P25 Announcement Group behavior question (Motorola Subscriber Unit vs. Scanner)

[KTR722]

I have a question below for those in the know on P25 programming.

Quick background: When a P25 radio affiliates with a site, it will be given approval for both the TGID and also assigned to an Announcement Group by the controller.

You can see this in the logs from SDRtrunk (and other programs):

ACCEPT AFFILIATION GROUP: 7265 (LOCAL) ANNOUNCEMENT GROUP:0

On most of the systems I monitor, the announcement group is zero which means the AG is not being used. Looks like they are seldom used and why I cannot see to find much out there about them on the interwebs.

However, there are a few Talkgroups on one of the systems that I monitor that does use Announcement Groups.

ACCEPT AFFILIATION GROUP: 1265 (LOCAL) ANNOUNCEMENT GROUP:1260

My question is confirming the behavior of a subscriber radio in regard to announcement groups vice how your scanner behaves.

First, if i understand this correctly, the Announcement Group is nothing more than just another TGID; in other words, the 1260 above comes from the same pool of available TGID numbers that the 1265 came from. There is not a separate pool of Announcement group ids.

For a Motorola subscriber radio, the announcement group is given to the radio by the controller during affiliation (as described above) and effectively it will "scan" that additional talkgroup (1260) in addition to the 1265 talkgroup that the radio is currently set to via the selector knob. If you key up the subscriber radio, it will only key up on 1265. but if someone keys up on 1260, the motorola radio will hear it also - NOT because the message is being transmitted over 1265, but because the subscriber radio was scanning 1260.

For a scanner listener, if you are also listening to talkgroup 1265 (and not scanning any other talkgroups) on your scanner, and someone keys up on the 1260 announcement group, the scanner will not hear it. This is because 1) the scanner does not affiliate with the controller and therefore does not receive announcement group assignments, and 2) scanners do not have any programming fields setup in them for you to plug in an announcement group that is associated with every TGID.

So if a scanner user wants to hear radio traffic on 1265 just like a motorola subscriber radio would hear it, the scanner has to be scanning both 1260 and 1265.

Do I have this correct? or am I missing something?

Reason for the ask was that I was originally told that Announcement Groups, when used, would transmit over any of the associated TGIDs in the AG.

e.g. if TGIDs 1261-1266 all had TGID 1260 configured as their AG, when a dispatcher keyed up on TGID 1260, it would simulcast on all the associated TGIDs (1260-1266).

But when I went to verify that behavior, that was not the case.

 

[crazyboy]

First, if i understand this correctly, the Announcement Group is nothing more than just another TGID; in other words, the 1260 above comes from the same pool of available TGID numbers that the 1265 came from. There is not a separate pool of Announcement group ids.

Correct they all come from one pool but are configured differently in the system.

For a Motorola subscriber radio, the announcement group is given to the radio by the controller during affiliation (as described above) and effectively it will "scan" that additional talkgroup (1260) in addition to the 1265 talkgroup that the radio is currently set to via the selector knob. If you key up the subscriber radio, it will only key up on 1265. but if someone keys up on 1260, the motorola radio will hear it also - NOT because the message is being transmitted over 1265, but because the subscriber radio was scanning 1260.

The announcement group is programmed into the subscriber radio not given out by the system. Each trunking personality can have any single announcement group programmed into it or no announcement group. So how it is configured is pretty adjustable.

It is not scanning per say, but yes the radio is monitoring for activity on both the selected talkgroup as well as the AG.

So if a scanner user wants to hear radio traffic on 1265 just like a motorola subscriber radio would hear it, the scanner has to be scanning both 1260 and 1265.

Correct, AFAIK.

Reason for the ask was that I was originally told that Announcement Groups, when used, would transmit over any of the associated TGIDs in the AG.

e.g. if TGIDs 1261-1266 all had TGID 1260 configured as their AG, when a dispatcher keyed up on TGID 1260, it would simulcast on all the associated TGIDs (1260-1266).

As you discovered this is not the case. This would be the result of a multiselect on the console, which would actually form a supergroup comprised of those talkgroups provided the talkgroups are not set to prevent that.

The nice thing about Announcement Groups is it does not matter what talkgroup the subscriber is on they will hear the announcement, depending how it is set up. Ex: A PD radio has a PD announcement group in all trunking personalities, even if the radio is selected on the fire or public works talkgroup the radio will get the PD announcement without the dispatcher knowing they are on a different talkgroup and selecting it and without PD traffic going over the fire or public works talkgroup.

 

[KTR722]

Thanks, and yes i was using the wrong terminology completely in that example. I should be using 'monitoring' instead of 'scanning'.

 

[mcjones2013]

The announcement group is programmed into the subscriber radio not given out by the system. Each trunking personality can have any single announcement group programmed into it or no announcement group. So how it is configured is pretty adjustable.

I have no doubt you're correct about it being programmed in the radio, but just like the OP described, I've seen in SDRTrunk where the announcement group is shown when a radio affiliates to a specific talk group, so it would seem it *can* also be announced by the system? I too am trying to better figure out how those ATG channels work.

 

[crazyboy]

I have no doubt you're correct about it being programmed in the radio, but just like the OP described, I've seen in SDRTrunk where the announcement group is shown when a radio affiliates to a specific talk group, so it would seem it *can* also be announced by the system? I too am trying to better figure out how those ATG channels work.

Correct it is shown. The same way you see joins to standard talk groups on the control channel. The system isn't telling the radio what talk group to join, the radio is essentially asking permission to be a part of said talkgroup, or ATG in this instance, and the system is either allowing it or denying it. You are seeing the data changes to accomplish this on the control channel decode.