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TRUNKED APCO P25 Voice/Data Stream

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naSTI

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Okay,

I know for a fact there is more data being transmitted on the data-substream along in the voice channel on a Motorola APCO-16 or even APCO-25 trunked system, than typical through a conventional P25 repeater.

So, is there anyone that can, with a high degree of certainty or authority (please don't speculate if you don't know), know if there's some or all of the CC data stream is also being retransmitted on each of the voice channels to facilitate radios that use Priority scanning or things of that nature to change TGs based on activity on other talkgroups?

I can look up the APCO 25 draft and read for myself the layers within the voice channel transmission, and, I know that 9600bps = voice + data @ *xyz* symbols/secs each stream. But has the question been answered as to what extra data typically is transmitted on a trunked voice channel, besides the usual Transmitting UnitID Number, NAC Code, and radio-specific auxiliary data (like the numeric "Custom Status" numeric message ID, etc..) found on conventional channels?

Thanks,
naSTI
 

DaveNF2G

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I'm not sure if my level of "certainty" is high enough, but I will stop short of speculation.

Here's what I know - the original PRO-92 scanner model (without any suffix) tracked Motorola P16 systems by using subaudible data received over the voice channels. There was some talk about how that data could have been used by actual trunking radios, such as priority talkgroups.
 

SCPD

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naSTI said:
I can look up the APCO 25 draft and read for myself the layers within the voice channel transmission
Read the section on "Link Control". That should answer your questions.
 

naSTI

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Unitrunker said:
Read the section on "Link Control". That should answer your questions.
Right. It does only partially. Now let me rephrase by asking that in practice/experience, what are the "most common" found on these streams? I.E. Is there any evidence that all of the "Future Use" bits are actually containing information on some of the more optioned and advanced installations like GPS coords, etc.. or is it common to see all of those bits in those frames to be NULLs or ZEROed-out?

Since I don't have a P25 data decoder or terminal for that matter, I was wondering if anyone with an Anritsu, Agilent or Motorola IFR/Analyzer could comment on the "typical" type of information found.


QUOTE FROM APCO-25 Study:
[...] "If the addressee is a talk-group, the link control information consists of 1 octet describing the type of information (e.g., talk-group type of transmission), 1 octet containing the manufacturer identifier, 1 bit indicating whether or not the transmission is of an emergency nature, 15 bits reserved for future use, 2 octets for the talk-group address, and 3 octets for the transmitting radio's identifier.
If the addressee is another radio, the last 7 of the 9 octets consist of : 1 octet reserved for future use, 3 octets for the destination radio's identifier, and 3 octets for the transmitting radio's identifier. This information is protected with 168 bits of error corrective coding." [...]
 

wlmr

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naSTI said:
So, is there anyone that can, with a high degree of certainty or authority (please don't speculate if you don't know), know if there's some or all of the CC data stream is also being retransmitted on each of the voice channels to facilitate radios that use Priority scanning or things of that nature to change TGs based on activity on other talkgroups?
Thanks,
naSTI
(9600 P25 voice channel data) Not all the CC data gets broadcast. Only talkgroups with the priority scan bit set in the system (that are actively talking) are listed in the voice channel low speed data. The main thing to know it is SLOW low speed data so a big list would never fully get broadcast. Was described as kinda like the text scroll at the bottom of some news channels. This is the ONLY way to get a radio on the system that's actively scanning a different talkgroup to go back to the talkgroup it's supposed to be listening to. (Scanning other talkgroups by radios on the system that are supposed to be doing something important is fraught with hazzards.)

(Don't know if the priority talkgroups are broadcasted on any site's voice channels if no radios are affiliated on the priority talkgroup(s) at that site.

As far as any other things happening along with voice on the voice channels, - I don't know.
 
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naSTI

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Nice. This seems plausible.

All of the /\/\oneyrola and other trunked radios I have, will interrupt the current scanned TG and jump to the Priority TG before the voice frames begin. It's super fast. Even in most cases will beat the GRE or Uniden scanners that are stopped on that particular TG.

This is how this whole thing came up, however, it does this on the 3600 bps moto system as well.
 
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