--- a/src/devices/wimax/bs-scheduler-rtps.cc	Fri Mar 19 20:07:09 2010 +0100
+++ b/src/devices/wimax/bs-scheduler-rtps.cc	Fri Mar 19 20:58:23 2010 +0100
@@ -226,7 +226,6 @@
       packet = connection->GetQueue ()->Peek (hdr);
       nrSymbolsRequired = GetBs ()->GetPhy ()->GetNrSymbols (packet->GetSize (), modulationType);
 
-      // PIRO: check for fragmentation
       if (availableSymbols < nrSymbolsRequired
           && !CheckForFragmentation (connection, availableSymbols, modulationType))
         {
@@ -460,15 +459,18 @@
 
           nrSymbolsRequired = connection->GetServiceFlow ()->GetRecord ()->GetGrantSize ();
 
-          // PIRO: packet fragmentation for UGS connections has not been implemented yet!
+          // Packet fragmentation for UGS connections has not been implemented yet!
           if (availableSymbols > nrSymbolsRequired)
             {
               availableSymbols -= nrSymbolsRequired;
               burst = CreateUgsBurst (connection->GetServiceFlow (), modulationType, nrSymbolsRequired);
-              AddDownlinkBurst (connection, diuc, modulationType, burst);
-              currentTime = Simulator::Now ();
-              serviceFlowRecord->SetDlTimeStamp (currentTime);
-              burst = Create<PacketBurst> ();
+              if (burst->GetNPackets () != 0)
+                {
+                  AddDownlinkBurst (connection, diuc, modulationType, burst);
+                  currentTime = Simulator::Now ();
+                  serviceFlowRecord->SetDlTimeStamp (currentTime);
+                  burst = Create<PacketBurst> ();
+                }
             }
         }
     }
@@ -562,6 +564,7 @@
   // Downlink Bandwidth Allocation
   for (int i = 0; i < nbConnection; i++)
     {
+
       packet = rtPSConnection[i]->GetQueue ()->Peek (hdr);
       uint32_t symbolsForPacketTransmission = 0;
       burst = Create<PacketBurst> ();

--- a/src/devices/wimax/packet-burst.cc	Fri Mar 19 20:07:09 2010 +0100
+++ b/src/devices/wimax/packet-burst.cc	Fri Mar 19 20:58:23 2010 +0100
@@ -76,7 +76,10 @@
 void
 PacketBurst::AddPacket (Ptr<Packet> packet)
 {
-  m_packets.push_back (packet);
+  if (packet)
+    {
+      m_packets.push_back (packet);
+    }
 }
 
 std::list<Ptr<Packet> >