FreeCalypso > hg > gsm-net-reveng

annotate doc/TFO-xform/Theory @ 34:35d38348c880

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
doc/TFO-xform/FRv1: article written
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 01 Sep 2024 06:28:35 +0000
parents e828468b0afd
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
33
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
1 TFO transform from uplink to downlink
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
2 =====================================
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
3
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
4 With all 3 classic GSM codecs (FRv1, HRv1, EFR) the original architecture calls
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
6 implications are:
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
7
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
9 mobile-generated speech into 64 kbit/s G.711. The Rx DTX handler, a subblock
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
10 of that speech decoder in the TRAU, handles error concealment (substitution
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
12 and once this speech stream has been transcoded to G.711, all trace of these
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
13 GSM-specific effects disappears.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
14
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
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>
parents:
diff changeset
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>
parents:
diff changeset
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>
parents:
diff changeset
19 with DTXd, it is a mixture of good speech and valid SID frames.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
20
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
22 Bob), and you wish to eliminate the quality-degrading effect of double or tandem
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
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>
parents:
diff changeset
25 contain BFI frame gaps that are never allowed in DL, and if the network deploys
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
27 small-capacity single-carrier cells), the representation of DTXu pauses coming
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
29 suitable for direct passing to the DL of the opposite call leg.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
30
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
31 The solution offered in the TFO spec (GSM 08.62) is a special transform from
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
33 could find, but I call it "TFO transform". In the original GSM 08.62 spec (up
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
34 to R99) this TFO transform is described in sections 8.2.1 and 8.2.2; when the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
36 UMTS), the description of TFO transform for classic GSM codecs moved to section
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
37 C.3.2.1.1.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
38
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
39 However, both spec versions only say what "shall" be done without any guidance
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
40 on how to do it algorithmically: the spec language is "subject to manufacturer
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
41 dependent future improvements and is not part of this recommendation."
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
43 of logical transform on codec frames (and a stateful one at that!) that never
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
44 appeared previously anywhere in classic GSM architecture, is not mentioned in
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
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>
parents:
diff changeset
46 sources. This new transform is implemented only in the TFO block in TRAUs and
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
47 nowhere else (in classic GSM architecture), and can be exercised only by
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
48 establishing a TFO call between two interworking TRAUs.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
49
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
50 There are 3 main parts to this TFO transform, 3 main areas where anyone who
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
51 seeks to implement this transform has to think hard and come up with an
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
52 innovative solution:
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
53
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
54 1) Error concealment in non-DTX speech: if an errored frame (BFI) appears after
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
55 non-SID speech frames (meaning non-DTX speech), the transform has to fill in
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
56 substitution/muting "speech" frames (meaning codec frames that look like
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
57 valid speech frames) in the stream going to call leg B DL.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
58
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
59 2) Comfort noise insertion: if the incoming frame stream from call leg A UL
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
60 contains SID frames (DTXu) but the same are not allowed on call leg B DL
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
61 (no DTXd), the transform has to insert "speech" frames (in the same
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
62 parenthetical meaning) that represent comfort noise, as intended by Alice's
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
63 phone that transmitted SID with certain CN parameters.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
64
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
65 3) Comfort noise muting: handling the case where the incoming UL frame stream
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
66 goes into CN insertion state (via one or more SID frames), but then goes
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
67 total BFI, with no more SID update frames appearing in TAF positions. In
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
68 the case of a single codec leg from a source encoder to an end decoder,
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
69 standard decoders are required by their respective DTX specs to gradually
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
70 mute their CN output, to indicate channel breakdown to the user - the TFO
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
71 transform has to produce the same effect.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
72
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
73 All 3 of the just-listed functions are explicitly called out in the TFO spec, in
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
74 each case with the same language of "shall" followed by "subject to manufacturer
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
75 dependent future improvements and is not part of this recommendation."
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
76
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
77 DTXd or no DTXd
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
78 ===============
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
79
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
80 When the destination call leg operates without DTXd, the TFO transform can only
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
81 emit frames that are well-formed speech frames for the respective codec, no SID
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
82 frames. In this case the transform has to do "everything", all 3 of the listed
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
83 functions, although the last function of CN muting may be either separate or
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
84 absorbed into CN generation function depending on the codec.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
85
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
86 OTOH, when call leg B has DTXd enabled/allowed, there is more room for
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
87 additional complexity. The simplest solution would be to not make use of DTXd
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
88 capability and always emit speech frames - but the problem with this simple
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
89 approach is teleological. If a GSM network operator runs with DTXd enabled,
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
90 presumably that operator seeks to reap the benefits of DTXd as in reduction of
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
91 radio interference, in which case a TFO transform that fails to make use of DTXd
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
92 capability would defeat the purpose. Hence if someone sets out to implement a
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
93 TFO transform that supports full utilization of DTXd, they would have to do
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
94 additional work:
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
95
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
96 * The function of CN insertion in the transform _mostly_ goes away: if a valid
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
97 SID frame comes, the TRAU caches it and repeats it continuously until the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
98 next SID update, allowing the BTS to select which SID frames it will actually
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
99 transmit based on its SACCH alignment. But more complex handling is still
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
100 needed if the first SID frame (the one that begins CN insertion period) came
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
101 in as invalid SID, and the function of CN muting takes on new significance.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
102
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
103 * CN muting: when the cached SID expires and no new SID updates arrive in TAF
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
104 positions, the TFO transform has to indicate somehow to Bob that Alice's call
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
105 leg is having trouble, which will be easy or difficult depending on what rules
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
106 are specified in the codec specs for SID interpolation in the final receiver.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
107
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
108 * Error concealment in non-DTX speech: at first glance this function appears to
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
109 be exactly the same whether DTXd is used or not. But consider the case of
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
110 total channel breakdown, such that the incoming frame stream becomes all BFI:
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
111 how should this case be handled? In the absence of DTXd, the output of the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
112 TFO transform becomes a stream of silence frames, meaning some kind of
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
113 "speech" frames that produce total silence at the end decoder. But if the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
114 network operates with DTXd with the aim of reducing radio interference, these
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
115 silence "speech" frames should be replaced with SIDs whose parameters are
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
116 chosen to produce silent output.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
117
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
118 Current approach in Themyscira libraries
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
119 ========================================
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
120
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
121 There is a desire to implement TFO transform for all 3 classic GSM codecs in
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
122 Themyscira Wireless GSM codec libraries suite, and the first question to be
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
123 decided is the policy with regard to DTXd.
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
124
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
125 The current approach is to not implement any DTXd support, i.e., implement the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
126 TFO transform only in its no-DTXd basic form. The reason for this decision is
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
127 based on the reality of small-capacity single-carrier cells: given that the
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
128 total number of humans who actually _want_ to use GSM (as opposed to whatever
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
129 latest 4G/5G/etc is peddled by Big Tech mafia) is vanishingly small, there is
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
130 currently no justification for building higher-capacity GSM cells that use more
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
131 than a single 200 kHz radio carrier. And if each GSM cell consists of only one
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
132 radio carrier (the BCCH carrier, also called C0 in the specs), then physical
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
133 DTXd (as in actually turning off radio Tx, as opposed to "logical" DTXd where
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
134 that effect is merely faked for the MS by transmitting dummy bursts or
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
135 induced-BFI frames) is simply impossible. Therefore, in the present state of
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
136 human condition, there is no justification for expending the effort to implement
e828468b0afd doc/TFO-xform/Theory: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
137 additional complexity for proper DTXd.