Add support for Access Control Class ramping.

Access Control Class (ACC) ramping is used to slowly make the cell
available to an increasing number of MS. This avoids overload at
startup time in cases where a lot of MS would discover the new
cell and try to connect to it all at once.

Ramping behaviour can be configured with new VTY commands:

  [no] access-control-class-ramping
  access-control-class-ramping-step-interval (<30-600>|dynamic)
  access-control-class-ramping-step-size (<1-10>)

(The minimum and maximum values for these parameters are hard-coded,
but could be changed if they are found to be inadequate.)

The VTY command 'show bts' has been extended to display the
current ACC ramping configuration.

By default, ACC ramping is disabled.

When enabled, the default behaviour is to enable one ACC per
ramping step with a 'dynamic' step interval. This means the
ramping interval (time between steps) is scaled to the channel
load average of the BTS, i.e. the number of used vs. available
channels measured over a certain amount of time.

Below is an example of debug log output with ACC ramping enabled,
while many 'mobile' programs are concurrently trying to connect
to the network via an osmo-bts-virtual BTS. Initially, all ACCs
are barred, and then only one class is allowed. Then the current
BTS channel load average is consulted for scheduling the next
ramping step. While the channel load average is low, ramping
proceeds faster, and while it is is high, ramping proceeds slower:

(bts=0) ACC RAMP: barring Access Control Class 0
(bts=0) ACC RAMP: barring Access Control Class 1
(bts=0) ACC RAMP: barring Access Control Class 2
(bts=0) ACC RAMP: barring Access Control Class 3
(bts=0) ACC RAMP: barring Access Control Class 4
(bts=0) ACC RAMP: barring Access Control Class 5
(bts=0) ACC RAMP: barring Access Control Class 6
(bts=0) ACC RAMP: barring Access Control Class 7
(bts=0) ACC RAMP: barring Access Control Class 8
(bts=0) ACC RAMP: barring Access Control Class 9
(bts=0) ACC RAMP: allowing Access Control Class 0
(bts=0) ACC RAMP: step interval set to 30 seconds based on 0% channel load average
(bts=0) ACC RAMP: allowing Access Control Class 1
(bts=0) ACC RAMP: step interval set to 354 seconds based on 59% channel load average
(bts=0) ACC RAMP: allowing Access Control Class 2
(bts=0) ACC RAMP: step interval set to 30 seconds based on 0% channel load average
(bts=0) ACC RAMP: allowing Access Control Class 3
(bts=0) ACC RAMP: step interval set to 30 seconds based on 0% channel load average

Port of osmo-bsc commit a5c1e8727c391bc56847a00b2ecc08787573b91f

Change-Id: Idd5c4fd7ea2e10086d9b26deee3a71f9469d1280
Related: OS#2591

3 files changed, 156 insertions, 0 deletions

diff --git a/openbsc/include/openbsc/Makefile.am b/openbsc/include/openbsc/Makefile.am
index 126b115ed..1fa30d56c 100644
--- a/openbsc/include/openbsc/Makefile.am
+++ b/openbsc/include/openbsc/Makefile.am
@@ -2,6 +2,7 @@ noinst_HEADERS = \
 	abis_nm.h \
 	abis_om2000.h \
 	abis_rsl.h \
+	acc_ramp.h \
 	arfcn_range_encode.h \
 	auth.h \
 	bsc_msc.h \
diff --git a/openbsc/include/openbsc/acc_ramp.h b/openbsc/include/openbsc/acc_ramp.h
new file mode 100644
index 000000000..cb063bf53
--- /dev/null
+++ b/openbsc/include/openbsc/acc_ramp.h
@@ -0,0 +1,149 @@
+/* (C) 2018 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * Author: Stefan Sperling <ssperling@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include <osmocom/core/timer.h>
+#include <osmocom/gsm/protocol/gsm_04_08.h>
+
+/*!
+ * Access control class (ACC) ramping is used to slowly make the cell available to
+ * an increasing number of MS. This avoids overload at startup time in cases where
+ * a lot of MS would discover the new cell and try to connect to it all at once.
+ */
+
+#define ACC_RAMP_STEP_SIZE_MIN 1 /* allow at most 1 new ACC per ramp step */
+#define ACC_RAMP_STEP_SIZE_DEFAULT ACC_RAMP_STEP_SIZE_MIN
+#define ACC_RAMP_STEP_SIZE_MAX 10 /* allow all ACC in one step (effectively disables ramping) */
+
+#define ACC_RAMP_STEP_INTERVAL_MIN 30	/* 30 seconds */
+#define ACC_RAMP_STEP_INTERVAL_MAX 600	/* 10 minutes */
+
+/*!
+ * Data structure used to manage ACC ramping. Please avoid setting or reading fields
+ * in this structure directly. Use the accessor functions below instead.
+ */
+struct acc_ramp {
+	struct gsm_bts *bts; /*!< backpointer to BTS using this ACC ramp */
+
+	bool acc_ramping_enabled; /*!< whether ACC ramping is enabled */
+
+	/*!
+	 * Bitmask which keeps track of access control classes that are currently denied
+	 * access. The function acc_ramp_apply() uses this mask to modulate bits from
+	 * octets 2 and 3 in RACH Control Parameters (see 3GPP 44.018 10.5.2.29).
+	 * Ramping is only concerned with ACCs 0-9. While any of the bits 0-9 is set,
+	 * the corresponding ACC is barred.
+	 * ACCs 11-15 should always be allowed, and ACC 10 denies emergency calls for
+	 * all ACCs from 0-9 inclusive; these ACCs are ignored in this implementation.
+	 */
+	uint16_t barred_accs;
+
+	/*!
+	 * This controls the maximum number of ACCs to allow per ramping step (1 - 10).
+	 * The compile-time default value is ACC_RAMP_STEP_SIZE_DEFAULT.
+	 * This value can be changed by VTY configuration.
+	 * A value of ACC_RAMP_STEP_SIZE_MAX effectively disables ramping.
+	 */
+	unsigned int step_size;
+
+	/*!
+	 * Ramping step interval in seconds.
+	 * This value depends on the current BTS channel load average, unless
+	 * it has been overriden by VTY configuration.
+	 */
+	unsigned int step_interval_sec;
+	bool step_interval_is_fixed;
+	struct osmo_timer_list step_timer;
+};
+
+/*!
+ * Return true if ACC ramping is currently enabled, else false.
+ * \param[in] acc_ramp Pointer to acc_ramp structure.
+ */
+static inline bool acc_ramp_is_enabled(struct acc_ramp *acc_ramp)
+{
+	return acc_ramp->acc_ramping_enabled;
+}
+
+/*!
+ * Return the current ACC ramp step size.
+ * \param[in] acc_ramp Pointer to acc_ramp structure.
+ */
+static inline unsigned int acc_ramp_get_step_size(struct acc_ramp *acc_ramp)
+{
+	return acc_ramp->step_size;
+}
+
+/*!
+ * Return the current ACC ramp step interval (in seconds)
+ * \param[in] acc_ramp Pointer to acc_ramp structure.
+ */
+static inline unsigned int acc_ramp_get_step_interval(struct acc_ramp *acc_ramp)
+{
+	return acc_ramp->step_interval_sec;
+}
+
+/*!
+ * If the step interval is dynamic, return true, else return false.
+ * \param[in] acc_ramp Pointer to acc_ramp structure.
+ */
+static inline bool acc_ramp_step_interval_is_dynamic(struct acc_ramp *acc_ramp)
+{
+	return !(acc_ramp->step_interval_is_fixed);
+}
+
+/*!
+ * Return bitmasks which correspond to access control classes that are currently
+ * denied access. Ramping is only concerned with those bits which control access
+ * for ACCs 0-9, and any of the other bits will always be set to zero in these masks, i.e.
+ * it is safe to OR these bitmasks with the corresponding fields in struct gsm48_rach_control.
+ * \param[in] acc_ramp Pointer to acc_ramp structure.
+ */
+static inline uint8_t acc_ramp_get_barred_t2(struct acc_ramp *acc_ramp)
+{
+	return ((acc_ramp->barred_accs >> 8) & 0x03);
+};
+static inline uint8_t acc_ramp_get_barred_t3(struct acc_ramp *acc_ramp)
+{
+	return (acc_ramp->barred_accs & 0xff);
+}
+
+/*!
+ * Potentially mark certain Access Control Classes (ACCs) as barred in accordance to ACC ramping.
+ * \param[in] rach_control RACH control parameters in which barred ACCs will be configured.
+ * \param[in] acc_ramp Pointer to acc_ramp structure.
+ */
+static inline void acc_ramp_apply(struct gsm48_rach_control *rach_control, struct acc_ramp *acc_ramp)
+{
+	rach_control->t2 |= acc_ramp_get_barred_t2(acc_ramp);
+	rach_control->t3 |= acc_ramp_get_barred_t3(acc_ramp);
+}
+
+void acc_ramp_init(struct acc_ramp *acc_ramp, bool enable, struct gsm_bts *bts);
+int acc_ramp_set_step_size(struct acc_ramp *acc_ramp, unsigned int step_size);
+int acc_ramp_set_step_interval(struct acc_ramp *acc_ramp, unsigned int step_interval);
+void acc_ramp_set_step_interval_dynamic(struct acc_ramp *acc_ramp);
+void acc_ramp_start(struct acc_ramp *acc_ramp);
+void acc_ramp_abort(struct acc_ramp *acc_ramp);
diff --git a/openbsc/include/openbsc/gsm_data_shared.h b/openbsc/include/openbsc/gsm_data_shared.h
index 47c5faef4..ddd59918e 100644
--- a/openbsc/include/openbsc/gsm_data_shared.h
+++ b/openbsc/include/openbsc/gsm_data_shared.h
@@ -24,6 +24,8 @@
 #include <osmocom/gsm/lapdm.h>
 #endif
 
+#include <openbsc/acc_ramp.h>
+
 /* 16 is the max. number of SI2quater messages according to 3GPP TS 44.018 Table 10.5.2.33b.1:
    4-bit index is used (2#1111 = 10#15) */
 #define SI2Q_MAX_NUM 16
@@ -845,6 +847,9 @@ struct gsm_bts {
 	int force_combined_si;
 	int bcch_change_mark;
 
+	/* access control class ramping */
+	struct acc_ramp acc_ramp;
+
 #ifdef ROLE_BSC
 	/* Abis NM queue */
 	struct llist_head abis_queue;
@@ -915,6 +920,7 @@ struct gsm_bts {
 	/* Periodic channel load measurements are used to maintain T3122. */
 	struct load_counter chan_load_samples[7];
 	int chan_load_samples_idx;
+	uint8_t chan_load_avg; /* current channel load average in percent (0 - 100). */
 
 #endif /* ROLE_BSC */
 	void *role;