396XT & P25 Conventional channels

[natedawg1604]

So I live in the Broomfield area and I'm a fairly new user of the incredibly, amazingly awesome 396XT. Until recently I've been using my XT mostly to scan Public safety-related trunked systems in the Denver-Metro Area, namely local DTRS sites, Denver EDACS, Aurora EDACS, Lakewood P25 and Westy EDACS.

However I recently started running the scanner in conventional search mode on the 763-960 MHz range with the P25 NAC search filter enabled. Within a short time I began picking up a HUGE number of frequencies with NAC codes. All of the frequencies I've picked up so far come through very clearly, and it sounds like the radio units are all transmitting P25 digital voice.

My understanding is that whenever I'm running the XT in search mode/w NAC filter and I see a NAC code associated with a particular frequency, this always means I found a discrete P25 CONVENTIONAL Frequency (and NOT a "orphan" frequency belonging to a larger group of frequencies within a trunked system)? Is this correct? Because it sounds like I might be listening to individual DTRS site channels (or perhaps other P25 systems), but that doesn't make sense if NAC codes are only associated with Conventional P25 channels...

So far none of the frequency/NAC code combinations I found in the 763-960 MHz range are listed in the RR database. Take, for example, 851.9750/NAC D14 (hex). The RR database does list "851.97500" (with an additional fifth digit after the decimal point) as being associated with one or more trunked systems. But there is no 851.9750 listed with an associated NAC code.

 

[Spitfire8520]

The search you are running picks up any P25 voice transmission, which includes both trunked and conventional. All trunked systems still use certain assigned frequencies to carry voice transmissions, and are typically the frequencies you find in the trunked database that are not the control channel. From my understanding of digital trunking, all sites have a specific NAC ID assigned to them to use for P25 voice. Given the nature of Colorado, I would expect that almost all of the traffic you are picking up are apart of DTRS, or possibly from the Lakewood/West Metro/Wheat Ridge system.

The 851.9750 example you provided is associated with the West Creek (Denver Metro) site located in Southern Douglas County and does indeed use a NAC D14 (shown in the link). From the pattern I've seen, any NAC ID with the prefix of D1 is DTRS voice, most likely determined by the System ID D1C. The last value of the NAC ID seems to be the last value of each site's hexadecimal ID (West Creek's hex ID is 4).

I hope I was able to help you understand a little more.

 

[natedawg1604]

Thanks, that was definitely helpful. On a related note, why do most or all the of 700/800 MHz frequencies in Colorado get used for trunked systems, as opposed to conventional systems? Does any agency anywhere in Colorado used "Conventional" channels on 700/800, or is that considered a "waste" of valuable spectrum resources?

 

[Spitfire8520]

I wasn't here when Colorado had trunked systems popping up, so I can only make speculations based on information I've gathered. I would imagine the reason many agencies went to the trunked system rather than try to build their own conventional system (especially with DTRS) is to join a system that doesn't require a massive investment of time, money, and effort into building their own individual system. I think DTRS was started rather early in the whole nationwide 800MHz/trunked system movement, so many agencies at the time were not seeking to make the change just yet. I also think that since many agencies did move to DTRS, some saw DTRS as an effective way for interagency communications, as well as a way of improving radio coverage for areas outside of their immediate jurisdictions.

Now I think the biggest appeal of any trunked system as opposed to a conventional system is the number of "channels" agencies can have as opposed to having a conventional frequency dedicated to a channel that may not be used all the time. This can be easily seen in the database with some agencies having several dozen talkgroups (virtual channels), only needing to be supported by several frequencies. An example of this is Denver Public Safety Trunking System which has about 200 known channels/talkgroups for Police/Fire/EMS/Sheriff/Mutual Aid (not including stuff that isn't known, schools, etc). For those 200 channel/talkgroups, the system only utilizes 20 frequencies. If Denver stayed on a conventional system, they would end up not having the freedom of so many "channels" to use.

As for your final question, a limited amount of agencies do have individual licenses to 700/800MHz conventional frequencies. The more likely agencies to do so appear to be fire departments for simplex communications when trunked radio communications has poor reception (examples being South Metro, Littleton, and Douglas County). Otherwise, I don't believe I know of any agency that primarily operate on a 700/800MHz conventional system. The database lists a few, but I don't know whether or not the database is accurate as everyone who does 700/800MHz operate primarily on one of the five (soon to be six or seven) major trunked systems. There are also some agencies that have retained their older VHF/UHF equipment to use if they ever had a need for conventional communications, which is a possibility of why you don't see too many conventional 700/800MHz frequencies.

 

[ecanderson]

"There are also some agencies that have retained their older VHF/UHF equipment to use if they ever had a need for conventional communications"

Longmont is certainly an interesting example of this, though they've taken a somewhat different approach. Like most everyone else, they were on VHF prior to the introduction of the state DTRS system, but rather than scrap the VHF, they continue to simulcast several of their channels over both. What's interesting is that the VHF channels remain used as they were before, with a primary dispatch channel and a couple of others whose function varies depending upon need. When something unexpected (unplanned) gets 'hot' up here, and since the activity for such things begins on 'Channel 1', they leave that activity on 1 and move regular traffic to '2'. They've assigned TGs to each of their VHF channels (and added a couple of new ones) so that the parallel is clean in the simulcast.

What's amusing is that there are times when 151.22 is encrypted on DTRS TG #9518, but remains in the clear on VHF. More than once I've noted that during revenue enhancement ... er .. traffic enforcement 'events'.

 

[greenthumb]

What's amusing is that there are times when 151.22 is encrypted on DTRS TG #9518, but remains in the clear on VHF. More than once I've noted that during revenue enhancement ... er .. traffic enforcement 'events'.

Yep, a console patch will cause the transmission to be decrypted and then sent over the conventional channel in the clear.

Related to 800 MHz conventional use - keep in mind that most of the people involved in the regional planning committees (RPCs) here in Colorado for 700 MHz and 800 MHz are users of some kind of trunked system, so reserving a pair through those committees in the NPSPAC or 700 MHz bands for exclusive conventional use instead of shared trunked use will be more difficult to justify and have approved.