Instead of explaining because I'm at work, I copied a Harris employee trying to explain it on a thread for an agency near here that uses it, installed by Harris
" Amherst County Dispatchers have pre-programmed paging schemes in their consoles.
If the Agency being alerted HAS retained their VHF-Highband pagers, the 155.520 MHz (192.8 Hz) transmitter AND that Agency’s ‘quiet’ talkgroup (one group the range of TGID 1111 through 1120) will be simulselected, and the first set of paging tones transmitted. Then, a second or two later, the paging tones will be transmitted a second time on the Fire Dispatch talkgroup (TGID 1100), with a voice announcement of the type of call and details.
If the Agency being alerted HAS NOT retained their VHF-Highband pagers, the Agency’s ‘quiet’ talkgroup (one group the range of TGID 1111 through 1120) will be simulselected, and the first set of paging tones transmitted. Then, a second or two later, the paging tones will be transmitted a second time on the Fire Dispatch talkgroup (TGID 1100), with a voice announcement of the type of call and details.
Keep in mind, the simulselect does not transmit the call on two different talkgroups simultaneously. What actually occurs is when two or more talkgroups are paired in a simulselect, the system randomly chooses an open SAID (System Assigned talkgroup ID), and the call is actually transmitted over the SAID group, and not the individual talkgroups in the simulselect.
Ditto when two or more talkgroups are patched together. The actual calls are made on a SAID group, and not the individual talkgroups in the patch. This the technique that was utilized in APCO Project-16 (EDACS, LTR, Moto various flavors, etc.). It worked correctly, it still does today in Project-25 systems.
Actual user radios on the system constantly monitor the Control Channel for group call assignments and other information. When a normal group call is made, the user radios receive the broadcast on the control channel to shift to working channel X, to receive a group call on talkgroup X.
In the case of a simulselect or patch, the user radios receive the broadcast on the control channel that if you (user radio) is monitoring talkgroup X, then to shift to working channel X, to receive a group call on SAID talkgroup X. Actual system user radios do this automatically, and always have.
BUT, for some goofy reason, the Scanner Manufacturers have never been able to wrap their collective minds around this simple concept and implement it into their products! This causes you to have Listeners report hearing patched (or simulselect) calls on all sorts of wild talkgroups, usually above 50,000.
In the average P-25 systems I have installed, there are several hundred talkgroups reserved into a pool for SAID assignments. Some System Engineers allocated these talkgroups as Static (IE: same group used for specific patch), while other Engineers opted for greater flexibility and used Dynamic.
I personally always opted for Dynamic. "
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