Make "waiting indicator" of IMMEDIATE ASSIGN REJECT dynamic.

The IMMEDIATE ASSIGN REJECT message contains a wait indicator which
tells an MS requesting a channel to wait for a specified amount of
time before trying to request a channel again, i.e. the wait indicator
controls the T3122 timeout value in the MS.

Previously, the wait indicator was fixed to 10 seconds.
This is not sufficient if there are a lot of MS requesting channels
because the MS will retry too soon. Instead of using a fixed value,
maintain a dynamic wait indicator value based on average channel load.

The load (used vs. available channels on a BTS) is sampled once per
second, and once 8 samples have been collected we update a BTS-specific
T3122 wait indicator based on the measured load.

While the wait indicator could go up to 255 seconds, this initial
implementation keeps it in the range from 10 to 128 seconds.

Further experimentation and testing will show whether higher wait
indicator values are desirable, if the sampling rate needs to change,
or if the function mapping the load measurement to a wait indicator
value should change (currently we map the load average linearly into
the range [10, 128] inclusive).

Change-Id: I57e38f6d6ba3b23cc6e1f9520b90261dbb1f1cec
Related: OS#2592

3 files changed, 17 insertions, 5 deletions

diff --git a/include/osmocom/bsc/chan_alloc.h b/include/osmocom/bsc/chan_alloc.h
index 748e9cd9f..98568a50f 100644
--- a/include/osmocom/bsc/chan_alloc.h
+++ b/include/osmocom/bsc/chan_alloc.h
@@ -37,17 +37,13 @@ void lchan_reset(struct gsm_lchan *lchan);
 /* Release the given lchan */
 int lchan_release(struct gsm_lchan *lchan, int sacch_deact, enum rsl_rel_mode release_mode);
 
-struct load_counter {
-	unsigned int total;
-	unsigned int used;
-};
-
 struct pchan_load {
 	struct load_counter pchan[_GSM_PCHAN_MAX];
 };
 
 void bts_chan_load(struct pchan_load *cl, const struct gsm_bts *bts);
 void network_chan_load(struct pchan_load *pl, struct gsm_network *net);
+void bts_update_t3122_chan_load(struct gsm_bts *bts);
 
 bool trx_is_usable(const struct gsm_bts_trx *trx);
 bool ts_is_usable(const struct gsm_bts_trx_ts *ts);
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index bdf7cfba7..25dee7876 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -308,6 +308,9 @@ struct gsm_network {
 	/* Periodic location update default value */
 	uint8_t t3212;
 
+	/* Timer for periodic channel load measurements to maintain each BTS's T3122. */
+	struct osmo_timer_list t3122_chan_load_timer;
+
 	struct {
 		struct mgcp_client_conf *conf;
 		struct mgcp_client *client;
diff --git a/include/osmocom/bsc/gsm_data_shared.h b/include/osmocom/bsc/gsm_data_shared.h
index 86c5ca928..504b42a17 100644
--- a/include/osmocom/bsc/gsm_data_shared.h
+++ b/include/osmocom/bsc/gsm_data_shared.h
@@ -573,6 +573,12 @@ struct bts_location {
 	double height;
 };
 
+/* Channel load counter */
+struct load_counter {
+	unsigned int total;
+	unsigned int used;
+};
+
 /* One BTS */
 struct gsm_bts {
 	/* list header in net->bts_list */
@@ -802,6 +808,13 @@ struct gsm_bts {
 	struct rate_ctr_group *bts_ctrs;
 
 	struct handover_cfg *ho;
+
+	/* BTS-specific overrides for timer values from struct gsm_network. */
+	uint8_t T3122;	/* ASSIGMENT REJECT wait indication */
+
+	/* Periodic channel load measurements are used to maintain T3122. */
+	struct load_counter chan_load_samples[7];
+	int chan_load_samples_idx;
 };