Control Channel Only P25 Scanning Question

[jonwienke]

I use GPS to avoid scanning distant sites. Not scanning a site at all saves even more time than scanning only the control channels.

Since the active control channel is cached, there's basically zero benefit to deleting the voice channel frequencies when scanning sites in range.

 

[JoeBearcat]

I use GPS to avoid scanning distant sites. Not scanning a site at all saves even more time than scanning only the control channels.

Since the active control channel is cached, there's basically zero benefit to deleting the voice channel frequencies when scanning sites in range.

That's true, not evaluating a site at all is faster. And when it's in range, it should retain the Control Channel to return there first, but there is a point where you want to scan sites that are not currently in range and not far enough to be excluded by GPS. So the practice is still valid even if you use other mitigation methods. It's like disconnecting your feedline even though you have a lightning protection attached. Either practice is good. Both are better.

 

[jonwienke]

For the x36 models, the scan time difference is ~12.5ms * the number of voice channels the scanner scans before hitting the control channel the first time the site is scanned. For the SDS models, maybe 20ms/voice channel scanned. So worst-case scenario is maybe an extra half-second of scanning when you first boot the scanner or start scanning the system. That doesn't seem like a particularly big deal to me, given that it insures you against any control channel swaps in return.

 

[jr3792]

I use GPS to avoid scanning distant sites. Not scanning a site at all saves even more time than scanning only the control channels.

Since the active control channel is cached, there's basically zero benefit to deleting the voice channel frequencies when scanning sites in range.

I also use the GPS most of the time just trying to see if I can make it that much faster when traveling around. I have found even with GPS its still a slower scan but I like having the State PD and DOT on to stay on top of road conditions.

Dose the cache clear every time the scanner turns off?

Well if that makes me feel better if its recommended by you Joe.

 

[JoeBearcat]

For the x36 models, the scan time difference is ~12.5ms * the number of voice channels the scanner scans before hitting the control channel the first time the site is scanned. For the SDS models, maybe 20ms/voice channel scanned. So worst-case scenario is maybe an extra half-second of scanning when you first boot the scanner or start scanning the system. That doesn't seem like a particularly big deal to me, given that it insures you against any control channel swaps in return.

But that is only if all sites are in range. Otherwise, the time apples to every scan cycle for those sites not in range.

And if there is activity on these frequencies, that 12.5 mS goes up significantly while they are evaluated.

Aside from Harris and Explicit systems, there is no need for the voice-only channels. Some use the argument that a voice channel could be changed to a control channel, but the odds are greater a new frequency will be added to the system for that use. Either way the system radios have to be reprogrammed, so changing the control channels will be rare.

I admit this is a fine line, but anything that helps speed up scanning is a good thing - including your GPS.

 

[GTR8000]

Aside from Harris and Explicit systems, there is no need for the voice-only channels.

P25 systems using explicit signaling do not require all traffic channels to be in the scanner's programming. Those systems will still scan fine as 'control channel only', as the OSP provides the information on which frequency to tune to for a voice grant. That's the entire point of explicit signaling. Explicit signaling = DFA (Direct Frequency Assignment)

 

[JoeBearcat]

P25 systems using explicit signaling do not require all traffic channels to be in the scanner's programming. Those systems will still scan fine as 'control channel only', as the OSP provides the information on which frequency to tune to for a voice grant. That's the entire point of explicit signaling. Explicit signaling = DFA (Direct Frequency Assignment)

Implicit/explicit is one of those terms I can never keep straight. The one that needs all the frequencies programmed uses one-frequency-per-table like an LCN matrix, and any frequencies not programmed will not be scanned - ever. Maybe those are IMplicit systems.

The vast majority of systems are the ones that do not need the voice-only channels.

 

[GTR8000]

Implicit/explicit is one of those terms I can never keep straight. The one that needs all the frequencies programmed uses one-frequency-per-table like an LCN matrix, and any frequencies not programmed will not be scanned - ever. Maybe those are IMplicit systems.

Nope. Implicit systems are the majority of P25 in the UHF and 700-900 bands where the offsets are standard. Implicit system use the band plan in the subscriber and/or that is broadcast over the air.

Aeroflex has a PDF describing implicit and explicit in good detail, should be easy to find.

 

[JoeBearcat]

Nope. Implicit systems are the majority of P25 in the UHF and 700-900 bands where the offsets are standard. Implicit system use the band plan in the subscriber and/or that is broadcast over the air.

Aeroflex has a PDF describing implicit and explicit in good detail, should be easy to find.

OK. I was right the first time then.

EXplicit systems need all the frequencies (table) programmed. If you have a table that is not programmed with a frequency, and that table is assigned voice traffic, the scanner will not know where to tune to hear it. Again, it's like LTR which is not surprising given EF Johnson started that.

 

[KevinC]

OK. I was right the first time then.

EXplicit systems need all the frequencies (table) programmed. If you have a table that is not programmed with a frequency, and that table is assigned voice traffic, the scanner will not know where to tune to hear it. Again, it's like LTR which is not surprising given EF Johnson started that.

You're still wrong.

Neither needs all the frequencies programmed. I'm currently listening to a system with explicit entries on a SDS100 and I only have one CC entered and did not populate the bandplan (it got that info from the CC).

 

[JoeBearcat]

You're still wrong.

Neither needs all the frequencies programmed. I'm currently listening to a system with explicit entries on a SDS100 and I only have one CC entered and did not populate the bandplan (it got that info from the CC).

I wonder where is gets the info when the system says "go to table 5" and it somehow ends up on the correct frequency.

I guess it's possible they changed how explicit systems work. Or maybe there is more than one type of explicit system.

The one I am familiar with uses the trunking tables programmed with single frequencies, and the control channel never assigns anything other than the table number.

But again, the spec might have been updated.

Is the one you monitor designed by E.F. Johnson (who is now owned by Kenwood, so maybe that is when it changed)

Wait - if it got a BANDPLAN from the control channel, that is an implicit system. Explicit systems do not send those.

 

[KevinC]

I wonder where is gets the info when the system says "go to table 5" and it somehow ends up on the correct frequency.

I guess it's possible they changed how explicit systems work. Or maybe there is more than one type of explicit system.

The one I am familiar with uses the trunking tables programmed with single frequencies, and the control channel never assigns anything other than the table number.

But again, the spec might have been updated.

Is the one you monitor designed by E.F. Johnson (who is now owned by Kenwood, so maybe that is when it changed)

Negative on EFJ, very large MSI system.

 

[JoeBearcat]

Can you post a RR link to it? If not, can you PM me the link?

To my knowledge, MSI never made explicit systems.

Or is GTR wrong and the vast majority of systems are EXplicit and it is IMplicit systems that are the rare ones?

While I have freely admitted my confusion, it seems there is conflicting info above.

 

[JoeBearcat]

Part of the confusion might be that we could be talking about two separate areas of programming.

The rare P25 type requires all frequencies to be programmed IN THE TRUNKING TABLE (AKA bandplan).

Now, is that implicit or explicit?

The common P25 type gets the bandplan automatically from the control channel. This is the vast majority of systems out there.

Is this type implicit or explicit?

According to Aeroflex, IMplicit systems must be preprogrammed with the system frequencies. Only the table number is transmitted and the radio is programmed to tune to that specific frequency.

So, above it would be in the order: IMplicit, and the common P25 systems are EXplicit.

That means IMplicit systems must be programmed with all system frequencies. Otherwise, when it is told to tune to table (LCN) 5, and no frequency is programmed in slot 5, it will not tune to the correct frequency and you will not hear the voice traffic on that channel.

 

[GTR8000]

You are making a big mess of this whole implicit vs explicit thing. I don't even know where to begin, but you are interpreting a few things incorrectly. This applies to all P25 systems, regardless of manufacturer.

Implicit: All traffic channels are assigned via the channel ID plan, aka the band plan. This band plan is normally programmed into the radio and is transmitted over the control channel. Traffic channels are assigned via a single block TSBK, referencing only the Ch ID. Implicit messaging is the most common form of P25 system, by virtue of the UHF, T-Band, 700, 800, and 900 bands all using standard offsets, which are normally handled pretty easily within a 16 slot band plan.

Explicit: Traffic channels are assigned via a multi block TSBK, which explicitly spells out the RX and TX frequencies, and does not rely on a pre-defined band plan entry. Explicit signaling is most commonly used with VHF systems, which lack standard offsets, and where a 16 slot band plan is simply not enough to cover all possibilities.

Note that a system can use a combination of implicit and explicit signaling. The PA-STARNet system, for example, uses implicit signaling for the UHF, 700, and 800 MHz sites within the system, whereas the majority of VHF sites are using explicit signaling. There are some VHF entries in the band plan, however that does not cover all of the possible channels used on the system, and thus explicit signaling must also be used.

It's all spelled out on Page 3 of this document: https://www.viavisolutions.com/en-u...nctions-discontinued-application-notes-en.pdf

 

[GTR8000]

Let me put it a different way, since this thread is about a Uniden product scanning a P25 system.

As far as Uniden is concerned, if the scanner is capable of "control channel only" scanning of a P25 system, then nothing else needs to be done. Where implicit signaling is used, the scanner will receive the band plan from the control channel, and will use those Channel ID's to tune to traffic channels. Where explicit signaling is used, the scanner will decode the multi-block TSBK and will know what traffic channel to tune to based on that OSP.

Let's not over-complicate this by bringing other system types or various P25 manufacturers into the mix.

 

[KevinC]

Can you post a RR link to it? If not, can you PM me the link?

To my knowledge, MSI never made explicit systems.

Or is GTR wrong and the vast majority of systems are EXplicit and it is IMplicit systems that are the rare ones?

While I have freely admitted my confusion, it seems there is conflicting info above.

Greater Austin/Travis Regional Radio System (GATRRS) Trunking System, Austin, Texas

Greater Austin/Travis Regional Radio System (GATRRS) Trunking System Profile

www.radioreference.com

 

[JoeBearcat]

You are making a big mess of this whole implicit vs explicit thing. I don't even know where to begin, but you are interpreting a few things incorrectly. This applies to all P25 systems, regardless of manufacturer.

Implicit: All traffic channels are assigned via the channel ID plan, aka the band plan. This band plan is normally programmed into the radio and is transmitted over the control channel. Traffic channels are assigned via a single block TSBK, referencing only the Ch ID. Implicit messaging is the most common form of P25 system, by virtue of the UHF, T-Band, 700, 800, and 900 bands all using standard offsets, which are normally handled pretty easily within a 16 slot band plan.

Explicit: Traffic channels are assigned via a multi block TSBK, which explicitly spells out the RX and TX frequencies, and does not rely on a pre-defined band plan entry. Explicit signaling is most commonly used with VHF systems, which lack standard offsets, and where a 16 slot band plan is simply not enough to cover all possibilities.

Note that a system can use a combination of implicit and explicit signaling. The PA-STARNet system, for example, uses implicit signaling for the UHF, 700, and 800 MHz sites within the system, whereas the majority of VHF sites are using explicit signaling. There are some VHF entries in the band plan, however that does not cover all of the possible channels used on the system, and thus explicit signaling must also be used.

It's all spelled out on Page 3 of this document: https://www.viavisolutions.com/en-u...nctions-discontinued-application-notes-en.pdf

Neither of those are the system type I am talking about. The type I'm talking about assigns voice channels by a simple assignment to "Table X". No bandplan is sent over the air and the radios must be programmed with the frequency for each table.

In RR, the system frequencies have a small number next to the frequency like an LCN that says which table that frequency belongs to.

It seems the terms 'implicit/explicit' changed in the past few years since neither of your definitions seems to apply to these systems.

Most systems were one type, but others were the rare other type.

 

[JoeBearcat]

Let me put it a different way, since this thread is about a Uniden product scanning a P25 system.

As far as Uniden is concerned, if the scanner is capable of "control channel only" scanning of a P25 system, then nothing else needs to be done. Where implicit signaling is used, the scanner will receive the band plan from the control channel, and will use those Channel ID's to tune to traffic channels. Where explicit signaling is used, the scanner will decode the multi-block TSBK and will know what traffic channel to tune to based on that OSP.

Let's not over-complicate this by bringing other system types or various P25 manufacturers into the mix.

If you program the system type I'm talking about with the control channel only without programming the table, it will never know where to tune to hear the voice traffic. The "Base frequency" is the specific frequency used by the system and only one frequency per "bandplan" is used.

 

[JoeBearcat]

Greater Austin/Travis Regional Radio System (GATRRS) Trunking System, Austin, Texas

Greater Austin/Travis Regional Radio System (GATRRS) Trunking System Profile

www.radioreference.com

That looks like a regular P25 system that uses bandplans.