On Sunday, May 6, 2018 at 2:36:42 PM UTC-4, Mr. Luddite wrote:
On 5/6/2018 2:31 PM, Its Me wrote:
On Sunday, May 6, 2018 at 12:57:02 PM UTC-4, Mr. Luddite wrote:
On 5/6/2018 12:08 PM, wrote:
On Sun, 6 May 2018 11:20:24 -0400, "Mr. Luddite"
wrote:

On 5/6/2018 10:34 AM, Keyser Soze wrote:
Mr. Luddite wrote:


Describes the process:

https://www.yahoo.com/news/toughest-gun-law-america-120137258.html


Seems perfectly reasonable...



I think so too. The one thing I don't think they mention in the article
that would probably offend some is that when you buy a gun from a FFL
or privately (and do it legally) a record of that gun, it's model and
serial number is entered into a MA database so the state knows who owns
what ... again, assuming it was purchased legally. That's why I have
mentioned the chain of custody thing so often.

I recently bought a scanner mainly to monitor the recreational aircraft
that fly from a nearby airport. I also programed the local police and
fire frequencies in it as well. It's interesting that when a traffic
stop is made, the license plate (tag) and driver's license is radioed to
the dispatcher who checks to make sure registration and license is
current and for any warrants, etc. The dispatcher also reports back
to the cop who stopped the car if the person with that license also has
a license to carry. I didn't know they did that.

It is interesting that a scanner still works there. Cops are starting
to encrypt their feed and most are using frequency hopping digital
packet radio. My old Bearcat stopped working decades ago.


Some, like the MA State Police have used the freq hopping trunking
system for years but most of the smaller towns and cities still have
dedicated, assigned freq channels in the 400 to 500 Mhz UHF band. I
have my town, Plymouth and several other nearby towns programed and
receive them all. Bought a small UHF whip (only about 16") and mounted
it on the eve of the roof of the single story house. Works great.
Can hear both dispatch and the individual cop cars 5 by 5.

Yeah, most of the public safety traffic has gone to P25 radio systems which are usually digital and encrypted, although they can be analog and unencrypted. As you say, there are local municipalities that are still on old analog UHF, but they will be moving to P25 as funds free up to move them.

The "frequency hopping" you mention is actually called trunking. You have a number of assigned frequencies (say, 15) and the radio system has a smart controller. One of the frequencies is used as a control channel. When a field unit wants to key up and talk, it is actually a request out over the control channel for a freq. The controller assigns that talk freq, and the unit then keys up on that freq. Instead of a radio being tuned to a specific channel, it tunes to a talkgroup. A talkgroup is just a collection of field radios, like police, fire, water, roads, admin, etc. When a radio that is tuned to the police talkgroup keys, it may be told to transmit on channel (freq) 8. All other radios tuned to the police TG are told over the control channel to also tune to freq. 8. They all then have their conversation. After a period of silence, freq 8 is freed up for other TGs to use..

For example this way you could have 25 TGs, but only 10 freqs. You essentially share the freqs. There are only so many freqs to go around, and the spectrum is crowded. This is more efficient, but the danger is that if it gets busy and all of the freqs are in use, you can get a busy tone when you try to key and not be able to talk for a few seconds.

There are several different "flavors" of trunking including MOTOTRBO, NXDN, P25, DMR, Tetra, etc. P25 is the "cadillac" open standard here in the US for public safety.



Thanks for the explanation. I knew that trunking utilized freq hopping
but I never knew exactly how it worked. Learned something.

Are the freqs they use still in the UHF bands?

Most of it are in the 800-900 bands, but there were (still are?) trunked system in lower bands as well.

Most of the old radio systems used 25Mhz channel spacings to broadcast their FM (frequency modulation) signals. They needed that much room to swing the frequency around to get the signal out.

Now with P25 for example, you can send your signal out over 12.5Mhz, and with Phase II P25, 6.25Mhz (narrowbanding). You can pack four voice transmissions in the bandwidth that used to carry only one. In heavily populated areas the spectrum is *that* crowded.