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annotate doc/TCH-bit-access @ 619:f82551c77e58

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libserial-newlnx: ASYNC_LOW_LATENCY patch reverted Reports from Das Signal indicate that loadtools performance on Debian is about the same as on Slackware, and that including or omitting the ASYNC_LOW_LATENCY patch from Serg makes no difference. Because the patch in question does not appear to be necessary, it is being reverted until and unless someone other than Serg reports an actual real-world system on which loadtools operation times are slowed compared to the Mother's Slackware reference and on which Slackware-like performance can be restored by setting the ASYNC_LOW_LATENCY flag.
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 27 Feb 2020 01:09:48 +0000
parents 2748f257312b
children 3de3b34189be
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2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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1 It has been discovered that the DSP ROM in the Calypso GSM baseband processor
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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2 implements one nifty feature which is not used at all in standard phone or modem
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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3 operation, but which can be used for all kinds of interesting hacks: the traffic
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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4 channel (TCH) bits coming out of the GSM 05.03 channel decoder in the downlink
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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5 direction (to be fed to one of several speech decoders) can be read out of the
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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6 DSP's API RAM in real time, and in the uplink direction the user can feed her
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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7 own bits to the input of the GSM 05.03 channel encoder, effectively suppressing
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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8 the output of the internal vocoder.
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9
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10 The DSP mechanism in question is known to work in TCH/FS and TCH/EFS channel
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Mychaela Falconia <falcon@freecalypso.org>
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11 modes, corresponding to the FR and EFR codecs; it is not currently known if it
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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12 can also be used in TCH/HS, data traffic or AMR modes. I am aware though of an
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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13 anecdotal report that someone tried to make this feature work with AMR, but was
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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14 unsuccessful - hence we should be prepared for the possibility that the hack
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Mychaela Falconia <falcon@freecalypso.org>
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15 is not possible with AMR.
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16
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Mychaela Falconia <falcon@freecalypso.org>
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17 In order to make use of this TCH bit access feature, one needs 3 things:
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18
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19 1) Firmware on the Calypso (the ARM part) that reads downlink bits from the DSP
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Mychaela Falconia <falcon@freecalypso.org>
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20 and writes uplink bits into it while doing everything else that a GSM fw must
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21 do in order to operate the MS;
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22
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23 2) Some protocol for passing these TCH bits into and out of the Calypso device;
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24
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25 3) A source for TCH UL bits and a sink for TCH DL bits on the external host.
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26
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27 In the case of FreeCalypso, we have defined our own protocol for passing TCH
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Mychaela Falconia <falcon@freecalypso.org>
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28 bits into and out of Calypso GSM devices running one of our firmwares in the
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Mychaela Falconia <falcon@freecalypso.org>
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29 form of an extension to TI's RVTMUX interface, i.e., we have defined a new
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Mychaela Falconia <falcon@freecalypso.org>
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30 RVTMUX channel for this TCH interface and defined the packet types and formats
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Mychaela Falconia <falcon@freecalypso.org>
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31 to be sent over the wire. On the Calypso side this interface is implemented in
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Mychaela Falconia <falcon@freecalypso.org>
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32 FreeCalypso GSM firmwares (currently only in Citrine, but we can also make a
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Mychaela Falconia <falcon@freecalypso.org>
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33 special version of TCS211 with this feature added if the need arises), and on
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Mychaela Falconia <falcon@freecalypso.org>
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34 the host tools side there is support in rvinterf and fc-shell.
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35
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36 The TCH bit access mechanism in FreeCalypso has been designed with an objective
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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37 of presenting to the user exactly what TI's DSP presents to us, i.e., standing
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Mychaela Falconia <falcon@freecalypso.org>
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38 out of the way as much as possible. TI's DSP presents TCH downlink bits and
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Mychaela Falconia <falcon@freecalypso.org>
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39 accepts TCH uplink bits in the form of an array of 16-bit words; the bit order
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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40 within these words corresponds to the GSM 05.03 channel encoder bit order (with
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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41 a couple of TI-specific quirks documented below) and NOT that of the GSM 06.10
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Mychaela Falconia <falcon@freecalypso.org>
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42 or EFR codecs. On the RVTMUX serial interface between the Calypso device and
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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43 the external host we transfer each TCH frame as a block of 33 bytes; our Calypso
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Mychaela Falconia <falcon@freecalypso.org>
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44 firmwares translate between these bytes and the DSP's 16-bit words, but do not
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
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45 reorder or change the bits in any way.
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46
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47 On the host tools side our fc-shell utility provides user commands to save TCH
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Mychaela Falconia <falcon@freecalypso.org>
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48 DL bits into a file and to play TCH UL bits from a file; in the present version
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Mychaela Falconia <falcon@freecalypso.org>
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49 these files are written and read in an ASCII-based hex format. In these ASCII
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Mychaela Falconia <falcon@freecalypso.org>
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50 files each TCH frame is represented as a string of 66 hexadecimal digits, and
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Mychaela Falconia <falcon@freecalypso.org>
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51 these hex digits correspond directly to the 33 bytes being read out of or
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Mychaela Falconia <falcon@freecalypso.org>
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52 written into DSP API words. Therefore, in order to generate and/or interpret
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Mychaela Falconia <falcon@freecalypso.org>
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53 these hexadecimal strings correctly, you (the user) need to understand the bit
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Mychaela Falconia <falcon@freecalypso.org>
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54 order and mapping used by TI's implementation of the GSM 05.03 channel encoder.
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55
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56 Recall from the GSM specs that the 260 bits which comprise one speech frame are
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Mychaela Falconia <falcon@freecalypso.org>
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57 not all treated equally, instead they are divided into 182 class 1 bits which
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Mychaela Falconia <falcon@freecalypso.org>
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58 are protected by a convolutional encoder and 78 class 2 bits which are
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Mychaela Falconia <falcon@freecalypso.org>
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59 transmitted without any forward error correction. Furthermore, the first 50 of
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Mychaela Falconia <falcon@freecalypso.org>
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60 the class 1 bits are also protected by a CRC. The order in which the bits
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Mychaela Falconia <falcon@freecalypso.org>
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61 appear on TI's DSP interface corresponds to this division, known as the order of
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Mychaela Falconia <falcon@freecalypso.org>
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62 subjective importance.
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63
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64 Now let's look at the actual bit order and mapping which you need to understand
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Mychaela Falconia <falcon@freecalypso.org>
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65 in order to make sense of the hex strings in tch record and tch play files.
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66 The bit numbering is from the most significant bit to the least significant bit,
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Mychaela Falconia <falcon@freecalypso.org>
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67 i.e., in our string of 66 hex digits the most significant bit of the leftmost
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Mychaela Falconia <falcon@freecalypso.org>
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68 digit is bit 0 and the least significant bit of the rightmost digit is bit 263.
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Mychaela Falconia <falcon@freecalypso.org>
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69 TI's DSP assigns these bits as follows:
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70
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71 * Bits 0 through 181 correspond to the 182 protected (class 1) bits in the
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Mychaela Falconia <falcon@freecalypso.org>
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72 standard GSM 05.03 order;
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73
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74 * Bits 182 through 185 are unused - put zeros there if you are generating them
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75 yourself;
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76
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77 * Bits 186 through 263 correspond to the 78 unprotected (class 2) bits in the
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Mychaela Falconia <falcon@freecalypso.org>
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78 standard GSM 05.03 order.
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79
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80 Uplink testing
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81 ==============
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82
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83 The uplink sending mechanism can be exercised as follows:
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84
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85 1. Record a speech sample in the GSM 06.10 codec format using any Unix/Linux
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86 audio tool that can write the de facto standard libgsm format. For example,
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87 using SoX:
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88
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89 rec -c1 recording.gsm
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90
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91 SoX will write the recording in the GSM 06.10 libgsm format based on the
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Mychaela Falconia <falcon@freecalypso.org>
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92 .gsm suffix at the end of the recording file name; the -c1 option is needed
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93 to disable stereo, otherwise the recording will be slowed down 2x.
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94
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95 2. Convert it from libgsm standard format into our ad hoc hex strings for
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96 playing into the TCH uplink:
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97
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98 fc-fr2tch recording.gsm recording.tch-ul
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99
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100 3. In fc-shell, when you are in an established voice call, issue this command:
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101
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102 tch play recording.tch-ul
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103
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104 You should now hear the speech sample you recorded in step 1 above on the other
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105 end of the GSM call. Please note, though, that for this example to work, the
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Mychaela Falconia <falcon@freecalypso.org>
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106 call must be connected in the FR codec mode, not EFR. If you try it on an EFR
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Mychaela Falconia <falcon@freecalypso.org>
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107 call, the vocoder on the other end of the call will try to interpret your FR
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Mychaela Falconia <falcon@freecalypso.org>
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108 codec bits per EFR, resulting in garbage. In order to make this procedure work
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109 properly with EFR, you would need to generate a speech sample in EFR format and
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110 then put it in the correct hex string form with the correct bit order for
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111 feeding to TI's GSM 05.03 channel encoder implementation, and at the present
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112 moment the tools to do this feat are lacking.
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113
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114 The fc-fr2tch step above is new with the current version of FreeCalypso host
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115 tools. My original implementation of the tch play command in fc-shell read
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116 libgsm GSM 06.10 speech files directly, but I changed it to read hex strings
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117 which correspond literally to what goes into TI's implementation of the 05.03
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118 channel encoder to make it more general:
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119
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120 1) to support playing EFR speech bits into TCH/EFS channels;
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
121 2) to support people who may want to pass totally non-speech data over traffic
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
122 channels in TFO mode - see below.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
123
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
124 Downlink testing
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
125 ================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
126
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
127 When you are in an established voice call in fc-shell, you can record the
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
128 downlink TCH bits as follows:
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
129
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
130 tch record <name of file to put the recording into>
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
131
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
132 If you would like to record an entire call from the beginning, issue the
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
133 tch record command as above before the ATD or ATA command that dials or answers
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
134 the call. Either way, whether you are recording a call from the beginning or
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
135 from the middle, you need to eventually stop your recording with this command:
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
136
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
137 tch record stop
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
138
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
139 You can issue this stop command before or after the call is terminated, but
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
140 until you issue this tch record stop command, the output file is not closed and
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
141 thus may not be written to the file system.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
142
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
143 The recording is written in an ASCII format that is similar but not identical
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
144 to what the tch play command takes as its input. Here is an example of what
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
145 you might get from tch record:
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
146
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
147 C214 281D 0063 81C008000008200046000000000000000000000000000007056608060B0A010B09
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
148 C204 0540 003A 9391480D3051F4BD81DF5EB35069BEBC4AEDF756351C4C19689BB1CA4EA5D4F5F5
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
149 C204 05A2 0047 65B80F9E690F7C8CE4ED80DEF69DA6436518AB99ABCE5815E6B562C5CE4EAC5DC5
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
150 C204 04CA 0044 A4483744B04371ED0334ECB350AF28C639B7F095519EF0242D299B6405124F77A5
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
151 C214 2544 0066 83800400000C1000E80000000000000000000000000000300674A07070F080D0B2
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
152 C200 4E8A 0000 07071DF0F83B9521EE61CFF095AA8C0E560300F6A5573C31F3E00601ED4AAE7E2F
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
153 C200 4C94 0000 077385ADE20450B2E410961D6C5B0A173ACF9E2D38D77C28CED8495D88AA4DE72E
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
154 C200 4ABC 0000 077F0CF86004132DA5C0A6D5A4B0BD4B28159A07D8F4282DC6AAAB27503BC02701
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
155
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
156 The example above is quoted from an actual recording made from a call to WWV at
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
157 +1-303-499-7111, a time of day service. This example shows 5 garbled frames
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
158 followed by 3 good ones.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
159
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
160 In each line the first 3 space-separated 16-bit words are status words from the
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
161 DSP, and the rest is a hex string of 66 digits giving the 260-bit frame with 4
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
162 extra dummy bits at the boundary between the protected and unprotected portions.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
163
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
164 Now here comes the unpleasant part: radio systems are inherently error-prone,
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
165 hence if you are naively expecting every received downlink frame to be a valid
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
166 speech frame that matches exactly what the other end transmitted, you will be
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
167 disappointed. The downlink frame stream will always consist of both valid and
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
168 invalid frames, and in the standard processing chain inside TI's DSP the speech
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
169 decoder block that receives these frames from the channel decoder will look at
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
170 the status words written by the latter in order to decide what to do with each
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
171 frame. But unfortunately I don't know the details, as the DSP is a mostly
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
172 undocumented black box.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
173
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
174 Our FreeCalypso firmwares forward the downlink frame status words from the DSP
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
175 to the external host and my current implementation of the tch record command
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
176 writes them into the file because it is valuable information which should not
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
177 be discarded, but unfortunately I don't know their meaning. The first status
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
178 word consists of bit flags, the last one seems to be some kind of error count
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
179 (what kind of errors? I don't know), and the middle one is totally unknown.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
180
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
181 There is also an fc-tch2fr utility that does sort of an inverse of fc-fr2tch,
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
182 except that fc-tch2fr expects files written by tch record which contain the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
183 status words at the beginning - the files written by fc-fr2tch and read by
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
184 tch play don't have these status words; they just have 66 hex digits per line
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
185 giving the bits to be fed to the GSM 05.03 block in the DSP. The fc-tch2fr
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
186 utility works by disregarding the status words; the result will be a .gsm file
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
187 which you can play with SoX etc, but it will produce horrible sounds wherever
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
188 some garbled frames were received on the call downlink.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
189
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
190 Passing non-speech data over TCH
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
191 ================================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
192
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
193 If you are incredibly lucky and you happen to live in a part of the world where
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
194 the local GSM network implements TFO (tandem-free operation, see GSM 02.53),
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
195 i.e., if your GSM network transparently passes codec frames from one end of the
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
196 call to the other without transcoding back and forth in the middle, you might
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
197 be able to send arbitrary non-speech data bits over this TCH connection. If
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
198 you are going to attempt such a feat, you are mostly on your own as the GSM
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
199 network in my part of the world does not support TFO, but here are some general
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
200 ideas:
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
201
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
202 * Heed the difference between the 182 protected bits and the 78 unprotected
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
203 bits of each 260-bit frame. The bits in the first class are less likely to
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
204 be corrupted by radio errors. Also the first 50 out of these class 1 bits
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
205 are protected by a CRC, so if they get corrupted, there will be a flag bit
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
206 somewhere in the DSP status words - but I don't know which bit it is...
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
207
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
208 * Given the lack of detailed information about the exact meaning of the DSP
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
209 status words, implement some error checking of your own, e.g., an in-band
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
210 CRC within your frame payload so you can check on the receiving end if you
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
211 got an uncorrupted frame from your data source or not.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
212
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
213 * Expect to lose some frames - even if your radio environment is pristine and
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
214 error-free, some frames will still be lost because their timeslots had to be
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
215 stolen by FACCH on either end of the call. You will need to implement
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
216 acknowledgements, retransmissions and retries - the usual stuff.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
217
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
218 EFR differences
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
219 ===============
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
220
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
221 TCH/FS and TCH/EFS channel modes corresponding to the FR and EFR codecs,
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
222 respectively, share the same channel encoder that takes 260-bit frames as input.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
223 An EFR speech frame is only 244 bits, but as the GSM 05.03 spec explains, each
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
224 EFR frame gets expanded to 260 bits by adding some CRC and repetition bits.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
225 One would thus expect that the behaviour of the channel encoder block should be
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
226 strictly identical between TCH/FS and TCH/EFS channel modes, but it appears that
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
227 TI's implementation exhibits one difference in behaviour between the two modes.
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
228
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
229 It appears that TI's implementation of the GSM 05.03 channel encoder computes
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
230 the EFR-specific CRC bits internally, such that bits [65,72] of each uplink
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
231 frame fed to the DSP in the TCH/EFS mode need to be 0 and will be replaced by
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
232 the DSP with CRC bits computed per the standard. It also appears that their
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
233 implementation of the channel decoder verifies this CRC, sets a status bit
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
234 indicating whether the check succeeded or failed, and then zeroes these bits
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
235 out, i.e., the original bits in these CRC bit positions are lost. No problem
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
236 if what you are passing over TCH/EFS is indeed EFR speech frames, but it will
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
237 be a problem if you are passing non-speech data over your TCH and put something
2748f257312b doc/TCH-bit-access: new write-up
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
238 else in those bits which the spec allocates to CRC.