The RTP stream is generated or forwarded by OpenBSC to nanoBTS. Due to switching of streams (hold/retrieve call), packet loss or even stalling of sender's process, the time stamp must be corrected. If outdated packets are received, they get dropped. --- openbsc/src/libtrau/rtp_proxy.c | 82 ++++++++++++++++++++++++++++++----------- 1 file changed, 61 insertions(+), 21 deletions(-)

diff --git a/openbsc/src/libtrau/rtp_proxy.c b/openbsc/src/libtrau/rtp_proxy.c index 2284798..88753b6 100644 --- a/openbsc/src/libtrau/rtp_proxy.c +++ b/openbsc/src/libtrau/rtp_proxy.c @@ -236,6 +236,12 @@ static int rtp_decode(struct msgb *msg, uint32_t callref, struct msgb **data, in return 0; }

+#define USEC_1S 1000000 +#define USEC_10MS 10000 +#define USEC_20MS 20000 +#define SAMPLES_1S 8000 +#define USEC_SAMPLE 125 + /* "to - from" */ static void tv_difference(struct timeval *diff, const struct timeval *from, const struct timeval *__to) @@ -244,13 +250,62 @@ static void tv_difference(struct timeval *diff, const struct timeval *from,

if (to->tv_usec < from->tv_usec) { to->tv_sec -= 1; - to->tv_usec += 1000000; + to->tv_usec += USEC_1S; }

diff->tv_usec = to->tv_usec - from->tv_usec; diff->tv_sec = to->tv_sec - from->tv_sec; }

+/* add sec,usec to tv */ +static void tv_add(struct timeval *tv, int sec, int usec) +{ + + while (usec < 0) { + usec += USEC_1S; + sec--; + } + tv->tv_sec += sec; + tv->tv_usec += usec; + while (tv->tv_usec >= USEC_1S) { + tv->tv_sec++; + tv->tv_usec -= USEC_1S; + } +} + +static int correct_timestamp(struct rtp_socket *rs, int duration) +{ + struct timeval tv, tv_diff; + long int usec_diff, frame_diff; + + gettimeofday(&tv, NULL); + tv_difference(&tv_diff, &rs->transmit.last_tv, &tv); + tv_add(&rs->transmit.last_tv, 0, USEC_20MS); + + usec_diff = tv_diff.tv_sec * USEC_1S + tv_diff.tv_usec; + frame_diff = ((usec_diff + (USEC_10MS)) / USEC_20MS); /* round */ + + if (abs(frame_diff - 1) > 1) { + long int frame_diff_excess = frame_diff - 1; + long int sample_diff_excess = frame_diff_excess * duration; + + /* correct last_tv */ + tv_add(&rs->transmit.last_tv, sample_diff_excess / SAMPLES_1S, + (sample_diff_excess % SAMPLES_1S) * USEC_SAMPLE); + /* drop frame, if time stamp is in the past */ + if (frame_diff_excess < 0) + return -1; + LOGP(DLMUX, LOGL_NOTICE, + "Correcting timestamp difference of %ld frames " + "(to %s)\n", frame_diff_excess, + (rs->rx_action == RTP_RECV_L4) ? "app" : "BTS"); + rs->transmit.sequence += frame_diff_excess; + rs->transmit.timestamp += sample_diff_excess; + } + + return 0; +} + /*! \brief encode and send a rtp frame * \param[in] rs RTP socket through which we shall send * \param[in] frame GSM RTP frame to be sent @@ -265,6 +320,7 @@ int rtp_send_frame(struct rtp_socket *rs, struct gsm_data_frame *frame) int duration; /* in samples */ int amr = 0; uint8_t dynamic_pt = 0; + int rc;

if (rs->rx_action == RTP_RECV_L4) dynamic_pt = rs->receive.payload_type; @@ -275,6 +331,7 @@ int rtp_send_frame(struct rtp_socket *rs, struct gsm_data_frame *frame) rs->transmit.ssrc = rand(); rs->transmit.sequence = random(); rs->transmit.timestamp = random(); + gettimeofday(&rs->transmit.last_tv, NULL); }

switch (frame->msg_type) { @@ -313,26 +370,9 @@ int rtp_send_frame(struct rtp_socket *rs, struct gsm_data_frame *frame) return -EINVAL; }

- { - struct timeval tv, tv_diff; - long int usec_diff, frame_diff; - - gettimeofday(&tv, NULL); - tv_difference(&tv_diff, &rs->transmit.last_tv, &tv); - rs->transmit.last_tv = tv; - - usec_diff = tv_diff.tv_sec * 1000000 + tv_diff.tv_usec; - frame_diff = (usec_diff / 20000); - - if (abs(frame_diff) > 1) { - long int frame_diff_excess = frame_diff - 1; - - LOGP(DLMUX, LOGL_NOTICE, - "Correcting frame difference of %ld frames\n", frame_diff_excess); - rs->transmit.sequence += frame_diff_excess; - rs->transmit.timestamp += frame_diff_excess * duration; - } - } + rc = correct_timestamp(rs, duration); + if (rc) + return 0;

if (frame->msg_type == GSM_BAD_FRAME) return 0;