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When is a trunked channel deallocated?

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lukekb

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I am trying to follow P25 talkgroup frequency allocations on trunked system using an SDR. I have looked through all of the P25 standards, but I can't figure out if anything is sent out on the control channel when a talkgroup is no longer associated with a frequency.

Does the association just time out if a update has been sent after X seconds? I am noticing that I am getting some double assignments to frequencies. Is there an Outbound message I can look for on the control channel that signals a disassociation?
 

n0nhp

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As soon as the radio un-keys. The next radio has to key up on the control channel and ask for a new allocation. Using Pro96COM you can watch the preferred channel change TGs with every transmission if the site is busy.
No signal, just a lack of signal releases the channel for the next user.

Bruce
 

mikey60

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There is nothing on the control channel to indicate when the transmission ends. It's just a timeout based on when the last update message was received. Unless you monitor the voice channel, that's your only option.

Mike
 

slicerwizard

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I am trying to follow P25 talkgroup frequency allocations on trunked system using an SDR. I have looked through all of the P25 standards, but I can't figure out if anything is sent out on the control channel when a talkgroup is no longer associated with a frequency.
There is nothing in the standards because none of the subscriber radios need that information. Ask yourself how a radio could use that information - do any use cases come to mind?
 

lukekb

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That makes sense. If a subscriber unit see another TG being broadcast on the frequency it is tuned, it would just go back to monitoring the Control channel, right?

Any idea what the timeout is with P25? Does one of the system status messages specify it or is it configured out of band?

I used 5 seconds and it seemed to work pretty well.
 

krokus

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That makes sense. If a subscriber unit see another TG being broadcast on the frequency it is tuned, it would just go back to monitoring the Control channel, right?

Any idea what the timeout is with P25? Does one of the system status messages specify it or is it configured out of band?

I used 5 seconds and it seemed to work pretty well.

The time out is determined by the system administrator, and programmed into the relevant equipment. Five seconds is pretty typical, some use three seconds.

AFAIK, it is not part of P25. That is just the modulation, like FM.

Sent via Tapatalk
 
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