dexter has uploaded this change for review.
abis_rsl: fix frame number calculation
The formula that is used to recover the (relative) frame number from the
T1, T2, T3 parameters matches the definition in the spec, but since the
partial term t3-t2 can be negative special precaution is required when
performing the MOD 26 operation. This is due to the modulo
implementation in C/C++, which has a very specific understanding on how
to deal with negative input parameters.
Change-Id: I5fb2b0ada8d409730ac22963741fb4ab0026abdd
Related: OS#5198
---
M src/osmo-bsc/abis_rsl.c
1 file changed, 3 insertions(+), 1 deletion(-)
git pull ssh://gerrit.osmocom.org:29418/osmo-bsc refs/changes/22/30922/1
diff --git a/src/osmo-bsc/abis_rsl.c b/src/osmo-bsc/abis_rsl.c
index 640ff4d..fe5cfc6 100644
--- a/src/osmo-bsc/abis_rsl.c
+++ b/src/osmo-bsc/abis_rsl.c
@@ -1817,10 +1817,12 @@
rqd_ta = rqd->ta;
/* See also 3GPP TS 04.08, section 10.5.2.38 and 3GPP TS 08.58, section 9.3.8 */
+ /* NOTE: The forumula to compute fn was slightly altered to account for the C/C++
+ * specific modulo definition. */
t1 = rqd->ref.t1;
t2 = rqd->ref.t2;
t3 = rqd->ref.t3_low | (rqd->ref.t3_high << 3);
- fn = (51 * ((t3-t2) % 26) + t3 + 51 * 26 * t1);
+ fn = 51 * ((((t3-t2) % 26) + 26) % 26) + t3 + 51 * 26 * t1;
LOG_BTS(rqd->bts, DRSL, LOGL_INFO, "CHAN RQD: fn(t1=%u,t3=%u,t2=%u) = %u\n", t1, t3, t2, fn);
}
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