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<H1 align=center>IV. The first Tcl script</H1>
<P>[<A href="http://www.isi.edu/nsnam/ns/tutorial/nsbasic.html">Previous 
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to the index</A>] </P>
<P>In this section, you are going to develop a Tcl script for ns which simulates 
a simple topology. You are going to learn how to set up nodes and links, how to 
send data from one node to another, how to monitor a queue and how to start nam 
from your simulation script to visualize your simulation. </P>
<HR>
<A name=first>
<P><STRONG>IV.1. How to start</STRONG><BR>Now we are going to write a 'template' 
that you can use for all of the first Tcl scripts. You can write your Tcl 
scripts in any text editor like joe or emacs. I suggest that you call this first 
example 'example1.tcl'. </P>
<P>First of all, you need to create a simulator object. This is done with the 
command 
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set ns [new Simulator]</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>Now we open a file for writing that is going to be used for the nam trace 
data. 
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set nf [open out.nam w]
$ns namtrace-all $nf
</PRE></CODE></TD></TR></TBODY></TABLE>The first line opens the file 'out.nam' 
for writing and gives it the file handle 'nf'. In the second line we tell the 
simulator object that we created above to write all simulation data that is 
going to be relevant for nam into this file. </P>
<P>The next step is to add a 'finish' procedure that closes the trace file and 
starts nam. 
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proc finish {} {
        global ns nf
        $ns flush-trace
        close $nf
        exec nam out.nam &amp;
        exit 0
}
</PRE></CODE></TD></TR></TBODY></TABLE>You don't really have to understand all 
of the above code yet. It will get clearer to you once you see what the code 
does. </P>
<P>The next line tells the simulator object to execute the 'finish' procedure 
after 5.0 seconds of simulation time. 
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$ns at 5.0 "finish"
</PRE></CODE></TD></TR></TBODY></TABLE>You probably understand what this line 
does just by looking at it. ns provides you with a very simple way to schedule 
events with the 'at' command. </P>
<P>The last line finally starts the simulation. 
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$ns run
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>You can actually save the file now and try to run it with 'ns example1.tcl'. 
You are going to get an error message like 'nam: empty trace file out.nam' 
though, because until now we haven't defined any objects (nodes, links, etc.) or 
events. We are going to define the objects in <A 
href="http://www.isi.edu/nsnam/ns/tutorial/nsscript1.html#second">section 2</A> 
and the events in <A 
href="http://www.isi.edu/nsnam/ns/tutorial/nsscript1.html#third">section 3</A>. 
</P>
<P>You will have to use the code from this section as starting point in the 
other sections. You can download it <A 
href="http://www.isi.edu/nsnam/ns/tutorial/examples/template.tcl">here</A>. </P>
<HR>
<A name=second>
<P><STRONG>IV.2. Two nodes, one link</STRONG><BR>In this section we are going to 
define a very simple topology with two nodes that are connected by a link. The 
following two lines define the two nodes. (Note: You have to insert the code in 
this section <STRONG>before</STRONG> the line '$ns run', or even better, before 
the line '$ns at 5.0 "finish"'). 
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set n0 [$ns node]
set n1 [$ns node]
</PRE></CODE></TD></TR></TBODY></TABLE>A new node object is created with the 
command '$ns node'. The above code creates two nodes and assigns them to the 
handles 'n0' and 'n1'. </P>
<P>The next line connects the two nodes. 
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$ns duplex-link $n0 $n1 1Mb 10ms DropTail
</PRE></CODE></TD></TR></TBODY></TABLE>This line tells the simulator object to 
connect the nodes n0 and n1 with a duplex link with the bandwidth 1Megabit, a 
delay of 10ms and a DropTail queue. </P>
<P>Now you can save your file and start the script with 'ns example1.tcl'. nam 
will be started automatically and you should see an output that resembles the 
picture below. </P>
<P><IMG height=199 alt="Nam snap shot" src="nsscript1.files/namss2.gif" 
width=334> </P>
<P>You can download the complete example <A 
href="http://www.isi.edu/nsnam/ns/tutorial/examples/example1a.tcl">here</A> if 
it doesn't work for you and you think you might have made a mistake. </P>
<HR>
<A name=third>
<P><STRONG>IV.3 Sending data</STRONG><BR>Of course, this example isn't very 
satisfying yet, since you can only look at the topology, but nothing actually 
happens, so the next step is to send some data from node n0 to node n1. In ns, 
data is always being sent from one 'agent' to another. So the next step is to 
create an agent object that sends data from node n0, and another agent object 
that receives the data on node n1. 
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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
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>These lines create a UDP agent and attach it to the node n0, then attach a 
CBR traffic generatot to the UDP agent. CBR stands for 'constant bit rate'. Line 
7 and 8 should be self-explaining. The packetSize is being set to 500 bytes and 
a packet will be sent every 0.005 seconds (i.e. 200 packets per second). You can 
find the relevant parameters for each agent type in the <A 
href="http://www.isi.edu/nsnam/ns/doc/index.html">ns manual page</A><BR></P>
<P>The next lines create a Null agent which acts as traffic sink and attach it 
to node n1. 
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set null0 [new Agent/Null] 
$ns attach-agent $n1 $null0
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>Now the two agents have to be connected with each other. 
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$ns connect $udp0 $null0
</PRE></CODE></TD></TR></TBODY></TABLE></P>
<P>And now we have to tell the CBR agent when to send data and when to stop 
sending. Note: It's probably best to put the following lines just before the 
line '$ns at 5.0 "finish"'. 
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$ns at 0.5 "$cbr0 start"
$ns at 4.5 "$cbr0 stop"
</PRE></CODE></TD></TR></TBODY></TABLE>This code should be self-explaining 
again. </P>
<P>Now you can save the file and start the simulation again. When you click on 
the 'play' button in the nam window, you will see that after 0.5 simulation 
seconds, node 0 starts sending data packets to node 1. You might want to slow 
nam down then with the 'Step' slider. </P>
<P><IMG height=199 alt="Nam snap shot" src="nsscript1.files/namss3.gif" 
width=334> </P>
<P>I suggest that now you start some experiments with nam and the Tcl script. 
You can click on any packet in the nam window to monitor it, and you can also 
click directly on the link to get some graphs with statistics. I also suggest 
that you try to change the 'packetsize_' and 'interval_' parameters in the Tcl 
script to see what happens. You can download the full example <A 
href="http://www.isi.edu/nsnam/ns/tutorial/examples/example1b.tcl">here</A>. 
</P>
<P>Most of the information that I needed to be able to write this Tcl script was 
taken directly from the example files in the 'tcl/ex/' directory, while I 
learned which CBR agent arguments (packetSize_, interval_) I had to set from the 
<A href="http://www.isi.edu/nsnam/ns/doc/index.html">ns manual page</A>. </P>
<HR>

<P>[<A href="http://www.isi.edu/nsnam/ns/tutorial/nsbasic.html">Previous 
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href="mailto:ns-users@isi.edu">ns-users@isi.edu</A></ADDRESS></BODY></HTML>
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