P25 Trunked Systems

[HHD1]

Just bought my Katie a Uniden sds100 for her B-Day.  She literally cried she was soooo happy.  I got it pretty well programmed with some help from YouTube.  But I've been trying to research p25 and trunked systems just to try to learn about it, and DUUUUUDE!!!!  Most of the info I find, I'll just say, I need to do research on the research.  I'm not sure why it's so hard for me to find info that's not designed to make things more confusing.  Maybe something with illustrations and not some monotone nerd staring at the camera whilst droning on like he reads a dictionary for fun.  

Anyhow, I'm super intrigued by p25 and trunked systems.  And I have a feeling it's easier to understand than what I'm finding so far. Does anyone here have any links to good sources of info to learn about these systems?

[tweiss3]

I guess what do you want to know about them? Does trunking radio make sense to you, or are you stuck on how that is done?

[HHD1]

The idea of trunking is confusing to me. Control frequencies, talk frequencies, DEC's... not sure how it all works.

[UncleYoda]

Maybe you're too young to remember the old phone system when AT&T (and then regional Bells) owned everything.  But back then every town of any size had a local phone office.  And there were more copper wire lines coming in from customers than there were between offices.  The lines between offices were the "trunk".  And it's one reason why we got the message all circuits are busy.  Public service and 911 dispatch have more officers in the field than they have frequencies for.  So trunking systems choose an available frequency for the officers/firemen/EMS to communicate without them having to switch around to find an open channel.  But, trunking isn't something we need or would even benefit from.  We are better off with our independent stations and the frequencies we have.  I knew firemen in my hometown who were buying UV5Rs to get around some of the limitations of their official radios.

[Lscott]

This is a training guide.

 

P25 Training Guide Codan.pdf

[WROQ359]

Not so much info to learn about the trunking systems, but an almost complete listing can be found here for your local area https://www.radioreference.com/ There is a forum as well like here with plenty of info on the radio systems in use and tips for programing your new toy. 

[HHD1]

32 minutes ago, WROQ359 said:

Not so much info to learn about the trunking systems, but an almost complete listing can be found here for your local area https://www.radioreference.com/ There is a forum as well like here with plenty of info on the radio systems in use and tips for programing your new toy. 

Radio Reference is great.  I subscribe.  

[HHD1]

On 9/12/2025 at 4:03 PM, UncleYoda said:

Maybe you're too young to remember the old phone system when AT&T (and then regional Bells) owned everything.  But back then every town of any size had a local phone office.  And there were more copper wire lines coming in from customers than there were between offices.  The lines between offices were the "trunk".  And it's one reason why we got the message all circuits are busy.  Public service and 911 dispatch have more officers in the field than they have frequencies for.  So trunking systems choose an available frequency for the officers/firemen/EMS to communicate without them having to switch around to find an open channel.  But, trunking isn't something we need or would even benefit from.  We are better off with our independent stations and the frequencies we have.  I knew firemen in my hometown who were buying UV5Rs to get around some of the limitations of their official radios.

That's kinda what I was thinking.  So a set of frequencies, and a bunch of ID's, and a controller to decide which ID's use which frequency?

[nokones]

I bet "some people" more like a lot, probably don't even know the meaning of P25 and where/how/why it originated?

[tweiss3]

Yea, to put it in very simple boxes: Trunked radio has a handful of frequencies (channels) at a site, one way or the other, you have a "control channel" which assigns traffic to an available channel. When all channels are in use and you try to key up, you get bonked (the radio makes a bonk tone, different than the proceed-to-talk tone that you get when your voice traffic is granted).

P25 is just an implementation of that, but there are other digital versions using DMR, NXDN and dPMR. There were also analog versions of trunking as well, like Motorola Type II (Viking16 is a currently compatible Kenwood option) where traffic could be analog or P25 digital, or LTR, where the control data was in the subaudible tone portion of the analog signal directing voice traffic to the correct channel.

[HHD1]

After playing around with the Sentinel software,  I started to see how it all works.  Very cool.  You all are providing such great feedback.  Man, I love this place. . Lately I feel like I'm getting assimilated, like the Borg.  I am no longer Jake... I am WSHD560. LOL. Resistance is futile. The radio dorkness is taking hold.   Randy!!!!!  Please Help!!!! 

[WRZS246]

The Association of Public-Safety Communications Officials (APCO) developed a trunked standard called Project 25, or P25 for short. It is widely used in the U.S. by police and emergency responders.

[tweiss3]

Yea, the Sentinel software makes it way easier. You don't have to understand exactly how the trunked systems work, but you do need to know what you intend to listen to.

 

I need to remember to update the SD card on the Trucks SDS200 now that I think about it.

[Lscott]

On 11/29/2025 at 11:01 PM, WRZS246 said:

The Association of Public-Safety Communications Officials (APCO) developed a trunked standard called Project 25, or P25 for short. It is widely used in the U.S. by police and emergency responders.

To make it more confusing there is a Phase-1 and Phase-2 version of the protocol. The Phase-2 version is for the moment used only for trunking.

[PACNWComms]

Had to specify radios for the MH-139A Grey Wolf helicopter, and its civilian affiliated missions......original specifications only stated "P25 compatible". Took a year to get some Collins engineers to understand: analog-conventional, Frequency Division Multiple Access (Phase 1), and Time Division Multiple Access (Phase 2). 

Once the Air Force got involved (and they realized I was retired and knew the Huey/and variant predecessor well), they let me go for the jugular and carotid.....educated a few engineers on trunked radio standards. 

18 helicopters fielded and I am now educating some other engineers on "Flash"(ing) Motorola and affiliated radio hardware, for specific options (TDMA for example). The AeroConnect RT-7000 has a Motorola APX8000 card in it, with all the associated drama of the big /\/\ involved (as well as Cobham/AeroConnect). 

FRS/GMRS is being added to a zone for search and rescue, trunked P25 Phase 1/2 for interop with law enforcement, fire, and emergency medical in another, and other zones dictated by the helicopters home base. 

The last lesson will be the NIFOG (National Interoperability Field Operations Guide). 

[nokones]

Back in my day in designing Avionics for Public Safety aircraft like the Hughes 500D, Bell Rangers and Long Rangers, A-Stars, Cessna 180 and 185s, Wolfsburg radios for the FM stuff and King Silver Crown for the AM stuff and and a bunch of custom audio mixer panels and other required flight equipment was the hot ticket and the aircraft was very heavy laden. The weight and balance was a bit of a challenge the flight crews had to manage the weight of the fuel loads.

In those days, the radio interoperability on Trunk 800 Meg systems had to be with appropriate portable radios from the respective agencies and the audio was interfaced through the audio mixtures because Wulfsburg did not have trunk capable RT units.