Queuing Models in NS2

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M/M/1 is a system with poisson arrival time, servicing exponentially and a queue of unlimited capacity and type of FIFO Queue. This is the simplest queuing system.  NS2 supports various distributions like pareto, exponential, constant, unifrom, etc to handle the network dynamics and metrics. So it is very easy to test the given network link to monitor a given queue using any of these queuing models. The listing 3 and 4 are monitoring the link when DropTail queue is used with a capacity of finite and infinite. Listing 13.3 uses infinite capacity and Listing 13.4 uses Finite capacity The output screen shot is shown below the scripts for further understanding
Listing 3 – M/M/1 Queuing Model #new Simulator creation set ns [new Simulator] #trace file creation for capturing the UDP data set tf [open out.tr w] $ns trace-all $tf
#setting the exponential distribution param set lambda 30.0 set mu     33.0
#creation of nodes set n1 [$ns node] set n2 [$ns node] #The queue limit is 1Lakh as the capacity is infin…

Error "bailing in Tcl::eval"

In ns2, if you ever encounter a error like "bailing in Tcl::eval", then it could be due the size of the buffer that Tcl:: eval may offer
By default the buffer size is 1024 and it can be increased to 2048 (for example)

Usually this error will be encountered when you increase the number of nodes in the simulation (for example more than 100 nodes might give you this error)

bailing in Tcl::eval
bailing in Tcl::eval
How to overcome this error? Here is a small workaround
Open the command prompt and go the ns-allinone-2.35/tclcl-1.xx/ folder and open the file Tcl.cc

$] cd ns-allinone-2.35/tclcl-1.20/Tcl.cc (in my case)

in the following function (Line number: 202, change 1024 to 2048) as shown below

void Tcl::eval()
{
char* p = bp_;
bp_ = p + strlen(p) + 1;
/*XXX*/
if (bp_ >= &buffer_[2048]) {
fprintf(stderr, "bailing in Tcl::eval\n");
assert(0);
exit(1);
}
eval(p);
bp_ = p;
}

Once the changes are made, reinstall ns2 itself

$] cd ns-allinone-2.35/
$] ./install 

T S Pradeep Kumar

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