P25 Digital Simplex: Multiple Talkpaths (Same Freq; but Different Talkgroup ID's)

[cmpsa]

All:

Curious question about P25 digital. I know that Motorola TRBO, in conjunction with a repeater only, you can have 2 talkpaths per frequency (though this is not the case in direct/simplex mode).

INTRO:
We have Kenwood & Vertex Standard P25 digital mobiles & portables that we use. I noticed in the programming software, the default setting for Vertex “Talkgroup ID" is "0001" (meaning talkgroup 1) and default setting for Kenwood is “Talkgroup ID” is “None” (meaning talkgroup 0).

So we decided to play around, create “talkgroup 1” and “talkgroup 2” in both radios using the same frequency (on purpose). When we talk on either talkgroup, the radio also receives the audio from the opposite talkgroup (which I thought it would not do that). So what I was trying to test here was, using the same freq {example 151.625 simplex NAC 300} is ch1 (t/g 1) & ch2 (t/g 2) and because it was the same NAC, ch1 & ch2 both receive (though each has a different talkgroup number). I’m sure if I was trunked, my theory would work the way I was thinking.

I was just playing around trying to see if it was possible for 2 people to talk on say 151.625 simplex NAC 300, and have another set of staff use talkgroup 2 for another conversation (still on 151.625 simplex NAC 300) and the conversations remain separate because of 2 separate talkgroups.

Thanks in advance for your input


QUESTIONS:
1. So am I confused that I thought P25 digital could do more than 1 talkpath (i.e. talkgroup ID #) on the same frequency?

2. If above is true, then the talkgroup setting in both radios would primarily be for a trunked system, not for direct/simplex mode?

 

[Thunderknight]

Yes, you can have multiple talkgroups on p25 conventional (simplex or repeated). Of course it's fdma, so only one person can talk at a time (1 talk path), but the conversations will be separated, like if the NACs were different..
Note that default on a lot of radios is to unmute on nac alone and ignore TG, so you need to set the digital squelch to nac AND talkgroup. It might be called selective squelch or something like that.

MotoTRBO is tdma 2-slot, hence 2 talk paths.

 

[jaaredmond]

Agree with Knight. I know that in the moto world that "normal squelch" opens to and TGID with matching NAC, where "selective squelch" opens with matching NAC AND TGID.

There should be a TGID that will open and transmit to all other TGs using the same NAC. So you could set up several TGs and simulcast to all of them.....or monitor all. I think moto uses "63533" off the top of my head.

 

[cmpsa]

awesome... Thank you BOTH for the valuable information; which has helped me understand what I was trying to do doesn't work...

 

[Jay911]

Agree with Knight. I know that in the moto world that "normal squelch" opens to and TGID with matching NAC, where "selective squelch" opens with matching NAC AND TGID.

There should be a TGID that will open and transmit to all other TGs using the same NAC. So you could set up several TGs and simulcast to all of them.....or monitor all. I think moto uses "63533" off the top of my head.

That seems to be an oddball value to use. I would suspect it would be more like 65535, which is a more "round" number when dealing with hexadecimal values (which is what most if not all radio systems "talk in" more or less).

 

[cg]

From this site:
http://www.danelec.com/pdfs/TG-001-3-0-0 P25 Training Guide.pdf

TGID $0001 default TGID where only 1 TGID is used
TGID $0000 group with no users, used for I-Calls
TGID $FFFF Reserved as a TG where everybody is included (65535)

chris