IV. The first Tcl script

IV.1. How to start
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'.

First of all, you need to create a simulator object. This is done with the command

set ns [new Simulator]

Now we open a file for writing that is going to be used for the nam trace data.

set nf [open out.nam w] $ns namtrace-all $nf

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.

The next step is to add a 'finish' procedure that closes the trace file and starts nam.

proc finish {} { global ns nf $ns flush-trace close $nf exec nam out.nam & exit 0 }

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.

The next line tells the simulator object to execute the 'finish' procedure after 5.0 seconds of simulation time.

$ns at 5.0 "finish"

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.

The last line finally starts the simulation.

$ns run

IV.2. Two nodes, one link
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 before the line '$ns run', or even better, before the line '$ns at 5.0 "finish"').

set n0 [$ns node] set n1 [$ns node]

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'.

The next line connects the two nodes.

$ns duplex-link $n0 $n1 1Mb 10ms DropTail

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.

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.

IV.3 Sending data
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.

#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