Random variables is an important concept in
networks as the modeling of network traffic and other packet arrival times are
mostly random models. Hence there is a necessity of modeling such metrics in
ns2. NS2 supports various random models using different seed generations.
Seeds are numbers that are helpful in
generating the random numbers. The seed number 0 indicates that the random
number order changes every time the simulation is running. But other than 0,
the order in which the random number generated are same.
The following listing 1 shows the random
number generation for various distributions. This listing will just tell you
how to create random number generation for various distributions. For seeing
the output, refer to listing 2
Listing 1 – Random Number Generation Sample
#create the random number generation using RNG
set rand1 [new RNG]
set rand2 [new RNG]
set rand3 [new RNG]
#$repli is the value set already and it can be either 1
or <1 or >1
for {set i 1} {$i < $repli} {incr i} {
$rand1 nextsubstream;
$rand2 nextsubstream;
$rand3 nextsubstream;
}
Here are the distributions
Pareto Distribution
set r1 [new RandomVariable/Pareto]
$r1 userng $rand1
$r1 set avg_ 10.0
$r1 set shape_ 1.2
Constant – Specifies the constant number
set r2 [new RandomVariable/Constant]
$r2 userng $rand2
$r2 set val_ 5.0
Uniform distribution – Specifies the min and
max number
set r3 [new RandomVariable/Uniform]
$r3 userng $rand3
$r3 set min_ 0.0
$r3 set max_ 10.0
Exponential distribution. Specified the
average value:
set r4 [new RandomVariable/Exponential]
$r4 userng $rand1
$r4 set avg_ 5
Hyperexponential distribution
set r5 [new RandomVariable/HyperExponential]
$r5 userng $rand2
$r5 set avg_ 1.0
$r5 set cov_ 4.0

The following listing 2 creates the random number
generation for two different distributions uniform and exponential. The
replication number is supplied through the command line option. If no
replication number is supplied, it will be taken as 0 and it a value less than
1 is supplied then, it will be running with value 1. So the following example
uses the replication number as 0, 1 or any higher number > 1 and it uses a
seed value of 9999
The syntax for setting the seed value is
$defaultRNG seed N
Where N is a interger which may take 0 or 1 or any positive
number
Listing 2 – Random number generation – Case 1
#filename : randtest1.tcl
#
# Usage: ns randtest.tcl [replication number]
#
if {$argc > 1} {
puts
"Usage: ns randtest1.tcl \[replication number\]"
exit
}
set run 1
if {$argc == 1} {
set run [lindex
$argv 0]
}
if {$run < 1} {
set run 1
}
# seed the default RNG
global defaultRNG
$defaultRNG seed 9999
# create the RNGs and set them to the correct substream
set arrivaldist [new RNG]
set size [new RNG]
for {set j 1} {$j < $run} {incr j} {
$arrivaldist nextsubstream
$size
nextsubstream
}
# arrival_ is a exponential random variable describing
the time between
# consecutive packet arrivals
set arrival_ [new RandomVariable/Exponential]
$arrival_ set avg_ 5
$arrival_ userng $arrivaldist
# size_ is a uniform random variable describing packet
sizes
set size_ [new RandomVariable/Uniform]
$size_ set min_ 100
$size_ set max_ 5000
$size_ userng $size
# print the first 5 arrival times and sizes
for {set j 0} {$j < 5} {incr j} {
puts [format
"%8.3f %4d" [$arrival_
value] \
[expr round([$size_ value])]]
}

Here is the sample output for above
file
Prompt]
~/nsallinone2.35/ns2.35/tcl/ex $ ns randtest1.tcl 0
6.358 4783
5.828 1732
1.469 2188
0.732 3076
4.002 626
Prompt]~/nsallinone2.35/ns2.35/tcl/ex
$ ns randtest1.tcl 1
6.358 4783
5.828 1732
1.469 2188
0.732 3076
4.002 626
Prompt]~/nsallinone2.35/ns2.35/tcl/ex
$ ns randtest1.tcl 2
2.091 153
12.085 4732
3.588 2329
1.201 230
5.161 2980
Prompt]~/nsallinone2.35/ns2.35/tcl/ex
$ ns randtest1.tcl 3
2.515 1119
3.154 3118
9.673 1201
13.346 2515
7.052 2115
Prompt]~/nsallinone2.35/ns2.35/tcl/ex
$ ns randtest1.tcl 100
8.940 2149
0.888 4870
0.998 4860
1.801 1205
18.224 2534
Prompt]~/nsallinone2.35/ns2.35/tcl/ex
$ ns randtest1.tcl 0.5
6.358 4783
5.828 1732
1.469 2188
0.732 3076
4.002 626
