Q on P25 800mhz digital system operations.

[Slyster]

Was wondering.. when I am listing to my local P25 digital system... I know the tower (control channel freq) sends out control information... but what else?

Does the tower send out just the dispatcher's voice..? and all the PD cars I hear are from THEIR mobile antennas to my scanner?... or is it first sent to the tower and the rebroadcasted like some sort of repeater or amplifier? (I'm talking the home towers.. not the simulcasts where available)

Just confused on the actual workings of the system.

I also notice the VC channels are on a large number of frequencies... but where are these signals actually coming from.. the mobile/hh antenna? Or from the 'tower'

[BaLa]

Trunking Systems, are just a number of repeaters linked together.

[mikeydcg2003]

From what i know of 800 mhz systems. The Control channel send out the ID of the TG and what freq it is assigning to the conversation.
The Wiki site linked below provide good detail of how the different trunking systems work. I am still waiting for a good digital system to come online where i live to really test it out. I hope to get out to visit my friend one of these days to really understand it better. He lives in a city that moved to all digital.

http://wiki.radioreference.com/index.php/Trunking

[Stick0413]

Originally Posted by Slyster

.
I also notice the VC channels are on a large number of frequencies... but where are these signals actually coming from.. the mobile/hh antenna? Or from the 'tower'

Dispatch to Tower to units Mobile/Handheld (and your Scanner)... Units Mobile/Handheld to the Tower to Dispatch (and your Scanner)

[Slyster]

Tx.. I'll read the wiki.

So.. when I hear mobile/handheld.. it really is going from their small antenna to mine and not through the systems 'tower' I am programmed for?

[n2mdk]

No everyone is going through the system then to your scanner. Lets see if I can explain a simple repeater. It listens on one freq. known as the input and transmits on another in the RR Db called the frequency. All radios in the field go into the receiver then get retransmitted out.
Dispatch can work the same way or through a local mike and directly out of the transmitter. Most dispatches aren't at the transmitter site so they can go through the system like the field units do, go through a link frequency into the system, or even over special dedicated phone lines.

[n4voxgill]

the control channel has no voice on it. It only sends signals that direct radios in talk groups to various frequencies. When the dispatcher of a talk group keys up the mike, the systems selects the frequency that will be used and sends a signal to the dispatchers equipment and all mobiles and portables on that talk group telling them all what frequency to use. All of this happens in a blink of the eye. Thus when the dispatcher talks, the voice goes out on the frequency that was assigned, not on the control channel, and the units in the talk group all hear the transmission. The same thing happens if a portable initiates the conversation. The portable keys up and the system selects the frequency and sends the command to everyone with that talk group selected and they can hear and talk.

[DiGiTaLD]

Originally Posted by Slyster

Does the tower send out just the dispatcher's voice..? and all the PD cars I hear are from THEIR mobile antennas to my scanner?... or is it first sent to the tower and the rebroadcasted like some sort of repeater or amplifier?

You are correct the second time. You are not hearing the signal directly from the subscriber radio. You are hearing it through the system.

Originally Posted by Slyster

I also notice the VC channels are on a large number of frequencies... but where are these signals actually coming from.. the mobile/hh antenna? Or from the 'tower'

From the tower. The input channels that the repeaters listen to the mobiles and portables on are usually 45 MHz lower if on 800 MHz.

This site provides some good information about how a trunked system operates. You can also find some good stuff in the wiki.

[loumaag]

Everything asked and explained in this thread so far is covered in the RR Wiki Trunking Basics article, which is the first suggestion in the Trunking article linked above by mikeydcg2003. Wayne's site (linked by DiGiTaLD) in the previous post is a great site for more detail and will provide much information especially on Motorola systems.

[Stick0413]

Originally Posted by Slyster

Tx.. I'll read the wiki.

So.. when I hear mobile/handheld.. it really is going from their small antenna to mine and not through the systems 'tower' I am programmed for?

No when I said "and your scanner" in my earlier post it was meant a units base/handheld and your scanner from the tower. It would go to the tower then your scanner

From dispatch TO the tower TO your scanner / units handheld / mobile

From the units handheld / mobile TO the tower TO your scanner / dispatch.

[N_Jay]

They don't call 'em "repeaters" for nothin!

[Slyster]

One reason I ask.. is that is SEEMS that the distpatcher can be nice and clear.. but the responses (mobile unit) will be weak...

if all was coming from the tower.. wouldn't both always come it clear?

[bigdaddyduke]

it could be that the signal into the repeater (mobile unit) is weak. The repeater will 'repeat' what it hears - either a strong signal or a weak signal

Chris

[Slyster]

ic. i thought it would boost it to 'full signal'... but that could explain it.

[DiGiTaLD]

Originally Posted by Slyster

ic. i thought it would boost it to 'full signal'... but that could explain it.

It can't boost what isn't there. It repeats what it hears. Think of it like this: its giving a tiny portable radio a huge transmitter to use to extend its coverage area, but if that tiny portable radio isn't making it in to the repeater clearly, the repeater will repeat the crap signal it hears. Fortunately, repeaters (and trunked systems) should have their antennas located at an optimum location to receive the weak signals of portables and mobiles clearly - more clearly than a mobile or portable talking direct to another mobile or portable several miles apart.

Of course, the more complex the system, the less simplistic this explanation becomes. Single-site conventional repeaters repeat received audio locally. When you get into conventional systems with multiple repeaters, you could be getting audio from a number of remote receiver sites. Simulcast, be it conventional or trunked, gets even more complex. Then you have your Smartzone Omnilink and other wide area networked systems, where in some cases, no audio is repeated locally (unless the system is site trunking) but all of that is well beyond the scope of this post.

[N_Jay]

Originally Posted by Slyster

ic. i thought it would boost it to 'full signal'... but that could explain it.

Yes, if you receive the tower "full signal", the repeated portable will also be "full signal", but the information it is carrying will be only as good or bad as the tower is receiving.


Hmmm, just what field of science do you teach?

[Slyster]

biology

I think I am thinking of this on a deeper level than some of you assume.

These is a DIGITAL system.. it would rebroadcast BETTER (i.e. more strength=my scanner can decode it better) since it isn't the data that's not coming through it's the strength of that data.. so my scanner has decoding/garbled issues (not for dispatcher, but mobile units) see what I mean? My scanner might be garbled and I hear the data (hiss) if the signal isn't STRONG enough... therefore.. the repeater should retransmit it at full strength.. therefore I would hear it just fine. The repeater wouldn't rebroadcast "weak signals".... in this way it's very different than analog

(EDIT: of course, if the actual repeater is too far from the mobile so it receives the message garbled, nothings going to help that)

[loumaag]

Originally Posted by Slyster

biology

I think I am thinking of this on a deeper level than some of you assume.

These is a DIGITAL system.. it would rebroadcast BETTER (i.e. more strength=my scanner can decode it better) since it isn't the data that's not coming through it's the strength of that data.. so my scanner has decoding/garbled issues (not for dispatcher, but mobile units) see what I mean? My scanner might be garbled and I hear the data (hiss) if the signal isn't STRONG enough... therefore.. the repeater should retransmit it at full strength.. therefore I would hear it just fine. The repeater wouldn't rebroadcast "weak signals".... in this way it's very different than analog

(EDIT: of course, if the actual repeater is too far from the mobile so it receives the message garbled, nothings going to help that)

With the edit you are correct. Either the repeater (with error correction) is getting a good signal or it is not. There will be the case as in when the signal level drops low enough that you will hear a garbled signal (often times the user call it "pixelating") just before the repeater drops the signal. Now if you hear it normally, than it is your radio not the repeater. I don't see where you say what scanner you are using, if it is a Uniden, try adjusting the decode parameters, if it is a RS ... oh well.

[N_Jay]

Originally Posted by Slyster

biology

I think I am thinking of this on a deeper level than some of you assume.

These is a DIGITAL system.. it would rebroadcast BETTER (i.e. more strength=my scanner can decode it better) since it isn't the data that's not coming through it's the strength of that data.. so my scanner has decoding/garbled issues (not for dispatcher, but mobile units) see what I mean? My scanner might be garbled and I hear the data (hiss) if the signal isn't STRONG enough... therefore.. the repeater should retransmit it at full strength.. therefore I would hear it just fine. The repeater wouldn't rebroadcast "weak signals".... in this way it's very different than analog

(EDIT: of course, if the actual repeater is too far from the mobile so it receives the message garbled, nothings going to help that)

Hu?

You usually don't get "hiss" with a weak digital signal.

Lets say the repeaters receiver gets a weak signal. Some of the packets are corrected to perfect, and some are corrected to close to perfect (but with errors).
The repeater transmits the corrected packets, some match the original perfectly, others are approximations. This signal picks up further errors when received by your scanner.
You scanner tries to present the best reproduction possible, but it is not perfect.

You have to think about the signal as in layers. RF Signal strength is only one portion of the system.

[loumaag]

Originally Posted by N_Jay

Hu?

You usually don't get "hiss" with a weak digital signal.

Lets say the repeaters receiver gets a weak signal. Some of the packets are corrected to perfect, and some are corrected to close to perfect (but with errors).
The repeater transmits the corrected packets, some match the original perfectly, others are approximations. This signal picks up further errors when received by your scanner.
You scanner tries to present the best reproduction possible, but it is not perfect.

You have to think about the signal as in layers. RF Signal strength is only one portion of the system.

That's what I said...well almost...well sort of anyway.