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annotate doc/RTP-BFI-extension @ 183:452c1d5a6268

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libgsmefr BFI w/o data: emit zero output after decoder reset In real-life usage, each EFR decoder session will most likely begin with lots of BFI frames before the first real frame arrives. However, because the spec-defined home state of the decoder is speech rather than CN, our regular logic for BFI w/o data would have to feed pseudorandom noise to the decoder (in the "fixed codebook excitation pulses" part), which is silly to do at the beginning of the decoder session right out of reset. Therefore, let's check reset_flag_old, and if we are still in the reset state, simply emit zero output.
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 03 Jan 2023 00:12:18 +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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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
106 FR decoders would accept it as a valid SID frame, not as BFI. The situation is
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
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107 likely to be even worse with EFR, where a frame of all zeros would not be
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
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108 treated as SID (EFR SID code word is 95 ones instead of 95 zeros) and would
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
109 probably produce garbage at the decoder output.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
110
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
111 Themyscira Wireless implemented solution
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
112 ========================================
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
113
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
114 We have invented our own non-standard extension to RTP payload format for GSM
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
115 FR and EFR codecs. Our extension is as follows: wherever a BTS needs to send a
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
116 BFI marker in the place of a traffic frame, instead of sending a 33-byte payload
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
117 beginning with 0xD nibble or a 31-byte payload beginning with 0xC nibble, it
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
118 needs to send a 2-byte payload formatted as follows:
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
119
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
120 byte 0: 0xBF signature;
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
121 byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81,
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
122 section 6.1.1 in both documents; other bits are reserved.
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
123
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Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
124 In the uplink direction, with an RTP stream going from a BTS to our "soft TRAU"
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
125 MGW, our themwi-mgw recognizes these BFI packets and acts accordingly, feeding
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
126 BFI and TAF to the spec-prescribed Rx DTX handler for FR or EFR. However, if a
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
127 BTS receives these BFI marker packets in the downlink direction as a result of
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
128 TrFO (the RTP stream comes from the uplink of another GSM call), it simply
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
129 discards them without any processing - because a BTS always runs on its own TDMA
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
130 timing, there is no difference between receiving a BFI packet vs receiving no
2b3f612a5fe5 doc/RTP-BFI-extension: import from themwi-system-sw,
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
131 RTP packet at all for that 20 ms frame.