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📁 介绍了网络仿真软件NS2的使用
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<H1 align=center>V. Making it more interesting</H1>
<P>[<A href="http://www.isi.edu/nsnam/ns/tutorial/nsscript1.html">Previous 
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to the index</A>] </P>
<P>In this section we are going to define a topology with four nodes in which 
one node acts as router that forwards the data that two nodes are sending to the 
fourth node. I will explain find a way to distinguish the data flows from the 
two nodes from each other, and I will show how a queue can be monitored to see 
how full it is, and how many packets are being discarded. </P>
<HR>
<A name=first>
<P><STRONG>V.1. The topology</STRONG><BR>As always, the first step is to define 
the topology. You should create a file 'example2.tcl', using the <A 
href="http://www.isi.edu/nsnam/ns/tutorial/examples/template.tcl">code</A> from 
<A href="http://www.isi.edu/nsnam/ns/tutorial/nsscript1.html#first">section 
IV.1</A> as a template. As I said before, this code will always be similar. You 
will always have to create a simulator object, you will always have to start the 
simulation with the same command, and if you want to run nam automatically, you 
will always have to open a trace file, initialize it, and define a procedure 
which closes it and starts nam. </P>
<P>Now insert the following lines into the code to create four nodes. 
<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
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    <TD><CODE><PRE>
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>The following piece of Tcl code creates three duplex links between the nodes. 

<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
  <TR>
    <TD><CODE><PRE>
$ns duplex-link $n0 $n2 1Mb 10ms DropTail
$ns duplex-link $n1 $n2 1Mb 10ms DropTail
$ns duplex-link $n3 $n2 1Mb 10ms DropTail
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>You can save and start the script now. You might notice that the topology 
looks a bit awkward in nam. You can hit the 're-layout' button to make it look 
better, but it would be nice to have some more control over the layout. Add the 
next three lines to your Tcl script and start it again. 
<TABLE cellPadding=5 bgColor=#eeeeee>
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    <TD><CODE><PRE>
$ns duplex-link-op $n0 $n2 orient right-down      
$ns duplex-link-op $n1 $n2 orient right-up 
$ns duplex-link-op $n2 $n3 orient right 
</PRE></CODE></TD></TR></TBODY></TABLE>You will probably understand what this 
code does when you look at the topology in the nam window now. It should look 
like the picture below. </P>
<P><IMG height=199 alt="Nam snap shot" src="nsscript2.files/namss4.gif" 
width=300> </P>
<P>Note that the autolayout related parts of nam are gone, since now you have 
taken the layout into your own hands. The options for the orientation of a link 
are right, left, up, down and combinations of these orientations. You can 
experiment with these settings later, but for now please leave the topology the 
way it is. </P>
<HR>
<A name=second>
<P><STRONG>V.2. The events</STRONG><BR>Now we create two UDP agents with CBR 
traffic sources and attach them to the nodes n0 and n1. Then we create a Null 
agent and attach it to node n3. 
<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
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    <TD><CODE><PRE>
#Create a UDP agent and attach it to node n0
set udp0 [new Agent/UDP]
$ns attach-agent $n0 $udp0

# Create a CBR traffic source and attach it to udp0
set cbr0 [new Application/Traffic/CBR]
$cbr0 set packetSize_ 500
$cbr0 set interval_ 0.005
$cbr0 attach-agent $udp0

#Create a UDP agent and attach it to node n1
set udp1 [new Agent/UDP]
$ns attach-agent $n1 $udp1

# Create a CBR traffic source and attach it to udp1
set cbr1 [new Application/Traffic/CBR]
$cbr1 set packetSize_ 500
$cbr1 set interval_ 0.005
$cbr1 attach-agent $udp1

set null0 [new Agent/Null] 
$ns attach-agent $n3 $null0 
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>The two CBR agents have to be connected to the Null agent. 
<TABLE cellPadding=5 bgColor=#eeeeee>
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    <TD><CODE><PRE>
$ns connect $udp0 $null0 
$ns connect $udp1 $null0
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>We want the first CBR agent to start sending at 0.5 seconds and to stop at 
4.5 seconds while the second CBR agent starts at 1.0 seconds and stops at 4.0 
seconds. 
<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
  <TR>
    <TD><CODE><PRE>
$ns at 0.5 "$cbr0 start" 
$ns at 1.0 "$cbr1 start"
$ns at 4.0 "$cbr1 stop"
$ns at 4.5 "$cbr0 stop"

</PRE></CODE></TD></TR></TBODY></TABLE></P>When you start the script now with 
'ns example2.tcl', you will notice that there is more traffic on the links from 
n0 to n2 and n1 to n2 than the link from n2 to n3 can carry. A simple 
calculation confirms this: We are sending 200 packets per second on each of the 
first two links and the packet size is 500 bytes. This results in a bandwidth of 
0.8 megabits per second for the links from n0 to n2 and from n1 to n2. That's a 
total bandwidth of 1.6Mb/s, but the link between n2 and n3 only has a capacity 
of 1Mb/s, so obviously some packets are being discarded. But which ones? Both 
flows are black, so the only way to find out what is happening to the packets is 
to monitor them in nam by clicking on them. In the next two sections I'm going 
to show you how to distinguish between different flows and how to see what is 
actually going on in the queue at the link from n2 to n3. 
<P></P>
<HR>
<A name=third>
<P><STRONG>V.3. Marking flows</STRONG><BR>Add the following two lines to your 
CBR agent definitions. 
<TABLE cellPadding=5 bgColor=#eeeeee>
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    <TD><CODE><PRE>
$udp0 set class_ 1
$udp1 set class_ 2

</PRE></CODE></TD></TR></TBODY></TABLE>The parameter 'fid_' stands for 'flow 
id'. </P>
<P>Now add the following piece of code to your Tcl script, preferably at the 
beginning after the simulator object has been created, since this is a part of 
the simulator setup. 
<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
  <TR>
    <TD><CODE><PRE>
$ns color 1 Blue
$ns color 2 Red

</PRE></CODE></TD></TR></TBODY></TABLE>This code allows you to set different 
colors for each flow id. </P>
<P><IMG height=199 alt="Nam snap shot" src="nsscript2.files/namss5.gif" 
width=300> </P>
<P>Now you can start the script again and one flow should be blue, while the 
other one is red. Watch the link from node n2 to n3 for a while, and you will 
notice that after some time the distribution between blue and red packets isn't 
too fair anymore (at least that's the way it is on my system). In the next 
section I'll show you how you can look inside this link's queue to find out what 
is going on there. </P>
<HR>
<A name=fourth>
<P><STRONG>V.4. Monitoring a queue</STRONG><BR>You only have to add the 
following line to your code to monitor the queue for the link from n2 to n3. 
<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
  <TR>
    <TD><CODE><PRE>
$ns duplex-link-op $n2 $n3 queuePos 0.5
</PRE></CODE></TD></TR></TBODY></TABLE>Start ns again and you will see a picture 
similar to the one below after a few moments. </P>
<P><IMG height=199 alt="Nam snap shot" src="nsscript2.files/namss6.gif" 
width=300> </P>You can see the packets in the queue now, and after a while you 
can even see how the packets are being dropped, though (at least on my system, I 
guess it might be different in later or earlier releases) only blue packets are 
being dropped. But you can't really expect too much 'fairness' from a simple 
DropTail queue. So let's try to improve the queueing by using a SFQ (stochastic 
fair queueing) queue for the link from n2 to n3. Change the link definition for 
the link between n2 and n3 to the following line. 
<TABLE cellPadding=5 bgColor=#eeeeee>
  <TBODY>
  <TR>
    <TD><CODE><PRE>
$ns duplex-link $n3 $n2 1Mb 10ms SFQ     
</PRE></CODE></TD></TR></TBODY></TABLE>The queueing should be 'fair' now. The 
same amount of blue and red packets should be dropped. 
<P></P>
<P><IMG height=199 alt="Nam snap shot" src="nsscript2.files/namss7.gif" 
width=300> </P>
<P>You can download the full example <A 
href="http://www.isi.edu/nsnam/ns/tutorial/examples/example2.tcl">here</A>. </P>
<HR>

<P>[<A href="http://www.isi.edu/nsnam/ns/tutorial/nsscript1.html">Previous 
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