diff --git a/modem/arq_session.py b/modem/arq_session.py
index 5c1dfcfb..ca2694ea 100644
--- a/modem/arq_session.py
+++ b/modem/arq_session.py
@@ -6,11 +6,24 @@ from modem_frametypes import FRAME_TYPE
 
 class ARQSession():
 
-    MODE_BY_SPEED = [
-        codec2.FREEDV_MODE.datac4,
-        codec2.FREEDV_MODE.datac3,
-        codec2.FREEDV_MODE.datac1,
-    ]
+    SPEED_LEVEL_DICT = {
+        0: {
+            'mode': codec2.FREEDV_MODE.datac4,
+            'min_snr': -10,
+            'duration_per_frame': 5.17,
+        },
+        1: {
+            'mode': codec2.FREEDV_MODE.datac3,
+            'min_snr': 0,
+            'duration_per_frame': 3.19,
+        },
+        2: {
+            'mode': codec2.FREEDV_MODE.datac1,
+            'min_snr': 3,
+            'duration_per_frame': 4.18,
+        },
+    }
+
 
     def __init__(self, config: dict, tx_frame_queue: queue.Queue, dxcall: str):
         self.logger = structlog.get_logger(type(self).__name__)
@@ -36,7 +49,7 @@ class ARQSession():
         logger(msg)
 
     def get_mode_by_speed_level(self, speed_level):
-        return self.MODE_BY_SPEED[speed_level]
+        return self.SPEED_LEVEL_DICT[speed_level]["mode"]
 
     def transmit_frame(self, frame: bytearray):
         self.log("Transmitting frame")
@@ -55,7 +68,7 @@ class ARQSession():
     def get_data_payload_size(self):
         return self.frame_factory.get_available_data_payload_for_mode(
             FRAME_TYPE.ARQ_BURST_FRAME,
-            self.MODE_BY_SPEED[self.speed_level]
+            self.SPEED_LEVEL_DICT[self.speed_level]["mode"]
             )
 
     def set_details(self, snr, frequency_offset):
diff --git a/modem/arq_session_irs.py b/modem/arq_session_irs.py
index 5542658f..3df6f5af 100644
--- a/modem/arq_session_irs.py
+++ b/modem/arq_session_irs.py
@@ -52,6 +52,8 @@ class ARQSessionIRS(arq_session.ARQSession):
         self.received_bytes = 0
         self.received_crc = None
 
+        self.transmitted_acks = 0
+
     def set_modem_decode_modes(self, modes):
         pass
 
@@ -65,6 +67,13 @@ class ARQSessionIRS(arq_session.ARQSession):
         self.transmit_frame(frame)
         self.log(f"Waiting {timeout} seconds...")
         if not self.event_frame_received.wait(timeout):
+            # use case: data burst got lost, we want to send a NACK with updated speed level
+            if self.state in [self.STATE_BURST_REPLY_SENT, self.STATE_INFO_ACK_SENT]:
+                self.transmitted_acks = 0
+                self.calibrate_speed_settings()
+                self.send_burst_nack()
+                return
+
             self.log("Timeout waiting for ISS. Session failed.")
             self.set_state(self.STATE_FAILED)
 
@@ -94,6 +103,12 @@ class ARQSessionIRS(arq_session.ARQSession):
         self.launch_transmit_and_wait(info_ack, self.TIMEOUT_CONNECT)
         self.set_state(self.STATE_INFO_ACK_SENT)
 
+    def send_burst_nack(self):
+        self.calibrate_speed_settings()
+        nack = self.frame_factory.build_arq_burst_ack(self.id, self.received_bytes, self.speed_level, self.frames_per_burst, self.snr[0])
+        self.transmit_and_wait(nack)
+
+
     def process_incoming_data(self, frame):
         if frame['offset'] != self.received_bytes:
             self.logger.info(f"Discarding data frame due to wrong offset", frame=self.frame_received)
@@ -116,12 +131,14 @@ class ARQSessionIRS(arq_session.ARQSession):
 
     def receive_data(self, burst_frame):
         self.process_incoming_data(burst_frame)
-
+        self.calibrate_speed_settings()
         ack = self.frame_factory.build_arq_burst_ack(
             self.id, self.received_bytes, 
             self.speed_level, self.frames_per_burst, self.snr[0])
 
         if not self.all_data_received():
+            # increase ack counter
+            self.transmitted_acks += 1
             self.transmit_and_wait(ack)
             self.set_state(self.STATE_BURST_REPLY_SENT)
             return
@@ -136,17 +153,12 @@ class ARQSessionIRS(arq_session.ARQSession):
             self.set_state(self.STATE_FAILED)
 
     def calibrate_speed_settings(self):
-        return
-    
-        # decrement speed level after the 2nd retry
-        if self.RETRIES_TRANSFER - self.retries >= 2:
-            self.speed -= 1
-            self.speed = max(self.speed, 0)
-            return
+        # if we have two ACKS, then consider increasing speed level
+        if self.transmitted_acks >= 2:
+            self.transmitted_acks = 0
+            new_speed_level = min(self.speed_level + 1, len(self.SPEED_LEVEL_DICT) - 1)
 
-        # increment speed level after 2 successfully sent bursts and fitting snr
-        # TODO
+            # check first if the next mode supports the actual snr
+            if self.snr >= self.SPEED_LEVEL_DICT[new_speed_level]["min_snr"]:
+                self.speed_level = new_speed_level
 
-
-        self.speed = self.speed
-        self.frames_per_burst = self.frames_per_burst
diff --git a/modem/arq_session_iss.py b/modem/arq_session_iss.py
index 72fda0f6..fdb6580d 100644
--- a/modem/arq_session_iss.py
+++ b/modem/arq_session_iss.py
@@ -105,7 +105,7 @@ class ARQSessionISS(arq_session.ARQSession):
             offset = self.confirmed_bytes
             payload = self.data[offset : offset + payload_size]
             data_frame = self.frame_factory.build_arq_burst_frame(
-                self.MODE_BY_SPEED[self.speed_level],
+                self.SPEED_LEVEL_DICT[self.speed_level]["mode"],
                 self.id, self.confirmed_bytes, payload)
             burst.append(data_frame)