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doc/Calypso-TCH-downlink: grammar fix
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 20 May 2024 01:24:22 +0000
parents e512f0d25409
children 18aca50d68df
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1 It has been discovered that the implementation of standard signal processing
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2 chains for speech TCH downlink and uplink in the DSP ROM in the Calypso GSM
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3 baseband processor allows these signal processing chains to be tapped at certain
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4 points, as detailed in the TCH-tap-modes article in our freecalypso-docs Hg
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5 repository. There is a mechanism to capture the stream of received traffic
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6 frames on TCH DL, and there is another mechanism by which an externally supplied
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7 stream can be "played" into TCH UL.
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8
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9 I (Mother Mychaela) previously played with this functionality back in 2016, and
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10 it's been mostly shelved since then. This functionality became interesting
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11 once again in late 2022: now that we have a proper set of codec libraries (the
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12 present package) and a proper understanding of Rx DTX handling requirements, we
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13 can take another shot at decoding TCH downlink captures taken from Calypso GSM
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14 MS.
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15
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16 The overall functionality is described in the TCH-tap-modes article in
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17 freecalypso-docs; the mechanism for capturing TCH DL bits from Calypso DSP is
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18 split between FreeCalypso GSM MS firmware (added to FC Tourmaline as of
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19 2022-12-13) and the fc-shell utility in the FC host tools package, updated as
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20 of fc-host-tools-r18 to support the new FreeCalypso fw. There is also a set of
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21 utilities included in the present GSM codec libraries & utilities package for
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22 parsing and decoding these Calypso TCH DL captures; the present document
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23 describes these utilities.
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24
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25 As explained in the TCH-tap-modes article in freecalypso-docs, the mechanism
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26 for capturing TCH DL is currently implemented for TCH/FS, TCH/HS and TCH/EFS,
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27 corresponding to FR1, HR1 and EFR codecs. However, further parsing and decoding
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28 support has only been implemented for FR1 and EFR codecs in the present package,
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29 in the form of the following utilities:
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30
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31 gsmfr-dlcap-parse This program reads a TCH/FS DL capture file and parses
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32 it for human analysis. All input fields are passed
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33 through to the output, but the program also computes
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34 the ternary SID flag of GSM 06.31 section 6.1.1 from
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35 the payload bits (for comparison against what the DSP
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36 wrote in its status word 0) and prints all broken-down
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37 parameter fields of each GSM 06.10 FR1 codec frame.
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38
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39 gsmfr-dlcap-gsmx This program reads a TCH/FS DL capture file and converts
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40 it into an extended-libgsm (gsmx) file containing a mix
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41 of FR1 codec frames and Themyscira BFI markers. The
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42 latter BFI markers will be emitted in those frame
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43 positions where FACCH was received instead of speech,
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44 or where the DSP otherwise indicated BFI=1. The gsmx
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45 output from this utility needs to be fed to gsmfr-decode
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46 from the present package, so that our FR1 Rx DTX
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47 preprocessor will take care of SIDs and BFIs, completing
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48 the required GSM MS processing chain for TCH/FS DL.
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49
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50 gsmefr-dlcap-parse This program reads a TCH/EFS DL capture file and parses
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51 it for human analysis. All input fields are passed
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52 through to the output, but the program also computes
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53 the ternary SID flag of GSM 06.81 section 6.1.1 from
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54 the payload bits (for comparison against what the DSP
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55 wrote in its status word 0) and prints all broken-down
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56 parameter fields of each EFR codec frame. Finally, each
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57 triplicated bit group of GSM 05.03 section 3.1.1.2 is
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58 printed as an octal digit, to aid human analysis of how
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59 the DSP writes these bits in its a_dd_0 buffer.
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61 gsmefr-dlcap-gsmx This program reads a TCH/EFS DL capture file and
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62 converts it into a gsmx binary file, containing a mix
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63 of EFR codec frames and Themyscira BFI markers. The
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64 latter BFI markers will be emitted in those frame
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65 positions where FACCH was received instead of speech,
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66 or where the DSP otherwise indicated BFI=1. The gsmx
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67 output from this utility needs to be fed to
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68 gsmefr-decode (or gsmefr-decode-r) from the present
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69 package.
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71 gsmefr-dlcap-dec This program reads a TCH/EFS DL capture file and feeds
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72 it directly to the EFR reference decoder implemented in
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73 libgsmefr, without going through a gsmx intermediary.
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74
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75 Additional notes:
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76
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77 * The new gsmfr-dlcap-gsmx utility described above replaces the old fc-tch2fr
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78 utility from FC host tools - the latter should now be considered a bogon.
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79 The required GSM MS processing chain for TCH/FS DL includes the step of Rx
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80 DTX handler between the output of GSM 05.03 channel decoder and the input of
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81 GSM 06.10 speech decoder; the old chain of fc-tch2fr followed by libgsm
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82 decoding omitted this critical step and thus produced very unkind-on-ears
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83 sounds.
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84
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85 * gsmefr-dlcap-dec has been written as a bold attempt to replicate the complete
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86 Rx DTX handler and speech decoder (the part of TCH DL processing chain that
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87 sits past the a_dd_0 buffer) as they are implemented inside TI's DSP. Such a
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88 feat won't be possible for FR1 codec (other than by a Herculean effort of full
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89 static reversing of the DSP ROM) because there is no bit-exact definition of
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90 FR1 Rx DTX functions in GSM specs, but for EFR there is a bit-exact reference
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91 implementation from ETSI. *If* TI's DSP matches this bit-exact reference
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92 (there are some aspects of Rx DTX handling where this bit-exact reference is
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93 considered to be an example rather than normative, see GSM 06.61), then there
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94 is a chance we could replicate TI's DSP chain externally - but only if we can
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95 figure out exactly how the bits of a_dd_0[0] drive the logic of their Rx DTX
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96 handler. The Mother's plan is to capture the DSP's decoded speech output from
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97 MCSI on an FCDEV3B using a small FPGA board with a PCM-to-UART logic function,
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98 while simultaneously capturing TCH DL bits in the a_dd_0 buffer, then run
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99 gsmefr-dlcap-dec on the captured TCH DL booty and see if we can replicate the
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100 DSP's end output - but until then, this gsmefr-dlcap-dec program should be
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101 treated as an unfinished experiment in progress.
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102
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103 * In the case of FR1 codec, there is no prescribed bit-exact definition for the
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104 Rx DTX handler (GSM 06.11, 06.12 and 06.31 specs define general requirements,
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105 but aren't bit-exact in most aspects), and the way in which we (Themyscira
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106 Wireless) have implemented our FR1 Rx DTX handler (libgsmfr2 in the present
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107 package) perfectly matches our gsmx binary file format for good vs bad frames.
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108 Therefore, in the case of FR1 codec there is nothing to be gained by skipping
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109 gsmx and calling library functions directly, and thus there is no FR1
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110 counterpart to gsmefr-dlcap-dec - just use gsmfr-dlcap-gsmx followed by
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111 gsmfr-decode or gsmfr-decode-r.
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112
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113 * In addition to TCH DL capture files, gsmfr-dlcap-parse also accepts the hex
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114 output from fc-vm2hex, originating from TCS211 voice memo recordings,
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115 including fc-vm2hex output in the case of VM recordings made in DTX mode.
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116 However, if the objective is to play that VM recording and not just look at
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117 parsed bits, the correct approach is to convert the VM file to gsmx with
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118 fc-vm2gsmx, and then decode with gsmfr-decode. Using fc-vm2hex followed by
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119 gsmfr-dlcap-gsmx instead of fc-vm2gsmx won't work!
475
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120
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121 Catching the output of the network-side speech encoder
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122 ======================================================
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123
482
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124 The set of FR1 test sequences included with later versions of GSM 06.10 spec
475
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125 and the set of EFR test sequences in GSM 06.54 include special synchronization
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126 sequences that can be fed to the G.711 PCMA or PCMU input of the TRAU in the
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127 downlink direction, and the set of 160 possible speech encoder outputs for each
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128 codec that can result from the TRAU processing that DL input, depending on the
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129 alignment between the input and the location of 20 ms frame boundaries for the
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130 encoder. In the case of EFR, there is a second dimension of uncertainty when
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131 experimenting with GSM networks that aren't your own: in addition to the unknown
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132 alignment of G.711 input (160 possibilities), there is the unknown of whether
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133 the network transcoder implements classic EFR or an AMR-EFR hybrid - see our
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134 AMR-EFR-philosophy and AMR-EFR-hybrid-emu articles. However, thanks to the work
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135 we did in vband-misc Hg repository, we now have a fully backward-compatible
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136 extended version of ETSI's seqsync[au].inp TRAU DL inputs (the last frame of
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137 160 samples that isn't EHF is simply repeated twice) that allows us to
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138 distinguish between two possible styles of EFR implementation in the network
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139 transcoder, producing 320 possible outputs on GSM Um DL for 160 possible
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140 alignments times two possible EFR implementation options.
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141
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142 However, tools are still needed on the GSM MS side of the test setup, reading
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143 the TCH DL capture produced with FreeCalypso tools and detecting which of the
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144 possible 160 (FR1) or 320 (EFR) encoded frames have been produced. (320 or
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145 even 160 possible frames is too many to check by hand!) These tools are
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146 provided in gsmfr-dlcap-sync and gsmefr-dlcap-sync, added to Themyscira GSM
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147 codec libraries and utilities suite as of gsm-codec-lib-r3. Each of these
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148 utilities takes two command line arguments: the name of TCH DL capture file to
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149 read and analyze, and an "alaw" or "ulaw" keyword argument selecting the match
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150 table to use. Specify alaw if you are feeding seqsynca.inp to a PCMA-native
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151 TRAU or other GSM network speech transcoder, or ulaw if you are feeding
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152 seqsyncu.inp to a PCMU-native network. The program will read the entire TCH DL
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153 capture, looking for matches, and will report any matches it finds.
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154
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155 Both gsmfr-dlcap-sync and gsmefr-dlcap-sync implement the logic of looking for
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156 the respective codec's DHF followed by one of 160 (FR1) or 320 (EFR) distinct
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157 encoded frames. In the case of EFR, if the network transcoder implements
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158 AMR-EFR and the alignment shift happens to be in the [120,159] range, there
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159 will also be an MR122 DHF sandwiched between the standard EFR DHF and the
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160 distinct encoded frame (unique for each of the 40 possible alignments in this
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161 range) if the AMR-EFR hybrid is implemented like our amr_dhf_subst_efr2()
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162 function, matching the network of T-Mobile USA. gsmefr-dlcap-sync looks for
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163 both EFR and MR122 DHF; in the case of matches to AMR-EFR offset [120,159], the
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164 tool's indication whether the unique frame was preceded by EFR or MR122 DHF
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165 indicates how the alien network transcoder implements its DHF transformation;
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166 in the case of other matches, seeing MR122 DHF is an unexpected error condition,
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167 and it is reported as such.
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168
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169 These tools cover just one step in the workflow of reverse-engineering an alien
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170 GSM network's speech transcoder and confirming if it matches standard EFR or
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171 the AMR-EFR hybrid as currently found in the wild. The complete workflow in
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172 the GSM downlink direction will typically be as follows:
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173
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174 1) Using sipout-test-voice utility from the sipout-test-utils suite, establish
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175 a test call from IP-PSTN to a test MS served by the GSM network under study.
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176 AT%SPVER will typically need to be used to cause the network to assign the
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177 desired codec on this call.
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178
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179 2) While making a TCH DL recording on the FreeCalypso MS used in this test,
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180 play seqsync[au].inp (or the extended version with the last frame sent twice)
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181 into the G.711 PCM stream from IP-PSTN side, using 'play' command of
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182 sipout-test-voice.
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183
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184 3) Run gsmfr-dlcap-sync or gsmefr-dlcap-sync on the DL recording from the
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185 previous step, as appropriate.
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186
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187 4) Once the alignment is known, use 'play-offset' command in sipout-test-voice
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188 to play a longer test sequence into the same call, and have another TCH DL
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189 recording running on the test MS.
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190
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191 5) If the longer test sequence begins with the same seqsync[au].inp preamble
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192 (which is recommended), playing it with the correct offset from IP-PSTN side
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193 should result in gsm[e]fr-dlcap-sync reporting zero offset on the new DL
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194 recording. However, gsm[e]fr-dlcap-sync on this second DL capture should
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195 indicate the line number where the interesting part begins.
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196
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197 6) Extract the part of interest identified in the previous step, convert it to
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198 gsmx format with gsm[e]fr-dlcap-gsmx, and compare it against the expected
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199 encoded frame sequence.