diff --git a/AWSIoT/Internal/MQTTSDK/AWSMQTTSession.m b/AWSIoT/Internal/MQTTSDK/AWSMQTTSession.m
index fccec15e9ce..7a1f455ab2c 100644
--- a/AWSIoT/Internal/MQTTSDK/AWSMQTTSession.m
+++ b/AWSIoT/Internal/MQTTSDK/AWSMQTTSession.m
@@ -63,7 +63,7 @@ - (UInt16)nextMsgId;
 
 @property (strong,atomic) NSMutableArray* queue; //Queue to temporarily hold messages if encoder is busy sending another message
 @property (strong,atomic) NSMutableArray* timerRing; // circular array of 60. Each element is a set that contains the messages that need to be retried.
-@property (strong,nonatomic) dispatch_semaphore_t drainSenderQueueSemaphore;
+@property (nonatomic, strong) dispatch_queue_t drainSenderSerialQueue;
 
 @end
 
@@ -96,7 +96,7 @@ - (id)initWithClientId:(NSString*)theClientId
     
     if (self = [super init]) {
         clientId = theClientId;
-        _drainSenderQueueSemaphore = dispatch_semaphore_create(1);
+        _drainSenderSerialQueue = dispatch_queue_create("com.amazon.aws.iot.drain-sender-queue", DISPATCH_QUEUE_SERIAL);
         keepAliveInterval = theKeepAliveInterval;
         connectMessage = msg;
         _publishRetryThrottle = publishRetryThrottle;
@@ -345,22 +345,13 @@ - (void)encoder:(AWSMQTTEncoder*)sender handleEvent:(AWSMQTTEncoderEvent) eventC
                         break;
                     case AWSMQTTSessionStatusConnecting:
                         break;
-                    case AWSMQTTSessionStatusConnected:
-                        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ waiting on drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-                        dispatch_semaphore_wait(self.drainSenderQueueSemaphore, DISPATCH_TIME_FOREVER);
-                        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ passed  drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-                        
-                        if ([self.queue count] > 0) {
-                            AWSDDLogDebug(@"Sending message from session queue" );
-                            AWSMQTTMessage *msg = [self.queue objectAtIndex:0];
-                            [self.queue removeObjectAtIndex:0];
-                            [encoder encodeMessage:msg];
-                        }
-                        
-                        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ signaling on drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-                        dispatch_semaphore_signal(self.drainSenderQueueSemaphore);
-                        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ finshed draining messages", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
+                    case AWSMQTTSessionStatusConnected: {
+                        dispatch_assert_queue_not(self.drainSenderSerialQueue);
+                        dispatch_sync(self.drainSenderSerialQueue, ^{
+                            [self queueNextMessage];
+                        });
                         break;
+                    }
                     case AWSMQTTSessionStatusError:
                         break;
                 }
@@ -684,17 +675,10 @@ - (void)send:(AWSMQTTMessage*)msg {
         [encoder encodeMessage:msg];
     }
     else {
-        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ waiting on drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-        dispatch_semaphore_wait(self.drainSenderQueueSemaphore, DISPATCH_TIME_FOREVER);
-        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ passed  drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-        
-        AWSDDLogDebug(@"<<%@>>: MQTTSession.send added msg to queue to send later", [NSThread currentThread]);
-        [self.queue addObject:msg];
-        
-        
-        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ signaling drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-        dispatch_semaphore_signal(self.drainSenderQueueSemaphore);
-        AWSDDLogVerbose(@"%s [Line %d], Thread:%@ finished draining messages", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
+        dispatch_assert_queue_not(self.drainSenderSerialQueue);
+        dispatch_sync(self.drainSenderSerialQueue, ^{
+            [self queueMessage:msg];
+        });
     }
 }
 
@@ -711,23 +695,45 @@ - (BOOL)isReadyToPublish {
     return encoder && [encoder status] == AWSMQTTEncoderStatusReady;
 }
 
--(void) drainSenderQueue {
-    AWSDDLogVerbose(@"%s [Line %d], Thread:%@ waiting on drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-    dispatch_semaphore_wait(self.drainSenderQueueSemaphore, DISPATCH_TIME_FOREVER);
-    AWSDDLogVerbose(@"%s [Line %d], Thread:%@ passed drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
+- (void)drainSenderQueue {
+    dispatch_assert_queue_not(self.drainSenderSerialQueue);
+    dispatch_sync(self.drainSenderSerialQueue, ^{
+        [self drainAllMessages];
+    });
+}
+
+# pragma mark - private/serial functions -
+
+- (void)queueNextMessage {
+    dispatch_assert_queue(self.drainSenderSerialQueue);
+
+    if ([self.queue count] > 0) {
+        AWSDDLogDebug(@"Sending message from session queue");
+        AWSMQTTMessage *msg = [self.queue objectAtIndex:0];
+        [self.queue removeObjectAtIndex:0];
+        [encoder encodeMessage:msg];
+    }
+}
+
+- (void)queueMessage:(AWSMQTTMessage*)msg {
+    dispatch_assert_queue(self.drainSenderSerialQueue);
+
+    [self.queue addObject:msg];
+}
+
+- (void)drainAllMessages {
+    dispatch_assert_queue(self.drainSenderSerialQueue);
 
     int count = 0;
-    while ([self.queue count] > 0 && count < _publishRetryThrottle && [self isReadyToPublish]) {
+    while (self.queue.count > 0 && count < _publishRetryThrottle && self.isReadyToPublish) {
         AWSDDLogDebug(@"Sending message from session queue" );
         AWSMQTTMessage *msg = [self.queue objectAtIndex:0];
         [self.queue removeObjectAtIndex:0];
         [encoder encodeMessage:msg];
         count = count + 1;
     }
-    
-    AWSDDLogVerbose(@"%s [Line %d], Thread:%@ signaling on drainSenderQueueSemaphore", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
-    dispatch_semaphore_signal(self.drainSenderQueueSemaphore);
-    AWSDDLogVerbose(@"%s [Line %d], Thread:%@ finished draining messages", __PRETTY_FUNCTION__, __LINE__, [NSThread currentThread]);
 }
+
+
 @end