Understanding P25 trunking in DSD+

[garys]

I am the first to admit that I don't understand much about how DSD+ processes P25 trunking. Especially, I'm not sure about a lot of what I'm seeing in the CC.bat console window.

I entered the control channel on a 700Mhz P2 x2 TDMA channel. I understand that DSD+ doesn't process Phase 2 as of yet, so that's not a problem.

Here is what I'm seeing that I don't understand.

I get all of the system voice and control channels. Those coincide with the information in the Wiki and other sources.

The display looks like this,

1-2012 774.58125 CC

The rest of channels follow that same format. the 1-xxxx numbers are the preassigned channel designations for specific frequencies.

I also see this,

3-1340 770.15625-1
3-1341 770.15625-2

From the appearance, these seem to be Phase 2 TDMA channels. The only problem is that these aren't licensed to the system. They don't show up on my x36HP display, which does process Phase 2. There isn't a lot of Phase 2 activity on this system and what there is is mostly encrypted.

I have no idea what these frequencies represent.

I will also occasionally see one of the active frequencies display in bright blue, italicized format, with L/O next to it. That also shows individual RIDS, but no activity in the VC.bat console window.

I've read through the notes.txt files in DSD+, but don't see any information on any of this.

One other thing I've noted is that after a while, P25 decoding will stop. I have to restart FMPA and DSD+ to restart decoding. Occasionally, the CC.bat console window will show NEXEDGE96 and nothing will display in the console window.

Any ideas will be appreciated.

Thanks.

 

[GTR8000]

I entered the control channel on a 700Mhz P2 x2 TDMA channel.

A bit of housekeeping: Phase II TDMA and X2 TDMA are two different protocols that can't coexist on the same system. A system is one or the other, not both. X2 TDMA was Motorola's early implementation of TDMA, prior to the final standard for APCO's Phase II TDMA being adopted. There are little to no X2 TDMA systems left; they have almost certainly been converted to Phase II by now.

1-2012 774.58125 CC

3-1340 770.15625-1
3-1341 770.15625-2

Those frequencies are part of the allocation of the 700 MHz spectrum designated for the states, so it's safe to assume that you are monitoring a statewide system.

From the appearance, these seem to be Phase 2 TDMA channels.

They are indeed TDMA channels. The -1 and -2 indicate the time slot for the frequency.

The only problem is that these aren't licensed to the system.

You're not going to find licenses because the state 700 MHz allocations are covered by blanket SL licenses. In other words, the states are not required to license each frequency/site combination, something rather unique to the 700 MHz band. The states have a lot of leeway as to what frequencies can be provisioned at whatever sites throughout the state they choose. It essentially comes down to them coordinating with neighboring states to prevent interference.

They don't show up on my x36HP display, which does process Phase 2. There isn't a lot of Phase 2 activity on this system and what there is is mostly encrypted. I have no idea what these frequencies represent.

If those frequencies are not showing up on your scanner, then they are likely encrypted voice calls that the scanner is ignoring.

I will also occasionally see one of the active frequencies display in bright blue, italicized format, with L/O next to it. That also shows individual RIDS, but no activity in the VC.bat console window.

Most likely data calls.


Your best bet would be to decode this system/site(s) with Unitrunker to get a clearer picture of what is really going on.

 

[garys]

A bit of housekeeping: Phase II TDMA and X2 TDMA are two different protocols that can't coexist on the same system. A system is one or the other, not both. X2 TDMA was Motorola's early implementation of TDMA, prior to the final standard for APCO's Phase II TDMA being adopted. There are little to no X2 TDMA systems left; they have almost certainly been converted to Phase II by now.

Thank you for clarifying this. I've looked for this information, but haven't been able to find it. A related question, maybe more appropriate for the Uniden forum, is why the RR downloads into ARC536 set the system type to X2 TDMA. I do most of my listening to this system on one of my x36HP scanners, which is why I ask. On DSD+ the console just shows P25. As I said, probably a question better asked on the Uniden forum.

Those frequencies are part of the allocation of the 700 MHz spectrum designated for the states, so it's safe to assume that you are monitoring a statewide system.

This correct. It's a statewide system. Or will be if they ever get it built out.

They are indeed TDMA channels. The -1 and -2 indicate the time slot for the frequency.

I see that now. When I did a restart of DSD+, the console window shows them under the heading of TDMA as opposed to Channel. I don't know why it didn't display that way earlier.

You're not going to find licenses because the state 700 MHz allocations are covered by blanket SL licenses. In other words, the states are not required to license each frequency/site combination, something rather unique to the 700 MHz band. The states have a lot of leeway as to what frequencies can be provisioned at whatever sites throughout the state they choose. It essentially comes down to them coordinating with neighboring states to prevent interference.

That makes sense. Since Phase 2 use on this system is currently limited, it's not easy to catch these things on a scanner. It's interesting to know that there are TDMA only frequencies. Which probably makes sense from a technical standpoint.

If those frequencies are not showing up on your scanner, then they are likely encrypted voice calls that the scanner is ignoring.

For whatever reason, the x36HP scanners don't mute or ignore encrypted talk groups on P25 systems.

Most likely data calls.

Your best bet would be to decode this system/site(s) with Unitrunker to get a clearer picture of what is really going on.

I haven't used Unitrunker for a while as I mostly use my set up to follow DMR systems. I'll have to go back and look at Unitrunker, download the latest version and see if I can configure it.

Thanks for the information, it's helped a lot.