CHP and crazy Nac's

P25Radio

Member
Premium Subscriber
Joined
Feb 6, 2006
Messages
340
769.18125 799.18125 RMDAE NACTAN1 EXT PTan 1 - Extender - Primary P25 Law Dispatch
769.66875 799.66875 RMDB5 NACTAN1 EXT ATan 1 - Extender - Alternate P25 Law Dispatch

Never seen a NAC DAE or DB5 do they cross reference to something? I think this is the first time I have seen that.
 

K6CDO

Member
Premium Subscriber
Joined
Jan 12, 2003
Messages
1,194
Location
San Diego, CA
NAC - The RadioReference Wiki

"The Network Access Code or NAC is a feature of Project 25 digital radios similar to CTCSS or DCS for analog radios. That is, radios can be programmed to only break squelch when receiving the correct NAC. NACs are programmed as a 3 digit hexadecimal code that is broadcast along with the digital signal being transmitted.

Since the NAC is 3 digit hexadecimal number (12 bits), it gives 4096 possible NAC's for programming, which far exceeds its analog counterparts combined. It should be noted that 3 of the possible NACs have special meaning:
  • 0x293 ($293) - the default NAC
  • 0xf7e ($F7E) - a receiver set for this NAC will unsquelch on any NAC received
  • 0xf7f ($F7F) - a repeater receiver set for this NAC will allow all incoming signals and the repeater transmitter will retransmit the received NAC.
Note that the above is true for commercial grade radios only; setting a scanner to NAC F7E is not the same as setting it to 'NAC Search'. "

$DAE and $DB5 are well within the range.
 

Anderegg

Enter text in this field
Premium Subscriber
Joined
Mar 7, 2010
Messages
2,042
Location
San Diego
The CHP have 16 normal extender frequencies...each has a specific NAC. When an extender channel is resused, the NAC doesn't change, so the NAC always is the same for the specified frequency.

I didn't see an extender frequency for the Blue, but a few months ago there was a big street racers thing in Encinitas requiring units to use the Blue 1, and discovered Blue 1 uses the same extender channel as Orange San Diego...same NAC and everything.

Paul
 

P25Radio

Member
Premium Subscriber
Joined
Feb 6, 2006
Messages
340
While on the subject of CHP ect, What channel is use when Borrego Springs or Ocotillo wells get dispatched by CHP.
 

Anderegg

Enter text in this field
Premium Subscriber
Joined
Mar 7, 2010
Messages
2,042
Location
San Diego
I am interested in the El Cajon vs Resident talkgroups...they are always patched, but the hit logs for north and west seem to show El Cajon tg gets a lot more affiliations than resident on West, but I think less on North? I am not sure how patching works out in the sticks on the N/E and rural sites.

Paul
 

d119

Member
Joined
Dec 19, 2002
Messages
68
Location
The Internet
Most likely the selected channel in the car is what is heard on the extender channel. One NAC is used for STATION, One NAC is used for CAR, and one NAC is used for portable-to-portable.

At least thats what I would guess. I don't think they are using channel steering on their hand helds for anything other than STATION and CAR (repeat and talkaround).

If the car radio is set to ORANGE-1, you hear ORANGE-1. If it's set to BLUE-1 you hear BLUE-1. Extender frequency doesn't change.
 

Anderegg

Enter text in this field
Premium Subscriber
Joined
Mar 7, 2010
Messages
2,042
Location
San Diego
Well, if I was only hearing the Orange extender in Oceansides area where all the units from SD, Oside, and El Cajon were converging, there must be some sort of preprogrammed slaving going on right, or I would have been hearing more than one extender frequency, and what if the unit switches to the Orange to talk but the extender is in use for Blue? Maybe I will ask a CHP officer.

I was going off the RRdb for all 16 extender freqs having set NACs.

Paul
 

d119

Member
Joined
Dec 19, 2002
Messages
68
Location
The Internet
Well, I have a few theories on how the "new" CHP extender system works.

Again, these are only theories, but have SOME backing based on conversations I've had with state communications techs in the course of my employment.

It doesn't appear that the P25 extenders have any sort of "priority hierarchy" like the VHF units did, in other words, they are full-duplex units that don't "unkey" periodically to check for handheld activity, the handheld can just key up at any time and get on the air, much like a traditional repeater.

That being the case, my guess is that the first unit on scene turns on their extender, and additional arriving units just work through the existing active extender.

-OR-

Each vehicle is "assigned" an extender channel number, and the officers portables are set accordingly. Or perhaps the handhelds are assigned to the vehicle?

Whatever the case may be, there is DEFINITELY a hellacious problem with multiple extenders being on at the same time, as it's much more common to hear heterodynes when an officer is talking on his or her portable than used to be. THAT is an officer safety issue.

Something tells me the state isn't getting the mileage out of this 700MHz P25 extender thing that they thought they would. At least it works most of the time.

Oh, and Paul, probably time to change that post signature, what with UPman going 10-7 and all.
 

scannerboy02

Member
Joined
Nov 16, 2004
Messages
1,342
This is something I have been trying to figure out for many years. I was VERY familiar with the old analog extender system and that system would transmit a tone when an officer activated the extender that would turn off all other extenders at a scene and the extender that transmitted the tone would be the only active unit. I have not noticed this 'tone' system with the new P25 VRS. If you program the VRS frequencies into a scanner that has One Frequency Trunk (OFT) capability you will see that the system seems to act like a private call between the portable and the car so my guess is they are using the radio ID to direct the portables transmission to the correct car. How they keep from having interference on a transmission from several cars to the portables I don't know.

On the Consolidated Patrol Vehicle Environment (CPVE) touchscreen they have the ability to set a primary VRS channel and a secondary VRS channel in the car. From my understanding each patrol car has the ability to transmit two different VRS channels, one primary and one secondary. So as an example, the car can re-transmit the CHP low band channel on the primary VRS channel and also transmit a local PD channel on the secondary VRS channel and the officer can change channels on the portable to hear the traffic. I have been told the officer can also select what the VRS re-transmits from the car by switching channels on the portable and pressing the PTT to make the CPVE select the channel the officer wants to hear. This is done by transmitting a different NAC on the VRS 'input' frequency.
 

Anderegg

Enter text in this field
Premium Subscriber
Joined
Mar 7, 2010
Messages
2,042
Location
San Diego
A CHP officer cleared things up for me. Their radios are zone oriented, so there would be a San Diego area office zone, an Oceanside area office zone etc, and the VRS assignments would be preprogrammed into the corrosponding area zone. The portables will operate on the VRS assigned to the area main they are on, and sounds like they can change portable channel to access Blue, which the car radio would steer to, then drop back to the main after a timeout with an alert. Doesn't sound like they can switch mains or areas from the radio (can't steer except to Blue), but there is a generic 16 channel VRS zone that they can manually hit all the main VRS frequencies if they can't get to the car for whatever reason to switch the car radio over...or if the car explodes or something. There is also a Division VRS, maybe that is "BOR BASE" in the RRdb, that allows an officer to stay on the base VRS (on portable) while changing the car radio to whatever.

Scannerboy...they have many instances where multiple cars are extendering at the same time and makes it impossible for the dispatcher to hear them...they have to tell them to go turn off someones extender.

Paul
 
Top