wifi: Schedule channel access request upon enqueuing a packet

This allows us to request channel access after that all the packets of a burst
are enqueued, instead of requesting channel access right after the first packet

src/wifi/model/txop.cc

@@ -545,7 +545,14 @@ Txop::Queue(Ptr<WifiMpdu> mpdu)
     m_queue->Enqueue(mpdu);
     for (const auto linkId : linkIds)
     {
-        StartAccessIfNeeded(linkId);
+        // schedule a call to StartAccessIfNeeded() to request channel access after that all the
+        // packets of a burst have been enqueued, instead of requesting channel access right after
+        // the first packet. The call to StartAccessIfNeeded() is scheduled only after the first
+        // packet
+        if (auto& event = GetLink(linkId).accessRequest.event; !event.IsRunning())
+        {
+            event = Simulator::ScheduleNow(&Txop::StartAccessIfNeeded, this, linkId);
+        }
     }
 }
 

src/wifi/model/txop.h

@@ -509,6 +509,12 @@ class Txop : public Object
         uint8_t aifsn{0};                          //!< the AIFSN
         Time txopLimit{0};                         //!< the TXOP limit time
         ChannelAccessStatus access{NOT_REQUESTED}; //!< channel access status
+
+        mutable class
+        {
+            friend void Txop::Queue(Ptr<WifiMpdu>);
+            EventId event;
+        } accessRequest; //!< access request event, to be used by Txop::Queue() only
     };
 
     /**