wifi: (fixes #1205) Make EDCA more standard compliant

RELEASE_NOTES

@@ -55,6 +55,7 @@ New user-visible features
 Bugs fixed
 ----------
 - Bug 1132 - useless for loops in block-ack-test-suite.cc
+- Bug 1205 - EDCA is incorrectly modelled as DCF
 - Bug 1571 - TCP zero-window and flow control window updates by the receiver
 - Bug 1761 - Rounding with olsr::EmfToSeconds
 - Bug 1954 - Serialized size of wifi-net-device differ for TX and RX trace

src/wifi/model/dca-txop.cc

@@ -48,7 +48,10 @@ public:
     : m_txop (txop)
   {
   }
-
+  virtual bool IsEdca (void) const
+  {
+    return false;
+  }
 private:
   virtual void DoNotifyAccessGranted (void)
   {

src/wifi/model/dcf-manager.cc

@@ -645,6 +645,21 @@ DcfManager::UpdateBackoff (void)
         {
           uint32_t nus = (Simulator::Now () - backoffStart).GetMicroSeconds ();
           uint32_t nIntSlots = nus / m_slotTimeUs;
+          /*
+           * EDCA behaves slightly different to DCA. For EDCA we
+           * decrement once at the slot boundary at the end of AIFS as
+           * well as once at the end of each clear slot
+           * thereafter. For DCA we only decrement at the end of each
+           * clear slot after DIFS. We account for the extra backoff
+           * by incrementing the slot count here in the case of
+           * EDCA. The if statement whose body we are in has confirmed
+           * that a minimum of AIFS has elapsed since last busy
+           * medium.
+           */
+          if (state->IsEdca ())
+            {
+              nIntSlots++;
+            }
           uint32_t n = std::min (nIntSlots, state->GetBackoffSlots ());
           MY_DEBUG ("dcf " << k << " dec backoff slots=" << n);
           Time backoffUpdateBound = backoffStart + MicroSeconds (n * m_slotTimeUs);

src/wifi/model/dcf-manager.h

@@ -49,6 +49,16 @@ public:
   DcfState ();
   virtual ~DcfState ();
 
+  /**
+   * \return whether this DCF state is an EDCA state
+   *
+   * This method, which must be overridden in derived classes,
+   * indicates whether DCF or EDCAF rules should be used for this
+   * channel access function. This affects the behavior of DcfManager
+   * when dealing with this instance. 
+   */
+  virtual bool IsEdca (void) const = 0;
+
   /**
    * \param aifsn the number of slots which make up an AIFS for a specific DCF.
    *        a DIFS corresponds to an AIFSN = 2.

src/wifi/model/edca-txop-n.cc

@@ -51,6 +51,11 @@ public:
   {
   }
 
+  virtual bool IsEdca (void) const
+  {
+    return true;
+  }
+
 private:
   virtual void DoNotifyAccessGranted (void)
   {

src/wifi/test/dcf-manager-test.cc

@@ -31,6 +31,7 @@ class DcfStateTest : public DcfState
 public:
   DcfStateTest (DcfManagerTest *test, uint32_t i);
   void QueueTx (uint64_t txTime, uint64_t expectedGrantTime);
+  bool IsEdca (void) const;
 
 
 private:
@@ -113,6 +114,12 @@ DcfStateTest::DcfStateTest (DcfManagerTest *test, uint32_t i)
 {
 }
 
+bool
+DcfStateTest::IsEdca (void) const
+{
+  return false;
+}
+
 void
 DcfStateTest::QueueTx (uint64_t txTime, uint64_t expectedGrantTime)
 {