diff --git a/include/openthread-types.h b/include/openthread-types.h
index 792ef927a..344c31503 100644
--- a/include/openthread-types.h
+++ b/include/openthread-types.h
@@ -62,6 +62,18 @@ typedef enum ThreadError
     kThreadError_NotImplemented = 13,
     kThreadError_InvalidState = 14,
     kThreadError_NoTasklets = 15,
+
+    /**
+     * No acknowledgment was received after macMaxFrameRetries (IEEE 802.15.4-2006).
+     */
+    kThreadError_NoAck = 16,
+
+    /**
+     * A transmission could not take place due to activity on the channel, i.e., the CSMA-CA mechanism has failed
+     * (IEEE 802.15.4-2006).
+     */
+    kThreadError_ChannelAccessFailure = 17,
+
     kThreadError_Error = 255,
 } ThreadError;
 
diff --git a/src/core/mac/mac.cpp b/src/core/mac/mac.cpp
index 2a7b6012a..9c6626454 100644
--- a/src/core/mac/mac.cpp
+++ b/src/core/mac/mac.cpp
@@ -55,7 +55,7 @@ static Tasklet sTransmitDoneTask(&Mac::TransmitDoneTask, NULL);
 
 void Mac::StartCsmaBackoff(void)
 {
-    uint32_t backoffExponent = kMinBE + mCsmaAttempts;
+    uint32_t backoffExponent = kMinBE + mTransmitAttempts + mCsmaAttempts;
     uint32_t backoff;
 
     if (backoffExponent > kMaxBE)
@@ -84,6 +84,7 @@ Mac::Mac(ThreadNetif &aThreadNetif):
 
     mRxOnWhenIdle = true;
     mCsmaAttempts = 0;
+    mTransmitAttempts = 0;
     mTransmitBeacon = false;
     mBeacon.Init();
 
@@ -499,17 +500,16 @@ void Mac::TransmitDoneTask(void)
 
     mAckTimer.Stop();
 
-    if (error != kThreadError_None)
+    if (error == kThreadError_ChannelAccessFailure &&
+        mCsmaAttempts < kMaxCSMABackoffs)
     {
-        if (mCsmaAttempts < kMaxCSMABackoffs)
-        {
-            mCsmaAttempts++;
-        }
-
+        mCsmaAttempts++;
         StartCsmaBackoff();
         ExitNow();
     }
 
+    mCsmaAttempts = 0;
+
     switch (mState)
     {
     case kStateActiveScan:
@@ -530,7 +530,7 @@ void Mac::TransmitDoneTask(void)
             mReceiveTimer.Stop();
         }
 
-        SentFrame(true);
+        SentFrame(error == kThreadError_None);
         break;
 
     default:
@@ -619,9 +619,9 @@ void Mac::SentFrame(bool aAcked)
         {
             otDumpDebgMac("NO ACK", mSendFrame.GetHeader(), 16);
 
-            if (mCsmaAttempts < kMaxCSMABackoffs)
+            if (mTransmitAttempts < kMaxFrameAttempts)
             {
-                mCsmaAttempts++;
+                mTransmitAttempts++;
                 StartCsmaBackoff();
                 ExitNow();
             }
@@ -634,7 +634,7 @@ void Mac::SentFrame(bool aAcked)
             }
         }
 
-        mCsmaAttempts = 0;
+        mTransmitAttempts = 0;
 
         sender = mSendHead;
         mSendHead = mSendHead->mNext;
diff --git a/src/core/mac/mac.hpp b/src/core/mac/mac.hpp
index 015090c33..d3f6dbd35 100644
--- a/src/core/mac/mac.hpp
+++ b/src/core/mac/mac.hpp
@@ -65,19 +65,21 @@ namespace Mac {
  */
 enum
 {
-    kMinBE                = 3,       ///< macMinBE (IEEE 802.15.4-2006)
-    kMaxBE                = 6,       ///< macMaxBE (IEEE 802.15.4-2006)
-    kMaxCSMABackoffs      = 12,      ///< macMaxCSMABackoffs (IEEE 802.15.4-2006)
-    kUnitBackoffPeriod    = 20,      ///< Number of symbols (IEEE 802.15.4-2006)
+    kMinBE                = 3,                     ///< macMinBE (IEEE 802.15.4-2006)
+    kMaxBE                = 6,                     ///< macMaxBE (IEEE 802.15.4-2006)
+    kMaxCSMABackoffs      = 4,                     ///< macMaxCSMABackoffs (IEEE 802.15.4-2006)
+    kMaxFrameRetries      = 15,                    ///< macMaxFrameRetries (IEEE 802.15.4-2006)
+    kUnitBackoffPeriod    = 20,                    ///< Number of symbols (IEEE 802.15.4-2006)
 
-    kMinBackoff           = 16,      ///< Minimum backoff (milliseconds).
+    kMinBackoff           = 16,                    ///< Minimum backoff (milliseconds).
+    kMaxFrameAttempts     = kMaxFrameRetries + 1,  ///< Number of transmission attempts.
 
-    kAckTimeout           = 16,      ///< Timeout for waiting on an ACK (milliseconds).
-    kDataPollTimeout      = 100,     ///< Timeout for receivint Data Frame (milliseconds).
-    kNonceSize            = 13,      ///< Size of IEEE 802.15.4 Nonce (bytes).
+    kAckTimeout           = 16,                    ///< Timeout for waiting on an ACK (milliseconds).
+    kDataPollTimeout      = 100,                   ///< Timeout for receivint Data Frame (milliseconds).
+    kNonceSize            = 13,                    ///< Size of IEEE 802.15.4 Nonce (bytes).
 
-    kScanChannelsAll      = 0xffff,  ///< All channels.
-    kScanDurationDefault  = 200,     ///< Default interval between channels (milliseconds).
+    kScanChannelsAll      = 0xffff,                ///< All channels.
+    kScanDurationDefault  = 200,                   ///< Default interval between channels (milliseconds).
 };
 
 /**
@@ -440,6 +442,7 @@ private:
     uint8_t mDataSequence;
     bool mRxOnWhenIdle;
     uint8_t mCsmaAttempts;
+    uint8_t mTransmitAttempts;
     bool mTransmitBeacon;
 
     bool mActiveScanRequest;