wifi: Update backoff before checking for immediate access

src/wifi/model/channel-access-manager.cc

@@ -242,6 +242,56 @@ ChannelAccessManager::IsBusy (void) const
   return false;
 }
 
+bool
+ChannelAccessManager::NeedBackoffUponAccess (Ptr<Txop> txop)
+{
+  NS_LOG_FUNCTION (this << txop);
+
+  // the Txop might have a stale value of remaining backoff slots
+  UpdateBackoff ();
+
+  /*
+   * From section 10.3.4.2 "Basic access" of IEEE 802.11-2016:
+   *
+   * A STA may transmit an MPDU when it is operating under the DCF access
+   * method, either in the absence of a PC, or in the CP of the PCF access
+   * method, when the STA determines that the medium is idle when a frame is
+   * queued for transmission, and remains idle for a period of a DIFS, or an
+   * EIFS (10.3.2.3.7) from the end of the immediately preceding medium-busy
+   * event, whichever is the greater, and the backoff timer is zero. Otherwise
+   * the random backoff procedure described in 10.3.4.3 shall be followed.
+   *
+   * From section 10.22.2.2 "EDCA backoff procedure" of IEEE 802.11-2016:
+   *
+   * The backoff procedure shall be invoked by an EDCAF when any of the following
+   * events occurs:
+   * a) An MA-UNITDATA.request primitive is received that causes a frame with that AC
+   *    to be queued for transmission such that one of the transmit queues associated
+   *    with that AC has now become non-empty and any other transmit queues
+   *    associated with that AC are empty; the medium is busy on the primary channel
+   */
+  if (!txop->HasFramesToTransmit () && !txop->GetLow ()->IsCfPeriod () && txop->GetBackoffSlots () == 0)
+    {
+      if (!IsBusy ())
+        {
+          // medium idle. If this is a DCF, use immediate access (we can transmit
+          // in a DIFS if the medium remains idle). If this is an EDCAF, update
+          // the backoff start time kept by the EDCAF to the current time in order
+          // to correctly align the backoff start time at the next slot boundary
+          // (performed by the next call to ChannelAccessManager::RequestAccess())
+          Time delay = (txop->IsQosTxop () ? Seconds (0)
+                                           : m_sifs + txop->GetAifsn () * m_slot);
+          txop->UpdateBackoffSlotsNow (0, Simulator::Now () + delay);
+        }
+      else
+        {
+          // medium busy, backoff is neeeded
+          return true;
+        }
+    }
+  return false;
+}
+
 void
 ChannelAccessManager::RequestAccess (Ptr<Txop> state, bool isCfPeriod)
 {

src/wifi/model/channel-access-manager.h

@@ -112,6 +112,15 @@ public:
    */
   void Add (Ptr<Txop> dcf);
 
+  /**
+   * Determine if a new backoff needs to be generated when a packet is queued
+   * for transmission.
+   *
+   * \param txop the Txop requesting to generate a backoff
+   * \return true if backoff needs to be generated, false otherwise
+   */
+  bool NeedBackoffUponAccess (Ptr<Txop> txop);
+
   /**
    * \param state a Txop
    * \param isCfPeriod flag whether it is called during the CF period

src/wifi/model/txop.cc

@@ -312,50 +312,6 @@ Txop::HasFramesToTransmit (void)
   return ret;
 }
 
-void
-Txop::GenerateBackoffUponAccessIfNeeded (void)
-{
-  NS_LOG_FUNCTION (this);
-  /*
-   * From section 10.3.4.2 "Basic access" of IEEE 802.11-2016:
-   *
-   * A STA may transmit an MPDU when it is operating under the DCF access
-   * method, either in the absence of a PC, or in the CP of the PCF access
-   * method, when the STA determines that the medium is idle when a frame is
-   * queued for transmission, and remains idle for a period of a DIFS, or an
-   * EIFS (10.3.2.3.7) from the end of the immediately preceding medium-busy
-   * event, whichever is the greater, and the backoff timer is zero. Otherwise
-   * the random backoff procedure described in 10.3.4.3 shall be followed.
-   *
-   * From section 10.22.2.2 "EDCA backoff procedure" of IEEE 802.11-2016:
-   *
-   * The backoff procedure shall be invoked by an EDCAF when any of the following
-   * events occurs:
-   * a) An MA-UNITDATA.request primitive is received that causes a frame with that AC
-   *    to be queued for transmission such that one of the transmit queues associated
-   *    with that AC has now become non-empty and any other transmit queues
-   *    associated with that AC are empty; the medium is busy on the primary channel
-   */
-  if (!HasFramesToTransmit () && !m_low->IsCfPeriod () && m_backoffSlots == 0)
-    {
-      if (!m_channelAccessManager->IsBusy ())
-        {
-          // medium idle. If this is a DCF, use immediate access (we can transmit
-          // in a DIFS if the medium remains idle). If this is an EDCAF, update
-          // the backoff start time kept by the EDCAF to the current time in order
-          // to correctly align the backoff start time at the next slot boundary
-          // (performed by the next call to ChannelAccessManager::RequestAccess())
-          Time delay = (IsQosTxop () ? Seconds (0) : m_low->GetSifs () + GetAifsn () * m_low->GetSlotTime ());
-          UpdateBackoffSlotsNow (0, Simulator::Now () + delay);
-        }
-      else
-        {
-          // medium busy, generate backoff
-          GenerateBackoff ();
-        }
-    }
-}
-
 void
 Txop::Queue (Ptr<const Packet> packet, const WifiMacHeader &hdr)
 {
@@ -365,7 +321,10 @@ Txop::Queue (Ptr<const Packet> packet, const WifiMacHeader &hdr)
   SocketPriorityTag priorityTag;
   packetCopy->RemovePacketTag (priorityTag);
   m_stationManager->PrepareForQueue (hdr.GetAddr1 (), &hdr, packetCopy);
-  GenerateBackoffUponAccessIfNeeded ();
+  if (m_channelAccessManager->NeedBackoffUponAccess (this))
+    {
+      GenerateBackoff ();
+    }
   m_queue->Enqueue (Create<WifiMacQueueItem> (packetCopy, hdr));
   StartAccessIfNeeded ();
 }

src/wifi/model/txop.h

@@ -376,10 +376,6 @@ protected:
    * \return true if the DCF has frames to transmit.
    */
   virtual bool HasFramesToTransmit (void);
-  /**
-   * Generate a new backoff, if needed, when a packet is queued for transmission.
-   */
-  virtual void GenerateBackoffUponAccessIfNeeded (void);
   /**
    * Generate a new backoff now.
    */

src/wifi/test/channel-access-manager-test.cc

@@ -679,7 +679,10 @@ void
 ChannelAccessManagerTest<TxopType>::DoAccessRequest (uint64_t txTime, uint64_t expectedGrantTime,
                                                      Ptr<TxopTest<TxopType>> state)
 {
-  state->GenerateBackoffUponAccessIfNeeded ();
+  if (m_ChannelAccessManager->NeedBackoffUponAccess (state))
+    {
+      state->GenerateBackoff ();
+    }
   state->QueueTx (txTime, expectedGrantTime);
   m_ChannelAccessManager->RequestAccess (state);
 }

src/wifi/test/wifi-test.cc

@@ -460,7 +460,6 @@ DcfImmediateAccessBroadcastTestCase::NotifyPhyTxBegin (Ptr<const Packet> p, doub
 {
   if (m_numSentPackets == 0)
     {
-      NS_ASSERT_MSG (Simulator::Now () == Time (Seconds (1)), "Packet 0 not transmitted at 1 second");
       m_numSentPackets++;
       m_firstTransmissionTime = Simulator::Now ();
     }
@@ -536,13 +535,17 @@ DcfImmediateAccessBroadcastTestCase::DoRun (void)
   Simulator::Run ();
   Simulator::Destroy ();
 
+  // First packet is transmitted a DIFS after the packet is queued. A DIFS
+  // is 2 slots (2 * 9 = 18 us) plus a SIFS (16 us), i.e., 34 us
+  Time expectedFirstTransmissionTime = Seconds (1.0) + MicroSeconds (34);
+
   //First packet has 1408 us of transmit time.   Slot time is 9 us.
   //Backoff is 1 slots.  SIFS is 16 us.  DIFS is 2 slots = 18 us.
   //Should send next packet at 1408 us + (1 * 9 us) + 16 us + (2 * 9) us
   //1451 us after the first one.
   uint32_t expectedWait1 = 1408 + (1 * 9) + 16 + (2 * 9);
-  Time expectedSecondTransmissionTime = MicroSeconds (expectedWait1) + MilliSeconds (1000);
-  NS_TEST_ASSERT_MSG_EQ (m_firstTransmissionTime, MilliSeconds (1000), "The first transmission time not correct!");
+  Time expectedSecondTransmissionTime = expectedFirstTransmissionTime + MicroSeconds (expectedWait1);
+  NS_TEST_ASSERT_MSG_EQ (m_firstTransmissionTime, expectedFirstTransmissionTime, "The first transmission time not correct!");
 
   NS_TEST_ASSERT_MSG_EQ (m_secondTransmissionTime, expectedSecondTransmissionTime, "The second transmission time not correct!");
 }