diff -r c1bd4ffb5e47 src/devices/wifi/dcf-manager.cc
--- a/src/devices/wifi/dcf-manager.cc	Tue Sep 29 10:16:17 2009 -0700
+++ b/src/devices/wifi/dcf-manager.cc	Wed Sep 30 01:22:41 2009 +0400
@@ -511,6 +511,7 @@
 void
 DcfManager::UpdateBackoff (void)
 {
+  long double slotNanoSeconds = m_slotTime.GetNanoSeconds ();
   uint32_t k = 0;
   for (States::const_iterator i = m_states.begin (); i != m_states.end (); i++, k++)
     {
@@ -519,8 +520,9 @@
       Time backoffStart = GetBackoffStartFor (state);
       if (backoffStart <= Simulator::Now ())
         {
-          Scalar nSlots = (Simulator::Now () - backoffStart) / m_slotTime;
-          uint32_t nIntSlots = lrint (nSlots.GetDouble ());          
+          long double inBackoffNanoSeconds = Simulator::Now ().GetNanoSeconds () - backoffStart.GetNanoSeconds ();
+          long double nSlots = inBackoffNanoSeconds / slotNanoSeconds;
+          uint32_t nIntSlots = lrintl (nSlots);
           uint32_t n = std::min (nIntSlots, state->GetBackoffSlots ());
           MY_DEBUG ("dcf " << k << " dec backoff slots=" << n);
           Time backoffUpdateBound = backoffStart + Scalar (n) * m_slotTime;