Queuing Models in NS2

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…

nohup command in Linux

Often you come across a situation where you try to open a machine remotely using ssh and try to start a server or a run a command indefinitely.

But once you close the ssh session, your session also terminates and the background process also terminates. So here is a solution.

To open ssh remotely,

prompt $] ssh username@machinename
Ex: ssh root@
Ex: ssh root@example.com 
prompt $] ssh -X username @machinename 
Ex. ssh -X root@ 

(this -X indicates the remote session can be opened in X window (GUI) mode)

Assume we need to start a httpd server in the remote machine.
we can issue the command like this

prompt$] nohup command 
if any error or log information may be stored in the nohup.out file. if you want to redirect to a file use the redirected symbol (1>  indicates standard output and 2> indicates standard Error). Specify the filenames for the output. Here is the typical nohup command to start the httpd server.

prompt $] nohup  /etc/init.d/httpd start  1> file.out 2> file.err & 
nohup is the command simply tells "no hangup". This will send the output to non-tty.
The above command will start the httpd server and any error or output will be written on to the files file.err and file.out respectively (The & indicates the background process that closes the current terminal and prompts for further inputs).
nohup in Linux

This command I learnt when i try to deploy Sage math libraries in a IBM Blade Server. We were really struggling to run the notebook in sagemath library. But finally accomplished using the nohup command.

The alternative to nohup is crontab or cronjob can be used.

 T S Pradeep Kumar

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