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bts: TC_pcu_interf_ind: check reporting interval instead of absolute FN

Recent osmo-bts commit 53f0ec29 broke TC_pcu_interf_ind:

  Verdict: fail reason: Odd TDMA frame number := 935

There's no requirement that periodic interference reports must land
on a TDMA frame number satisfying 'fn mod 104 == 0' - only that they
keep arriving every configured averaging period.  Instead, verify
that the interval between consecutive reports equals 104 TDMA frames
(1 SACCH period), using the new f_gsm_fn_sub() helper.

Change-Id: Ia96f2926400bae8c0a6fb8f81eb56b48ca5f2de9
Related: 53f0ec29 ("l1sap: fix duplicate RF RESOURCE INDICATION on clock bootstrap")
---
M bts/BTS_Tests.ttcn
1 file changed, 11 insertions(+), 4 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-ttcn3-hacks refs/changes/16/42916/1

diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 54238f5..328919b 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -6664,6 +6664,7 @@
 	var template PCUIF_Message tr_interf_ind;
 	var integer interf_ind_num := 0;
 	var boolean first := true;
+	var integer prev_fn := -1;
 	var PCUIF_Message msg;
 	timer T;
 
@@ -6691,6 +6692,7 @@
 	alt {
 	/* The first interference report may contain unreliable values, so we ignore it */
 	[first] as_pcuif_msg(msg, tr_PCUIF_INTERF_IND(0, 0)) {
+		prev_fn := msg.u.interf_ind.fn;
 		/* 4 SACCH periods => 4 reports (plus some guard time) */
 		T.start(0.480 * 4.0 + 0.480 / 2.0);
 		first := false;
@@ -6698,11 +6700,16 @@
 		}
 	/* Subsequent interference reports shall match our expectations */
 	[not first] as_pcuif_msg(msg, tr_interf_ind) {
-		/* Check TDMA frame number period */
-		if (msg.u.interf_ind.fn mod 104 != 0) {
-			setverdict(fail, "Odd TDMA frame number := ",
-				   msg.u.interf_ind.fn);
+		/* Check the reporting interval: there is no requirement on the absolute
+		 * TDMA frame number a report arrives at, only that reports keep arriving
+		 * every 'avg-period' (here: 1) SACCH period(s), i.e. every 104 frames. */
+		var integer fn_diff := f_gsm_fn_sub(msg.u.interf_ind.fn, prev_fn);
+		if (fn_diff != 104) {
+			setverdict(fail, "Unexpected TDMA frame number interval := ",
+				   fn_diff, " (fn := ", msg.u.interf_ind.fn,
+				   ", prev_fn := ", prev_fn, ")");
 		}
+		prev_fn := msg.u.interf_ind.fn;
 		interf_ind_num := interf_ind_num + 1;
 		if (interf_ind_num < 4)
 			{ repeat; }

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