FreeCalypso > hg > freecalypso-citrine
annotate doc/TCH-special-feature @ 37:5487db285025
LICENSE explanatory file added
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sat, 26 Nov 2016 01:03:01 +0000 |
parents | 3362a76ab432 |
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documentation write-ups imported from freecalypso-sw and updated for Citrine
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1 FreeCalypso Citrine 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; only FR and EFR codecs are supported) can be non- |
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8 invasively intercepted and forwarded to the external host connected to the |
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9 RVTMUX serial interface; |
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10 |
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11 * Using the same serial interface, the external host can supply substitute |
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12 uplink TCH bits which will be transmitted in the place of the built-in |
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13 vocoder output, i.e., the latter can be effectively suppressed. |
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14 |
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15 In order to use this feature, you need to compile our firmware in the voice+SMS |
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16 pseudo-modem configuration, i.e., the configuration in which the fw expects to |
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17 be controlled via AT commands wrapped in the RVTMUX binary packet serial |
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18 interface. You can use a target GSM device that has just one accessible serial |
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19 port (Mot C1xx and Pirelli DP-L10) or one that has two Calypso UARTs (Openmoko |
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20 GTA02 or our future FCDEV3B), but in the latter case you will be using only one |
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21 UART - whichever one you have configured to be RVTMUX. |
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22 |
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23 Whatever system you are building that will act as the source and sink for TCH |
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24 bits will need to interface to the FreeCalypso GSM device via a serial port in |
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25 the RVTMUX binary packet format. Your system will need to send RVTMUX packets |
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26 with AT commands inside them in order to command the FC GSM device to register |
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27 with the network and to dial and/or answer calls, and you will need to send |
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28 RVTMUX packets of a different kind in order to supply the uplink TCH bits |
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29 during calls. In the other direction, your system will receive responses to |
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30 the AT commands you send, asynchronous notifications of incoming calls and SMS, |
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31 downlink TCH bits and various debug trace output from our FreeCalypso firmware. |
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32 The last part (debug trace output) can be simply ignored and discarded if you |
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33 wish, but we strongly recommend that you provide a way to view and/or log it |
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34 for debugging purposes. |
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35 |
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36 Please see the RVTMUX document in the FreeCalypso host tools package for general |
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37 background information regarding the RVTMUX binary packet interface; this |
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38 document should be considered required reading for anyone interested in working |
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39 with the TCH rerouting special feature. |
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40 |
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41 All packets transferred over the RVTMUX interface begin and end with 0x02. If |
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42 a payload byte within a packet equals 0x02 or 0x10, it needs to be prepended |
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43 with 0x10 as a transparency escape; all other payload bytes are sent literally. |
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44 The first byte within each RVTMUX packet after the opening 0x02 is the packet |
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45 type; the two packet types you will need to handle (both generate and receive) |
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46 are 0x1A for AT commands and 0x1C for TCH configuration commands. |
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47 |
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48 To send an AT command to the FreeCalypso GSM device, prepend the 0x1A packet |
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49 type in front of the "AT" characters, wrap the packet with 0x02 bytes on both |
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50 ends, and send it to the modem. Responses to AT commands and asynchronous |
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51 notification messages such as "RING" for incoming calls will be sent to the |
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52 host as RVTMUX packets also beginning with the 0x1A packet type; they will be |
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53 interspersed among other packet types, mostly debug trace output. Your system |
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54 will need to receive the RVTMUX serial byte stream continuously, parsing the |
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55 packet structure and looking at the type of each packet (the first byte after |
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56 the opening 0x02) in order to detect if the modem has sent something you may be |
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57 interested in. |
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58 |
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59 If you wish to receive a copy of all downlink TCH bits on the serial channel, |
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60 you will need to send the following 5-byte command packet to the modem: |
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61 |
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62 0x02: opening flag |
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63 0x1C: RVTMUX packet type for TCH |
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64 0x11: TCH_CONFIG_REQ command opcode |
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65 0x01: payload byte indicating that the "forward downlink" state should be |
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66 set to enabled |
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67 0x02: closing flag |
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68 |
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69 The modem will respond with a TCH_CONFIG_CONF confirmation message (opcode |
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70 0x12), and then during all voice calls your external host will receive the |
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71 following packet every 20 ms: |
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72 |
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73 0x02: opening flag |
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74 0x1C: RVTMUX packet type for TCH |
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75 0x15: TCH_DLBITS_IND opcode |
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76 - 40 (decimal) bytes of payload - |
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77 0x02: closing flag |
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78 |
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79 The 40 bytes of payload sent in every TCH_DLBITS_IND packet directly correspond |
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80 to the 20 16-bit words provided by the Calypso DSP in its a_dd_0 buffer. The |
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81 first 3 words (6 bytes) contain the DSP's own status information (not fully |
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82 understood by us yet, but we let you see what the DSP tells us without redacting |
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83 anything out), and the remaining 17 words (34 bytes) contain the TCH bits |
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84 received from the GSM network in the GSM 05.03 bit order - see TCH-bit-access |
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85 write-up in the FreeCalypso host tools package for more information. Each DSP |
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86 API word is sent in the big-endian byte order, i.e., the most significant byte |
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87 followed by the least significant byte. |
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88 |
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89 If you wish to send your own TCH uplink bits, replacing the output of the |
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90 built-in vocoder with your own alternate uplink data, you will need to send |
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91 your uplink TCH bits to the modem in packets of the following format: |
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92 |
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93 0x02: opening flag |
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94 0x1C: RVTMUX packet type for TCH |
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95 0x13: TCH_ULBITS_REQ opcode |
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96 - 33 or 34 (decimal) bytes of payload - |
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97 0x02: closing flag |
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98 |
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99 Sending 260 bits requires only 33 bytes, but the DSP operates in terms of |
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100 16-bit words, hence 17 of those words are used. The least significant byte of |
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101 the last word (i.e., the very last byte with our big-endian transmission order) |
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102 is not expected to be used, but if you send 34 bytes rather than 33, you will |
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103 have control over every bit going into the DSP API RAM in this case. |
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104 |
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105 There is a queue inside the firmware in which these TCH uplink data blocks are |
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106 stored; this queue is filled by the serial packet receiving handler and drained |
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107 by the L1S (synchronous) code that executes at the right times in the GSM TDMA |
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108 multiplex when uplink TCH transmission is expected. Up to 4 blocks can be |
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109 queued up; as each queued-up block is transmitted on the air (more precisely, |
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110 as it is passed to the DSP for channel encoding and transmission), a |
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111 TCH_ULBITS_CONF short packet (consisting of just the opcode and nothing more) |
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112 is sent to the host. These confirmation packets can be used to pace the sending |
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113 of further TCH_ULBITS_REQs. |
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114 |
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115 Testing |
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116 ======= |
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117 |
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118 See TCH-bit-access write-up in the freecalypso-tools Hg repository. |
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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 own system for talking to FreeCalypso GSM |
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124 pseudo-modems via the RVTMUX binary packet interface, we strongly recommend |
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125 that you use our rvinterf and fc-shell Unix/Linux host utilities as your |
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126 starting point. You can find their source code in the freecalypso-tools Hg |
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127 repository on Bitbucket. |