Help Understanding Trunking Details

[fletch97]

I'm just looking to better understand trunking, in particular the Motorola Type 2 version. When I look at a list of frequencies for a system that uses this type of setup, I notice a control channel, several alternate control channels, and then several frequencies.

My main question is about the alternate control channels--what role do they play? Are they only used when something is wrong with the primary control channel? Perhaps they're used when someone tries to transmit and the control channel is in use? When not being used as a control channel, can they be used to transmit voice data? Do they have some other role?

Other than that, if anyone wants to correct my basic understanding of how it works, feel free--this is what I think happens. . .
1. Someone pushes to transmit
2. A signal is sent along the control channel (or possibly an alternate control channel?) to some controlling piece of hardware (I'll call it a server for lack of a better term).
3. This server then determines if there is an available voice frequency. If not, it sends a signal back to the transmitter denying a transmission (user might get a certain beep or tone). If so, a frequency is reserved for whatever talk group the user is transmitting on, and the user gets another tone indicating that they can talk.
4. Any voice communications while the transmit to talk button continues to be depressed is sent via this reserved frequency. Either this frequency or the control channel (?) tells all other users what talk group this frequency is associated with thus determining who hears it (based on what talk groups they are set up to listen to).
5. While talking, as other frequencies are used for different talk groups, their info is sent out over the control channel, possibly causing an interruption in what the end user is hearing based on the priority of talk groups on the user's radio.
6. As soon as the transmit to talk button is released, the server releases the frequency and it's open for someone else to use. Any one else responding on the same talk group may or may not end up using the same frequency depending on the server.

Am I way off? Thanks for helping someone new to the field try to understand what's going on!

 

[mlevin]

Actually you're very close. just a few corrections to get you pointed in the right directions.

Some trunking systems rotate control channels on a regular basis. Some trunking systems have other frequencies that can be used as the control channel, but aren't used on a regular basis. A system can only have one conetrol channel in use at a time. (Unless it's a Smartzone system.) If a channel that is designated for the control channel rotation, or an alternate control channel is not in use, it is used as a regular voice frequency.

In addition...

5) Every agency operating on a trunked system is given a priority. For example Police would have the highest priority, while the fire department will have second and publi works the third. If lets say a police unit wants to talk and all the voice channels are currently being used, the control channel will bump the lowest priority user off that particular voice channel, and "open it up" for the higher priority police unit.

6) That's correct, however there's a more scientific way of deciding if the next reply will be on the same voice channel or on another voice channel. When a unit stops keying up, the control channel will keep the repeater open for a few seconds awaiting a possible reply. If no one replys (keys up) with in the the alooeted "delay time" the control channel will revert all the radios on that takgroups back to the control channel. Now if anyone wants to talk, the whole process begins anew.

Other then this, everything else seemed correct to me. Once you program a trunked system into your scanner, it'll become much clearer how these systems work.

Welcome to the hobby

 

[fletch97]

Thanks

mlevin--thanks for taking the time to read and reply to my post. I greatly appreciate your insight! Thanks!

 

[mlevin]

Your Welcome. All this stuff is kinda confusing at first, but as soon as you start programming and listening it all makes sense. Unfortunately too many new enthusiasts are discouraged by the complexity that this hobby has taken on since the trunking evolution.

 

[loumaag]

fletch:
One other thing, and you may well understand this, it just seemed that saying it wouldn't hurt. The control channel, just like every other channel on the Trunking system is a pair of channels. When the user presses his PTT button, he initially transmits a "channel request" on the input of the CC currently in use. While the PTT is still depressed, the radio goes into receive mode listening on the CC output for either a "channel grant" or other answer to the request. If the "channel grant" is there, then the radio (and all other radios on the same TG and your scanner) takes the channel grant information, changes frequencies as required and goes into trasmit mode on the newly granted channel pair (voice channel) and notifies the user to talk with a short beep or bonk or whatever that model radio does to say "go ahead and talk." This all takes considerably less time to do that to even think it through much less explain it.

 

[fletch97]

loumaag,

Thanks for the additional info! I'm a volunteer EMT, so I've used the radio plenty, but I've never had any one teach me exactly how it works--I just know to select a channel, push the button, and talk. But, now I picked up a scanner and actually want to learn how it works and hopefully be able to make sure the new folks at the fire station actually know a little more than I was ever taught!

So, with that in mind, no, I can't say that I was distinctly aware of the channel pairs. I think I understand what you're saying, but at the same time the concept of having a pair of channels (input/output) doesn't make sense to me. That is, in a conventional, non-trunking scenario where you use only one frequency, you transmit and receive on this same frequency, right? That is, there is no sense of channels, is there? Maybe it's simplifying things too much, but it seems to me that after the trunking system decides what frequency to use, you're kind of back to that same idea--you're just using one frequency to transmit and to receive so I do not understand why you need a pair of channels, unless maybe you're saying that it's essentially full duplex and can receive and transmit simultaneously??? Does each channel use a slightly different frequency?

Anyway, thanks for trying to make me smart (even if it that is an impossible task!).

 

[BoxAlarm187]

Not exactly ... if you were to transmit AND receive on one frequency only, this is "simplex". When you go to the talkaround channel on your trunked radio system, you've simply gone to a simplex channel. Simplex range is limited mostly to the transmit power of the two radios that are speaking (think CB radio, or Loudoun's old 46MHz system).

Both conventional and trunked radio systems share one thing - repeaters. They get thier name from the fact that they are "repeating" what is being said from one frequency onto another. I'll explain it on a conventional system, though the idea is similar for a trunked system. (for the purpose of this illustration, all frequencies and agencies names have been changed to protect the innocent).

Let's say that Ambulance 1 is riding down the road and wants to talk to Ambulance 2. When A-1 keys the radio, the radio transmits on 154.000 to the repeater, and the repeater takes the transmission, and repeats it on 155.000, then A-2 receives it. When A-2 is ready to talk back to A-1, he will again transmit on 154.000, but A-1 will hear it on 155.000.

When a radio is just sitting there, it's always in receive mode, listening to the output frequency of the repeater. However, when you hit the PTT button, it transmits on the input frequency of the repeater, and immediately goes back to receive mode when you let the PTT button go.

A trunked system works in basically the same way, except the computer determines what the input and output frequencies are going to be.

Don't know if this helped or not, let us know!

 

[loumaag]

BoxAlarm:
That seems like a pretty good explanation.

Fletch:
As BoxAlarm says, mostly (these days) although you think you are talking and hearing on the same frequency, that really isn't the case. Thanks to those clever radio techs, you are listening on one frequency and talking on another. In any case, check out this RR Wiki Link for the basics of Trunking. I think the guy who wrote it knew what he was talking about. :roll:

 

[WayneH]

Fletch, take a look at my page on Motorola trunking. I've written a lot on operation and the process the radios go through for receiving, transmitting, etc... http://www.thebriarpatch.org/trunking/

-Wayne

 

[loumaag]

I've written a lot on operation and the process the radios go through for receiving, transmitting, etc...

Wayne, would you go to the Wiki and see if it's okay. I am pretty sure it is right, but it wouldn't hurt to have you review it.

 

[fletch97]

Thanks

Thanks to all for their continued assistance!