Change in osmo-bts[master]: chan activ: activate DL SACCH only when TA is known

[neels]

neels has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-bts/+/22509 )


Change subject: chan activ: activate DL SACCH only when TA is known
......................................................................

chan activ: activate DL SACCH only when TA is known

A channel activation for handover to another cell does not know the
Timing Advance until the handover RACH is received. It does not make
much sense to enable downlink SACCH without an accurate TA.

If the BSC omits the Access Delay IE (a.k.a. the Timing Advance), do not
enable downlink SACCH. This is expected to happen only for inter-cell
handover. In all other situations, the TA should be known either from a
Channel Request RACH for Immediate Assignment, or from the previous
lchan on the same cell upon Assignment / intra-cell handover.

Related: OS#4008 OS#4009 SYS#5192
Change-Id: I170b63c9856230d5f1a10654a9d950ada8e730d7
---
M src/common/rsl.c
1 file changed, 14 insertions(+), 2 deletions(-)



  git pull ssh://gerrit.osmocom.org:29418/osmo-bts refs/changes/09/22509/1

diff --git a/src/common/rsl.c b/src/common/rsl.c
index b4df22c..cbd6d41 100644
--- a/src/common/rsl.c
+++ b/src/common/rsl.c
@@ -1545,11 +1545,23 @@
 	}
 
 	/* Indicate which SAPIs should be enabled before the first RACH is received, for handover. See 3GPP TS 48.058
-	 * 4.1.3 and 4.1.4. */
+	 * 4.1.3 and 4.1.4.
+	 *
+	 *          |          | Access ||  transmit         |  activate         |  This implementation
+	 *          | MS Power | Delay  ||  on main channel  |  dl SACCH         |  activates DL SACCH
+	 * -----------------------------------------------------------------------------------------
+	 * async ho   no         *     -->  yes                 no                  no
+	 * async ho   yes        *     -->  yes                 may be started      no
+	 * async ho   yes        yes   -->  yes                 may be started      yes
+	 * sync ho    no         no    -->  yes                 no                  no
+	 * sync ho    yes        no    -->  yes                 may be started      no
+	 * sync ho    yes        yes   -->  yes                 shall be started    yes
+	 */
 	switch (type) {
 	case RSL_ACT_INTER_ASYNC:
 	case RSL_ACT_INTER_SYNC:
-		lchan->want_dl_sacch_active = TLVP_PRES_LEN(&tp, RSL_IE_MS_POWER, 1);
+		lchan->want_dl_sacch_active = (TLVP_PRES_LEN(&tp, RSL_IE_MS_POWER, 1)
+					       && TLVP_PRES_LEN(&tp, RSL_IE_ACCESS_DELAY, 1))
 		break;
 	default:
 		lchan->want_dl_sacch_active = true;

-- 
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Gerrit-Project: osmo-bts
Gerrit-Branch: master
Gerrit-Change-Id: I170b63c9856230d5f1a10654a9d950ada8e730d7
Gerrit-Change-Number: 22509
Gerrit-PatchSet: 1
Gerrit-Owner: neels <nhofmeyr at sysmocom.de>
Gerrit-MessageType: newchange
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