diff -r a9796fc4a88c src/devices/wifi/dca-txop.cc
--- a/src/devices/wifi/dca-txop.cc	Thu Oct 02 16:33:58 2008 +0200
+++ b/src/devices/wifi/dca-txop.cc	Fri Oct 03 03:32:39 2008 +0200
@@ -305,12 +305,6 @@
   return station->NeedFragmentation (m_currentPacket);
 }
 
-uint32_t
-DcaTxop::GetNFragments (void)
-{
-  WifiRemoteStation *station = GetStation (m_currentHdr.GetAddr1 ());
-  return station->GetNFragments (m_currentPacket);
-}
 void
 DcaTxop::NextFragment (void)
 {
@@ -337,12 +331,19 @@
   return station->GetFragmentSize (m_currentPacket, m_fragmentNumber + 1);
 }
 
+uint32_t
+DcaTxop::GetFragmentOffset (void) 
+{
+  WifiRemoteStation *station = GetStation (m_currentHdr.GetAddr1 ());
+  return station->GetFragmentOffset (m_currentPacket, m_fragmentNumber);
+}
+
 Ptr<Packet>
 DcaTxop::GetFragmentPacket (WifiMacHeader *hdr)
 {
   *hdr = m_currentHdr;
   hdr->SetFragmentNumber (m_fragmentNumber);
-  uint32_t startOffset = m_fragmentNumber * GetFragmentSize ();
+  uint32_t startOffset = GetFragmentOffset ();
   Ptr<Packet> fragment;
   if (IsLastFragment ()) 
     {
diff -r a9796fc4a88c src/devices/wifi/dca-txop.h
--- a/src/devices/wifi/dca-txop.h	Thu Oct 02 16:33:58 2008 +0200
+++ b/src/devices/wifi/dca-txop.h	Fri Oct 03 03:32:39 2008 +0200
@@ -137,9 +137,9 @@
   bool NeedRtsRetransmission (void);
   bool NeedDataRetransmission (void);
   bool NeedFragmentation (void);
-  uint32_t GetNFragments (void);
   uint32_t GetNextFragmentSize (void);
   uint32_t GetFragmentSize (void);
+  uint32_t GetFragmentOffset (void);
   WifiRemoteStation *GetStation (Mac48Address to) const;
   bool IsLastFragment (void);
   void NextFragment (void);
diff -r a9796fc4a88c src/devices/wifi/wifi-remote-station-manager.cc
--- a/src/devices/wifi/wifi-remote-station-manager.cc	Thu Oct 02 16:33:58 2008 +0200
+++ b/src/devices/wifi/wifi-remote-station-manager.cc	Fri Oct 03 03:32:39 2008 +0200
@@ -656,6 +656,14 @@
       return GetManager ()->GetFragmentationThreshold ();
     }
 }
+uint32_t
+WifiRemoteStation::GetFragmentOffset (Ptr<const Packet> packet, uint32_t fragmentNumber)
+{
+  uint32_t nFragment = GetNFragments (packet);
+  NS_ASSERT (fragmentNumber < nFragment);
+  uint32_t fragmentOffset = fragmentNumber * GetManager ()->GetFragmentationThreshold ();
+  return fragmentOffset;
+}
 
 bool
 WifiRemoteStation::IsLastFragment (Ptr<const Packet> packet, uint32_t fragmentNumber) 
diff -r a9796fc4a88c src/devices/wifi/wifi-remote-station-manager.h
--- a/src/devices/wifi/wifi-remote-station-manager.h	Thu Oct 02 16:33:58 2008 +0200
+++ b/src/devices/wifi/wifi-remote-station-manager.h	Fri Oct 03 03:32:39 2008 +0200
@@ -228,15 +228,16 @@
   virtual bool NeedFragmentation (Ptr<const Packet> packet);
   /**
    * \param packet the packet to send
-   * \returns the number of fragments which should be used for this packet.
-   */
-  virtual uint32_t GetNFragments (Ptr<const Packet> packet);
-  /**
-   * \param packet the packet to send
    * \param fragmentNumber the fragment index of the next fragment to send (starts at zero).
    * \returns the size of the corresponding fragment.
    */
   virtual uint32_t GetFragmentSize (Ptr<const Packet> packet, uint32_t fragmentNumber);
+  /**
+   * \param packet the packet to send
+   * \param fragmentNumber the fragment index of the next fragment to send (starts at zero).
+   * \returns the offset within the original packet where this fragment starts.
+   */
+  virtual uint32_t GetFragmentOffset (Ptr<const Packet> packet, uint32_t fragmentNumber);
   /**
    * \param packet the packet to send
    * \param fragmentNumber the fragment index of the next fragment to send (starts at zero).
@@ -262,6 +263,7 @@
   virtual Ptr<WifiRemoteStationManager> GetManager (void) const = 0;
   virtual WifiMode DoGetDataMode (uint32_t size) = 0;
   virtual WifiMode DoGetRtsMode (void) = 0;
+  uint32_t GetNFragments (Ptr<const Packet> packet);
 protected:
   virtual void DoReportRtsFailed (void) = 0;
   virtual void DoReportDataFailed (void) = 0;