FreeCalypso > hg > e1-fake-trau

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add README
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 29 Aug 2024 14:56:30 +0000
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Mychaela Falconia <falcon@freecalypso.org>
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1 The hack program in this repository is intended to run on a lab machine (which
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Mychaela Falconia <falcon@freecalypso.org>
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2 can be RPi etc) that drives an E1 BTS by way of an icE1usb adapter and osmo-e1d.
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Mychaela Falconia <falcon@freecalypso.org>
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3
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Mychaela Falconia <falcon@freecalypso.org>
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4 When a traditional E1 BTS activates a speech or data TCH, it starts transmitting
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Mychaela Falconia <falcon@freecalypso.org>
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5 TRAU-UL frames on the E1 subslot corresponding to the activated TDMA ts, and it
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Mychaela Falconia <falcon@freecalypso.org>
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6 expects to receive back TRAU-DL frames shortly thereafter. If those TRAU-DL
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Mychaela Falconia <falcon@freecalypso.org>
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7 frames don't appear as expected, the BTS declares remote transcoder failure and
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Mychaela Falconia <falcon@freecalypso.org>
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8 ends the call attempt - hence a lab test setup has to include a TRAU emulator
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Mychaela Falconia <falcon@freecalypso.org>
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9 to keep the BTS happy, even if the only objective is to keep the call open and
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Mychaela Falconia <falcon@freecalypso.org>
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10 capture TRAU-UL output.
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Mychaela Falconia <falcon@freecalypso.org>
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11
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Mychaela Falconia <falcon@freecalypso.org>
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12 The standard solution in Osmocom is to run OsmoMGW in E1 interface mode.
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Mychaela Falconia <falcon@freecalypso.org>
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13 However, my primary objective was to capture the byte stream coming from the
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Mychaela Falconia <falcon@freecalypso.org>
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14 TRAU-frame-carrying E1 timeslot and record it into a file for later analysis
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Mychaela Falconia <falcon@freecalypso.org>
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15 offline - and I found it too difficult to hack this function into OsmoMGW.
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Mychaela Falconia <falcon@freecalypso.org>
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16 Therefore, I took a different approach:
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Mychaela Falconia <falcon@freecalypso.org>
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17
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Mychaela Falconia <falcon@freecalypso.org>
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18 * Because OsmoBSC still needs its associated OsmoMGW, I do run the latter, but
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Mychaela Falconia <falcon@freecalypso.org>
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19 I point it at a fake E1 interface (vpair in osmo-e1d) instead of the real one.
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Mychaela Falconia <falcon@freecalypso.org>
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20 (OsmoBSC still talks to the real E1 for OML and RSL.)
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Mychaela Falconia <falcon@freecalypso.org>
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21
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Mychaela Falconia <falcon@freecalypso.org>
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22 * The hack program contained in this repository opens the real timeslot on the
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Mychaela Falconia <falcon@freecalypso.org>
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23 real E1 and acts as a fake TRAU, reacting to TRAU-UL output from the BTS and
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Mychaela Falconia <falcon@freecalypso.org>
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24 responding with TRAU-DL frames. This faketrau program also implements the
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Mychaela Falconia <falcon@freecalypso.org>
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25 recording function.
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Mychaela Falconia <falcon@freecalypso.org>
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26
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Mychaela Falconia <falcon@freecalypso.org>
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27 The only version currently implemented is faketrau-16k: it opens a single E1 ts
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Mychaela Falconia <falcon@freecalypso.org>
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28 and treats it as four 16 kbit/s subslots. It acts like real TRAUs in that it
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Mychaela Falconia <falcon@freecalypso.org>
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29 needs to receive TRAU-UL frames from the BTS in order to activate a channel,
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Mychaela Falconia <falcon@freecalypso.org>
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30 and the TRAU-UL frame type in bits C1-C5 indicates the codec type (FR or EFR),
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Mychaela Falconia <falcon@freecalypso.org>
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31 selecting the type of TRAU-DL frames that need to be sent back to the BTS.
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Mychaela Falconia <falcon@freecalypso.org>
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32
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Mychaela Falconia <falcon@freecalypso.org>
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33 The stream of TRAU-DL frames sent to the BTS by faketrau-16k consists of
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Mychaela Falconia <falcon@freecalypso.org>
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34 GSM 06.11 silence frames in the case of FRv1 or decoder homing frames in the
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Mychaela Falconia <falcon@freecalypso.org>
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35 case of EFR.