dissolve libbsc: move all to src/osmo-bsc, link .o files

Move all of libbsc/ into osmo-bsc/, and separate/move some implementations to
allow linking from utils/* and ipaccess/* without pulling in unccessary
dependencies.

Some utilities use gsm_network and gsm_bts structs, which already include data
structures for fairly advanced uses. Move initialization that only osmo-bsc
needs into new bsc_network_init() and bsc_bts_alloc_register() functions, so
that the leaner tools can use the old gsm_* versions without the need to link
everything (e.g. handover and lchan alloc code).

In some instances, there need to be stubs if to cut off linking "just before
the RSL level" and prevent dependencies from creeping in.
- abis_rsl_rcvmsg(): the only program currently interpreting RSL messages is
  osmo-bsc, the utils are merely concerned with OML, if at all.
- paging_flush_bts(): ip.access nanobts models call this when the RSL link is
  dropped. Only osmo-bsc actually needs to do anything there.
- on_gsm_ts_init(): the mechanism to trigger timeslot initialization is related
  to OML, while this action to take on init would pull in RSL dependencies.
utils/ and ipaccess/ each have a stubs.c file to implement these stubs. Tests
implement stubs inline where required.

From src/utils/, src/ipaccess/ and tests/*/, link in .o files from osmo-bsc/.
In order for this to work, the osmo-bsc subdir must be built before the other
source trees. (An alternative would be to include the .c files as sources, but
that would re-compile them in every source tree. Not a large burden really, but
unless linking .o files gives problems, let's have the quicker build.)

Minor obvious cleanups creep in with this patch, I will not bother to name them
individually now unless code review asks me to.

Rationale:

1) libbsc has been separate to use it for osmo-nitb and osmo-bsc in the old
openbsc.git. This is no longer required, and spreading over libbsc and osmo-bsc
is distracting.

2) Recently, ridiculous linking requirements have made adding new functions
cumbersome, because libbsc has started depending on osmo-bsc/*.c
implementations: on gscon FSM and bssap functions. For example, neither
bs11_config nor ipaccess-config nor bts_test need handover_cfg or BSSMAP
message composition. It makes no sense to link the entire osmo-bsc to it, nor
do we want to keep adding stubs to each linking realm.

Change-Id: I36a586726f5818121abe54d25654819fc451d3bf

1 files changed, 1830 insertions, 0 deletions

diff --git a/src/osmo-bsc/handover_decision_2.c b/src/osmo-bsc/handover_decision_2.c
new file mode 100644
index 000000000..7ac54df95
--- /dev/null
+++ b/src/osmo-bsc/handover_decision_2.c
@@ -0,0 +1,1830 @@
+/* Handover Decision Algorithm 2 for intra-BSC (inter-BTS) handover, public API for OsmoBSC. */
+
+/* (C) 2009 by Andreas Eversberg <jolly@eversberg.eu>
+ * (C) 2017-2018 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * All Rights Reserved
+ *
+ * Author: Andreas Eversberg <jolly@eversberg.eu>
+ *         Neels Hofmeyr <nhofmeyr@sysmocom.de>
+ *
+ * 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/>.
+ */
+
+#include <stdbool.h>
+#include <errno.h>
+
+#include <osmocom/bsc/debug.h>
+#include <osmocom/bsc/gsm_data.h>
+#include <osmocom/bsc/handover.h>
+#include <osmocom/bsc/handover_decision.h>
+#include <osmocom/bsc/handover_decision_2.h>
+#include <osmocom/bsc/handover_cfg.h>
+#include <osmocom/bsc/bsc_subscriber.h>
+#include <osmocom/bsc/chan_alloc.h>
+#include <osmocom/bsc/signal.h>
+#include <osmocom/bsc/penalty_timers.h>
+
+#define LOGPHOBTS(bts, level, fmt, args...) \
+	LOGP(DHODEC, level, "(BTS %u) " fmt, bts->nr, ## args)
+
+#define LOGPHOLCHAN(lchan, level, fmt, args...) \
+	LOGP(DHODEC, level, "(lchan %u.%u%u%u %s) (subscr %s) " fmt, \
+	     lchan->ts->trx->bts->nr, \
+	     lchan->ts->trx->nr, \
+	     lchan->ts->nr, \
+	     lchan->nr, \
+	     gsm_pchan_name(lchan->ts->pchan), \
+	     bsc_subscr_name(lchan->conn? lchan->conn->bsub : NULL), \
+	     ## args)
+
+#define LOGPHOLCHANTOBTS(lchan, new_bts, level, fmt, args...) \
+	LOGP(DHODEC, level, "(lchan %u.%u%u%u %s)->(BTS %u) (subscr %s) " fmt, \
+	     lchan->ts->trx->bts->nr, \
+	     lchan->ts->trx->nr, \
+	     lchan->ts->nr, \
+	     lchan->nr, \
+	     gsm_pchan_name(lchan->ts->pchan), \
+	     new_bts->nr, \
+	     bsc_subscr_name(lchan->conn? lchan->conn->bsub : NULL), \
+	     ## args)
+
+#define REQUIREMENT_A_TCHF	0x01
+#define REQUIREMENT_B_TCHF	0x02
+#define REQUIREMENT_C_TCHF	0x04
+#define REQUIREMENT_A_TCHH	0x10
+#define REQUIREMENT_B_TCHH	0x20
+#define REQUIREMENT_C_TCHH	0x40
+#define REQUIREMENT_TCHF_MASK	(REQUIREMENT_A_TCHF | REQUIREMENT_B_TCHF | REQUIREMENT_C_TCHF)
+#define REQUIREMENT_TCHH_MASK	(REQUIREMENT_A_TCHH | REQUIREMENT_B_TCHH | REQUIREMENT_C_TCHH)
+#define REQUIREMENT_A_MASK	(REQUIREMENT_A_TCHF | REQUIREMENT_A_TCHH)
+#define REQUIREMENT_B_MASK	(REQUIREMENT_B_TCHF | REQUIREMENT_B_TCHH)
+#define REQUIREMENT_C_MASK	(REQUIREMENT_C_TCHF | REQUIREMENT_C_TCHH)
+
+struct ho_candidate {
+	struct gsm_lchan *lchan;	/* candidate for whom */
+	struct gsm_bts *bts;		/* target BTS */
+	uint8_t requirements;		/* what is fulfilled */
+	int avg;			/* average RX level */
+};
+
+enum ho_reason {
+	HO_REASON_INTERFERENCE,
+	HO_REASON_BAD_QUALITY,
+	HO_REASON_LOW_RXLEVEL,
+	HO_REASON_MAX_DISTANCE,
+	HO_REASON_BETTER_CELL,
+	HO_REASON_CONGESTION,
+};
+
+static const struct value_string ho_reason_names[] = {
+	{ HO_REASON_INTERFERENCE,	"interference (bad quality)" },
+	{ HO_REASON_BAD_QUALITY,	"bad quality" },
+	{ HO_REASON_LOW_RXLEVEL,	"low rxlevel" },
+	{ HO_REASON_MAX_DISTANCE,	"maximum allowed distance" },
+	{ HO_REASON_BETTER_CELL,	"better cell" },
+	{ HO_REASON_CONGESTION,		"congestion" },
+	{0, NULL}
+};
+
+static const char *ho_reason_name(int value)
+{
+        return get_value_string(ho_reason_names, value);
+}
+
+
+static bool hodec2_initialized = false;
+static enum ho_reason global_ho_reason;
+
+static void congestion_check_cb(void *arg);
+
+/* This function gets called on ho2 init, whenever the congestion check interval is changed, and also
+ * when the timer has fired to trigger again after the next congestion check timeout. */
+static void reinit_congestion_timer(struct gsm_network *net)
+{
+	int congestion_check_interval_s;
+	bool was_active;
+
+	/* Don't setup timers from VTY config parsing before the main program has actually initialized
+	 * the data structures. */
+	if (!hodec2_initialized)
+		return;
+
+	was_active = net->hodec2.congestion_check_timer.active;
+	if (was_active)
+		osmo_timer_del(&net->hodec2.congestion_check_timer);
+
+	congestion_check_interval_s = net->hodec2.congestion_check_interval_s;
+	if (congestion_check_interval_s < 1) {
+		if (was_active)
+			LOGP(DHODEC, LOGL_NOTICE, "HO algorithm 2: Disabling congestion check\n");
+		return;
+	}
+
+	LOGP(DHODEC, LOGL_DEBUG, "HO algorithm 2: next periodical congestion check in %u seconds\n",
+	     congestion_check_interval_s);
+
+	osmo_timer_setup(&net->hodec2.congestion_check_timer,
+			 congestion_check_cb, net);
+	osmo_timer_schedule(&net->hodec2.congestion_check_timer,
+			    congestion_check_interval_s, 0);
+}
+
+void hodec2_on_change_congestion_check_interval(struct gsm_network *net, unsigned int new_interval)
+{
+	net->hodec2.congestion_check_interval_s = new_interval;
+	reinit_congestion_timer(net);
+}
+
+static void conn_penalty_time_add(struct gsm_subscriber_connection *conn, struct gsm_bts *bts,
+				   int penalty_time)
+{
+	if (!conn->hodec2.penalty_timers) {
+		conn->hodec2.penalty_timers = penalty_timers_init(conn);
+		OSMO_ASSERT(conn->hodec2.penalty_timers);
+	}
+	penalty_timers_add(conn->hodec2.penalty_timers, bts, penalty_time);
+}
+
+static unsigned int conn_penalty_time_remaining(struct gsm_subscriber_connection *conn,
+						struct gsm_bts *bts)
+{
+	if (!conn->hodec2.penalty_timers)
+		return 0;
+	return penalty_timers_remaining(conn->hodec2.penalty_timers, bts);
+}
+
+/* did we get a RXLEV for a given cell in the given report? Mark matches as MRC_F_PROCESSED. */
+static struct gsm_meas_rep_cell *cell_in_rep(struct gsm_meas_rep *mr, uint16_t arfcn, uint8_t bsic)
+{
+	int i;
+
+	for (i = 0; i < mr->num_cell; i++) {
+		struct gsm_meas_rep_cell *mrc = &mr->cell[i];
+
+		if (mrc->arfcn != arfcn)
+			continue;
+		if (mrc->bsic != bsic)
+			continue;
+
+		return mrc;
+	}
+	return NULL;
+}
+
+/* obtain averaged rxlev for given neighbor */
+static int neigh_meas_avg(struct neigh_meas_proc *nmp, int window)
+{
+	unsigned int i, idx;
+	int avg = 0;
+
+	/* reduce window to the actual number of existing measurements */
+	if (window > nmp->rxlev_cnt)
+		window = nmp->rxlev_cnt;
+	/* this should never happen */
+	if (window <= 0)
+		return 0;
+
+	idx = calc_initial_idx(ARRAY_SIZE(nmp->rxlev),
+			       nmp->rxlev_cnt % ARRAY_SIZE(nmp->rxlev),
+			       window);
+
+	for (i = 0; i < window; i++) {
+		int j = (idx+i) % ARRAY_SIZE(nmp->rxlev);
+
+		avg += nmp->rxlev[j];
+	}
+
+	return avg / window;
+}
+
+/* Find empty slot or the worst neighbor. */
+static struct neigh_meas_proc *find_unused_or_worst_neigh(struct gsm_lchan *lchan)
+{
+	struct neigh_meas_proc *nmp_worst = NULL;
+	int worst;
+	int j;
+
+	/* First try to find an empty/unused slot. */
+	for (j = 0; j < ARRAY_SIZE(lchan->neigh_meas); j++) {
+		struct neigh_meas_proc *nmp = &lchan->neigh_meas[j];
+		if (!nmp->arfcn)
+			return nmp;
+	}
+
+	/* No empty slot found. Return worst neighbor to be evicted. */
+	worst = 0; /* (overwritten on first loop, but avoid compiler warning) */
+	for (j = 0; j < ARRAY_SIZE(lchan->neigh_meas); j++) {
+		struct neigh_meas_proc *nmp = &lchan->neigh_meas[j];
+		int avg = neigh_meas_avg(nmp, MAX_WIN_NEIGH_AVG);
+		if (nmp_worst && avg >= worst)
+			continue;
+		worst = avg;
+		nmp_worst = nmp;
+	}
+
+	return nmp_worst;
+}
+
+/* process neighbor cell measurement reports */
+static void process_meas_neigh(struct gsm_meas_rep *mr)
+{
+	int i, j, idx;
+
+	/* For each reported cell, try to update measurements we already have from previous reports. */
+	for (j = 0; j < ARRAY_SIZE(mr->lchan->neigh_meas); j++) {
+		struct neigh_meas_proc *nmp = &mr->lchan->neigh_meas[j];
+		unsigned int idx;
+		struct gsm_meas_rep_cell *mrc;
+
+		/* skip unused entries */
+		if (!nmp->arfcn)
+			continue;
+
+		mrc = cell_in_rep(mr, nmp->arfcn, nmp->bsic);
+		idx = nmp->rxlev_cnt % ARRAY_SIZE(nmp->rxlev);
+		if (mrc) {
+			nmp->rxlev[idx] = mrc->rxlev;
+			nmp->last_seen_nr = mr->nr;
+			LOGPHOLCHAN(mr->lchan, LOGL_DEBUG, "neigh %u rxlev=%d last_seen_nr=%u\n",
+				    nmp->arfcn, mrc->rxlev, nmp->last_seen_nr);
+			mrc->flags |= MRC_F_PROCESSED;
+		} else {
+			nmp->rxlev[idx] = 0;
+			LOGPHOLCHAN(mr->lchan, LOGL_DEBUG, "neigh %u not in report (last_seen_nr=%u)\n",
+				    nmp->arfcn, nmp->last_seen_nr);
+		}
+		nmp->rxlev_cnt++;
+	}
+
+	/* Add cells that we don't know about yet, if necessary overwriting previous records that reflect
+	 * cells with worse receive levels */
+	for (i = 0; i < mr->num_cell; i++) {
+		struct gsm_meas_rep_cell *mrc = &mr->cell[i];
+		struct neigh_meas_proc *nmp;
+
+		if (mrc->flags & MRC_F_PROCESSED)
+			continue;
+
+		nmp = find_unused_or_worst_neigh(mr->lchan);
+
+		nmp->arfcn = mrc->arfcn;
+		nmp->bsic = mrc->bsic;
+
+		nmp->rxlev_cnt = 0;
+		idx = nmp->rxlev_cnt % ARRAY_SIZE(nmp->rxlev);
+		nmp->rxlev[idx] = mrc->rxlev;
+		nmp->rxlev_cnt++;
+		nmp->last_seen_nr = mr->nr;
+		LOGPHOLCHAN(mr->lchan, LOGL_DEBUG, "neigh %u new in report rxlev=%d last_seen_nr=%u\n",
+			    nmp->arfcn, mrc->rxlev, nmp->last_seen_nr);
+
+		mrc->flags |= MRC_F_PROCESSED;
+	}
+}
+
+static bool codec_type_is_supported(struct gsm_subscriber_connection *conn,
+				    enum gsm0808_speech_codec_type type)
+{
+	int i;
+	struct gsm0808_speech_codec_list *clist = &conn->codec_list;
+
+	if (!conn->codec_list_present) {
+		/* We don't have a list of supported codecs. This should never happen. */
+		LOGPHOLCHAN(conn->lchan, LOGL_ERROR,
+			    "No Speech Codec List present, accepting all codecs\n");
+		return true;
+	}
+
+	for (i = 0; i < clist->len; i++) {
+		if (clist->codec[i].type == type) {
+			LOGPHOLCHAN(conn->lchan, LOGL_DEBUG, "%s supported\n",
+				    gsm0808_speech_codec_type_name(type));
+			return true;
+		}
+	}
+	LOGPHOLCHAN(conn->lchan, LOGL_DEBUG, "Codec not supported by MS or not allowed by MSC: %s\n",
+		    gsm0808_speech_codec_type_name(type));
+	return false;
+}
+
+/*
+ * Check what requirements the given cell fulfills.
+ * A bit mask of fulfilled requirements is returned.
+ *
+ * Target cell requirement A -- ability to service the call
+ *
+ * In order to successfully handover/assign to a better cell, the target cell
+ * must be able to continue the current call. Therefore the cell must fulfill
+ * the following criteria:
+ *
+ *  * The handover must be enabled for the target cell, if it differs from the
+ *    originating cell.
+ *  * The assignment must be enabled for the cell, if it equals the current
+ *    cell.
+ *  * The handover penalty timer must not run for the cell.
+ *  * If FR, EFR or HR codec is used, the cell must support this codec.
+ *  * If FR or EFR codec is used, the cell must have a TCH/F slot type
+ *    available.
+ *  * If HR codec is used, the cell must have a TCH/H slot type available.
+ *  * If AMR codec is used, the cell must have a TCH/F slot available, if AFS
+ *    is supported by mobile and BTS.
+ *  * If AMR codec is used, the cell must have a TCH/H slot available, if AHS
+ *    is supported by mobile and BTS.
+ *  * osmo-nitb with built-in MNCC application:
+ *     o If AMR codec is used, the cell must support AMR codec with equal codec
+ *       rate or rates. (not meaning TCH types)
+ *  * If defined, the number of maximum unsynchronized handovers to this cell
+ *    may not be exceeded. (This limits processing load for random access
+ *    bursts.)
+ *
+ *
+ * Target cell requirement B -- avoid congestion
+ *
+ * In order to prevent congestion of a target cell, the cell must fulfill the
+ * requirement A, but also:
+ *
+ *  * The minimum free channels, that are defined for that cell must be
+ *    maintained after handover/assignment.
+ *  * The minimum free channels are defined for TCH/F and TCH/H slot types
+ *    individually.
+ *
+ *
+ * Target cell requirement C -- balance congestion
+ *
+ * In order to balance congested cells, the target cell must fulfill the
+ * requirement A, but also:
+ *
+ *  * The target cell (which is congested also) must have more or equal free
+ *    slots after handover/assignment.
+ *  * The number of free slots are checked for TCH/F and TCH/H slot types
+ *    individually.
+ */
+static uint8_t check_requirements(struct gsm_lchan *lchan, struct gsm_bts *bts, int tchf_count, int tchh_count)
+{
+	int count;
+	uint8_t requirement = 0;
+	unsigned int penalty_time;
+	struct gsm_bts *current_bts = lchan->ts->trx->bts;
+
+	/* Requirement A */
+
+	/* the handover/assignment must not be disabled */
+	if (current_bts == bts) {
+		if (!ho_get_hodec2_as_active(bts->ho)) {
+			LOGPHOLCHAN(lchan, LOGL_DEBUG, "Assignment disabled\n");
+			return 0;
+		}
+	} else {
+		if (!ho_get_ho_active(bts->ho)) {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "not a candidate, handover is disabled in target BTS\n");
+			return 0;
+		}
+	}
+
+	/* the handover penalty timer must not run for this bts */
+	penalty_time = conn_penalty_time_remaining(lchan->conn, bts);
+	if (penalty_time) {
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "not a candidate, target BTS still in penalty time"
+				 " (%u seconds left)\n", penalty_time);
+		return 0;
+	}
+
+	LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "tch_mode='%s' type='%s'\n",
+			 get_value_string(gsm48_chan_mode_names, lchan->tch_mode),
+			 gsm_lchant_name(lchan->type));
+
+	/* compatibility check for codecs.
+	 * if so, the candidates for full rate and half rate are selected */
+	switch (lchan->tch_mode) {
+	case GSM48_CMODE_SPEECH_V1:
+		switch (lchan->type) {
+		case GSM_LCHAN_TCH_F: /* mandatory */
+			requirement |= REQUIREMENT_A_TCHF;
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "tch_mode='%s' type='%s' supported\n",
+					 get_value_string(gsm48_chan_mode_names, lchan->tch_mode),
+					 gsm_lchant_name(lchan->type));
+			break;
+		case GSM_LCHAN_TCH_H:
+			if (!bts->codec.hr) {
+				LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+						 "tch_mode='%s' type='%s' not supported\n",
+						 get_value_string(gsm48_chan_mode_names,
+								  lchan->tch_mode),
+						 gsm_lchant_name(lchan->type));
+				break;
+			}
+			if (codec_type_is_supported(lchan->conn, GSM0808_SCT_HR1))
+				requirement |= REQUIREMENT_A_TCHH;
+			break;
+		default:
+			LOGPHOLCHAN(lchan, LOGL_ERROR, "Unexpected channel type: neither TCH/F nor TCH/H for %s\n",
+				    get_value_string(gsm48_chan_mode_names, lchan->tch_mode));
+			return 0;
+		}
+		break;
+	case GSM48_CMODE_SPEECH_EFR:
+		if (!bts->codec.efr) {
+			LOGPHOBTS(bts, LOGL_DEBUG, "EFR not supported\n");
+			break;
+		}
+		if (codec_type_is_supported(lchan->conn, GSM0808_SCT_FR2))
+			requirement |= REQUIREMENT_A_TCHF;
+		break;
+	case GSM48_CMODE_SPEECH_AMR:
+		if (!bts->codec.amr) {
+			LOGPHOBTS(bts, LOGL_DEBUG, "AMR not supported\n");
+			break;
+		}
+		if (codec_type_is_supported(lchan->conn, GSM0808_SCT_FR3))
+			requirement |= REQUIREMENT_A_TCHF;
+		if (codec_type_is_supported(lchan->conn, GSM0808_SCT_HR3))
+			requirement |= REQUIREMENT_A_TCHH;
+		break;
+	default:
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "Not even considering: src is not a SPEECH mode lchan\n");
+		return 0;
+	}
+
+	/* no candidate, because new cell is incompatible */
+	if (!requirement) {
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "not a candidate, because codec of MS and BTS are incompatible\n");
+		return 0;
+	}
+
+	/* remove slot types that are not available */
+	if (!tchf_count && requirement & REQUIREMENT_A_TCHF) {
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+				 "removing TCH/F, since all TCH/F lchans are in use\n");
+		requirement &= ~(REQUIREMENT_A_TCHF);
+	}
+	if (!tchh_count && requirement & REQUIREMENT_A_TCHH) {
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+				 "removing TCH/H, since all TCH/H lchans are in use\n");
+		requirement &= ~(REQUIREMENT_A_TCHH);
+	}
+
+	if (!requirement) {
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "not a candidate, because no suitable slots available\n");
+		return 0;
+	}
+
+	/* omit same channel type on same BTS (will not change anything) */
+	if (bts == current_bts) {
+		switch (lchan->type) {
+		case GSM_LCHAN_TCH_F:
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "removing TCH/F, already on TCH/F in this cell\n");
+			requirement &= ~(REQUIREMENT_A_TCHF);
+			break;
+		case GSM_LCHAN_TCH_H:
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "removing TCH/H, already on TCH/H in this cell\n");
+			requirement &= ~(REQUIREMENT_A_TCHH);
+			break;
+		default:
+			break;
+		}
+
+		if (!requirement) {
+			LOGPHOLCHAN(lchan, LOGL_DEBUG,
+				    "Reassignment within cell not an option, no differing channel types available\n");
+			return 0;
+		}
+	}
+
+#ifdef LEGACY
+	// This was useful in osmo-nitb. We're in osmo-bsc now and have no idea whether the osmo-msc does
+	// internal or external call control. Maybe a future config switch wants to add this behavior?
+	/* Built-in call control requires equal codec rates. Remove rates that are not equal. */
+	if (lchan->tch_mode == GSM48_CMODE_SPEECH_AMR
+	    && current_bts->network->mncc_recv != mncc_sock_from_cc) {
+		switch (lchan->type) {
+		case GSM_LCHAN_TCH_F:
+			if ((requirement & REQUIREMENT_A_TCHF)
+			    && !!memcmp(&current_bts->mr_full, &bts->mr_full,
+					sizeof(struct amr_multirate_conf)))
+				requirement &= ~(REQUIREMENT_A_TCHF);
+			if ((requirement & REQUIREMENT_A_TCHH)
+			    && !!memcmp(&current_bts->mr_full, &bts->mr_half,
+					sizeof(struct amr_multirate_conf)))
+				requirement &= ~(REQUIREMENT_A_TCHH);
+			break;
+		case GSM_LCHAN_TCH_H:
+			if ((requirement & REQUIREMENT_A_TCHF)
+			    && !!memcmp(&current_bts->mr_half, &bts->mr_full,
+					sizeof(struct amr_multirate_conf)))
+				requirement &= ~(REQUIREMENT_A_TCHF);
+			if ((requirement & REQUIREMENT_A_TCHH)
+			    && !!memcmp(&current_bts->mr_half, &bts->mr_half,
+					sizeof(struct amr_multirate_conf)))
+				requirement &= ~(REQUIREMENT_A_TCHH);
+			break;
+		default:
+			break;
+		}
+
+		if (!requirement) {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "not a candidate, cannot provide identical codec rate\n");
+			return 0;
+		}
+	}
+#endif
+
+	/* the maximum number of unsynchonized handovers must no be exceeded */
+	if (current_bts != bts
+	    && bsc_ho_count(bts, true) >= ho_get_hodec2_ho_max(bts->ho)) {
+		LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+				 "not a candidate, number of allowed handovers (%d) would be exceeded\n",
+				 ho_get_hodec2_ho_max(bts->ho));
+		return 0;
+	}
+
+	/* Requirement B */
+
+	/* the minimum free timeslots that are defined for this cell must
+	 * be maintained _after_ handover/assignment */
+	if (requirement & REQUIREMENT_A_TCHF) {
+		if (tchf_count - 1 >= ho_get_hodec2_tchf_min_slots(bts->ho)) {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/F would not be congested after HO\n");
+			requirement |= REQUIREMENT_B_TCHF;
+		} else {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/F would be congested after HO\n");
+		}
+	}
+	if (requirement & REQUIREMENT_A_TCHH) {
+		if (tchh_count - 1 >= ho_get_hodec2_tchh_min_slots(bts->ho)) {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/H would not be congested after HO\n");
+			requirement |= REQUIREMENT_B_TCHH;
+		} else {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/H would be congested after HO\n");
+		}
+	}
+
+	/* Requirement C */
+
+	/* the nr of free timeslots of the target cell must be >= the
+	 * free slots of the current cell _after_ handover/assignment */
+	count = bts_count_free_ts(current_bts,
+				  (lchan->type == GSM_LCHAN_TCH_H) ?
+				  	GSM_PCHAN_TCH_H : GSM_PCHAN_TCH_F);
+	if (requirement & REQUIREMENT_A_TCHF) {
+		if (tchf_count - 1 >= count + 1) {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/F would be less congested in target than source cell after HO\n");
+			requirement |= REQUIREMENT_C_TCHF;
+		} else {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/F would not be less congested in target than source cell after HO\n");
+		}
+	}
+	if (requirement & REQUIREMENT_A_TCHH) {
+		if (tchh_count - 1 >= count + 1) {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/H would be less congested in target than source cell after HO\n");
+			requirement |= REQUIREMENT_C_TCHH;
+		} else {
+			LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG,
+					 "TCH/H would not be less congested in target than source cell after HO\n");
+		}
+	}
+
+	LOGPHOLCHANTOBTS(lchan, bts, LOGL_DEBUG, "requirements=0x%x\n", requirement);
+
+	/* return mask of fulfilled requirements */
+	return requirement;
+}
+
+/* Trigger handover or assignment depending on the target BTS */
+static int trigger_handover_or_assignment(struct gsm_lchan *lchan, struct gsm_bts *new_bts, uint8_t requirements)
+{
+	struct gsm_bts *current_bts = lchan->ts->trx->bts;
+	int afs_bias = 0;
+	bool full_rate = false;
+
+	if (current_bts == new_bts)
+		LOGPHOLCHAN(lchan, LOGL_NOTICE, "Triggering Assignment\n");
+	else
+		LOGPHOLCHANTOBTS(lchan, new_bts, LOGL_NOTICE, "Triggering Handover\n");
+
+	/* afs_bias becomes > 0, if AFS is used and is improved */
+	if (lchan->tch_mode == GSM48_CMODE_SPEECH_AMR)
+		afs_bias = ho_get_hodec2_afs_bias_rxlev(new_bts->ho);
+
+	/* select TCH rate, prefer TCH/F if AFS is improved */
+	switch (lchan->type) {
+	case GSM_LCHAN_TCH_F:
+		/* keep on full rate, if TCH/F is a candidate */
+		if ((requirements & REQUIREMENT_TCHF_MASK)) {
+			if (current_bts == new_bts) {
+				LOGPHOLCHAN(lchan, LOGL_INFO, "Not performing assignment: Already on target type\n");
+				return 0;
+			}
+			full_rate = true;
+			break;
+		}
+		/* change to half rate */
+		if (!(requirements & REQUIREMENT_TCHH_MASK)) {
+			LOGPHOLCHANTOBTS(lchan, new_bts, LOGL_ERROR,
+					 "neither TCH/F nor TCH/H requested, aborting ho/as\n");
+			return -EINVAL;
+		}
+		break;
+	case GSM_LCHAN_TCH_H:
+		/* change to full rate if AFS is improved and a candidate */
+		if (afs_bias > 0 && (requirements & REQUIREMENT_TCHF_MASK)) {
+			full_rate = true;
+			LOGPHOLCHAN(lchan, LOGL_DEBUG, "[Improve AHS->AFS]\n");
+			break;
+		}
+		/* change to full rate if the only candidate */
+		if ((requirements & REQUIREMENT_TCHF_MASK)
+		    && !(requirements & REQUIREMENT_TCHH_MASK)) {
+			full_rate = true;
+			break;
+		}
+		/* keep on half rate */
+		if (!(requirements & REQUIREMENT_TCHH_MASK)) {
+			LOGPHOLCHANTOBTS(lchan, new_bts, LOGL_ERROR,
+					 "neither TCH/F nor TCH/H requested, aborting ho/as\n");
+			return -EINVAL;
+		}
+		if (current_bts == new_bts) {
+			LOGPHOLCHAN(lchan, LOGL_INFO, "Not performing assignment: Already on target type\n");
+			return 0;
+		}
+		break;
+	default:
+		LOGPHOLCHANTOBTS(lchan, new_bts, LOGL_ERROR, "lchan is neither TCH/F nor TCH/H, aborting ho/as\n");
+		return -EINVAL;
+	}
+
+	/* trigger handover or assignment */
+	if  (current_bts == new_bts)
+		LOGPHOLCHAN(lchan, LOGL_NOTICE, "Triggering assignment to %s, due to %s\n",
+			    full_rate ? "TCH/F" : "TCH/H",
+			    ho_reason_name(global_ho_reason));
+	else
+		LOGPHOLCHANTOBTS(lchan, new_bts, LOGL_NOTICE,
+				 "Triggering handover to %s, due to %s\n",
+				 full_rate ? "TCH/F" : "TCH/H",
+				 ho_reason_name(global_ho_reason));
+
+	return bsc_handover_start(HODEC2, lchan, current_bts == new_bts? NULL : new_bts,
+				  full_rate? GSM_LCHAN_TCH_F : GSM_LCHAN_TCH_H);
+}
+
+/* debug collected candidates */
+static inline void debug_candidate(struct ho_candidate *candidate,
+	int neighbor, int8_t rxlev, int tchf_count, int tchh_count)
+{
+	if (neighbor)
+		LOGP(DHODEC, LOGL_DEBUG, " - neighbor BTS %d, RX level "
+			"%d -> %d\n", candidate->bts->nr, rxlev2dbm(rxlev),
+			rxlev2dbm(candidate->avg));
+	else
+		LOGP(DHODEC, LOGL_DEBUG, " - current BTS %d, RX level %d\n",
+			candidate->bts->nr, rxlev2dbm(candidate->avg));
+
+	LOGP(DHODEC, LOGL_DEBUG, "   o free TCH/F slots %d, minimum required "
+		"%d\n", tchf_count, ho_get_hodec2_tchf_min_slots(candidate->bts->ho));
+	LOGP(DHODEC, LOGL_DEBUG, "   o free TCH/H slots %d, minimum required "
+		"%d\n", tchh_count, ho_get_hodec2_tchh_min_slots(candidate->bts->ho));
+
+	if ((candidate->requirements & REQUIREMENT_TCHF_MASK))
+		LOGP(DHODEC, LOGL_DEBUG, "   o requirement ");
+	else
+		LOGP(DHODEC, LOGL_DEBUG, "   o no requirement ");
+	if ((candidate->requirements & REQUIREMENT_A_TCHF))
+		LOGPC(DHODEC, LOGL_DEBUG, "A ");
+	if ((candidate->requirements & REQUIREMENT_B_TCHF))
+		LOGPC(DHODEC, LOGL_DEBUG, "B ");
+	if ((candidate->requirements & REQUIREMENT_C_TCHF))
+		LOGPC(DHODEC, LOGL_DEBUG, "C ");
+	LOGPC(DHODEC, LOGL_DEBUG, "fulfilled for TCHF");
+	if (!(candidate->requirements & REQUIREMENT_TCHF_MASK)) /* nothing */
+		LOGPC(DHODEC, LOGL_DEBUG, " (no %s possible)\n",
+			(neighbor) ? "handover" : "assignment");
+	else if ((candidate->requirements & REQUIREMENT_TCHF_MASK)
+					== REQUIREMENT_A_TCHF) /* only A */
+		LOGPC(DHODEC, LOGL_DEBUG, " (more congestion after %s)\n",
+			(neighbor) ? "handover" : "assignment");
+	else if ((candidate->requirements & REQUIREMENT_B_TCHF)) /* B incl. */
+		LOGPC(DHODEC, LOGL_DEBUG, " (not congested after %s)\n",
+			(neighbor) ? "handover" : "assignment");
+	else /* so it must include C */
+		LOGPC(DHODEC, LOGL_DEBUG, " (less or equally congested after "
+			"%s)\n", (neighbor) ? "handover" : "assignment");
+
+	if ((candidate->requirements & REQUIREMENT_TCHH_MASK))
+		LOGP(DHODEC, LOGL_DEBUG, "   o requirement ");
+	else
+		LOGP(DHODEC, LOGL_DEBUG, "   o no requirement ");
+	if ((candidate->requirements & REQUIREMENT_A_TCHH))
+		LOGPC(DHODEC, LOGL_DEBUG, "A ");
+	if ((candidate->requirements & REQUIREMENT_B_TCHH))
+		LOGPC(DHODEC, LOGL_DEBUG, "B ");
+	if ((candidate->requirements & REQUIREMENT_C_TCHH))
+		LOGPC(DHODEC, LOGL_DEBUG, "C ");
+	LOGPC(DHODEC, LOGL_DEBUG, "fulfilled for TCHH");
+	if (!(candidate->requirements & REQUIREMENT_TCHH_MASK)) /* nothing */
+		LOGPC(DHODEC, LOGL_DEBUG, " (no %s possible)\n",
+			(neighbor) ? "handover" : "assignment");
+	else if ((candidate->requirements & REQUIREMENT_TCHH_MASK)
+					== REQUIREMENT_A_TCHH) /* only A */
+		LOGPC(DHODEC, LOGL_DEBUG, " (more congestion after %s)\n",
+			(neighbor) ? "handover" : "assignment");
+	else if ((candidate->requirements & REQUIREMENT_B_TCHH)) /* B incl. */
+		LOGPC(DHODEC, LOGL_DEBUG, " (not congested after %s)\n",
+			(neighbor) ? "handover" : "assignment");
+	else /* so it must include C */
+		LOGPC(DHODEC, LOGL_DEBUG, " (less or equally congested after "
+			"%s)\n", (neighbor) ? "handover" : "assignment");
+}
+
+/* add candidate for re-assignment within the current cell */
+static void collect_assignment_candidate(struct gsm_lchan *lchan, struct ho_candidate *clist,
+					 unsigned int *candidates, int av_rxlev)
+{
+	struct gsm_bts *bts = lchan->ts->trx->bts;
+	int tchf_count, tchh_count;
+	struct ho_candidate *c;
+
+	tchf_count = bts_count_free_ts(bts, GSM_PCHAN_TCH_F);
+	tchh_count = bts_count_free_ts(bts, GSM_PCHAN_TCH_H);
+
+	c = &clist[*candidates];
+	c->lchan = lchan;
+	c->bts = bts;
+	c->requirements = check_requirements(lchan, bts, tchf_count, tchh_count);
+	c->avg = av_rxlev;
+	debug_candidate(c, 0, 0, tchf_count, tchh_count);
+	(*candidates)++;
+}
+
+/* add candidates for handover to all neighbor cells */
+static void collect_handover_candidate(struct gsm_lchan *lchan, struct neigh_meas_proc *nmp,
+				       struct ho_candidate *clist, unsigned int *candidates,
+				       bool include_weaker_rxlev, int av_rxlev,
+				       int *neighbors_count)
+{
+	struct gsm_bts *bts = lchan->ts->trx->bts;
+	int tchf_count, tchh_count;
+	struct gsm_bts *neighbor_bts;
+	int avg;
+	struct ho_candidate *c;
+	int min_rxlev;
+
+	/* skip empty slots */
+	if (nmp->arfcn == 0)
+		return;
+
+	if (neighbors_count)
+		(*neighbors_count)++;
+
+	/* skip if measurement report is old */
+	if (nmp->last_seen_nr != lchan->meas_rep_last_seen_nr) {
+		LOGPHOLCHAN(lchan, LOGL_DEBUG, "neighbor ARFCN %u measurement report is old"
+			    " (nmp->last_seen_nr=%u lchan->meas_rep_last_seen_nr=%u)\n",
+			    nmp->arfcn, nmp->last_seen_nr, lchan->meas_rep_last_seen_nr);
+		return;
+	}
+
+	neighbor_bts = bts_by_arfcn_bsic(bts->network, nmp->arfcn, nmp->bsic);
+	if (!neighbor_bts) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "neighbor ARFCN %u does not belong to this network\n",
+			  nmp->arfcn);
+		return;
+	}
+
+	/* in case we have measurements of our bts, due to misconfiguration */
+	if (neighbor_bts == bts) {
+		LOGPHOBTS(bts, LOGL_ERROR, "Configuration error: this BTS appears as its own neighbor\n");
+		return;
+	}
+
+	/* caculate average rxlev for this cell over the window */
+	avg = neigh_meas_avg(nmp, ho_get_hodec2_rxlev_neigh_avg_win(bts->ho));
+
+	/* Heed rxlev hysteresis only if the RXLEV/RXQUAL/TA levels of the MS aren't critically bad and
+	 * we're just looking for an improvement. If levels are critical, we desperately need a handover
+	 * and thus skip the hysteresis check. */
+	if (!include_weaker_rxlev) {
+		unsigned int pwr_hyst = ho_get_hodec2_pwr_hysteresis(bts->ho);
+		if (avg <= (av_rxlev + pwr_hyst)) {
+			LOGPHOLCHAN(lchan, LOGL_DEBUG,
+				    "BTS %d is not a candidate, because RX level (%d) is lower"
+				    " or equal than current RX level (%d) + hysteresis (%d)\n",
+				    neighbor_bts->nr, rxlev2dbm(avg), rxlev2dbm(av_rxlev), pwr_hyst);
+			return;
+		}
+	}
+
+	/* if the minimum level is not reached */
+	min_rxlev = ho_get_hodec2_min_rxlev(neighbor_bts->ho);
+	if (rxlev2dbm(avg) < min_rxlev) {
+		LOGPHOLCHAN(lchan, LOGL_DEBUG,
+			    "BTS %d is not a candidate, because RX level (%d) is lower"
+			    " than its minimum required RX level (%d)\n",
+			    neighbor_bts->nr, rxlev2dbm(avg), min_rxlev);
+		return;
+	}
+
+	tchf_count = bts_count_free_ts(neighbor_bts, GSM_PCHAN_TCH_F);
+	tchh_count = bts_count_free_ts(neighbor_bts, GSM_PCHAN_TCH_H);
+	c = &clist[*candidates];
+	c->lchan = lchan;
+	c->bts = neighbor_bts;
+	c->requirements = check_requirements(lchan, neighbor_bts, tchf_count,
+					     tchh_count);
+	c->avg = avg;
+	debug_candidate(c, 1, av_rxlev, tchf_count, tchh_count);
+	(*candidates)++;
+}
+
+static void collect_candidates_for_lchan(struct gsm_lchan *lchan,
+					 struct ho_candidate *clist, unsigned int *candidates,
+					 int *_av_rxlev, bool include_weaker_rxlev)
+{
+	struct gsm_bts *bts = lchan->ts->trx->bts;
+	int av_rxlev;
+	unsigned int candidates_was;
+	bool assignment;
+	bool handover;
+	int neighbors_count = 0;
+	unsigned int rxlev_avg_win = ho_get_hodec2_rxlev_avg_win(bts->ho);
+
+	OSMO_ASSERT(candidates);
+	candidates_was = *candidates;
+
+	/* caculate average rxlev for this cell over the window */
+	av_rxlev = get_meas_rep_avg(lchan,
+				    ho_get_hodec2_full_tdma(bts->ho) ?
+				    MEAS_REP_DL_RXLEV_FULL : MEAS_REP_DL_RXLEV_SUB,
+				    rxlev_avg_win);
+	if (_av_rxlev)
+		*_av_rxlev = av_rxlev;
+
+	/* in case there is no measurment report (yet) */
+	if (av_rxlev < 0) {
+		LOGPHOLCHAN(lchan, LOGL_DEBUG, "Not collecting candidates, not enough measurements"
+			    " (got %d, want %u)\n",
+			    lchan->meas_rep_count, rxlev_avg_win);
+		return;
+	}
+
+	assignment = ho_get_hodec2_as_active(bts->ho);
+	handover = ho_get_ho_active(bts->ho);
+
+	LOGPHOLCHAN(lchan, LOGL_DEBUG, "Collecting candidates for%s%s%s\n",
+		    assignment ? " Assignment" : "",
+		    assignment && handover ? " and" : "",
+		    handover ? " Handover" : "");
+
+	if (assignment)
+		collect_assignment_candidate(lchan, clist, candidates, av_rxlev);
+
+	if (handover) {
+		int i;
+		for (i = 0; i < ARRAY_SIZE(lchan->neigh_meas); i++) {
+			collect_handover_candidate(lchan, &lchan->neigh_meas[i],
+						   clist, candidates,
+						   include_weaker_rxlev, av_rxlev, &neighbors_count);
+		}
+	}
+
+	LOGPHOLCHAN(lchan, LOGL_DEBUG, "adding %u candidates from %u neighbors, total %u\n",
+		    *candidates - candidates_was, neighbors_count, *candidates);
+}
+
+/*
+ * Search for a alternative / better cell.
+ *
+ * Do not trigger handover/assignment on slots which have already ongoing
+ * handover/assignment processes. If no AFS improvement offset is given, try to
+ * maintain the same TCH rate, if available.
+ * Do not perform this process, if handover and assignment are disabled for
+ * the current cell.
+ * Do not perform handover, if the minimum acceptable RX level
+ * is not reched for this cell.
+ *
+ * If one or more 'better cells' are available, check the current and neighbor
+ * cell measurements in descending order of their RX levels (down-link):
+ *
+ *  * Select the best candidate that fulfills requirement B (no congestion
+ *    after handover/assignment) and trigger handover or assignment.
+ *  * If no candidate fulfills requirement B, select the best candidate that
+ *    fulfills requirement C (less or equally congested cells after handover)
+ *    and trigger handover or assignment.
+ *  * If no candidate fulfills requirement C, do not perform handover nor
+ *    assignment.
+ *
+ * If the RX level (down-link) or RX quality (down-link) of the current cell is
+ * below minimum acceptable level, or if the maximum allowed timing advance is
+ * reached or exceeded, check the RX levels (down-link) of the current and
+ * neighbor cells in descending order of their levels: (bad BTS case)
+ *
+ *  * Select the best candidate that fulfills requirement B (no congestion after
+ *    handover/assignment) and trigger handover or assignment.
+ *  * If no candidate fulfills requirement B, select the best candidate that
+ *    fulfills requirement C (less or equally congested cells after handover)
+ *    and trigger handover or assignment.
+ *  * If no candidate fulfills requirement C, select the best candidate that
+ *    fulfills requirement A (ignore congestion after handover or assignment)
+ *    and trigger handover or assignment.
+ *  * If no candidate fulfills requirement A, do not perform handover nor
+ *    assignment.
+ *
+ * RX levels (down-link) of current and neighbor cells:
+ *
+ *  * The RX levels of the current cell and neighbor cells are improved by a
+ *    given offset, if AFS (AMR on TCH/F) is used or is a candidate for
+ *    handover/assignment.
+ *  * If AMR is used, the requirement for handover is checked for TCH/F and
+ *    TCH/H. Both results (if any) are used as a candidate.
+ *  * If AMR is used, the requirement for assignment to a different TCH slot
+ *    rate is checked. The result (if available) is used as a candidate.
+ *
+ * If minimum RXLEV, minimum RXQUAL or maximum TA are exceeded, the caller should pass
+ * include_weaker_rxlev=true so that handover is performed despite congestion.
+ */
+static int find_alternative_lchan(struct gsm_lchan *lchan, bool include_weaker_rxlev)
+{
+	struct gsm_bts *bts = lchan->ts->trx->bts;
+	int ahs = (lchan->tch_mode == GSM48_CMODE_SPEECH_AMR
+		   && lchan->type == GSM_LCHAN_TCH_H);
+	int av_rxlev;
+	struct ho_candidate clist[1 + ARRAY_SIZE(lchan->neigh_meas)];
+	unsigned int candidates = 0;
+	int i;
+	struct ho_candidate *best_cand = NULL;
+	unsigned int best_better_db;
+	bool best_applied_afs_bias = false;
+	int better;
+
+	/* check for disabled handover/assignment at the current cell */
+	if (!ho_get_hodec2_as_active(bts->ho)
+	    && !ho_get_ho_active(bts->ho)) {
+		LOGP(DHODEC, LOGL_INFO, "Skipping, Handover and Assignment both disabled in this cell\n");
+		return 0;
+	}
+
+	collect_candidates_for_lchan(lchan, clist, &candidates, &av_rxlev, include_weaker_rxlev);
+
+	/* If assignment is disabled and no neighbor cell report exists, or no neighbor cell qualifies,
+	 * we may not even have any candidates. */
+	if (!candidates)
+		goto no_candidates;
+
+	/* select best candidate that fulfills requirement B: no congestion after HO */
+	best_better_db = 0;
+	for (i = 0; i < candidates; i++) {
+		int afs_bias;
+		if (!(clist[i].requirements & REQUIREMENT_B_MASK))
+			continue;
+
+		better = clist[i].avg - av_rxlev;
+		/* Apply AFS bias? */
+		afs_bias = 0;
+		if (ahs && (clist[i].requirements & REQUIREMENT_B_TCHF))
+			afs_bias = ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+		better += afs_bias;
+		if (better > best_better_db) {
+			best_cand = &clist[i];
+			best_better_db = better;
+			best_applied_afs_bias = afs_bias? true : false;
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (best_cand) {
+		LOGPHOLCHANTOBTS(lchan, best_cand->bts, LOGL_INFO, "Best candidate, RX level %d%s\n",
+				 rxlev2dbm(best_cand->avg),
+				 best_applied_afs_bias ? " (applied AHS -> AFS rxlev bias)" : "");
+		return trigger_handover_or_assignment(lchan, best_cand->bts,
+						      best_cand->requirements & REQUIREMENT_B_MASK);
+	}
+
+	/* select best candidate that fulfills requirement C: less or equal congestion after HO */
+	best_better_db = 0;
+	for (i = 0; i < candidates; i++) {
+		int afs_bias;
+		if (!(clist[i].requirements & REQUIREMENT_C_MASK))
+			continue;
+
+		better = clist[i].avg - av_rxlev;
+		/* Apply AFS bias? */
+		afs_bias = 0;
+		if (ahs && (clist[i].requirements & REQUIREMENT_C_TCHF))
+			afs_bias = ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+		better += afs_bias;
+		if (better > best_better_db) {
+			best_cand = &clist[i];
+			best_better_db = better;
+			best_applied_afs_bias = afs_bias? true : false;
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (best_cand) {
+		LOGPHOLCHANTOBTS(lchan, best_cand->bts, LOGL_INFO, "Best candidate, RX level %d%s\n",
+				 rxlev2dbm(best_cand->avg),
+				 best_applied_afs_bias? " (applied AHS -> AFS rxlev bias)" : "");
+		return trigger_handover_or_assignment(lchan, best_cand->bts,
+						      best_cand->requirements & REQUIREMENT_C_MASK);
+	}
+
+	/* we are done in case the MS RXLEV/RXQUAL/TA aren't critical and we're avoiding congestion. */
+	if (!include_weaker_rxlev)
+		goto no_candidates;
+
+	/* Select best candidate that fulfills requirement A: can service the call.
+	 * From above we know that there are no options that avoid congestion. Here we're trying to find
+	 * *any* free lchan that has no critically low RXLEV and is able to handle the MS. */
+	best_better_db = 0;
+	for (i = 0; i < candidates; i++) {
+		int afs_bias;
+		if (!(clist[i].requirements & REQUIREMENT_A_MASK))
+			continue;
+
+		better = clist[i].avg - av_rxlev;
+		/* Apply AFS bias? */
+		afs_bias = 0;
+		if (ahs && (clist[i].requirements & REQUIREMENT_A_TCHF))
+			afs_bias = ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+		better += afs_bias;
+		if (better > best_better_db) {
+			best_cand = &clist[i];
+			best_better_db = better;
+			best_applied_afs_bias = afs_bias? true : false;
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (best_cand) {
+		LOGPHOLCHANTOBTS(lchan, best_cand->bts, LOGL_INFO, "Best candidate, RX level %d"
+			" with greater congestion found%s\n",
+			rxlev2dbm(best_cand->avg),
+			best_applied_afs_bias ? " (applied AHS -> AFS rxlev bias)" : "");
+		return trigger_handover_or_assignment(lchan, best_cand->bts,
+						      best_cand->requirements & REQUIREMENT_A_MASK);
+	}
+
+	/* Damn, all is congested, has too low RXLEV or cannot service the voice call due to codec
+	 * restrictions or because all lchans are taken. */
+
+no_candidates:
+	if (include_weaker_rxlev)
+		LOGPHOLCHAN(lchan, LOGL_INFO, "No alternative lchan found\n");
+	else
+		LOGPHOLCHAN(lchan, LOGL_INFO, "No better/less congested neighbor cell found\n");
+
+	return 0;
+}
+
+/*
+ * Handover/assignment check, if measurement report is received
+ *
+ * Do not trigger handover/assignment on slots which have already ongoing
+ * handover/assignment processes.
+ *
+ * In case of handover triggered because maximum allowed timing advance is
+ * exceeded, the handover penalty timer is started for the originating cell.
+ *
+ */
+static void on_measurement_report(struct gsm_meas_rep *mr)
+{
+	struct gsm_lchan *lchan = mr->lchan;
+	struct gsm_bts *bts = lchan->ts->trx->bts;
+	int av_rxlev = -EINVAL, av_rxqual = -EINVAL;
+	unsigned int pwr_interval;
+
+	/* we currently only do handover for TCH channels */
+	switch (mr->lchan->type) {
+	case GSM_LCHAN_TCH_F:
+	case GSM_LCHAN_TCH_H:
+		break;
+	default:
+		return;
+	}
+
+	if (log_check_level(DHODEC, LOGL_DEBUG)) {
+		int i;
+		LOGPHOLCHAN(lchan, LOGL_DEBUG, "MEASUREMENT REPORT (%d neighbors)\n",
+			    mr->num_cell);
+		for (i = 0; i < mr->num_cell; i++) {
+			struct gsm_meas_rep_cell *mrc = &mr->cell[i];
+			LOGPHOLCHAN(lchan, LOGL_DEBUG,
+				    "  %d: arfcn=%u bsic=%u neigh_idx=%u rxlev=%u flags=%x\n",
+				    i, mrc->arfcn, mrc->bsic, mrc->neigh_idx, mrc->rxlev, mrc->flags);
+		}
+	}
+
+	/* parse actual neighbor cell info */
+	if (mr->num_cell > 0 && mr->num_cell < 7)
+		process_meas_neigh(mr);
+
+	/* check for ongoing handover/assignment */
+	if (!lchan->conn) {
+		LOGPHOLCHAN(lchan, LOGL_ERROR, "Skipping, No subscriber connection???\n");
+		return;
+	}
+	if (lchan->conn->secondary_lchan) {
+		LOGPHOLCHAN(lchan, LOGL_INFO, "Skipping, Initial Assignment is still ongoing\n");
+		return;
+	}
+	if (lchan->conn->ho) {
+		LOGPHOLCHAN(lchan, LOGL_INFO, "Skipping, Handover already triggered\n");
+		return;
+	}
+
+	LOGPHOLCHAN(lchan, LOGL_DEBUG, "HODEC2: evaluating measurement report\n");
+
+	/* get average levels. if not enought measurements yet, value is < 0 */
+	av_rxlev = get_meas_rep_avg(lchan,
+				    ho_get_hodec2_full_tdma(bts->ho) ?
+				    MEAS_REP_DL_RXLEV_FULL : MEAS_REP_DL_RXLEV_SUB,
+				    ho_get_hodec2_rxlev_avg_win(bts->ho));
+	av_rxqual = get_meas_rep_avg(lchan,
+				     ho_get_hodec2_full_tdma(bts->ho) ?
+				     MEAS_REP_DL_RXQUAL_FULL : MEAS_REP_DL_RXQUAL_SUB,
+				     ho_get_hodec2_rxqual_avg_win(bts->ho));
+	if (av_rxlev < 0 && av_rxqual < 0) {
+		LOGPHOLCHAN(lchan, LOGL_INFO, "Skipping, Not enough recent measurements\n");
+		return;
+	}
+	if (av_rxlev >= 0) {
+		LOGPHOLCHAN(lchan, LOGL_DEBUG, "Measurement report: average RX level = %d\n",
+			    rxlev2dbm(av_rxlev));
+	}
+	if (av_rxqual >= 0) {
+		LOGPHOLCHAN(lchan, LOGL_DEBUG, "Measurement report: average RX quality = %d\n",
+			    av_rxqual);
+	}
+
+	/* improve levels in case of AFS, if defined */
+	if (lchan->type == GSM_LCHAN_TCH_F
+	 && lchan->tch_mode == GSM48_CMODE_SPEECH_AMR) {
+		int rxlev_bias = ho_get_hodec2_afs_bias_rxlev(bts->ho);
+		int rxqual_bias = ho_get_hodec2_afs_bias_rxqual(bts->ho);
+		if (av_rxlev >= 0 && rxlev_bias) {
+			int imp = av_rxlev + rxlev_bias;
+			LOGPHOLCHAN(lchan, LOGL_INFO, "Virtually improving RX level from %d to %d,"
+				    " due to AFS bias\n", rxlev2dbm(av_rxlev), rxlev2dbm(imp));
+			av_rxlev = imp;
+		}
+		if (av_rxqual >= 0 && rxqual_bias) {
+			int imp = av_rxqual - rxqual_bias;
+			if (imp < 0)
+				imp = 0;
+			LOGPHOLCHAN(lchan, LOGL_INFO, "Virtually improving RX quality from %d to %d,"
+				    " due to AFS bias\n", rxlev2dbm(av_rxqual), rxlev2dbm(imp));
+			av_rxqual = imp;
+		}
+	}
+
+	/* Bad Quality */
+	if (av_rxqual >= 0 && av_rxqual > ho_get_hodec2_min_rxqual(bts->ho)) {
+		if (rxlev2dbm(av_rxlev) > -85) {
+			global_ho_reason = HO_REASON_INTERFERENCE;
+			LOGPHOLCHAN(lchan, LOGL_INFO, "Trying handover/assignment"
+				    " due to interference (bad quality)\n");
+		} else {
+			global_ho_reason = HO_REASON_BAD_QUALITY;
+			LOGPHOLCHAN(lchan, LOGL_INFO, "Trying handover/assignment due to bad quality\n");
+		}
+		find_alternative_lchan(lchan, true);
+		return;
+	}
+
+	/* Low Level */
+	if (av_rxlev >= 0 && rxlev2dbm(av_rxlev) < ho_get_hodec2_min_rxlev(bts->ho)) {
+		global_ho_reason = HO_REASON_LOW_RXLEVEL;
+		LOGPHOLCHAN(lchan, LOGL_INFO, "Attempting handover/assignment due to low rxlev\n");
+		find_alternative_lchan(lchan, true);
+		return;
+	}
+
+	/* Max Distance */
+	if (lchan->meas_rep_count > 0
+	    && lchan->rqd_ta > ho_get_hodec2_max_distance(bts->ho)) {
+		global_ho_reason = HO_REASON_MAX_DISTANCE;
+		LOGPHOLCHAN(lchan, LOGL_INFO, "Attempting handover due to high TA\n");
+		/* start penalty timer to prevent comming back too
+		 * early. it must be started before selecting a better cell,
+		 * so there is no assignment selected, due to running
+		 * penalty timer. */
+		conn_penalty_time_add(lchan->conn, bts, ho_get_hodec2_penalty_max_dist(bts->ho));
+		find_alternative_lchan(lchan, true);
+		return;
+	}
+
+	/* pwr_interval's range is 1-99, clarifying that no div-zero shall happen in modulo below: */
+	pwr_interval = ho_get_hodec2_pwr_interval(bts->ho);
+	OSMO_ASSERT(pwr_interval);
+
+	/* try handover to a better cell */
+	if (av_rxlev >= 0 && (mr->nr % pwr_interval) == 0) {
+		LOGPHOLCHAN(lchan, LOGL_INFO, "Looking whether a cell has better RXLEV\n");
+		global_ho_reason = HO_REASON_BETTER_CELL;
+		find_alternative_lchan(lchan, false);
+	}
+}
+
+/*
+ * Handover/assignment check after timer timeout:
+ *
+ * Even if handover process tries to prevent a congestion, a cell might get
+ * congested due to new call setups or handovers to prevent loss of radio link.
+ * A cell is congested, if not the minimum number of free slots are available.
+ * The minimum number can be defined for TCH/F and TCH/H individually.
+ *
+ * Do not perform congestion check, if no minimum free slots are defined for
+ * a cell.
+ * Do not trigger handover/assignment on slots which have already ongoing
+ * handover/assignment processes. If no AFS improvement offset is given, try to
+ * maintain the same TCH rate, if available.
+ * Do not perform this process, if handover and assignment are disabled for
+ * the current cell.
+ * Do not perform handover, if the minimum acceptable RX level
+ * is not reched for this cell.
+ * Only check candidates that will solve/reduce congestion.
+ *
+ * If a cell is congested, all slots are checked for all their RX levels
+ * (down-link) of the current and neighbor cell measurements in descending
+ * order of their RX levels:
+ *
+ *  * Select the best candidate that fulfills requirement B (no congestion after
+ *    handover/assignment), trigger handover or assignment. Candidates that will
+ *    cause an assignment from AHS (AMR on TCH/H) to AFS (AMR on TCH/F) are
+ *    omitted.
+ *     o This process repeated until the minimum required number of free slots
+ *       are restored or if all cell measurements are checked. The process ends
+ *       then, otherwise:
+ *  * Select the worst candidate that fulfills requirement B, trigger
+ *    assignment. Note that only assignment candidates for changing from AHS to
+ *    AFS are left.
+ *     o This process repeated until the minimum required number of free slots
+ *       are restored or if all cell measurements are checked. The process ends
+ *       then, otherwise:
+ *  * Select the best candidates that fulfill requirement C (less or equally
+ *    congested cells after handover/assignment), trigger handover or
+ *    assignment. Candidates that will cause an assignment from AHS (AMR on
+ *    TCH/H) to AFS (AMR on TCH/F) are omitted.
+ *     o This process repeated until the minimum required number of free slots
+ *       are restored or if all cell measurements are checked. The process ends
+ *       then, otherwise:
+ *  * Select the worst candidate that fulfills requirement C, trigger
+ *    assignment. Note that only assignment candidates for changing from AHS to
+ *    AFS are left.
+ *     o This process repeated until the minimum required number of free slots
+ *       are restored or if all cell measurements are checked.
+ */
+static int bts_resolve_congestion(struct gsm_bts *bts, int tchf_congestion, int tchh_congestion)
+{
+	struct gsm_lchan *lc;
+	struct gsm_bts_trx *trx;
+	struct gsm_bts_trx_ts *ts;
+	int i, j;
+	struct ho_candidate *clist;
+	unsigned int candidates;
+	struct ho_candidate *best_cand = NULL, *worst_cand = NULL;
+	struct gsm_lchan *delete_lchan = NULL;
+	unsigned int best_avg_db, worst_avg_db;
+	int avg;
+	int rc = 0;
+	int any_ho = 0;
+	int is_improved = 0;
+
+	if (tchf_congestion < 0)
+		tchf_congestion = 0;
+	if (tchh_congestion < 0)
+		tchh_congestion = 0;
+
+	LOGPHOBTS(bts, LOGL_INFO, "congested: %d TCH/F and %d TCH/H should be moved\n",
+		  tchf_congestion, tchh_congestion);
+
+	/* allocate array of all bts */
+	clist = talloc_zero_array(tall_bsc_ctx, struct ho_candidate,
+		bts->num_trx * 8 * 2 * (1 + ARRAY_SIZE(lc->neigh_meas)));
+	if (!clist)
+		return 0;
+
+	candidates = 0;
+
+	/* loop through all active lchan and collect candidates */
+	llist_for_each_entry(trx, &bts->trx_list, list) {
+		if (!trx_is_usable(trx))
+			continue;
+
+		for (i = 0; i < 8; i++) {
+			ts = &trx->ts[i];
+			if (!ts_is_usable(ts))
+				continue;
+
+			/* (Do not consider dynamic TS that are in PDCH mode) */
+			switch (ts_pchan(ts)) {
+			case GSM_PCHAN_TCH_F:
+				lc = &ts->lchan[0];
+				/* omit if channel not active */
+				if (lc->type != GSM_LCHAN_TCH_F
+				    || lc->state != LCHAN_S_ACTIVE)
+					break;
+				/* omit if there is an ongoing ho/as */
+				if (!lc->conn || lc->conn->secondary_lchan
+				    || lc->conn->ho)
+					break;
+				/* We desperately want to resolve congestion, ignore rxlev when
+				 * collecting candidates by passing include_weaker_rxlev=true. */
+				collect_candidates_for_lchan(lc, clist, &candidates, NULL, true);
+				break;
+			case GSM_PCHAN_TCH_H:
+				for (j = 0; j < 2; j++) {
+					lc = &ts->lchan[j];
+					/* omit if channel not active */
+					if (lc->type != GSM_LCHAN_TCH_H
+					    || lc->state != LCHAN_S_ACTIVE)
+						continue;
+					/* omit of there is an ongoing ho/as */
+					if (!lc->conn
+					    || lc->conn->secondary_lchan
+					    || lc->conn->ho)
+						continue;
+					/* We desperately want to resolve congestion, ignore rxlev when
+					 * collecting candidates by passing include_weaker_rxlev=true. */
+					collect_candidates_for_lchan(lc, clist, &candidates, NULL, true);
+				}
+				break;
+			default:
+				break;
+			}
+		}
+	}
+
+	if (!candidates) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "No neighbor cells qualify to solve congestion\n");
+		goto exit;
+	}
+	if (log_check_level(DHODEC, LOGL_DEBUG)) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "Considering %u candidates to solve congestion:\n", candidates);
+		for (i = 0; i < candidates; i++) {
+			LOGPHOLCHANTOBTS(clist[i].lchan, clist[i].bts, LOGL_DEBUG,
+					 "#%d: req=0x%x avg-rxlev=%d\n",
+					 i, clist[i].requirements, clist[i].avg);
+		}
+	}
+
+#if 0
+next_b1:
+#endif
+	/* select best candidate that fulfills requirement B,
+	 * omit change from AHS to AFS */
+	best_avg_db = 0;
+	for (i = 0; i < candidates; i++) {
+		/* delete subscriber that just have handovered */
+		if (clist[i].lchan == delete_lchan)
+			clist[i].lchan = NULL;
+		/* omit all subscribers that are handovered */
+		if (!clist[i].lchan)
+			continue;
+
+		if (!(clist[i].requirements & REQUIREMENT_B_MASK))
+			continue;
+		/* omit assignment from AHS to AFS */
+		if (clist[i].lchan->ts->trx->bts == clist[i].bts
+		 && clist[i].lchan->type == GSM_LCHAN_TCH_H
+		 && (clist[i].requirements & REQUIREMENT_B_TCHF))
+			continue;
+		/* omit candidates that will not solve/reduce congestion */
+		if (clist[i].lchan->type == GSM_LCHAN_TCH_F
+		 && tchf_congestion <= 0)
+			continue;
+		if (clist[i].lchan->type == GSM_LCHAN_TCH_H
+		 && tchh_congestion <= 0)
+			continue;
+
+		avg = clist[i].avg;
+		/* improve AHS */
+		if (clist[i].lchan->tch_mode == GSM48_CMODE_SPEECH_AMR
+		 && clist[i].lchan->type == GSM_LCHAN_TCH_H
+		 && (clist[i].requirements & REQUIREMENT_B_TCHF)) {
+			avg += ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+			is_improved = 1;
+		} else
+			is_improved = 0;
+		LOGP(DHODEC, LOGL_DEBUG, "candidate %d: avg=%d best_avg_db=%d\n", i, avg, best_avg_db);
+		if (avg > best_avg_db) {
+			best_cand = &clist[i];
+			best_avg_db = avg;
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (best_cand) {
+		any_ho = 1;
+		LOGPHOLCHAN(best_cand->lchan, LOGL_INFO,
+			    "Best candidate BTS %u (RX level %d) without congestion found\n",
+			    best_cand->bts->nr, rxlev2dbm(best_cand->avg));
+		if (is_improved)
+			LOGP(DHODEC, LOGL_INFO, "(is improved due to "
+				"AHS -> AFS)\n");
+		trigger_handover_or_assignment(best_cand->lchan, best_cand->bts,
+			best_cand->requirements & REQUIREMENT_B_MASK);
+#if 0
+		/* if there is still congestion, mark lchan as deleted
+		 * and redo this process */
+		if (best_cand->lchan->type == GSM_LCHAN_TCH_H)
+			tchh_congestion--;
+		else
+			tchf_congestion--;
+		if (tchf_congestion > 0 || tchh_congestion > 0) {
+			delete_lchan = best_cand->lchan;
+			best_cand = NULL;
+			goto next_b1;
+		}
+#else
+		/* must exit here, because triggering handover/assignment
+		 * will cause change in requirements. more check for this
+		 * bts is performed in the next iteration.
+		 */
+#endif
+		goto exit;
+	}
+
+	LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a best candidate that fulfills requirement B"
+		  " (omitting change from AHS to AFS)\n");
+
+#if 0
+next_b2:
+#endif
+	/* select worst candidate that fulfills requirement B,
+	 * select candidates that change from AHS to AFS only */
+	if (tchh_congestion > 0) {
+		/* since this will only check half rate channels, it will
+		 * only need to be checked, if tchh is congested */
+		worst_avg_db = 999;
+		for (i = 0; i < candidates; i++) {
+			/* delete subscriber that just have handovered */
+			if (clist[i].lchan == delete_lchan)
+				clist[i].lchan = NULL;
+			/* omit all subscribers that are handovered */
+			if (!clist[i].lchan)
+				continue;
+
+			if (!(clist[i].requirements & REQUIREMENT_B_MASK))
+				continue;
+			/* omit all but assignment from AHS to AFS */
+			if (clist[i].lchan->ts->trx->bts != clist[i].bts
+			 || clist[i].lchan->type != GSM_LCHAN_TCH_H
+			 || !(clist[i].requirements & REQUIREMENT_B_TCHF))
+				continue;
+
+			avg = clist[i].avg;
+			/* improve AHS */
+			if (clist[i].lchan->tch_mode == GSM48_CMODE_SPEECH_AMR
+			 && clist[i].lchan->type == GSM_LCHAN_TCH_H) {
+				avg += ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+				is_improved = 1;
+			} else
+				is_improved = 0;
+			LOGP(DHODEC, LOGL_DEBUG, "candidate %d: avg=%d worst_avg_db=%d\n", i, avg,
+			     worst_avg_db);
+			if (avg < worst_avg_db) {
+				worst_cand = &clist[i];
+				worst_avg_db = avg;
+			}
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (worst_cand) {
+		any_ho = 1;
+		LOGP(DHODEC, LOGL_INFO, "Worst candidate for assignment "
+			"(RX level %d) from TCH/H -> TCH/F without congestion "
+			"found\n", rxlev2dbm(worst_cand->avg));
+		if (is_improved)
+			LOGP(DHODEC, LOGL_INFO, "(is improved due to "
+				"AHS -> AFS)\n");
+		trigger_handover_or_assignment(worst_cand->lchan,
+			worst_cand->bts,
+			worst_cand->requirements & REQUIREMENT_B_MASK);
+#if 0
+		/* if there is still congestion, mark lchan as deleted
+		 * and redo this process */
+		tchh_congestion--;
+		if (tchh_congestion > 0) {
+			delete_lchan = worst_cand->lchan;
+			best_cand = NULL;
+			goto next_b2;
+		}
+#else
+		/* must exit here, because triggering handover/assignment
+		 * will cause change in requirements. more check for this
+		 * bts is performed in the next iteration.
+		 */
+#endif
+		goto exit;
+	}
+
+	LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a worst candidate that fulfills requirement B,"
+		  " selecting candidates that change from AHS to AFS only\n");
+
+#if 0
+next_c1:
+#endif
+	/* select best candidate that fulfills requirement C,
+	 * omit change from AHS to AFS */
+	best_avg_db = 0;
+	for (i = 0; i < candidates; i++) {
+		/* delete subscriber that just have handovered */
+		if (clist[i].lchan == delete_lchan)
+			clist[i].lchan = NULL;
+		/* omit all subscribers that are handovered */
+		if (!clist[i].lchan)
+			continue;
+
+		if (!(clist[i].requirements & REQUIREMENT_C_MASK))
+			continue;
+		/* omit assignment from AHS to AFS */
+		if (clist[i].lchan->ts->trx->bts == clist[i].bts
+		 && clist[i].lchan->type == GSM_LCHAN_TCH_H
+		 && (clist[i].requirements & REQUIREMENT_C_TCHF))
+			continue;
+		/* omit candidates that will not solve/reduce congestion */
+		if (clist[i].lchan->type == GSM_LCHAN_TCH_F
+		 && tchf_congestion <= 0)
+			continue;
+		if (clist[i].lchan->type == GSM_LCHAN_TCH_H
+		 && tchh_congestion <= 0)
+			continue;
+
+		avg = clist[i].avg;
+		/* improve AHS */
+		if (clist[i].lchan->tch_mode == GSM48_CMODE_SPEECH_AMR
+		 && clist[i].lchan->type == GSM_LCHAN_TCH_H
+		 && (clist[i].requirements & REQUIREMENT_C_TCHF)) {
+			avg += ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+			is_improved = 1;
+		} else
+			is_improved = 0;
+		LOGP(DHODEC, LOGL_DEBUG, "candidate %d: avg=%d best_avg_db=%d\n", i, avg, best_avg_db);
+		if (avg > best_avg_db) {
+			best_cand = &clist[i];
+			best_avg_db = avg;
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (best_cand) {
+		any_ho = 1;
+		LOGP(DHODEC, LOGL_INFO, "Best candidate BTS %d (RX level %d) "
+			"with less or equal congestion found\n",
+			best_cand->bts->nr, rxlev2dbm(best_cand->avg));
+		if (is_improved)
+			LOGP(DHODEC, LOGL_INFO, "(is improved due to "
+				"AHS -> AFS)\n");
+		trigger_handover_or_assignment(best_cand->lchan, best_cand->bts,
+			best_cand->requirements & REQUIREMENT_C_MASK);
+#if 0
+		/* if there is still congestion, mark lchan as deleted
+		 * and redo this process */
+		if (best_cand->lchan->type == GSM_LCHAN_TCH_H)
+			tchh_congestion--;
+		else
+			tchf_congestion--;
+		if (tchf_congestion > 0 || tchh_congestion > 0) {
+			delete_lchan = best_cand->lchan;
+			best_cand = NULL;
+			goto next_c1;
+		}
+#else
+		/* must exit here, because triggering handover/assignment
+		 * will cause change in requirements. more check for this
+		 * bts is performed in the next iteration.
+		 */
+#endif
+		goto exit;
+	}
+
+	LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a best candidate that fulfills requirement C"
+		  " (omitting change from AHS to AFS)\n");
+
+#if 0
+next_c2:
+#endif
+	/* select worst candidate that fulfills requirement C,
+	 * select candidates that change from AHS to AFS only */
+	if (tchh_congestion > 0) {
+		/* since this will only check half rate channels, it will
+		 * only need to be checked, if tchh is congested */
+		worst_avg_db = 999;
+		for (i = 0; i < candidates; i++) {
+			/* delete subscriber that just have handovered */
+			if (clist[i].lchan == delete_lchan)
+				clist[i].lchan = NULL;
+			/* omit all subscribers that are handovered */
+			if (!clist[i].lchan)
+				continue;
+
+			if (!(clist[i].requirements & REQUIREMENT_C_MASK))
+				continue;
+			/* omit all but assignment from AHS to AFS */
+			if (clist[i].lchan->ts->trx->bts != clist[i].bts
+			 || clist[i].lchan->type != GSM_LCHAN_TCH_H
+			 || !(clist[i].requirements & REQUIREMENT_C_TCHF))
+				continue;
+
+			avg = clist[i].avg;
+			/* improve AHS */
+			if (clist[i].lchan->tch_mode == GSM48_CMODE_SPEECH_AMR
+			 && clist[i].lchan->type == GSM_LCHAN_TCH_H) {
+				avg += ho_get_hodec2_afs_bias_rxlev(clist[i].bts->ho);
+				is_improved = 1;
+			} else
+				is_improved = 0;
+			LOGP(DHODEC, LOGL_DEBUG, "candidate %d: avg=%d worst_avg_db=%d\n", i, avg,
+			     worst_avg_db);
+			if (avg < worst_avg_db) {
+				worst_cand = &clist[i];
+				worst_avg_db = avg;
+			}
+		}
+	}
+
+	/* perform handover, if there is a candidate */
+	if (worst_cand) {
+		any_ho = 1;
+		LOGP(DHODEC, LOGL_INFO, "Worst candidate for assignment "
+			"(RX level %d) from TCH/H -> TCH/F with less or equal "
+			"congestion found\n", rxlev2dbm(worst_cand->avg));
+		if (is_improved)
+			LOGP(DHODEC, LOGL_INFO, "(is improved due to "
+				"AHS -> AFS)\n");
+		trigger_handover_or_assignment(worst_cand->lchan,
+			worst_cand->bts,
+			worst_cand->requirements & REQUIREMENT_C_MASK);
+#if 0
+		/* if there is still congestion, mark lchan as deleted
+		 * and redo this process */
+		tchh_congestion--;
+		if (tchh_congestion > 0) {
+			delete_lchan = worst_cand->lchan;
+			worst_cand = NULL;
+			goto next_c2;
+		}
+#else
+		/* must exit here, because triggering handover/assignment
+		 * will cause change in requirements. more check for this
+		 * bts is performed in the next iteration.
+		 */
+#endif
+		goto exit;
+	}
+	LOGPHOBTS(bts, LOGL_DEBUG, "Did not find a worst candidate that fulfills requirement C,"
+		  " selecting candidates that change from AHS to AFS only\n");
+
+
+exit:
+	/* free array */
+	talloc_free(clist);
+
+	if (tchf_congestion <= 0 && tchh_congestion <= 0)
+		LOGP(DHODEC, LOGL_INFO, "Congestion at BTS %d solved!\n",
+			bts->nr);
+	else if (any_ho)
+		LOGP(DHODEC, LOGL_INFO, "Congestion at BTS %d reduced!\n",
+			bts->nr);
+	else
+		LOGP(DHODEC, LOGL_INFO, "Congestion at BTS %d can't be reduced/solved!\n", bts->nr);
+
+	return rc;
+}
+
+static void bts_congestion_check(struct gsm_bts *bts)
+{
+	int min_free_tchf, min_free_tchh;
+	int tchf_count, tchh_count;
+
+	global_ho_reason = HO_REASON_CONGESTION;
+
+	/* only check BTS if TRX 0 is usable */
+	if (!trx_is_usable(bts->c0)) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "No congestion check: TRX 0 not usable\n");
+		return;
+	}
+
+	/* only check BTS if handover or assignment is enabled */
+	if (!ho_get_hodec2_as_active(bts->ho)
+	    && !ho_get_ho_active(bts->ho)) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "No congestion check: Assignment and Handover both disabled\n");
+		return;
+	}
+
+	min_free_tchf = ho_get_hodec2_tchf_min_slots(bts->ho);
+	min_free_tchh = ho_get_hodec2_tchh_min_slots(bts->ho);
+
+	/* only check BTS with congestion level set */
+	if (!min_free_tchf && !min_free_tchh) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "No congestion check: no minimum for free TCH/F nor TCH/H set\n");
+		return;
+	}
+
+	tchf_count = bts_count_free_ts(bts, GSM_PCHAN_TCH_F);
+	tchh_count = bts_count_free_ts(bts, GSM_PCHAN_TCH_H);
+	LOGPHOBTS(bts, LOGL_INFO, "Congestion check: (free/want-free) TCH/F=%d/%d TCH/H=%d/%d\n",
+		  tchf_count, min_free_tchf, tchh_count, min_free_tchh);
+
+	/* only check BTS if congested */
+	if (tchf_count >= min_free_tchf && tchh_count >= min_free_tchh) {
+		LOGPHOBTS(bts, LOGL_DEBUG, "Not congested\n");
+		return;
+	}
+
+	LOGPHOBTS(bts, LOGL_DEBUG, "Attempting to resolve congestion...\n");
+	bts_resolve_congestion(bts, min_free_tchf - tchf_count, min_free_tchh - tchh_count);
+}
+
+void hodec2_congestion_check(struct gsm_network *net)
+{
+	struct gsm_bts *bts;
+
+	llist_for_each_entry(bts, &net->bts_list, list)
+		bts_congestion_check(bts);
+}
+
+static void congestion_check_cb(void *arg)
+{
+	struct gsm_network *net = arg;
+	hodec2_congestion_check(net);
+	reinit_congestion_timer(net);
+}
+
+void on_ho_chan_activ_nack(struct bsc_handover *ho)
+{
+	struct gsm_bts *new_bts = ho->new_lchan->ts->trx->bts;
+
+	LOGPHO(ho, LOGL_ERROR, "Channel Activate Nack for %s, starting penalty timer\n", ho->inter_cell? "Handover" : "Assignment");
+
+	/* if channel failed, wait 10 seconds before allowing to retry handover */
+	conn_penalty_time_add(ho->old_lchan->conn, new_bts, 10); /* FIXME configurable */
+}
+
+void on_ho_failure(struct bsc_handover *ho)
+{
+	struct gsm_bts *old_bts = ho->old_lchan->ts->trx->bts;
+	struct gsm_bts *new_bts = ho->new_lchan->ts->trx->bts;
+	struct gsm_subscriber_connection *conn = ho->old_lchan->conn;
+
+	if (!conn) {
+		LOGPHO(ho, LOGL_ERROR, "HO failure, but no conn");
+		return;
+	}
+
+	if (conn->hodec2.failures >= ho_get_hodec2_retries(old_bts->ho)) {
+		int penalty = ho->inter_cell
+			? ho_get_hodec2_penalty_failed_ho(old_bts->ho)
+			: ho_get_hodec2_penalty_failed_as(old_bts->ho);
+		LOGPHO(ho, LOGL_NOTICE, "%s failed, starting penalty timer (%d s)\n",
+		       ho->inter_cell ? "Handover" : "Assignment",
+		       penalty);
+		conn->hodec2.failures = 0;
+		conn_penalty_time_add(conn, new_bts, penalty);
+	} else {
+		conn->hodec2.failures++;
+		LOGPHO(ho, LOGL_NOTICE, "%s failed, allowing handover decision to try again"
+		       " (%d/%d attempts)\n",
+		       ho->inter_cell ? "Handover" : "Assignment",
+		       conn->hodec2.failures, ho_get_hodec2_retries(old_bts->ho));
+	}
+}
+
+struct handover_decision_callbacks hodec2_callbacks = {
+	.hodec_id = 2,
+	.on_measurement_report = on_measurement_report,
+	.on_ho_chan_activ_nack = on_ho_chan_activ_nack,
+	.on_ho_failure = on_ho_failure,
+};
+
+void hodec2_init(struct gsm_network *net)
+{
+	handover_decision_callbacks_register(&hodec2_callbacks);
+	hodec2_initialized = true;
+	reinit_congestion_timer(net);
+}