gsm: Introduce functions to convert between FN and RFN (Reduced FN)
Implementation ported from osmo-pcu.git
e98b315d12fb009359410809f4169f9380f3d933, function bts_rfn_to_fn().
This functionality can be used by osmo-pcu, libosmo-gprs or any other
related code which needs to handle RFNs.
Change-Id: Ib71e8da976f6cc84c3a4ab17b0a8c2101492e243
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index 06b2406..bb40339 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -80,6 +80,7 @@
#include <osmocom/gsm/gsm_utils.h>
#include <osmocom/gsm/meas_rep.h>
#include <osmocom/gsm/protocol/gsm_04_08.h>
+#include <osmocom/gsm/gsm0502.h>
#include <stdlib.h>
#include <stdint.h>
@@ -924,6 +925,45 @@
return osmo_dump_gsmtime_buf(buf, 64, tm);
}
+#define GSM_RFN_THRESHOLD (GSM_RFN_MODULUS / 2)
+uint32_t gsm_rfn2fn(uint16_t rfn, uint32_t curr_fn)
+{
+ uint32_t curr_rfn;
+ uint32_t fn_rounded;
+ const uint32_t rfn32 = rfn; /* used as 32bit for calculations */
+
+ /* Ensure that all following calculations are performed with the
+ * relative frame number */
+ OSMO_ASSERT(rfn32 < GSM_RFN_MODULUS);
+
+ /* Compute an internal relative frame number from the full internal
+ frame number */
+ curr_rfn = gsm_fn2rfn(curr_fn);
+
+ /* Compute a "rounded" version of the internal frame number, which
+ * exactly fits in the RFN_MODULUS raster */
+ fn_rounded = GSM_TDMA_FN_SUB(curr_fn, curr_rfn);
+
+ /* If the delta between the internal and the external relative frame
+ * number exceeds a certain limit, we need to assume that the incoming
+ * request belongs to a the previous rfn period. To correct this,
+ * we roll back the rounded frame number by one RFN_MODULUS */
+ if (GSM_TDMA_FN_DIFF(rfn32, curr_rfn) > GSM_RFN_THRESHOLD) {
+ /* Race condition between rfn and curr_fn detected: rfn belongs
+ to the previous RFN_MODULUS cycle, wrapping... */
+ if (fn_rounded < GSM_RFN_MODULUS) {
+ /* Corner case detected: wrapping crosses GSM_MAX_FN border */
+ fn_rounded = GSM_TDMA_FN_SUB(GSM_MAX_FN, (GSM_TDMA_FN_SUB(GSM_RFN_MODULUS, fn_rounded)));
+ } else {
+ fn_rounded = GSM_TDMA_FN_SUB(fn_rounded, GSM_RFN_MODULUS);
+ }
+ }
+
+ /* The real frame number is the sum of the rounded frame number and the
+ * relative framenumber computed via RACH */
+ return GSM_TDMA_FN_SUM(fn_rounded, rfn32);
+}
+
/*! append range1024 encoded data to bit vector
* \param[out] bv Caller-provided output bit-vector
* \param[in] r Input Range1024 sructure */