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.

 

[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.

[gortex2]

I foget your in Maine ? Where abouts ? 

 

[LeoG]

Connecticut.

[WRKC935]

An 'In the weeds" explanation of a trunked radio system, at least the MSI P25 version. 

The way this all works, at least at 700/800 Mhz, is every frequency is assigned by the FCC a channel number.  These channel numbers are set in steps of a specific channel spacing and start frequency.  With VHF it's not that way, VHF requires the channels to be programmed into the radio with TX and RX frequencies and the channel number is assigned by that programming.

So as mentioned there is a 'control channel' that is effectively a data channel.  All radios use this to connect to the system.

When a radio is turned on it finds the control channel of the closest site and then announces it's presence.  The system will acknowledge that radio and do one of three things.  It will OK the radio to be on the system, it will deny access or it will if told to by the system admin send a stun / kill command to that radio and disable the radio.

If the radio is allowed on the system, that system will process the radios request for access to a specific talkgroup.  The radio talkgroup is remembered by the system and the site it's connected to.  When that talk group becomse active the control channel will tell the radio to go to the specific assigned channel for the traffic and begin listening.

It will pick the channel in b ased on a couple things.  First is the standard round robin assignment.  The next available channel on the site is used.  But certain channels can be set to NOT use that talk group, so it can skip certain channels.  It can also use lowest channel TX time to equalize the TX loading on each channel based on total time of transmit for each channel at a site.

Now, since you didn't mention that you had to reprogram your scanner, your not hearing different conversations on a specific channel that was only for police previously, or any of the other stuff.  I am gonna say that you are probably NOT listening to a trunked radio system. 

Back to the programming and channel assignment.

When you setup your scanner, if it's 700/800, you will only need to put in the control channels and the stepping information.  The scanner will know what frequencies are assigned to what channel numbers with that information.  If they used VHF or UHF frequencies, you may / will need to program all used frequencies manually. 

Refer to Radio Reference for programming information on your specific system for more information

 

[LeoG]

The scanner is generally my Tidradio H3 or my KG1000+.  I just went online and searched the scanner frequencies for my town and it said some frequencies which I programmed in and have been listening to for a few months.

No real idea what kind of a system it is.