diff --git a/tests/Makefile.am b/tests/Makefile.am
index a24f4ea..a03528c 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,7 +1,7 @@
 AM_CPPFLAGS = $(STD_DEFINES_AND_INCLUDES) $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGB_CFLAGS) $(LIBOSMOGSM_CFLAGS) -I$(top_srcdir)/src/
 AM_LDFLAGS = -lrt
 
-check_PROGRAMS = rlcmac/RLCMACTest alloc/AllocTest tbf/TbfTest types/TypesTest ms/MsTest llist/LListTest llc/LlcTest codel/codel_test edge/EdgeTest bitcomp/BitcompTest
+check_PROGRAMS = rlcmac/RLCMACTest alloc/AllocTest tbf/TbfTest types/TypesTest ms/MsTest llist/LListTest llc/LlcTest codel/codel_test edge/EdgeTest bitcomp/BitcompTest fn/FnTest
 noinst_PROGRAMS = emu/pcu_emu
 
 rlcmac_RLCMACTest_SOURCES = rlcmac/RLCMACTest.cpp
@@ -90,6 +90,14 @@
 	$(LIBOSMOCORE_LIBS) \
 	$(COMMON_LA)
 
+fn_FnTest_SOURCES = fn/FnTest.cpp
+fn_FnTest_LDADD = \
+	$(top_builddir)/src/libgprs.la \
+	$(LIBOSMOGB_LIBS) \
+	$(LIBOSMOGSM_LIBS) \
+	$(LIBOSMOCORE_LIBS) \
+	$(COMMON_LA)
+
 # The `:;' works around a Bash 3.2 bug when the output is not writeable.
 $(srcdir)/package.m4: $(top_srcdir)/configure.ac
 	:;{ \
@@ -119,7 +127,8 @@
 	llc/LlcTest.ok llc/LlcTest.err \
 	llist/LListTest.ok llist/LListTest.err \
 	codel/codel_test.ok \
-	edge/EdgeTest.ok
+	edge/EdgeTest.ok \
+	fn/FnTest.ok
 
 DISTCLEANFILES = atconfig
 
diff --git a/tests/fn/FnTest.cpp b/tests/fn/FnTest.cpp
new file mode 100644
index 0000000..279903c
--- /dev/null
+++ b/tests/fn/FnTest.cpp
@@ -0,0 +1,175 @@
+/* Frame number calculation test */
+
+/* (C) 2016 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * Author: Philipp Maier
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "bts.h"
+#include <string.h>
+#include <stdio.h>
+
+extern "C" {
+#include <osmocom/core/application.h>
+#include <osmocom/gsm/gsm_utils.h>
+#include <osmocom/core/talloc.h>
+#include <osmocom/core/utils.h>
+}
+
+#define RFN_MODULUS 42432
+
+/* globals used by the code */ void *tall_pcu_ctx;
+int16_t spoof_mnc = 0, spoof_mcc = 0;
+
+static uint32_t calc_fn(BTS * bts, uint32_t rfn)
+{
+	uint32_t fn;
+	fn = bts->rfn_to_fn(rfn);
+	printf("rfn=%i ==> fn=%i\n", rfn, fn);
+	return fn;
+}
+
+static void set_fn(BTS * bts, uint32_t fn)
+{
+	printf("\n");
+	bts->set_current_frame_number(fn);
+	printf("bts: fn=%i\n", fn);
+}
+
+static void run_test()
+{
+	BTS bts;
+	uint32_t fn;
+
+	printf("RFN_MODULUS=%i\n",RFN_MODULUS);
+	printf("GSM_MAX_FN=%i\n",GSM_MAX_FN);
+
+
+	/* Test with a collection of real world examples,
+	 * all all of them are not critical and do not
+	 * assume the occurence of any race contions */
+	set_fn(&bts, 1320462);
+	fn = calc_fn(&bts, 5066);
+	OSMO_ASSERT(fn == 1320458);
+
+	set_fn(&bts, 8246);
+	fn = calc_fn(&bts, 8244);
+	OSMO_ASSERT(fn == 8244);
+
+	set_fn(&bts, 10270);
+	fn = calc_fn(&bts, 10269);
+	OSMO_ASSERT(fn == 10269);
+
+	set_fn(&bts, 311276);
+	fn = calc_fn(&bts, 14250);
+	OSMO_ASSERT(fn == 311274);
+
+
+	/* Now lets assume a case where the frame number
+	 * just wrapped over a little bit above the
+	 * modulo 42432 raster, but the rach request
+	 * occurred before the wrapping */
+	set_fn(&bts, RFN_MODULUS + 30);
+	fn = calc_fn(&bts, RFN_MODULUS - 10);
+	OSMO_ASSERT(fn == 42422);
+
+	set_fn(&bts, RFN_MODULUS + 1);
+	fn = calc_fn(&bts, RFN_MODULUS - 1);
+	OSMO_ASSERT(fn == 42431);
+
+	set_fn(&bts, RFN_MODULUS * 123 + 16);
+	fn = calc_fn(&bts, RFN_MODULUS - 4);
+	OSMO_ASSERT(fn == 5219132);
+
+	set_fn(&bts, RFN_MODULUS * 123 + 451);
+	fn = calc_fn(&bts, RFN_MODULUS - 175);
+	OSMO_ASSERT(fn == 5218961);
+
+
+	/* Lets check a special cornercase. We assume that
+	 * the BTS just wrapped its internal frame number
+	 * but we still get rach requests with high relative
+	 * frame numbers. */
+	set_fn(&bts, 0);
+	fn = calc_fn(&bts, RFN_MODULUS - 13);
+	OSMO_ASSERT(fn == 2715635);
+
+	set_fn(&bts, 453);
+	fn = calc_fn(&bts, RFN_MODULUS - 102);
+	OSMO_ASSERT(fn == 2715546);
+
+	set_fn(&bts, 10);
+	fn = calc_fn(&bts, RFN_MODULUS - 10);
+	OSMO_ASSERT(fn == 2715638);
+
+	set_fn(&bts, 23);
+	fn = calc_fn(&bts, RFN_MODULUS - 42);
+	OSMO_ASSERT(fn == 2715606);
+
+
+	/* Also check with some corner case
+	 * values where Fn and RFn reach its
+	 * maximum/minimum valid range */
+	set_fn(&bts, GSM_MAX_FN);
+	fn = calc_fn(&bts, RFN_MODULUS-1);
+	OSMO_ASSERT(fn == GSM_MAX_FN-1);
+
+	set_fn(&bts, 0);
+	fn = calc_fn(&bts, RFN_MODULUS-1);
+	OSMO_ASSERT(fn == GSM_MAX_FN-1);
+
+	set_fn(&bts, GSM_MAX_FN);
+	fn = calc_fn(&bts, 0);
+	OSMO_ASSERT(fn == GSM_MAX_FN);
+
+	set_fn(&bts, 0);
+	fn = calc_fn(&bts, 0);
+	OSMO_ASSERT(fn == 0);
+}
+
+int main(int argc, char **argv)
+{
+	tall_pcu_ctx = talloc_named_const(NULL, 1, "fn test context");
+	if (!tall_pcu_ctx)
+		abort();
+
+	msgb_talloc_ctx_init(tall_pcu_ctx, 0);
+	osmo_init_logging(&gprs_log_info);
+	log_set_use_color(osmo_stderr_target, 0);
+	log_set_print_filename(osmo_stderr_target, 0);
+	log_set_log_level(osmo_stderr_target, LOGL_DEBUG);
+
+	run_test();
+	return EXIT_SUCCESS;
+}
+
+/*
+ * stubs that should not be reached
+ */
+extern "C" {
+	void l1if_pdch_req() {
+		abort();
+	} void l1if_connect_pdch() {
+		abort();
+	}
+	void l1if_close_pdch() {
+		abort();
+	}
+	void l1if_open_pdch() {
+		abort();
+	}
+}
diff --git a/tests/fn/FnTest.ok b/tests/fn/FnTest.ok
new file mode 100644
index 0000000..be6400f
--- /dev/null
+++ b/tests/fn/FnTest.ok
@@ -0,0 +1,50 @@
+RFN_MODULUS=42432
+GSM_MAX_FN=2715648
+
+bts: fn=1320462
+rfn=5066 ==> fn=1320458
+
+bts: fn=8246
+rfn=8244 ==> fn=8244
+
+bts: fn=10270
+rfn=10269 ==> fn=10269
+
+bts: fn=311276
+rfn=14250 ==> fn=311274
+
+bts: fn=42462
+rfn=42422 ==> fn=42422
+
+bts: fn=42433
+rfn=42431 ==> fn=42431
+
+bts: fn=5219152
+rfn=42428 ==> fn=5219132
+
+bts: fn=5219587
+rfn=42257 ==> fn=5218961
+
+bts: fn=0
+rfn=42419 ==> fn=2715635
+
+bts: fn=453
+rfn=42330 ==> fn=2715546
+
+bts: fn=10
+rfn=42422 ==> fn=2715638
+
+bts: fn=23
+rfn=42390 ==> fn=2715606
+
+bts: fn=2715648
+rfn=42431 ==> fn=2715647
+
+bts: fn=0
+rfn=42431 ==> fn=2715647
+
+bts: fn=2715648
+rfn=0 ==> fn=2715648
+
+bts: fn=0
+rfn=0 ==> fn=0
diff --git a/tests/testsuite.at b/tests/testsuite.at
index e42a0fd..d8f8f9a 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -70,3 +70,9 @@
 cat $abs_srcdir/codel/codel_test.ok > expout
 AT_CHECK([$OSMO_QEMU $abs_top_builddir/tests/codel/codel_test], [0], [expout], [ignore])
 AT_CLEANUP
+
+AT_SETUP([fn])
+AT_KEYWORDS([fn])
+cat $abs_srcdir/fn/FnTest.ok > expout
+AT_CHECK([$OSMO_QEMU $abs_top_builddir/tests/fn/FnTest], [0], [expout], [ignore])
+AT_CLEANUP