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TCH special feature documentation update
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 01 Jun 2016 02:06:44 +0000
parents 759b3cbf46aa
children
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1 FreeCalypso GSM firmware implements an optional special feature (needs to be
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2 explicitly enabled at compile time) which we call TCH rerouting. When this
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3 feature is enabled, it applies the following special handling to GSM voice
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4 traffic channels (TCH):
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5
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6 * All downlink TCH bits passing from the channel decoder to the vocoder block
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7 (260 bits every 20 ms) can be non-invasively intercepted and forwarded to
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8 the external host connected to the RVTMUX serial interface;
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9
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10 * Using the same serial interface, the external host can supply substitute
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11 uplink TCH bits which will be transmitted in the place of the built-in
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12 vocoder output, i.e., the latter can be effectively suppressed.
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13
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14 In order to use this feature, you need to compile our firmware in the voice+SMS
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15 pseudo-modem configuration, i.e., the configuration in which the fw expects to
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16 be controlled via AT commands wrapped in the RVTMUX binary packet serial
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17 interface. You can use a target GSM device that has just one accessible serial
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18 port (Mot C1xx and Pirelli DP-L10) or one that has two Calypso UARTs (Openmoko
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19 GTA02 or our future FCDEV3B), but in the latter case you will be using only one
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20 UART - whichever one you have configured to be RVTMUX.
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21
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22 Whatever system you are building that will act as the source and sink for TCH
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23 bits will need to interface to the FreeCalypso GSM device via a serial port in
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24 the RVTMUX binary packet format. Your system will need to send RVTMUX packets
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25 with AT commands inside them in order to command the FC GSM device to register
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26 with the network and to dial and/or answer calls, and you will need to send
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27 RVTMUX packets of a different kind in order to supply the uplink TCH bits
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28 during calls. In the other direction, your system will receive responses to
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29 the AT commands you send, asynchronous notifications of incoming calls and SMS,
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30 downlink TCH bits and various debug trace output from our FreeCalypso firmware.
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31 The last part (debug trace output) can be simply ignored and discarded if you
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32 wish, but we strongly recommend that you provide a way to view and/or log it
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33 for debugging purposes.
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34
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35 Please see the RVTMUX document for general background information regarding the
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36 RVTMUX binary packet interface; this document should be considered required
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37 reading for anyone interested in working with the TCH rerouting special feature.
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38
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39 All packets transferred over the RVTMUX interface begin and end with 0x02. If
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40 a payload byte within a packet equals 0x02 or 0x10, it needs to be prepended
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41 with 0x10 as a transparency escape; all other payload bytes are sent literally.
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42 The first byte within each RVTMUX packet after the opening 0x02 is the packet
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43 type; the two packet types you will need to handle (both generate and receive)
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44 are 0x1A for AT commands and 0x1C for TCH configuration commands.
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45
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46 To send an AT command to the FreeCalypso GSM device, prepend the 0x1A packet
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47 type in front of the "AT" characters, wrap the packet with 0x02 bytes on both
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48 ends, and send it to the modem. Responses to AT commands and asynchronous
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49 notification messages such as "RING" for incoming calls will be sent to the
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50 host as RVTMUX packets also beginning with the 0x1A packet type; they will be
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51 interspersed among other packet types, mostly debug trace output. Your system
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52 will need to receive the RVTMUX serial byte stream continuously, parsing the
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53 packet structure and looking at the type of each packet (the first byte after
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54 the opening 0x02) in order to detect if the modem has sent something you may be
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55 interested in.
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56
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57 If you wish to receive a copy of all downlink TCH bits on the serial channel,
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58 you will need to send the following 5-byte command packet to the modem:
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59
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60 0x02: opening flag
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61 0x1C: RVTMUX packet type for TCH
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62 0x11: TCH_CONFIG_REQ command opcode
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63 0x01: payload byte indicating that the "forward downlink" state should be
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64 set to enabled
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65 0x02: closing flag
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66
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67 The modem will respond with a TCH_CONFIG_CONF confirmation message (opcode
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68 0x12), and then during all voice calls your external host will receive the
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69 following packet every 20 ms:
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70
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71 0x02: opening flag
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72 0x1C: RVTMUX packet type for TCH
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73 0x15: TCH_DLBITS_IND opcode
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74 - 40 (decimal) bytes of payload -
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75 0x02: closing flag
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76
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77 The 40 bytes of payload sent in every TCH_DLBITS_IND packet directly correspond
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78 to the 20 16-bit words provided by the Calypso DSP in its a_dd_0 buffer. The
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79 first 3 words (6 bytes) contains the DSP's own status information (not fully
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80 understood by us yet, but we let you see what the DSP tells us without redacting
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81 anything out), and the remaining 17 words (34 bytes) are supposed to contain
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82 the TCH bits received from the GSM network in the FR codec format. Each DSP
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83 API word is sent in the big-endian byte order, i.e., the most significant byte
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84 followed by the least significant byte.
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85
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86 If you wish to send your own TCH uplink bits, replacing the output of the
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87 built-in vocoder with your own alternate uplink data, you will need to send
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88 your uplink TCH bits to the modem in packets of the following format:
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89
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90 0x02: opening flag
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91 0x1C: RVTMUX packet type for TCH
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92 0x13: TCH_ULBITS_REQ opcode
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93 - 33 or 34 (decimal) bytes of payload -
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94 0x02: closing flag
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95
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96 Sending 260 bits requires only 33 bytes, but the DSP operates in terms of
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97 16-bit words, hence 17 of those words are used. The least significant byte of
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98 the last word (i.e., the very last byte with our big-endian transmission order)
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99 is not expected to be used, but if you send 34 bytes rather than 33, you will
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100 have control over every bit going into the DSP API RAM in this case.
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101
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102 There is a queue inside the firmware in which these TCH uplink data blocks are
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103 stored; this queue is filled by the serial packet receiving handler and drained
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104 by the L1S (synchronous) code that executes at the right times in the GSM TDMA
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105 multiplex when uplink TCH transmission is expected. Up to 4 blocks can be
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106 queued up; as each queued-up block is transmitted on the air (more precisely,
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107 as it is passed to the DSP for channel encoding and transmission), a
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108 TCH_ULBITS_CONF short packet (consisting of just the opcode and nothing more)
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109 is sent to the host. These confirmation packets can be used to pace the sending
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110 of further TCH_ULBITS_REQs.
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111
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112 Testing
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113 =======
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114
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115 The just-described mechanism has been tested as follows:
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116
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117 1. I placed a call to WWV (+1-303-499-7111), and after verifying with my ear
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118 that the downlink audio was good, I recorded the downlink TCH bits on that
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119 call into a file with the tch record command in fc-shell.
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120
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121 2. I placed a call to another phone (running over a live commercial GSM network)
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122 and played the saved recording from WWV into the call uplink with the
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123 tch play command in fc-shell.
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124
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125 3. The audio heard on the other end of the call in the previous step: the
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126 recording from WWV was definitely recognizable, but it didn't sound perfect,
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127 i.e., it was rather garbled.
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128
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129 Further debugging of this mechanism will require two things which I currently
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130 lack: (1) proper understanding of the workings of the GSM 06.10 FR codec and
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131 (2) a test GSM network (as in OpenBTS/OpenBSC/etc) that could be used instead
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132 of live commercial ones, so we could see exactly what the test MS is
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133 transmitting on the air and what the BTS transmits in the downlink.
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134
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135 Host side reference implementation
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136 ==================================
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137
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138 If you are going to implement your system for talking to FreeCalypso GSM pseudo-
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139 modems via the RVTMUX binary packet interface, we strongly recommend that you
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140 use our rvinterf and fc-shell Unix/Linux host utilities as your starting point.
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141 You can find their source code in the rvinterf subtree.
1033
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142
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143 The following test commands have been added to fc-shell for exercising the
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144 experimental TCH rerouting mechanism:
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145
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146 tch record <filename>
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147
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148 Sends a TCH_CONFIG_REQ packet to the target, commanding the firmware to
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149 start forwarding TCH downlink bits to the external host, and starts
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150 recording the bits it receives in the named file. The file is written
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151 with the same ordering of GSM 06.10 bits as used by the popular libgsm
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152 implementation of this codec, i.e., the bits received from the GSM
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153 device (ultimately coming from TI's DSP) are reordered before being
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154 written into the file. It is only a reordering of bits with no change
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155 in the information content.
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156
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157 I was hoping that the resulting files could be played with the SoX play
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158 command under Slackware Linux, but all I got was garbled audio, and my
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159 audio-fu is not good enough to figure out what is wrong.
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160
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161 tch record stop
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162
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163 Stops TCH downlink recording and closes the file into which the bytes
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164 were being written; until the file is thus closed, it may not be
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165 actually written out to the file system.
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166
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167 tch play <filename>
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168
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169 Plays GSM 06.10 FR speech frames from the named file in libgsm format
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170 (same as written by the tch record command) into the call uplink.
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171
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172 tch play stop
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173
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174 Terminates the TCH UL play-from-file operation. This command is
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175 normally not needed, as the play session will end automatically when
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176 the end of file is reached.