diff -r bb1eea10412f src/devices/wifi/dcf-manager.cc
--- a/src/devices/wifi/dcf-manager.cc	Tue Nov 24 10:45:08 2009 +0100
+++ b/src/devices/wifi/dcf-manager.cc	Tue Nov 24 12:26:02 2009 +0100
@@ -243,7 +243,7 @@
     m_lastSwitchingStart (MicroSeconds (0)), 
     m_lastSwitchingDuration (MicroSeconds (0)), 
     m_rxing (false),
-    m_slotTime (Seconds (0.0)),
+    m_slotTimeUs (0),
     m_sifs (Seconds (0.0)),
     m_phyListener (0),
     m_lowListener (0)
@@ -273,7 +273,7 @@
 void 
 DcfManager::SetSlot (Time slotTime)
 {
-  m_slotTime = slotTime;
+  m_slotTimeUs = slotTime.GetMicroSeconds ();
 }
 void 
 DcfManager::SetSifs (Time sifs)
@@ -497,7 +497,7 @@
 DcfManager::GetBackoffStartFor (DcfState *state)
 {
   Time mostRecentEvent = MostRecent (state->GetBackoffStart (),
-                                     GetAccessGrantStart () + Scalar (state->GetAifsn ()) * m_slotTime);
+                                     GetAccessGrantStart () + MicroSeconds (state->GetAifsn () * m_slotTimeUs));
   
   return mostRecentEvent;
 }
@@ -505,7 +505,7 @@
 Time
 DcfManager::GetBackoffEndFor (DcfState *state)
 {
-  return GetBackoffStartFor (state) + Scalar (state->GetBackoffSlots ()) * m_slotTime;
+  return GetBackoffStartFor (state) + MicroSeconds (state->GetBackoffSlots () * m_slotTimeUs);
 }
 
 void
@@ -519,11 +519,11 @@
       Time backoffStart = GetBackoffStartFor (state);
       if (backoffStart <= Simulator::Now ())
         {
-          Scalar nSlots = (Simulator::Now () - backoffStart) / m_slotTime;
-          uint32_t nIntSlots = lrint (nSlots.GetDouble ());          
+          uint32_t nus = (Simulator::Now () - backoffStart).GetMicroSeconds ();
+          uint32_t nIntSlots = nus / m_slotTimeUs;
           uint32_t n = std::min (nIntSlots, state->GetBackoffSlots ());
           MY_DEBUG ("dcf " << k << " dec backoff slots=" << n);
-          Time backoffUpdateBound = backoffStart + Scalar (n) * m_slotTime;
+          Time backoffUpdateBound = backoffStart + MicroSeconds (n * m_slotTimeUs);
           state->UpdateBackoffSlotsNow (n, backoffUpdateBound);
         }
     }
diff -r bb1eea10412f src/devices/wifi/dcf-manager.h
--- a/src/devices/wifi/dcf-manager.h	Tue Nov 24 10:45:08 2009 +0100
+++ b/src/devices/wifi/dcf-manager.h	Tue Nov 24 12:26:02 2009 +0100
@@ -323,7 +323,7 @@
   bool m_sleeping;
   Time m_eifsNoDifs;
   EventId m_accessTimeout;
-  Time m_slotTime;
+  uint32_t m_slotTimeUs;
   Time m_sifs;
   class PhyListener *m_phyListener;
   class LowDcfListener *m_lowListener;