Sándor Rácz

Flow level performance analysis of a multi-service system supporting elastic and adaptive services

We consider a multi-rate loss system where two types of non-peak allocated traffic flows receive service. Both elastic and adaptive flows are associated with a peak-and a minimum bandwidth requirement and they tolerate bandwidth compression while in service. The holding time of elastic flows depends on their received throughput, while the holding time of the adaptive flows is insensitive to that. Unfortunately, while this system is Markovian under quite non-restrictive assumptions (that are often used in the literature), it is not reversible. We propose a method whereby the approximation of this system by a reversible system is possible. We derive recursive formulas for determining the occupancy distribution and the mean number of flows in the system. By using a continuous approximation of the discrete state space, we also derive an explicit formula for the average throughputs that is independent of the size of the state space. The recursive formulas and the continuous approximation together provide a powerful tool for the performance analysis of this quite general system in the sense that they allow the calculation of the blocking probabilities and the mean throughputs in medium and large systems as well.

Providing Congestion Control in the Iub Transport Network for HSDPA

The introduction of High Speed Downlink Packet Access (HSDPA) presents new challenges to be solved in the UMTS Terrestrial Radio Access Network. Bandwidth reservation for HSDPA is not efficient and TCP cannot efficiently resolve a congestion situation because lower layer retransmissions hide the congestion situations from TCP. An HSDPA Flow Control algorithm was introduced by 3GPP to control congestion. This HSDPA Flow Control algorithm was originally intended to control radio scheduler queues in Node B. In this paper we propose an algorithm that also provides congestion control in the transport network. The performance analysis concentrates on transport network limited scenarios and shows that the algorithm can achieve high end-user perceived throughput, while maintaining low delay and loss in the transport network. The analysis also shows the advantages of the newly introduced congestion detection functionality.

Call level performance analysis of 3/sup rd/ generation mobile core networks

We develop a call level model of UMTS core networks where calls belonging to one of the four UMTS service classes arrive randomly. Arriving calls are granted service depending on the calls service class, the required maximum and minimum-bandwidth, and the available network resources at the arrival instance. We use a Markov model of transmission links to derive GoS (blocking probability) and QoS (throughput) measures under two reasonable and technologically feasible bandwidth sharing policies. We conclude that one of these policies is able to provide GoS/QoS guarantees for a wide range of traffic mixes. We argue that the results are applicable to the all IP/MPLS based new UMTS architecture.

Numerical analysis of large Markov reward models

Abstract A first analysis of Markov Reward Models (MRM) resulted in a double transform expression, whose numerical solution is based on the inverse transformations both in time and reward variable domain. Better numerical methods were proposed based on the time domain properties of these models, such as the set of partial differential equations describing the process evolution in time. This paper introduces an effective numerical method for the analysis of MRMs based on the transform domain description of the system, which allows the evaluation of models with large state space (∼106 states). The proposed method provides the moments of reward measures on the same computational cost and memory requirement as the transient analysis of the underlying Continuous Time Markov Chain and benefits from the advantages of the randomization method, which avoids numerical instabilities and provides global error bound in advance of the computation. Implementation notes and numerical examples demonstrate the numerical properties of the proposed method are also provided.

Link Capacity Sharing between Guaranteed- and Best Effort Services on an ATM Transmission Link under GoS Constraints

While link allocation policies in multi-rate circuit switched loss models have drawn much attention in recent years, it is still an open question how to share the link capacity between service classes in a fair manner. In particular, when an ATM link is offered calls from service classes with/without strict QoS guarantees one is interested in link capacity sharing policies that maximize throughput and keep the per-class blocking probabilities under some GoS constraints. In this paper we propose a model and associated computational technique for an ATM transmission link to which CBR/VBR and ABR classes offer calls. We also propose a simple link allocation rule which takes into account blocking probability constraints for the CBR/VBR calls and a throughput constraint for the ABR calls and attempts to minimize the blocking probability of ABR calls. Numerical examples demonstrate the effectiveness of the policy and of the applied computational technique.

HSUPA Transport Network Congestion Control

The introduction of high speed uplink packet access (HSUPA) greatly improves achievable uplink bitrate but it presents new challenges to be solved in the radio access network. In the transport network, bandwidth reservation for HSUPA is not efficient and TCP cannot efficiently resolve congestion because of lower layer retransmissions. This paper proposes an HSUPA transport network flow control algorithm that handles congestion situations efficiently and supports quality of service differentiation. In the radio network controller (RNC), transport network congestion is detected. Relying on the standardized control frame the RNC notifies the Node B about transport network congestion. In case of transport network congestion the Node B part of the HSUPA flow control instructs the air interface scheduler to reduce the bitrate of the flow to eliminate congestion. The performance analysis concentrates on transport network limited scenarios. It is shown that TCP cannot provide efficient congestion control. The proposed algorithm can achieve high end-user perceived throughput, while maintaining low delay, loss and good fairness in the transport network.

Connection admission control in UMTS radio access networks

On transport links of UMTS Terrestrial Radio Access Networks, but especially on those connecting base stations and radio network controllers (i.e. on the Iub interface), resource allocation is complex, because packet delay and loss requirements are strict and the amount of transmission resources is relatively low. In this paper a novel connection admission control (CAC) algorithm is provided, which is applicable on the Iub interface with both ATM/AAL2 and IP transport options. The CAC algorithm is validated by mathematical analysis and computer simulation.

On providing blocking probability and throughput guarantees in a multi‐service environment

As the Internet evolves from a packet network supporting a single best effort service class towards an integrated infrastructure supporting several service classes—some with QoS guarantees—there is a growing interest in the introduction of admission control and in devising bandwidth sharing strategies, which meet the diverse needs of QoS-assured and elastic services. In this paper we show that the extension of the classical multi-rate loss model is possible in a way that makes it useful in the performance analysis of a future admission control based Internet that supports traffic with peak rate guarantee as well as elastic traffic. After introducing the model, it is applied for the analysis of a single link, where it sheds light on the trade-off between blocking probability and throughput. For the investigation of this trade-off, we introduce the throughput-threshold constraint, which bounds the probability that the throughput of a traffic flow drops below a predefined threshold. Finally, we use the model to determine the optimal parameter set of the popular partial overlap link allocation policy: we propose a computationally efficient algorithm that provides blocking probability- and throughput guarantees. We conclude that the model and the numerical results provide important insights in traffic engineering in the Internet. Copyright

A moments based distribution bounding method

We present a numerical procedure to bound distributions based on their finite number of moments. Numerical examples demonstrate the properties of the proposed method.

Fairness-optimal initial shaping rate for HSDPA transport network congestion control

In the HSDPA (high-speed downlink packet access) Iub transport network (TN) which connects the radio network controller with base stations (node Bs), congestion control is needed. Because of the TN¿s often narrow resources, fairness of resource sharing is an important issue. Basic Additive Increase multiplicative decrease (AIMD) congestion control guarantees convergence to fairness in the long run; all flows converge to an equal share of resources in steady state, where no flows join or leave. However, incoming flows decrease the level of fairness, that is why transient fairness should be taken into account as well. This paper proposes a method which provides fairness-optimal initial rate for incoming HSDPA flows. The proposed method improves average fairness. The method can be applied in a rate based congestion control where flows share the same bottleneck. The paper also gives a general solution for fairness-optimal initial rate in case of second-order fairness measures.