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data belch

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nycap

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monitoring a freq which is know to use P25 exclusively. what does it mean when my radio is constantly making what sounds like a short undecoded "P25 belch" and the radio (BC396t) displays "DATA" and "LNK" but does not display "P25"?

this has been been going on constantly and i cant figure? could it be P25 radios texting? but then why does my my receivever not display "P25". i am stumped here any help is much appreciated?
 

nycap

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nope; no voice at all. just a "burp" that sounds like P25 when its not decoded; then the receiver displays "DATA" and "LNK" maybe for a second or two and thats it? the receiver dosnt even show "P25".
 

nycap

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sometimes there is not even a "data burp", but ill look over becuase i can hear the electronics sort of make a click to open the scquelch. the speaker i think is engaged but there is no sound.
 

W8RMH

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Sounds like it may be encryption.
 

nycap

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It would show ENC after figuring out that the TG is in encrypted mode.
Exactly, the display would show "ENC" but it does not. Like i said it dosnt even show 'P25". and the transmitions are very short, not long enough to be a voice conversation. I mean the whole thing takes place in less than 1 to 2 seconds; faster than even those speed talking pilots when they announce themsleves on UNICOM knowing that there is not even a response coming.
 

nycap

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but i know that on this channel only P25 radios are used. so i am guessing that it could be text messaging in P25 or GPS data transmissions? but i dont want to have to guess; for me the hobby sometimes is driven by that curiousity of "just having to know". and i know there are other members on here that have P25 radios so they would be familiar with how these transmitions would come across if not not properly decoded. and i know that other members have SDRs that can analyze the P25 packets so they would know. thats why i would respectfully suggest to keep the thread here.
 

WayneH

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Maybe the mods should move this to the Uniden forum...
This isn't Uniden specific.

nycap, are these pulses random or do you see them come in evenly spaced? I've heard of there being P25 pulses on conventional channels and we've yet to ascertain exactly what causes them. In some cases I've guessed it's a desktop console type of device that's connected over the air and it uses the pulses to test for connectivity. Basically a heartbeat check by a device for some reason.
 

MattSR

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Certain P25 frame types trigger this behaviour on scanners. TSBKs (trunking control channel packets) PDU (Packet data units) or TDU packets (terminator data unit) or TDULC (terminator data unit with link control) are interpreted as DATA or LNK by the radios.

PDUs are pure data packets for things like GPS location or data terminals. TSBKs are used for conventional inhibit and trunking control commands. the TDU and TDULC packets are used as a digital "tail" to keep receivers in sync with the C4FM modulator for a couple of seconds at the end of a transmission, so that when another radio replies within in that time frame, the receivers are all still trained to the tx'er (as per any asynchronous digital protocol)

If the OP can send me a disc-tapped sample of the transmission I'll invesitgate further when I get time.
 

nycap

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This isn't Uniden specific.

nycap, are these pulses random or do you see them come in evenly spaced? I've heard of there being P25 pulses on conventional channels and we've yet to ascertain exactly what causes them. In some cases I've guessed it's a desktop console type of device that's connected over the air and it uses the pulses to test for connectivity. Basically a heartbeat check by a device for some reason.
i have found using a stop watch (i dont have logging capabilities) that for the short data burst is TXing exactly every two minutes. but its not 24/7, it quits at night and starts in the morning; and there are some times when the activity increases (i.e. more and longer buzzes).
 

WayneH

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i have found using a stop watch (i dont have logging capabilities) that for the short data burst is TXing exactly every two minutes. but its not 24/7, it quits at night and starts in the morning; and there are some times when the activity increases (i.e. more and longer buzzes).
Is encryption used on the channel?
 

wlmr

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P25 can be used for various types of data transmissions as well as voice. Unless you know what types of capabilities were purchased for the system you'd be guessing. Everything from data terminals in vehicles, encryption key changes, and remotely reprogramming radios are possibilities as well as likely some I haven't thought of at the moment.

Short answer - if the radio is calling it data it's likely that's what it is.
 

nycap

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It is a repeater pulsing TDUs without link control. Does anyone know why a repeater would to this?
 

MattSR

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How long does the noise last?

The Motorola Quantar transmits TDU packets during the repeater 'hang time' (aka tail) The length of this is configured in the "drop out delay" field in the programming software.

Cheers,
Matt
 

nycap

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the transmition takes a split second. the length ofthe transmition seems random. sometimes one frame, sometimes two, sometimes three, sometimes whatever upto maybe eight. goes on all day and night. intervals seem random as well. only one byte changes sequentially with each frame. the sequence continues with each discrete transmition of however many frames, even if there is alot of time inbetween. the tdu doesnt seem to have anything in it other than the nac.
 

mancow

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There is a Federal band repeater here that does the same thing but the intervals are around 10 minutes. I don't know what it is either.
 

WayneH

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One of the local BATFE repeaters has multiple devices that pulse at timed rates. Since they're TXing through the input I can verify there's more than one. Anything sent by these devices is repeated through the repeater (hence the quick data packets).

Normally I see one single timed packet, which I believe is for voting scan.
 
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