falconia has uploaded this change for review.
trau_rtp_conv: document API functions
The two API functions that constitute TRAU<->RTP conversion layer
of libosmotrau, osmo_trau2rtp() and osmo_rtp2trau(), date back to
the reintroduction of E1 BTS support in Osmocom in 2020, and their
functionality has been greatly extended in previous Themyscira
patches. However, they have not been properly documented until
now - fix this defect.
Change-Id: Ib7ee2466aaab47e47f78ff94c5179b90cf393d0e
---
M src/trau/trau_rtp_conv.c
1 file changed, 99 insertions(+), 0 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/libosmo-abis refs/changes/20/39620/1
diff --git a/src/trau/trau_rtp_conv.c b/src/trau/trau_rtp_conv.c
index 9bee640..328221e 100644
--- a/src/trau/trau_rtp_conv.c
+++ b/src/trau/trau_rtp_conv.c
@@ -1456,6 +1456,38 @@
return false;
}
+/*! convert received TRAU-UL frame to RTP payload
+ * \param[out] out Buffer for the output RTP payload
+ * \param[in] out_len Size of buffer pointed to by \ref out
+ * \param[in] tf Osmo-decoded TRAU frame
+ * \param[in] st State/config structure
+ * \returns length of converted RTP payload if successful; negative on error
+ *
+ * This function is intended to operate on TRAU-UL or TFO frames, either
+ * received from an E1 BTS or extracted from lsbs of PCM samples in TFO.
+ * Supported TRAU frame types are FR & EFR speech, HRv1 speech in both
+ * 16k and 8k formats, and all defined CSD frame types up to 14.4 kbit/s
+ * extended data frames.
+ *
+ * In the case of FR/EFR speech, the output format is either RFC 3551 or
+ * TW-TS-001; in the case of HRv1 speech, the output format is either RFC 5993
+ * or TW-TS-002. st->rtp_extensions field selects the use or non-use of
+ * Themyscira RTP extensions; the structure passed in \ref st currently
+ * has no other uses in the TRAU->RTP direction.
+ *
+ * The following TRAU frame types are _not_ supported:
+ *
+ * - TRAU-DL frames: the direction field of the parsed frame structure is
+ * ignored and all frames are processed as if they were TRAU-UL. In the
+ * case of FR, EFR, HR-16k and extended data (E-TRAU) frames some control
+ * bits are different between TRAU-UL and TRAU-DL, such that this
+ * misinterpretation will produce invalid results.
+ *
+ * - D144 sync frames: these special frames are not convertible to RTP;
+ * their synchronization function needs to be handled by the application.
+ *
+ * - AMR speech frames: not currently implemented.
+ */
int osmo_trau2rtp(uint8_t *out, size_t out_len, const struct osmo_trau_frame *tf,
struct osmo_trau2rtp_state *st)
{
@@ -1481,6 +1513,73 @@
}
}
+/*! convert RTP payload to TRAU-UL or TRAU-DL frame
+ * \param[out] tf Osmocom-defined structure for TRAU frame bits
+ * \param[in] rtp Payload to be converted
+ * \param[in] rtp_len Length of payload in \ref rtp
+ * \param[in] st State/config structure
+ * \returns 0 in case of success; negative on error
+ *
+ * This function can be used to generate both TRAU-UL and TRAU-DL frames.
+ * TRAU-DL output is needed when feeding traffic to an E1 BTS; TRAU-UL output
+ * is needed in more specialized applications that emulate an E1 BTS or
+ * implement in-band TFO.
+ *
+ * The set of supported codecs and frame types is the same as osmo_trau2rtp();
+ * st->type selects the TRAU frame type to be emitted. Additionally, if the
+ * TRAU frame type to be generated is OSMO_TRAU16_FT_DATA, st->interm_rate_16k
+ * needs to be set to true for 16 kbit/s IR or false for 8 kbit/s IR.
+ *
+ * In the output structure pointed to by \ref tf, the caller MUST set dir
+ * member prior to calling the present function; additionally, dl_ta_usec
+ * member MUST be set (usually to 0) prior to calling osmo_trau_frame_encode().
+ * All other required bits are filled correctly by the present function.
+ *
+ * With FR/HR/EFR speech codecs, semantically appropriate RTP payloads are
+ * different between UL and DL output applications. Considerations for
+ * TRAU-DL output:
+ *
+ * - RTP payload formats of RFC 5993, TW-TS-001 and TW-TS-002 are accepted
+ * by the function - however, all metadata flags carried by the header octet
+ * of these extended formats are ignored/dropped in the DL direction.
+ *
+ * - The most native RTP input formats for conversion to TRAU-DL are those
+ * defined in ETSI TS 101 318 for FR, HR and EFR; the ones for FR and EFR
+ * are also duplicated in RFC 3551. In the case of HR codec, RFC 5993 input
+ * is also appropriate as specified in 3GPP TS 48.103 - as long as the user
+ * remembers that the extra header octet is ignored.
+ *
+ * - The only correct way to implement TrFO for GSM, accepting FR/HR/EFR from
+ * call leg A uplink in TW-TS-001 or TW-TS-002 format and generating TRAU-DL
+ * frames for call leg B, is to apply the TFO transform of TS 28.062 section
+ * C.3.2.1.1, then feed the output of that transform to the present function.
+ *
+ * - The provision whereby a zero-length RTP payload is not treated as an error
+ * like other invalid RTP inputs, but is converted to an idle speech frame
+ * in TRAU-DL output should be considered a bogon. This condition never
+ * occurs when the just-mentioned TFO transform is applied immediately prior
+ * to TRAU-DL output, nor does it ever occur in the original GSM architecture
+ * where the TRAU either free-runs a speech encoder or applies the same TFO
+ * transform - hence it is unlikely to be handled well by real E1 BTSes.
+ * Furthermore, this code path in libosmotrau is currently broken (the
+ * intended idle speech frame gets turned into a "regular" but invalid FR/EFR
+ * speech frame) and should be considered for removal.
+ *
+ * Considerations for TRAU-UL output:
+ *
+ * - For FR and EFR codecs, the only correct RTP format for conversion to
+ * TRAU-UL (TFO) is TW-TS-001 - the basic RTP format of TS 101 318 or
+ * RFC 3551 lacks the necessary metadata flags.
+ *
+ * - TRAU-UL output for HR codec is not currently implemented; when we do
+ * implement it in the future, TW-TS-002 will be required in this path
+ * for the same reason as above.
+ *
+ * - TRAU-UL output for CSD 14.4 kbit/s mode is not currently implemented
+ * (C-bits are always set according to the rules for TRAU-DL) - but the
+ * primary application for TRAU-UL frame output via libosmotrau is TFO,
+ * which does not include CSD.
+ */
int osmo_rtp2trau(struct osmo_trau_frame *tf, const uint8_t *rtp, size_t rtp_len,
struct osmo_trau2rtp_state *st)
{
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