[ns] On-Off traffic source

[Daniel Ratton Figueiredo <ratton@cs.umass.edu>]

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.