GRN Data Channels...?

Mike Alpha
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GRN Data Channels...?

Post by Mike Alpha » Mon Dec 28, 2020 12:19 pm

I don't know if they are new or I just haven't noticed them before but I've been noticing these data channels on most sites of the GRN in Sydney. Not the control channel, a separate data stream that radios affiliate to.

In Unitrunker I notice a RID will pop up on a channel with no talk group and in the Svc column it shows DAT. If you tune a scanner to that frequency there is a continuous data transmission. Some sites have a dedicated channel just for this data stream. GPT uses 416.3750MHz and Horsley Park 419.5875MHz. On some sites the frequency changes.

Cartman put the data stream into DSD+ and it decoded it with LC_ADJ_STS_BCST_EXP and various other RSSI site info. Googling LC_ADJ_STS_BCST_EXP shows it stands for 'Adjacent Site Status Broadcast - Explicit' which is part of the TIA P25 messaging standard.

Are these data channels something new or have they been around for a while?

Mike

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JAFO
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Re: GRN Data Channels...?

Post by JAFO » Mon Dec 28, 2020 6:31 pm

Mike Alpha wrote:I don't know if they are new or I just haven't noticed them before but I've been noticing these data channels on most sites of the GRN in Sydney. Not the control channel, a separate data stream that radios affiliate to.

In Unitrunker I notice a RID will pop up on a channel with no talk group and in the Svc column it shows DAT. If you tune a scanner to that frequency there is a continuous data transmission. Some sites have a dedicated channel just for this data stream. GPT uses 416.3750MHz and Horsley Park 419.5875MHz. On some sites the frequency changes.

Cartman put the data stream into DSD+ and it decoded it with LC_ADJ_STS_BCST_EXP and various other RSSI site info. Googling LC_ADJ_STS_BCST_EXP shows it stands for 'Adjacent Site Status Broadcast - Explicit' which is part of the TIA P25 messaging standard.

Are these data channels something new or have they been around for a while?

Mike
I don't know if it is related, but I've notice a couple of times while doing Control Channel Search at nights, one Frequency that the 369 stops on as if it was a CC, but only displays P25. I haven't taken much notice of it, thinking it may had just been a test CC given the current up-grade in progress.

Is it anything like that?
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Mike Alpha
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Re: GRN Data Channels...?

Post by Mike Alpha » Mon Dec 28, 2020 7:06 pm

JAFO wrote:I don't know if it is related, but I've notice a couple of times while doing Control Channel Search at nights, one Frequency that the 369 stops on as if it was a CC, but only displays P25. I haven't taken much notice of it, thinking it may had just been a test CC given the current up-grade in progress.

Is it anything like that?
Yes, like that. I've not noticed it before so I think it's something new, although the TIA Standard paper says this secondary data information has been around since 2004.

I always thought adjacent site info was broadcast in the main control channel.

Mike

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Re: GRN Data Channels...?

Post by JAFO » Mon Dec 28, 2020 7:30 pm

this is the one I get
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Mike Alpha
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Re: GRN Data Channels...?

Post by Mike Alpha » Tue Dec 29, 2020 11:41 am

JAFO wrote:this is the one I get
Yep. That's the Kurrajong Heights "data channel"

Mike

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Re: GRN Data Channels...?

Post by citabria » Tue Dec 29, 2020 11:55 am

Mike Alpha wrote:Cartman put the data stream into DSD+ and it decoded it with LC_ADJ_STS_BCST_EXP and various other RSSI site info. Googling LC_ADJ_STS_BCST_EXP shows it stands for 'Adjacent Site Status Broadcast - Explicit' which is part of the TIA P25 messaging standard.
Those are just Control Channel TSBK packets that transmit the adjacent site status info. Nothing to do with P25 Packet Data, and they aren't heard on the "data channels" at all.

If you tune to the data channel when it's in action, you will see a steady stream of TDULC (Terminator Data Units with link control) packets that keep the channel alive, and the occasional PDU (Packet Data Unit) that is a simple "ACK" back to the radio to tell it that it's data packets were received by the FNE (Fixed network Equipment).

Basically put, yes these are data channels, they can appear on any GRN base freq on a site, but most sites have a primary channel, and no, there's no useful data coming off the repeater. It's used for stuff like GPS etc...

Oh, they've been in used since the early days of P25 GRN, but have only recently becoem more active.

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Re: GRN Data Channels...?

Post by Mike Alpha » Tue Dec 29, 2020 5:43 pm

Thanks Citabria. I think I understand it a bit more now... :?

But if anyone has the inclination, you can read all about it in the TIA Standard document here - PROJECT 25 LINK CONTROL WORD FORMATS AND MESSAGES NEW TECHNOLOGY STANDARDS PROJECT - DIGITAL RADIO TECHNICAL STANDARDS

Mike

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Re: GRN Data Channels...?

Post by JAFO » Thu Dec 31, 2020 4:59 pm

citabria wrote: It's used for stuff like GPS etc...
Citabria,

When you say used for stuff like GPS, is that for like Calculating the Position of a Mobile Radio Unit?
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Re: GRN Data Channels...?

Post by citabria » Thu Dec 31, 2020 6:14 pm

Mike Alpha wrote:Thanks Citabria. I think I understand it a bit more now... :?

But if anyone has the inclination, you can read all about it in the TIA Standard document here - PROJECT 25 LINK CONTROL WORD FORMATS AND MESSAGES NEW TECHNOLOGY STANDARDS PROJECT - DIGITAL RADIO TECHNICAL STANDARDS

Mike
Ummmn, that document has nothing to do with P25 Packet Data at all. It refers to link control protocols for voice traffic only. TIA-102.BAEF is probably more relevant to the topic at hand.

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Re: GRN Data Channels...?

Post by citabria » Thu Dec 31, 2020 6:16 pm

JAFO wrote:
citabria wrote: It's used for stuff like GPS etc...
Citabria,

When you say used for stuff like GPS, is that for like Calculating the Position of a Mobile Radio Unit?
Yes, except the position is not calculated, the radio has an embedded GPS, the host firmware gets the GPS co-ordinates, embeds them in a P25 LRRP PDU packet, and explicitly transmits them up to the network. Quite a few agencies are using this feature already, RFS being one of them.

Page 33 of this document has more detail:- http://www.project25.org/images/stories ... e_v1.7.pdf

Cheers,
Matt

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