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fc-shell: tch command framework started
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 30 May 2016 23:19:30 +0000
parents 759b3cbf46aa
children 5ab737ac3ad7
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759b3cbf46aa doc/TCH-special-feature: document written
Mychaela Falconia <falcon@ivan.Harhan.ORG>
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1 FreeCalypso GSM firmware implements an optional special feature (needs to be
759b3cbf46aa doc/TCH-special-feature: document written
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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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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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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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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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17 interface. You can use a target GSM device that has just one accessible serial
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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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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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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29 the AT commands you send, asynchronous notifications of incoming calls and SMS,
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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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.
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83
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84 If you wish to send your own TCH uplink bits, replacing the output of the
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85 built-in vocoder with your own alternate uplink data, you will need to send
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86 your uplink TCH bits to the modem in packets of the following format:
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87
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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88 0x02: opening flag
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89 0x1C: RVTMUX packet type for TCH
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90 0x13: TCH_ULBITS_REQ opcode
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91 - 33 or 34 (decimal) bytes of payload -
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92 0x02: closing flag
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93
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94 Sending 260 bits requires only 33 bytes, but the DSP operates in terms of
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95 16-bit words, hence 17 of those words are used. The upper byte of the last
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96 word is not expected to be used, but if you send 34 bytes rather than 33, you
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97 will have control over every bit going into the DSP API RAM in this case.
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98
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99 There is a queue inside the firmware in which these TCH uplink data blocks are
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100 stored; this queue is filled by the serial packet receiving handler and drained
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101 by the L1S (synchronous) code that executes at the right times in the GSM TDMA
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102 multiplex when uplink TCH transmission is expected. Up to 4 blocks can be
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103 queued up; as each queued-up block is transmitted on the air (more precisely,
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104 as it is passed to the DSP for channel encoding and transmission), a
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105 TCH_ULBITS_CONF short packet (consisting of just the opcode and nothing more)
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106 is sent to the host. These confirmation packets can be used to pace the sending
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107 of further TCH_ULBITS_REQs.
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108
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109 Testing
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110 =======
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111
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112 The just-described mechanism has been tested to the following extent: I have
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113 sent TCH_CONFIG_REQ packets to the modem, requesting that TCH downlink
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114 forwarding be enabled or disabled, and observed that the flow of TCH_DLBITS_IND
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115 packets from the modem to the host starts and stops as expected. I have not
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116 done any further work to verify the correctness of the FR codec payload bits
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117 within these packets. However, the TCH uplink substitution mechanism has not
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118 been tested at all, as I do not have a source of FR codec bits for testing.
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119
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120 Host side reference implementation
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121 ==================================
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122
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123 If you are going to implement your system for talking to FreeCalypso GSM pseudo-
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124 modems via the RVTMUX binary packet interface, we strongly recommend that you
759b3cbf46aa doc/TCH-special-feature: document written
Mychaela Falconia <falcon@ivan.Harhan.ORG>
parents:
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125 use our rvinterf and fc-shell Unix/Linux host utilities as your starting point.
759b3cbf46aa doc/TCH-special-feature: document written
Mychaela Falconia <falcon@ivan.Harhan.ORG>
parents:
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126 You can find their source code in the rvinterf subtree.