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% One simple example
% If someone could clean up and add comments to it, it would be great...
\chapter{Introduction}
\begin{quote}
Let's start at the very beginning, \\
a very nice place to start, \\
when you sing, you begin with A, B, C, \\
when you simulate, you begin with the topology,\footnote{%
with apologies to Rodgers and Hammerstein}\\
\ldots
\end{quote}
This document (\emph{ns Notes and Documentation}) provides reference
documentation for ns.
Although we begin with a simple simulation script,
resources like Marc Greis's tutorial web pages
(originally at his web site,
now at \url{http://www.isi.edu/nsnam/ns/tutorial/})
or the slides from one of the ns tutorials
are problably better places to begin for the ns novice.
We first begin by showing a simple simulation script.
This script is also available in the sources in
\nsf{tcl/ex/simple.tcl}.
This script defines a simple topology of four nodes,
and two agents, a UDP agent with a CBR traffic generator, and a TCP agent.
The simulation runs for $3s$. The output is two trace files,
\code{out.tr} and \code{out.nam}.
When the simulation completes at the end of $3s$,
it will attempt to run a nam visualisation of the simulation on your
screen.
\begin{program}
{\cf # The preamble}
set ns [new Simulator] \; initialise the simulation;
{\cf # Predefine tracing}
set f [open out.tr w]
$ns trace-all $f
set nf [open out.nam w]
$ns namtrace-all $nf
\clearpage
{\cf # so, we lied. now, we define the topology}
{\cf #}
{\cf #} n0
{\cf #} \bs
{\cf #} 5Mb \bs
{\cf #} 2ms \bs
{\cf #} \bs
{\cf #} n2 --------- n3
{\cf #} / 1.5Mb
{\cf #} 5Mb / 10ms
{\cf #} 2ms /
{\cf #} /
{\cf #} n1
{\cf #}
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
$ns duplex-link $n0 $n2 5Mb 2ms DropTail
$ns duplex-link $n1 $n2 5Mb 2ms DropTail
$ns duplex-link $n2 $n3 1.5Mb 10ms DropTail
{\cf # Some agents.}
set udp0 [new Agent/UDP] \; A UDP agent;
$ns attach-agent $n0 $udp0 \; on node $n0;
set cbr0 [new Application/Traffic/CBR] \; A CBR traffic generator agent;
$cbr0 attach-agent $udp0 \; attached to the UDP agent;
$udp0 set class_ 0 \; actually, the default, but\ldots;
set null0 [new Agent/Null] \; Its sink;
$ns attach-agent $n3 $null0 \; on node $n3;
$ns connect $udp0 $null0
$ns at 1.0 "$cbr0 start"
puts [$cbr0 set packetSize_]
puts [$cbr0 set interval_]
{\cf # A FTP over TCP/Tahoe from $n1 to $n3, flowid 2}
set tcp [new Agent/TCP]
$tcp set class_ 1
$ns attach-agent $n1 $tcp
set sink [new Agent/TCPSink]
$ns attach-agent $n3 $sink
set ftp [new Application/FTP] \; TCP does not generate its own traffic;
$ftp attach-agent $tcp
$ns at 1.2 "$ftp start"
$ns connect $tcp $sink
$ns at 1.35 "$ns detach-agent $n0 $tcp ; $ns detach-agent $n3 $sink"
\clearpage
{\cf # The simulation runs for \(3s\).}
{\cf # The simulation comes to an end when the scheduler invokes the finish\{\} procedure below.}
{\cf # This procedure closes all trace files, and invokes nam visualization on one of the trace files.}
$ns at 3.0 "finish"
proc finish \{\} \{
global ns f nf
$ns flush-trace
close $f
close $nf
puts "running nam..."
exec nam out.nam &
exit 0
\}
{\cf # Finally, start the simulation.}
$ns run
\end{program}
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