Introduce VTY option to forbid use of TCH for non-voicecall signalling

Usual allocation mechansim, when some signalling channel is needed,
first tries to reserve an SDCCH, and if all of them are exhausted, then
attempts to reserve a TCH as a last resort.
This, however, may cause TCH starvation under certain situations, for
instance if there high load on other services (LU, SMS, etc.).
Hence, it may be desirable for the operator to forbid reservation
of TCH slots once SDCCH become exhausted. This commit is thus adding a
VTY command which allows forbidding it. The default behavior (allow using
TCH timeslots when SDCCHs are exhausted) is kept as before.

The above mentioned prohibition is applied only to non-voicecall related
signalling services. That's because voicecall services will end up
requiring a TCH anyway, and forbidding reservation of TCH straighaway
when SDCCHs are exhausted would mean no voice calls could be initiated
while still TCHs would be available.

Related: SYS#5548
Change-Id: Ib08027125145df26602069bfb51847063b0ccc0c

3 files changed, 36 insertions, 11 deletions

diff --git a/src/osmo-bsc/abis_rsl.c b/src/osmo-bsc/abis_rsl.c
index 8e4916dc0..d5b0d532e 100644
--- a/src/osmo-bsc/abis_rsl.c
+++ b/src/osmo-bsc/abis_rsl.c
@@ -2015,17 +2015,19 @@ void abis_rsl_chan_rqd_queue_poll(struct gsm_bts *bts)
 	 * in the code below, all other channel requests will get an SDCCH first
 	 * (if possible). */
 
-	if (!lchan) {
-		LOG_BTS(bts, DRSL, LOGL_NOTICE, "CHAN RQD[%s]: no resources for %s 0x%x, retrying with %s\n",
-			get_value_string(gsm_chreq_descs, rqd->reason), gsm_lchant_name(GSM_LCHAN_SDCCH),
-			rqd->ref.ra, gsm_lchant_name(GSM_LCHAN_TCH_H));
-		lchan = lchan_select_by_type(bts, GSM_LCHAN_TCH_H);
-	}
-	if (!lchan) {
-		LOG_BTS(bts, DRSL, LOGL_NOTICE, "CHAN RQD[%s]: no resources for %s 0x%x, retrying with %s\n",
-			get_value_string(gsm_chreq_descs, rqd->reason), gsm_lchant_name(GSM_LCHAN_SDCCH),
-			rqd->ref.ra, gsm_lchant_name(GSM_LCHAN_TCH_F));
-		lchan = lchan_select_by_type(bts, GSM_LCHAN_TCH_F);
+	if (gsm_chreq_reason_is_voicecall(rqd->reason) || bts->chan_alloc_allow_tch_for_signalling) {
+		if (!lchan) {
+			LOG_BTS(bts, DRSL, LOGL_NOTICE, "CHAN RQD[%s]: no resources for %s 0x%x, retrying with %s\n",
+				get_value_string(gsm_chreq_descs, rqd->reason), gsm_lchant_name(GSM_LCHAN_SDCCH),
+				rqd->ref.ra, gsm_lchant_name(GSM_LCHAN_TCH_H));
+			lchan = lchan_select_by_type(bts, GSM_LCHAN_TCH_H);
+		}
+		if (!lchan) {
+			LOG_BTS(bts, DRSL, LOGL_NOTICE, "CHAN RQD[%s]: no resources for %s 0x%x, retrying with %s\n",
+				get_value_string(gsm_chreq_descs, rqd->reason), gsm_lchant_name(GSM_LCHAN_SDCCH),
+				rqd->ref.ra, gsm_lchant_name(GSM_LCHAN_TCH_F));
+			lchan = lchan_select_by_type(bts, GSM_LCHAN_TCH_F);
+		}
 	}
 	if (!lchan) {
 		LOG_BTS(bts, DRSL, LOGL_NOTICE, "CHAN RQD[%s]: no resources for %s 0x%x\n",
diff --git a/src/osmo-bsc/bts.c b/src/osmo-bsc/bts.c
index ec29ac80e..c0d663420 100644
--- a/src/osmo-bsc/bts.c
+++ b/src/osmo-bsc/bts.c
@@ -245,6 +245,7 @@ struct gsm_bts *gsm_bts_alloc(struct gsm_network *net, struct gsm_bts_sm *bts_sm
 	bts->neigh_list_manual_mode = NL_MODE_AUTOMATIC;
 	bts->early_classmark_allowed_3g = true; /* 3g Early Classmark Sending controlled by bts->early_classmark_allowed param */
 	bts->si_unused_send_empty = true;
+	bts->chan_alloc_allow_tch_for_signalling = true;
 	bts->si_common.cell_sel_par.cell_resel_hyst = 2; /* 4 dB */
 	bts->si_common.cell_sel_par.rxlev_acc_min = 0;
 	bts->si_common.si2quater_neigh_list.arfcn = bts->si_common.data.earfcn_list;
diff --git a/src/osmo-bsc/bts_vty.c b/src/osmo-bsc/bts_vty.c
index b8b9f6c39..3695d2838 100644
--- a/src/osmo-bsc/bts_vty.c
+++ b/src/osmo-bsc/bts_vty.c
@@ -565,6 +565,25 @@ DEFUN_ATTR(cfg_bts_chan_alloc_interf,
 	return CMD_SUCCESS;
 }
 
+DEFUN_ATTR(cfg_bts_chan_alloc_allow_tch_for_signalling,
+	   cfg_bts_chan_alloc_allow_tch_for_signalling_cmd,
+	   "channel allocator allow-tch-for-signalling (0|1)",
+	   "Channel Allocator\n" "Channel Allocator\n"
+	   "Configure whether TCH/H or TCH/F channels can be used to serve non-call-related signalling if SDCCHs are exhausted\n"
+	   "Forbid use of TCH for non-call-related signalling purposes\n"
+	   "Allow use of TCH for non-call-related signalling purposes (default)\n",
+	   CMD_ATTR_IMMEDIATE)
+{
+	struct gsm_bts *bts = vty->index;
+
+	if (!strcmp(argv[0], "0"))
+		bts->chan_alloc_allow_tch_for_signalling = false;
+	else
+		bts->chan_alloc_allow_tch_for_signalling = true;
+
+	return CMD_SUCCESS;
+}
+
 #define RACH_STR "Random Access Control Channel\n"
 
 DEFUN_USRATTR(cfg_bts_rach_tx_integer,
@@ -3736,6 +3755,8 @@ static void config_write_bts_single(struct vty *vty, struct gsm_bts *bts)
 		VTY_NEWLINE);
 	if (bts->chan_alloc_avoid_interf)
 		vty_out(vty, "  channel allocator avoid-interference 1%s", VTY_NEWLINE);
+	if (!bts->chan_alloc_allow_tch_for_signalling)
+		vty_out(vty, "  channel allocator allow-tch-for-signalling 0%s", VTY_NEWLINE);
 	vty_out(vty, "  rach tx integer %u%s",
 		bts->si_common.rach_control.tx_integer, VTY_NEWLINE);
 	vty_out(vty, "  rach max transmission %u%s",
@@ -4018,6 +4039,7 @@ int bts_vty_init(void)
 	install_element(BTS_NODE, &cfg_bts_oml_e1_tei_cmd);
 	install_element(BTS_NODE, &cfg_bts_challoc_cmd);
 	install_element(BTS_NODE, &cfg_bts_chan_alloc_interf_cmd);
+	install_element(BTS_NODE, &cfg_bts_chan_alloc_allow_tch_for_signalling_cmd);
 	install_element(BTS_NODE, &cfg_bts_rach_tx_integer_cmd);
 	install_element(BTS_NODE, &cfg_bts_rach_max_trans_cmd);
 	install_element(BTS_NODE, &cfg_bts_rach_max_delay_cmd);