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doc/BFI-payload-fill: article written
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 25 May 2024 23:07:13 +0000
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15
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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1 The traditional TRAU-UL frame format of GSM 08.60 & 08.61 for FR/HR/EFR codecs
86a10ba0a1f8 doc/BFI-payload-fill: article written
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2 always includes a full payload in every frame, even those marked as BFI=1.
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3 Hence a question naturally arises: what does a traditional T1/E1 BTS put in the
86a10ba0a1f8 doc/BFI-payload-fill: article written
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4 payload bits of its TRAU-UL output when there are no "received, but erroneous"
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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5 bits for this frame, i.e., when the speech frame position was stolen for FACCH,
86a10ba0a1f8 doc/BFI-payload-fill: article written
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6 or when nothing at all was received (no burst midambles detected), with nothing
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7 to run the channel decoder on? A satisfactory answer to this question will be
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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8 needed by anyone who sets out to do either of the following:
86a10ba0a1f8 doc/BFI-payload-fill: article written
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9
86a10ba0a1f8 doc/BFI-payload-fill: article written
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10 a) build a new (FOSS-based) T1/E1 BTS;
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11
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12 b) implement GSM 08.62 TFO (in-band) in a soft transcoder that works with an
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13 IP-based GSM RAN.
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14
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15 Case (a) is probably only a theoretical thought exercise, but case (b) is quite
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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16 real: I (Mother Mychaela) do very much desire to bring back in-band TFO, and do
86a10ba0a1f8 doc/BFI-payload-fill: article written
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17 so under current economic conditions that call for IP transport within GSM RAN
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18 and an IP interface to G.711 PSTN. For the latter problem, the enhancements of
86a10ba0a1f8 doc/BFI-payload-fill: article written
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19 TW-TS-001 & TW-TS-002 help greatly (and are required for proper implementation
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20 of TFO), but the question still remains of what should be emitted in TFO frames
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21 when a no-data BFI comes in. (Most BFIs on OsmoBTS are the no-data kind; there
86a10ba0a1f8 doc/BFI-payload-fill: article written
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22 is currently little opportunity for BFI-with-data UL output.)
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23
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24 In order to get better insight into this question, we (the small community of
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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25 people who care about this topic) need to look at the Abis UL output from real
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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26 historical E1 (or T1) BTS implementations, and see what they actually put out
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27 under these conditions. I (Mother Mychaela) do not currently own any T1/E1 BTS
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Mychaela Falconia <falcon@freecalypso.org>
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28 hardware, but thankfully there are some published capture files in Osmocom which
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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29 we can look at. The two binary files in trau-files directory of this repository
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30 have been copied from libosmo-abis/contrib/trau2rtp, where they were committed
86a10ba0a1f8 doc/BFI-payload-fill: article written
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31 some time in Anno Horribilis (2020).
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32
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33 Analysis of Osmocom contrib/trau2rtp files
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34 ==========================================
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35
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36 The README file in libosmo-abis/contrib/trau2rtp describes the origin of
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37 e1_ts2_efr.bin and e1_ts2_fr.bin thusly:
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38
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39 > The input data (*.log.bz2) was generated using strace on an osmo-nitb process
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40 > while a MO-to-MT call was running on two sub-slots of TS2.
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41 >
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42 > The strace log is converted to a binary stream of the raw 64bit E1 slot
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43 > using strace-write-parse.py
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44
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45 Unfortunately there is no annotation as to what make/model of E1 BTS was used
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46 in the OsmoNITB test call setup that produced these logs. The same README file
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47 later says:
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48
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49 > The code has been tested against BS-11 and RBS6000/DUG20 in both modes
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50 > (loop vs. RTP) and for FR and EFR.
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51
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52 but that note seems to refer to the workflow of the trau2rtp.c program operating
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53 directly on a "live" DAHDI timeslot, not to the provenance of the included
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54 capture files.
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55
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56 However, despite not knowing the specific BTS model that produced the TRAU-UL
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57 frame streams that have been preserved for posterity in those Osmocom-published
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58 capture files, we can make some observations after parsing them with our
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59 trau-parse utility in trau-decode directory:
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60
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61 * In both FR and EFR captures, sub-timeslot 1 comes to life first (the idle fill
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62 pattern of 2'b01 changes to TRAU-UL frames), then sub-timeslot 2. Thus we can
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63 infer that subslot 1 is the MO leg of the test call, and subslot 2 is the MT
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64 leg.
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65
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66 * The FR capture looks like I expected: every TRAU-UL frame (right from the
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67 point where the sub-timeslot comes to life) is of type FR UL, and various bits
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68 are set as I expected from the reading of GSM 08.60 spec.
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69
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70 * The EFR capture exhibits this strange oddity: each subslot starts out emitting
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71 FR UL frames, then switches to emitting EFR frames some significant time
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72 later. (920 ms later on the MO leg and 740 ms later on the MT leg in this
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73 test call capture.) I have no clue why this oddity is occuring: is it perhaps
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74 an artifact of OsmoNITB initially activating the GSM timeslot as FR and then
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75 switching to EFR? Or is it a quirk intrinsic to whatever BTS model these
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76 captures came from?
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77
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78 BFI output seen in these Osmocom-published captures
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79 ===================================================
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80
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81 Having done the preliminary analysis presented above, we can return to our
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Mychaela Falconia <falcon@freecalypso.org>
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82 original question: what did that model-unknown E1 BTS emit in its BFI frames,
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83 those marked with C12=1? The evidence in the artifacts under examination
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84 indicates that at least this particular E1 BTS model uses the buffer method,
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85 similar to what is done on the mobile side of GSM Um in the well-studied
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86 Calypso DSP.
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87
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88 What I mean by "the buffer method" is that the Rx Radio Subsystem (as defined
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89 in GSM 06.31, 06.41 and 06.81 specs) maintains a buffer, potentially persisting
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90 from one Rx frame position to the next, that holds the output of GSM 05.03
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91 channel decoder block and the associated SID detector. Under good Rx
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92 conditions, those that lead to BFI=0 output, this buffer gets overwritten with
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93 new bits on every frame, and its potentially persistent nature is not apparent.
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94 However, when no new traffic bits were received, neither good nor bad, the
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95 buffer holds its previous content - and this buffer content gets emitted in
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96 subsequent BFI=1 frames. How can we tell by black-box observation whether a
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97 given implementation uses this buffer method? When the buffer method is used,
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98 the following output should be visible at the observed interface (TRAU-UL frames
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99 on the BTS side or Calypso DSP a_dd_0[] buffer on the MS side):
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100
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101 * When a certain frame position gets stolen for FACCH in the middle of what is
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102 otherwise a stream of good traffic frames (the transmitter is not cut off as
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103 in DTX), the BFI=1 output corresponding to the FACCH position will have the
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104 same bit content as the previous good traffic frame.
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105
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106 * During times of prolonged absence of radio Rx (DTX pauses, or times during
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107 channel activation or shutdown), there will be occasional appearances of new
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108 bits (frame positions in which some radio noise was decoded as a frame),
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109 followed by exact repetitions of previous "garbage" bit content.
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110
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111 The behavior just described is indeed what we see in the TRAU-UL captures under
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112 examination, with one exception. The exception is that in the case of plain FR
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113 (not in EFR), whenever this BTS emitted a repetition of previous buffer content
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114 (no new decoded bits, good or bad), 4 out of 260 bits got corrupted, or rather
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115 overwritten with a fixed pattern. These 4 bits are those that appear at the
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116 very end of class 2 portion in GSM 05.03 bit order; in the state of "old buffer
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117 output" they get replaced with 4'b1001. When reordered from GSM 05.03 into TRAU
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118 frame bit order (the natural bit order of codec parameters), these 4 bits end up
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119 in disparate places, hence when we look at the output of trau-parse, we see what
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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120 looks like corruption in the lsb of 4 different speech parameters:
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121
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122 * The lsb of 2nd LARc becomes 0;
86a10ba0a1f8 doc/BFI-payload-fill: article written
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123 * The lsb of 6th LARc becomes 1;
86a10ba0a1f8 doc/BFI-payload-fill: article written
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124 * The lsb of second-to-last Xmc becomes 1;
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125 * The lsb of the very last Xmc becomes 0.
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126
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127 The presence of this corruption (can't really call it a bug, as the output from
86a10ba0a1f8 doc/BFI-payload-fill: article written
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128 the BTS under these conditions is officially undefined garbage) does not
86a10ba0a1f8 doc/BFI-payload-fill: article written
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129 invalidate the buffer hypothesis, but on the contrary, further confirms that
86a10ba0a1f8 doc/BFI-payload-fill: article written
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130 this hypothesis is most likely correct.
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131
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132 EFR CRC observations
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133 ====================
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134
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135 Taking advantage of EFR being more compact than the original FR (244 bits
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136 instead of 260), the TRAU frame format for EFR adds 5 CRC-3 fields, covering
86a10ba0a1f8 doc/BFI-payload-fill: article written
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137 some of the "most important" bits of the payload. I was wondering if perhaps
86a10ba0a1f8 doc/BFI-payload-fill: article written
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138 some T1/E1 BTS may be transmitting intentionally bad CRC to indicate that they
86a10ba0a1f8 doc/BFI-payload-fill: article written
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139 got no payload bits at all, i.e., BFI with no data - but at least the (unknown)
86a10ba0a1f8 doc/BFI-payload-fill: article written
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140 BTS model from which these Osmocom-published captured were taken always emitted
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141 good CRC in every TRAU-UL frame, including buffer-output frame positions which
86a10ba0a1f8 doc/BFI-payload-fill: article written
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142 were clearly "BFI with no new bits".
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143
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144 Ideas for software implementation of TFO
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145 ========================================
86a10ba0a1f8 doc/BFI-payload-fill: article written
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146
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147 The main motivator for seeking answers to the question of BFI payload fill is
86a10ba0a1f8 doc/BFI-payload-fill: article written
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148 deciding what to put in those payload bits in the case of TFO frame output when
86a10ba0a1f8 doc/BFI-payload-fill: article written
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149 the RTP-based source is BFI with no data. From the standpoint of implementation
86a10ba0a1f8 doc/BFI-payload-fill: article written
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150 costs (both the effort of implementation and CPU load in operation), the buffer
86a10ba0a1f8 doc/BFI-payload-fill: article written
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151 method looks very attractive. One implementation strategy would be to have an
86a10ba0a1f8 doc/BFI-payload-fill: article written
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152 array of 320 bits (with each bit expanded to a byte at this stage) in the
86a10ba0a1f8 doc/BFI-payload-fill: article written
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153 per-call state structure, holding the TFO frame to be transmitted. This way
86a10ba0a1f8 doc/BFI-payload-fill: article written
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154 bits that never change (sync pattern, frame type bits etc) would only be
86a10ba0a1f8 doc/BFI-payload-fill: article written
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155 initialized once and then reused for the duration of the call; the frame output
86a10ba0a1f8 doc/BFI-payload-fill: article written
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156 function executing every 20 ms would fill in the new Dn bits most of the time,
86a10ba0a1f8 doc/BFI-payload-fill: article written
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157 set new C12-15, and do the final step of packing the content of this buffer
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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158 into the two lsbs of outgoing PCM samples. With this approach, the processing
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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159 of BFI-no-data frames would entail simply skipping the step that writes the new
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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160 Dn bits (and associated C13-14), leaving the old bits in place - apparently the
86a10ba0a1f8 doc/BFI-payload-fill: article written
Mychaela Falconia <falcon@freecalypso.org>
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161 exact same approach used by T1/E1 BTS and MS DSP implementors.