FreeCalypso > hg > themwi-system-sw

annotate doc/RTP-BFI-extension @ 248:257da5474e77

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
sip-in: fix bug in incoming number validation for non-plus
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 14 Aug 2023 22:56:20 -0800
parents 185225722714
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
207
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
1 NOTE: This document describes the old BFI representation format that was used
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
2 by Themyscira Wireless for a few months from late 2022 into early 2023. We no
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
3 longer use this format, instead we have a new, bolder extended payload format
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
4 for FR & EFR codecs, defined and described in the new RTP-TRAUlike-format
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
5 document.
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
6
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
7 Original document follows
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
8 =========================
185225722714 doc: new extended RTP format
Mychaela Falconia <falcon@freecalypso.org>
parents: 177
diff changeset
9
173
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
10 We (Themyscira Wireless) have invented our own non-standard extension to the
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
11 generally accepted standard for RTP-based transport of GSM FR and EFR traffic
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
12 within a GSM RAN, on stretches running from a BTS to a TRAU-like component.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
13
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
14 The fundamental question is: when the radio subsystem of the BTS does not have
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
15 any good traffic frame to send in a given 20 ms window, what should it do? The
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
16 generally accepted standard behavior is that no packet is sent, an intentional
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
17 gap is created in the RTP stream (the next time an RTP packet does go out, the
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
18 timestamp increments over the gap while the sequence number increments only by
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
19 1, indicating an intentional gap rather than packet loss), and apparently the
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
20 intent was/is that this gap in the RTP stream serves as the BFI (bad frame
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
21 indication).
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
22
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
23 The problem with this generally accepted gap-as-BFI approach is that it deprives
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
24 the downstream transcoding MGW (a "soft TRAU" of sorts) of its timing source.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
25 If the TRAU-like entity on the receiving end of the RTP stream originating from
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
26 the BTS were an RTP to TDM gateway, there would be no problem - such a gateway
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
27 would have to buffer received RTP packets in order to synchronize to fixed TDM
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
28 timing, and the absence of an RTP packet arriving in time would serve just fine
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
29 as the BFI marker, signaling BFI condition to the Rx DTX handler. But what if
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
30 the G.711 interface on the 64 kbps side of the TRAU is also an RTP stream, this
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
31 time going to a PSTN-via-SIP connectivity provider? Now the TRAU-like component
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
32 becomes a transcoding RTP forwarding MGW without any inherently fixed timing.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
33
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
34 If the desire is to implement a traditional TRAU in every way except for an
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
35 RTP-based implementation instead of TDM-based, i.e., if the desire is to emit a
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
36 fully continuous G.711 RTP stream from the MGW toward PSTN with comfort noise
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
37 generation and in-band DTMF insertion happening inside the MGW, rather than
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
38 emit gaps in the outgoing stream or punt CN generation (and DTMF) to VoIP
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
39 network elements, this task becomes dramatically easier if the BTS can be
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
40 forced to send an RTP packet in every 20 ms window, be it rain or shine,
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
41 conveying either a good traffic frame or a BFI marker.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
42
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
43 Representing BFI markers in an RTP stream
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
44 =========================================
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
45
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
46 In the case of AMR codec, the existing standard RTP payload format already
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
47 provides an obvious way to send a BFI marker: it is the NO_DATA frame type,
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
48 i.e., FT=15 - see RFC 4867 section 4.3.2. That same section also categorizes
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
49 what we seek to do here as a "SHOULD NOT":
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
50
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
51 Note that packets containing only NO_DATA frames SHOULD NOT be
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
52 transmitted in any payload format configuration, [...]
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
53
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
54 However, the just-quoted directive is a SHOULD NOT rather than a MUST NOT,
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
55 and RFC 2119 states:
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
56
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
57 SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
58 there may exist valid reasons in particular circumstances when the
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
59 particular behavior is acceptable or even useful, but the full
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
60 implications should be understood and the case carefully weighed
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
61 before implementing any behavior described with this label.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
62
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
63 Our situation is just that: in our particular circumstance (desire to implement
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
64 a traditional GSM TRAU in an RTP-to-RTP environment with no TDM network to act
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
65 as a rigid timing governor) a valid reason exists why this "SHOULD NOT" behavior
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
66 is not only acceptable, but becomes necessary. Thus in the case of AMR, we are
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
67 good - there is no need to invent our own totally non-standard extensions to
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
68 RTP payload format, it just needs to be a configurable option in the IP-based
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
69 BTS or in OsmoMGW converting from an E1-based BTS to RTP.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
70
177
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
71 The same situation holds for the rarely-used HR1 codec: RFC 5993 extends GSM-HR
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
72 RTP representation with a ToC byte modeled after the one defined for AMR in RFC
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
73 4867. Just like in AMR, GSM-HR ToC byte allows the possibility of a No_Data
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
74 frame (FT=7 for GSM-HR), with exactly the same semantics - and exactly the same
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
75 argument as above applies for sending such No_Data frames against the general
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
76 SHOULD NOT.
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
77
173
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
78 But what about the older FR and EFR codecs? In the case of existing standard
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
79 RTP payload formats for FR and EFR, there is no defined way to represent a BFI
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
80 condition as distinct from any possible good traffic frame, and there lies our
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
81 challenge.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
82
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
83 Inventing an RTP BFI marker for FR and EFR
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
84 ==========================================
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
85
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
86 The existing code in osmo-bts-trx (but not in the osmo-bts-sysmo version of
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
87 interest to us) already contains a partial implementation of what we seek to do
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
88 here: it runs its own ECU instance in the case of a BFI from the channel
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
89 decoding layer, and if there is still no luck, there is code present to send a
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
90 BFI packet. The implemented behavior is not useful for us because RTP output
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
91 is still fully suppressed when the uplink is expected to be in DTX, and there
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
92 is a higher-level check in common/l1sap.c (l1sap_tch_ind() function) that also
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
93 suppresses RTP output, but still, the point is that someone did already write
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
94 code for sending an RTP packet intended to serve as a BFI. In the case of AMR,
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
95 that code sends out the expected NO_DATA (aka AMR_BAD) frame type - but what
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
96 about FR and EFR?
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
97
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
98 The existing code in osmo-bts-trx sends its FR codec BFI as a valid-looking FR
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
99 frame with all 260 content bits set to 0, and it sends its EFR codec BFI as a
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
100 valid-looking EFR frame with all 244 content bits set to 0. I (Mother Mychaela)
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
101 have given consideration to using this all-zeros in-band BFI representation as
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
102 our RTP BFI marker for ThemWi, but then rejected this idea and decided to
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
103 implement our own non-standard extension to RTP payload format instead,
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
104 described further below.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
105
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
106 The fundamental philosophical problem which I (Mother Mychaela) have with this
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
107 in-band BFI representation is that in the world of ETSI and 3GPP standards, BFI
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
108 has always been meant to be out-of-band, not in-band. In the TRAU frame format
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
109 defined in GSM 08.60 there is an explicit control bit that carries BFI - the
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
110 condition is NOT to be derived from the 260 or 244 traffic frame bits carried
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
111 in data bit positions. Abusing one particular bit pattern within the regular
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
112 260-bit or 244-bit frame, even if it happens to be all zeros, goes against the
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
113 spirit of classic GSM and 3GPP. Per the specs, an FR codec frame of all zeros
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
114 would be a SID frame with all LAR coefficients set to 0, and standards-compliant
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
115 FR decoders would accept it as a valid SID frame, not as BFI. The situation is
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
116 likely to be even worse with EFR, where a frame of all zeros would not be
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
117 treated as SID (EFR SID code word is 95 ones instead of 95 zeros) and would
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
118 probably produce garbage at the decoder output.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
119
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
120 Themyscira Wireless implemented solution
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
121 ========================================
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
122
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
123 We have invented our own non-standard extension to RTP payload format for GSM
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
124 FR and EFR codecs. Our extension is as follows: wherever a BTS needs to send a
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
125 BFI marker in the place of a traffic frame, instead of sending a 33-byte payload
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
126 beginning with 0xD nibble or a 31-byte payload beginning with 0xC nibble, it
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
127 needs to send a 2-byte payload formatted as follows:
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
128
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
129 byte 0: 0xBF signature;
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
130 byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81,
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
131 section 6.1.1 in both documents; other bits are reserved.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
132
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
133 In the uplink direction, with an RTP stream going from a BTS to our "soft TRAU"
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
134 MGW, our themwi-mgw recognizes these BFI packets and acts accordingly, feeding
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
135 BFI and TAF to the spec-prescribed Rx DTX handler for FR or EFR. However, if a
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
136 BTS receives these BFI marker packets in the downlink direction as a result of
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
137 TrFO (the RTP stream comes from the uplink of another GSM call), it simply
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
138 discards them without any processing - because a BTS always runs on its own TDMA
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
139 timing, there is no difference between receiving a BFI packet vs receiving no
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
140 RTP packet at all for that 20 ms frame.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
141
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
142 Our patch to osmo-bts
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
143 =====================
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
144
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
145 Our current BTS is a sysmoBTS, as opposed to a T1/E1-based BTS interfaced via
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
146 OsmoBSC and OsmoMGW, hence the software component in need of patching is
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
147 osmo-bts. The patch in osmo-patches/osmo-bts-rtp-bfi.patch in the present
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
148 repository applies to the common part of osmo-bts, hence it should theoretically
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
149 work with all models, but it has only been tested with osmo-bts-sysmo. If
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
150 anyone wishes to play with this patch in an osmo-bts-trx environment, they will
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
151 probably need to remove TRX-specific ECU and BFI code from
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
152 osmo-bts-trx/sched_lchan_tchf.c, otherwise the two implementations will "fight":
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
153 the TRX-specific code will kick in on "unexpected" bad frames and times of FACCH
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
154 stealing, whereas the common code will fill in the new form of BFI during times
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
155 of uplink DTX.
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
156
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
157 Aside from the just-described incompatibility with osmo-bts-trx version, our
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
158 current patch has the following outstanding issues which will need to be fixed
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
159 before it can be proposed for merging:
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
160
177
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
161 * The code will need to be extended to support AMR, and possibly HR1 too - our
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
162 current patch always emits our FR/EFR BFI, implicitly assuming that the speech
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
163 codec is either FR or EFR, as that is how we currently run our network.
173
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
164
36cce9b0bbe2 doc/RTP-BFI-extension: article written
Mychaela Falconia <falcon@freecalypso.org>
parents:
diff changeset
165 * A configuration option will need to be implemented, to be controlled from
177
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
166 OsmoBSC or from the BTS' own vty, enabling or disabling the non-standard
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
167 behavior of sending explicit BFI packets instead of gaps in the RTP stream.
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
168 It would probably be best implemented as a tristate option, with the choices
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
169 being no BFI packets at all (default), sending BFI packets only for AMR
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
170 (where a standard encoding exists) or sending BFI packets for AMR and also
a851bde42d82 doc/RTP-BFI-extension: update for the current situation
Mychaela Falconia <falcon@freecalypso.org>
parents: 173
diff changeset
171 for FR & EFR, using Themyscira encoding for the latter.