diff --git a/src/simulator/simulator-impl.cc b/src/simulator/default-simulator-impl.cc
similarity index 80%
rename from src/simulator/simulator-impl.cc
rename to src/simulator/default-simulator-impl.cc
index 0bfdc9995..fca228dd3 100644
--- a/src/simulator/simulator-impl.cc
+++ b/src/simulator/default-simulator-impl.cc
@@ -19,7 +19,7 @@
  */
 
 #include "simulator.h"
-#include "simulator-impl.h"
+#include "default-simulator-impl.h"
 #include "scheduler.h"
 #include "event-impl.h"
 
@@ -30,23 +30,23 @@
 
 #include <math.h>
 
-NS_LOG_COMPONENT_DEFINE ("SimulatorImpl");
+NS_LOG_COMPONENT_DEFINE ("DefaultSimulatorImpl");
 
 namespace ns3 {
 
-NS_OBJECT_ENSURE_REGISTERED (SimulatorImpl);
+NS_OBJECT_ENSURE_REGISTERED (DefaultSimulatorImpl);
 
 TypeId
-SimulatorImpl::GetTypeId (void)
+DefaultSimulatorImpl::GetTypeId (void)
 {
-  static TypeId tid = TypeId ("ns3::SimulatorImpl")
+  static TypeId tid = TypeId ("ns3::DefaultSimulatorImpl")
     .SetParent<Object> ()
-    .AddConstructor<SimulatorImpl> ()
+    .AddConstructor<DefaultSimulatorImpl> ()
     ;
   return tid;
 }
 
-SimulatorImpl::SimulatorImpl ()
+DefaultSimulatorImpl::DefaultSimulatorImpl ()
 {
   m_stop = false;
   m_stopAt = 0;
@@ -62,7 +62,7 @@ SimulatorImpl::SimulatorImpl ()
   m_unscheduledEvents = 0;
 }
 
-SimulatorImpl::~SimulatorImpl ()
+DefaultSimulatorImpl::~DefaultSimulatorImpl ()
 {
   while (!m_events->IsEmpty ())
     {
@@ -72,7 +72,7 @@ SimulatorImpl::~SimulatorImpl ()
 }
 
 void
-SimulatorImpl::Destroy ()
+DefaultSimulatorImpl::Destroy ()
 {
   while (!m_destroyEvents.empty ()) 
     {
@@ -87,7 +87,7 @@ SimulatorImpl::Destroy ()
 }
 
 void
-SimulatorImpl::SetScheduler (Ptr<Scheduler> scheduler)
+DefaultSimulatorImpl::SetScheduler (Ptr<Scheduler> scheduler)
 {
   if (m_events != 0)
     {
@@ -101,20 +101,20 @@ SimulatorImpl::SetScheduler (Ptr<Scheduler> scheduler)
 }
 
 Ptr<Scheduler>
-SimulatorImpl::GetScheduler (void) const
+DefaultSimulatorImpl::GetScheduler (void) const
 {
   return m_events;
 }
 
 void
-SimulatorImpl::EnableLogTo (char const *filename)
+DefaultSimulatorImpl::EnableLogTo (char const *filename)
 {
   m_log.open (filename);
   m_logEnable = true;
 }
 
 void
-SimulatorImpl::ProcessOneEvent (void)
+DefaultSimulatorImpl::ProcessOneEvent (void)
 {
   EventId next = m_events->RemoveNext ();
 
@@ -133,13 +133,13 @@ SimulatorImpl::ProcessOneEvent (void)
 }
 
 bool 
-SimulatorImpl::IsFinished (void) const
+DefaultSimulatorImpl::IsFinished (void) const
 {
   return m_events->IsEmpty ();
 }
 
 uint64_t
-SimulatorImpl::NextTs (void) const
+DefaultSimulatorImpl::NextTs (void) const
 {
   NS_ASSERT (!m_events->IsEmpty ());
   EventId id = m_events->PeekNext ();
@@ -147,13 +147,13 @@ SimulatorImpl::NextTs (void) const
 }
 
 Time
-SimulatorImpl::Next (void) const
+DefaultSimulatorImpl::Next (void) const
 {
   return TimeStep (NextTs ());
 }
 
 void
-SimulatorImpl::Run (void)
+DefaultSimulatorImpl::Run (void)
 {
 
   while (!m_events->IsEmpty () && !m_stop && 
@@ -170,13 +170,13 @@ SimulatorImpl::Run (void)
 }
 
 void 
-SimulatorImpl::Stop (void)
+DefaultSimulatorImpl::Stop (void)
 {
   m_stop = true;
 }
 
 void 
-SimulatorImpl::Stop (Time const &time)
+DefaultSimulatorImpl::Stop (Time const &time)
 {
   NS_ASSERT (time.IsPositive ());
   Time absolute = Simulator::Now () + time;
@@ -184,7 +184,7 @@ SimulatorImpl::Stop (Time const &time)
 }
 
 EventId
-SimulatorImpl::Schedule (Time const &time, const Ptr<EventImpl> &event)
+DefaultSimulatorImpl::Schedule (Time const &time, const Ptr<EventImpl> &event)
 {
   NS_ASSERT (time.IsPositive ());
   NS_ASSERT (time >= TimeStep (m_currentTs));
@@ -202,7 +202,7 @@ SimulatorImpl::Schedule (Time const &time, const Ptr<EventImpl> &event)
 }
 
 EventId
-SimulatorImpl::ScheduleNow (const Ptr<EventImpl> &event)
+DefaultSimulatorImpl::ScheduleNow (const Ptr<EventImpl> &event)
 {
   EventId id (event, m_currentTs, m_uid);
   if (m_logEnable) 
@@ -217,7 +217,7 @@ SimulatorImpl::ScheduleNow (const Ptr<EventImpl> &event)
 }
 
 EventId
-SimulatorImpl::ScheduleDestroy (const Ptr<EventImpl> &event)
+DefaultSimulatorImpl::ScheduleDestroy (const Ptr<EventImpl> &event)
 {
   EventId id (event, m_currentTs, 2);
   m_destroyEvents.push_back (id);
@@ -231,13 +231,13 @@ SimulatorImpl::ScheduleDestroy (const Ptr<EventImpl> &event)
 }
 
 Time
-SimulatorImpl::Now (void) const
+DefaultSimulatorImpl::Now (void) const
 {
   return TimeStep (m_currentTs);
 }
 
 Time 
-SimulatorImpl::GetDelayLeft (const EventId &id) const
+DefaultSimulatorImpl::GetDelayLeft (const EventId &id) const
 {
   if (IsExpired (id))
     {
@@ -250,7 +250,7 @@ SimulatorImpl::GetDelayLeft (const EventId &id) const
 }
 
 void
-SimulatorImpl::Remove (const EventId &ev)
+DefaultSimulatorImpl::Remove (const EventId &ev)
 {
   if (ev.GetUid () == 2)
     {
@@ -281,7 +281,7 @@ SimulatorImpl::Remove (const EventId &ev)
 }
 
 void
-SimulatorImpl::Cancel (const EventId &id)
+DefaultSimulatorImpl::Cancel (const EventId &id)
 {
   if (!IsExpired (id))
     {
@@ -290,7 +290,7 @@ SimulatorImpl::Cancel (const EventId &id)
 }
 
 bool
-SimulatorImpl::IsExpired (const EventId &ev) const
+DefaultSimulatorImpl::IsExpired (const EventId &ev) const
 {
   if (ev.GetUid () == 2)
     {
@@ -319,7 +319,7 @@ SimulatorImpl::IsExpired (const EventId &ev) const
 }
 
 Time 
-SimulatorImpl::GetMaximumSimulationTime (void) const
+DefaultSimulatorImpl::GetMaximumSimulationTime (void) const
 {
   // XXX: I am fairly certain other compilers use other non-standard
   // post-fixes to indicate 64 bit constants.
diff --git a/src/simulator/default-simulator-impl.h b/src/simulator/default-simulator-impl.h
new file mode 100644
index 000000000..a3c321018
--- /dev/null
+++ b/src/simulator/default-simulator-impl.h
@@ -0,0 +1,90 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/*
+ * Copyright (c) 2005,2006 INRIA
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Mathieu Lacage <mathieu.lacage@sophia.inria.fr>
+ */
+
+#ifndef DEFAULT_SIMULATOR_IMPL_H
+#define DEFAULT_SIMULATOR_IMPL_H
+
+#include "simulator-impl.h"
+
+#include "scheduler.h"
+#include "event-impl.h"
+
+#include "ns3/ptr.h"
+#include "ns3/assert.h"
+#include "ns3/log.h"
+
+#include <list>
+#include <fstream>
+
+namespace ns3 {
+
+  class DefaultSimulatorImpl : public SimulatorImpl
+{
+public:
+  static TypeId GetTypeId (void);
+
+  DefaultSimulatorImpl ();
+  ~DefaultSimulatorImpl ();
+
+  void Destroy ();
+
+  void EnableLogTo (char const *filename);
+
+  bool IsFinished (void) const;
+  Time Next (void) const;
+  void Stop (void);
+  void Stop (Time const &time);
+  EventId Schedule (Time const &time, const Ptr<EventImpl> &event);
+  EventId ScheduleNow (const Ptr<EventImpl> &event);
+  EventId ScheduleDestroy (const Ptr<EventImpl> &event);
+  void Remove (const EventId &ev);
+  void Cancel (const EventId &ev);
+  bool IsExpired (const EventId &ev) const;
+  void Run (void);
+  Time Now (void) const;
+  Time GetDelayLeft (const EventId &id) const;
+  Time GetMaximumSimulationTime (void) const;
+
+  void SetScheduler (Ptr<Scheduler> scheduler);
+  Ptr<Scheduler> GetScheduler (void) const;
+
+private:
+  void ProcessOneEvent (void);
+  uint64_t NextTs (void) const;
+
+  typedef std::list<EventId> DestroyEvents;
+  DestroyEvents m_destroyEvents;
+  uint64_t m_stopAt;
+  bool m_stop;
+  Ptr<Scheduler> m_events;
+  uint32_t m_uid;
+  uint32_t m_currentUid;
+  uint64_t m_currentTs;
+  std::ofstream m_log;
+  std::ifstream m_inputLog;
+  bool m_logEnable;
+  // number of events that have been inserted but not yet scheduled,
+  // not counting the "destroy" events; this is used for validation
+  int m_unscheduledEvents;
+};
+
+} // namespace ns3
+
+#endif /* DEFAULT_SIMULATOR_IMPL_H */
diff --git a/src/simulator/simulator-impl.h b/src/simulator/simulator-impl.h
index f72a71952..86b871d58 100644
--- a/src/simulator/simulator-impl.h
+++ b/src/simulator/simulator-impl.h
@@ -23,6 +23,7 @@
 
 #include "scheduler.h"
 #include "event-impl.h"
+#include "nstime.h"
 
 #include "ns3/ptr.h"
 #include "ns3/assert.h"
@@ -36,51 +37,24 @@ namespace ns3 {
 class SimulatorImpl : public Object
 {
 public:
-  static TypeId GetTypeId (void);
-
-  SimulatorImpl ();
-  ~SimulatorImpl ();
-
-  void Destroy ();
-
-  void EnableLogTo (char const *filename);
-
-  bool IsFinished (void) const;
-  Time Next (void) const;
-  void Stop (void);
-  void Stop (Time const &time);
-  EventId Schedule (Time const &time, const Ptr<EventImpl> &event);
-  EventId ScheduleNow (const Ptr<EventImpl> &event);
-  EventId ScheduleDestroy (const Ptr<EventImpl> &event);
-  void Remove (const EventId &ev);
-  void Cancel (const EventId &ev);
-  bool IsExpired (const EventId &ev) const;
-  void Run (void);
-  Time Now (void) const;
-  Time GetDelayLeft (const EventId &id) const;
-  Time GetMaximumSimulationTime (void) const;
-
-  void SetScheduler (Ptr<Scheduler> scheduler);
-  Ptr<Scheduler> GetScheduler (void) const;
-
-private:
-  void ProcessOneEvent (void);
-  uint64_t NextTs (void) const;
-
-  typedef std::list<EventId> DestroyEvents;
-  DestroyEvents m_destroyEvents;
-  uint64_t m_stopAt;
-  bool m_stop;
-  Ptr<Scheduler> m_events;
-  uint32_t m_uid;
-  uint32_t m_currentUid;
-  uint64_t m_currentTs;
-  std::ofstream m_log;
-  std::ifstream m_inputLog;
-  bool m_logEnable;
-  // number of events that have been inserted but not yet scheduled,
-  // not counting the "destroy" events; this is used for validation
-  int m_unscheduledEvents;
+  virtual void Destroy () = 0;
+  virtual void EnableLogTo (char const *filename) = 0;
+  virtual bool IsFinished (void) const = 0;
+  virtual Time Next (void) const = 0;
+  virtual void Stop (void) = 0;
+  virtual void Stop (Time const &time) = 0;
+  virtual EventId Schedule (Time const &time, const Ptr<EventImpl> &event) = 0;
+  virtual EventId ScheduleNow (const Ptr<EventImpl> &event) = 0;
+  virtual EventId ScheduleDestroy (const Ptr<EventImpl> &event) = 0;
+  virtual void Remove (const EventId &ev) = 0;
+  virtual void Cancel (const EventId &ev) = 0;
+  virtual bool IsExpired (const EventId &ev) const = 0;
+  virtual void Run (void) = 0;
+  virtual Time Now (void) const = 0;
+  virtual Time GetDelayLeft (const EventId &id) const = 0;
+  virtual Time GetMaximumSimulationTime (void) const = 0;
+  virtual void SetScheduler (Ptr<Scheduler> scheduler) = 0;
+  virtual Ptr<Scheduler> GetScheduler (void) const = 0;
 };
 
 } // namespace ns3
diff --git a/src/simulator/simulator.cc b/src/simulator/simulator.cc
index 7d20ec4d9..2abaf218a 100644
--- a/src/simulator/simulator.cc
+++ b/src/simulator/simulator.cc
@@ -20,12 +20,15 @@
 
 #include "simulator.h"
 #include "simulator-impl.h"
+#include "default-simulator-impl.h"
 #include "scheduler.h"
 #include "map-scheduler.h"
 #include "event-impl.h"
 
 #include "ns3/ptr.h"
-#include "ns3/pointer.h"
+#include "ns3/string.h"
+#include "ns3/object-factory.h"
+#include "ns3/global-value.h"
 #include "ns3/assert.h"
 #include "ns3/log.h"
 
@@ -39,38 +42,56 @@ NS_LOG_COMPONENT_DEFINE ("Simulator");
 
 namespace ns3 {
 
+GlobalValue g_simTypeImpl = GlobalValue ("SimulatorImplementationType", 
+  "The object class to use as the simulator implementation",
+  StringValue ("ns3::DefaultSimulatorImpl"),
+  MakeStringChecker ());
+
+Ptr<SimulatorImpl> Simulator::m_impl = 0;
+
 #ifdef NS3_LOG_ENABLE
+
+//
+// Note:  Calls that take TimePrinter as a parameter are defined as nothing
+// in the logging module if NS3_LOG_ENABLE is not defined.
+// 
+
 static void
 TimePrinter (std::ostream &os)
 {
   os << Simulator::Now ();
 }
+
 #endif /* NS3_LOG_ENABLE */
 
-Ptr<SimulatorImpl> Simulator::m_impl = 0;
-
-Ptr<SimulatorImpl>
+SimulatorImpl *
 Simulator::GetImpl (void)
 {
   NS_LOG_FUNCTION_NOARGS ();
 
   if (m_impl == 0) 
     {
-      /* Note: we call LogSetTimePrinter below _after_ calling CreateObject 
-       * because CreateObject can trigger calls to the logging framework which
-       * would call the TimePrinter function above which would call 
-       * Simulator::Now which would call Simulator::GetImpl, and, thus, get 
-       * us in an infinite recursion until the stack explodes.
-       */
-      m_impl = CreateObject<SimulatorImpl> ();
+      ObjectFactory factory;
+      StringValue s;
+
+      g_simTypeImpl.GetValue (s);
+      factory.SetTypeId (s.Get ());
+      m_impl = factory.Create<SimulatorImpl> ();
 
       Ptr<Scheduler> scheduler = CreateObject<MapScheduler> ();
       m_impl->SetScheduler (scheduler);
 
+//
+// Note: we call LogSetTimePrinter _after_ creating the implementation
+// object because the act of creation can trigger calls to the logging 
+// framework which would call the TimePrinter function which would call 
+// Simulator::Now which would call Simulator::GetImpl, and, thus, get us 
+// in an infinite recursion until the stack explodes.
+//
       LogSetTimePrinter (&TimePrinter);
     }
   NS_LOG_LOGIC ("priv " << m_impl);
-  return m_impl;
+  return PeekPointer (m_impl);
 }
 
 void
diff --git a/src/simulator/simulator.h b/src/simulator/simulator.h
index 5ceaf27e8..8b833dede 100644
--- a/src/simulator/simulator.h
+++ b/src/simulator/simulator.h
@@ -21,18 +21,19 @@
 #ifndef SIMULATOR_H
 #define SIMULATOR_H
 
-#include <stdint.h>
 #include "event-id.h"
 #include "event-impl.h"
 #include "nstime.h"
 #include "scheduler.h"
+
 #include "ns3/type-traits.h"
 
+#include <stdint.h>
+#include <string>
+
 namespace ns3 {
 
-
 class SimulatorImpl;
-class SchedulerFactory;
 
 /**
  * \ingroup simulator
@@ -55,6 +56,20 @@ class SchedulerFactory;
 class Simulator 
 {
 public:
+  /**
+   * \param impl a new simulator implementation
+   *
+   * The simulator provides a mechanism to swap out different implementations.
+   * For example, the default implementation is a single-threaded simulator
+   * that performs no realtime synchronization.  By calling this method, you
+   * can substitute in a new simulator implementation that might be multi-
+   * threaded and synchronize events to a realtime clock.
+   *
+   * The simulator implementation can be set when the simulator is not 
+   * running.
+   */
+  static void SetImplementation (Ptr<SimulatorImpl> impl);
+
   /**
    * \param scheduler a new event scheduler
    *
@@ -91,7 +106,6 @@ public:
    */
   static void Destroy (void);
 
-
   /**
    * If there any any events lefts to be scheduled, return
    * true. Return false otherwise.
@@ -113,12 +127,14 @@ public:
    *     is greater than or equal to the stop time.
    */
   static void Run (void);
+
   /**
    * If an event invokes this method, it will be the last
    * event scheduled by the Simulator::run method before
    * returning to the caller.
    */
   static void Stop (void);
+
   /**
    * Force the Simulator::run method to return to the caller when the
    * expiration time of the next event to be processed is greater than
@@ -140,6 +156,7 @@ public:
    */
   template <typename MEM, typename OBJ>
   static EventId Schedule (Time const &time, MEM mem_ptr, OBJ obj);
+
   /**
    * @param time the relative expiration time of the event.
    * @param mem_ptr member method pointer to invoke
@@ -149,6 +166,7 @@ public:
    */
   template <typename MEM, typename OBJ, typename T1>
   static EventId Schedule (Time const &time, MEM mem_ptr, OBJ obj, T1 a1);
+
   /**
    * @param time the relative expiration time of the event.
    * @param mem_ptr member method pointer to invoke
@@ -159,6 +177,7 @@ public:
    */
   template <typename MEM, typename OBJ, typename T1, typename T2>
   static EventId Schedule (Time const &time, MEM mem_ptr, OBJ obj, T1 a1, T2 a2);
+
   /**
    * @param time the relative expiration time of the event.
    * @param mem_ptr member method pointer to invoke
@@ -171,6 +190,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1, typename T2, typename T3>
   static EventId Schedule (Time const &time, MEM mem_ptr, OBJ obj, T1 a1, T2 a2, T3 a3);
+
   /**
    * @param time the relative expiration time of the event.
    * @param mem_ptr member method pointer to invoke
@@ -184,6 +204,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1, typename T2, typename T3, typename T4>
   static EventId Schedule (Time const &time, MEM mem_ptr, OBJ obj, T1 a1, T2 a2, T3 a3, T4 a4);
+
   /**
    * @param time the relative expiration time of the event.
    * @param mem_ptr member method pointer to invoke
@@ -205,6 +226,7 @@ public:
    * @returns an id for the scheduled event.
    */
   static EventId Schedule (Time const &time, void (*f) (void));
+
   /**
    * @param time the relative expiration time of the event.
    * @param f the function to invoke
@@ -213,6 +235,7 @@ public:
    */
   template <typename U1, typename T1>
   static EventId Schedule (Time const &time, void (*f) (U1), T1 a1);
+
   /**
    * @param time the relative expiration time of the event.
    * @param f the function to invoke
@@ -222,6 +245,7 @@ public:
    */
   template <typename U1, typename U2, typename T1, typename T2>
   static EventId Schedule (Time const &time, void (*f) (U1,U2), T1 a1, T2 a2);
+
   /**
    * @param time the relative expiration time of the event.
    * @param f the function to invoke
@@ -232,6 +256,7 @@ public:
    */
   template <typename U1, typename U2, typename U3, typename T1, typename T2, typename T3>
   static EventId Schedule (Time const &time, void (*f) (U1,U2,U3), T1 a1, T2 a2, T3 a3);
+
   /**
    * @param time the relative expiration time of the event.
    * @param f the function to invoke
@@ -244,6 +269,7 @@ public:
   template <typename U1, typename U2, typename U3, typename U4, 
             typename T1, typename T2, typename T3, typename T4>
   static EventId Schedule (Time const &time, void (*f) (U1,U2,U3,U4), T1 a1, T2 a2, T3 a3, T4 a4);
+
   /**
    * @param time the relative expiration time of the event.
    * @param f the function to invoke
@@ -258,7 +284,6 @@ public:
             typename T1, typename T2, typename T3, typename T4, typename T5>
   static EventId Schedule (Time const &time, void (*f) (U1,U2,U3,U4,U5), T1 a1, T2 a2, T3 a3, T4 a4, T5 a5);
 
-
   /**
    * Schedule an event to expire Now. All events scheduled to
    * to expire "Now" are scheduled FIFO, after all normal events
@@ -269,6 +294,7 @@ public:
    */
   template <typename MEM, typename OBJ>
   static EventId ScheduleNow (MEM mem_ptr, OBJ obj);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -277,6 +303,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1>
   static EventId ScheduleNow (MEM mem_ptr, OBJ obj, T1 a1);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -286,6 +313,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1, typename T2>
   static EventId ScheduleNow (MEM mem_ptr, OBJ obj, T1 a1, T2 a2);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -296,6 +324,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1, typename T2, typename T3>
   static EventId ScheduleNow (MEM mem_ptr, OBJ obj, T1 a1, T2 a2, T3 a3);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -325,6 +354,7 @@ public:
    * @param f the function to invoke
    */
   static EventId ScheduleNow (void (*f) (void));
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -332,6 +362,7 @@ public:
   template <typename U1,
             typename T1>
   static EventId ScheduleNow (void (*f) (U1), T1 a1);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -340,6 +371,7 @@ public:
   template <typename U1, typename U2,
             typename T1, typename T2>
   static EventId ScheduleNow (void (*f) (U1,U2), T1 a1, T2 a2);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -349,6 +381,7 @@ public:
   template <typename U1, typename U2, typename U3,
             typename T1, typename T2, typename T3>
   static EventId ScheduleNow (void (*f) (U1,U2,U3), T1 a1, T2 a2, T3 a3);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -359,6 +392,7 @@ public:
   template <typename U1, typename U2, typename U3, typename U4,
             typename T1, typename T2, typename T3, typename T4>
   static EventId ScheduleNow (void (*f) (U1,U2,U3,U4), T1 a1, T2 a2, T3 a3, T4 a4);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -371,7 +405,6 @@ public:
             typename T1, typename T2, typename T3, typename T4, typename T5>
   static EventId ScheduleNow (void (*f) (U1,U2,U3,U4,U5), T1 a1, T2 a2, T3 a3, T4 a4, T5 a5);
 
-
   /**
    * Schedule an event to expire at Destroy time. All events 
    * scheduled to expire at "Destroy" time are scheduled FIFO, 
@@ -383,6 +416,7 @@ public:
    */
   template <typename MEM, typename OBJ>
   static EventId ScheduleDestroy (MEM mem_ptr, OBJ obj);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -391,6 +425,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1>
   static EventId ScheduleDestroy (MEM mem_ptr, OBJ obj, T1 a1);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -400,6 +435,7 @@ public:
   template <typename MEM, typename OBJ,
             typename T1, typename T2>
   static EventId ScheduleDestroy (MEM mem_ptr, OBJ obj, T1 a1, T2 a2);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -410,6 +446,7 @@ public:
   template <typename MEM, typename OBJ, 
             typename T1, typename T2, typename T3>
   static EventId ScheduleDestroy (MEM mem_ptr, OBJ obj, T1 a1, T2 a2, T3 a3);
+
   /**
    * @param mem_ptr member method pointer to invoke
    * @param obj the object on which to invoke the member method
@@ -439,6 +476,7 @@ public:
    * @param f the function to invoke
    */
   static EventId ScheduleDestroy (void (*f) (void));
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -446,6 +484,7 @@ public:
   template <typename U1,
             typename T1>
   static EventId ScheduleDestroy (void (*f) (U1), T1 a1);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -454,6 +493,7 @@ public:
   template <typename U1, typename U2,
             typename T1, typename T2>
   static EventId ScheduleDestroy (void (*f) (U1,U2), T1 a1, T2 a2);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -463,6 +503,7 @@ public:
   template <typename U1, typename U2, typename U3,
             typename T1, typename T2, typename T3>
   static EventId ScheduleDestroy (void (*f) (U1,U2,U3), T1 a1, T2 a2, T3 a3);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -473,6 +514,7 @@ public:
   template <typename U1, typename U2, typename U3, typename U4,
             typename T1, typename T2, typename T3, typename T4>
   static EventId ScheduleDestroy (void (*f) (U1,U2,U3,U4), T1 a1, T2 a2, T3 a3, T4 a4);
+
   /**
    * @param f the function to invoke
    * @param a1 the first argument to pass to the function to invoke
@@ -497,6 +539,7 @@ public:
    * @param id the event to remove from the list of scheduled events.
    */
   static void Remove (const EventId &id);
+
   /**
    * Set the cancel bit on this event: the event's associated function
    * will not be invoked when it expires. 
@@ -510,6 +553,7 @@ public:
    * @param id the event to cancel
    */
   static void Cancel (const EventId &id);
+
   /**
    * This method has O(1) complexity.
    * Note that it is not possible to test for the expiration of
@@ -523,10 +567,12 @@ public:
    * @returns true if the event has expired, false otherwise.
    */
   static bool IsExpired (const EventId &id);
+
   /**
    * Return the "current simulation time".
    */
   static Time Now (void);
+
   /**
    * \param id the event id to analyse
    * \returns the delay left until the input event id expires.
@@ -542,6 +588,7 @@ public:
    * The returned value will always be bigger than or equal to Simulator::Now.
    */
   static Time GetMaximumSimulationTime (void);
+
   /**
    * \param time delay until the event expires
    * \param event the event to schedule
@@ -551,6 +598,7 @@ public:
    * to delegate events to their own subclass of the EventImpl base class.
    */
   static EventId Schedule (Time const &time, const Ptr<EventImpl> &event);  
+
   /**
    * \param event the event to schedule
    * \returns a unique identifier for the newly-scheduled event.
@@ -559,6 +607,7 @@ public:
    * to delegate events to their own subclass of the EventImpl base class.
    */
   static EventId ScheduleDestroy (const Ptr<EventImpl> &event);
+
   /**
    * \param event the event to schedule
    * \returns a unique identifier for the newly-scheduled event.
@@ -606,8 +655,10 @@ private:
             typename T1, typename T2, typename T3, typename T4, typename T5>
   static Ptr<EventImpl> MakeEvent (void (*f) (U1,U2,U3,U4,U5), T1 a1, T2 a2, T3 a3, T4 a4, T5 a5);
 
-  static Ptr<SimulatorImpl> GetImpl (void);
+  static SimulatorImpl *GetImpl (void);
   static Ptr<SimulatorImpl> m_impl;
+  static std::string m_typeImpl;
+  static std::string m_typeSched;
 };
 
 /**
diff --git a/src/simulator/wscript b/src/simulator/wscript
index 9bdcef84f..1b4d92003 100644
--- a/src/simulator/wscript
+++ b/src/simulator/wscript
@@ -58,7 +58,7 @@ def build(bld):
         'heap-scheduler.cc',
         'event-impl.cc',
         'simulator.cc',
-        'simulator-impl.cc',
+        'default-simulator-impl.cc',
         'timer.cc',
         'watchdog.cc',
         ]
@@ -72,6 +72,7 @@ def build(bld):
         'event-impl.h',
         'simulator.h',
         'simulator-impl.h',
+        'default-simulator-impl.h',
         'scheduler.h',
         'list-scheduler.h',
         'map-scheduler.h',