Publication list Omar AïtHellal
Publication list Omar AïtHellal
Here you find my list of publications, and the links to postscript files (papers submitted to conferences, short versions).
To obtain a copy of the original publication (or to give me some feedback), just send me an email.
Redundancy
 O. AitHellal, E. Altman, Alain.JeanMarie, Irina Kurkova, "On Loss Probabilities in Presence of Redundant Packets and Several Traffic Sources", Submitted to INFOC
OMM'99.
The purpose of this paper is to study the effect of
adding redundancy to an input stream on the losses that occur due to
buffer overflow. We consider several sessions
that generate trafic into a finite capacity queue.
In most of the paper, the input trafic of each session
is modeled by a Poisson input.
We use multidimensional probability generating functions
for solving the recursions
introduced by Cidon, Khamisy and Sidi for computing the loss
probabilities and derive analytical formulas. Using asymptotic
analysis (for large n and small $\rho$),
we obtain good approximations with very low complexity
of computation and memory.
Our analysis allows us to investigate when does adding
redundancy decrease the loss probabilities.
In many cases, adding capacity is seen to
degrade the loss probabilities, which can be explained
by the fact that the gain in adding redundancy is
not sufficient to compensate the additional losses
due to the fact that the trafic load is increased
when redundancy is added.
We show, however, that it is possible to decrease
loss probabilities if a sufficiently large amount of redundancy is added.
Indeed, we show that for an arbitrary stationary
ergodic input process, if $\rho<1$
then redundancy can
improve the performance and reduce loss probabilities
to an arbitrarily small value. We compute the rate of
additional redundancy required for the case Poisson input
processes.
To get the paper, click here.
Flow Control and Stability
 O. AitHellal, E. Altman, "Analysis of TCPVegas and TCPReno" IEEE International Conference on Communications (ICC'97) Montreal, 812 June 1997.
In this paper we use
an analytic fluid approach in order to analyze the
different features of both Vegas and Reno TCP versions.
We then use simulations to confirm our analytic results.
When the available bandwidth is high, indeed Vegas can retransmit
less than onefifth as much data as Reno does, so that the higher
the available bandwidth is, the more efficient Vegas is. However, under
heavy congestion Vegas behaves like Reno and does not
manage to make efficient use of its new mechanism for congestion detection.
The analytic results that we obtain are the evolution
of the window size, round trip times and their averages, and the average
throughput.
To get a short version of the paper, click here.
 O. AitHellal, E. Altman, ``Problems in TCPVegas and TCPReno'', DNAC (De Nouvelles Architectures pour les Communications), UVSQ,
Paris, 35 Décembre 1996.
This paper outlines some problems observed, by simulation in Reno
and Vegas TCP versions. We describe the two protocols and give
examples (obtained by simulation) where these problems can occur. The
most serious problem causes in an erroneous estimation of the RTT,
and can occur in both Vegas and Reno. Also, we show by an
example that the technic which consists on retransmitting packets
when we receive the first or second acknowledgment after
retransmission can't work in many cases when the retransmission timer
(coarse grain timeout) is not sufficiently large (very small as many papers suggested it for ATM networks).
To get a short version of the paper, click here.

Corrected TCP versions for REAL network simulator

TCP Tahoe, click tahoe.c to get the source.
TCP Reno, click reno.c to get the source.
TCP Vegas, click vegas.c to get the source.

Get also the indispensable file tcp.h

TCP versions for REAL network simulator 5.0

TCP Tahoe, click tahoe.c to get the source.
TCP Reno, click reno.c to get the source.
TCP Vegas, click vegas.c to get the source.
TCP NewReno, click new_reno.c to get the source.

Get also the indispensable file tcp.h
 O. AitHellal, E. Altman, T. Basar, ``Rate based flow control
with bandwidth information'', (invited paper) European Trans. on Telecom. , special issue on ABR, pp. 5566, 1996. (invited paper) the proceedings of the 35th IEEE Conference on Decision and Control, Kobe, Japan, Dec. 1996.
The ATM Forum has chosen the ratebased approach for flow control of
ABR traffic in ATM, and has specified the behavior of the source and
destination, as well as the manner in which feedback information
should be conveyed back to the source. The decision on the precise
control mechanism, however, has been left to the designer of the
switches. We propose in this paper a reactive control scheme that is
based only on information on the available bandwidth. We analyze its
stability, and test its performance by simulations in the
presence of other higher priority CBR or VBR traffic.
 O. AitHellal, E. Altman, D. Elwadghiri and M. Erramdani, "Performance Evaluation of the RateBased Flow Control Mechanism for ABR Service", INRIA Research Report No. RR3131. Second IFIP Workshop on Traffic Management and synthesis of ATM networks, Montreal, 2426 Sept. 1997. To appear in Telecommunication systems, special issue.
In this paper we investigate the performances of the EFCIbased
(Explicit Forward Congestion Indication) and ERbased (Explicit Rate)
algorithms for the ratebased flow control of the ABR (available Bit
Rate) traffic in an ATM network. We consider the case of two switches in tandem. We present several definitions of bottleneck, and provide conditions that determine whether the first, the second
or both queues are bottleneck. We show that it is not necessarily the queue with
the slowest transmission rate that is ``responsible'' for a bottleneck. We derive
analytic formulas for the maximum queue length. We compare our results to those
obtained by approximating a network by a simpler one, containing only the bottleneck
switch. We show that the maximum queue lengths under the approximating approach may
largely underestimate the ones obtained in the real network.
To get the paper, click here.
 O. AitHellal, E. Altman, "Performance Evaluation of the RateBased Flow Control Mechanism for ABR Service: Generalization", IEEE INFOCOMM'99, NewYork, Feb. 1999.
In this this work, we investigate the performances of the EFCIbased
(Explicit Forward Congestion Indication) and ERbased (Explicit Rate)
algorithms for the ratebased flow control of the ABR
(Available Bit Rate) traffic in an ATM network.
We consider the case of multiple switches in tandem. We present
several definitions of
a bottleneck, and provide conditions that determine which queue is the
bottleneck. We show that it is not necessarily
the queue with the slowest transmission rate that is ``responsible'' for
a bottleneck. We derive analytic formulas for the maximum queue length.
To get the paper, click here.
 O. AitHellal, "Stability Analysis of ABR Congestion Control Algorithms", Submitted.
In this paper, we investigate performances
(stability, convergence ...) of the Fixed Point
, ERAQLES and ERICA+
ABR congestion control algorithms. By using the stability theory of
delayed differential equations, we get the stability conditions for
each algorithm and examples where these algorithms are not stable are given.
We propose parameters for which ERICA+ is always stable.
We show that for a range of available bandwidths neither the
Fixed Point nor ERAQLES algorithms are stable,
whatever the other parameters are (round trip times, ...).
To get the paper, click here.
Identification
 O. AitHellal, E. Altman, T. Basar, "A robust identification algorithm for traffic models in telecommunications"., submitted to
International Conference on Broadband Communications (BC'98), April
98, Stuttgart, Germany.
There has been a growing interest in MA, AR and ARMA models
of traffic in telecommunications networks in recent years.
Such models allow not only to identify the characteristics and understand
the behavior of traffic, but also allow for
(i) the queueing performance analysis
and (ii) optimization and
control issues. The goal of this paper is to propose a new robust
identification approach for the parameters of AR
stochastic processes, and use these for analyzing
telecommunication traffic data.
To get the paper, click here.
Omar AïtHellal / Last update February 1998