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Re: [ns] On-Off traffic source




I didn't read the paper carefully, so correct me if I'm wrong.

some questions:

1. why is the case that "There is always at least
one packet being sent per on period" for NS EXPOO? 
Actually if the On period is shorter than the interval,
it won't generate any packet.

2. So the difference between NS and the "Deterministic"
models is just by the "round off errors". I can't see
why by using round off we wouldn't have a "well-defined
behavior". Can you define (or rephrase) this concept 
more clearly? And even if it is
not well-defined, a simple modification
like removing the round-off would work to bring it
back to "well-defined" models.


On Tue, 27 Jun 2000, Daniel Ratton Figueiredo wrote:

> Hello All,
> 
> The following problem came to our attention when we tried to match
> analytical and simulation performance results with the ones obtained using
> NS. It turned out that the results were not consistent on models using the
> NS Traffic/Expoo source.
> 
> The problem arrises when using the NS Expoo source, which is an On-Off
> source, in a simulation model and then validating or comparing the
> performance results obtained with other simulators or analytical models.
> The implementation of the NS Expoo source is different from most commonly
> used On-Off sources, and its specification is also not well documented.
> This confusion can lead to significant discrepancies in the results due to
> different source behavior. In the Tech Report below, we point out the how
> the NS Expoo source implementation differs from commom models, and show
> that this small difference can impact the performance evaluation of
> models. Finally, we suggest a small modification to the NS Expoo traffic
> source to comply with a more traditional and well known On-Off source.
> 
> Thanks for your attention,
> Daniel and Benyuan
> 
> 
> The complete Technical Report, incluiding the modified source code,
> can be found at:
> ftp://ftp.cs.umass.edu/pub/techrept/techreport/2000/UM-CS-2000-025.ps
> 
> Tech Report Title: 
> On the Specification of NS and other Known On-Off Sources
> 
>                            ABSTRACT 
> 
> Traffic source is of great importance to networks modeling. Different
> traffic characteristics can cause other components to behave
> differently, resulting in different network performance measures. 
> On-off source model has been widely adopted in networks research to
> capture the bursty nature of the network traffic; its behavior is
> precisely defined. In this work, we carefully analyzed the common
> structure and the behavior of on-off sources and show that the on-off
> source provided by ns simulator does not correspond to any of the known 
> on-off models. This inconsistency between on-off sources has a direct 
> impact on queueing performance measures. Modelers must be aware of this 
> fact when using the ns simulator to compare or validate results with other 
> modeling frameworks. We suggest that the ns on-off source be replaced by a
> consistent deterministic on-off source; the implementation and analysis 
> were provided. 
> 
> 
> 

Guo, Liang 

[email protected]                     Dept. of Comp. Sci., Boston Univ.,
(617)353-8924 (O)                  111 Cummington St., MCS-217,
(617)375-9206 (H)                  Boston, MA 02215