diff -r 10ae5ec6a903 src/wifi/model/dcf-manager.h
--- a/src/wifi/model/dcf-manager.h	Tue Sep 13 16:14:13 2016 -0700
+++ b/src/wifi/model/dcf-manager.h	Thu Sep 15 13:19:51 2016 -0700
@@ -140,16 +140,16 @@
    */
   bool IsAccessRequested (void) const;
 
-
-private:
-  friend class DcfManager;
-
   /**
    * Return the current number of backoff slots.
    *
    * \return the current number of backoff slots
    */
   uint32_t GetBackoffSlots (void) const;
+
+private:
+  friend class DcfManager;
+
   /**
    * Return the time when the backoff procedure started.
    *
@@ -427,6 +427,14 @@
    */
   void NotifyCtsTimeoutResetNow ();
 
+  /**
+   * Check if the device is busy sending or receiving,
+   * or NAV busy.
+   *
+   * \return true if the device is busy,
+   *         false otherwise
+   */
+  bool IsBusy (void) const;
 
 private:
   /**
@@ -527,14 +535,6 @@
    * Grant access to DCF
    */
   void DoGrantAccess (void);
-  /**
-   * Check if the device is busy sending or receiving,
-   * or NAV busy.
-   *
-   * \return true if the device is busy,
-   *         false otherwise
-   */
-  bool IsBusy (void) const;
 
   /**
    * typedef for a vector of DcfStates
diff -r 10ae5ec6a903 src/wifi/model/edca-txop-n.cc
--- a/src/wifi/model/edca-txop-n.cc	Tue Sep 13 16:14:13 2016 -0700
+++ b/src/wifi/model/edca-txop-n.cc	Thu Sep 15 13:19:51 2016 -0700
@@ -718,8 +718,7 @@
     {
       m_dcf->UpdateFailedCw ();
     }
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   RestartAccessIfNeeded ();
 }
 
@@ -727,8 +726,7 @@
 EdcaTxopN::NotifyCollision (void)
 {
   NS_LOG_FUNCTION (this);
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   RestartAccessIfNeeded ();
 }
 
@@ -862,8 +860,7 @@
       m_dcf->UpdateFailedCw ();
       m_cwTrace = m_dcf->GetCw ();
     }
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   RestartAccessIfNeeded ();
 }
 
@@ -899,7 +896,18 @@
   NS_LOG_FUNCTION (this << packet << &hdr);
   m_stationManager->PrepareForQueue (hdr.GetAddr1 (), &hdr, packet);
   m_queue->Enqueue (packet, hdr);
-  StartAccessIfNeeded ();
+  if (m_dcf->GetBackoffSlots () == 0 && m_manager->IsBusy () && !HasTxop ())
+    {
+      NS_LOG_DEBUG ("Starting backoff; slots=" << m_dcf->GetBackoffSlots () << " busy= " << m_manager->IsBusy ());
+      m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
+      NS_LOG_DEBUG ("Drawing new backoff " << m_backoffTrace);
+      m_dcf->StartBackoffNow (m_backoffTrace);
+      RestartAccessIfNeeded ();
+    }
+  else
+    {
+      StartAccessIfNeeded ();
+    }
 }
 
 void
@@ -938,11 +946,10 @@
         }
       m_currentPacket = 0;
       m_dcf->ResetCw ();
-      if (!HasTxop ())
+      m_cwTrace = m_dcf->GetCw ();
+      if (HasPendingQosMpduWithSameTidAndAddress () && !HasTxop () && m_dcf->GetBackoffSlots () == 0)
         {
-          m_cwTrace = m_dcf->GetCw ();
-          m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-          m_dcf->StartBackoffNow (m_backoffTrace);
+          StartBackoffIfNeeded ();
           RestartAccessIfNeeded ();
         }
     }
@@ -1012,8 +1019,7 @@
       m_dcf->UpdateFailedCw ();
       m_cwTrace = m_dcf->GetCw ();
     }
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   RestartAccessIfNeeded ();
 }
 
@@ -1087,8 +1093,7 @@
       m_dcf->ResetCw ();
       m_cwTrace = m_dcf->GetCw ();
     }
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   RestartAccessIfNeeded ();
 }
 
@@ -1260,8 +1265,17 @@
                                                                  &tstamp);
   if (peekedPacket == 0)
     {
+      NS_LOG_DEBUG ("No pending packet; returning");
       return;
     }
+  if (!HasTxop () && m_dcf->GetBackoffSlots () == 0)
+    {
+      NS_LOG_DEBUG ("No TXOP left; start backoff again");
+      m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
+      NS_LOG_DEBUG ("Drawing new backoff " << m_backoffTrace);
+      m_dcf->StartBackoffNow (m_backoffTrace);
+      RestartAccessIfNeeded ();
+    }
     
   MacLowTransmissionParameters params;
   params.DisableOverrideDurationId ();
@@ -1295,6 +1309,18 @@
     }
 }
 
+void
+EdcaTxopN::StartBackoffIfNeeded (void)
+{
+  NS_LOG_FUNCTION (this);
+  if (m_dcf->GetBackoffSlots () == 0)
+    {
+      m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
+      NS_LOG_DEBUG ("Drawing new backoff " << m_backoffTrace);
+      m_dcf->StartBackoffNow (m_backoffTrace);
+    }
+}
+
 Time
 EdcaTxopN::GetTxopRemaining (void)
 {
@@ -1353,6 +1379,35 @@
   return (GetTxopRemaining () >= Low ()->CalculateOverallTxTime (peekedPacket, &hdr, params));
 }
 
+bool
+EdcaTxopN::HasPendingQosMpduWithSameTidAndAddress (void) const
+{
+  NS_LOG_FUNCTION (this);
+  WifiMacHeader hdr;
+  Time tstamp;
+  if (!m_currentHdr.IsQosData ())
+    {
+      NS_LOG_DEBUG ("Current header is not QoS data; returning false");
+      return false;
+    }
+
+  Ptr<const Packet> peekedPacket = m_queue->PeekByTidAndAddress (&hdr,
+                                                                 m_currentHdr.GetQosTid (),
+                                                                 WifiMacHeader::ADDR1,
+                                                                 m_currentHdr.GetAddr1 (),
+                                                                 &tstamp);
+  if (peekedPacket == 0)
+    {
+      NS_LOG_DEBUG ("No pending MPDU matching current header TID and address; returning false");
+      return false;
+    }
+  else
+    {
+      NS_LOG_DEBUG ("Pending MPDU found; returning true");
+      return true;
+    }
+}
+
 void
 EdcaTxopN::Cancel (void)
 {
@@ -1368,8 +1423,7 @@
   m_currentPacket = 0;
   m_dcf->ResetCw ();
   m_cwTrace = m_dcf->GetCw ();
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   StartAccessIfNeeded ();
 }
 
@@ -1552,8 +1606,7 @@
   m_currentPacket = 0;
   m_dcf->ResetCw ();
   m_cwTrace = m_dcf->GetCw ();
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   RestartAccessIfNeeded ();
 }
 
@@ -1830,8 +1883,7 @@
   NS_LOG_FUNCTION (this);
   m_dcf->ResetCw ();
   m_cwTrace = m_dcf->GetCw ();
-  m_backoffTrace = m_rng->GetNext (0, m_dcf->GetCw ());
-  m_dcf->StartBackoffNow (m_backoffTrace);
+  StartBackoffIfNeeded ();
   ns3::Dcf::DoInitialize ();
 }
 
diff -r 10ae5ec6a903 src/wifi/model/edca-txop-n.h
--- a/src/wifi/model/edca-txop-n.h	Tue Sep 13 16:14:13 2016 -0700
+++ b/src/wifi/model/edca-txop-n.h	Thu Sep 15 13:19:51 2016 -0700
@@ -547,10 +547,20 @@
   /*
    * Check if the station has TXOP granted for the next MPDU.
    *
-   * \return true if the station has TXOP granted for the next MPDU,
+   * \return true if the station has another MPDU ready and TXOP granted for it,
    *         false otherwise
    */
   bool HasTxop (void);
+  /*
+   * Check if the station has a MPDU pending with the same TID and address
+   * as the current packet.
+   *
+   * \return true if the station has a MPDU pending with the same TID and
+   *         address as the current packet, false otherwise 
+   */
+  bool HasPendingQosMpduWithSameTidAndAddress (void) const;
+
+  void StartBackoffIfNeeded (void);
 
   AcIndex m_ac;
   class Dcf;