@c ========================================================================
@c Begin document body here
@c ========================================================================

@c ========================================================================
@c PART:  Building Topologies
@c ========================================================================
@c The below chapters are under the major heading "Building Topologies"
@c This is similar to the Latex \part command
@c
@c ========================================================================
@c Building Topologies
@c ========================================================================
@node Building Topologies
@chapter Building Topologies

@menu
* Building a Bus Network Topology::
* Building a Wireless Network Topology::
@end menu

@c ========================================================================
@c Building a Bus Network Topology
@c ========================================================================
@node Building a Bus Network Topology
@section Building a Bus Network Topology

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

The @command{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 @command{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 @code{examples} directory.  This script
builds on the @code{first.cc} script and adds a CSMA network to the 
point-to-point simulation we've already considered.  Go ahead and open 
@code{examples/second.cc} in your favorite editor.  You will have already seen
enough @command{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 @code{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
@code{first.cc} example.

@verbatim
  #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"
@end verbatim

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:

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

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

@verbatim  
  using namespace ns3;
  
  NS_LOG_COMPONENT_DEFINE ("SecondScriptExample");
@end verbatim

The main program begins with a slightly different twist.  We use a verbose
flag to determine whether or not the @code{UdpEchoClientApplication} and
@code{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.

@verbatim
  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;
@end verbatim

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

@verbatim
  NodeContainer p2pNodes;
  p2pNodes.Create (2);
@end verbatim

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

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

The next line of code @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 @emph{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
@code{PointToPointHelper} and set the associated default @code{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.

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

  NetDeviceContainer p2pDevices;
  p2pDevices = pointToPoint.Install (p2pNodes);
@end verbatim

We then instantiate a @code{NetDeviceContainer} to keep track of the 
point-to-point net devices and we @code{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 @code{CsmaHelper} works just
like a @code{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 @emph{channel} @code{Attribute} instead of a device 
@code{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 @code{Attribute} using 
its native data type.

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

  NetDeviceContainer csmaDevices;
  csmaDevices = csma.Install (csmaNodes);
@end verbatim

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

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

@verbatim
  InternetStackHelper stack;
  stack.Install (p2pNodes.Get (0));
  stack.Install (csmaNodes);
@end verbatim

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

Just as in the @code{first.cc} example script, we are going to use the 
@code{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.

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

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.

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

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

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

@verbatim
  UdpEchoServerHelper echoServer (9);

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

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

The client application is set up exactly as we did in the @code{first.cc}
example script.  Again, we provide required @code{Attributes} to the 
@code{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.

@verbatim
  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));
@end verbatim

Since we have actually built an internetwork here, we need some form of 
internetwork routing.  @command{ns-3} provides what we call a global route 
manager to set up the routing tables on nodes.  This route manager has a 
global function that runs though the nodes created for the simulation and does
the hard work of setting up routing for you.  

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:

@verbatim
  GlobalRouteManager::PopulateRoutingTables ();
@end verbatim

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.

@verbatim
  PointToPointHelper::EnablePcapAll ("second");
  CsmaHelper::EnablePcap ("second", csmaDevices.Get (0), true);
@end verbatim

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 @code{tcpdump}, for example, works.  That final 
parameter tells the CSMA helper whether or not 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 @code{tcpdump} would do.  If you were on a Linux machine
you might do something like @code{tcpdump -i eth0} to get the trace.  
In this case, we specify the device using @code{csmaDevices.Get(0)}, 
which selects the zeroth 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 @code{first.cc} example.

@verbatim
    Simulator::Run ();
    Simulator::Destroy ();
    return 0;
  }
@end verbatim

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

@verbatim
  cp examples/second.cc scratch/mysecond.cc
  ./waf
@end verbatim

Warning:  We use the file @code{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 
@code{second} already exists in the project.  To avoid any confusion
about what you are executing, please do the renaming to @code{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.

@verbatim
  export NS_LOG=
  ./waf --run scratch/mysecond
#end verbatim

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

@verbatim
  Entering directory `repos/ns-3-allinone/ns-3-dev/build'
  Build finished successfully (00:00:00)
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.4
@end verbatim

Recall that the first message, @code{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, @code{Received 1024 
bytes from 10.1.1.1}, is from the UDP echo server, generated when it receives
the echo packet.  The final message, @code{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:

@verbatim
  second-0-0.pcap  second-1-0.pcap  second-2-0.pcap
@end verbatim

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

@verbatim
  tcpdump -nn -tt -r second-0-0.pcap
@end verbatim

You should see the contents of the pcap file displayed:

@verbatim
  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
@end verbatim

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:

@verbatim
  tcpdump -nn -tt -r second-1-0.pcap
@end verbatim

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

@verbatim
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
@end verbatim

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.

@verbatim
  tcpdump -nn -tt -r second-2-0.pcap
@end verbatim

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

@verbatim
  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
@end verbatim

As you can see, the link type is now ``Ethernet''.  Something new has appeared,
though.  The bus network needs @code{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,

@verbatim
  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
@end verbatim

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

@verbatim
  2.003801 IP 10.1.1.1.49153 > 10.1.2.4.9: UDP, length 1024
@end verbatim

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.

@verbatim
  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
@end verbatim

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

@verbatim
  2.003915 IP 10.1.2.4.9 > 10.1.1.1.49153: UDP, length 1024
@end verbatim

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

@verbatim
  tcpdump -nn -tt -r second-1-0.pcap
@end verbatim

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

@verbatim
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
@end verbatim

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

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

@verbatim
  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
@end verbatim

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
@code{first.cc} example.  Try running the program with the number of ``extra'' 
devices set to four:

@verbatim
  ./waf --run "scratch/mysecond --nCsma=4"
@end verbatim

You should now see,

@verbatim
  Entering directory `repos/ns-3-allinone/ns-3-dev/build'
  Build finished successfully (00:00:00)
  Sent 1024 bytes to 10.1.2.5
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.5
@end verbatim

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,

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

@verbatim
  PointToPointHelper::EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);
  CsmaHelper::EnablePcap ("second", csmaNodes.Get (nCsma)->GetId (), 0, false);
  CsmaHelper::EnablePcap ("second", csmaNodes.Get (nCsma-1)->GetId (), 0, false);
@end verbatim

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 @code{nCsma + 1}.  This approach can become 
annoyingly difficult in larger simulations.  

An alternate way, which we use here, is to realize that the
@code{NodeContainers} contain pointers to @command{ns-3} @code{Node} Objects.
The @code{Node} Object has a method called @code{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 @code{Node} and locate that method, which is further down in 
the @command{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 @code{Node} documentation by
looking through at the ``Classes'' tab and scrolling down the ``Class List'' 
until you find @code{ns3::Node}.  Select @code{ns3::Node} and you will be taken
to the documentation for the @code{Node} class.  If you now scroll down to the
@code{GetId} method and select it, you will be taken to the detailed 
documentation for the method.  Using the @code{GetId} method can make 
determining node numbers much easier in complex topologies.

If you build the new script and run the simulation setting @code{nCsma} to 100,

@verbatim
  ./waf --run "scratch/mysecond --nCsma=100"
@end verbatim

you will see the following output:

@verbatim
  Entering directory `repos/ns-3-allinone/ns-3-dev/build'
  Build finished successfully (00:00:00)
  Sent 1024 bytes to 10.1.2.101
  Received 1024 bytes from 10.1.1.1
  Received 1024 bytes from 10.1.2.101
@end verbatim

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,

@verbatim
  second-0-0.pcap  second-100-0.pcap  second-101-0.pcap
@end verbatim

The trace file @code{second-0-0.pcap} is the ``leftmost'' point-to-point device
which is the echo packet source.  The file @code{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 @code{tcpdump} for @code{second-10-0.pcap}.

@verbatim
  tcpdump -nn -tt -r second-100-0.pcap
@end verbatim

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.

@verbatim
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
@end verbatim

@c ========================================================================
@c Building a Wireless Network Topology
@c ========================================================================
@node Building a Wireless Network Topology
@section Building a Wireless Network Topology

@cindex topology
@cindex wireless network topology
In this section we are going to further expand our knowledge of @command{ns-3}
network devices and channels to cover an example of a wireless network.  
@command{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 @code{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 @code{examples} directory.  This script
builds on the @code{second.cc} script and adds a Wifi network.  Go ahead and
open @code{examples/third.cc} in your favorite editor.  You will have already
seen enough @command{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 @code{second.cc} example (and in all @command{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 
@code{second.cc} script case, if you change @code{nCsma}, it will give you a 
number of ``extra'' CSMA nodes.  Similarly, you can set @code{nWifi} to 
control how many @code{STA} (station) nodes are created in the simulation.
There will always be one @code{AP} (access point) node on the wireless 
network.  By default there are three ``extra'' CSMA nodes and three wireless 
@code{STA} nodes.

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

@verbatim
#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"
@end verbatim

The network topology illustration follows:

@verbatim
  // 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
@end verbatim

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 @code{ns-3} namespace is @code{used} and a logging
component is defined.  This should all be quite familiar by now.

@verbatim
  using namespace ns3;
  
  NS_LOG_COMPONENT_DEFINE ("ThirdScriptExample");
@end verbatim

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

@verbatim
  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);
    }
@end verbatim

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.  

@verbatim
  NodeContainer p2pNodes;
  p2pNodes.Create (2);
@end verbatim

Next, we see an old friend.  We instantiate a @code{PointToPointHelper} and 
set the associated default @code{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 @code{Intall} the devices
on the nodes and the channel between them.

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

  NetDeviceContainer p2pDevices;
  p2pDevices = pointToPoint.Install (p2pNodes);
@end verbatim

Next, we declare another @code{NodeContainer} to hold the nodes that will be
part of the bus (CSMA) network.

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

The next line of code @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 @code{CsmaHelper} and set its @code{Attributes} as we did
in the previous example.  We create a @code{NetDeviceContainer} to keep track of
the created CSMA net devices and then we @code{Install} CSMA devices on the 
selected nodes.

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

  NetDeviceContainer csmaDevices;
  csmaDevices = csma.Install (csmaNodes);
@end verbatim

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.

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

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:

@verbatim
  YansWifiChannelHelper channel = YansWifiChannelHelper::Default ();
  YansWifiPhyHelper phy = YansWifiPhyHelper::Default ();
@end verbatim

For simplicity, this code uses the default PHY layer configuration and
channel models which are documented in the API doxygen documentation for
the @code{YansWifiChannelHelper::Default} and @code{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 @code{YansWifiPhyHelper}
all share the same underlying channel, that is, they share the same
wireless medium and can communication and interfere:

@verbatim
  phy.SetChannel (channel.Create ());
@end verbatim

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

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

The @code{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:

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

This code first creates an 802.11 service set identifier (SSID) object that 
will be used to set the value of the ``Ssid'' @code{Attribute} of the MAC
layer implementation.  The particular kind of MAC layer is specified by
@code{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'' @code{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 @code{Install} method to 
create the wifi devices of these stations:

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

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 @code{Attributes} of the @code{WifiHelper} to reflect the 
requirements of the AP.

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

In this case, the @code{WifiHelper} is going to create MAC layers of the 
``ns3::NqapWifiMac'' (Non-Qos Access Point) type.  We set the 
``BeaconGeneration'' @code{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
@code{Attributes} (and channel) as the stations:

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

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 @code{MobilityHelper} to make this easy for us.
First, we instantiate a @code{MobilityHelper} object and set some 
@code{Attributes} controlling the ``position allocator'' functionality.

@verbatim
  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"));
@end verbatim

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 
@code{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 @code{RandomWalk2dMobilityModel} which has the 
nodes move in a random direction at a random speed around inside a bounding 
box.

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

We now tell the @code{MobilityHelper} to install the mobility models on the 
STA nodes.

@verbatim
  mobility.Install (wifiStaNodes);
@end verbatim

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 
@code{ns3::ConstantPositionMobilityModel}:

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

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 @code{InternetStackHelper}
to install these stacks.

@verbatim
  InternetStackHelper stack;
  stack.Install (csmaNodes);
  stack.Install (wifiApNode);
  stack.Install (wifiStaNodes);
@end verbatim

Just as in the @code{second.cc} example script, we are going to use the 
@code{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.

@verbatim
  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);
@end verbatim

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

@verbatim
  UdpEchoServerHelper echoServer (9);

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

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.

@verbatim
  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));
@end verbatim

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

@verbatim
  GlobalRouteManager::PopulateRoutingTables ();
@end verbatim

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.

@verbatim
  Simulator::Stop (Seconds (10.0));
@end verbatim

We create just enough tracing to cover all three networks:

@verbatim
  PointToPointHelper::EnablePcapAll ("third");
  YansWifiPhyHelper::EnablePcap ("third", apDevices.Get (0));
  CsmaHelper::EnablePcap ("third", csmaDevices.Get (0), true);
@end verbatim

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.

@verbatim
    Simulator::Run ();
    Simulator::Destroy ();
    return 0;
  }
@end verbatim

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

@verbatim
  cp examples/third.cc scratch/mythird.cc
  ./waf
  ./waf --run scratch/mythird
@end verbatim

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

@verbatim
  Entering directory `repos/ns-3-allinone-dev/ns-3-dev/build'
  Build finished successfully (00:00:00)
  Sent 1024 bytes to 10.1.2.4
  Received 1024 bytes from 10.1.3.3
  Received 1024 bytes from 10.1.2.4
@end verbatim

Recall that the first message, @code{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, 
@code{Received 1024 bytes from 10.1.3.3}, is from the UDP echo server, 
generated when it receives the echo packet.  The final message, 
@code{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, two from node zero and two from node one:

@verbatim
third-0-0.pcap  third-0-1.pcap  third-1-0.pcap  third-1-1.pcap
@end verbatim

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.

@verbatim
  tcpdump -nn -tt -r third-0-1.pcap
@end verbatim

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

@verbatim
  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
@end verbatim

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,

@verbatim
  tcpdump -nn -tt -r third-0-0.pcap
@end verbatim

Again, you should see some familiar looking contents:

@verbatim
  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
@end verbatim

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,

@verbatim
  tcpdump -nn -tt -r third-1-0.pcap
@end verbatim

Again, you should see some familiar looking contents:

@verbatim
  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
@end verbatim

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:

@verbatim
  tcpdump -nn -tt -r third-1-1.pcap
@end verbatim

You should see some familiar looking contents:

@verbatim
  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
@end verbatim

This should be easily understood.  If you've forgotten, go back and look at
the discussion in @code{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.  Let's do this by hooking into the 
@code{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 @command{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 @code{scratch/mythird.cc} script, add the 
following function:

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

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 
@code{Config::Connect} function.  Add the following lines of code to the
script just before the @code{Simulator::Run} call.

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

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

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 @code{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,

@verbatim
  /NodeList/7/$ns3::MobilityModel/CourseChange
@end verbatim

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 @code{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 @code{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.

@verbatim
  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
@end verbatim

If you are feeling brave, there is a list of all trace sources in the 
@uref{http://www.nsnam.org/doxygen-release/index.html,,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 @code{Attributes} via the command line as we have previously 
discussed.

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

-- The @command{ns-3} development team.