--- a/src/devices/wifi/wifi-mac-trailer.cc	Fri Jul 23 13:39:46 2010 +0200
+++ a/src/devices/wifi/wifi-mac-trailer.cc	Fri Jul 23 18:42:49 2010 +0200
@@ -51,18 +51,18 @@ 
 uint32_t 
 WifiMacTrailer::GetSerializedSize (void) const
 {
-  return 4;
+  return WIFI_MAC_FCS_LENGTH;
 }
 void 
 WifiMacTrailer::Serialize (Buffer::Iterator start) const
 {
-  start.Prev (4);
+  start.Prev (WIFI_MAC_FCS_LENGTH);
   start.WriteU32 (0);
 }
 uint32_t
 WifiMacTrailer::Deserialize (Buffer::Iterator start)
 {
-  return 4;
+  return WIFI_MAC_FCS_LENGTH;
 }
 
 } // namespace ns3
--- a/src/devices/wifi/wifi-mac-trailer.h	Fri Jul 23 13:39:46 2010 +0200
+++ a/src/devices/wifi/wifi-mac-trailer.h	Fri Jul 23 18:42:49 2010 +0200
@@ -25,6 +25,11 @@ 
 
 namespace ns3 {
 
+/** 
+ * The length in octects of the IEEE 802.11 MAC FCS field
+ */
+static const uint16_t WIFI_MAC_FCS_LENGTH = 4;
+
 class WifiMacTrailer : public Trailer 
 {
 public:
--- a/src/devices/wifi/wifi-net-device.cc	Fri Jul 23 13:39:46 2010 +0200
+++ a/src/devices/wifi/wifi-net-device.cc	Fri Jul 23 18:42:49 2010 +0200
@@ -43,7 +43,7 @@ 
     .SetParent<NetDevice> ()
     .AddConstructor<WifiNetDevice> ()
     .AddAttribute ("Mtu", "The MAC-level Maximum Transmission Unit",
-                   UintegerValue (MAX_MSDU_SIZE),
+                   UintegerValue (MAX_MSDU_SIZE - LLC_SNAP_HEADER_LENGTH),
                    MakeUintegerAccessor (&WifiNetDevice::SetMtu,
                                          &WifiNetDevice::GetMtu),
                    MakeUintegerChecker<uint16_t> (1,MAX_MSDU_SIZE))
--- a/src/devices/wifi/wifi-remote-station-manager.cc	Fri Jul 23 13:39:46 2010 +0200
+++ a/src/devices/wifi/wifi-remote-station-manager.cc	Fri Jul 23 18:42:49 2010 +0200
@@ -29,6 +29,7 @@ 
 #include "ns3/wifi-phy.h"
 #include "ns3/trace-source-accessor.h"
 #include "wifi-mac-header.h"
+#include "wifi-mac-trailer.h"
 
 NS_LOG_COMPONENT_DEFINE ("WifiRemoteStationManager");
 
@@ -151,14 +152,16 @@ 
                    UintegerValue (7),
                    MakeUintegerAccessor (&WifiRemoteStationManager::m_maxSlrc),
                    MakeUintegerChecker<uint32_t> ())
-    .AddAttribute ("RtsCtsThreshold", "If a data packet is bigger than this value, we use an RTS/CTS handshake"
-                   " before sending the data. This value will not have any effect on some rate control algorithms.",
+    .AddAttribute ("RtsCtsThreshold", "If  the size of the data packet + LLC header + MAC header + FCS trailer is bigger than "
+                   "this value, we use an RTS/CTS handshake before sending the data, as per IEEE Std. 802.11-2007, Section 9.2.6. "
+                   "This value will not have any effect on some rate control algorithms.",
                    UintegerValue (1500),
                    MakeUintegerAccessor (&WifiRemoteStationManager::m_rtsCtsThreshold),
                    MakeUintegerChecker<uint32_t> ())
-    .AddAttribute ("FragmentationThreshold", "If a data packet is bigger than this value, we fragment it such that"
-                   " the size of the fragments are equal or smaller than this value. This value will not have any effect"
-                   " on some rate control algorithms.",
+    .AddAttribute ("FragmentationThreshold", "If the size of the data packet + LLC header + MAC header + FCS trailer is bigger"
+                   "than this value, we fragment it such that the size of the fragments are equal or smaller "
+                   "than this value, as per IEEE Std. 802.11-2007, Section 9.4. "
+                   "This value will not have any effect on some rate control algorithms.",
                    UintegerValue (1500),
                    MakeUintegerAccessor (&WifiRemoteStationManager::m_fragmentationThreshold),
                    MakeUintegerChecker<uint32_t> ())
@@ -458,7 +461,7 @@ 
     {
       return false;
     }
-  bool normally = packet->GetSize () > GetRtsCtsThreshold ();
+  bool normally = (packet->GetSize () + header->GetSize () + WIFI_MAC_FCS_LENGTH) > GetRtsCtsThreshold ();
   return DoNeedRts (Lookup (address, header), packet, normally);
 }
 bool
@@ -488,7 +491,7 @@ 
       return false;
     }
   WifiRemoteStation *station = Lookup (address, header);
-  bool normally = packet->GetSize () > GetFragmentationThreshold ();
+  bool normally = (packet->GetSize () + header->GetSize () + WIFI_MAC_FCS_LENGTH) > GetFragmentationThreshold ();
   return DoNeedFragmentation (station, packet, normally);
 }
 uint32_t
--- a/src/node/llc-snap-header.cc	Fri Jul 23 13:39:46 2010 +0200
+++ a/src/node/llc-snap-header.cc	Fri Jul 23 18:42:49 2010 +0200
@@ -43,7 +43,7 @@ 
 uint32_t 
 LlcSnapHeader::GetSerializedSize (void) const
 {
-  return 1 + 1 + 1 + 3 + 2;
+  return LLC_SNAP_HEADER_LENGTH;
 }
 
 TypeId 
--- a/src/node/llc-snap-header.h	Fri Jul 23 13:39:46 2010 +0200
+++ a/src/node/llc-snap-header.h	Fri Jul 23 18:42:49 2010 +0200
@@ -27,6 +27,11 @@ 
 
 namespace ns3 {
 
+/** 
+ * The length in octects of the LLC/SNAP header
+ */
+static const uint16_t LLC_SNAP_HEADER_LENGTH = 8;
+
 /**
  * \ingroup node
  *