6.2 Building a Wireless Network Topology

In this section we are going to further expand our knowledge of ns-3 network devices and channels to cover an example of a wireless network. Ns-3 provides a set of 802.11 models that attempt to provide an accurate MAC-level implementation of the 802.11 specification and a “not-so-slow” PHY-level model of the 802.11a specification.

Just as we have seen both point-to-point and CSMA topology helper objects when constructing point-to-point topologies, we will see equivalent Wifi topology helpers in this section. The appearance and operation of these helpers should look quite familiar to you.

We provide an example script in our examples directory. This script builds on the second.cc script and adds a Wifi network. Go ahead and open examples/third.cc in your favorite editor. You will have already seen enough ns-3 code to understand most of what is going on in this example, but there are a few new things, so we will go over the entire script and examine some of the output.

Just as in the second.cc example (and in all ns-3 examples) the file begins with an emacs mode line and some GPL boilerplate.

Take a look at the ASCII art (reproduced below) that shows the default network topology constructed in the example. You can see that we are going to further extend our example by hanging a wireless network off of the left side. Notice that this is a default network topology since you can actually vary the number of nodes created on the wired and wireless networks. Just as in the second.cc script case, if you change nCsma, it will give you a number of “extra” CSMA nodes. Similarly, you can set nWifi to control how many STA (station) nodes are created in the simulation. There will always be one AP (access point) node on the wireless network. By default there are three “extra” CSMA nodes and three wireless STA nodes.

The code begins by loading module include files just as was done in the second.cc example. There are a couple of new includes corresponding to the Wifi module and the mobility module which we will discuss below.

#include "ns3/core-module.h"
#include "ns3/simulator-module.h"
#include "ns3/node-module.h"
#include "ns3/helper-module.h"
#include "ns3/global-routing-module.h"
#include "ns3/wifi-module.h"
#include "ns3/mobility-module.h"

The network topology illustration follows:

  // Default Network Topology
  //
  //   Wifi 10.1.3.0
  //                 AP
  //  *    *    *    *
  //  |    |    |    |    10.1.1.0
  // n5   n6   n7   n0 -------------- n1   n2   n3   n4
  //                   point-to-point  |    |    |    |
  //                                   ================
  //                                     LAN 10.1.2.0

You can see that we are adding a new network device to the node on the left side of the point-to-point link that becomes the access point for the wireless network. A number of wireless STA nodes are created to fill out the new 10.1.3.0 network as shown on the left side of the illustration.

After the illustration, the ns-3 namespace is used and a logging component is defined. This should all be quite familiar by now.

  using namespace ns3;
  
  NS_LOG_COMPONENT_DEFINE ("ThirdScriptExample");

As has become the norm in this tutorial, the main program begins by enabling the UdpEchoClientApplication and UdpEchoServerApplication logging components at INFO level so we can see some output when we run the simulation.

  int
  main (int argc, char *argv[])
  {
    LogComponentEnable("UdpEchoClientApplication", LOG_LEVEL_INFO);
    LogComponentEnable("UdpEchoServerApplication", LOG_LEVEL_INFO);

A fixed seed is provided to the random number generators so that they will generate repeatable results.

  RandomVariable::UseGlobalSeed (1, 1, 2, 3, 5, 8);

Next, you will see more familiar code that will allow you to change the number of devices on the CSMA and Wifi networks via command line argument.

  uint32_t nCsma = 3;
  uint32_t nWifi = 3;
  CommandLine cmd;
  cmd.AddValue ("nCsma", "Number of \"extra\" CSMA nodes/devices", nCsma);
  cmd.AddValue ("nWifi", "Number of wifi STA devices", nWifi);
  cmd.Parse (argc,argv);

Just as in all of the previous examples, the next step is to create two nodes that we will connect via the point-to-point link.

  NodeContainer p2pNodes;
  p2pNodes.Create (2);

Next, we see an old friend. We instantiate a PointToPointHelper and set the associated default attributes so that we create a five megabit per second transmitter on devices created using the helper and a two millisecond delay on channels created by the helper. We then Intall the devices on the nodes and the channel between them.

  PointToPointHelper pointToPoint;
  pointToPoint.SetDeviceAttribute ("DataRate", StringValue ("5Mbps"));
  pointToPoint.SetChannelAttribute ("Delay", StringValue ("2ms"));

  NetDeviceContainer p2pDevices;
  p2pDevices = pointToPoint.Install (p2pNodes);

Next, we delare another NodeContainer to hold the nodes that will be part of the bus (CSMA) network.

  NodeContainer csmaNodes;
  csmaNodes.Add (p2pNodes.Get (1));
  csmaNodes.Create (nCsma);

The next line of code Gets the first node (as in having an index of one) from the point-to-point node container and adds it to the container of nodes that will get CSMA devices. The node in question is going to end up with a point-to-point device and a CSMA device. We then create a number of “extra” nodes that compose the remainder of the CSMA network.

We then instantiate a CsmaHelper and set its attributes as we did in the previous example. We create a NetDeviceContainer to keep track of the created CSMA net devices and then we Install CSMA devices on the selected nodes.

  CsmaHelper csma;
  csma.SetChannelAttribute ("DataRate", StringValue ("100Mbps"));
  csma.SetChannelAttribute ("Delay", TimeValue (NanoSeconds (6560)));

  NetDeviceContainer csmaDevices;
  csmaDevices = csma.Install (csmaNodes);

Next, we are going to create the nodes that will be part of the Wifi network. We are going to create a number of “station” nodes as specified by the command line argument, and we are going to use the “leftmost” node of the point-to-point link as the node for the access point.

  NodeContainer wifiStaNodes;
  wifiStaNodes.Create (nWifi);
  NodeContainer wifiApNode = p2pNodes.Get (0);

The next bit of code constructs the wifi devices and the interconnection channel between these wifi nodes. First, we configure the PHY and channel helpers:

  YansWifiChannelHelper channel = YansWifiChannelHelper::Default ();
  YansWifiPhyHelper phy = YansWifiPhyHelper::Default ();

For simplicity, this code uses the default PHY layer configuration and channel models which are documented in the API doxygen documentation for the YansWifiChannelHelper::Default and YAnsWifiPhyHelper::Default methods. Once these objects are created, we create a channel object and associate it to our PHY layer object manager to make sure that all the PHY objects created layer by the YansWifiPhyHelper all share the same underlying channel, that is, they share the same wireless medium and can communication and interfere:

  phy.SetChannel (channel.Create ());

Once the PHY helper is configured, we can focus on the MAC layer:

  WifiHelper wifi = WifiHelper::Default ();
  wifi.SetRemoteStationManager ("ns3::AarfWifiManager");

The SetRemoteStationManager method tells the helper the type of rate control algorithm to use. Here, it is asking the helper to use the AARF algorithm — details are, of course, available in Doxygen.

Next, we configure the SSID of the infrastructure network we want to setup and make sure that our stations don't perform active probing:

  Ssid ssid = Ssid ("ns-3-ssid");
  wifi.SetMac ("ns3::NqstaWifiMac",
    "Ssid", SsidValue (ssid),
    "ActiveProbing", BooleanValue (false));

This code first creates an 802.11 service set identifier (SSID) object that will be used to set the value of the “Ssid” Attribute of the MAC layer implementation. The particular kind of MAC layer is specified by Attribute as being of the "ns3::NqstaWifiMac" type. This means that the MAC will use a “non-QoS station” (nqsta) state machine. Finally, the “ActiveProbing” attribute is set to false. This means that probe requests will not be sent by MACs created by this helper.

Once all the station-specific parameters are fully configured, both at the MAC and PHY layers, we can invoke our now-familiar Install method to create the wifi devices of these stations:

  NetDeviceContainer staDevices;
  staDevices = wifi.Install (phy, wifiStaNodes);

We have configured Wifi for all of our STA nodes, and now we need to configure the AP (access point) node. We begin this process by changing the default Attributes of the WifiHelper to reflect the requirements of the AP.

  wifi.SetMac ("ns3::NqapWifiMac", 
    "Ssid", SsidValue (ssid),
    "BeaconGeneration", BooleanValue (true),
    "BeaconInterval", TimeValue (Seconds (2.5)));

In this case, the WifiHelper is going to create MAC layers of the “ns3::NqapWifiMac” (Non-Qos Access Point) type. We set the “BeaconGeneration” attribute to true and also set an interval between beacons of 2.5 seconds.

The next lines create the single AP which shares the same set of PHY-level attributes (and channel) as the stations:

  NetDeviceContainer apDevices;
  apDevices = wifi.Install (phy, wifiApNode);

Now, we are going to add mobility models. We want the STA nodes to be mobile, wandering around inside a bounding box, and we want to make the AP node stationary. We use the MobilityHelper to make this easy for us. First, we instantiate a MobilityHelper obejct and set some attributes controlling the “position allocator” functionality.

  MobilityHelper mobility;

  mobility.SetPositionAllocator ("ns3::GridPositionAllocator",
    "MinX", DoubleValue (0.0),
    "MinY", DoubleValue (0.0),
    "DeltaX", DoubleValue (5.0),
    "DeltaY", DoubleValue (10.0),
    "GridWidth", UintegerValue (3),
    "LayoutType", StringValue ("RowFirst"));

This code tells the mobility helper to use a two-dimensional grid to initially place the STA nodes. Feel free to explore the Doxygen for class ns3::GridPositionAllocator to see exactly what is being done.

We have aranged our nodes on an initial grid, but now we need to tell them how to move. We choose the RandomWalk2dMobilityModel which has the nodes move in a random direction at a random speed around inside a bounding box.

  mobility.SetMobilityModel ("ns3::RandomWalk2dMobilityModel",
    "Bounds", RectangleValue (Rectangle (-50, 50, -50, 50)));

We now tell the MobilityHelper to install the mobility models on the STA nodes.

  mobility.Install (wifiStaNodes);

We want the access point to remain in a fixed position during the simulation. We accomplish this by setting the mobility model for this node to be the ns3::StaticMobilityModel:

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

We now have our nodes, devices and channels created, and mobility models chosen for the Wifi nodes, but we have no protocol stacks present. Just as we have done previously many times, we will use the InternetStackHelper to install these stacks.

  InternetStackHelper stack;
  stack.Install (csmaNodes);
  stack.Install (wifiApNode);
  stack.Install (wifiStaNodes);

Just as in the second.cc example script, we are going to use the Ipv4AddressHelper to assign IP addresses to our device interfaces. First we use the network 10.1.1.0 to create the two addresses needed for our two point-to-point devices. Then we use network 10.1.2.0 to assign addresses the the CSMA network and then we assign addresses from network 10.1.3.0 to both the STA devices and the AP on the wireless network.

  Ipv4AddressHelper address;

  address.SetBase ("10.1.1.0", "255.255.255.0");
  Ipv4InterfaceContainer p2pInterfaces;
  p2pInterfaces = address.Assign (p2pDevices);

  address.SetBase ("10.1.2.0", "255.255.255.0");
  Ipv4InterfaceContainer csmaInterfaces;
  csmaInterfaces = address.Assign (csmaDevices);

  address.SetBase ("10.1.3.0", "255.255.255.0");
  address.Assign (staDevices);
  address.Assign (apDevices);

We put the echo server on the “rightmost” node in the illustration at the start of the file. We have done this before.

  UdpEchoServerHelper echoServer (9);

  ApplicationContainer serverApps = echoServer.Install (csmaNodes.Get (nCsma));
  serverApps.Start (Seconds (1.0));
  serverApps.Stop (Seconds (10.0));

And we put the echo client on the last STA node we created, pointing it to the server on the CSMA network. We have also seen similar operations before.

  UdpEchoClientHelper echoClient (csmaInterfaces.GetAddress (nCsma), 9);
  echoClient.SetAttribute ("MaxPackets", UintegerValue (1));
  echoClient.SetAttribute ("Interval", TimeValue (Seconds (1.)));
  echoClient.SetAttribute ("PacketSize", UintegerValue (1024));

  ApplicationContainer clientApps =
    echoClient.Install (wifiStaNodes.Get (nWifi - 1));
  clientApps.Start (Seconds (2.0));
  clientApps.Stop (Seconds (10.0));

Since we have built an internetwork here, we need enable internetwork routing just as we did in the second.cc example script.

  GlobalRouteManager::PopulateRoutingTables ();

One thing that can surprise some users is the fact that the simulation we just created will never “naturally” stop. This is because we asked the wireless access point to generate beacons. It will generate beacons forever, so we must tell the simulator to stop even though it may have beacon generation events scheduled. The following line of code tells the simulator to stop so that we don't simulate beacons forever and enter what is essentially an endless loop.

  Simulator::Stop (Seconds (10.0));

We use the same trick as in the second.cc script to only generate pcap traces from the nodes we find interesting. Note that we use the same “formula” to get pcap tracing enabled on Wifi devices as we did on the CSMA and point-to-point devices.

  WifiHelper::EnablePcap ("third",
    wifiStaNodes.Get (nWifi - 1)->GetId (), 0);
  CsmaHelper::EnablePcap ("third",
    csmaNodes.Get (nCsma)->GetId (), 0);

Finally, we actually run the simulation, clean up and then exit the program.

    Simulator::Run ();
    Simulator::Destroy ();
    return 0;
  }

In order to run this example, you have to copy the third.cc example script into the scratch directory and use Waf to build just as you did with the second.cc example. If you are in the top-level directory of the repository you would type,

  cp examples/third.cc scratch/
  ./waf
  ./waf --run scratch/third

Since we have set up the UDP echo applications just as we did in the second.cc script, you will see similar output.

  ~/repos/ns-3-dev > ./waf --run scratch/third
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.3.3
  Received 1024 bytes from 10.1.2.4
  ~/repos/ns-3-dev >

Recall that the first message, Sent 1024 bytes to 10.1.2.4 is the UDP echo client sending a packet to the server. In this case, the client is on the wireless network (10.1.3.0). The second message, Received 1024 bytes from 10.1.3.3, is from the UDP echo server, generated when it receives the echo packet. The final message, Received 1024 bytes from 10.1.2.4 is from the echo client, indicating that it has received its echo back from the server.

If you now go and look in the top level directory, you will find two trace files:

  ~/repos/ns-3-dev > ls *.pcap
  third-4-0.pcap  third-7-0.pcap
  ~/repos/ns-3-dev >

The file “third-4-0.pcap” corresponds to the pcap trace for node four - device zero. This is the CSMA network node that acted as the echo server. Take a look at the tcpdump for this device:

  ~/repos/ns-3-dev > tcpdump -r third-4-0.pcap -nn -tt
  reading from file third-4-0.pcap, link-type EN10MB (Ethernet)
  2.005855 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.005855 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
  2.005859 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.005859 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.005861 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.005861 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024
  ~/repos/ns-3-dev >

This should be familiar and easily understood. If you've forgotten, go back and look at the discussion in second.cc. This is the same sequence.

Now, take a look at the other trace file, “third-7-0.pcap.” This is the trace file for the wireless STA node that acts as the echo client.

  ~/repos/ns-3-dev > tcpdump -r third-7-0.pcap -nn -tt
  reading from file third-7-0.pcap, link-type IEEE802_11 (802.11)
  0.000146 Beacon (ns-3-ssid) ...
  H: 0
  0.000180 Assoc Request (ns-3-ssid) ...
  0.000336 Acknowledgment RA:00:00:00:00:00:07
  0.000454 Assoc Response AID(0) :: Succesful
  0.000514 Acknowledgment RA:00:00:00:00:00:0a
  0.000746 Assoc Request (ns-3-ssid) ...
  0.000902 Acknowledgment RA:00:00:00:00:00:09
  0.001020 Assoc Response AID(0) :: Succesful
  0.001036 Acknowledgment RA:00:00:00:00:00:0a
  0.001219 Assoc Request (ns-3-ssid) ...
  0.001279 Acknowledgment RA:00:00:00:00:00:08
  0.001478 Assoc Response AID(0) :: Succesful
  0.001538 Acknowledgment RA:00:00:00:00:00:0a
  2.000000 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3
  2.000172 Acknowledgment RA:00:00:00:00:00:09
  2.000318 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3
  2.000581 arp reply 10.1.3.4 is-at 00:00:00:00:00:0a
  2.000597 Acknowledgment RA:00:00:00:00:00:0a
  2.000693 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.002229 Acknowledgment RA:00:00:00:00:00:09
  2.009663 arp who-has 10.1.3.3 (ff:ff:ff:ff:ff:ff) tell 10.1.3.4
  2.009697 arp reply 10.1.3.3 is-at 00:00:00:00:00:09
  2.009869 Acknowledgment RA:00:00:00:00:00:09
  2.011487 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024
  2.011503 Acknowledgment RA:00:00:00:00:00:0a
  2.500112 Beacon[|802.11]
  5.000112 Beacon[|802.11]
  7.500112 Beacon[|802.11]
  ~/repos/ns-3-dev >

You can see that the link type is now 802.11 as you would expect. We leave it as an exercise to parse the dump and trace packets across the internetwork.

Now, we spent a lot of time setting up mobility models for the wireless network and so it would be a shame to finish up without even showing that the STA nodes are actually moving around. Let's do this by hooking into the MobilityModel course change trace source. This is usually considered a fairly advanced topic, but let's just go for it.

As mentioned in the Tweaking Ns-3 section, the ns-3 tracing system is divided into trace sources and trace sinks, and we provide functions to connect the two. We will use the mobility model predefined course change trace source to originate the trace events. We will need to write a trace sink to connect to that source that will display some pretty information for us. Despite its reputation as being difficult, it's really quite simple. Just before the main program of the scratch/third.cc script, add the following function:

  void
  CourseChange (std::string context, Ptr<const MobilityModel> model)
  {
    Vector position = model->GetPosition ();
    NS_LOG_UNCOND (context << 
      " x = " << position.x << ", y = " << position.y);
  }

This code just pulls the position information from the mobility model and unconditionally logs the x and y position of the node. We are going to arrange for this function to be called every time the wireless node with the echo client changes its position. We do this using the Config::Connect function. Add the following lines of code to the script just before the Simulator::Run call.

  std::ostringstream oss;
  oss <<
    "/NodeList/" << wifiStaNodes.Get (nWifi - 1)->GetId () <<
    "/$ns3::MobilityModel/CourseChange";

  Config::Connect (oss.str (), MakeCallback (&CourseChange));

What we do here is to create a string containing the tracing namespace path of the event to which we want to connect. First, we have to figure out which node it is we want using the GetId method as described earlier. In the case of the default number of CSMA and wireless nodes, this turns out to be node seven and the tracing namespace path to the mobility model would look like,

  /NodeList/7/$ns3::MobilityModel/CourseChange

Based on the discussion in the tracing section, you can easily infer that this trace path references the seventh node in the NodeList. It specifies what is called an aggregated object of type ns3::MobilityModel. The dollar sign prefix implies that the MobilityModel is aggregated to node seven. The last component of the path means that we are hooking into the “CourseChange” event of that model.

We make a connection between the trace source in node seven with our trace sink by calling Config::Connect and passing this namespace path. Once this is done, every course change event on node seven will be hooked into our trace sink, which will in turn print out the new position.

If you now run the simulation, you will see the course changes displayed as they happen.

  ~/repos/ns-3-dev > ./waf --run scratch/third
  Entering directory `/home/craigdo/repos/ns-3-dev/build'
  Compilation finished successfully
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 10, y = 0
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.1304, y = 0.493761
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.70417, y = 1.39837
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.94799, y = 2.05274
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.82597, y = 1.57404
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.3003, y = 0.723347
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.3.3
  Received 1024 bytes from 10.1.2.4
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.74083, y = 1.62109
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.00146, y = 0.655647
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.98731, y = 0.823279
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.50206, y = 1.69766
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.68108, y = 2.26862
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.25992, y = 1.45317
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.55655, y = 0.742346
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.21992, y = 1.68398
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.81273, y = 0.878638
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.83171, y = 1.07256
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.60027, y = 0.0997156
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.45367, y = 0.620978
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.68484, y = 1.26043
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.53659, y = 0.736479
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.51876, y = 0.548502
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.89778, y = 1.47389
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.98984, y = 1.893
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.91524, y = 1.51402
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.98761, y = 1.14054
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.16617, y = 0.570239
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.02954, y = 1.56086
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.09551, y = 2.55868
  ~/repos/ns-3-dev >

If you are feeling brave, there is a list of all trace sources in the ns-3 Doxygen which you can find in the “Modules” tab. Under the “core” section, you will find a link to “The list of all trace sources.” You may find it interesting to try and hook some of these traces yourself. Additionally in the “Modules” documentation, there is a link to “The list of all attributes.” You can set the default value of any of these attributes via the command line as we have previously discussed.

We have just scratched the surface of ns-3 in this tutorial, but we hope we have covered enough to get you started doing useful work.

– The ns-3 development team.

This document was generated on December, 19 2008 using texi2html 1.78.