Control Channel Only P25 Scanning Question

[KevinC]

One more thing, if you look at the site controller codeplug for any of the sites using VHF I can 1,000% guarantee you it will show "explicit" as the channel assignment type for the VHF frequencies. You probably don't have access to that, but if you did you'd see I'm correct.

 

[GTR8000]

I want to clarify a couple of things that I don't think I clearly stated in previous posts. Hopefully this will clear things up a bit, I'll try to be short and to the point.

• All sites of a P25 system broadcast the same band plan (Channel ID, "16 entry table", etc.) throughout the entire system, regardless of whether implicit or explicit signaling is used
• Both implicit and explicit signaling use that band plan in some form to allow the subscribers to calculate channel frequencies
• Implicit signaling means that when there is a channel grant, the control channel need only assign the repeater's TX frequency (i.e. the subscriber's RX frequency). In other words, the repeater's input frequency is implied, as it follows standard offsets for the band (UHF, 700, 800,900)
• Explicit signaling means that when there is a channel grant, the control channel must assign both the repeater's TX frequency, as well as the repeater's RX frequency (i.e. the subscriber's TX frequency). In other words, the repeater's input frequency is explicitly provided to the subscriber, as there is no standard offset for that band (VHF)
• Both methods of signaling will use the band plan to calculate the appropriate frequencies.
  • An 800 MHz implicit channel grant would give only the repeater's TX frequency, e.g. 00-027 (851.250), and the input is automatically determined to be 806.250, as the standard offset for 800 MHz is -45 MHz.
  • A VHF explicit channel grant would give both the repeater's TX frequency, e.g. 07-66C (152.055), as well as the input frequency, e.g. 09-AFC (158.515), as there are no standard offsets in the VHF band, so both must be provided
• Implicit grants only require a single block, whereas explicit grants require two blocks. In plain English, it's the difference between a one line or two line message going out over the control channel. Explicit signaling is obviously less efficient because it needs two lines to get its message out.

To circle back to the original sticking point, the plain fact is that most sites of most P25 systems are using implicit signaling, because most sites of most trunked systems happen to be in the UHF, 700, and 800 bands. Explicit signaling is used less simply because there are less VHF P25 trunked systems out there, although as evidenced by a few very large statewide systems, that number is growing.

It's not correct to assume that all Motorola P25 system are implicit, or that all Harris P25 systems are explicit, or anything like that. P25 systems for the most part follow at least the basic TIA-102 standards, and as such both methods of signaling are available and may be used on any P25 system, regardless of manufacturer.

That's it in a nutshell. Does any of this even have any relevance to Uniden's Control Channel Only scanning? Probably not so much, as all modern P25 systems broadcast and all modern Uniden scanners receive all of this info, and know exactly what to do with it, simply by decoding the control channel messages.

 

[GTR8000]

I wish I could direct you to the system type I'm talking about, but the one I knew of seems to have been updated to a Harris system.

The P25 frequencies had those little black numbers next to them like an LTR system so you knew which slot to program the frequency in.
Do you recall any of those?

I don't think I can search other systems, but other ones (if they still exist) are likely to be in the UHF Fedband.

Joe, is it possible that you're referring to an EDACS system? Most of those have been or are being replaced with Harris P25 systems, seeing as Harris inherited EDACS from what used to be GE. EDACS systems have LCNs which are noted in the RRDB with those small numbers next to each frequency.

 

[JoeBearcat]

I very much doubt it was a P25 system if it didn't transmit IDEN_UP messages, aka a Channel ID plan, aka a Band Plan, aka the 16 channel table.

It was absolutely a P25 system. It was sold as such, and decodes as such.

You are forgetting about one key fact, which is that VHF does not use standard offsets (inputs), and in many cases sites are using a variety of inputs that don't necessarily jive up with that broadcast band plan. You may have a repeater output of 155.670 in the western part of the state using 159.08 as the input, whereas a repeater output of 155.670 in the eastern part of the state uses 151.8175 as the input. Now, do you see the problem with that 16 channel band plan, and how it could not possibly cover every permutation of output/input for VHF? The other bands aren't an issue...+5 on UHF, +30 on 700, -45 on 800...done.

I'm not forgetting that. I realize there are separate TX frequency broadcasts.

But that still uses the same bandplan for those calculations. It might say RX is 10-234 (155.295) while TX is 10-567 (155.71125). Obviously those are made up numbers for the channel, but the point is that it uses the same bandplan for both TX and RX, so that one plan does cover all frequencies and all offsets that are reasonable to expect.

Run Unitrunker or your favorite P25 decoding software on your local STARNet VHF site(s), and enable control channel logging. Look at the raw log whenever there is a TDMA transmission on VHF, and look for the multi-block signaling. That is explicit mode, using two TSBK's to provide both the RX and TX frequencies for the channel.

I view those all the time.

If you're lucky enough to be within range of an 800 site, which I believe that you are, do the same and watch how all of the channel grants are single-block signaling referencing the Ch ID from the band plan. That is implicit mode, using one TSBK to direct the radio to reference the channel plan/band plan/16 channel table.

Agreed that there is no TX broadcast.

Band plan = "16 table entry". You are literally describing the same thing using different terms. Any site in a system that relies solely on the band plan/16 table entry/Channel ID plan/etc. is using implicit signaling, period. If the channel assignments are not able to be covered by the 16 entries in the table/band plan, then the system must use explicit signaling to some degree.

Well, I consider one criteria of a bandplan to be a band segment and not an individual frequency. "Bandplans"that consist of single frequencies I call a frequency.

Since the 16 entries cover all system requirements (like the Starnet example), I see the need for explicit signalling (as you have described it) to be virtually non-existent.

If you want to take this discussion off the forum so we can go into more detail, feel free to PM me and we'll either chat there or via email. This is probably getting a bit tedious for most users on the forum.

I think the feelings expressed by Mstep might be shared by many. It would be nice to get this resolved once and for all.

Years ago when this EFJ system was being discussed, the P25 experts said it was an implicit system and was very rare and that most systems used explicit channel assignments. (again, I may have the terms implicit/explicit reversed).

Now, you are telling me those experts were wrong. It's a hard pill to swallow even if you have proof, as there is proof on their side, too.

Personally, I just want to understand this.

 

[JoeBearcat]

@JoeBearcat If I'm coming across as impatient or salty, I apologize. I'll take a few deep breaths and check back tomorrow to see if I've clarified anything or if we need to go through some more examples, or a deep dive to try to find out what system you might be thinking about from yesteryear.

I understand your position and I appreciate you sticking with this and the civil discussion.

There may be some of the more technical aspects I might like a PM on - perhaps you can explain the basic P25 system and we can go from there. I realize that systems using bands with no standard offset require separate TX assignment broadcasts, but maybe you can put this in the technical terms you want to use. If you want to do that in this thread, that is fine too.

 

[JoeBearcat]

In your example how does the system calculate what TX frequency the subscriber should use for VHF? It can't as VHF doesn't have a standard TX/RX split, whereas 7/800 and UHF do. So the TX OTA assignment must be coming form somewhere else and that somewhere else is the multi-block messages (which have both the TX and RX frequencies the subscriber is to use for that channel assignment in them, hence explicit).

The OTA assignment is a separate 'packet' (may or may not be the correct technical term) but uses the same bandplan.

You may be on to something, though, as the system in question is in the Fedband that traditionally used "non-standard offsets" (this may be one of the first systems to use the standard offset - I do not recall. I will have to look that up). It may have gone on using separate TX frequency assignments, and maybe that is what made it implicit (or explicit).

 

[JoeBearcat]

Joe, is it possible that you're referring to an EDACS system? Most of those have been or are being replaced with Harris P25 systems, seeing as Harris inherited EDACS from what used to be GE. EDACS systems have LCNs which are noted in the RRDB with those small numbers next to each frequency.

Definitely not. They had a compatibility issue with Motorola P25 radios. They worked for talkgroups, but not for icalls, for example.

BTW, I have not read your post #65 yet - I will do that when I can devote more time to 'absorb' it. I have skimmed over it.

 

[GTR8000]

Since the 16 entries cover all system requirements (like the Starnet example), I see the need for explicit signalling (as you have described it) to be virtually non-existent.

Explicit signaling is needed on the STARNet VHF sites because there is no standard input offset for the VHF frequencies. That is why both the RX and TX channel assignments must be broadcast in explicit mode. My post # 65 breaks that down.

Years ago when this EFJ system was being discussed, the P25 experts said it was an implicit system and was very rare and that most systems used explicit channel assignments. (again, I may have the terms implicit/explicit reversed).

Now, you are telling me those experts were wrong. It's a hard pill to swallow even if you have proof, as there is proof on their side, too.

I never said "the experts were wrong", and in fact I'm telling you the exact same thing: that IMPLICIT systems are the most common, with EXPLICIT being less common and normally only found where VHF is in the mix, ala STARNet.

Of course it's not that cut and dry, as STARNet actually uses a mix of implicit AND explicit signaling, exactly as one would expect to find on a system that is mixed band. VHF = explicit, UHF/700/800 = implicit.

Easiest way to keep it straight is as follows:

IMplicit - the repeater inputs are IMplied because they use standard offsets
EXplicit - the repeater inputs are EXplicitly broadcast as there are no standard offsets (VHF)

 

[JoeBearcat]

Explicit signaling is needed on the STARNet VHF sites because there is no standard input offset for the VHF frequencies. That is why both the RX and TX channel assignments must be broadcast in explicit mode. My post # 65 breaks that down.

My comment (first quote) was using what I have always understood 'explicit' to mean.

Let me absorb post 65 and catch up.

 

[KevinC]

Of course it's not that cut and dry, as STARNet actually uses a mix of implicit AND explicit signaling, exactly as one would expect to find on a system that is mixed band. VHF = explicit, UHF/700/800 = implicit.

As does the system I linked earlier in the thread. You "could" actually have a mix of both within the same site if you needed to.

 

[GTR8000]

As does the system I linked earlier in the thread. You "could" actually have a mix of both within the same site if you needed to.

Indeed, seen it in action!

Without getting too into the weeds until some basic concepts are understood, it's also the reason why the maximum active calls per site can vary once explicit signaling is in the mix. The more explicit channels that are required, the lower that maximum, as you run up against the limitations of the control channel's bandwidth and ability to keep up with all of the grants and continuation messages. Explicit signaling can become very inefficient very quickly at a large site with lots of explicit channels! (I know that you already know this, just mentioning it for everyone else reading)

 

[JoeBearcat]

OK. I've read your post #65 (now #62). I went back to reconcile that with the system in question, and I can see why I am confused. System X only sends a table number for the voice assignment, and the radio knows table 2 for example is 407.3875, and tunes there. Now, I do not know if the system sent a TX channel assignment or if it was programmed to use the (NOW) standard offset (+9 MHz). This system went on around the time the standard was created. So, it may have been implicit or explicit based on the terms in post #62.

Here is what I remember about it:
1. It was made by EF Johnson (now replaced with a Harris system and probably a different P25 scheme)
2. It was P25
3. It has 5 frequencies
4. It would not populate the bandplan automatically
4a. No system decoders ever showed that a bandplan was transmitted (it would not be needed)
5. The spacing in the table did not matter because each frequency was a SEPARATE table entry
6. Since this was an EFJ system, I can understand why they wanted to "LTR-ize" it - it was their hallmark.
6. The Uniden scanners would not trunk it unless you programmed the table as follows (I can post this since it's in the RR database now):



Now, obviously with a traditional P25 system they COULD have used one table for this system (such as 406 MHz base and 12.5 or 6.25 kHz step) IF it used a traditional bandplan scheme. (which would traditionally be in table 4 since 0-3 are the 700/800 MHz standard entries)

The system operator called this system Explicit (I think), and perhaps it was sending separate TX assignments if it was put on before the standard bandplan was used - or even if it came on about the same time since all previous FedBand systems would have had to be Explicit.

It does appear to be assigned frequencies per the revised bandplan, but again maybe it was developed under the 'old scheme' when the offsets were not standard.

So, based on the above posts (from GTR and Kevin) this appears to be what would be an "Explicit LTR" flavor of P25 trunk system. Explicit in that the TX frequencies are (assumed) to be distributed on the control channel separate from the RX, and LTR because there is only one frequency per table.

Does this make sense? Let me rephrase that... does it appear I have this correct? Because I agree from engineering this does not make sense to use one frequency per table ALA LTR. Maybe that is why EFJ was bought out.

AND IF this system type is known by some other name, please let me know.

 

[GTR8000]

I would just point out that when you're looking at the band plan that a Uniden scanner decodes and saves, you're not seeing the full picture. The actual band plan includes things like the Transmit Offset +/-, the Transmit Offset MHz, and whether the channel is FDMA or TDMA.

It's entirely possible with that EJF system you reference that none of those five frequencies had a standard offset, and so they had to create a custom table to account for each repeater output and the corresponding input. If that were the case, then yes the system would be considered implicit because all of the information regarding the TX offsets would've fit neatly within the channel plan, and would not have required the use of explicit channel grants.

Or maybe they just did something silly, because hey...why not. Certainly not the norm, however.


Let's continue with that premise and look at the STARNet system again, and maybe we can understand why the VHF channels have to use explicit signaling, whereas the UHF, 700, 800 do not.

This what the STARNet system's ASTRO 25 Channel ID table (aka band plan) actually looks like in full when it's programmed into an APX subscriber. Pay particular attention to the fact that all of the VHF entries in the table have +3.2 MHz transmit offsets. I think we can agree that it's fairly obvious that there is zero chance that every VHF repeater in that system has the input exactly +3.2 MHz above the repeater output! This is precisely why some explicit signaling is required, i.e. why the control channel must specify both the TX and RX Ch ID for sites that use VHF channels, because it's not nearly enough to imply (implicit mode) that the offset is going to be any sort of standard +/-.

 

[JoeBearcat]

I would just point out that when you're looking at the band plan that a Uniden scanner decodes and saves, you're not seeing the full picture. The actual band plan includes things like the Transmit Offset +/-, the Transmit Offset MHz, and whether the channel is FDMA or TDMA.

It's entirely possible with that EJF system you reference that none of those five frequencies had a standard offset, and so they had to create a custom table to account for each repeater output and the corresponding input. If that were the case, then yes the system would be considered implicit because all of the information regarding the TX offsets would've fit neatly within the channel plan, and would not have required the use of explicit channel grants.

Or maybe they just did something silly, because hey...why not. Certainly not the norm, however.


Let's continue with that premise and look at the STARNet system again, and maybe we can understand why the VHF channels have to use explicit signaling, whereas the UHF, 700, 800 do not.

This what the STARNet system's ASTRO 25 Channel ID table (aka band plan) actually looks like in full when it's programmed into an APX subscriber. Pay particular attention to the fact that all of the VHF entries in the table have +3.2 MHz transmit offsets. I think we can agree that it's fairly obvious that there is zero chance that every VHF repeater in that system has the input exactly +3.2 MHz above the repeater output! This is precisely why some explicit signaling is required, i.e. why the control channel must specify both the TX and RX Ch ID for sites that use VHF channels, because it's not nearly enough to imply (implicit mode) that the offset is going to be any sort of standard +/-.

View attachment 101562

Yes - that is the same table info I posted above (currently post #54). I just didn't include the TX offset.

Good point about the offset perhaps being different in each table. I don't have the TX data (at least I can't find it using a quick search).

But you can see how System X is (was) very much unlike nearly any other P25 system.

 

[GTR8000]

So then, are we finally good here?

 

[JoeBearcat]

I think so. Back 10 years ago or so, that was one of the only explicit (I think) systems around, and I was told that was because the bandplans were not used. The REAL reason is because the TX frequencies were not standardized. (which maybe they were - maybe they weren't).

Now, either here or via PM can you explain the various terms used to send the data on the control channels? I know these systems from a user and a programmer standpoint, but not from an engineering standpoint (that is for the engineers to know since they write the code, but I'm curious).

Oh, and what would you call System X? Explicit LTR P25 system?
How do we classify a system that does not use bandplans to differentiate it from nearly every other P25 system?
(and no, I don't think calling it a CF is appropriate - even if accurate)

 

[GTR8000]

I thought you got it, but it appears you didn't. You keep going back to stuff like "does not use bandplans" and calling it "explicit", when we pretty much determined (as best we could with the limited info) that it was indeed implicit if it was using those 5 entries in the bandplan. And no, let's not call it an "LTR P25 system", as there is not and never has been any such thing...implicit, explicit, or otherwise. We're not going to start inventing terms just to classify something that was, in all likelihood, not really that big of an anomaly.

Perhaps @KevinC and myself are not the ones to try to explain this to you, as we seem to be going around in circles here.

 

[JoeBearcat]

I thought you got it, but it appears you didn't. You keep going back to stuff like "does not use bandplans" and calling it "explicit", when we pretty much determined (as best we could with the limited info) that it was indeed implicit if it was using those 5 entries in the bandplan. And no, let's not call it an "LTR P25 system", as there is not and never has been any such thing...implicit, explicit, or otherwise. We're not going to start inventing terms just to classify something that was, in all likelihood, not really that big of an anomaly.

Perhaps @KevinC and myself are not the ones to try to explain this to you, as we seem to be going around in circles here.

I did not tie the lack of bandplans to explicit or implicit. I only said I WAS TOLD THAT 10 years ago. That is a fact. I was told that. I asked what type of system do we call one that does not use bandplans (but rather individual frequencies in the table). I ask that because that system type requires that the table be manually programmed - UNLIKE virtually all other P25 systems.

What I was asking is how we classify something that does not fit any other existing definition.

Again:
What would you call System X? Explicit LTR P25 system?
How do we classify a system that does not use bandplans to differentiate it from nearly every other P25 system?
(and no, I don't think calling it a CF is appropriate - even if accurate)

If you do not have an answer for that, that is fine.