[e2e] Simulator for wireless network

S. Keshav keshav at uwaterloo.ca
Sun Apr 15 07:18:13 PDT 2007

	The discussions on this topic should convince you, I hope, that  
before using simulations, their role has to be clearly understood.

For physical systems, such as planes, cars, and sailboats, the  
primary  operational parameters are the laws of physics, which can be  
modeled to as great a degree of accuracy as desired. In other words,  
I can overlay a 2D or 3D grid on the underlying system and apply the  
laws of physics at each grid point. This is why it's possible to  
accurately design physical systems from computer simulations.

For computer systems, where a single line of code can completely  
change the behavior of the system, one has to confront the fact that  
no simulation is ever going to be accurate. As has been pointed out  
already, the issue is not just of radio propagation modeling, which  
is hard enough, but the problem is that there are many layers of  
software that intervene from the receipt of a radio signal and a user- 
perceptible effect. A slight change in any of these can materially  
affect the result. For instance, a slight change in the card firmware  
can change the way in which packets are handed to the driver, which  
can change the timing at which packets are received by an  
application, which may result in user-perceptible audio effects for  
VOIP over WLAN. It is practically impossible to model these with any  
accuracy, and even if you do, a patch to the firmware, driver, OS, or  
application will invalidate your results.

This is the reason why 'proof by simulation', for computer systems,  
at least, is farcical. Not only are simulators known to be buggy, but  
they are also simulating a system that is too loosely coupled to be  
adequately modeled.

So, does this mean that simulation is useless? Not really.  
Simulations are useful in helping to form intuitions about the  
underlying system. They can also help explore the parameter space  
systematically. But, one has to realize that it is a coarse tool, and  
necessarily so. As long as you go in with your eyes open, simulations  
are a reasonable first step (but only the first step).


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