FreeCalypso > hg > gsm-net-reveng

annotate doc/BFI-payload-fill @ 69:cf60172895fe default tip

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