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author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 13 Aug 2023 22:01:25 -0800
parents 185225722714
children
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1 NOTE: This document describes the old BFI representation format that was used
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2 by Themyscira Wireless for a few months from late 2022 into early 2023. We no
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3 longer use this format, instead we have a new, bolder extended payload format
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4 for FR & EFR codecs, defined and described in the new RTP-TRAUlike-format
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5 document.
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6
185225722714 doc: new extended RTP format
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7 Original document follows
185225722714 doc: new extended RTP format
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8 =========================
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9
173
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10 We (Themyscira Wireless) have invented our own non-standard extension to the
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11 generally accepted standard for RTP-based transport of GSM FR and EFR traffic
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12 within a GSM RAN, on stretches running from a BTS to a TRAU-like component.
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13
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14 The fundamental question is: when the radio subsystem of the BTS does not have
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15 any good traffic frame to send in a given 20 ms window, what should it do? The
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16 generally accepted standard behavior is that no packet is sent, an intentional
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17 gap is created in the RTP stream (the next time an RTP packet does go out, the
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18 timestamp increments over the gap while the sequence number increments only by
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19 1, indicating an intentional gap rather than packet loss), and apparently the
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20 intent was/is that this gap in the RTP stream serves as the BFI (bad frame
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21 indication).
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22
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23 The problem with this generally accepted gap-as-BFI approach is that it deprives
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24 the downstream transcoding MGW (a "soft TRAU" of sorts) of its timing source.
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25 If the TRAU-like entity on the receiving end of the RTP stream originating from
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26 the BTS were an RTP to TDM gateway, there would be no problem - such a gateway
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27 would have to buffer received RTP packets in order to synchronize to fixed TDM
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28 timing, and the absence of an RTP packet arriving in time would serve just fine
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29 as the BFI marker, signaling BFI condition to the Rx DTX handler. But what if
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30 the G.711 interface on the 64 kbps side of the TRAU is also an RTP stream, this
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31 time going to a PSTN-via-SIP connectivity provider? Now the TRAU-like component
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32 becomes a transcoding RTP forwarding MGW without any inherently fixed timing.
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33
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34 If the desire is to implement a traditional TRAU in every way except for an
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35 RTP-based implementation instead of TDM-based, i.e., if the desire is to emit a
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36 fully continuous G.711 RTP stream from the MGW toward PSTN with comfort noise
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37 generation and in-band DTMF insertion happening inside the MGW, rather than
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38 emit gaps in the outgoing stream or punt CN generation (and DTMF) to VoIP
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39 network elements, this task becomes dramatically easier if the BTS can be
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40 forced to send an RTP packet in every 20 ms window, be it rain or shine,
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41 conveying either a good traffic frame or a BFI marker.
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42
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43 Representing BFI markers in an RTP stream
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44 =========================================
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45
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46 In the case of AMR codec, the existing standard RTP payload format already
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47 provides an obvious way to send a BFI marker: it is the NO_DATA frame type,
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48 i.e., FT=15 - see RFC 4867 section 4.3.2. That same section also categorizes
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49 what we seek to do here as a "SHOULD NOT":
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50
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51 Note that packets containing only NO_DATA frames SHOULD NOT be
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52 transmitted in any payload format configuration, [...]
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53
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54 However, the just-quoted directive is a SHOULD NOT rather than a MUST NOT,
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55 and RFC 2119 states:
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56
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57 SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that
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58 there may exist valid reasons in particular circumstances when the
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59 particular behavior is acceptable or even useful, but the full
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60 implications should be understood and the case carefully weighed
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61 before implementing any behavior described with this label.
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62
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63 Our situation is just that: in our particular circumstance (desire to implement
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64 a traditional GSM TRAU in an RTP-to-RTP environment with no TDM network to act
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65 as a rigid timing governor) a valid reason exists why this "SHOULD NOT" behavior
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66 is not only acceptable, but becomes necessary. Thus in the case of AMR, we are
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67 good - there is no need to invent our own totally non-standard extensions to
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68 RTP payload format, it just needs to be a configurable option in the IP-based
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69 BTS or in OsmoMGW converting from an E1-based BTS to RTP.
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70
177
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71 The same situation holds for the rarely-used HR1 codec: RFC 5993 extends GSM-HR
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72 RTP representation with a ToC byte modeled after the one defined for AMR in RFC
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73 4867. Just like in AMR, GSM-HR ToC byte allows the possibility of a No_Data
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74 frame (FT=7 for GSM-HR), with exactly the same semantics - and exactly the same
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75 argument as above applies for sending such No_Data frames against the general
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76 SHOULD NOT.
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77
173
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78 But what about the older FR and EFR codecs? In the case of existing standard
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79 RTP payload formats for FR and EFR, there is no defined way to represent a BFI
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80 condition as distinct from any possible good traffic frame, and there lies our
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81 challenge.
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82
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83 Inventing an RTP BFI marker for FR and EFR
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84 ==========================================
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85
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86 The existing code in osmo-bts-trx (but not in the osmo-bts-sysmo version of
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87 interest to us) already contains a partial implementation of what we seek to do
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88 here: it runs its own ECU instance in the case of a BFI from the channel
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89 decoding layer, and if there is still no luck, there is code present to send a
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90 BFI packet. The implemented behavior is not useful for us because RTP output
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91 is still fully suppressed when the uplink is expected to be in DTX, and there
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92 is a higher-level check in common/l1sap.c (l1sap_tch_ind() function) that also
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93 suppresses RTP output, but still, the point is that someone did already write
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94 code for sending an RTP packet intended to serve as a BFI. In the case of AMR,
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95 that code sends out the expected NO_DATA (aka AMR_BAD) frame type - but what
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96 about FR and EFR?
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97
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98 The existing code in osmo-bts-trx sends its FR codec BFI as a valid-looking FR
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99 frame with all 260 content bits set to 0, and it sends its EFR codec BFI as a
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100 valid-looking EFR frame with all 244 content bits set to 0. I (Mother Mychaela)
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101 have given consideration to using this all-zeros in-band BFI representation as
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102 our RTP BFI marker for ThemWi, but then rejected this idea and decided to
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103 implement our own non-standard extension to RTP payload format instead,
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104 described further below.
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105
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106 The fundamental philosophical problem which I (Mother Mychaela) have with this
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107 in-band BFI representation is that in the world of ETSI and 3GPP standards, BFI
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108 has always been meant to be out-of-band, not in-band. In the TRAU frame format
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109 defined in GSM 08.60 there is an explicit control bit that carries BFI - the
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110 condition is NOT to be derived from the 260 or 244 traffic frame bits carried
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111 in data bit positions. Abusing one particular bit pattern within the regular
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112 260-bit or 244-bit frame, even if it happens to be all zeros, goes against the
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113 spirit of classic GSM and 3GPP. Per the specs, an FR codec frame of all zeros
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114 would be a SID frame with all LAR coefficients set to 0, and standards-compliant
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115 FR decoders would accept it as a valid SID frame, not as BFI. The situation is
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116 likely to be even worse with EFR, where a frame of all zeros would not be
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117 treated as SID (EFR SID code word is 95 ones instead of 95 zeros) and would
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118 probably produce garbage at the decoder output.
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119
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120 Themyscira Wireless implemented solution
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121 ========================================
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122
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123 We have invented our own non-standard extension to RTP payload format for GSM
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124 FR and EFR codecs. Our extension is as follows: wherever a BTS needs to send a
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125 BFI marker in the place of a traffic frame, instead of sending a 33-byte payload
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126 beginning with 0xD nibble or a 31-byte payload beginning with 0xC nibble, it
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127 needs to send a 2-byte payload formatted as follows:
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128
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129 byte 0: 0xBF signature;
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130 byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81,
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131 section 6.1.1 in both documents; other bits are reserved.
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132
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133 In the uplink direction, with an RTP stream going from a BTS to our "soft TRAU"
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134 MGW, our themwi-mgw recognizes these BFI packets and acts accordingly, feeding
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135 BFI and TAF to the spec-prescribed Rx DTX handler for FR or EFR. However, if a
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136 BTS receives these BFI marker packets in the downlink direction as a result of
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137 TrFO (the RTP stream comes from the uplink of another GSM call), it simply
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138 discards them without any processing - because a BTS always runs on its own TDMA
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139 timing, there is no difference between receiving a BFI packet vs receiving no
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140 RTP packet at all for that 20 ms frame.
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141
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142 Our patch to osmo-bts
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143 =====================
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144
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145 Our current BTS is a sysmoBTS, as opposed to a T1/E1-based BTS interfaced via
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146 OsmoBSC and OsmoMGW, hence the software component in need of patching is
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147 osmo-bts. The patch in osmo-patches/osmo-bts-rtp-bfi.patch in the present
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148 repository applies to the common part of osmo-bts, hence it should theoretically
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149 work with all models, but it has only been tested with osmo-bts-sysmo. If
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150 anyone wishes to play with this patch in an osmo-bts-trx environment, they will
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151 probably need to remove TRX-specific ECU and BFI code from
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152 osmo-bts-trx/sched_lchan_tchf.c, otherwise the two implementations will "fight":
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153 the TRX-specific code will kick in on "unexpected" bad frames and times of FACCH
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154 stealing, whereas the common code will fill in the new form of BFI during times
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155 of uplink DTX.
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156
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157 Aside from the just-described incompatibility with osmo-bts-trx version, our
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158 current patch has the following outstanding issues which will need to be fixed
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159 before it can be proposed for merging:
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160
177
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161 * The code will need to be extended to support AMR, and possibly HR1 too - our
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162 current patch always emits our FR/EFR BFI, implicitly assuming that the speech
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163 codec is either FR or EFR, as that is how we currently run our network.
173
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164
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165 * A configuration option will need to be implemented, to be controlled from
177
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166 OsmoBSC or from the BTS' own vty, enabling or disabling the non-standard
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167 behavior of sending explicit BFI packets instead of gaps in the RTP stream.
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168 It would probably be best implemented as a tristate option, with the choices
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169 being no BFI packets at all (default), sending BFI packets only for AMR
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170 (where a standard encoding exists) or sending BFI packets for AMR and also
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171 for FR & EFR, using Themyscira encoding for the latter.