Catálogo de publicaciones - libros

This paper presents a study of the Quadratic optimization Problem (QP) lying on the learning process of Support Vector Machines (SVM). Taking the Karush-Kuhn-Tucker (KKT) optimality conditions, we present the strategy of implementation of the SVM-QP following two classical approaches: i) active set, also divided in primal and dual spaces, methods and ii) interior point methods. We also present the general extension to treat large scale applications consisting in a general decomposition of the QP problem into smaller ones. In the same manner, we discuss some considerations to take into account to start the general learning process. We compare the performances of the optimization strategies using some well-known benchmark databases.

The IEEE 802.15.3 standard does not specify how to schedule CTA (Channel Time Allocation)s in CTAP (Channel Time Allocation Period) of a superframe. In this paper, we propose a novel scheduling algorithm for channel time allocation of 802.15.3-based HR-WPAN (High Rate Wireless Personal Area Networks), named WFCTA (Weighted Fair Channel Time Allocation). The proposed WFCTA inherently has properties which are necessary for various QoS provisioning, because it is based on TDMA, applied by WFQ algorithm. We explain and compare four implementation cases of WFCTA . We also analyze its worst-case performance in terms of both link utilization and fairness and verify it by simulation.

In recent years, diverse solutions for service discovery in mobile ad hoc networks have been proposed. These solutions basically follow two approaches: the first one considers routing and service discovery as separate activities (i.e. implementing independent protocols for each activity); the second one integrates routing and service discovery into a single protocol. Based on this second approach we extend the AODV routing protocol to also perform service discovery. We analyze and compare the performance of both approaches. The results show that the integration of routing and service discovery reduces the amount of traffic generated and the time necessary to find a service.

In DCF of 802.11-based multi-hop wireless networks, a channel access sequence of 4-way handshake with RTS-CTS-DATA-ACK is used to forward a single packet. For multi-hop packet forwarding, multiple channel accesses with this 4-way handshake are required, and this might entail multiple collisions, which, in turn, results in poor end-to-end performance. To eliminate the overhead of these multiple channel accesses and, as a result, to reduce the possible number of collisions, the DCMA (Data-driven Cut-through Medium Access) protocol has been proposed[5]. We name the scheme used in the DCMA protocol the forward cut-through scheme . In this paper, we propose BCTMA (Bi-directional Cut-Through Medium Access) protocol for high performance 802.11-based multi-hop wireless networks by combining both forward and backward cut-through schemes . We have implemented BCTMA protocol in ns-2 [9]. We conducted a comprehensive simulation to study the performance improvements. The simulation results indicate that the performance is significantly improved and the number of dropped packets due to collisions can be significantly reduced as much as a half.

Wireless systems have found wide acceptance in many industries such as military and healthcare. These systems appear under a variety of architectures including fixed networks, cellular networks, and ad hoc networks. We survey some security problems, of interest to researchers, in wireless systems when used in these environments. Most studies of the security of these systems emphasize cryptographic aspects, we concentrate on other security aspects, such as operating systems, access control, web services, and location awareness.

The aim of this paper is to show some solutions for key management in ad hoc networks. The major problem in providing security services in such infrastructure less networks is how to manage the cryptographic keys that are needed. In order to design practical and sufficient key management systems it is necessary to understand the characteristics of ad hoc networks and why traditional key management systems cannot be used.

Collision due to the hidden terminals in contention-based medium access control protocols is one of the main reasons of the poor end-to-end performance of wireless networks. In this paper, we present a novel approach for MAC protocol improvements for 802.11-based multi-hop ad-hoc wireless networks to reduce unnecessary collisions, resulting in performance enhancement. Our main idea is to reduce unnecessary collisions by introducing a new time interval, named CAI (Collision Avoidance Interval) for the node in the carrier sensing zone[6]. Specifically, in the context of 802.11, the four-way handshake is specified as a collision avoidance medium access protocol and the protocol behavior for the node in the carrier sensing zone is required to be modified when it senses a signal for CTS, such that the node defers transmitting a frame during CAI , unlike during EIFS in the 802.11 specification. We carry in-depth analysis of the protocol behavior and have conducted a comprehensive simulation to study the performance improvements. The simulation results show that the end-to-end performance is greatly increased and the number of dropped frames due to collision is significantly reduced as much as one half, and so is the throughput cost.

As mobile devices roam between wireless cells, performing handovers, mobility is managed by protocols like Mobile IP and micro-mobility protocols. However, these protocols do not manage how to transfer context from cell to cell as the devices move. Context transfer mechanisms aim for performance improvements in the process of handover, so that the applications running on mobile nodes can operate with minimal disruption, thus allowing seamless mobility. In this paper we propose mechanisms that enable context transfer between access routers offering internet connectivity for mobile nodes; we use an actual implementation to test the performance of context transfer for different services and analyze the benefits that can be obtained using these mechanisms.

This paper provides a brief introduction to evolutionary algorithms including some of their applications. Our discussion includes short descriptions of genetic algorithms, evolution strategies, evolutionary programming and genetic programming. Then, a few case studies involving applications of evolutionary algorithms in real-world problems are analyzed. In the final part of the paper, some of the current research directions in this area are provided.

In this paper we present an introduction to computing anticipatory systems. The internals aspects of anticipation will be explained. The concepts of incursion are proposed to model anticipatory systems. A simple example of computing anticipatory systems will be simulated on computer that includes an anticipatory model.