Ok first of all, don't get your panties in a wad.. It's just a trunked system. I too am learning about trunking systems rather quickly. I know you're very confused right now, and that's cool, because when I got started in trunking, I thought I'd never understand it either. If you've got some time to kill, I reccomend checking out the Radio Reference Wiki. It will give you a brief rundow of what trunking is all about, better than I can explain it. But for the sake of typing, I will give you my best, and easiest description of how it works.
Ok, you say your friend drives an ambulance. Now whether he knows it or not, when he's in that ambulance talking on the radio, there's LOTS of things going on, that are enabling him to communicate.
Instead of an assigned frequency, as in an conventional system, the ambulance service is assigned a set of "codes". Known as talkgroups. One talkgroup may be for Dispatch to the ambulance, another may be so the ambulance can talk with the hospital or ER.
Here's where it gets confusing. At any one time, the ambulance can be on DIFFERENT frequencies. These frequencies will most likely be in the 800Mhz range. Let's say the ambulance gets dispatched. Ok, so someone falls off their roof, and calls 911. The dispatcher will dispatch the ambulance and they roll out of the garage. Let's say for the sake of talking, they're transmitting on 857.6375. Ok, down the road, dispatch calls the ambulance again, to tell them the victim is having a seizure. This time, their frequency might be 859.6375. All the while, the police are talking about taking a donut break, and they're on 857.6375, where the ambulance was just heard!
How's it all done? Talkgroups. There is a central "know it all" computer, that chooses which frequency gets transmitted on. Your BCT 8 scanner is designed to listen to the control channel (You won't hear nothing but a bunch of warbles here) and decode what it's "saying".
Confused yet?
