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doc: new extended RTP format
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 06 Apr 2023 21:30:33 -0800
parents
children f0b90591f67c
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1 TRAU-UL-like RTP transport format for FR & EFR codecs
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2 =====================================================
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3
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4 The generally accepted industry standard format for RTP transport of FR and EFR
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5 codec frames in an IP-based GSM RAN is given in ETSI TS 101 318; the same format
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6 is also codified in IETF RFC 3551. However, when compared to the classic
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7 TRAU-UL format of 3GPP TS 48.060, the standard RTP format of RFC 3551 exhibits
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8 the following two shortcomings:
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9
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10 1) no way to indicate a BFI condition and still send frame data bits;
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11 2) no way to transport the Time Alignment Flag (TAF).
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12
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13 Both of these shortcomings will be explained in detail further in this document;
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14 however, the primary purpose of this document is to propose a new, regrettably
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15 non-standard, RTP transport format for FR & EFR codecs, for use only within a
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16 GSM RAN and the immediately attached CN transcoder ("soft TRAU"), that provides
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17 the same functionality as the classic TRAU-UL format of TS 48.060, but is
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18 carried over RTP in IP rather than a 16 kbps TDM subchannel.
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19
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20 The non-standard RTP transport format presented in this document is implemented
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21 in OsmoBTS on a private feature branch:
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22
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23 https://cgit.osmocom.org/osmo-bts/log/?h=falconia/rtp_traulike
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24
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25 OsmoBTS versions that include this code always accept TRAUlike FR/EFR packets
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26 on their RTP input, following the principle of being liberal in what you accept
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27 while being conservative in what you send, but emit such packets on their RTP
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28 output only when this non-default vty config option is given:
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29
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30 rtp fr-efr-traulike
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31
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32 The recently added (mainline) "rtp continuous-streaming" vty config option also
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33 needs to be enabled.
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34
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35 The present document serves as the formal specification for the TRAUlike RTP
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36 transport format for FR and EFR.
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37
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38 Detailed description of shortcomings of standard RTP transport for FR & EFR
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39 ===========================================================================
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40
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41 These shortcomings are solved in the TRAUlike RTP transport format defined in
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42 this document; understanding these shortcomings provides the essential rationale
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43 for TRAU-like RTP.
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44
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45 Indicating BFI along with data bits
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46 -----------------------------------
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47
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48 The only way to indicate a BFI condition in standard RTP (for FR/EFR) is to
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49 either send no packet at all in the 20 ms window in question (industry standard
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50 behavior and OsmoBTS default) or send an RTP packet with a zero-length payload
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51 ("rtp continuous-streaming" option in OsmoBTS). The latter option provides a
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52 timing tick for a CN-attached transcoder relying on the BTS-originating RTP
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53 stream as its timing source, but there is still no way to send a frame of
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54 marked-erroneous data bits. Contrast with TS 48.060 TRAU-UL format: in this
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55 format the Dn bits carrying FR or EFR frame bits and the C12 bit carrying BFI
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56 are orthogonal.
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57
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58 Why would one care about known-bad or deemed-to-be-bad frame data bits? They
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59 do matter at least in the case of EFR: the official reference C-source EFR
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60 decoder from ETSI makes use of the "fixed codebook excitation pulses" portion
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61 of its EFR frame bits input (140 bits out of 244) even when BFI=1. This
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62 portion of reference C-source behavior is declared to be a non-normative example
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63 by the text of GSM 06.61 spec, thus there may be other compliant EFR decoder
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64 implementations that never look at marked-erroneous data bits - but given the
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65 ease of simply using the C code from ETSI as-is, or recoding it more efficiently
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66 but keeping unchanged all bit-exact algorithms, including non-normative ones,
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67 we should expect that the behavior of ETSI reference code is retained in many
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68 production implementations and deployments.
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69
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70 Consider the case where a traditional E1-based BTS with a classic TRAU interface
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71 is attached to an IP-based Osmocom RAN by way of OsmoMGW, and the resulting RTP
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72 stream then (after passing through another OsmoMGW instance at the MSC) goes to
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73 a "soft TRAU" transcoder (TC) in the CN. The TC will feed its RTP input to FR
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74 and EFR decoders, and at least the EFR decoder makes use of "fixed codebook
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75 excitation pulses" bits from erroneous frames. Furthermore, the TC may
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76 implement in-band TFO (3GPP TS 28.062) inside its G.711 RTP output, in which
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77 case it will need to insert a slightly modified TRAU-UL frame into that output.
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78 The bits that would ideally be fed to the ETSI EFR decoder and emitted to the
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79 outside world in TFO frames already exist at the output of the E1-based BTS,
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80 but they get lost in the RTP transport when the industry standard RTP payload
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81 format is used.
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82
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83 Consider another case where OsmoBTS does have an FR/EFR traffic frame that
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84 could potentially be sent out, but it is suppressed by the
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85 (tch_ind->lqual_cb >= bts->min_qual_norm) check in l1sap_tch_ind() in
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86 src/common/l1sap.c. In this case it would be ideal to send out that frame
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87 along with a BFI=1 indication, if the RTP transport format were to allow such
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88 representation.
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89
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90 Lack of TAF bit in standard RTP transport
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91 -----------------------------------------
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92
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93 The TRAU-UL frame format of TS 48.060 for FR and EFR includes a bit called TAF,
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94 for Time Alignment Flag. Per the specs (TS 48.060 refers to TS 46.031 for
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95 definition and coding of frame indicators) this bit shall be set to 1 in one
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96 particular position in the 480 ms SACCH multiframe (the particular 20 ms frame
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97 position in which a valid frame is always transmitted, even during DTX pauses)
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98 and set to 0 in all other frames. This flag factors into the Rx DTX handler
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99 logic prescribed in GSM 06.31 and 06.81 specs for FR and EFR, respectively, and
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100 there exist production decoders for these codecs that implement their Rx DTX
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101 handler function exactly to the letter of the specs, including the use of TAF
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102 bit when deciding what to do with a BFI=1 frame received in the comfort noise
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103 generation state. (These spec-compliant decoders include the reference ETSI
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104 C-source decoder for EFR and Themyscira libgsmfrp for FR.)
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105
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106 This TAF bit does not exist in the standard RTP transport for FR & EFR. The
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107 lack of this TAF bit causes the following problems for the CN-attached "soft
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108 TRAU" transcoder:
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109
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110 1) The ability to implement spec-compliant handling of GSM 06.11 or 06.61
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111 section 5.4 requirement (same section in both specs) is lost;
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112
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113 2) The TC won't know when to set the TAF bit in its outgoing TFO frames, if it
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114 implements in-band TFO per 3GPP TS 28.062.
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115
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116 The TFO problem is particularly concerning because these TFO frames are emitted
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117 to the outside world, outside of administrative and technical control of the
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118 party implementing the Osmocom-based GSM network and the TC at its edge. The
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119 resulting G.711 octet stream with TFO frames embedded inside can be carried
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120 half-way around the world by the international toll telephone network, and there
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121 is no telling what kind of implementation may be receiving and decoding these
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122 bits on the other end. For this reason, "poor man's" workarounds in the
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123 RTP-fed, TFO-generating TC are very unattractive:
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124
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125 * If the TC were to set TAF=0 in all TFO frames it generates, the receiver's
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126 expectation of seeing TAF=1 in every 24th frame will be violated.
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127
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128 * If the TC were to arbitrarily set TAF=1 in every 24th frame by its own free-
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129 running count, without knowledge of the actual SACCH alignment in the original
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130 GSM call leg, these TAF-marked frames won't coincide with those frame
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131 positions where the MS sends its SID frames, and the resulting TFO frame
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132 stream will be invalid to the receiving Rx DTX handler on the far end.
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133
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134 The knowledge of which frames need to be marked with TAF=1 exists inside the
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135 entity that generates the FR/EFR RTP stream: if this entity is a converter from
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136 E1-based Abis to RTP, the TRAU-UL frames from the BTS contain this TAF bit, and
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137 if the RTP-generating entity is a native IP BTS, it knows the frame number for
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138 which it generates each RTP packet. The only problem is that there is no place
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139 to insert this TAF bit in the standard RTP transport format of TS 101 318.
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140
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141 Why TRAU-UL and not TRAU-DL
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142 ===========================
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143
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144 The present document argues the case that the industry standard RTP transport
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145 format for FR & EFR is functionally crippled compared to the TRAU-UL transport
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146 format of 3GPP TS 48.060, and defines an alternative RTP transport format that
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147 can be used by those who desire TRAU-UL-like functionality badly enough to
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148 accept the price of going totally non-standard in their IP RAN transport. The
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149 new RTP transport format defined in this document explicitly mimics the
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150 functionality and semantics of TS 48.060 TRAU-UL for FR and EFR.
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151
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152 At this point a reader may reasonably ask: why TRAU-UL and not TRAU-DL? The
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153 answer is TFO: 3GPP TS 28.062 and its predecessor GSM 08.62 define the TFO frame
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154 format as being based on TRAU-UL frames with only a few bits changed, and no
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155 change in semantics of any of the frame indicator bits of TRAU-UL (C12 through
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156 C17). Whereas the Abis interface is inherently asymmetric (TRAU-UL frames in
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157 one direction, TRAU-DL frames in the other direction), end-to-end TFO is
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158 directionally symmetric. If we imagine a TFO call between Alice in America and
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159 Bob in Britain, there will be TRAU-UL frames flowing in both directions of the
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160 trans-oceanic G.711 toll connection, one set coming almost unchanged from
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161 Alice's BTS CCU and the other coming almost unchanged from Bob's BTS CCU. Of
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162 course each party's GSM call DL will require TRAU-DL frames to be fed to it,
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163 not TRAU-UL, but the necessary UL-to-DL conversion is the responsibility of the
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164 TFO receiver on each end.
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165
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166 The general rules for turning a TRAU-UL frame into one for TRAU-DL are specified
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167 in TS 28.062 section C.3.2.1.1; it should be noted that this section spells out
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168 the requirements of what the UL-to-DL converter must do, but does not specify
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169 exactly how to do it algorithmically - the wording it uses is "subject to
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170 manufacturer dependent future improvements and is not part of this
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171 recommendation." Implementing all of these section C.3.2.1.1 rules (hereafter
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172 called C3211 rules for short) exactly to the letter is quite easy for the FR
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173 codec (Themyscira libgsmfrp does everything that is needed, and is a simple and
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174 lightweight FLOSS function library), but much harder for EFR. At the present
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175 time it is unclear to the author of this document whether real historical T1/E1
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176 TRAU implementations for which GSM 08.62 TFO was originally specified really did
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177 implement C3211 rules to the letter, particularly for EFR, or if they cut some
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178 corners.
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179
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180 Because the TRAUlike RTP transport format defined in this document is
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181 semantically equivalent to TRAU-UL, any entity that receives such RTP packets
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182 but internally needs to generate either TRAU-DL or some private functional
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183 equivalent thereof will need to perform the same UL-to-DL conversion as called
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184 for in TFO. The lack of a readily available function library that implements
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185 the onerous rules of C3211 for EFR is certainly an obstacle, but it is also
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186 possible to "cut corners" by doing the following:
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187
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188 1) Ignore Table C.3.2.1-1 case 1 and treat it like case 2, at least for EFR:
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189 whenever SID frames are received on the incoming TRAU-UL or TRAUlike RTP
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190 interface, forward them to call leg B even when that destination call leg
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191 has no DTXd. Given that DTX and SID support has been an integral part of
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192 the EFR codec from the beginning, as opposed to an after-addition in the
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193 case of FR, every GSM MS that supports EFR can be expected to understand
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194 SID frames on the downlink.
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195
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196 2) During speech pauses following transmission of a SID frame on call leg B DL,
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197 if real DTXd (turning off Tx) is not allowed, do "fake DTXd" by transmitting
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198 dummy FACCH with an L2 fill frame in the same 20 ms traffic frame windows in
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199 which real DTXd would have been exercised if it were allowed.
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200
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201 3) Whenever a BFI condition is encountered in the incoming TRAU-UL or TRAUlike
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202 RTP frame stream outside of SID, i.e., the case described in the first
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203 paragraph of section C.3.2.1.1, induce an intentional BFI condition in the
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204 receiving GSM MS by transmitting a dummy FACCH frame as above, instead of
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205 trying to devise a parameter-level ECU for EFR.
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206
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207 It should be noted that the just-outlined "cut corners" method is exactly what
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208 OsmoBTS (and a "pure" Osmocom network in general) does currently, hence nothing
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209 is lost and no regression is introduced by continuing to do the same.
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210
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211 Seen another way, by making our RTP transport semantically equivalent to
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212 TRAU-UL, we achieve harmonization between TFO and TrFO. TrFO (Transcoder-Free
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213 Operation) is a scenario in which the RTP output from one IP BTS for call leg A
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214 goes directly to the RTP input of another IP BTS for call leg B, possibly
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215 passing through simple RTP forwarders like OsmoMGW, but never passing through
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216 any transcoder. TrFO is what happens in a self-contained Osmocom network
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217 without any external MNCC connected to OsmoMSC. The principal rules of what
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218 transformations are inherently necessary in order to produce a fully proper DL
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219 for call leg B from the UL of call leg A remain the same whether the transport
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220 in between is old-fashioned TFO or modern TrFO, hence the same conversions that
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221 are codified in TS 28.062 section C.3.2.1.1 are still needed - the only question
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222 is where in the network are they to be performed. The original TDM-based GSM
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223 designers at ETSI gave us a superb architecture end to end; by employing an RTP
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224 transport that is semantically equivalent to TRAU-UL, we can preserve that whole
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225 architecture fully intact in an all-IP implementation.
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226
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227 Specification for TRAUlike RTP payload format for FR and EFR
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228 ============================================================
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229
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230 The modified RTP payload format shall consist of a single octet called TRAUlike
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231 Extension Header (TEH), followed (most of the time) by the standard (same as in
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232 RFC 3551) 33 octets for FR or 31 octets for EFR. The TEH octet has the
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233 following structure:
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234
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235 +----+----+----+----+----+----+----+----+
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236 Hex mask | 0xF0 |0x08|0x04|0x02|0x01|
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237 +----+----+----+----+----+----+----+----+
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238 Meaning | signature |DTXd|NDF |BFI |TAF |
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239 +----+----+----+----+----+----+----+----+
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240
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241 (Bit numbers are identified by hex masks in order to avoid getting into an
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242 argument over which bit numbering convention should be used.)
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243
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244 The following bit fields are defined within the TEH octet:
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245
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246 signature: the upper nibble of the TEH octet shall be set to 0xE. This
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247 signature allows RTP packet receivers to identify the payload format by the
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248 upper nibble of the first octet: if it equals 0xC, the format is EFR without
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249 TEH, if it equals 0xD, the format is FR without TEH, and if it equals 0xE, then
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250 the first octet is TEH.
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251
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252 DTXd: this bit is strictly identical with TRAU-UL frame bit C17.
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253
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254 No_Data flag (NDF): this bit shall be set to 1 if the TRAUlike payload consists
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255 solely of TEH, with the standard 33-octet FR frame or 31-octet EFR frame
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256 entirely omitted, and shall be 0 otherwise.
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257
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258 BFI: this bit is strictly identical with TRAU-UL frame bit C12.
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259
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260 TAF: this bit is strictly identical with TRAU-UL frame bit C15.
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261
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262 There are two possibilities for full composition of a TRAUlike RTP payload:
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263
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264 Possibility 1: TEH with NDF=0 is followed by a standard 33-octet FR frame or a
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265 standard 31-octet EFR frame. The signature in the upper nibble of the octet
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266 immediately following TEH shall be correct: 0xD for FR or 0xC for EFR.
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267
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268 Possibility 2: TEH with NDF=1 constitutes the entirety of the RTP payload for
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269 the 20 ms time window in question.
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270
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271 If the No_Data flag is set, BFI must also be set: the combination of NDF=1 and
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272 BFI=0 is invalid.
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273
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274 Per this specification, the sender of a BFI packet has the choice of sending it
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275 in one of two forms: with or without presumed-erroneous frame bits. If the
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276 TRAUlike RTP packet is generated from bits received in an actual TRAU-UL frame
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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277 (E1 Abis or TFO), erroneous frame bits shall be included, unchanged from the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
278 TRAU-UL source. However, if the entity generating the TRAUlike RTP packet is
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parents:
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279 the ultimate point of origin (e.g., a native IP BTS), then it shall choose one
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parents:
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280 form or the other based on the situation at hand:
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Mychaela Falconia <falcon@freecalypso.org>
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281
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282 a) if the sender does have an FR or EFR frame "on hand" but that frame is
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parents:
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283 considered to be erroneous (for example, the link quality check in
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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284 l1sap_tch_ind() in OsmoBTS), the long form of BFI shall be sent, with the
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285 presumed-erroneous frame bits included.
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286
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287 b) if the sender does not have any FR or EFR frame at all that could be sent
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288 (for example, if the reason for the BFI condition is because FACCH was
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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289 successfully received and decoded instead of a traffic frame), then the
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Mychaela Falconia <falcon@freecalypso.org>
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290 No_Data form of BFI shall be sent.
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parents:
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291
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Mychaela Falconia <falcon@freecalypso.org>
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292 The option of No_Data BFI is provided in this RTP transport format specification
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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293 because if this option were disallowed, senders would be tasked with an
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Mychaela Falconia <falcon@freecalypso.org>
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294 additional burden of having to artificially generate dummy or "garbage" frame
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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295 bits. This task is slightly complicated, as explained in the following section,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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296 and the present design moves that task from all senders to only those receivers
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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297 that need it.
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Mychaela Falconia <falcon@freecalypso.org>
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298
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299 Lack of SID classification bits matching TRAU-UL C13 & C14
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parents:
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300 ----------------------------------------------------------
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301
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parents:
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302 TRAU-UL frame format includes two bits C13 & C14 that carry the ternany SID flag
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parents:
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303 (0, 1 or 2) as defined in GSM 06.31 and 06.81 section 6.1.1 (same section in
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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304 both specs). No equivalent bits are included in the TRAUlike RTP transport
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
305 format as defined by this specification - however, these bits are redundant.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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306 The rules of section 6.1.1 in GSM 06.31 and 06.81, hereafter called S611 rules,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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307 specify a strictly deterministic, unambiguous formula by which these C13 & C14
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
308 bits derive their values from the bit content of the FR/EFR frame payload -
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
309 thus if a TRAU-UL frame is received in which these C13 & C14 bits fail to match
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
310 the S611 value derived from the contained payload, then that TRAU-UL frame is
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
311 defective. There is no need to include such redundant bits in our TRAUlike RTP
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
312 format, only to create confusion for receivers as to which source of SID S611
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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313 classification they should use.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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314
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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315 Feeding received TRAUlike BFI frames to an EFR decoder
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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316 ======================================================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
317
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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318 If an EFR decoder implementation is based on the reference C source from ETSI,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
319 this decoder requires that _some_ frame bits input be fed to it at all times,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
320 even when BFI=1. But what if the BFI packet came in as No_Data? In that case
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
321 the receiver must synthesize its own fake "bad data" bits to feed to the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
322 standard decoder. When synthesizing "bad data" bits in this manner, the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
323 following rules should be observed:
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
324
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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325 * The 140 bits corresponding to "fixed codebook excitation pulses" (35 bits in
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
326 each of the 4 subframes) shall be filled using a PRNG. These bits are the
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
327 ones used by the standard decoder when its internal state, based on previous
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
328 good frames, puts it in GSM 06.61 substitution/muting mode as opposed to
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
329 GSM 06.62 comfort noise generation mode.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
330
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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331 * The remaining 104 bits of the EFR frame shall be set to 0. These bits are
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
332 never used by the standard decoder under the condition of BFI=1, and setting
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
333 them to 0 prevents the possibility of S611 rules classifying the frame as SID
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
334 even if the PRNG output in the other 140 bits happens to be all 1s in those
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
335 SID codeword bit positions (70 out of 140) that fall within the "fixed
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
336 codebook excitation pulses" portion.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
337
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
338 Converting from TRAU-UL to TRAUlike RTP
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
339 =======================================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
340
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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341 There will be a need to convert from standard TS 48.060 TRAU-UL frames to our
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
342 TRAUlike RTP format in the following two scenarios:
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
343
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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344 1) When interfacing an E1 BTS to Osmocom RAN, when and if such support is to be
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
345 added to OsmoMGW;
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
346
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
347 2) In the CN transcoder operating in TFO mode, when forwarding received TFO
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
348 frames to the local RAN.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
349
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
350 In both cases the conversion is straightforward:
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
351
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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352 * Always generate full-length TRAUlike RTP payloads, never generate No_Data in
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
353 the case of a properly received TRAU-UL speech (not idle) frame.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
354
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
355 * Forward the payload bits directly from TRAU-UL to TRAUlike RTP, for both good
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
356 and bad frames.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
357
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
358 * Directly forward BFI, TAF and DTXd frame indicator bits from TRAU-UL C-bits
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
359 to TEH octet bits.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
360
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
361 * Ignore TRAU-UL C13 & C14 bits.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
362
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
363 Converting from TRAUlike RTP to TRAU-UL
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
364 =======================================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
365
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
366 This direction of conversion will need to be performed in the CN transcoder when
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
367 emitting TFO frames toward the outside world. The following rules will need to
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
368 be applied:
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
369
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
370 * If the incoming TRAUlike RTP payload is full-length, as opposed to No_Data,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
371 simply copy the payload bits into the constructed TRAU-UL frame, for both
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
372 good (BFI=0) and bad (BFI=1) frames.
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
373
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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374 * If the incoming TRAUlike RTP payload is No_Data, put the following filler in
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
375 the data bits portion of the TRAU-UL frame:
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
376
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
377 - For FR codec, use the silence frame of 3GPP TS 46.011 Table 1 as the filler.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
378
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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379 - For EFR codec, perform the same PRNG procedure as detailed earlier in this
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
380 document for the case of feeding a No_Data BFI packet to the standard ETSI
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
381 decoder for EFR. Given that a TFO-frame-emitting transcoder still needs to
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
382 run its regular speech decoder in order to fill the upper 6 bits of each
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
383 outgoing G.711 sample octet, the same No_Data PRNG handler will typically
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
384 be run just once for both internal decoding and TFO frame output.
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
385
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
386 * Algorithmically set C13 & C14 bits in the generated TRAU-UL frame per the
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
387 rules of S611. This step can be done using osmo_{fr,efr}_sid_classify()
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
388 functions proposed in this Gerrit patch submission:
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
389
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
390 https://gerrit.osmocom.org/c/libosmocore/+/32183
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
391
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
392 or using equivalent functions in Themyscira libgsmefr and libgsmfrp.
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
393
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
394 * Directly forward BFI, TAF and DTXd frame indicator bits from TEH octet bits
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
395 to TRAU-UL C12, C15 and C17, respectively.
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
396
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
397 Mixing standard RFC 3551 and TRAUlike RTP payloads
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
398 ==================================================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
399
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
400 An RTP stream receiver for FR/EFR codecs that supports the present non-standard
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
401 extension to the RTP payload format shall behave gracefully when it receives a
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
402 mixture of standard RFC 3551 payloads and TRAUlike payloads in the same RTP
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
403 stream. A receiver that has no interest in the additional information carried
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
404 in the TRAUlike Extension Header shall simply strip the TEH octet when one is
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
405 received, reducing the received payload to standard RFC 3551; if a BFI or
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
406 No_Data payload is received, treat it the same as if nothing at all was
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
407 received. A receiver that is interested in the TRAUlike Extension Header but
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
408 receives an FR/EFR payload without one should behave as if it received a TEH
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
409 with BFI=0, TAF=0, and a received zero-length RTP payload should be treated the
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
410 same as receiving a No_Data TRAUlike payload with TAF=0.
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
411
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
412 There may even be cases when an RTP sender may alternate between sending
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
413 standard RFC 3551 payloads and TRAUlike payloads in the same session: for
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
414 example, a TFO-supporting CN transcoder may emit "plain" RFC 3551 payloads when
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
415 supplying the output of its free-running speech encoder, but switch to sending
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
416 TRAUlike payloads when it switches to forwarding bits received in TFO frames
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
417 from the far end.