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annotate Speech-codec-selection @ 99:c28a1518d268

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Speech-codec-selection: document AT%SPVER
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 06 Jun 2023 03:54:53 +0000
parents 915ff61137ee
children
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98
915ff61137ee Speech-codec-selection: document MSCAP
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1 There exist a total of 5 speech TCH modes in classic GSM, outside of exotic
915ff61137ee Speech-codec-selection: document MSCAP
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2 additions (did anyone ever implement them?) of AMR-WB and TCH-8PSK; these
915ff61137ee Speech-codec-selection: document MSCAP
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3 classic 5 speech TCH modes are:
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4
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5 TCH/FS (FRv1 codec)
915ff61137ee Speech-codec-selection: document MSCAP
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6 TCH/HS (HRv1 codec)
915ff61137ee Speech-codec-selection: document MSCAP
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7 TCH/EFS (EFR codec)
915ff61137ee Speech-codec-selection: document MSCAP
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8 TCH/AFS (AMR-FR)
915ff61137ee Speech-codec-selection: document MSCAP
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9 TCH/AHS (AMR-HR)
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10
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11 The selection of TCH type (full-rate or half-rate) and speech version (specific
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12 codec) assigned to each voice call is always done by the GSM network - the MS
915ff61137ee Speech-codec-selection: document MSCAP
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13 has no direct control over these decisions. However, the MS tells the network
915ff61137ee Speech-codec-selection: document MSCAP
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14 which codecs it supports by way of Bearer capability IE (information element)
915ff61137ee Speech-codec-selection: document MSCAP
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15 in CC (call control) Layer 3 messages, and networks that are designed to support
915ff61137ee Speech-codec-selection: document MSCAP
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16 a wide range of GSM MS technology generations (as opposed to, for example,
915ff61137ee Speech-codec-selection: document MSCAP
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17 operating with a policy that excludes non-AMR-capable MS) heed these MS-declared
915ff61137ee Speech-codec-selection: document MSCAP
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18 speech codec capabilities when deciding which codec and TCH type to use.
915ff61137ee Speech-codec-selection: document MSCAP
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19
915ff61137ee Speech-codec-selection: document MSCAP
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20 Per the specs, every type-approved GSM MS is required to support FRv1 codec,
915ff61137ee Speech-codec-selection: document MSCAP
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21 but support for all other speech codecs is optional in any combination.
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22
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23 In TI's GSM MS firmware architecture there is a configuration record in the
915ff61137ee Speech-codec-selection: document MSCAP
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24 Condat PCM layer that tells the protocol stack's CC component which speech
915ff61137ee Speech-codec-selection: document MSCAP
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25 versions it should list as supported in the Bearer capability IE. This PCM
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26 configuration record is called EF_MSCAP; normally the firmware runs with its
915ff61137ee Speech-codec-selection: document MSCAP
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27 compiled-in default for this record, and that default is crafted to match the
915ff61137ee Speech-codec-selection: document MSCAP
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28 capabilities of Calypso L1. However, if there is a file named /pcm/MSCAP in
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29 FFS, that FFS file overrides the compiled-in default for EF_MSCAP; this MSCAP
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30 override mechanism can be used to artificially restrict which codecs the MS
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31 will declare to the network as supported, thereby influencing the actual codec
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32 selection by the network.
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33
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34 As of 2023-06-05 FreeCalypso Tourmaline firmware offers 3 different methods for
915ff61137ee Speech-codec-selection: document MSCAP
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35 manipulating the speech version list sent to the network in the Bearer
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36 capability IE for voice calls:
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37
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38 * Writing /pcm/MSCAP with fc-fsio;
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39 * Writing to EF_MSCAP more conveniently with AT%MSCAP;
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40 * Setting the speech version list directly with AT%SPVER.
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41
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42 MSCAP record manipulation
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43 =========================
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44
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45 Writing /pcm/MSCAP with fc-fsio is the oldest method for artificially
915ff61137ee Speech-codec-selection: document MSCAP
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46 restricting the selection of speech codec for voice calls - this method has been
915ff61137ee Speech-codec-selection: document MSCAP
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47 available since the earliest FreeCalypso firmware versions. However, this
915ff61137ee Speech-codec-selection: document MSCAP
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48 method is very inconvenient: the firmware checks FFS for the presence of
915ff61137ee Speech-codec-selection: document MSCAP
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49 /pcm/MSCAP only once on boot, when the Condat PCM layer is initialized, and
915ff61137ee Speech-codec-selection: document MSCAP
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50 this file is never re-read afterward. Therefore, if you create or modify or
915ff61137ee Speech-codec-selection: document MSCAP
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51 delete this file with fc-fsio, your change won't have any effect whatsoever on
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52 the already-booted firmware session, instead it will only take effect on the
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53 next firmware boot cycle.
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54
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55 The addition of AT%MSCAP private AT command solves this problem: this command
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56 updates the EF_MSCAP record through the firmware's pcm_WriteFile() API, which
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57 not only writes to FFS, but also updates the in-RAM copy in the PCM layer which
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58 takes effect on the next voice call. This command supports both set and query
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59 forms:
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60
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61 AT%MSCAP=byte1,byte2,byte3[,byte4,byte5,byte6] -- set form
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62 AT%MSCAP? -- query form
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63
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64 This private AT command is unusual (differs from 3GPP standard commands) in that
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65 each of the byte arguments is hexadecimal. The hexadecimal form is "raw", i.e.,
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66 just the two hexadecimal digits without any prefix or suffix. For example, the
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67 following command restores the standard EF_MSCAP setting for CSD-enabled
915ff61137ee Speech-codec-selection: document MSCAP
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68 firmware builds on Calypso C035 platfotms with AMR-capable DSP ROM versions:
915ff61137ee Speech-codec-selection: document MSCAP
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69
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70 AT%MSCAP=B7,C7,00
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71
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72 The full EF_MSCAP record consists of 6 bytes, but only the first 3 bytes contain
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73 defined bits; the last 3 bytes are unused and should be set to 0. The set form
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74 of AT%MSCAP command requires the first 3 bytes to be fully specified (there is
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75 no default), but the last 3 unused bytes default to 0 if left unspecified.
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76
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77 The first byte of EF_MSCAP indicates supported speech codec versions, and the
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78 following two bytes specify CSD capabilities. The firmware's compiled-in MSCAP
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79 standard configurations are:
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80
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81 Firmware build w/o CSD Firmware build with CSD
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82 Early Calypso silicon w/o AMR 07,00,00 87,C7,00
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83 Final Calypso silicon with AMR 37,00,00 B7,C7,00
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84
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85 The first byte of EF_MSCAP is called chnMode and has the following bit
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86 definitions (ASCII art diagrams copied from TI's source comments):
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87
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88 _______________________________________________________
915ff61137ee Speech-codec-selection: document MSCAP
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89 |8 |7 |6 |5 |4 |3 |2 |1 |
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90 |------|------|------|------|------|------|------|------|
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91 |L1 |Tm |afs |ahs |spV3 |efrV2 |hr |spV1 |
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92 -------------------------------------------------------
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93
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94 (Note the same bit numbering convention as is used in the GSM 11.11 spec for
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95 EFs in the SIM file system.)
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96
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97 Bit 1: indicates support for FRv1, must always be set
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98 Bit 2: indicates support for HRv1
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99 Bit 3: indicates support for EFR
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100 Bit 4: defined as indicating support for "EFR v3", but there is no such thing
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101 Bit 5: indicates support for AMR-HR
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102 Bit 6: indicates support for AMR-FR
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103 Bit 7: defined as meaning "Test Mobile", not actually used
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104 Bit 8: defined as meaning "L1 Version", not actually used
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105
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106 The second byte is called datCap1 and has the following bit definitions:
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107
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108 _______________________________________________________
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109 |8 |7 |6 |5 |4 |3 |2 |1 |
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110 |------|------|------|------|------|------|------|------|
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111 |14.4 |tfax |ntfax |tsyn |syn |asyn |rlp |ds |
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112 -------------------------------------------------------
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113
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114 Specific bit meanings per TI's source comments:
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115
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116 Bit 1: Data support
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117 Bit 2: RLP data (NT Async)
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118 Bit 3: T Async data support
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119 Bit 4: NT Sync data support
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120 Bit 5: T Sync data support
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121 Bit 6: NT Fax support
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122 Bit 7: T Fax support
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123 Bit 8: Data rate 14.4 support
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124
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125 The third byte is called datCap2 and has the following bit definitions:
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126
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127 _______________________________________________________
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128 |8 |7 |6 |5 |4 |3 |2 |1 |
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129 |------|------|------|------|------|------|------|------|
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130 |DHR | | |NAS |TPD |NTPD |TP |NTP |
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131 -------------------------------------------------------
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132
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133 Specific bit meanings per TI's source comments:
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134
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135 Bit 1: NT Packet Service
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136 Bit 2: T Packet Service
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137 Bit 3: NT PAD Access Service
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138 Bit 4: T PAD Access Service
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139 Bit 5: No Alternate Services
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140 Bit 8: Data Halfrate support
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141
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142 Note that Calypso Layer1 as delivered by TI does not support CSD on TCH/H, only
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143 on TCH/F, and this lack of CSD half-rate support is reflected in the compiled-in
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144 EF_MSCAP configuration. However, the DSP does have channel mode definitions for
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145 TCH/H4.8 and TCH/H2.4, suggesting at least partial support for half-rate CSD -
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146 perhaps this support is incomplete and/or non-functional. ARM L1 support for
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147 half-rate CSD (called IDS in the code) is very obviously missing, though. But
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148 on TCH/F our TI-inherited GSM MS solution does support CSD modes from F2.4
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149 through F14.4 inclusive, with bearer services of transparent async,
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150 non-transparent RLP and fax.
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151
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152 Playing with AT%MSCAP and cleaning up afterward
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153 -----------------------------------------------
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154
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155 The set form of AT%MSCAP command issues a pcm_WriteFile() internal API call,
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156 and that operation writes /pcm/MSCAP into FFS in addition to updating the record
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157 inside the PCM layer. Therefore, MSCAP changes made with this command persist
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158 across firmware boot cycles. There is no way to delete /pcm/MSCAP via AT%MSCAP,
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159 and thus no way to return to the standard system design state where the active
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160 MSCAP record depends on the firmware build variant on development boards where
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161 different fw build configurations are often being run. Therefore, the following
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162 clean-up procedure is recommended:
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163
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164 Step 1: set AT%MSCAP to the standard config for your currently running firmware
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165 version, usually AT%MSCAP=37,00,00 or AT%MSCAP=B7,C7,00.
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166
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167 Step 2: execute 'rm /pcm/MSCAP' in fc-fsio, deleting the file from FFS.
99
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168
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169 Speech version preference list
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170 ==============================
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171
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172 Whenever a GSM MS supports any additional speech codecs beyond the always-
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173 required FRv1, it needs to indicate this support to the network in the Bearer
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174 capability IE. However, the encoding of the relevant bits in this IE indicates
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175 not only which codecs the MS supports, but also their order of preference.
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176 Indicating the order of preference between supported codecs is not optional:
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177 the MS always has to transmit _some_ order-of-preference list toward the
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178 network. (For GSM MS supporting FRv1 and HRv1 but no others, there is an
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179 abbreviated version of the Bearer capability IE with octet 3a list omitted, but
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180 even then the order of preference between the two supported codecs is still
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181 indicated via the two radio channel requirement bits.)
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182
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183 Our current FreeCalypso TCS2/TCS3 hybrid firmware indicates the following order
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184 of preference to the network, between the 5 potentially supported codecs:
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185
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186 AMR-FR > AMR-HR > EFR > FRv1 > HRv1
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187
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188 (The '>' character in the above line indicates that the codec on the left is
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189 preferred over the codec on the right.)
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190
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191 The actual Bearer capability IE is constructed by first masking off each of the
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192 5 possible codecs as supported or not-supported per EF_MSCAP (see previous
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193 section), but those codecs that remain in the supported set are sent to the
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194 network in the order of preference following the above diagram.
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195
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196 It appears that most GSM networks ignore the speech version preference order
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197 indicated by the MS, treating the MS-provided speech version list as an
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198 unordered checklist of supported or non-supported, and impose their own
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199 preference order set by network policy. Such behavior is implemented in Osmocom
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200 CNI software, and is also observed on the legacy GSM network of T-Mobile USA as
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201 of this writing. However, the world as a whole is big, there are many different
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202 GSM networks in different countries, and I (Mother Mychaela) deem it valuable to
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203 provide a mechanism whereby FreeCalypso-equipped GSM community members can test
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204 experimentally how their countries' GSM networks react to different speech
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205 version lists indicated by the MS, including peculiar preference orders.
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206
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207 To facilitate such testing, FreeCalypso Tourmaline firmware provides a private
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208 debug-oriented AT command controlling the speech version list: AT%SPVER. This
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209 command takes the following forms:
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210
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211 AT%SPVER=0 -- restore standard behavior
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212 AT%SPVER=1,sv1[,sv2[,sv3[,sv4[,sv5]]]]] -- set custom speech version list
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213 AT%SPVER? -- query current setting
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214
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215 In the second listed form of AT%SPVER command, arguments sv1 through sv5 are
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216 speech version codes as transmitted over the air in the Bearer capability IE,
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217 as given in Table 10.5.103/3GPP TS 24.008:
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218
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219 0 = FRv1
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220 1 = HRv1
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221 2 = EFR
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222 4 = AMR-FR
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223 5 = AMR-HR
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224
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225 The speech version list sent to the network will be exactly as given in the
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226 AT%SPVER command, in that order. You can list just one speech version, or all
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227 5 of them, or any intermediate subset - but FRv1 (sv code 0) must always be
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228 included, or the AT%SPVER command will be rejected. Some examples:
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229
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230 AT%SPVER=1,0 -- declare support for FRv1 only
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231 AT%SPVER=1,2,0,1 -- declare support for EFR, FRv1 and HRv1 in this order
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232 AT%SPVER=1,1,0 -- prefer HRv1 over FRv1
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233
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234 Unlike AT%MSCAP, the setting made with AT%SPVER is volatile: it is not saved
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235 anywhere in FFS, and thus does not persist across firmware boot cycles.
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236
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237 In order to affect the Bearer capability IE in the Call confirmed message for
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parents: 98
diff changeset
238 MT calls, the AT%SPVER command needs to be given before the MT Setup message
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parents: 98
diff changeset
239 arrives from the network. For MO call testing, it is sufficient to set AT%SPVER
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parents: 98
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240 before dialing the MO call with ATD command.