Random Generation of bi-directional Graphs in Omnet

Omar Usman

The Omnet usman.html manual has code for generation of random topologies. The type of network generated can contain uni-directional, or bi-directional edges. Usually, people working with networks require generation of bi-directional graphs only. What you will see here is a small hack to the Omnet Engine, which can enable you to do that. (Note: I am not an expert on Omnet so most probably there is another simpler way of doing this. If you do, please leave a comment).

The files you need to modify are:

1. src/sim/distrib.cc
2. include/distrib.h
3. src/sim/nedfunctions.cc

Modifications to distrib.cc

You need to create a new function (runiform). What this does is that it retains the random number generated from a previous call to uniform(). The random number is generated every 2-calls.

double runiform(int rng) {
  static int r = 0;
  static double l = 0;
  if(r%2 == 0) l = uniform(0,1);
  r++;
  return l;
}

Modifications to distrib.h

Just one line to be modified here:

SIM_API double runiform(int rng=0);

Modifications to nedfunctions.cc

The current usman.html manual (July 4, 2012) shows a different way of doing this. The code in the manual is not consistent with what I found in the source (4.2.2). You need to add the following code to where the other continuous random variate generation code is present.

DEF(nedf_runiform,
  "quantity runiform(int rng?)",
  "random/continuous",
  "Retains the value generated from a previous call to uniform()",
  {
    int rng = argc==2 ? (int)argv[1] : 0;
    return cNEDValue(runiform(rng), argv[0].getUnit());
})

Compilation and Usage

If you have already compiled omnet once, you can simply run make and compile the engine again. When complete, fire up your editor and open your .NED file. You can now generate bi-directional graphs using the following code from the manual (which is adopted to this hack):

module RandomGraph {
    parameters:
        int count;
        double connectedness; // 0.0    submodules:
        node[count]: Node {
            gates:
                in[count];
                out[count];
        }
    connections allowunconnected:
        for i=0..count-1, for j=0..count-1 {
            node[i].out[j] --> node[j].in[i]
                if i<j && runiform() > connectedness;
            node[j].out[i] --> node[i].in[j]
                if i<j && runiform() > connectedness;
        }
}