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Thread: TDU only transmitions

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    nycap No Longer Registered

    Default TDU only transmitions

    There are some repeaters that intermittently transmit only TDUs without link control. What is the purpose of this?


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    That's what the Quantar sends during it's "hang time" or "drop out delay" after a mobile transmission. You're probably hearing the hang time TDU's following a single data packet transmitted by mobile/portable. The data packet that keyed the repeater could be a call alert/page, data registration request, OTAR message, DTMF "Data 12" dialing digits, or some other general data packet.

    The P25 standard doesn't address what a repeater should send during the "hang time". Motorola chose to send those TDU's without link control. It's purpose is pretty much the same as hang time on an analog repeater. It just slightly increases performance with a weak mobile's signal that is fading in and out near the decode threshold.

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    nycap No Longer Registered

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    the length of the transmition seems random; sometimes one frame, sometimes two, sometimes three, sometimes whatever, upto maybe ten or more. this 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. other than that, the TDUs dont seem to have anything else in them except of course the nac. Seldom there will be a PDU frame transmitted which does reveal it is a motorola system.

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    It sounds like there's some active data exchange with the infrastructure going on there. Possibly OTAR. The "hang time" with TDU frames is only present after a mobile/portable transmits. The repeater will unkey immediately after the infrastructure sends a data packet, so that's probably why you're only hearing the TDU frames sometimes for varying durations.

    What equipment and/or software are you using to decode this stuff?

    From what I remember, the registration message for Motorola's OTAR system will have the Manufacturer ID value set to 90 hex, which is Motorola's number. This usually means it's a proprietary message of some type. According to the P25 standard, the Manufacturer ID should be set to 0 on all transmissions that are fully compliant with the standard.


    Quote Originally Posted by nycap View Post
    the length of the transmition seems random; sometimes one frame, sometimes two, sometimes three, sometimes whatever, upto maybe ten or more. this 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. other than that, the TDUs dont seem to have anything else in them except of course the nac. Seldom there will be a PDU frame transmitted which does reveal it is a motorola system.

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    Many conventional systems have a long series of TDU bursts. Motos theory on this is it allows late entry, I have never cared enough about it to research it any futher than that as it seems to apply only to conventional repeaters and not trunked systems.