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tmo/CSD-tests: additional experiments and historical notes
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 26 Nov 2024 20:56:33 +0000
parents 47947e25f922
children afebef67e8d4
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1 A series of experiments seeking to test CSD on the extant network of T-Mobile
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2 USA, to be performed while this network is still alive. The main objective is
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3 to test CSD calls between a single MS and analog (or PCMU-speaking) PSTN, rather
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4 than mobile to mobile.
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5
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6 All experiments detailed in the following sections were performed in 2024-11
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7 at the Mother's home base.
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8
70
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9 Test setup
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10 ==========
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11
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12 The test MS is an FCDEV3B with a legacy T-Mobile SIM. The AT command session
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13 will be driven through the dedicated UART, not the usual fc-shell, to enable
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14 full use of data functions. Rvinterf will still be used to capture logs; the
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15 following command needs to be issued in order to make the firmware emit traces
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16 indicating AT command activity:
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17
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18 sp MMI TRACECLASS 800050
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19
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20 Issue these two additional commands to trace the MMCM SAP between CC and MM
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21 entities of the protocol stack:
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22
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23 sp MM DUPLICATE CC PCO
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24 sp CC DUPLICATE MM PCO
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25
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26 The objective is to capture all CC protocol messages sent to and received from
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27 the network; tracing the exchange between CC and MM seems like the easiest way
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28 to accomplish this goal within our existing firmware architecture.
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29
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30 Experiment 1
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31 ============
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32
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33 In this experiment several MO CSD calls were made from the test MS to
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34 +13034944774, the number for USA taxpayer-funded NIST ACTS. Several modes were
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35 tested:
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36
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37 AT+CBST=7,0,1 -- TI GSM MS firmware default
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38 AT+CBST=7,0,0
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39 AT+CBST=6,0,0
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40 AT+CBST=4,0,0
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41 AT+CBST=2,0,0
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42
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43 Results were surprising:
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44
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45 * Transparent mode works beautifully at 9600 baud, as well as 4800 and 1200
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46 baud. Therefore, the previous hypothesis that CSD at 9600 baud no longer
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47 works as a result of GSM quality degradation or as a result of IP-fication of
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48 internal transports within USA PSTN is now disproven.
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49
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50 * Transparent mode at 2400 baud produced a different result: successful CONNECT
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51 followed by garbage and earlier disconnect, instead of 40 s of good ACTS
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52 output in other baud modes. Perhaps a defect in V.22bis implementation in
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53 the network IWF?
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54
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55 * All 3 data TCH modes were observed in L1 traces: TCH/F9.6, TCH/F4.8 and
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56 TCH/F2.4, and each of these produced a perfectly good data call in at least
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57 one mode.
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58
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59 * NT mode was only tested with AT+CBST=7,0,1 default, i.e., 9600 baud and asking
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60 the IWF for V.32 modem. As revealed by L1 and CC traces, the network does
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61 everything it should - but the MS returns NO CARRIER. However, further
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62 analysis of CC traces reveals that the DISCONNECT at CC level is initiated by
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63 the MS and not by the network! My current working hypothesis is that the MS
71
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64 is unhappy about something it sees at RLP level. [See further updates in
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65 Experiment 6.]
70
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66
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67 Additional observations:
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68
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69 * The MS issues a single SETUP message with BC indicating ITC of 3.1 kHz and the
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70 selected CSD mode. The network responds with CALL PROCEEDING, ALERTING and
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71 CONNECT in the same cadence as it would for a MO voice call. The timing of
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72 ALERTING message is consistent with my expectation of when PSTN plays the
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73 ringing tone, and the timing of CC CONNECT message is consistent with my
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74 expectation of when Answer Supervision occurs at PSTN level. The CC CONNECT
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75 step is reached in all tested modes, including the failing NT mode.
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76
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77 * The AT-command CONNECT response happens significantly later than CC CONNECT
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78 message. My current working hypothesis is that the network generates CC
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79 CONNECT when it gets Answer Supervision from PSTN just like it does for voice
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80 calls, well before modem connection sequence can happen. Perhaps the status
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81 of modem connection is then indicated via V.110 in-band flags, and perhaps
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82 the MS is using that in-band status to decide when to declare CONNECT.
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83
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84 * At the end of each call there was a network-initiated DISCONNECT with cause
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85 value 127 (interworking, unspecified) - but in this experiment there is
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86 insufficient information to tell if this action is initiated by the IWF upon
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87 loss of modem signal, or if it is merely passing along PSTN-level disconnect
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88 signaling from the called party. See Experiment 2.
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89
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90 Experiment 2
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91 ============
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92
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93 In this experiment several MO CSD call attempts were dialed from the test MS to
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94 Mother Mychaela's personal voice number on ThemWi; the latter is interconnected
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95 with USA PSTN via BulkVS. The objectives were: (1) to confirm the hypothesis
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96 about Answer Supervision on PSTN side being the criterion for signaling CC
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97 CONNECT to the MS, and (2) to hear what, if anything, the IWF transmits toward
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98 PSTN while waiting for modem connection.
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99
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100 Results:
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101
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102 * When I answered the call on the destination phone, I heard silence each time.
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103 The calling party received CC CONNECT signaling when I made that answer.
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104
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105 * In both tests where +CBST was set to 7,0,0 and I answered the call, the
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106 network (TMO) eventually ended the call, signaling cause 127 to the calling
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107 MS like in Experiment 1. (Thus we now know that it is the IWF taking this
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108 action, and not the called party hanging up at PSTN level.) In one of these
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109 two tests, there was a miraculous CONNECT response from the AT interpreter,
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110 some time well after Answer Supervision, but before the IWF-initiated
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111 disconnect. In the second test, there was no CONNECT response, only
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112 NO CARRIER at the time of IWF-initiated disconnect.
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113
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114 * In the test case where I didn't answer the destination phone (let the ringing
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115 time out), the calling MS received CC DISCONNECT message with cause 17 (user
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116 busy), as that is how PSTN middleboxes misinterpret SIP 480 error I return
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117 from ThemWi. The AT command interface generated BUSY response.
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118
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119 * In the final test case in this experiment series, I made the test call with
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120 +CBST=7,0,1 and answered the destination phone. The MS initiated DISCONNECT
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121 4 s after receiving CC CONNECT, just like it did when calling NIST ACTS.
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122
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123 Experiment 3
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124 ============
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125
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126 This experiment sought to observe how TMO GSM network handles MT calls from PSTN
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127 to a mobile subscriber, including the possiblity of MT CSD calls via AT+CSNS.
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128 An MT call was repeatedly dialed from Mother Mychaela's ThemWi phone to the test
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129 MS on TMO, with different AT+CSNS and AT+CBST settings established before each
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130 call attempt; logs were collected for subsequent analysis.
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131
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132 Observations:
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133
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134 * The initial SETUP message from the network to the MS contains _no_ bearer
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135 capability IE; the CALL CONFIRM response from the MS does include BC IE,
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136 filled with different content depending on AT+CSNS and AT+CBST settings.
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137
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138 * The network seems to always accept the BC supplied by the MS in that response,
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139 and assigns appropriate TCH type and mode.
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140
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141 * The PSTN caller hears the standard North American ringing tone while the MS
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142 emits RING messages on its AT command channel, subsequent to the MS sending
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143 CC ALERTING message to the network.
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144
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145 * When I answer the call with ATA, the MS sends CC CONNECT to the network, the
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146 PSTN caller gets Answer Supervision, and I hear modem answer tones from TMO's
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147 IWF.
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148
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149 * Only Transparent modes have been tested so far; the modem answer sequence
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150 heard by the PSTN caller depends on the specific modem emulation selected
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151 with AT+CBST. With AT+CBST=2,0,0 and AT+CBST=4,0,0 I heard a continuous (no
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152 phase reversals) V.25 ANS tone followed by a higher-pitched tone (presumably
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153 Unscramled Binary 1s signal of V.22), followed by disconnection; with
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154 AT+CBST=6,0,0 and AT+CBST=7,0,0 I heard a V.25 ANS tone *with* phase
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155 reversals, directly followed by disconnection (no AC tones).
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156
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157 Further tests were aborted (I didn't get to testing NT mode) because the network
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158 did something that caused the MS firmware (or perhaps the SIM) to go into a
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159 funky state; to be investigated as its own separate problem.
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160
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161 Experiment 4
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162 ============
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163
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164 In this experiment I used two of my legacy T-Mobile SIMs simulateneously: one
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165 in the FCDEV3B as in the preceding experiments, the other in a Pirelli DP-L10
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parents:
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166 phone. The Pirelli phone ran RAM-based FC Tourmaline firmware during these
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parents:
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167 tests. Test calls were dialed from the FCDEV3B to the Pirelli; the fw build
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168 running on the latter has CSD excluded, but the objective was to see the MT
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169 SETUP message as it arrives from the network, before destination MS firmware
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170 does anything with it.
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171
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172 Observation: _no_ BC IE was ever seen in the MT SETUP message, for both voice
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173 and data calls dialed from the other TMO subscriber line. It appears that all
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174 MO CSD calls were treated by the network in "convert to modem emulation" mode
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175 despite the dialed number being another T-Mobile line; the MT leg always
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176 received incoming voice calls.
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177
71
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178 Experiment 5
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179 ============
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180
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181 In this experiment I sought to capture TCH DL bits from the DSP in CSD modes in
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182 the same way how we do it in non-AMR speech modes. I added what seemed like
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183 the necessary support in FC Tourmaline fw (Hg commit 304:58c7961bd0b0) and in
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184 fc-shell TCH handling code (freecalypso-tools Hg commit 1014:961efadd530a), and
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185 made captures of CSD call sessions with +CBST set to (7,0,0), (6,0,0) and
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186 (2,0,0). However, as soon as I looked at the captured recordings, I was
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187 immediately disappointed: while the beginning of each capture shows what look
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188 like DL bits constituting idle frames (mostly all 1s, except where disturbed by
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parents: 70
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189 FACCH stealing), as soon as the IDS block kicks in, we read nothing but all 0s.
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parents: 70
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190 Is the IDS part of DSP ROM code overwriting the buffer with zeros after it groks
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
191 those bits?
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192
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193 Also in this experiment session I tried setting AT+CBST=14,0,0 to see if TMO
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parents: 70
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194 supports TCH/F14.4. The network responded to SETUP with CALL PROCEEDING, but
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parents: 70
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195 whereas in other modes channel assignment happens almost immediately afterward,
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196 here it stayed on SDCCH for several seconds, and then the network sent CC
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197 DISCONNECT message with cause 34. This cause value means "No circuit/channel
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parents: 70
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198 available" - so it looks like TMO's unloved GSM network does not support
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diff changeset
199 TCH/F14.4.
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200
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201 Experiment 6
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
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202 ============
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203
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
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204 As I was writing up notes regarding what seemed like non-working state of NT
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parents: 70
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205 mode while all Transparent modes up to 9600 bps work great, I decided to test
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parents: 70
diff changeset
206 with an older version of FC firmware, namely Magnetite l1reconst. In the
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
207 earlier years of FreeCalypso, I would frequently make CSD calls just for fun,
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
208 calling NIST ACTS, and it would always work - and being ignorant of T vs NT
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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209 and other mode settings, I never set AT+CBST and just dialed ATDnumber. Since
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
210 we now know that TI's firmware default is AT+CBST=7,0,1 and has always been
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
211 this way, we know that I was unknowingly exercising NT mode in those happy
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
212 years - hence I decided to try an older fw version from those years just to see
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
213 if it would have working NT mode. Tested - it worked!
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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214
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
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diff changeset
215 But then things got more interesting. I tested with Magnetite hybrid instead
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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216 of l1reconst - using the new source-enabled version of G23M PS. Result: it
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
217 seemed a little flaky, but it worked. Then I went back to current Tourmaline
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
218 fw, the same with which I started in Experiment 1. Did a call with
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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219 AT+CBST=7,0,0 serving as a control - it worked. Tried AT+CBST=7,0,1 - got a
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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220 NO CARRIER response; subsequent log analysis shows that the MS initiated CC
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
221 DISCONNECT just like in Experiment 1. Then I made another attempt, still with
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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222 AT+CBST=7,0,1 - and this time it worked beautifully: I got CONNECT followed by
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
223 the full 40 s of ACTS output.
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
224
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
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225 Thus we now know that NT mode working or not is not a question of firmware
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
226 version, but some kind of law of chance: time of day, space weather, phase of
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
227 the moon...
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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228
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229 NT mode: need for further debugging
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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230 ===================================
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
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diff changeset
231
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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232 The flaky nature of NT mode calls for further debugging: since we see
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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233 MS-initiated DISCONNECT on those tries when it fails, we really need to know
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
234 exactly what is making the MS unhappy. But this debugging is made difficult by
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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235 the misfeature of TI's DSP: in Experiment 5 I tried capturing TCH DL bits, but
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
236 all we get is zero fill once the IDS block is active. As next steps, we will
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
237 need to study the code that interfaces with this IDS block, and see what we can
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
238 observe at that interface level.
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
239
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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240 T vs NT mode: utility vs complexity
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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241 ===================================
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
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242
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
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diff changeset
243 Consider the IWF provided by the network, speaking CSD frame formats toward GSM
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
244 RAN and emulating analog modem signals toward 64 kbit/s PSTN, and consider the
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
245 work it has to do in T vs NT modes. It should be evident that NT mode is much
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
246 more complex: all of the work implementing V-series modulations is still needed
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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247 for both T and NT, but whereas T mode provides a direct path between the
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
248 emulated-modem "bit pump" and CSD V.110 frame bits, in NT mode the IWF has to
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
249 implement RLP on the GSM side, V.42 on the analog modem side, and make a high-
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
250 level data connection between these two engines of reliable transmission based
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
251 on acks, retries etc. Hence the "bug surface", the number of things that can
ed314cc25b8d tmo/CSD-tests: additional experiments and historical notes
Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
252 go wrong, is much greater for IWF in NT mode.
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253
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254 OTOH, utility considerations traditionally called for NT mode. There are
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255 *some* applications for which non-buffering, fixed-delay Transparent mode is
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256 the right choice: precision time distribution as in NIST ACTS, train engine
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257 communication in GSM-R etc - but such applications are more on the boutique
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258 side. Instead consider the more typical application of a business user dialing
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259 into his/her mainframe or UNIX minicomputer etc account while on the go: here
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260 you have a user with a text terminal, and a server/service somewhere that prints
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261 text for the user to read and expects text commands. In this case the user
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262 would certainly appreciate if his/her session is *not* disturbed by dropouts or
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263 garbage insertions from radio errors, from handover events or from line noise
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264 on the PSTN leg - such users would certainly benefit from NT mode. Outside of
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265 railways or other special environments, considering ordinary public GSM networks
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266 intended for consumers and business users, in the days when CSD was a feature
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267 of real user significance, NT mode was likely the more desired one for reasons
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268 just explained. Yet it is the one which is much more onerous to implement...
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269
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270 Lack of data-call indication from one T-Mobile MS to another
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271 ============================================================
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272
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273 Looking at the specs for GSM CSD, the intent of the original Creators seems
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274 clear: they intended CSD to run between an MS and services on the fixed network,
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275 perhaps going through an IWF. In contrast, the case of mobile-to-mobile CSD
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276 calls was NOT the canonical configuration - it was probably considered, but only
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277 as a rather contrived and unlikely use case. But if someone does make a CSD
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278 call addressed to another mobile, how should it work?
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279
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280 In order for a CSD call to be directly through-connected from Alice to Bob,
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281 Osmocom-style, without complex conversions and translations in the middle, two
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282 conditions would have to be met:
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283
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284 1) The connection element setting would have to be Transparent;
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285 2) ISDN/UDI mode would need to be selected, rather than 3.1 kHz modem emulation.
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286
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287 In this case the MO call will be turned into V.110 by the TRAU, no further IWF
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288 would be applied at the MSC, the ISDN call would connect from Alice to Bob, the
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289 destination MSC serving Bob would see that it's a data call, and it would
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290 indicate so to Bob's MS. So far, so good. But the moment you add NT mode or
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291 analog modem emulation, the above pass-through model breaks down:
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292
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293 1) If the bearer cap IE on the MO call indicates ITC of 3.1 kHz audio instead
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294 of UDI, Alice's MSC is supposed to pass the call through an IWF that does
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295 modem emulation. Once that modem emulation is applied, the call going out
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296 on ISDN is in 3.1 kHz audio mode, not digital data!
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297
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298 2) NT mode does not seem to have ever been intended to carry from mobile user
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299 Alice to mobile user Bob - instead it appears to have been intended to run
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300 between a single MS and a network IWF only. The IWF then converts to V.42
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301 in the case of 3.1 kHz modem emulation, or to V.1xx/X.75/etc protocols in
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302 the case of ISDN UDI, with or without V.42-like reliability through acks and
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303 retransmissions.
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304
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305 However, the above considerations stem from the architectural model _assumed_
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306 in ETSI GSM specs - whereas the question of what real-world GSM networks have
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307 actually implemented needs to be asked separately. In the case of T-Mobile USA,
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308 the behavior of the network upon own-subscriber Alice making a CSD call to own-
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309 subscriber Bob appears to have changed over the years:
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310
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311 * In my earlier years of FreeCalypso work, when I made CSD calls from FC GSM MS
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312 just for fun while being ignorant of T vs NT modes and other +CBST settings,
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313 I would call NIST ACTS to get a truly successful session. But I also
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314 experimented a little with mobile-to-mobile CSD. Unfortunately, I never put
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315 together a proper setup with two FC GSM MS boards powered up and connected to
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316 my laptop at the same time, fitted with two active T-Mobile SIMs, one calling
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317 the other - but I did dial CSD a few times from a single "proper" GSM MS
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318 (probably FCDEV3B, although I don't remember exactly) to my main personal-use
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319 T-Mobile line whose SIM sat in a Pirelli DP-L10 running its official fw.
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320 I don't remember if I got this degree of success reliably every time or if it
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321 was intermittent - but I did get an incoming call indication on the Pirelli
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322 that said "incoming data" or something to that effect. I don't remember
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323 exactly what happened afterward, but my vague recollection (subject to the
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324 caveats about wetware memory after many years of attention being shifted in
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325 very different directions) is that the phone would ring with this "incoming
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326 data" indication on the screen, but once I pressed the Answer button, the
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327 call attempt would drop - and curiously enough, turn into a "missed call"
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328 indication. I don't have good memory of what the call-originating FC GSM MS
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329 would show: I seem to recall it going directly to NO CARRIER upon me pressing
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330 Answer on the Pirelli, but I also seem to recall at least once getting a
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331 CONNECT response, presumably followed by NO CARRIER shortly afterward.
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332
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333 * In terms of memory reliability, the only part I do remember for certain is
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334 that the call-receiving Pirelli phone (running its official fw) _did_ display
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335 a "ringing" screen that said something along the lines of "incoming data" -
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336 IOW, it very clearly indicated the presence of an incoming call that was data
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337 rather than voice, even though of course this phone has no official Terminal
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338 Adaptor Function and no ability to meaningfully accept such calls. This
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339 indication implies that the MT SETUP message from the network must have
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340 included a bearer cap IE that indicated non-speech - although unfortunately
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341 I have no logs (back then I wasn't looking at CC or any other air interface
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342 messages), thus we don't know if it was a pass-through of BC from the MO leg
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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343 or altered in some way. The remaining memories of everything else that
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344 happened after this logically-deduced MT SETUP should be treated as
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345 unreliable.
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346
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347 * When I tried replicating in the present time (2024-11) the just-described
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348 result from many years ago, I had no success: see description of Experiment 4
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349 above. Now when I dial a CSD call from one of my legacy T-Mobile SIMs to
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
350 another, the receiving MS gets an MT SETUP message that contains no BC IE at
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parents: 70
diff changeset
351 all, same as with calls coming from outside PSTN, the call rings as voice,
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parents: 70
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352 and gets assigned TCH/AHS like other speech calls. And because the modem-
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
diff changeset
353 emulating CSD IWF in the MO direction initially emits silence while waiting
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Mychaela Falconia <falcon@freecalypso.org>
parents: 70
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354 for the answering modem, silence is what the receiving phone hears in this
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Mychaela Falconia <falcon@freecalypso.org>
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355 errant setup.