Catálogo de publicaciones - libros

This is a framework for resource allocation in a heterogeneous system composed of various access networks, for instance Third Generation wireless networks (3G) and WLAN, in the presence of multimedia traffic, namely voice and data. Our aim is to present a game theoretical modeling of routing and load-balancing strategies along with admission control and pricing in cooperative and non-cooperative settings.

Wireless Local Area Networks (WLANs) based on the IEEE 802.11 standard have become more and more popular in the last few years. In such networks, a handover has to be performed when moving from one cell to another. When considering small-scale scenarios like an office building, these handovers can be performed on the ISO/OSI layer two solely. However, the size of one subnet is rather restricted. Therefore, the handover has to be lifted to the network layer as well. The performance of this network layer handover will be shown in this paper.

During recent years, it has become evident that mobility functions will have a profound impact on current and future wireless networks. Users expect service connectivity anywhere and anytime without having to think about the underlying communication systems used at that particular moment in time.

On this background, this paper presents a ready-to-deploy implementation of a mobility framework that supports seamless communication and represents an important enabler for adaptive applications through its simple QoS feedback mechanism. The framework selects the best available network through a decision algorithm that takes advantage of both experience with different network types for certain types of services and a link performance monitoring concept. The impact of the proposed framework on performance in terms of processing and throughput overhead is also discussed.

This work introduces a new analytical method for performance evaluation of wireless packet-oriented networks. Unlike traditional call admission control procedures commonly used for performance evaluation of wireless networks, this paper deals with the problem of coupling connection and packet level QoS characteristics by analysis. At connection level we use the Blocked-Calls-Cleared (BCC) model, whereas at packet level we use the Blocked-Call-Interfered (BCI) model which has no immediate feedback from packet level to connection level about lost data. At connection level we use the convolution algorithm which defines the feasible state space at packet level. At packet level we take into consideration wireless interference (soft blocking). The traffic is modeled as multi-rate Binomial-Poisson-Pascal (BPP-) traffic at connection level and traffic at packet level. We obtain individual performance measures for each service, both at connection level and at packet level. By case studies we investigate the trade-off between the two levels to meet Grade of Service (GoS) requirements for cellular networks with WCDMA radio interface.

In this paper we develop approximate models for trunk reservation in multi-service systems with BPP (Binomial–Poisson–Pascal) multi-rate traffic streams. The approximation is a generalization of previous work by Tran-Gia & Hübner who assumed Poisson arrival processes. It is based on a generalized algorithm which allows for calculation of individual performance measures for each service, in particular the traffic congestion. The algorithm is numerically robust and requires a minimum of computer memory and computing time. The approximation is good when the services have equal mean service times.

The UMTS enhanced uplink or high speed uplink packet access (HSUPA) provides efficient mechanisms for the radio resource management of radio bearers for best effort traffic. The resources available for the enhanced uplink users depend on several factors like the spatial configuration of the mobiles in the cells, the number of QoS users and the implemented RRM strategy. In this work, we provide a model for the calculation of the resources assigned to the enhanced uplink users which also allows the inclusion of the maximum transmit power and down-grants for the reduction of the other-cell interference. We further show the impact of centralized and de-centralized radio resource management strategies on the feasible load region.

The paper presents an experimental analysis of the performance of TCP Westwood in AODV-routed ad hoc networks. Several tests have been carried out in both emulated and real environments, with varying network topologies, routing protocol settings and channel quality levels. As expected, TCP is deeply influenced by the settings of the routing protocol and the fluctuations in the signal to noise ratio. However, the results also reveal a quite novel aspect: AODV and TCP perceive the radio link quality in different ways and this effect is further enhanced by the presence of network interface cards based on the IEEE 802.11 standard. This aspect leads to some performance anomalies that are shown in the paper.

The fluid-flow approximation models investigate with much success the dynamics and stability of TCP/RED connections. Their main assumption is that the fluctuations of variables characterizing the behaviour of the connections are relatively small, that enables the linearization of model and the use of traditional control analysis tools to obtain such measures as Bode gain, phase margins, tracking error or delay margin. In this article, preserving linear fluid-flow model, we propose its extension to the case when a network is composed of wired and wireless part. We consider a variant of TCP algorithm (TCP-DCR or its new version TCP-NCR) and fluid-flow differential equations representing the size of congestion window, mean queue at the bottleneck router and loss probability at a RED queue are supplemented with terms representing constant loss probability due to transmission in wireless part and probability that a fraction of these errors is recovered by a link level mechanism. The decrease of congestion window due to TCP mechanism is delayed to allow the link protocol to deal with the errors. The model considers the presence of uncontrollable UDP flows.

Thin clients are lightweight devices from which all hardware, not related to input and output, is removed. Applications are executed on remote servers that render the graphical output and send it back to the client. As the reaction on user events can appear on the screen only after a two-way path delay, thin client computing can suffer from a high latency that degrades the user experience. We therefore propose that the application follows the user through the network by migrating to a server near enough to the user.

In this paper, a theoretical model and heuristics are presented to efficiently select servers for mobile users, in order to minimize the number of migrations and the corresponding application downtime. A sample scenario is presented, which clearly exposes the trade-off between the number of migrations and the average client-server latency. We then detail a theoretical model to determine the optimal allocation of applications to servers, in order to minimize the number of handovers. This model is based on the knowledge of the exact user movements and is only useful in an off-line setting. As this is impossible in real-time, several heuristics are presented. Their performance is compared and validated against the theoretical model.