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doc/TFO-xform/Theory: article written
author Mychaela Falconia <falcon@freecalypso.org>
date Sat, 31 Aug 2024 20:45:25 +0000
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e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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1 TFO transform from uplink to downlink
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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2 =====================================
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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3
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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4 With all 3 classic GSM codecs (FRv1, HRv1, EFR) the original architecture calls
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5 for a network-side transcoder (TRAU) on each individual call leg. The
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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6 implications are:
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7
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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8 * The uplink runs from the MS to the speech decoder in the TRAU that turns the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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9 mobile-generated speech into 64 kbit/s G.711. The Rx DTX handler, a subblock
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Mychaela Falconia <falcon@freecalypso.org>
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10 of that speech decoder in the TRAU, handles error concealment (substitution
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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11 and muting of lost frames) and comfort noise insertion during DTXu pauses,
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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12 and once this speech stream has been transcoded to G.711, all trace of these
e828468b0afd doc/TFO-xform/Theory: article written
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13 GSM-specific effects disappears.
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14
e828468b0afd doc/TFO-xform/Theory: article written
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15 * The downlink runs from the speech encoder in the TRAU to TCH DL radio output
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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16 from the BTS. Because the DL frame stream comes from a free-running speech
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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17 encoder, it never contains errored frames or invalid SID or any other
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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18 aberrations: without DTXd, this frame stream is 100% good speech frames, and
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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19 with DTXd, it is a mixture of good speech and valid SID frames.
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20
e828468b0afd doc/TFO-xform/Theory: article written
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21 But suppose you have two mobile call legs (mobile user Alice calls mobile user
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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22 Bob), and you wish to eliminate the quality-degrading effect of double or tandem
e828468b0afd doc/TFO-xform/Theory: article written
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23 transcoding by passing compressed speech frames directly from Alice to Bob and
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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24 vice-versa - what happens now? The UL frame stream from each call leg will
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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25 contain BFI frame gaps that are never allowed in DL, and if the network deploys
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26 DTX only in the UL direction (DTXu without DTXd, a very sensible choice for
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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27 small-capacity single-carrier cells), the representation of DTXu pauses coming
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Mychaela Falconia <falcon@freecalypso.org>
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28 from each call leg (SID frames followed by prolonged BFI gaps) is also not
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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29 suitable for direct passing to the DL of the opposite call leg.
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30
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31 The solution offered in the TFO spec (GSM 08.62) is a special transform from
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Mychaela Falconia <falcon@freecalypso.org>
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32 call leg A UL to call leg B DL. This transform has no official name that I
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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33 could find, but I call it "TFO transform". In the original GSM 08.62 spec (up
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34 to R99) this TFO transform is described in sections 8.2.1 and 8.2.2; when the
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Mychaela Falconia <falcon@freecalypso.org>
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35 spec changed to 28.062 with 3GPP Release 4 (adding AMR in GSM and AMR-only
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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36 UMTS), the description of TFO transform for classic GSM codecs moved to section
e828468b0afd doc/TFO-xform/Theory: article written
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37 C.3.2.1.1.
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38
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39 However, both spec versions only say what "shall" be done without any guidance
e828468b0afd doc/TFO-xform/Theory: article written
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40 on how to do it algorithmically: the spec language is "subject to manufacturer
e828468b0afd doc/TFO-xform/Theory: article written
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41 dependent future improvements and is not part of this recommendation."
e828468b0afd doc/TFO-xform/Theory: article written
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42 Distilling the problem to its essence, the addition of TFO introduces a new type
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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43 of logical transform on codec frames (and a stateful one at that!) that never
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44 appeared previously anywhere in classic GSM architecture, is not mentioned in
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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45 any other spec, and is not addressed at all by any of the reference codec
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
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46 sources. This new transform is implemented only in the TFO block in TRAUs and
e828468b0afd doc/TFO-xform/Theory: article written
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47 nowhere else (in classic GSM architecture), and can be exercised only by
e828468b0afd doc/TFO-xform/Theory: article written
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48 establishing a TFO call between two interworking TRAUs.
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49
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50 There are 3 main parts to this TFO transform, 3 main areas where anyone who
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51 seeks to implement this transform has to think hard and come up with an
e828468b0afd doc/TFO-xform/Theory: article written
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52 innovative solution:
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53
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54 1) Error concealment in non-DTX speech: if an errored frame (BFI) appears after
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Mychaela Falconia <falcon@freecalypso.org>
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55 non-SID speech frames (meaning non-DTX speech), the transform has to fill in
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56 substitution/muting "speech" frames (meaning codec frames that look like
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57 valid speech frames) in the stream going to call leg B DL.
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58
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59 2) Comfort noise insertion: if the incoming frame stream from call leg A UL
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60 contains SID frames (DTXu) but the same are not allowed on call leg B DL
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61 (no DTXd), the transform has to insert "speech" frames (in the same
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62 parenthetical meaning) that represent comfort noise, as intended by Alice's
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63 phone that transmitted SID with certain CN parameters.
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64
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65 3) Comfort noise muting: handling the case where the incoming UL frame stream
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66 goes into CN insertion state (via one or more SID frames), but then goes
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67 total BFI, with no more SID update frames appearing in TAF positions. In
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68 the case of a single codec leg from a source encoder to an end decoder,
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69 standard decoders are required by their respective DTX specs to gradually
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70 mute their CN output, to indicate channel breakdown to the user - the TFO
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71 transform has to produce the same effect.
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72
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73 All 3 of the just-listed functions are explicitly called out in the TFO spec, in
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74 each case with the same language of "shall" followed by "subject to manufacturer
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75 dependent future improvements and is not part of this recommendation."
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76
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77 DTXd or no DTXd
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78 ===============
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79
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80 When the destination call leg operates without DTXd, the TFO transform can only
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Mychaela Falconia <falcon@freecalypso.org>
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81 emit frames that are well-formed speech frames for the respective codec, no SID
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82 frames. In this case the transform has to do "everything", all 3 of the listed
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83 functions, although the last function of CN muting may be either separate or
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84 absorbed into CN generation function depending on the codec.
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85
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86 OTOH, when call leg B has DTXd enabled/allowed, there is more room for
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87 additional complexity. The simplest solution would be to not make use of DTXd
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88 capability and always emit speech frames - but the problem with this simple
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89 approach is teleological. If a GSM network operator runs with DTXd enabled,
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90 presumably that operator seeks to reap the benefits of DTXd as in reduction of
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91 radio interference, in which case a TFO transform that fails to make use of DTXd
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92 capability would defeat the purpose. Hence if someone sets out to implement a
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93 TFO transform that supports full utilization of DTXd, they would have to do
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94 additional work:
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95
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96 * The function of CN insertion in the transform _mostly_ goes away: if a valid
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97 SID frame comes, the TRAU caches it and repeats it continuously until the
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98 next SID update, allowing the BTS to select which SID frames it will actually
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99 transmit based on its SACCH alignment. But more complex handling is still
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100 needed if the first SID frame (the one that begins CN insertion period) came
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101 in as invalid SID, and the function of CN muting takes on new significance.
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102
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103 * CN muting: when the cached SID expires and no new SID updates arrive in TAF
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104 positions, the TFO transform has to indicate somehow to Bob that Alice's call
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105 leg is having trouble, which will be easy or difficult depending on what rules
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106 are specified in the codec specs for SID interpolation in the final receiver.
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107
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108 * Error concealment in non-DTX speech: at first glance this function appears to
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109 be exactly the same whether DTXd is used or not. But consider the case of
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110 total channel breakdown, such that the incoming frame stream becomes all BFI:
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111 how should this case be handled? In the absence of DTXd, the output of the
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112 TFO transform becomes a stream of silence frames, meaning some kind of
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113 "speech" frames that produce total silence at the end decoder. But if the
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114 network operates with DTXd with the aim of reducing radio interference, these
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115 silence "speech" frames should be replaced with SIDs whose parameters are
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116 chosen to produce silent output.
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117
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118 Current approach in Themyscira libraries
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119 ========================================
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120
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121 There is a desire to implement TFO transform for all 3 classic GSM codecs in
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122 Themyscira Wireless GSM codec libraries suite, and the first question to be
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123 decided is the policy with regard to DTXd.
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124
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125 The current approach is to not implement any DTXd support, i.e., implement the
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126 TFO transform only in its no-DTXd basic form. The reason for this decision is
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127 based on the reality of small-capacity single-carrier cells: given that the
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128 total number of humans who actually _want_ to use GSM (as opposed to whatever
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129 latest 4G/5G/etc is peddled by Big Tech mafia) is vanishingly small, there is
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130 currently no justification for building higher-capacity GSM cells that use more
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131 than a single 200 kHz radio carrier. And if each GSM cell consists of only one
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132 radio carrier (the BCCH carrier, also called C0 in the specs), then physical
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133 DTXd (as in actually turning off radio Tx, as opposed to "logical" DTXd where
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134 that effect is merely faked for the MS by transmitting dummy bursts or
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135 induced-BFI frames) is simply impossible. Therefore, in the present state of
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136 human condition, there is no justification for expending the effort to implement
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137 additional complexity for proper DTXd.