diff --git a/src/bts.cpp b/src/bts.cpp
index 61c9321..1d27284 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -238,6 +238,11 @@
 	m_pollController.expireTimedout(m_cur_fn, max_delay);
 }
 
+static inline int delta_fn(int fn, int to)
+{
+	return (fn + GSM_MAX_FN * 3 / 2 - to) % GSM_MAX_FN - GSM_MAX_FN/2;
+}
+
 void BTS::set_current_block_frame_number(int fn, unsigned max_delay)
 {
 	int delay = 0;
@@ -248,15 +253,14 @@
 	/* frame numbers in the received blocks are assumed to be strongly
 	 * monotonic. */
 	if (m_cur_blk_fn >= 0) {
-		int delta = (fn + 2715648 * 3 / 2 - m_cur_blk_fn) % 2715648 - 2715648/2;
+		int delta = delta_fn(fn, m_cur_blk_fn);
 		if (delta <= 0)
 			return;
 	}
 
 	/* Check block delay vs. the current frame number */
 	if (current_frame_number() != 0)
-		delay = (fn + 2715648 * 3 / 2 - current_frame_number()) % 2715648
-			- 2715648/2;
+		delay = delta_fn(fn, current_frame_number());
 	if (delay <= -late_block_delay_thresh) {
 		LOGP(DRLCMAC, LOGL_NOTICE,
 			"Late RLC block, FN delta: %d FN: %d curFN: %d\n",
@@ -814,7 +818,7 @@
 		tbf->trx->trx_no, tbf->trx->arfcn,
 		ts, tbf->ta(), poll ? tbf->poll_fn : -1);
 	plen = Encoding::write_immediate_assignment(tbf, immediate_assignment, 1, 125,
-		(tbf->pdch[ts]->last_rts_fn + 21216) % 2715648, tbf->ta(),
+		(tbf->pdch[ts]->last_rts_fn + 21216) % GSM_MAX_FN, tbf->ta(),
 		tbf->trx->arfcn, ts, tbf->tsc(), 7, poll,
 		tbf->poll_fn, m_bts.alpha, m_bts.gamma, -1);
 	if (plen >= 0) {
diff --git a/src/gprs_rlcmac_sched.cpp b/src/gprs_rlcmac_sched.cpp
index 97ee53e..a21c023 100644
--- a/src/gprs_rlcmac_sched.cpp
+++ b/src/gprs_rlcmac_sched.cpp
@@ -41,7 +41,7 @@
 	poll_fn = fn + 4;
 	if ((block_nr % 3) == 2)
 		poll_fn ++;
-	poll_fn = poll_fn % 2715648;
+	poll_fn = poll_fn % GSM_MAX_FN;
 	llist_for_each(pos, &bts->ul_tbfs()) {
 		ul_tbf = as_ul_tbf(pos->entry());
 		OSMO_ASSERT(ul_tbf);
diff --git a/src/pcu_utils.h b/src/pcu_utils.h
index d664446..3a64a47 100644
--- a/src/pcu_utils.h
+++ b/src/pcu_utils.h
@@ -20,6 +20,11 @@
 	return (msecs * (1024 * 1000 / 4615)) / 1024;
 }
 
+inline uint32_t next_fn(uint32_t fn, uint32_t offset)
+{
+	return (fn + offset) % GSM_MAX_FN;
+}
+
 inline void csecs_to_timeval(unsigned csecs, struct timeval *tv) {
 	tv->tv_sec  = csecs / 100;
 	tv->tv_usec = (csecs % 100) * 10000;
diff --git a/src/poll_controller.cpp b/src/poll_controller.cpp
index 54e3bc7..f8ab6c9 100644
--- a/src/poll_controller.cpp
+++ b/src/poll_controller.cpp
@@ -28,35 +28,39 @@
 	: m_bts(bts)
 {}
 
+static inline bool elapsed_fn_check(unsigned max_delay, int frame_number, uint32_t from)
+{
+	uint32_t elapsed = (frame_number + GSM_MAX_FN - from) % GSM_MAX_FN;
+
+	if (elapsed > max_delay && elapsed < 2715400)
+		return true;
+
+	return false;
+}
+
 void PollController::expireTimedout(int frame_number, unsigned max_delay)
 {
 	struct gprs_rlcmac_dl_tbf *dl_tbf;
 	struct gprs_rlcmac_ul_tbf *ul_tbf;
 	struct gprs_rlcmac_sba *sba, *sba2;
 	LListHead<gprs_rlcmac_tbf> *pos;
-	uint32_t elapsed;
 
 	llist_for_each(pos, &m_bts.ul_tbfs()) {
 		ul_tbf = as_ul_tbf(pos->entry());
 		if (ul_tbf->poll_state == GPRS_RLCMAC_POLL_SCHED) {
-			elapsed = (frame_number + 2715648 - ul_tbf->poll_fn)
-								% 2715648;
-			if (elapsed > max_delay && elapsed < 2715400)
+			if (elapsed_fn_check(max_delay, frame_number, ul_tbf->poll_fn))
 				ul_tbf->poll_timeout();
 		}
 	}
 	llist_for_each(pos, &m_bts.dl_tbfs()) {
 		dl_tbf = as_dl_tbf(pos->entry());
 		if (dl_tbf->poll_state == GPRS_RLCMAC_POLL_SCHED) {
-			elapsed = (frame_number + 2715648 - dl_tbf->poll_fn)
-								% 2715648;
-			if (elapsed > max_delay && elapsed < 2715400)
+			if (elapsed_fn_check(max_delay, frame_number, dl_tbf->poll_fn))
 				dl_tbf->poll_timeout();
 		}
 	}
 	llist_for_each_entry_safe(sba, sba2, &m_bts.sba()->m_sbas, list) {
-		elapsed = (frame_number + 2715648 - sba->fn) % 2715648;
-		if (elapsed > max_delay && elapsed < 2715400) {
+		if (elapsed_fn_check(max_delay, frame_number, sba->fn)) {
 			/* sba will be freed here */
 			m_bts.sba()->timeout(sba);
 		}
diff --git a/src/sba.cpp b/src/sba.cpp
index 5d75b17..56a7543 100644
--- a/src/sba.cpp
+++ b/src/sba.cpp
@@ -23,6 +23,7 @@
 #include <gprs_rlcmac.h>
 #include <gprs_debug.h>
 #include <bts.h>
+#include <pcu_utils.h>
 
 extern "C" {
 #include <osmocom/core/talloc.h>
@@ -75,7 +76,7 @@
 		return -EINVAL;
 	}
 
-	fn = (pdch->last_rts_fn + AGCH_START_OFFSET) % 2715648;
+	fn = next_fn(pdch->last_rts_fn, AGCH_START_OFFSET);
 
 	sba->trx_no = trx;
 	sba->ts_no = ts;
@@ -110,14 +111,13 @@
 
 uint32_t SBAController::sched(uint8_t trx, uint8_t ts, uint32_t fn, uint8_t block_nr)
 {
-	uint32_t sba_fn;
+	uint32_t sba_fn = fn + 4;
 	struct gprs_rlcmac_sba *sba;
 
 	/* check special TBF for events */
-	sba_fn = fn + 4;
 	if ((block_nr % 3) == 2)
-		sba_fn ++;
-	sba_fn = sba_fn % 2715648;
+		sba_fn++;
+	sba_fn = sba_fn % GSM_MAX_FN;
 	sba = find(trx, ts, sba_fn);
 	if (sba)
 		return sba_fn;
diff --git a/src/tbf.cpp b/src/tbf.cpp
index 70b8d61..48e8289 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -566,8 +566,7 @@
 int gprs_rlcmac_tbf::check_polling(uint32_t fn, uint8_t ts,
 	uint32_t *poll_fn_, unsigned int *rrbp_)
 {
-	uint32_t fn_offs = 13;
-	uint32_t new_poll_fn = (fn + fn_offs) % 2715648;
+	uint32_t new_poll_fn = next_fn(fn, 13);
 
 	if (!is_control_ts(ts)) {
 		LOGP(DRLCMAC, LOGL_DEBUG, "Polling cannot be "
@@ -581,7 +580,7 @@
 			name());
 		return -EBUSY;
 	}
-	if (bts->sba()->find(trx->trx_no, ts, (fn + 13) % 2715648)) {
+	if (bts->sba()->find(trx->trx_no, ts, next_fn(fn, 13))) {
 		LOGP(DRLCMAC, LOGL_DEBUG, "%s: Polling is already scheduled "
 			"for single block allocation at FN %d TS %d ...\n",
 			name(), new_poll_fn, ts);
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index d871c4d..24c6385 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -1170,30 +1170,27 @@
 		(llc_queue_size() > 0);
 }
 
-int gprs_rlcmac_dl_tbf::frames_since_last_poll(unsigned fn) const
+static inline int frames_since_last(int32_t last, unsigned fn)
 {
-	unsigned wrapped;
-	if (m_last_dl_poll_fn < 0)
+	unsigned wrapped = (fn + GSM_MAX_FN - last) % GSM_MAX_FN;
+
+	if (last < 0)
 		return -1;
 
-	wrapped = (fn + 2715648 - m_last_dl_poll_fn) % 2715648;
-	if (wrapped < 2715648/2)
+	if (wrapped < GSM_MAX_FN/2)
 		return wrapped;
-	else
-		return wrapped - 2715648;
+
+	return wrapped - GSM_MAX_FN;
+}
+
+int gprs_rlcmac_dl_tbf::frames_since_last_poll(unsigned fn) const
+{
+	return frames_since_last(m_last_dl_poll_fn, fn);
 }
 
 int gprs_rlcmac_dl_tbf::frames_since_last_drain(unsigned fn) const
 {
-	unsigned wrapped;
-	if (m_last_dl_drained_fn < 0)
-		return -1;
-
-	wrapped = (fn + 2715648 - m_last_dl_drained_fn) % 2715648;
-	if (wrapped < 2715648/2)
-		return wrapped;
-	else
-		return wrapped - 2715648;
+	return frames_since_last(m_last_dl_drained_fn, fn);
 }
 
 bool gprs_rlcmac_dl_tbf::keep_open(unsigned fn) const
diff --git a/tests/tbf/TbfTest.cpp b/tests/tbf/TbfTest.cpp
index 496437d..0db7fde 100644
--- a/tests/tbf/TbfTest.cpp
+++ b/tests/tbf/TbfTest.cpp
@@ -56,20 +56,6 @@
 		OSMO_ASSERT(tbf->T != 0);
 }
 
-/*
-static unsigned inc_fn(fn)
-{
-	unsigned next_fn;
-
-	next_fn = fn + 4;
-	if ((block_nr % 3) == 2)
-		next_fn ++;
-	next_fn = next_fn % 2715648;
-
-	return next_fn;
-}
-*/
-
 static void test_tbf_base()
 {
 
@@ -209,7 +195,7 @@
 	unsigned bn = fn2bn(fn) + blocks;
 	fn = fn - (fn % 52);
 	fn += bn * 4 + bn / 3;
-	return fn % 2715648;
+	return fn % GSM_MAX_FN;
 }
 
 static void request_dl_rlc_block(struct gprs_rlcmac_bts *bts,