Trunked Systems

[LeoG]

Just curious at what they are.  I've been using my radio to listen to the local police, fire and EMS frequencies that I've gotten off the interwebby and just recently they put in a new police station and the calls on the radio have decreased.  I see most the other towns are using a trunked system.

Not sure if I understand what it is. 

Is it where they have a pool of frequencies and when there is a communication need the trunked system picks out a frequency for the system to use that isn't currently busy?

If they had 10 different frequencies usually lets say the first 3 are normally used but when they are in use the system will send it out on an unused frequency?  I'm sure the receiving radios are setup to receive the communication from whatever frequency likely designated by some tone or digital system.

So if I wanted to listen and get all of the communications do I have to find out what all the trunked frequencies are and then scan all of them?  Or is this just way off base?

[BoxCar]

That's correct. Trunked systems pick an unused channel for each transmission. A conversation can have multiple transmissions. l

[LeoG]

So frequency hopping to boot?

[gortex2]

Sorta yes. In a P25 MSI system you would have 1 control channel that is alwasy control. Then the remainder of the system is available for voice traffic. However depending on the system channel capacity this gets challenging. For a 6 channel TLMR system you could have 3 channels set for control channels, 1 channel for BSI witch leaves 2 that are first used for voice or data calls. In reality you have 5 channels for use but it is managed to use certain channels before others. Now DMR is different but most P25 stuff is similar.

 

[WRXL702]

After the controller receives a valid / assigned ID for the unit transmitting (P25, LMR or conventional 800/900 trunking) - It allows the received transmission to be sent to the next available repeater for re-transmission, in that assigned talk group. One station (except UHF LMR) is commonly used as a control channel / station to continually communicate with all valid units for use in the system. 
 

P25 systems also evaluate the signal strength & may move the received transmission to a different tower location in the system, still in the same talk group for re-transmission.

[LeoG]

Not sure what the system is.  Everything I've been listening to has been in the 150-155Mhz range so far.  It's been very clear and has amazing range 15-30 miles I've heard it.

They just built a new police station and have been moving in.  I hear them doing lots of radio checks and sending out the EMS and Fire tones.  I've noticed that some of the communications have gotten weaker and have hiss and scratchiness to them when it use to be nearly perfect other than HT inside buildings.

Recently the traffic is down from what I'm used to and the wife isn't to happy about that as she enjoys listening to the chatter.  Like to keep the little lady happy and keep receiving the communications. 

I believe the old system was 3 transmitters on a single tower but that's just a guess.  The new system on top of the new building has multiple antennas that I haven't gotten a good look at yet.  All in a row, maybe 8 of them with a short (24"?) blue dipole on top.  Haven't noticed if they have a folded dipole on them or not.