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annotate doc/RTP-BFI-extension @ 143:195911f2211c

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document PCM format conversion utilities
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 14 Dec 2022 07:04:59 +0000
parents 6fd49f73b025
children
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1 We (Themyscira Wireless) have invented our own non-standard extension to the
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2 generally accepted standard for RTP-based transport of GSM FR and EFR traffic
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3 within a GSM RAN, on stretches running from a BTS to a TRAU-like component.
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4
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5 The fundamental question is: when the radio subsystem of the BTS does not have
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6 any good traffic frame to send in a given 20 ms window, what should it do? The
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7 generally accepted standard behavior is that no packet is sent, an intentional
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8 gap is created in the RTP stream (the next time an RTP packet does go out, the
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9 timestamp increments over the gap while the sequence number increments only by
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10 1, indicating an intentional gap rather than packet loss), and apparently the
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11 intent was/is that this gap in the RTP stream serves as the BFI (bad frame
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12 indication).
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13
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14 The problem with this generally accepted gap-as-BFI approach is that it deprives
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15 the downstream transcoding MGW (a "soft TRAU" of sorts) of its timing source.
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16 If the TRAU-like entity on the receiving end of the RTP stream originating from
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17 the BTS were an RTP to TDM gateway, there would be no problem - such a gateway
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18 would have to buffer received RTP packets in order to synchronize to fixed TDM
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19 timing, and the absence of an RTP packet arriving in time would serve just fine
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20 as the BFI marker, signaling BFI condition to the Rx DTX handler. But what if
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21 the G.711 interface on the 64 kbps side of the TRAU is also an RTP stream, this
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22 time going to a PSTN-via-SIP connectivity provider? Now the TRAU-like component
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23 becomes a transcoding RTP forwarding MGW without any inherently fixed timing.
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24
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25 If the desire is to implement a traditional TRAU in every way except for an
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26 RTP-based implementation instead of TDM-based, i.e., if the desire is to emit a
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27 fully continuous G.711 RTP stream from the MGW toward PSTN with comfort noise
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28 generation and in-band DTMF insertion happening inside the MGW, rather than
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29 emit gaps in the outgoing stream or punt CN generation (and DTMF) to VoIP
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30 network elements, this task becomes dramatically easier if the BTS can be
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31 forced to send an RTP packet in every 20 ms window, be it rain or shine,
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32 conveying either a good traffic frame or a BFI marker.
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33
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34 Representing BFI markers in an RTP stream
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35 =========================================
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36
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37 In the case of AMR codec, the existing standard RTP payload format already
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38 provides an obvious way to send a BFI marker: it is the NO_DATA frame type,
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39 i.e., FT=15 - see RFC 4867 section 4.3.2. That same section also categorizes
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40 what we seek to do here as a "SHOULD NOT":
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41
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42 Note that packets containing only NO_DATA frames SHOULD NOT be
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43 transmitted in any payload format configuration, [...]
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44
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45 However, the just-quoted directive is a SHOULD NOT rather than a MUST NOT,
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46 and RFC 2119 states:
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47
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48 SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that
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49 there may exist valid reasons in particular circumstances when the
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50 particular behavior is acceptable or even useful, but the full
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51 implications should be understood and the case carefully weighed
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52 before implementing any behavior described with this label.
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53
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54 Our situation is just that: in our particular circumstance (desire to implement
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55 a traditional GSM TRAU in an RTP-to-RTP environment with no TDM network to act
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56 as a rigid timing governor) a valid reason exists why this "SHOULD NOT" behavior
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57 is not only acceptable, but becomes necessary. Thus in the case of AMR, we are
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58 good - there is no need to invent our own totally non-standard extensions to
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59 RTP payload format, it just needs to be a configurable option in the IP-based
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60 BTS or in OsmoMGW converting from an E1-based BTS to RTP.
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61
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62 The same situation holds for the rarely-used HR1 codec: RFC 5993 extends GSM-HR
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63 RTP representation with a ToC byte modeled after the one defined for AMR in RFC
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64 4867. Just like in AMR, GSM-HR ToC byte allows the possibility of a No_Data
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65 frame (FT=7 for GSM-HR), with exactly the same semantics - and exactly the same
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66 argument as above applies for sending such No_Data frames against the general
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67 SHOULD NOT.
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68
125
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69 But what about the older FR and EFR codecs? In the case of existing standard
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70 RTP payload formats for FR and EFR, there is no defined way to represent a BFI
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71 condition as distinct from any possible good traffic frame, and there lies our
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72 challenge.
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73
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74 Inventing an RTP BFI marker for FR and EFR
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75 ==========================================
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76
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77 The existing code in osmo-bts-trx (but not in the osmo-bts-sysmo version of
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78 interest to us) already contains a partial implementation of what we seek to do
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79 here: it runs its own ECU instance in the case of a BFI from the channel
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80 decoding layer, and if there is still no luck, there is code present to send a
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81 BFI packet. The implemented behavior is not useful for us because RTP output
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82 is still fully suppressed when the uplink is expected to be in DTX, and there
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83 is a higher-level check in common/l1sap.c (l1sap_tch_ind() function) that also
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84 suppresses RTP output, but still, the point is that someone did already write
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85 code for sending an RTP packet intended to serve as a BFI. In the case of AMR,
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86 that code sends out the expected NO_DATA (aka AMR_BAD) frame type - but what
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87 about FR and EFR?
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88
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89 The existing code in osmo-bts-trx sends its FR codec BFI as a valid-looking FR
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90 frame with all 260 content bits set to 0, and it sends its EFR codec BFI as a
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91 valid-looking EFR frame with all 244 content bits set to 0. I (Mother Mychaela)
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92 have given consideration to using this all-zeros in-band BFI representation as
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93 our RTP BFI marker for ThemWi, but then rejected this idea and decided to
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94 implement our own non-standard extension to RTP payload format instead,
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95 described further below.
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96
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97 The fundamental philosophical problem which I (Mother Mychaela) have with this
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98 in-band BFI representation is that in the world of ETSI and 3GPP standards, BFI
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99 has always been meant to be out-of-band, not in-band. In the TRAU frame format
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100 defined in GSM 08.60 there is an explicit control bit that carries BFI - the
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101 condition is NOT to be derived from the 260 or 244 traffic frame bits carried
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102 in data bit positions. Abusing one particular bit pattern within the regular
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103 260-bit or 244-bit frame, even if it happens to be all zeros, goes against the
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104 spirit of classic GSM and 3GPP. Per the specs, an FR codec frame of all zeros
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105 would be a SID frame with all LAR coefficients set to 0, and standards-compliant
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106 FR decoders would accept it as a valid SID frame, not as BFI. The situation is
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107 likely to be even worse with EFR, where a frame of all zeros would not be
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Mychaela Falconia <falcon@freecalypso.org>
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108 treated as SID (EFR SID code word is 95 ones instead of 95 zeros) and would
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Mychaela Falconia <falcon@freecalypso.org>
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109 probably produce garbage at the decoder output.
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110
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111 Themyscira Wireless implemented solution
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Mychaela Falconia <falcon@freecalypso.org>
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112 ========================================
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Mychaela Falconia <falcon@freecalypso.org>
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113
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114 We have invented our own non-standard extension to RTP payload format for GSM
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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115 FR and EFR codecs. Our extension is as follows: wherever a BTS needs to send a
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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116 BFI marker in the place of a traffic frame, instead of sending a 33-byte payload
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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117 beginning with 0xD nibble or a 31-byte payload beginning with 0xC nibble, it
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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118 needs to send a 2-byte payload formatted as follows:
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Mychaela Falconia <falcon@freecalypso.org>
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119
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Mychaela Falconia <falcon@freecalypso.org>
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120 byte 0: 0xBF signature;
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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121 byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81,
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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122 section 6.1.1 in both documents; other bits are reserved.
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Mychaela Falconia <falcon@freecalypso.org>
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123
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Mychaela Falconia <falcon@freecalypso.org>
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124 In the uplink direction, with an RTP stream going from a BTS to our "soft TRAU"
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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125 MGW, our themwi-mgw recognizes these BFI packets and acts accordingly, feeding
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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126 BFI and TAF to the spec-prescribed Rx DTX handler for FR or EFR. However, if a
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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127 BTS receives these BFI marker packets in the downlink direction as a result of
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Mychaela Falconia <falcon@freecalypso.org>
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128 TrFO (the RTP stream comes from the uplink of another GSM call), it simply
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Mychaela Falconia <falcon@freecalypso.org>
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129 discards them without any processing - because a BTS always runs on its own TDMA
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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130 timing, there is no difference between receiving a BFI packet vs receiving no
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Mychaela Falconia <falcon@freecalypso.org>
parents:
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131 RTP packet at all for that 20 ms frame.