network: (merges !424) Fix instant send in SimpleNetDevice for empty queue

Also consolidate scheduling logic into ScheduleTransmit() rather than
repeating in both SendFrom() and TransmitComplete()

src/network/utils/simple-net-device.cc

@@ -446,39 +446,51 @@ SimpleNetDevice::SendFrom (Ptr<Packet> p, const Address& source, const Address&
 
   if (m_queue->Enqueue (p))
     {
-      if (m_queue->GetNPackets () == 1 && !TransmitCompleteEvent.IsRunning ())
+      if (m_queue->GetNPackets () == 1 && !FinishTransmissionEvent.IsRunning ())
         {
-          p = m_queue->Dequeue ();
-          p->RemovePacketTag (tag);
-          Time txTime = Time (0);
-          if (m_bps > DataRate (0))
-            {
-              txTime = m_bps.CalculateBytesTxTime (packet->GetSize ());
-            }
-          m_channel->Send (p, protocolNumber, to, from, this);
-          TransmitCompleteEvent = Simulator::Schedule (txTime, &SimpleNetDevice::TransmitComplete, this);
+          StartTransmission();
         }
       return true;
     }
 
 
-  m_channel->Send (packet, protocolNumber, to, from, this);
-  return true;
+  return false;
 }
 
-
 void
-SimpleNetDevice::TransmitComplete ()
+SimpleNetDevice::StartTransmission ()
 {
-  NS_LOG_FUNCTION (this);
-
   if (m_queue->GetNPackets () == 0)
     {
       return;
     }
-
+  NS_ASSERT_MSG (!FinishTransmissionEvent.IsRunning (),
+                 "Tried to transmit a packet while another transmission was in progress");
   Ptr<Packet> packet = m_queue->Dequeue ();
 
+  /**
+   * SimpleChannel will deliver the packet to the far end(s) of the link as soon as Send is called
+   * (or after its fixed delay, if one is configured). So we have to handle the rate of the link here,
+   * which we do by scheudling FinishTransmission (packetSize / linkRate) time in the future. While
+   * that event is running, the transmit path of this NetDevice is busy, so we can't send other packets. 
+   * 
+   * SimpleChannel doesn't have a locking mechanism, and doesn't check for collisions, so there's nothing
+   * we need to do with the channel until the transmisison has "completed" from the perspective of this
+   * NetDevice. 
+   */
+  Time txTime = Time (0);
+  if (m_bps > DataRate (0))
+    {
+      txTime = m_bps.CalculateBytesTxTime (packet->GetSize ());
+    }
+  FinishTransmissionEvent = Simulator::Schedule (txTime, &SimpleNetDevice::FinishTransmission, this, packet);
+}
+
+void
+SimpleNetDevice::FinishTransmission (Ptr<Packet> packet)
+{
+  NS_LOG_FUNCTION (this);
+
   SimpleTag tag;
   packet->RemovePacketTag (tag);
 
@@ -488,12 +500,7 @@ SimpleNetDevice::TransmitComplete ()
 
   m_channel->Send (packet, proto, dst, src, this);
 
-  Time txTime = Time (0);
-  if (m_bps > DataRate (0))
-    {
-      txTime = m_bps.CalculateBytesTxTime (packet->GetSize ());
-    }
-  TransmitCompleteEvent = Simulator::Schedule (txTime, &SimpleNetDevice::TransmitComplete, this);
+  StartTransmission();
 
   return;
 }
@@ -535,9 +542,9 @@ SimpleNetDevice::DoDispose (void)
   m_node = 0;
   m_receiveErrorModel = 0;
   m_queue->Flush ();
-  if (TransmitCompleteEvent.IsRunning ())
+  if (FinishTransmissionEvent.IsRunning ())
     {
-      TransmitCompleteEvent.Cancel ();
+      FinishTransmissionEvent.Cancel ();
     }
   NetDevice::DoDispose ();
 }

src/network/utils/simple-net-device.h

@@ -158,11 +158,13 @@ private:
    */
   TracedCallback<Ptr<const Packet> > m_phyRxDropTrace;
 
+  void StartTransmission(void);
+
   /**
-   * The TransmitComplete method is used internally to finish the process
+   * The FinishTransmission method is used internally to finish the process
    * of sending a packet out on the channel.
    */
-  void TransmitComplete (void);
+  void FinishTransmission (Ptr<Packet> packet);
 
   bool m_linkUp; //!< Flag indicating whether or not the link is up
 
@@ -174,7 +176,7 @@ private:
 
   Ptr<Queue<Packet> > m_queue; //!< The Queue for outgoing packets.
   DataRate m_bps; //!< The device nominal Data rate. Zero means infinite
-  EventId TransmitCompleteEvent; //!< the Tx Complete event
+  EventId FinishTransmissionEvent; //!< the Tx Complete event
 
   /**
    * List of callbacks to fire if the link changes state (up or down).