FreeCalypso > hg > freecalypso-tools
annotate doc/TCH-bit-access @ 821:16bee7b6bd6b
buzplayer: PWT needs to be turned on in order to not hang
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sun, 30 May 2021 04:01:33 +0000 |
parents | 2748f257312b |
children | 3de3b34189be |
rev | line source |
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doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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1 It has been discovered that the DSP ROM in the Calypso GSM baseband processor |
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2 implements one nifty feature which is not used at all in standard phone or modem |
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3 operation, but which can be used for all kinds of interesting hacks: the traffic |
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4 channel (TCH) bits coming out of the GSM 05.03 channel decoder in the downlink |
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5 direction (to be fed to one of several speech decoders) can be read out of the |
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6 DSP's API RAM in real time, and in the uplink direction the user can feed her |
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7 own bits to the input of the GSM 05.03 channel encoder, effectively suppressing |
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8 the output of the internal vocoder. |
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9 |
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10 The DSP mechanism in question is known to work in TCH/FS and TCH/EFS channel |
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11 modes, corresponding to the FR and EFR codecs; it is not currently known if it |
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12 can also be used in TCH/HS, data traffic or AMR modes. I am aware though of an |
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13 anecdotal report that someone tried to make this feature work with AMR, but was |
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14 unsuccessful - hence we should be prepared for the possibility that the hack |
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15 is not possible with AMR. |
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16 |
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17 In order to make use of this TCH bit access feature, one needs 3 things: |
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18 |
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19 1) Firmware on the Calypso (the ARM part) that reads downlink bits from the DSP |
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20 and writes uplink bits into it while doing everything else that a GSM fw must |
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21 do in order to operate the MS; |
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22 |
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23 2) Some protocol for passing these TCH bits into and out of the Calypso device; |
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24 |
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25 3) A source for TCH UL bits and a sink for TCH DL bits on the external host. |
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26 |
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27 In the case of FreeCalypso, we have defined our own protocol for passing TCH |
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28 bits into and out of Calypso GSM devices running one of our firmwares in the |
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29 form of an extension to TI's RVTMUX interface, i.e., we have defined a new |
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30 RVTMUX channel for this TCH interface and defined the packet types and formats |
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31 to be sent over the wire. On the Calypso side this interface is implemented in |
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32 FreeCalypso GSM firmwares (currently only in Citrine, but we can also make a |
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33 special version of TCS211 with this feature added if the need arises), and on |
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34 the host tools side there is support in rvinterf and fc-shell. |
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35 |
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36 The TCH bit access mechanism in FreeCalypso has been designed with an objective |
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37 of presenting to the user exactly what TI's DSP presents to us, i.e., standing |
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38 out of the way as much as possible. TI's DSP presents TCH downlink bits and |
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39 accepts TCH uplink bits in the form of an array of 16-bit words; the bit order |
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40 within these words corresponds to the GSM 05.03 channel encoder bit order (with |
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41 a couple of TI-specific quirks documented below) and NOT that of the GSM 06.10 |
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42 or EFR codecs. On the RVTMUX serial interface between the Calypso device and |
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43 the external host we transfer each TCH frame as a block of 33 bytes; our Calypso |
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44 firmwares translate between these bytes and the DSP's 16-bit words, but do not |
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45 reorder or change the bits in any way. |
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46 |
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47 On the host tools side our fc-shell utility provides user commands to save TCH |
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48 DL bits into a file and to play TCH UL bits from a file; in the present version |
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49 these files are written and read in an ASCII-based hex format. In these ASCII |
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50 files each TCH frame is represented as a string of 66 hexadecimal digits, and |
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51 these hex digits correspond directly to the 33 bytes being read out of or |
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52 written into DSP API words. Therefore, in order to generate and/or interpret |
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53 these hexadecimal strings correctly, you (the user) need to understand the bit |
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54 order and mapping used by TI's implementation of the GSM 05.03 channel encoder. |
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55 |
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56 Recall from the GSM specs that the 260 bits which comprise one speech frame are |
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57 not all treated equally, instead they are divided into 182 class 1 bits which |
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58 are protected by a convolutional encoder and 78 class 2 bits which are |
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59 transmitted without any forward error correction. Furthermore, the first 50 of |
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60 the class 1 bits are also protected by a CRC. The order in which the bits |
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61 appear on TI's DSP interface corresponds to this division, known as the order of |
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62 subjective importance. |
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63 |
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64 Now let's look at the actual bit order and mapping which you need to understand |
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65 in order to make sense of the hex strings in tch record and tch play files. |
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66 The bit numbering is from the most significant bit to the least significant bit, |
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67 i.e., in our string of 66 hex digits the most significant bit of the leftmost |
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68 digit is bit 0 and the least significant bit of the rightmost digit is bit 263. |
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69 TI's DSP assigns these bits as follows: |
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70 |
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71 * Bits 0 through 181 correspond to the 182 protected (class 1) bits in the |
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72 standard GSM 05.03 order; |
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73 |
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74 * Bits 182 through 185 are unused - put zeros there if you are generating them |
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75 yourself; |
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76 |
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77 * Bits 186 through 263 correspond to the 78 unprotected (class 2) bits in the |
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78 standard GSM 05.03 order. |
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79 |
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80 Uplink testing |
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81 ============== |
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82 |
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83 The uplink sending mechanism can be exercised as follows: |
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84 |
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85 1. Record a speech sample in the GSM 06.10 codec format using any Unix/Linux |
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86 audio tool that can write the de facto standard libgsm format. For example, |
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87 using SoX: |
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88 |
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89 rec -c1 recording.gsm |
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90 |
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91 SoX will write the recording in the GSM 06.10 libgsm format based on the |
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92 .gsm suffix at the end of the recording file name; the -c1 option is needed |
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93 to disable stereo, otherwise the recording will be slowed down 2x. |
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94 |
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95 2. Convert it from libgsm standard format into our ad hoc hex strings for |
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96 playing into the TCH uplink: |
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97 |
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98 fc-fr2tch recording.gsm recording.tch-ul |
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99 |
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100 3. In fc-shell, when you are in an established voice call, issue this command: |
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101 |
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102 tch play recording.tch-ul |
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103 |
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104 You should now hear the speech sample you recorded in step 1 above on the other |
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105 end of the GSM call. Please note, though, that for this example to work, the |
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106 call must be connected in the FR codec mode, not EFR. If you try it on an EFR |
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107 call, the vocoder on the other end of the call will try to interpret your FR |
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108 codec bits per EFR, resulting in garbage. In order to make this procedure work |
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109 properly with EFR, you would need to generate a speech sample in EFR format and |
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110 then put it in the correct hex string form with the correct bit order for |
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111 feeding to TI's GSM 05.03 channel encoder implementation, and at the present |
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112 moment the tools to do this feat are lacking. |
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113 |
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114 The fc-fr2tch step above is new with the current version of FreeCalypso host |
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115 tools. My original implementation of the tch play command in fc-shell read |
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116 libgsm GSM 06.10 speech files directly, but I changed it to read hex strings |
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117 which correspond literally to what goes into TI's implementation of the 05.03 |
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118 channel encoder to make it more general: |
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119 |
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120 1) to support playing EFR speech bits into TCH/EFS channels; |
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121 2) to support people who may want to pass totally non-speech data over traffic |
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122 channels in TFO mode - see below. |
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123 |
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124 Downlink testing |
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125 ================ |
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126 |
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127 When you are in an established voice call in fc-shell, you can record the |
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128 downlink TCH bits as follows: |
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129 |
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130 tch record <name of file to put the recording into> |
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131 |
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132 If you would like to record an entire call from the beginning, issue the |
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133 tch record command as above before the ATD or ATA command that dials or answers |
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134 the call. Either way, whether you are recording a call from the beginning or |
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135 from the middle, you need to eventually stop your recording with this command: |
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136 |
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137 tch record stop |
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138 |
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139 You can issue this stop command before or after the call is terminated, but |
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140 until you issue this tch record stop command, the output file is not closed and |
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141 thus may not be written to the file system. |
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142 |
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143 The recording is written in an ASCII format that is similar but not identical |
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144 to what the tch play command takes as its input. Here is an example of what |
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145 you might get from tch record: |
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146 |
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147 C214 281D 0063 81C008000008200046000000000000000000000000000007056608060B0A010B09 |
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148 C204 0540 003A 9391480D3051F4BD81DF5EB35069BEBC4AEDF756351C4C19689BB1CA4EA5D4F5F5 |
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149 C204 05A2 0047 65B80F9E690F7C8CE4ED80DEF69DA6436518AB99ABCE5815E6B562C5CE4EAC5DC5 |
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150 C204 04CA 0044 A4483744B04371ED0334ECB350AF28C639B7F095519EF0242D299B6405124F77A5 |
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151 C214 2544 0066 83800400000C1000E80000000000000000000000000000300674A07070F080D0B2 |
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152 C200 4E8A 0000 07071DF0F83B9521EE61CFF095AA8C0E560300F6A5573C31F3E00601ED4AAE7E2F |
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153 C200 4C94 0000 077385ADE20450B2E410961D6C5B0A173ACF9E2D38D77C28CED8495D88AA4DE72E |
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154 C200 4ABC 0000 077F0CF86004132DA5C0A6D5A4B0BD4B28159A07D8F4282DC6AAAB27503BC02701 |
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155 |
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156 The example above is quoted from an actual recording made from a call to WWV at |
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157 +1-303-499-7111, a time of day service. This example shows 5 garbled frames |
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158 followed by 3 good ones. |
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159 |
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160 In each line the first 3 space-separated 16-bit words are status words from the |
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161 DSP, and the rest is a hex string of 66 digits giving the 260-bit frame with 4 |
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162 extra dummy bits at the boundary between the protected and unprotected portions. |
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163 |
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164 Now here comes the unpleasant part: radio systems are inherently error-prone, |
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165 hence if you are naively expecting every received downlink frame to be a valid |
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166 speech frame that matches exactly what the other end transmitted, you will be |
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167 disappointed. The downlink frame stream will always consist of both valid and |
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168 invalid frames, and in the standard processing chain inside TI's DSP the speech |
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169 decoder block that receives these frames from the channel decoder will look at |
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170 the status words written by the latter in order to decide what to do with each |
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171 frame. But unfortunately I don't know the details, as the DSP is a mostly |
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172 undocumented black box. |
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173 |
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174 Our FreeCalypso firmwares forward the downlink frame status words from the DSP |
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175 to the external host and my current implementation of the tch record command |
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176 writes them into the file because it is valuable information which should not |
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177 be discarded, but unfortunately I don't know their meaning. The first status |
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178 word consists of bit flags, the last one seems to be some kind of error count |
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179 (what kind of errors? I don't know), and the middle one is totally unknown. |
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180 |
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181 There is also an fc-tch2fr utility that does sort of an inverse of fc-fr2tch, |
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182 except that fc-tch2fr expects files written by tch record which contain the |
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183 status words at the beginning - the files written by fc-fr2tch and read by |
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184 tch play don't have these status words; they just have 66 hex digits per line |
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185 giving the bits to be fed to the GSM 05.03 block in the DSP. The fc-tch2fr |
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186 utility works by disregarding the status words; the result will be a .gsm file |
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187 which you can play with SoX etc, but it will produce horrible sounds wherever |
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188 some garbled frames were received on the call downlink. |
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189 |
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190 Passing non-speech data over TCH |
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191 ================================ |
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192 |
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193 If you are incredibly lucky and you happen to live in a part of the world where |
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194 the local GSM network implements TFO (tandem-free operation, see GSM 02.53), |
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195 i.e., if your GSM network transparently passes codec frames from one end of the |
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196 call to the other without transcoding back and forth in the middle, you might |
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197 be able to send arbitrary non-speech data bits over this TCH connection. If |
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198 you are going to attempt such a feat, you are mostly on your own as the GSM |
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199 network in my part of the world does not support TFO, but here are some general |
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200 ideas: |
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201 |
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202 * Heed the difference between the 182 protected bits and the 78 unprotected |
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203 bits of each 260-bit frame. The bits in the first class are less likely to |
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204 be corrupted by radio errors. Also the first 50 out of these class 1 bits |
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205 are protected by a CRC, so if they get corrupted, there will be a flag bit |
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206 somewhere in the DSP status words - but I don't know which bit it is... |
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207 |
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208 * Given the lack of detailed information about the exact meaning of the DSP |
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209 status words, implement some error checking of your own, e.g., an in-band |
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210 CRC within your frame payload so you can check on the receiving end if you |
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211 got an uncorrupted frame from your data source or not. |
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212 |
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213 * Expect to lose some frames - even if your radio environment is pristine and |
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214 error-free, some frames will still be lost because their timeslots had to be |
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215 stolen by FACCH on either end of the call. You will need to implement |
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216 acknowledgements, retransmissions and retries - the usual stuff. |
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217 |
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218 EFR differences |
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219 =============== |
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220 |
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221 TCH/FS and TCH/EFS channel modes corresponding to the FR and EFR codecs, |
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222 respectively, share the same channel encoder that takes 260-bit frames as input. |
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223 An EFR speech frame is only 244 bits, but as the GSM 05.03 spec explains, each |
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224 EFR frame gets expanded to 260 bits by adding some CRC and repetition bits. |
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225 One would thus expect that the behaviour of the channel encoder block should be |
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226 strictly identical between TCH/FS and TCH/EFS channel modes, but it appears that |
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227 TI's implementation exhibits one difference in behaviour between the two modes. |
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228 |
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229 It appears that TI's implementation of the GSM 05.03 channel encoder computes |
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230 the EFR-specific CRC bits internally, such that bits [65,72] of each uplink |
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231 frame fed to the DSP in the TCH/EFS mode need to be 0 and will be replaced by |
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232 the DSP with CRC bits computed per the standard. It also appears that their |
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233 implementation of the channel decoder verifies this CRC, sets a status bit |
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234 indicating whether the check succeeded or failed, and then zeroes these bits |
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235 out, i.e., the original bits in these CRC bit positions are lost. No problem |
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236 if what you are passing over TCH/EFS is indeed EFR speech frames, but it will |
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237 be a problem if you are passing non-speech data over your TCH and put something |
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238 else in those bits which the spec allocates to CRC. |