Subject: Re: [manet] Ns-2 or Glomosim

From: "Emin Gun Sirer" <[email-masked]>
Date: Sat, 1 May 2004 11:09:55 -0400 (EDT)

Dear Ravi,

Three critical properties for network simulators are accuracy, 
speed, and scale. ns2 and glomosim are pretty much equivalent
on all three axis. On accuracy, the core simulators, the protocol 
implementations, and the physical propagation and energy consumption
models have been vetted by the community. On speed & scale, the two
simulators are equally bad. In scenarios commonly used for evaluating 
ad hoc routing protocols (e.g. [1]), ns2 and glomosim perform and
scale with O(N^2), where N is number of nodes, with constant
factors in the same ballpark. In short, you will likely end up
wasting a lot of time waiting for ns2 or glomosim to finish. The
choice between ns2 and glomosim thus comes down to convenience of
their respective programming environments.

I would like to point out SNS, an alternative simulator whose accuracy
is identical to ns2, but whose performance and scalability are 
significantly better. SNS is freely available for downloads here:
     http://www.cs.cornell.edu/people/egs/sns/

SNS is based on the same code base as ns2 - it shares the same
protocol implementations, the same physical propagation model and the
same energy consumption parameters. We have extensively checked its
correctness against ns2 (Note that simulations typically have some
amount of non-determinism, e.g. when packets are sent at precisely the
same time, simulators may pick a different global order. These effects
may cascade and lead to slight differences. When we force SNS and ns2
to follow the same global order, they compute bit-for-bit identical
results).

The main contribution in SNS is a restructuring of the internal events
of the simulation engine to cache and reuse computations. SNS 
eliminates redundant, expensive computations both within and across 
simulation runs through function caching and reuse. This enables SNS to
achieve performance and scale proportional to O(N) for common ad hoc
networking scenarios. SNS runs accurate 10000-node simulations in under
an hour.

The SNS programming environment is identical to ns2 - protocol
implementations written for SNS should work in ns2 without any
modifications, and vice versa.

Choosing a simulator is quite a difficult topic, open to religious
arguments of the kind one occasionally encounters in the comparison 
of programming languages. I hope this message pointed out three 
objective, scientific metrics for choosing a suitable simulator,
and suggested an alternative that improves two out of the three with
no loss of accuracy.

Gun.

--

[1] Josh Broch, David A. Maltz, David B. Johnson, Yih-Chun Hu, and Jorjeta
Jetcheva. A Performance Comparison of Multi-Hop Wireless Ad Hoc
Network Routing Protocols. In Proceedings of the Fourth Annual
ACM/IEEE International Conference on Mobile Computing and Networking,
ACM, Dallas, TX, October 1998. 

[2] Kevin Walsh, Emin Gun Sirer. Staged Simulation: A General Technique
for Improving Simulation Scale and Performance. To appear in ACM
Transactions on Modeling and Computer Simulation (TOMACS), April
2004. 

[3] Kevin Walsh, Emin Gun Sirer. Staged Simulation for Improving the Scale
and Performance of Wireless Network Simulations. In Procedings of the
Winter Simulation Conference, New Orleans, LA, December 2003. 

Ravi kumar writes:
>
>Hi all,
>
>         I am working on wireless security and i have
>to implement an algorithm on wireless networks, ad-hoc
>in particular. 
>
>         I have seen some discussion in this group on
>both Ns-2 and Glomosim.I'd like to find out which one
>is better in regard to functionality and ease of use.
>i know that ns-2 is a generally accepted standard for
>simulation  in the academic community. i dont know
>much about glomosim, anyway if i need a opinion i
>guess this is the place to ask.
>
>   waiting for your views. thanks in advance
>
>ravi  
>
>
>	
>		
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