Trunking system inputs?

[wtp]

How does trunking work? | Tait Radio Academy

Trunked networks get their name from the world of telephony. Traditionally two cities would bundle their connections together into a single thick line like the trunk of a tree, which is known as a trunked line. The local household loop lines operated like the branches of a tree, one line for...

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[Pr999]

Correct. Each repeater has 2 frequencies. One is an input(receive) the other is an output(transmit) so a site with 8 repeaters will have 8 pairs of frequencies. A site with 20 channels, will have 20 pairs of frequencies.

Yes, the controller decides what channels get used, and does this by listening to the subscribers on the control channel input, and sends commands to control the subscribers on the control channel output. One repeater is used for the control channel.

Yes, but how does it switch the radio's input frequency automatically? it just doesn't make sense to me.

 

[dlwtrunked]

Yes, but how does it switch the radio's input frequency automatically? it just doesn't make sense to me.

The control channel broadcasts which talkgroups are on which repeate (each repeater has its own output/input). This assignment of talgroups to which repeateris always changing in general. Whenever there is a pause between the conversation on a talkgroup, reception of the control channel by the mobile unit tells it which repeater/frequency to listen for that talkgroup next. this is a main function of the control channel. All radios on a talkgroup at any time.constantly change repeater frequencies as directed by the control channel. "Automatically" in your post is a vague term. a

 

[kayn1n32008]

Yes, but how does it switch the radio's input frequency automatically? it just doesn't make sense to me.

When the subscriber radio is on standby(Not actively transmitting or actively receiving) it monitors the control channel. When there is traffic for the TG the radio is affiliated to, the controller assigns a voice channel, and directs all radios affiliated to that TG to tune to the assigned voice channel. Once the traffic is done, and the repeater hang time expires, all radios that were actively receiving on that TG return to the control channel and resume standing by for the next transmission.

When a radio transmits, it sends a very short transmission on the control channel input asking for a voice channel to be assigned. When the controller assigned a channel, it tells the radio which channel to transmit on, the radio then tunes to that channel and transmits on the voice channel input. At the same time, the control channel also direct all other radios affiliated to the same TG to also tune to the assigned voice channel and pass the traffic to the radios speaker.

 

[TampaTyron]

A few items to note:
-radio frequency bands have bandb plans with pre-assigned inputs and output for every frequency/channel
-systems use these radio bands and use th eband plans according to convention
-inputs are mathmatically related to repeater outputs. Repeater 1 has output freq x and input frequency x-(offset for the band). So, when the system says "switch to repeater 1", the radio looks up repeater 1 in a table and "knows" the input based off the repeater output frequency less the band plan input frequency
-all of this can be changed base don the system admin and use cases, but the above is accurate 99% of the time

TT

 

[GTR8000]

A few items to note:
-radio frequency bands have bandb plans with pre-assigned inputs and output for every frequency/channel

Not applicable to the VHF band, which has no standard offsets. It's also not uncommon for P25 systems in the VHF band to use explicit signaling instead of implicit signaling to direct the subscribers to specific traffic channel frequency pairs, as the predefined 16 element band plan may not be able to cover all possible combinations of repeater pairs.

 

[wtp]

and just remember that a very fast computer takes care of it all.
so when it picks out the output, the input is automatically taken care of.
758 to 775 out
788 to 805 in

806 to 817 in
851 to 862 out

896 to 902 in
935 to 941 out

 

[Pr999]

So essentially, all the radios within the system have a whole band of frequencies stored on them for the input side?
Also, one more thing I'd like to ask is if the input side is also trunked too. because how do the transmissions not bump into each other if it gets busy with multiple units going simultaneously?

 

[GTR8000]

So essentially, all the radios within the system have a whole band of frequencies stored on them for the input side?
Also, one more thing I'd like to ask is if the input side is also trunked too. because how do the transmissions not bump into each other if it gets busy with multiple units going simultaneously?

1) No. There is a band plan for UHF, 700, 800, 900 MHz with common repeater offsets, making it very easy for the radio to calculate the input frequency for each traffic channel repeater.

2) "Trunking" refers to multiple frequencies being pooled together and randomly assigned by a controller, primarily for voice transmissions on a per talkgroup basis, but also for data transmissions if the system is capable. As the zone controller assigns a trunked channel (repeater) for a talkgroup call, the input frequency for that particular channel (repeater) is inherently assigned. It's not just random "oh pick a frequency and talk on that".

 

[Pr999]

I can't believe I forgot about a very important thing about this. Unit IDs. All trunking systems need to have IDs for the radio to be registered. Idk why I had forgotten about this. They transmit their ID before the beginning of the transmission and sometimes even continuously without the radio transmitting at all. This is how we get talkgroups. All IDs registered to one of many talkgroups get to transmit on either selected and when inputting voice or data, the CC collects the data and connects it to which every talkgroup is on.

 

[GTR8000]

Well...not exactly, no. A subscriber requests to transmit on a particular talkgroup when the PTT button is pressed, the system grants that request (or denies it for reasons such as the talkgroup is already busy, the subscriber is not authorized, etc.) If the grant is successful, then the subscriber gets to talk. There's a lot more to it than that, but since you're starting out on a very basic level of learning how trunking works, I won't get too technical. There are plenty of resources online where you can learn about the basics of trunking, which is what you should do before asking specific questions. I believe someone posted a link earlier in this thread.

 

[kayn1n32008]

I can't believe I forgot about a very important thing about this. Unit IDs. All trunking systems need to have IDs for the radio to be registered. Idk why I had forgotten about this. They transmit their ID before the beginning of the transmission and sometimes even continuously without the radio transmitting at all. This is how we get talkgroups. All IDs registered to one of many talkgroups get to transmit on either selected and when inputting voice or data, the CC collects the data and connects it to which every talkgroup is on.

Radios do not continously transmit thier ID, when not transmitting. It's RID is only transmitted, when the radio is transmitting.

Keep in mind, with a Trunked radio, the radio can, and does, transmit with out user intervention. When it is initially turned on, it scans for a valid control chs. El for the system it is assigned to, then when it finds a control channel, it will transmit data to the controller telling it its RID and the talk group currently selected. When the radio meets roaming threshold, it will scan for another site and then transmit to register and affiliate with the new site.
When a radio registers, and affiliates to a site and talk group, that info is stored in the controller. The controller keeps track of every radio.

 

[Pr999]

While I'm still somewhat confused, I also get it at the same. I would love to learn more about trunking, but looking at those lesson pages from Tait doesn't really help me understand it any better. Is there some kind of book or whole website to teach me on this stuff?

 

[AM909]

The details vary according to the technology type of the trunking system, but in most cases, for a given site, the radio builds a table of channels in its memory when you first select the site. Each channel is a pair of freqs, one TX and one RX. There is a corresponding repeater at that site for each of those channels.

When someone wants to talk, their radio sends a request to the trunking controller, which then assigns one of the channels in the table and tells that radio, and any others that need to hear it, which channel to go to.

If you want to understand the details, study one technology type at a time or it will be confusing. P25, DMR (different flavors), NXDN (and IDAS), and maybe LTR are likely to be encountered in the wild.

 

[Pr999]

On multichannel systems, they use a transmit combiner feeding a transmit antenna, and a receive multicoupler that used one RX antenna to feed all the receivers.

Sorry to bring this back up but it's kinda eating at me. Where could you fin one of these "transmit combiners?"

 

[GTR8000]

Sorry to bring this back up but it's kinda eating at me. Where could you fin one of these "transmit combiners?"

I'd start with Google, but that's just me. 😒

 

[KevinC]

Sorry to bring this back up but it's kinda eating at me. Where could you fin one of these "transmit combiners?"

My garage.

 

[KevinC]

And speaking of combiners, what I have too many of are short haul control station combiners. Engineers seem to miss the 30+ dB loss and when it doesn't work they trash them and have to start over.

 

[GTR8000]

And speaking of combiners, what I have too many of are short haul control station combiners. Engineers seem to miss the 30+ dB loss and when it doesn't work they trash them and have to start over.

Funny you mention those...literally just had a discussion earlier today about an issue with some APX control stations attached to one of those horrid things. 💩

 

[mmckenna]

Where could you fin one of these "transmit combiners?"

In the rack next to the trunked system.

Sorry, could not resist, mainly because others started it...

Transmitter Combiners - Telewave.io

www.telewave.com