FreeCalypso > hg > gsm-codec-lib
annotate doc/Calypso-TCH-downlink @ 556:18aca50d68df default tip
doc/Calypso-TCH-downlink: update for FR1 BFI-with-data
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Fri, 11 Oct 2024 01:54:00 +0000 |
parents | 5e2d849a4fbc |
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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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60 |
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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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70 |
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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 [Update: as of libgsmfr2 version 2.1.0, the above paragraph is no longer fully |
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114 correct. The new version of our Rx DTX handler does look at bad traffic frames |
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115 (BFI=1) in order to catch those that should be handled as invalid SID rather |
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116 than unusable frame classification, and our original gsmx binary file format |
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117 is thus no longer sufficient to match the full capabilities of our library. |
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118 The new hexadecimal file format of TW-TS-005 Annex A solves this shortcoming - |
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119 but the present suite of Calypso TCH downlink utilities is currently a low |
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120 priority, hence no code updates are currently being worked on in this area.] |
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121 |
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122 * In addition to TCH DL capture files, gsmfr-dlcap-parse also accepts the hex |
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123 output from fc-vm2hex, originating from TCS211 voice memo recordings, |
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124 including fc-vm2hex output in the case of VM recordings made in DTX mode. |
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125 However, if the objective is to play that VM recording and not just look at |
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126 parsed bits, the correct approach is to convert the VM file to gsmx with |
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127 fc-vm2gsmx, and then decode with gsmfr-decode. Using fc-vm2hex followed by |
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128 gsmfr-dlcap-gsmx instead of fc-vm2gsmx won't work! |
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129 |
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130 Catching the output of the network-side speech encoder |
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131 ====================================================== |
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132 |
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133 The set of FR1 test sequences included with later versions of GSM 06.10 spec |
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134 and the set of EFR test sequences in GSM 06.54 include special synchronization |
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135 sequences that can be fed to the G.711 PCMA or PCMU input of the TRAU in the |
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136 downlink direction, and the set of 160 possible speech encoder outputs for each |
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137 codec that can result from the TRAU processing that DL input, depending on the |
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138 alignment between the input and the location of 20 ms frame boundaries for the |
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139 encoder. In the case of EFR, there is a second dimension of uncertainty when |
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140 experimenting with GSM networks that aren't your own: in addition to the unknown |
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141 alignment of G.711 input (160 possibilities), there is the unknown of whether |
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142 the network transcoder implements classic EFR or an AMR-EFR hybrid - see our |
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143 AMR-EFR-philosophy and AMR-EFR-hybrid-emu articles. However, thanks to the work |
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144 we did in vband-misc Hg repository, we now have a fully backward-compatible |
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145 extended version of ETSI's seqsync[au].inp TRAU DL inputs (the last frame of |
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146 160 samples that isn't EHF is simply repeated twice) that allows us to |
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147 distinguish between two possible styles of EFR implementation in the network |
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148 transcoder, producing 320 possible outputs on GSM Um DL for 160 possible |
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149 alignments times two possible EFR implementation options. |
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150 |
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151 However, tools are still needed on the GSM MS side of the test setup, reading |
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152 the TCH DL capture produced with FreeCalypso tools and detecting which of the |
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153 possible 160 (FR1) or 320 (EFR) encoded frames have been produced. (320 or |
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154 even 160 possible frames is too many to check by hand!) These tools are |
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155 provided in gsmfr-dlcap-sync and gsmefr-dlcap-sync, added to Themyscira GSM |
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156 codec libraries and utilities suite as of gsm-codec-lib-r3. Each of these |
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157 utilities takes two command line arguments: the name of TCH DL capture file to |
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158 read and analyze, and an "alaw" or "ulaw" keyword argument selecting the match |
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159 table to use. Specify alaw if you are feeding seqsynca.inp to a PCMA-native |
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160 TRAU or other GSM network speech transcoder, or ulaw if you are feeding |
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161 seqsyncu.inp to a PCMU-native network. The program will read the entire TCH DL |
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162 capture, looking for matches, and will report any matches it finds. |
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163 |
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164 Both gsmfr-dlcap-sync and gsmefr-dlcap-sync implement the logic of looking for |
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165 the respective codec's DHF followed by one of 160 (FR1) or 320 (EFR) distinct |
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166 encoded frames. In the case of EFR, if the network transcoder implements |
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167 AMR-EFR and the alignment shift happens to be in the [120,159] range, there |
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168 will also be an MR122 DHF sandwiched between the standard EFR DHF and the |
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169 distinct encoded frame (unique for each of the 40 possible alignments in this |
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170 range) if the AMR-EFR hybrid is implemented like our amr_dhf_subst_efr2() |
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171 function, matching the network of T-Mobile USA. gsmefr-dlcap-sync looks for |
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172 both EFR and MR122 DHF; in the case of matches to AMR-EFR offset [120,159], the |
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173 tool's indication whether the unique frame was preceded by EFR or MR122 DHF |
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174 indicates how the alien network transcoder implements its DHF transformation; |
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175 in the case of other matches, seeing MR122 DHF is an unexpected error condition, |
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176 and it is reported as such. |
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177 |
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178 These tools cover just one step in the workflow of reverse-engineering an alien |
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179 GSM network's speech transcoder and confirming if it matches standard EFR or |
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180 the AMR-EFR hybrid as currently found in the wild. The complete workflow in |
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181 the GSM downlink direction will typically be as follows: |
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182 |
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183 1) Using sipout-test-voice utility from the sipout-test-utils suite, establish |
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184 a test call from IP-PSTN to a test MS served by the GSM network under study. |
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185 AT%SPVER will typically need to be used to cause the network to assign the |
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186 desired codec on this call. |
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187 |
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188 2) While making a TCH DL recording on the FreeCalypso MS used in this test, |
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189 play seqsync[au].inp (or the extended version with the last frame sent twice) |
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190 into the G.711 PCM stream from IP-PSTN side, using 'play' command of |
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191 sipout-test-voice. |
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192 |
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193 3) Run gsmfr-dlcap-sync or gsmefr-dlcap-sync on the DL recording from the |
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194 previous step, as appropriate. |
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195 |
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196 4) Once the alignment is known, use 'play-offset' command in sipout-test-voice |
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197 to play a longer test sequence into the same call, and have another TCH DL |
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198 recording running on the test MS. |
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199 |
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200 5) If the longer test sequence begins with the same seqsync[au].inp preamble |
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201 (which is recommended), playing it with the correct offset from IP-PSTN side |
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202 should result in gsm[e]fr-dlcap-sync reporting zero offset on the new DL |
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203 recording. However, gsm[e]fr-dlcap-sync on this second DL capture should |
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204 indicate the line number where the interesting part begins. |
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205 |
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206 6) Extract the part of interest identified in the previous step, convert it to |
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207 gsmx format with gsm[e]fr-dlcap-gsmx, and compare it against the expected |
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208 encoded frame sequence. |