wifi: Enable ChannelAccessManager to send alerts before backoff expiration

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

@@ -185,7 +185,20 @@ ChannelAccessManager::GetTypeId()
                           "and subsequently the medium becomes busy.",
                           BooleanValue(false),
                           MakeBooleanAccessor(&ChannelAccessManager::m_proactiveBackoff),
-                          MakeBooleanChecker());
+                          MakeBooleanChecker())
+            .AddAttribute("NSlotsLeft",
+                          "Fire the NSlotsLeftAlert trace source when the backoff counter with "
+                          "the minimum value among all ACs reaches this value or it is started "
+                          "with a value less than this attribute. If this value is zero, the "
+                          "trace source is never fired.",
+                          UintegerValue(0),
+                          MakeUintegerAccessor(&ChannelAccessManager::m_nSlotsLeft),
+                          MakeUintegerChecker<uint8_t>())
+            .AddTraceSource("NSlotsLeftAlert",
+                            "The backoff counter of the AC with the given index reached the "
+                            "threshold set through the NSlotsLeft attribute.",
+                            MakeTraceSourceAccessor(&ChannelAccessManager::m_nSlotsLeftCallback),
+                            "ns3::ChannelAccessManager::NSlotsLeftCallback");
     return tid;
 }
 
@@ -754,26 +767,43 @@ ChannelAccessManager::DoRestartAccessTimeoutIfNeeded()
      * Is there a Txop which needs to access the medium, and,
      * if there is one, how many slots for AIFS+backoff does it require ?
      */
-    bool accessTimeoutNeeded = false;
+    Ptr<Txop> nextTxop;
     auto expectedBackoffEnd = Simulator::GetMaximumSimulationTime();
     const auto accessGrantStart = GetAccessGrantStart();
+    const auto now = Simulator::Now();
     for (auto txop : m_txops)
     {
         if (txop->GetAccessStatus(m_linkId) == Txop::REQUESTED)
         {
-            Time tmp = GetBackoffEndFor(txop, accessGrantStart);
-            if (tmp > Simulator::Now())
+            if (auto backoffEnd = GetBackoffEndFor(txop, accessGrantStart);
+                backoffEnd > now && backoffEnd < expectedBackoffEnd)
             {
-                accessTimeoutNeeded = true;
-                expectedBackoffEnd = std::min(expectedBackoffEnd, tmp);
+                expectedBackoffEnd = backoffEnd;
+                nextTxop = txop;
             }
         }
     }
-    NS_LOG_DEBUG("Access timeout needed: " << accessTimeoutNeeded);
-    if (accessTimeoutNeeded)
+    NS_LOG_DEBUG("Access timeout needed: " << (nextTxop != nullptr));
+    if (nextTxop)
     {
-        NS_LOG_DEBUG("expected backoff end=" << expectedBackoffEnd);
-        Time expectedBackoffDelay = expectedBackoffEnd - Simulator::Now();
+        const auto aci = nextTxop->GetWifiMacQueue()->GetAc();
+        NS_LOG_DEBUG("expected backoff end=" << expectedBackoffEnd << " by " << aci);
+        auto expectedBackoffDelay = expectedBackoffEnd - now;
+
+        if (m_nSlotsLeft > 0)
+        {
+            if (const auto slots = m_nSlotsLeft * GetSlot(); expectedBackoffDelay > slots)
+            {
+                // make the timer expire when the specified number of slots are left
+                expectedBackoffDelay -= slots;
+            }
+            else
+            {
+                // notify that a number of slots less than or equal to the specified value are left
+                m_nSlotsLeftCallback(m_linkId, aci, expectedBackoffDelay);
+            }
+        }
+
         if (m_accessTimeout.IsPending() &&
             Simulator::GetDelayLeft(m_accessTimeout) > expectedBackoffDelay)
         {

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

@@ -15,6 +15,7 @@
 #include "ns3/event-id.h"
 #include "ns3/nstime.h"
 #include "ns3/object.h"
+#include "ns3/traced-callback.h"
 
 #include <algorithm>
 #include <map>
@@ -32,6 +33,7 @@ class WifiPhy;
 class PhyListener;
 class Txop;
 class FrameExchangeManager;
+enum AcIndex : uint8_t; // opaque enum declaration
 
 /**
  * \brief Manage a set of ns3::Txop
@@ -501,6 +503,23 @@ class ChannelAccessManager : public Object
     Ptr<WifiPhy> m_phy;                    //!< pointer to the unique active PHY
     Ptr<FrameExchangeManager> m_feManager; //!< pointer to the Frame Exchange Manager
     uint8_t m_linkId;                      //!< the ID of the link this object is associated with
+    uint8_t m_nSlotsLeft;                  //!< fire the NSlotsLeftAlert trace source when the
+                                           //!< backoff counter with the minimum value among all
+                                           //!< ACs reaches this value
+
+    /**
+     * TracedCallback signature for NSlotsLeft alerts.
+     *
+     * \param linkId the ID of this link
+     * \param aci the index of the AC that triggered the NSlotsLeft alert
+     * \param backoffDelay delay until backoff counts down to zero
+     */
+    typedef void (*NSlotsLeftCallback)(uint8_t linkId, AcIndex aci, const Time& backoffDelay);
+
+    /// TracedCallback for NSlotsLeft alerts typedef
+    using NSlotsLeftTracedCallback = TracedCallback<uint8_t, AcIndex, const Time&>;
+
+    NSlotsLeftTracedCallback m_nSlotsLeftCallback; //!< traced callback for NSlotsLeft alerts
 };
 
 } // namespace ns3