to use. As long as there is at least one radio affiliated with that tower site on that talkgroup, you'll hear the traffic that is on that talkgroup from that site.
As the user moves around, the radio is constantly monitoring the signal decode rate of the control channel stream. If it drops too low, it will search for another site with a better signal. P25 systems like the MPSCS make this pretty easy for the radio as the control channel transmits a list of nearby sites and their control channel frequencies for the radio to check so it doesn't have to do a lot of searching for control channels.
The site used will not always be the main site for the agency. Depending in access permissions and the location of the user, it could be any of the 200+ sites across the state. It's not unusual to be hearing an agency on a site one minute, and then not hear another transmission for a long time, occasionally even in mid sentence when a radio moves from that site to another site.
There's more to it, but those are the basics.
Mike