diff --git a/src/bts.h b/src/bts.h
index af66dfc8..b31db5fc 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -142,6 +142,9 @@ struct gprs_rlcmac_bts {
 	uint8_t alpha, gamma;
 	uint32_t dl_tbf_idle_msec; /* hold time for idle DL TBFs */
 	uint32_t ms_idle_sec;
+	uint8_t cs_adj_enabled;
+	uint8_t cs_adj_upper_limit;
+	uint8_t cs_adj_lower_limit;
 
 	/* TBF handling, make private or move into TBFController */
 	/* list of uplink TBFs */
diff --git a/src/gprs_ms.cpp b/src/gprs_ms.cpp
index dc8783c4..f5924fd8 100644
--- a/src/gprs_ms.cpp
+++ b/src/gprs_ms.cpp
@@ -351,3 +351,39 @@ void GprsMs::set_ms_class(uint8_t ms_class_)
 	m_ms_class = ms_class_;
 }
 
+void GprsMs::update_error_rate(gprs_rlcmac_tbf *tbf, int error_rate)
+{
+	struct gprs_rlcmac_bts *bts_data;
+
+	OSMO_ASSERT(m_bts != NULL);
+	bts_data = m_bts->bts_data();
+
+	if (error_rate < 0)
+		return;
+
+	/* TODO: Support different CS values for UL and DL */
+
+	if (error_rate > bts_data->cs_adj_upper_limit) {
+		if (m_current_cs_dl > 1) {
+			m_current_cs_dl -= 1;
+			m_current_cs_ul = m_current_cs_dl;
+			LOGP(DRLCMACDL, LOGL_INFO,
+				"MS (IMSI %s): High error rate %d%%, "
+				"reducing CS level to %d\n",
+				imsi(), error_rate, m_current_cs_dl);
+		}
+	} else if (error_rate < bts_data->cs_adj_lower_limit) {
+		if (m_current_cs_dl < 4) {
+			m_current_cs_dl += 1;
+			m_current_cs_ul = m_current_cs_dl;
+			LOGP(DRLCMACDL, LOGL_INFO,
+				"MS (IMSI %s): Low error rate %d%%, "
+				"increasing CS level to %d\n",
+				imsi(), error_rate, m_current_cs_dl);
+		}
+	} else {
+		LOGP(DRLCMACDL, LOGL_DEBUG,
+			"MS (IMSI %s): Medium error rate %d%%, ignored\n",
+			imsi(), error_rate);
+	}
+}
diff --git a/src/gprs_ms.h b/src/gprs_ms.h
index 01858366..27c4be40 100644
--- a/src/gprs_ms.h
+++ b/src/gprs_ms.h
@@ -86,6 +86,8 @@ public:
 
 	void detach_tbf(gprs_rlcmac_tbf *tbf);
 
+	void update_error_rate(gprs_rlcmac_tbf *tbf, int percent);
+
 	bool is_idle() const {return !m_ul_tbf && !m_dl_tbf && !m_ref;}
 
 	void* operator new(size_t num);
diff --git a/src/pcu_main.cpp b/src/pcu_main.cpp
index 9e96af08..2e4a970f 100644
--- a/src/pcu_main.cpp
+++ b/src/pcu_main.cpp
@@ -173,6 +173,9 @@ int main(int argc, char *argv[])
 	bts->n3105 = 8;
 	bts->alpha = 0; /* a = 0.0 */
 	bts->ms_idle_sec = 60; /* slightly above T3314 (default 44s, 24.008, 11.2.2) */
+	bts->cs_adj_enabled = 1;
+	bts->cs_adj_upper_limit = 33; /* Decrease CS if the error rate is above */
+	bts->cs_adj_lower_limit = 10; /* Increase CS if the error rate is below */
 
 	msgb_set_talloc_ctx(tall_pcu_ctx);
 
diff --git a/src/rlc.h b/src/rlc.h
index 55cae3ff..313f3c72 100644
--- a/src/rlc.h
+++ b/src/rlc.h
@@ -59,6 +59,8 @@ struct gprs_rlc_data {
 	uint8_t block[RLC_MAX_LEN];
 	/* block len of history */
 	uint8_t len;
+
+	uint8_t cs;
 };
 
 /*
diff --git a/src/tbf.h b/src/tbf.h
index f50c489a..8e4c3b98 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -363,6 +363,7 @@ protected:
 	bool dl_window_stalled() const;
 	void reuse_tbf(const uint8_t *data, const uint16_t len);
 	void start_llc_timer();
+	int analyse_errors(char *show_rbb, uint8_t ssn);
 
 	struct osmo_timer_list m_llc_timer;
 };
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 0aa41a26..0726ef79 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -412,6 +412,7 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 	/* now we still have untransmitted LLC data, so we fill mac block */
 	rlc_data = m_rlc.block(bsn);
 	data = rlc_data->prepare(block_data_len);
+	rlc_data->cs = cs;
 
 	rh = (struct rlc_dl_header *)data;
 	rh->pt = 0; /* Data Block */
@@ -669,6 +670,53 @@ struct msgb *gprs_rlcmac_dl_tbf::create_dl_acked_block(
 	return dl_msg;
 }
 
+static uint16_t bitnum_to_bsn(int bitnum, uint16_t ssn, uint16_t mod_sns)
+{
+	return (ssn - 1 - bitnum) & mod_sns;
+}
+
+int gprs_rlcmac_dl_tbf::analyse_errors(char *show_rbb, uint8_t ssn)
+{
+	gprs_rlc_data *rlc_data;
+	uint16_t lost = 0, received = 0, skipped = 0;
+
+	/* SSN - 1 is in range V(A)..V(S)-1 */
+	for (int bitpos = 0; bitpos < m_window.ws(); bitpos++) {
+		uint16_t bsn = bitnum_to_bsn(bitpos, ssn, m_window.mod_sns());
+
+		if (bsn == ((m_window.v_a() - 1) & m_window.mod_sns()))
+			break;
+
+		rlc_data = m_rlc.block(bsn);
+		if (!rlc_data)
+			continue;
+
+		if (rlc_data->cs != current_cs()) {
+			/* This block has already been encoded with a different
+			 * CS, so it doesn't help us to decide, whether the
+			 * current CS is ok. Ignore it. */
+			skipped += 1;
+			continue;
+		}
+
+		if (show_rbb[m_window.ws() - 1 - bitpos] == 'R') {
+			if (!m_window.m_v_b.is_acked(bsn))
+				received += 1;
+		} else {
+			lost += 1;
+		}
+	}
+
+	LOGP(DRLCMACDL, LOGL_DEBUG, "%s DL analysis, range=%d:%d, lost=%d, recv=%d, skipped=%d\n",
+		name(), m_window.v_a(), m_window.v_s(), lost, received, skipped);
+
+	if (lost + received == 0)
+		return -1;
+
+	return lost * 100 / (lost + received);
+}
+
+
 int gprs_rlcmac_dl_tbf::update_window(const uint8_t ssn, const uint8_t *rbb)
 {
 	int16_t dist; /* must be signed */
@@ -676,6 +724,7 @@ int gprs_rlcmac_dl_tbf::update_window(const uint8_t ssn, const uint8_t *rbb)
 	char show_rbb[65];
 	char show_v_b[RLC_MAX_SNS + 1];
 	const uint16_t mod_sns = m_window.mod_sns();
+	int error_rate;
 
 	Decoding::extract_rbb(rbb, show_rbb);
 	/* show received array in debug (bit 64..1) */
@@ -698,6 +747,11 @@ int gprs_rlcmac_dl_tbf::update_window(const uint8_t ssn, const uint8_t *rbb)
 		return 1; /* indicate to free TBF */
 	}
 
+	if (bts_data()->cs_adj_enabled && ms()) {
+		error_rate = analyse_errors(show_rbb, ssn);
+		ms()->update_error_rate(this, error_rate);
+	}
+
 	m_window.update(bts, show_rbb, ssn,
 			&lost, &received);