453 Finally, recall that we added the ability to control the number of CSMA devices

   454 in the simulation by command line argument.  You can change this argument in

   455 the same way as when we looked at changing the number of packets echoed in the

   456 @code{first.cc} example.  Try setting the number of ``extra'' devices to four:

   457 

   458 @verbatim

   459   ~/repos/ns-3-dev > ./waf --run "scratch/second --nCsma=4"

   460   Entering directory `/home/craigdo/repos/ns-3-dev/build'

   461   Compilation finished successfully

   462   Sent 1024 bytes to 10.1.2.5

   463   Received 1024 bytes from 10.1.1.1

   464   Received 1024 bytes from 10.1.2.5

   465   ~/repos/ns-3-dev >

   466 @end verbatim

   467 

   468 Notice that the echo server has now been relocated to the last of the CSMA

   469 nodes, which is 10.1.2.5 instead of the default case, 10.1.2.4.  You can 

   470 increase the number to your hearts content, but remember that you will get a

   471 pcap trace file for every node in the simulation.  One thing you can do to

   472 keep from getting all of those pcap traces with nothing but ARP exchanges in 

   473 them is to be more specific about which nodes and devices you want to trace.

   474 

   475 Let's take a look at @code{scratch/second.cc} and be more specific about which

   476 nodes and devices we want to trace.  The file we provide uses the 

   477 @code{EnablePcapAll} methods of the helpers to enable pcap on all devices.  

   478 We want to use the more specific method, @code{EnablePcap}, which takes

   479 a node number and device number as parameters.  Go ahead and replace the 

   480 @code{EnablePcapAll} calls with the calls below.

   481 

   482 @verbatim

   483   PointToPointHelper::EnablePcap ("second", p2pNodes.Get (0)->GetId (), 0);

   484   CsmaHelper::EnablePcap ("second", csmaNodes.Get (nCsma)->GetId (), 0);

   485 @end verbatim

   486 

   487 We know that we want to create a pcap file with the base name "second" and

   488 we also know that the device of interest in both cases is going to be zero,

   489 so those parameters are not really interesting.  In order to get the node

   490 number, you have two choices:  first, nodes are numbered in a monotonically

   491 increasing fashion starting from zero in the order in which you created them.

   492 One way to get a node number to pass to the helper is to figure this number

   493 out ``manually.''  If you take a look at the network topology illustration at

   494 the beginning of the file, you can see that the last CSMA node is going to be

   495 node number code{nCsma + 1}.  This can become annoyingly difficult in larger

   496 simulations.  An alternate way, which we use here, is to realize that the

   497 @code{NodeContainer}s contain pointers to ns-3 @code{Node} Objects.  The

   498 @code{Node} Object has a method called @code{GetId} which will return that

   499 node's ID.  Let's go take a look at the Doxygen for the @code{Node} and locate

   500 that method, which is further down in the code than we've seen so far.

   501 

   502 Go to the Doxygen documentation for your release (recall that you can find it

   503 on the project web site).  You can get to the @code{Node} documentation by

   504 looking through at the ``Classes'' tab in and scrolling down to 

   505 @code{ns3::Node} in the list.  Select @code{ns3::Node} and you will be taken

   506 to the documentation for the @code{Node} class.  If you now scroll down to the

   507 @code{GetId} method and select it, you will be taken to the detailed 

   508 documentation for the method.  Using the @code{GetId} method can make 

   509 determining node numbers much easier in complex topologies.

   510 

   511 Now that we have got some trace filtering in place, it is reasonable to start

   512 increasing the number of CSMA devices in our simulation.  If you run the 

   513 simulation setting @code{nCsma} to 100, you will see the following:

   514 

   515 @verbatim

   516   ~/repos/ns-3-dev > ./waf --run "scratch/second --nCsma=100"

   517   Entering directory `/home/craigdo/repos/ns-3-dev/build'

   518   Compilation finished successfully

   519   Sent 1024 bytes to 10.1.2.101

   520   Received 1024 bytes from 10.1.1.1

   521   Received 1024 bytes from 10.1.2.101

   522   ~/repos/ns-3-dev >

   523 @end verbatim

   524 

   525 Note that the echo server is now located at 10.1.2.101 which corresponds to

   526 100 ``extra'' CSMA nodes with the echo server on the last one.  If you list

   527 the pcap files in the top level directory,

   528 

   529 @verbatim

   530   ~/repos/ns-3-dev > ls *.pcap

   531   second-0-0.pcap  second-101-0.pcap

   532   ~/repos/ns-3-dev >

   533 @end verbatim

   534 

   535 you will see that we have, in fact, only created two trace files.  The trace

   536 file @code{second-0-0.pcap} is the ``leftmost'' point-to-point device which is

   537 the echo packet source.  The file @code{second-101-0.pcap} corresponds to the

   538 rightmost CSMA device which is where the echo server resides.

   539 

   540 

   541 

   542 

   543