MobilityHelper mobility;
  mobility.SetPositionAllocator ("ns3::RandomBoxPositionAllocator",
                                 "X", StringValue("ns3::UniformRandomVariable[Min=0|Max=200]"),
                                 "Y", StringValue("ns3::UniformRandomVariable[Min=0|Max=200]"),
                                 "Z", StringValue("ns3::UniformRandomVariable[Min=0|Max=0]"));

  mobility.SetMobilityModel ("ns3::ConstantPositionMobilityModel");
  mobility.Install (wifiStaNodes);

  mobility.SetPositionAllocator ("ns3::RandomBoxPositionAllocator",
                                 "X", StringValue("ns3::UniformRandomVariable[Min=0|Max=200]"),
                                 "Y", StringValue("ns3::UniformRandomVariable[Min=0|Max=200]"),
                                 "Z", StringValue("ns3::UniformRandomVariable[Min=0|Max=0]"));

  mobility.SetMobilityModel ("ns3::ConstantPositionMobilityModel");
  mobility.Install (wifiApNode);