[op25-dev] Re: Control channel questions

Andrew Porrett wap@ica.net [op25-dev] op25-dev at yahoogroups.com
Tue May 9 01:43:11 UTC 2017

At 08:44 PM 08-05-2017, gnorbury at bondcar.com [op25-dev] wrote:
>Operationally, when a user keys the PTT button on a P25 radio, a 
>series of messages are exchanged that either grants or denys access 
>to the requested TGID.  Typically the user hears a "beep" or a 
>"bong" and at that point they can start speaking and this is 
>transmitted over the uplink freq.  The repeater then turns the data 
>stream back around and retransmitts it on the downlink for other 
>affiliated radios to receive.

This is not correct.  You suggest that there is "a series of 
messages" followed by "turns the data stream around and retransmits 
it"; that's an odd way to describe a single inbound request and an 
outbound grant.

>   As a "fly on the wall" scanner user, OP25 can monitor for these 
> messages and use them to determine where to tune, what tgid and 
> timeslot etc.  When the PTT key is released, more messages are 
> passed and a message pops out on the control channel notifying 
> client radios that the call has ended.

How would radios sitting on a voice channel see this message on the 
control channel?  And why have we never seen evidence of such a "call 
is over" message on any P25 control channel?

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