Building Topologies
*******************

Building a Bus Network Topology
===============================

In this section we are going to expand our mastery of ns-3 network devices and channels to cover an example of a bus network.  Ns-3 provides a net device and channel we call CSMA (Carrier Sense Multiple Access).

The ns-3 CSMA device models a simple network in the spirit of Ethernet.  A real Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) scheme with exponentially increasing backoff to contend for the shared transmission medium.  The ns-3 CSMA device and channel models only a subset of this.

Just as we have seen point-to-point topology helper objects when constructing point-to-point topologies, we will see equivalent CSMA 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/tutorial directory.  This script builds on the first.cc script and adds a CSMA network to the point-to-point simulation we've already considered.  Go ahead and open examples/tutorial/second.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 we will go over the entire script and examine some of the output.

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

The actual code begins by loading module include files just as was done in the first.cc example.

  #include "ns3/core-module.h"
  #include "ns3/simulator-module.h"
  #include "ns3/node-module.h"
  #include "ns3/helper-module.h"

One thing that can be surprisingly useful is a small bit of ASCII art that shows a cartoon of the network topology constructed in the example.  You will find a similar ``drawing'' in most of our examples.

In this case, you can see that we are going to extend our point-to-point example (the link between the nodes n0 and n1 below) by hanging a bus network off of the right side.  Notice that this is the default network topology since you can actually vary the number of nodes created on the LAN.  If you set nCsma to one, there will be a total of two nodes on the LAN (CSMA channel) --- one required node and one ``extra'' node.  By default there are three ``extra'' nodes as seen below:

// Default Network Topology
//
//       10.1.1.0
// n0 -------------- n1   n2   n3   n4
//    point-to-point  |    |    |    |
//                    ================
//                      LAN 10.1.2.0

Then the ns-3 namespace is used and a logging component is defined. This is all just as it was in first.cc, so there is nothing new yet.

  using namespace ns3;

  NS_LOG_COMPONENT_DEFINE ("SecondScriptExample");

The main program begins with a slightly different twist.  We use a verbose flag to determine whether or not the UdpEchoClientApplication and UdpEchoServerApplication logging components are enabled.  This flag defaults to true (the logging components are enabled) but allows us to turn off logging during regression testing of this example.

You will see some familiar code that will allow you to change the number of devices on the CSMA network via command line argument.  We did something similar when we allowed the number of packets sent to be changed in the section on command line arguments.  The last line makes sure you have at least one ``extra'' node.

The code consists of variations of previously covered API so you should be entirely comfortable with the following code at this point in the tutorial.

  bool verbose = true;
  uint32_t nCsma = 3;

  CommandLine cmd;
  cmd.AddValue (``nCsma'', ``Number of \"extra\" CSMA nodes/devices'', nCsma);
  cmd.AddValue (``verbose'', ``Tell echo applications to log if true'', verbose);

  cmd.Parse (argc,argv);

  if (verbose)
    {
      LogComponentEnable(``UdpEchoClientApplication'', LOG_LEVEL_INFO);
      LogComponentEnable(``UdpEchoServerApplication'', LOG_LEVEL_INFO);
    }

  nCsma = nCsma == 0 ? 1 : nCsma;

The next step is to create two nodes that we will connect via the point-to-point link.  The NodeContainer is used to do this just as was done in first.cc.

  NodeContainer p2pNodes;
  p2pNodes.Create (2);

Next, we declare another NodeContainer to hold the nodes that will be part of the bus (CSMA) network.  First, we just instantiate the container object itself.

  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.  Since we already have one node in the CSMA network -- the one that will have both a point-to-point and CSMA net device, the number of ``extra'' nodes means the number nodes you desire in the CSMA section minus one.

The next bit of code should be quite familiar by now.  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.

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

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

We then instantiate a NetDeviceContainer to keep track of the point-to-point net devices and we Install devices on the point-to-point nodes.

We mentioned above that you were going to see a helper for CSMA devices and channels, and the next lines introduce them.  The CsmaHelper works just like a PointToPointHelper, but it creates and connects CSMA devices and channels.  In the case of a CSMA device and channel pair, notice that the data rate is specified by a channel Attribute instead of a device Attribute.  This is because a real CSMA network does not allow one to mix, for example, 10Base-T and 100Base-T devices on a given channel.  We first set the data rate to 100 megabits per second, and then set the speed-of-light delay of the channel to 6560 nano-seconds (arbitrarily chosen as 1 nanosecond per foot over a 100 meter segment).  Notice that you can set an Attribute using its native data type.

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

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

Just as we created a NetDeviceContainer to hold the devices created by the PointToPointHelper we create a NetDeviceContainer to hold the devices created by our CsmaHelper.  We call the Install method of the CsmaHelper to install the devices into the nodes of the csmaNodes NodeContainer.

We now have our nodes, devices and channels created, but we have no protocol stacks present.  Just as in the first.cc script, we will use the InternetStackHelper to install these stacks.

  InternetStackHelper stack;
  stack.Install (p2pNodes.Get (0));
  stack.Install (csmaNodes);

Recall that we took one of the nodes from the p2pNodes container and added it to the csmaNodes container.  Thus we only need to install the stacks on the remaining p2pNodes node, and all of the nodes in the csmaNodes container to cover all of the nodes in the simulation.

Just as in the first.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.

  Ipv4AddressHelper address;
  address.SetBase ("10.1.1.0", "255.255.255.0");
  Ipv4InterfaceContainer p2pInterfaces;
  p2pInterfaces = address.Assign (p2pDevices);

Recall that we save the created interfaces in a container to make it easy to pull out addressing information later for use in setting up the applications.

We now need to assign IP addresses to our CSMA device interfaces.  The operation works just as it did for the point-to-point case, except we now are performing the operation on a container that has a variable number of CSMA devices --- remember we made the number of CSMA devices changeable by command line argument.  The CSMA devices will be associated with IP addresses from network number 10.1.2.0 in this case, as seen below.

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

Now we have a topology built, but we need applications.  This section is going to be fundamentally similar to the applications section of first.cc but we are going to instantiate the server on one of the nodes that has a CSMA device and the client on the node having only a point-to-point device.

First, we set up the echo server.  We create a UdpEchoServerHelper and provide a required Attribute value to the constructor which is the server port number.  Recall that this port can be changed later using the SetAttribute method if desired, but we require it to be provided to the constructor.

  UdpEchoServerHelper echoServer (9);

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

Recall that the csmaNodes NodeContainer contains one of the nodes created for the point-to-point network and nCsma ``extra'' nodes. What we want to get at is the last of the ``extra'' nodes.  The zeroth entry of the csmaNodes container will be the point-to-point node.  The easy way to think of this, then, is if we create one ``extra'' CSMA node, then it will be at index one of the csmaNodes container.  By induction, if we create nCsma ``extra'' nodes the last one will be at index nCsma.  You see this exhibited in the Get of the first line of code.

The client application is set up exactly as we did in the first.cc example script.  Again, we provide required Attributes to the UdpEchoClientHelper in the constructor (in this case the remote address and port).  We tell the client to send packets to the server we just installed on the last of the ``extra'' CSMA nodes.  We install the client on the leftmost point-to-point node seen in the topology illustration.

  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 (p2pNodes.Get (0));
  clientApps.Start (Seconds (2.0));
  clientApps.Stop (Seconds (10.0));

Since we have actually built an internetwork here, we need some form of internetwork routing.  ns-3 provides what we call global routing to help you out.  Global routing takes advantage of the fact that the entire internetwork is accessible in the simulation and runs through the all of the nodes created for the simulation --- it does the hard work of setting up routing for you without having to configure routers.

Basically, what happens is that each node behaves as if it were an OSPF router that communicates instantly and magically with all other routers behind the scenes.  Each node generates link advertisements and communicates them directly to a global route manager which uses this global information to construct the routing tables for each node.  Setting up this form of routing is a one-liner:

  Ipv4GlobalRoutingHelper::PopulateRoutingTables ();

Next we enable pcap tracing.  The first line of code to enable pcap tracing in the point-to-point helper should be familiar to you by now.  The second line enables pcap tracing in the CSMA helper and there is an extra parameter you haven't encountered yet.

  pointToPoint.EnablePcapAll ("second");
  csma.EnablePcap ("second", csmaDevices.Get (1), true);

The CSMA network is a multi-point-to-point network.  This means that there can (and are in this case) multiple endpoints on a shared medium.  Each of these endpoints has a net device associated with it.  There are two basic alternatives to gathering trace information from such a network.  One way is to create a trace file for each net device and store only the packets that are emitted or consumed by that net device.  Another way is to pick one of the devices and place it in promiscuous mode.  That single device then ``sniffs'' the network for all packets and stores them in a single pcap file.  This is how tcpdump, for example, works.  That final parameter tells the CSMA helper whether or not to arrange to capture packets in promiscuous mode.

In this example, we are going to select one of the devices on the CSMA network and ask it to perform a promiscuous sniff of the network, thereby emulating what tcpdump would do.  If you were on a Linux machine you might do something like tcpdump -i eth0 to get the trace. In this case, we specify the device using csmaDevices.Get(1), which selects the first device in the container.  Setting the final parameter to true enables promiscuous captures.

The last section of code just runs and cleans up the simulation just like the first.cc example.

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

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

  cp examples/tutorial/second.cc scratch/mysecond.cc
  ./waf

Warning:  We use the file second.cc as one of our regression tests to verify that it works exactly as we think it should in order to make your tutorial experience a positive one.  This means that an executable named second already exists in the project.  To avoid any confusion about what you are executing, please do the renaming to mysecond.cc suggested above.

If you are following the tutorial religiously (you are, aren't you) you will still have the NS_LOG variable set, so go ahead and clear that variable and run the program.

  export NS_LOG=
  ./waf --run scratch/mysecond

Since we have set up the UDP echo applications to log just as we did in first.cc, you will see similar output when you run the script.

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.415s)
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.4

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 server is on a different network (10.1.2.0).  The second message, ``Received 1024 bytes from 10.1.1.1,'' 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 three trace files:

  second-0-0.pcap  second-1-0.pcap  second-2-0.pcap

Let's take a moment to look at the naming of these files.  They all have the same form, <name>-<node>-<device>.pcap.  For example, the first file in the listing is second-0-0.pcap which is the pcap trace from node zero, device zero.  This is the point-to-point net device on node zero.  The file second-1-0.pcap is the pcap trace for device zero on node one, also a point-to-point net device; and the file second-2-0.pcap is the pcap trace for device zero on node two.

If you refer back to the topology illustration at the start of the section, you will see that node zero is the leftmost node of the point-to-point link and node one is the node that has both a point-to-point device and a CSMA device.  You will see that node two is the first ``extra'' node on the CSMA network and its device zero was selected as the device to capture the promiscuous-mode trace.

Now, let's follow the echo packet through the internetwork.  First, do a tcpdump of the trace file for the leftmost point-to-point node --- node zero.

  tcpdump -nn -tt -r second-0-0.pcap

You should see the contents of the pcap file displayed:

  reading from file second-0-0.pcap, link-type PPP (PPP)
  2.000000 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
  2.007602 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

The first line of the dump indicates that the link type is PPP (point-to-point) which we expect.  You then see the echo packet leaving node zero via the device associated with IP address 10.1.1.1 headed for IP address 10.1.2.4 (the rightmost CSMA node).  This packet will move over the point-to-point link and be received by the point-to-point net device on node one.  Let's take a look:

  tcpdump -nn -tt -r second-1-0.pcap

You should now see the pcap trace output of the other side of the point-to-point link:

reading from file second-1-0.pcap, link-type PPP (PPP)
2.003686 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
2.003915 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

Here we see that the link type is also PPP as we would expect.  You see the packet from IP address 10.1.1.1 (that was sent at 2.000000 seconds) headed toward IP address 10.1.2.4 appear on this interface.  Now, internally to this node, the packet will be forwarded to the CSMA interface and we should see it pop out on that device headed for its ultimate destination.

Remember that we selected node 2 as the promiscuous sniffer node for the CSMA network so let's then look at second-2-0.pcap and see if its there.

  tcpdump -nn -tt -r second-2-0.pcap

You should now see the promiscuous dump of node two, device zero:

  reading from file second-2-0.pcap, link-type EN10MB (Ethernet)
  2.003696 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003707 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
  2.003801 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
  2.003811 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.003822 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.003915 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

As you can see, the link type is now ``Ethernet''.  Something new has appeared, though.  The bus network needs ARP, the Address Resolution Protocol. Node one knows it needs to send the packet to IP address 10.1.2.4, but it doesn't know the MAC address of the corresponding node.  It broadcasts on the CSMA network (ff:ff:ff:ff:ff:ff) asking for the device that has IP address 10.1.2.4.  In this case, the rightmost node replies saying it is at MAC address 00:00:00:00:00:06.  Note that node two is not directly involved in this exchange, but is sniffing the network and reporting all of the traffic it sees.

This exchange is seen in the following lines,

  2.003696 arp who-has 10.1.2.4 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003707 arp reply 10.1.2.4 is-at 00:00:00:00:00:06

Then node one, device one goes ahead and sends the echo packet to the UDP echo server at IP address 10.1.2.4.

  2.003801 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024

The server receives the echo request and turns the packet around trying to send it back to the source.  The server knows that this address is on another network that it reaches via IP address 10.1.2.1.  This is because we initialized global routing and it has figured all of this out for us.  But, the echo server node doesn't know the MAC address of the first CSMA node, so it has to ARP for it just like the first CSMA node had to do.

  2.003811 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.003822 arp reply 10.1.2.1 is-at 00:00:00:00:00:03

The server then sends the echo back to the forwarding node.

  2.003915 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

Looking back at the rightmost node of the point-to-point link,

  tcpdump -nn -tt -r second-1-0.pcap

You can now see the echoed packet coming back onto the point-to-point link as the last line of the trace dump.

reading from file second-1-0.pcap, link-type PPP (PPP)
2.003686 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
2.003915 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

Lastly, you can look back at the node that originated the echo
  tcpdump -nn -tt -r second-0-0.pcap

and see that the echoed packet arrives back at the source at 2.007602 seconds,

  reading from file second-0-0.pcap, link-type PPP (PPP)
  2.000000 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
  2.007602 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024

Finally, recall that we added the ability to control the number of CSMA devices in the simulation by command line argument.  You can change this argument in the same way as when we looked at changing the number of packets echoed in the first.cc example.  Try running the program with the number of ``extra'' devices set to four:

  ./waf --run "scratch/mysecond --nCsma=4"

You should now see,

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.405s)
  Sent 1024 bytes to 10.1.2.5
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.5

Notice that the echo server has now been relocated to the last of the CSMA nodes, which is 10.1.2.5 instead of the default case, 10.1.2.4.

It is possible that you may not be satisfied with a trace file generated by a bystander in the CSMA network.  You may really want to get a trace from a single device and you may not be interested in any other traffic on the network.  You can do this fairly easily.

Let's take a look at scratch/mysecond.cc and add that code enabling us to be more specific.  ns-3 helpers provide methods that take a node number and device number as parameters.  Go ahead and replace the EnablePcap calls with the calls below.

  pointToPoint.EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);
  csma.EnablePcap ("second", csmaNodes.Get (nCsma)->GetId (), 0, false);
  csma.EnablePcap ("second", csmaNodes.Get (nCsma-1)->GetId (), 0, false);

We know that we want to create a pcap file with the base name "second" and we also know that the device of interest in both cases is going to be zero, so those parameters are not really interesting.

In order to get the node number, you have two choices:  first, nodes are numbered in a monotonically increasing fashion starting from zero in the order in which you created them.  One way to get a node number is to figure this number out ``manually'' by contemplating the order of node creation. If you take a look at the network topology illustration at the beginning of the file, we did this for you and you can see that the last CSMA node is going to be node number nCsma + 1.  This approach can become annoyingly difficult in larger simulations.

An alternate way, which we use here, is to realize that the NodeContainers contain pointers to ns-3 Node Objects. The Node Object has a method called GetId which will return that node's ID, which is the node number we seek.  Let's go take a look at the Doxygen for the Node and locate that method, which is further down in the ns-3 core code than we've seen so far; but sometimes you have to search diligently for useful things.

Go to the Doxygen documentation for your release (recall that you can find it on the project web site).  You can get to the Node documentation by looking through at the ``Classes'' tab and scrolling down the ``Class List'' until you find ns3::Node.  Select ns3::Node and you will be taken to the documentation for the Node class.  If you now scroll down to the GetId method and select it, you will be taken to the detailed documentation for the method.  Using the GetId method can make determining node numbers much easier in complex topologies.

Let's clear the old trace files out of the top-level directory to avoid confusion about what is going on,

  rm *.pcap
  rm *.tr

If you build the new script and run the simulation setting nCsma to 100,

  ./waf --run "scratch/mysecond --nCsma=100"

you will see the following output:

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.407s)
  Sent 1024 bytes to 10.1.2.101
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.101

Note that the echo server is now located at 10.1.2.101 which corresponds to having 100 ``extra'' CSMA nodes with the echo server on the last one.  If you list the pcap files in the top level directory you will see,

  second-0-0.pcap  second-100-0.pcap  second-101-0.pcap

The trace file second-0-0.pcap is the ``leftmost'' point-to-point device which is the echo packet source.  The file second-101-0.pcap corresponds to the rightmost CSMA device which is where the echo server resides.  You may have noticed that the final parameter on the call to enable pcap tracing on the echo server node was false.  This means that the trace gathered on that node was in non-promiscuous mode.

To illustrate the difference between promiscuous and non-promiscuous traces, we also requested a non-promiscuous trace for the next-to-last node.  Go ahead and take a look at the tcpdump for second-100-0.pcap.

  tcpdump -nn -tt -r second-100-0.pcap

You can now see that node 100 is really a bystander in the echo exchange.  The only packets that it receives are the ARP requests which are broadcast to the entire CSMA network.

  reading from file second-100-0.pcap, link-type EN10MB (Ethernet)
  2.003696 arp who-has 10.1.2.101 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003811 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.101

Now take a look at the tcpdump for second-101-0.pcap.

  tcpdump -nn -tt -r second-101-0.pcap

You can now see that node 101 is really the participant in the echo exchange.

  reading from file second-101-0.pcap, link-type EN10MB (Ethernet)
  2.003696 arp who-has 10.1.2.101 (ff:ff:ff:ff:ff:ff) tell 10.1.2.1
  2.003696 arp reply 10.1.2.101 is-at 00:00:00:00:00:67
  2.003801 IP 10.1.1.1.49153 > 10.1.2.101.9: UDP, length 1024
  2.003801 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.101
  2.003822 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.003822 IP 10.1.2.101.9 > 10.1.1.1.49153: UDP, length 1024

Models, Attributes and Reality
==============================

This is a convenient place to make a small excursion and make an important point.  It may or may not be obvious to you, but whenever one is using a simulation, it is important to understand exactly what is being modeled and what is not.  It is tempting, for example, to think of the CSMA devices and channels used in the previous section as if they were real Ethernet devices; and to expect a simulation result to directly reflect what will happen in a real Ethernet.  This is not the case.

A model is, by definition, an abstraction of reality.  It is ultimately the responsibility of the simulation script author to determine the so-called ``range of accuracy'' and ``domain of applicability'' of the simulation as a whole, and therefore its constituent parts.

In some cases, like Csma, it can be fairly easy to determine what is not modeled.  By reading the model description (csma.h) you can find that there is no collision detection in the CSMA model and decide on how applicable its use will be in your simulation or what caveats you may want to include with your results.  In other cases, it can be quite easy to configure behaviors that might not agree with any reality you can go out and buy.  It will prove worthwhile to spend some time investigating a few such instances, and how easily you can swerve outside the bounds of reality in your simulations.

As you have seen, ns-3 provides Attributes which a user can easily set to change model behavior.  Consider two of the Attributes of the CsmaNetDevice:  Mtu and EncapsulationMode. The Mtu attribute indicates the Maximum Transmission Unit to the device.  This is the size of the largest Protocol Data Unit (PDU) that the device can send.

The MTU defaults to 1500 bytes in the CsmaNetDevice.  This default corresponds to a number found in RFC 894, ``A Standard for the Transmission of IP Datagrams over Ethernet Networks.''  The number is actually derived from the maximum packet size for 10Base5 (full-spec Ethernet) networks -- 1518 bytes.  If you subtract the DIX encapsulation overhead for Ethernet packets (18 bytes) you will end up with a maximum possible data size (MTU) of 1500 bytes.  One can also find that the MTU for IEEE 802.3 networks is 1492 bytes.  This is because LLC/SNAP encapsulation adds an extra eight bytes of overhead to the packet.  In both cases, the underlying hardware can only send 1518 bytes, but the data size is different.

In order to set the encapsulation mode, the CsmaNetDevice provides an Attribute called EncapsulationMode which can take on the values Dix or Llc.  These correspond to Ethernet and LLC/SNAP framing respectively.

If one leaves the Mtu at 1500 bytes and changes the encapsulation mode to Llc, the result will be a network that encapsulates 1500 byte PDUs with LLC/SNAP framing resulting in packets of 1526 bytes, which would be illegal in many networks, since they can transmit a maximum of 1518 bytes per packet.  This would most likely result in a simulation that quite subtly does not reflect the reality you might be expecting.

Just to complicate the picture, there exist jumbo frames (1500 < MTU <= 9000 bytes) and super-jumbo (MTU > 9000 bytes) frames that are not officially sanctioned by IEEE but are available in some high-speed (Gigabit) networks and NICs.  One could leave the encapsulation mode set to Dix, and set the Mtu Attribute on a CsmaNetDevice to 64000 bytes -- even though an associated CsmaChannel DataRate was set at 10 megabits per second. This would essentially model an Ethernet switch made out of vampire-tapped 1980s-style 10Base5 networks that support super-jumbo datagrams.  This is certainly not something that was ever made, nor is likely to ever be made, but it is quite easy for you to configure.

In the previous example, you used the command line to create a simulation that had 100 Csma nodes.  You could have just as easily created a simulation with 500 nodes.  If you were actually modeling that 10Base5 vampire-tap network, the maximum length of a full-spec Ethernet cable is 500 meters, with a minimum tap spacing of 2.5 meters.  That means there could only be 200 taps on a real network.  You could have quite easily built an illegal network in that way as well.  This may or may not result in a meaningful simulation depending on what you are trying to model.

Similar situations can occur in many places in ns-3 and in any simulator.  For example, you may be able to position nodes in such a way that they occupy the same space at the same time, or you may be able to configure amplifiers or noise levels that violate the basic laws of physics.

ns-3 generally favors flexibility, and many models will allow freely setting Attributes without trying to enforce any arbitrary consistency or particular underlying spec.

The thing to take home from this is that ns-3 is going to provide a super-flexible base for you to experiment with.  It is up to you to understand what you are asking the system to do and to  make sure that the simulations you create have some meaning and some connection with a reality defined by you.

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/tutorial directory.  This script builds on the second.cc script and adds a Wifi network.  Go ahead and open examples/tutorial/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/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");

The main program begins just like second.cc by adding some command line parameters for enabling or disabling logging components and for changing the number of devices created.

  bool verbose = true;
  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.AddValue (``verbose'', ``Tell echo applications to log if true'', verbose);

  cmd.Parse (argc,argv);

  if (verbose)
    {
      LogComponentEnable(``UdpEchoClientApplication'', LOG_LEVEL_INFO);
      LogComponentEnable(``UdpEchoServerApplication'', LOG_LEVEL_INFO);
    }

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 declare 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 layer objects created by the YansWifiPhyHelper 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. Here we choose to work with non-Qos MACs so we use a NqosWifiMacHelper object to set MAC parameters.

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

  NqosWifiMacHelper mac = NqosWifiMacHelper::Default ();

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 type of MAC, 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");
  mac.SetType ("ns3::StaWifiMac",
    "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 that will be created by the helper is specified by Attribute as being of the "ns3::StaWifiMac" type.  The use of NqosWifiMacHelper will ensure that the ''QosSupported'' Attribute for created MAC objects is set false. The combination of these two configurations means that the MAC instance next created will be a non-QoS non-AP station (STA) in an infrastructure BSS (i.e., a BSS with an AP).  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, mac, 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 NqosWifiMacHelper to reflect the requirements of the AP.

  mac.SetType ("ns3::ApWifiMac",
    "Ssid", SsidValue (ssid),
    "BeaconGeneration", BooleanValue (true),
    "BeaconInterval", TimeValue (Seconds (2.5)));

In this case, the NqosWifiMacHelper is going to create MAC layers of the ``ns3::ApWifiMac'', the latter specifying that a MAC instance configured as an AP should be created, with the helper type implying that the ''QosSupported'' Attribute should be set to false - disabling 802.11e/WMM-style QoS support at created APs.  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, mac, 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 object 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 arranged 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::ConstantPositionMobilityModel:

  mobility.SetMobilityModel ("ns3::ConstantPositionMobilityModel");
  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 to 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 to enable internetwork routing just as we did in the second.cc example script.

  Ipv4GlobalRoutingHelper::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, and this will result in simulator events being scheduled into the future indefinitely, 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 create just enough tracing to cover all three networks:

  pointToPoint.EnablePcapAll ("third");
  phy.EnablePcap ("third", apDevices.Get (0));
  csma.EnablePcap ("third", csmaDevices.Get (0), true);

These three lines of code will start pcap tracing on both of the point-to-point nodes that serves as our backbone, will start a promiscuous (monitor) mode trace on the Wifi network, and will start a promiscuous trace on the CSMA network.  This will let us see all of the traffic with a minimum number of trace files.

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/mythird.cc
  ./waf
  ./waf --run scratch/mythird

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

  Waf: Entering directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  Waf: Leaving directory `/home/craigdo/repos/ns-3-allinone/ns-3-dev/build'
  'build' finished successfully (0.407s)
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.3.3
  Received 1024 bytes from 10.1.2.4

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 four trace files from this simulation, two from node zero and two from node one:

third-0-0.pcap  third-0-1.pcap  third-1-0.pcap  third-1-1.pcap

The file ``third-0-0.pcap'' corresponds to the point-to-point device on node zero -- the left side of the ``backbone''.  The file ``third-1-0.pcap'' corresponds to the point-to-point device on node one -- the right side of the ``backbone''.  The file ``third-0-1.pcap'' will be the promiscuous (monitor mode) trace from the Wifi network and the file ``third-1-1.pcap'' will be the promiscuous trace from the CSMA network.  Can you verify this by inspecting the code?

Since the echo client is on the Wifi network, let's start there.  Let's take a look at the promiscuous (monitor mode) trace we captured on that network.

  tcpdump -nn -tt -r third-0-1.pcap

You should see some wifi-looking contents you haven't seen here before:

  reading from file third-0-1.pcap, link-type IEEE802_11 (802.11)
  0.000025 Beacon () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS
  0.000263 Assoc Request () [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit]
  0.000279 Acknowledgment RA:00:00:00:00:00:07
  0.000357 Assoc Response AID(0) :: Succesful
  0.000501 Acknowledgment RA:00:00:00:00:00:0a
  0.000748 Assoc Request () [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit]
  0.000764 Acknowledgment RA:00:00:00:00:00:08
  0.000842 Assoc Response AID(0) :: Succesful
  0.000986 Acknowledgment RA:00:00:00:00:00:0a
  0.001242 Assoc Request () [6.0 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit]
  0.001258 Acknowledgment RA:00:00:00:00:00:09
  0.001336 Assoc Response AID(0) :: Succesful
  0.001480 Acknowledgment RA:00:00:00:00:00:0a
  2.000112 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3
  2.000128 Acknowledgment RA:00:00:00:00:00:09
  2.000206 arp who-has 10.1.3.4 (ff:ff:ff:ff:ff:ff) tell 10.1.3.3
  2.000487 arp reply 10.1.3.4 is-at 00:00:00:00:00:0a
  2.000659 Acknowledgment RA:00:00:00:00:00:0a
  2.002169 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.002185 Acknowledgment RA:00:00:00:00:00:09
  2.009771 arp who-has 10.1.3.3 (ff:ff:ff:ff:ff:ff) tell 10.1.3.4
  2.010029 arp reply 10.1.3.3 is-at 00:00:00:00:00:09
  2.010045 Acknowledgment RA:00:00:00:00:00:09
  2.010231 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024
  2.011767 Acknowledgment RA:00:00:00:00:00:0a
  2.500000 Beacon () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS
  5.000000 Beacon () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS
  7.500000 Beacon () [6.0* 9.0 12.0 18.0 24.0 36.0 48.0 54.0 Mbit] IBSS

You can see that the link type is now 802.11 as you would expect.  You can probably understand what is going on and find the IP echo request and response packets in this trace.  We leave it as an exercise to completely parse the trace dump.

Now, look at the pcap file of the right side of the point-to-point link,

  tcpdump -nn -tt -r third-0-0.pcap

Again, you should see some familiar looking contents:

  reading from file third-0-0.pcap, link-type PPP (PPP)
  2.002169 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.009771 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024

This is the echo packet going from left to right (from Wifi to CSMA) and back again across the point-to-point link.

Now, look at the pcap file of the right side of the point-to-point link,

  tcpdump -nn -tt -r third-1-0.pcap

Again, you should see some familiar looking contents:

  reading from file third-1-0.pcap, link-type PPP (PPP)
  2.005855 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.006084 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024

This is also the echo packet going from left to right (from Wifi to CSMA) and back again across the point-to-point link with slightly different timings as you might expect.

The echo server is on the CSMA network, let's look at the promiscuous trace there:

  tcpdump -nn -tt -r third-1-1.pcap

You should see some familiar looking contents:

  reading from file third-1-1.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
  2.005877 arp reply 10.1.2.4 is-at 00:00:00:00:00:06
  2.005877 IP 10.1.3.3.49153 > 10.1.2.4.9: UDP, length 1024
  2.005980 arp who-has 10.1.2.1 (ff:ff:ff:ff:ff:ff) tell 10.1.2.4
  2.005980 arp reply 10.1.2.1 is-at 00:00:00:00:00:03
  2.006084 IP 10.1.2.4.9 > 10.1.3.3.49153: UDP, length 1024

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

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 during the simulation.  Let's do this by hooking into the MobilityModel course change trace source.  This is just a sneak peek into the detailed tracing section which is coming up, but this seems a very nice place to get an example in.

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/mythird.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 may infer that this trace path references the seventh node in the global 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.

  Build finished successfully (00:00:01)
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 10, y = 0
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 9.41539, y = -0.811313
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.46199, y = -1.11303
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.52738, y = -1.46869
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.67099, y = -1.98503
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 5.6835, y = -2.14268
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.70932, y = -1.91689
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.3.3
  Received 1024 bytes from 10.1.2.4
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 5.53175, y = -2.48576
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.58021, y = -2.17821
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.18915, y = -1.25785
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.7572, y = -0.434856
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.62404, y = 0.556238
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 4.74127, y = 1.54934
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 5.73934, y = 1.48729
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.18521, y = 0.59219
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.58121, y = 1.51044
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.27897, y = 2.22677
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.42888, y = 1.70014
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.40519, y = 1.91654
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.51981, y = 1.45166
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.34588, y = 2.01523
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.81046, y = 2.90077
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 6.89186, y = 3.29596
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.46617, y = 2.47732
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.05492, y = 1.56579
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 8.00393, y = 1.25054
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.00968, y = 1.35768
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.33503, y = 2.30328
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.18682, y = 3.29223
  /NodeList/7/$ns3::MobilityModel/CourseChange x = 7.96865, y = 2.66873