poll: Use the data_ind FN as time source for current frame

The FN of the data_ind taken from the DSP are monotonic, so latency
does not affect the detection of poll timeouts if these FN are used.
If the FN is larger than a poll_fn value, it can safely be assumed
that the poll response will not arrive later on.

Currently a max_delay of 60 frames is used, which has the drawback
that additional ~250ms will pass until a lost ACK is detected.

Using the data_ind's FN alone breaks the poll timeout detection if
there are no other MS sending data blocks.

This commit adds BTS::set_current_block_frame_number that is called
with the FN taken from data_ind messages. The max_delay is set to 0
which removes the additional delay, when this FN is used to detect
poll timeouts. So the average additional delay decreases with the
number of data_ind per time. The current_frame is updated unless it
seems to have been updated already (assumed if 0 < cur_fn - block_fn
< 500). Thus the time_ind has still priority to update the
current_frame value.

Sponsored-by: On-Waves ehf

src/bts.cpp

@@ -103,6 +103,7 @@ struct rate_ctr_group *bts_main_data_stats()
 
 BTS::BTS()
 	: m_cur_fn(0)
+	, m_cur_blk_fn(-1)
 	, m_pollController(*this)
 	, m_sba(*this)
 	, m_ms_store(this)
@@ -151,6 +152,38 @@ void BTS::set_current_frame_number(int fn)
 	m_pollController.expireTimedout(m_cur_fn, max_delay);
 }
 
+void BTS::set_current_block_frame_number(int fn)
+{
+	int delay = 0;
+	const int late_block_delay_thresh = 13;
+	const int fn_update_ok_min_delay = -500;
+	const int fn_update_ok_max_delay = 0;
+
+	/* frame numbers in the received blocks are assumed to be strongly
+	 * monotonic. */
+	if (m_cur_blk_fn >= 0) {
+		int delta = (fn + 2715648 * 3 / 2 - m_cur_blk_fn) % 2715648 - 2715648/2;
+		if (delta <= 0)
+			return;
+	}
+
+	/* Check block delay vs. the current frame number */
+	if (current_frame_number() != 0)
+		delay = (fn + 2715648 * 3 / 2 - current_frame_number()) % 2715648
+			- 2715648/2;
+	if (delay <= -late_block_delay_thresh)
+		LOGP(DRLCMAC, LOGL_NOTICE,
+			"Late RLC block, FN delta: %d FN: %d curFN: %d\n",
+			delay, fn, current_frame_number());
+
+	m_cur_blk_fn = fn;
+	if (delay < fn_update_ok_min_delay || delay > fn_update_ok_max_delay ||
+		current_frame_number() == 0)
+		m_cur_fn = fn;
+
+	m_pollController.expireTimedout(fn, 0);
+}
+
 int BTS::add_paging(uint8_t chan_needed, uint8_t *identity_lv)
 {
 	uint8_t l, trx, ts, any_tbf = 0;
@@ -1084,6 +1117,8 @@ int gprs_rlcmac_pdch::rcv_block(uint8_t *data, uint8_t len, uint32_t fn,
 	bitvec *block;
 	int rc = 0;
 
+	bts()->set_current_block_frame_number(fn);
+
 	switch (payload) {
 	case GPRS_RLCMAC_DATA_BLOCK:
 		rc = rcv_data_block_acknowledged(data, len, meas);

src/bts.h

@@ -244,6 +244,7 @@ public:
 
 	/** TODO: change the number to unsigned */
 	void set_current_frame_number(int frame_number);
+	void set_current_block_frame_number(int frame_number);
 	int current_frame_number() const;
 
 	/** add paging to paging queue(s) */
@@ -301,6 +302,7 @@ public:
 
 private:
 	int m_cur_fn;
+	int m_cur_blk_fn;
 	struct gprs_rlcmac_bts m_bts;
 	PollController m_pollController;
 	SBAController m_sba;

tests/tbf/TbfTest.err

@@ -1637,6 +1637,10 @@ TX: Immediate Assignment Uplink (AGCH)
 Sending data request: trx=0 ts=0 sapi=2 arfcn=0 fn=0 block=0 data=2d 06 3f 10 0f 00 00 73 8c f6 07 00 c0 0c 68 ab 2b 2b 2b 2b 2b 2b 2b 
 Searching for first unallocated TFI: TRX=0
  Found TFI=1.
+TBF(TFI=0 TLLI=0xf1223344 DIR=DL STATE=FINISHED) poll timeout for FN=2654292 (curr FN 2654335)
+- Timeout for polling PACKET DOWNLINK ACK.
+- Assignment was on PACCH
+- No downlink ACK received yet
 +++++++++++++++++++++++++ RX : Uplink Control Block +++++++++++++++++++++++++
 ------------------------- RX : Uplink Control Block -------------------------
 MS requests UL TBF in packet resource request of single block, so we provide one: