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view doc/RTP-BFI-extension @ 181:3a0ee08a3b9d
doc/EFR-rationale: typo fix
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Mon, 02 Jan 2023 20:09:07 +0000 |
parents | 6fd49f73b025 |
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We (Themyscira Wireless) have invented our own non-standard extension to the generally accepted standard for RTP-based transport of GSM FR and EFR traffic within a GSM RAN, on stretches running from a BTS to a TRAU-like component. The fundamental question is: when the radio subsystem of the BTS does not have any good traffic frame to send in a given 20 ms window, what should it do? The generally accepted standard behavior is that no packet is sent, an intentional gap is created in the RTP stream (the next time an RTP packet does go out, the timestamp increments over the gap while the sequence number increments only by 1, indicating an intentional gap rather than packet loss), and apparently the intent was/is that this gap in the RTP stream serves as the BFI (bad frame indication). The problem with this generally accepted gap-as-BFI approach is that it deprives the downstream transcoding MGW (a "soft TRAU" of sorts) of its timing source. If the TRAU-like entity on the receiving end of the RTP stream originating from the BTS were an RTP to TDM gateway, there would be no problem - such a gateway would have to buffer received RTP packets in order to synchronize to fixed TDM timing, and the absence of an RTP packet arriving in time would serve just fine as the BFI marker, signaling BFI condition to the Rx DTX handler. But what if the G.711 interface on the 64 kbps side of the TRAU is also an RTP stream, this time going to a PSTN-via-SIP connectivity provider? Now the TRAU-like component becomes a transcoding RTP forwarding MGW without any inherently fixed timing. If the desire is to implement a traditional TRAU in every way except for an RTP-based implementation instead of TDM-based, i.e., if the desire is to emit a fully continuous G.711 RTP stream from the MGW toward PSTN with comfort noise generation and in-band DTMF insertion happening inside the MGW, rather than emit gaps in the outgoing stream or punt CN generation (and DTMF) to VoIP network elements, this task becomes dramatically easier if the BTS can be forced to send an RTP packet in every 20 ms window, be it rain or shine, conveying either a good traffic frame or a BFI marker. Representing BFI markers in an RTP stream ========================================= In the case of AMR codec, the existing standard RTP payload format already provides an obvious way to send a BFI marker: it is the NO_DATA frame type, i.e., FT=15 - see RFC 4867 section 4.3.2. That same section also categorizes what we seek to do here as a "SHOULD NOT": Note that packets containing only NO_DATA frames SHOULD NOT be transmitted in any payload format configuration, [...] However, the just-quoted directive is a SHOULD NOT rather than a MUST NOT, and RFC 2119 states: SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label. Our situation is just that: in our particular circumstance (desire to implement a traditional GSM TRAU in an RTP-to-RTP environment with no TDM network to act as a rigid timing governor) a valid reason exists why this "SHOULD NOT" behavior is not only acceptable, but becomes necessary. Thus in the case of AMR, we are good - there is no need to invent our own totally non-standard extensions to RTP payload format, it just needs to be a configurable option in the IP-based BTS or in OsmoMGW converting from an E1-based BTS to RTP. The same situation holds for the rarely-used HR1 codec: RFC 5993 extends GSM-HR RTP representation with a ToC byte modeled after the one defined for AMR in RFC 4867. Just like in AMR, GSM-HR ToC byte allows the possibility of a No_Data frame (FT=7 for GSM-HR), with exactly the same semantics - and exactly the same argument as above applies for sending such No_Data frames against the general SHOULD NOT. But what about the older FR and EFR codecs? In the case of existing standard RTP payload formats for FR and EFR, there is no defined way to represent a BFI condition as distinct from any possible good traffic frame, and there lies our challenge. Inventing an RTP BFI marker for FR and EFR ========================================== The existing code in osmo-bts-trx (but not in the osmo-bts-sysmo version of interest to us) already contains a partial implementation of what we seek to do here: it runs its own ECU instance in the case of a BFI from the channel decoding layer, and if there is still no luck, there is code present to send a BFI packet. The implemented behavior is not useful for us because RTP output is still fully suppressed when the uplink is expected to be in DTX, and there is a higher-level check in common/l1sap.c (l1sap_tch_ind() function) that also suppresses RTP output, but still, the point is that someone did already write code for sending an RTP packet intended to serve as a BFI. In the case of AMR, that code sends out the expected NO_DATA (aka AMR_BAD) frame type - but what about FR and EFR? The existing code in osmo-bts-trx sends its FR codec BFI as a valid-looking FR frame with all 260 content bits set to 0, and it sends its EFR codec BFI as a valid-looking EFR frame with all 244 content bits set to 0. I (Mother Mychaela) have given consideration to using this all-zeros in-band BFI representation as our RTP BFI marker for ThemWi, but then rejected this idea and decided to implement our own non-standard extension to RTP payload format instead, described further below. The fundamental philosophical problem which I (Mother Mychaela) have with this in-band BFI representation is that in the world of ETSI and 3GPP standards, BFI has always been meant to be out-of-band, not in-band. In the TRAU frame format defined in GSM 08.60 there is an explicit control bit that carries BFI - the condition is NOT to be derived from the 260 or 244 traffic frame bits carried in data bit positions. Abusing one particular bit pattern within the regular 260-bit or 244-bit frame, even if it happens to be all zeros, goes against the spirit of classic GSM and 3GPP. Per the specs, an FR codec frame of all zeros would be a SID frame with all LAR coefficients set to 0, and standards-compliant FR decoders would accept it as a valid SID frame, not as BFI. The situation is likely to be even worse with EFR, where a frame of all zeros would not be treated as SID (EFR SID code word is 95 ones instead of 95 zeros) and would probably produce garbage at the decoder output. Themyscira Wireless implemented solution ======================================== We have invented our own non-standard extension to RTP payload format for GSM FR and EFR codecs. Our extension is as follows: wherever a BTS needs to send a BFI marker in the place of a traffic frame, instead of sending a 33-byte payload beginning with 0xD nibble or a 31-byte payload beginning with 0xC nibble, it needs to send a 2-byte payload formatted as follows: byte 0: 0xBF signature; byte 1: least-significant bit encoding TAF per GSM 06.31 or GSM 06.81, section 6.1.1 in both documents; other bits are reserved. In the uplink direction, with an RTP stream going from a BTS to our "soft TRAU" MGW, our themwi-mgw recognizes these BFI packets and acts accordingly, feeding BFI and TAF to the spec-prescribed Rx DTX handler for FR or EFR. However, if a BTS receives these BFI marker packets in the downlink direction as a result of TrFO (the RTP stream comes from the uplink of another GSM call), it simply discards them without any processing - because a BTS always runs on its own TDMA timing, there is no difference between receiving a BFI packet vs receiving no RTP packet at all for that 20 ms frame.