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…

String Functions in C

Like numbered arrays, C handles character arrays (strings).
  1. Each string is identified as a character array and ends with a ‘\0’(null) character (the compiler automatically adds the null character). So the end of string is identified as a null character.
  2. Being an array, all the elements of the string array stored in continuous memory locations
following is the declaration of character array
char name[50];
char name[]=”Pradeep Kumar”;
char name[20]=”Hello Pradeep”;
Strings can be handled or manipulated through loops or library functions. There are some library functions to handle strings are available at string.h
Some of them are
  • This is to find the length of the string and returns an integer
  • For example, if “Hello” is the string, the length will be 5 and “Hello “, the length is 6 (there is a blank space after o in hello)
strcat(String1, string2)
  • to concatenate two strings
  • One string will be appended to another string (the concatenation happens by inserting the new string at the null character ‘\0’ of the first string)
  • Example, if String1 is “Hello” and String2 is “Pradeep”, then strcat(string1, string2) will be “HelloPradeep”
  • Similarly,  if String1 is “Hello ” and String2 is “Pradeep”, then strcat(string1, string2) will be “Hello Pradeep”
strcpy(Destination, source)
  • This function is to copy one string to other
  • usually the copying string will be the 2nd parameter and copied string will be the first parameter
  • Example, if string1 is “Bad” and string2 is “Good” the strcpy(string1,string2) will give Good for string1
strcmp(Destination, source)
  • This function is to compare two strings whether they are equal or not
  • like numbers, two strings should not be compared directly like if(a==b)
  • If both the strings are equal, the function returns 0, else it will return the difference of ASCII value of the first non matching characters
  • Example, strcmp(“Their”, “There”) will return ASCII(i) – ASCII(r) (an integer will be returned)
Example Program
//program to demonstrate string handling functions
#include <stdio.h>
#include <conio.h>
int main()
    char a[50];
    char b[50];
    printf("Please the two strings one by one\n");
    printf("Length of String a is %d \n",strlen(a));
    printf("Length of String b is %d \n",strlen(b));
    if(!strcmp(a,b))  //Comparing two string will return 0, ! before strcmp() is needed
    printf("Both the strings are Equal");
    printf("Both the strings are not equal");
    strcat(a,b);  //Concatenation function
    printf("the concatenated String is:");
    printf("The reverse string is\n");
    return 0;
In the above program , instead of gets() function, scanf() can be used, but the disadvantage of scanf() being it truncates the blank space, new line, form feed, tab.
So for example, if the input give is “Hello pradeep”, then Hello only will be accepted, the string after the blank space will be omitted by the compiler, so gets() can be used.
Similarly, puts() or printf() can be used to display strings.
 T S Pradeep Kumar

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