P25 One-Frequency Trunk vs. Conventional P25

trunker · #1 (**permalink**) 07-12-2010, 03:26 PM

Can someone please explain the difference between a P25 One-Frequency Trunk system and a P25 conventional channel?

Does the 1 Tr system have a control channel?

W2SJW · #2 (**permalink**) 07-12-2010, 07:45 PM

I just tried looking some stuff on Google for you about this, and did not come up with much success. Uniden's TWIKI has this to say:
"Single-frequency P25 systems are almost identical to standard P25 systems, except they only have one site per system and the system can use a P25 network address."

mikey60 · #3 (**permalink**) 07-12-2010, 08:14 PM

Quote:

Originally Posted by trunker

Can someone please explain the difference between a P25 One-Frequency Trunk system and a P25 conventional channel?

Does the 1 Tr system have a control channel?

The difference between them is in the data that's transmitted along with the voice.

A P25 Conventional system basically puts all radios on the system on a single talkgroup using a NAC to control access.

A P25 single channel trunked system also adds the concept of conventional talkgroups, where you must have both the correct NAC and the correct Talkgroup ID to hear and participate in the conversation. There is no control channel on a single frequency trunked system, all signaling of talkgroups is done in the data stream that is transmitted along side the voice data.

Mike

W2SJW · #4 (**permalink**) 07-12-2010, 08:25 PM

That's what I thought as well - I have heard that the TSA does that with the numerous simplex channels that they have in their portables - multiple channels with the same frequency, but different NAC & talkgroups assigned to each...

trunker · #5 (**permalink**) 07-12-2010, 10:21 PM

Quote:

Originally Posted by W2SJW

That's what I thought as well - I have heard that the TSA does that with the numerous simplex channels that they have in their portables - multiple channels with the same frequency, but different NAC & talkgroups assigned to each...

That's my situation exactly. I have 3 TSA freqs (with a known NAC on one that is 001) programmed in my Pro-106. But I'll only get about 2 or 3 hits a day if I leave the radio in hold mode on the frequency.

I've just acquired a 996XT and I'm wondering how to program those freqs into the radio; as a conventional P25 or a P25 1 freq trunk system?

I could try both ways to see witch way works when they transmit, but only if I sat in front of the radio all day. Not sure if 'Auto Search and Store' would catch it correctly.

ecps92 · #6 (**permalink**) 07-13-2010, 06:37 AM

TSA has been known to use Simplex [might you be too far away?] vs the Repeater, also TSA has migrated many of their radios from the P25 [Supervisors] to [same band] Analog radios [ICOM] with Speech Inversion encryption.

Quote:

Originally Posted by trunker

That's my situation exactly. I have 3 TSA freqs (with a known NAC on one that is 001) programmed in my Pro-106. But I'll only get about 2 or 3 hits a day if I leave the radio in hold mode on the frequency.

I've just acquired a 996XT and I'm wondering how to program those freqs into the radio; as a conventional P25 or a P25 1 freq trunk system?

I could try both ways to see witch way works when they transmit, but only if I sat in front of the radio all day. Not sure if 'Auto Search and Store' would catch it correctly.

trunker · #7 (**permalink**) 07-13-2010, 09:55 AM

I'm about 4 miles from the airport and when I do get a hit it's loud and clear-voice.

Maybe if I just spent a few hours at the airport I'd get more hits.

AA1IR · #8 (**permalink**) 07-13-2010, 10:00 AM

Quote:

Originally Posted by trunker

I'm about 4 miles from the airport and when I do get a hit it's loud and clear-voice.

Maybe if I just spent a few hours at the airport I'd get more hits.

ecps92 mentioned that they may use analog and digital on the same channel. If you have the channel programmed with a NAC, or if it is programmed as a single-frequency trunked system, then I believe you will not pick up an analog transmissions on that channel.

KS4VT · #9 (**permalink**) 07-13-2010, 06:30 PM

APCO 25 also allows for conventional talk-groups. It is very similiar to having a muti-tone ctcss panel in an analog repeater. I have programmed APCO25 radios to do this in conventional and it does work as the users on a different talk-group doesn't hear the traffic, but someone in conventional listening to in digital carrier squelch will hear everything.

trunker · #10 (**permalink**) 07-13-2010, 09:01 PM

Quote:

Originally Posted by AA1IR

ecps92 mentioned that they may use analog and digital on the same channel. If you have the channel programmed with a NAC, or if it is programmed as a single-frequency trunked system, then I believe you will not pick up an analog transmissions on that channel.

Good point. I'll set the channel to 'All'.

BTW, went to the airport with the freqs programmed as conv P25 and that works.

fordcobra04 · #11 (**permalink**) 07-13-2010, 09:08 PM

Quote:

Originally Posted by KS4VT

APCO 25 also allows for conventional talk-groups. It is very similiar to having a muti-tone ctcss panel in an analog repeater. I have programmed APCO25 radios to do this in conventional and it does work as the users on a different talk-group doesn't hear the traffic, but someone in conventional listening to in digital carrier squelch will hear everything.

Do you still need a system key to program Astro25 radios this way?

trunker · #12 (**permalink**) 07-13-2010, 09:10 PM

Quote:

Originally Posted by KS4VT

APCO 25 also allows for conventional talk-groups. It is very similiar to having a muti-tone ctcss panel in an analog repeater. I have programmed APCO25 radios to do this in conventional and it does work as the users on a different talk-group doesn't hear the traffic, but someone in conventional listening to in digital carrier squelch will hear everything.

Talkgroups? Does that mean I can still program the freq as a 1 freq P25 system and see talkgroups? (already confirmed it works as conventional P25).

WayneH · #13 (**permalink**) 07-13-2010, 09:28 PM

I think it's ridiculous they named it that. There's no trunking involved and it's no different than conventional P25 operation. A "talkgroup" (naming shared between trunking and conventional, yet are two different things) will always be sent with P25 conventional operation, same as with the NAC. Generally it's 1 (Moto for example) but some other vendors default to 65535. And, generally, a majority of the radios will not be setup to listen for specific talkgroups. Think of a talkgroup as a NAC within a NAC that the repeater does not care about (and will pass any one).

Another default TG is 4095, which is the default for Motorola DIU's (Digital Interface Unit). They're used to vocode digital voice for a dispatch console (since there is technically no radio to do it), logging (voice recording) systems or telephone patches.

So what Uniden is really saying is that their equipment can now decode and segregate P25 conventional talkgroups (on top of NAC decoding). It's just marketing terminology.

Toss that around any radio tech and people will look at you strangely.

bezking · #14 (**permalink**) 07-13-2010, 09:32 PM

Quote:

Originally Posted by fordcobra04

Do you still need a system key to program Astro25 radios this way?

No, you shouldn't need a Syskey to program CONVENTIONAL P25 Talkgroups.

fordcobra04 · #15 (**permalink**) 07-13-2010, 09:41 PM

Quote:

Originally Posted by bezking

No, you shouldn't need a Syskey to program CONVENTIONAL P25 Talkgroups.

Sounds like I just need to mess with the CPS some more... Thanks...

AA1IR · #16 (**permalink**) 07-14-2010, 05:50 AM

Quote:

Originally Posted by trunker

Good point. I'll set the channel to 'All'.

BTW, went to the airport with the freqs programmed as conv P25 and that works.

Did you notice if they were using both digital and analog?

jim202 · #17 (**permalink**) 07-14-2010, 07:14 AM

Quote:

Originally Posted by trunker

Can someone please explain the difference between a P25 One-Frequency Trunk system and a P25 conventional channel?

Does the 1 Tr system have a control channel?

Let me make a simple statement. You don't have a trunked system with only one channel. You need a control channel to have a P25 trunked system. Then the control channel will assign the talkgroups to a voice channel for the conversations. Generally you won't see a trunked system with less than 4 channels. One for the control channel and then 3 for voice.

With that said, you can have a P25 conventional channel where the voice is in the digital mode. Motorola normally calls this "Astro" mode. You can have a NAC and different talkgroups in this type of operation. I expect that this digital conventional mode is what your trying to describe.

MattSR · #18 (**permalink**) 07-14-2010, 07:36 AM

There is also a talkgroup field in P25 Voice signals. In true trunking its used to transmit the current talkgroup (like the old sub-audible data in SmartZone) However it can be used in the same way a NAC is used on a conventional channel to create logical user groups.

AA1IR · #19 (**permalink**) 07-14-2010, 11:17 AM

Quote:

Originally Posted by MattSR

There is also a talkgroup field in P25 Voice signals. In true trunking its used to transmit the current talkgroup (like the old sub-audible data in SmartZone) However it can be used in the same way a NAC is used on a conventional channel to create logical user groups.

The P25 transmissions also transmit a Unit ID number, which some departments have set to the radio callsign. You can also assign alpha tags to the UID numbers. Either way, if you do it that way, the ID of the unit transmitting shows up on the screen. I think that's more of the attraction of being able to decode the stream, than actually trying to use the talkgroup ID for anything.

Quote:

Originally Posted by jim202

Let me make a simple statement. You don't have a trunked system with only one channel. You need a control channel to have a P25 trunked system. Then the control channel will assign the talkgroups to a voice channel for the conversations. Generally you won't see a trunked system with less than 4 channels. One for the control channel and then 3 for voice.

With that said, you can have a P25 conventional channel where the voice is in the digital mode. Motorola normally calls this "Astro" mode. You can have a NAC and different talkgroups in this type of operation. I expect that this digital conventional mode is what your trying to describe.

He was referring to it as P25 one-channel trunking because on the scanner we were discussing, the BCD396XT, in order to decode the digital information sent with the transmission (Talkgroup ID, Unit ID), you have to program the frequency in a system with the type designator of "P25 single-frequency trunk". Regardless of whether it's the correct terminology or not, he was using it because that's how Uniden refers to it.

asd6845 · #20 (**permalink**) 07-15-2010, 10:12 PM

Quote:

Originally Posted by fordcobra04

Do you still need a system key to program Astro25 radios this way?

With the P25 radios, I have found that CPS lets you program the conventional P25 as a "mixed mode" in the receive section tab in the Conventional Personalities which will allow the radio to determine the mode when it receives the signal. So if TSA uses analog signaling, you hear it. If they switch to P25, you still hear it.

I did it for my Astro Saber and it works great.