Catálogo de publicaciones - libros

In recent years, several wireless LAN technologies making use of TDMA/TDD MAC have been designed. In this type of environment, there is the need for a central controller responsible for allocating the bandwidth among all the active mobile terminals. In order to properly carry out this task, the use of simple but effective signaling and bandwidth allocation mechanisms is a must. In this way, the mobile terminals can let the controller know their needs in terms of the bandwidth to be allocated. In turn, the central controller has to properly allocate the available bandwidth among all the competing mobiles taking into account their QoS requirements. In this work, we undertake the design and performance evaluation of the signaling protocols and bandwidth allocation mechanisms. We have paid particular attention to studying the amount of overhead introduced by these mechanisms: an important feature when designing control mechanisms for wireless environments. We validate the effectiveness of our proposed schemes when supporting a multi-service environment comprising four types of services: video, voice, best-effort and background.

In this paper, multi-hop wireless infrastructures are identified as a way to increase user data rates and/or capacity of wireless systems by means of a high base station density without high base station interconnection costs. For such a system, a new routing algorithm, named Balanced Interference Routing Algorithm (BIRA), is proposed. One of the main features of this new routing algorithm is to take the interference between wirelessly transmitting nodes into account. In BIRA a link cost is calculated considering the interference level of a node and a fixed cost for each link. Based on this link cost, the Dijkstra algorithm is used to compute routes. From the performance analysis, we see that BIRA outperforms other algorithms in terms of obtained data rates for a given available spectrum. BIRA helps to reduce the interference in the network and to achieve higher throughput.

In this paper, we propose an efficient transmission slot selection scheme for Band Division Multi-Carrier-CDMA (BD-MC-CDMA) systems under the constraints of packet loss and delay bound for each individual session. By utilizing channel dynamics together with the delay deadline and loss history, one can determine whether to transmit or not on each time slot, based on the prediction of future channel variations. In this way, one can enhance the chance for transmitting packets with the best channel quality. To validate the efficiency of the proposed algorithm, we model each sub-band as a discrete time Markov Chain using a finite state Markov channel (FSMC) and derive the criteria required for transmission decision. Simulation results show that our proposed scheme can satisfy quality of service (QoS) requirements for real-time traffic with a minimum use of power, while increasing throughput of non-real-time traffic with the power saved from real-time traffic.

Several studies in literature have investigated the performance of the proposed 802.11 standard for o differentiation in , but most of them are limited both with respect to the range of the parameter settings and the considered traffic scenarios.The aim of the present study is to systematically investigate (by simulations) the impact of each of the o differentiation parameters, under more realistic traffic conditions. In particular, we investigate flow-level performance characteristics (e.g., file transfer times) in the situation that the number of active stations varies dynamically in time.

In the wireless network environment, the effect of transmission errors and losses on the video quality varies depending on the intensity of the burst and which parts of the video stream are lost. Among the existing transmission error control techniques, FEC and ARQ are good solutions for combating transmission errors, but they require redundant data. Although interleaving has no error correcting capability, it can improve subjective video quality without wasting additional bandwidth, because it allows the spreading of successive errors. In this paper, we propose a content-aware packet-level interleaving method, which uses a quantitative index to indicate the degree of content-importance of the video content, so that the effect of burst packet losses is distributed intelligently. The proposed scheme improves the overall video quality in comparison with content-blind interleaving methods.

For systems based on connection-oriented services, such as IEEE 802.16, call admission control (CAC) strategy is essential to provide a desired level of quality of service (QoS). Although many handoff-prioritized CAC schemes, which assume a fixed channel capacity, have been introduced, this assumption is not always valid for IEEE 802.16 that uses adaptive modulation and coding (AMC). With AMC, the modulation type of a user’s connection can be changed dynamically and the ongoing connection might fail due to the change of modulation. In this paper, we approach the AMC-induced CAC problem by focusing on the guaranteed connection. Three kinds of calls, new, handoff, and modulation-changed calls, are considered. We propose a modified guard channel CAC scheme that allows the modulation-changed and handoff calls to use the guard channel. Then we analyze a Markov model for the CAC scheme with long-term AMC in mind. According to the simulation results, the proposed approach reduces the call dropping probability for modulation-changed calls, which suggests the threshold of guard channels in IEEE 802.16 can be determined based on the proposed approach.

Today’s public Wireless LANs are restricted to hotspots. With the current technology, providers can only target a small audience and in turn charge high prices for their service to generate revenue. Also, providers can not react appropriate to dynamic changes of the demand. With multi-hop cellular networks the coverage area can be increased and the installation costs and investment risks for the provider can be reduced. However, the individual customers play an important role in such networks and their participation must be encouraged. Therefore, we propose a cooperation and accounting scheme which introduces monetary rewards. We compare our scheme called CASHnet with the Nuglet scheme using simulations under the criteria of network liveliness as well as goodput, overhead and packet error rate.

The last few years, research in wireless multi-hop networks is mainly driven by a search for efficient routing protocols. From an application point of view, nodes (users) will only setup connections with a specific goal, i.e. in order to use services and resources available in or reachable through the ad hoc network. Consequently, resource and service discovery (R&SD) protocols that allow nodes to learn available services in the network are indispensable. In this paper we compare the performance of two basic decentralized R&SD techniques, proactive and reactive discovery, through simulations and theoretical analysis. Our results show that the choice between them is not straightforward. It highly depends on the network and service characteristics and on the interaction with the underlying routing protocols. Therefore, our analysis provides some guidelines for developing new or extending existing R&SD protocols for operation in mobile ad hoc networks.

UMTS/WLAN integration offers considerable benefits for the users as well as for the mobile network. As soon as coverage is available, a mobile user should be able to switch seamlessly from the ubiquitous low bandwidth UMTS to a high bandwidth WLAN. This will improve his quality of service, but this also increases capacity in the cellular network. To be able to change between radio access technologies, an inter-RAT handover protocol is needed. If this handover protocol is not fast enough, the vehicular user will penetrate deep into the cell, underutilizing the available bandwidth with a lower overall performance as a result. In this paper we introduce a new vertical handover protocol which uses location information from the vehicle to predict where and when the handover should occur, thus optimizing user throughput and network performance. We consider its deployment on three UMTS/WLAN integration scenarios.

We present a framework for developing a traffic generator that produces massive, realistic network payloads. The techniques and methods in this article can be easily applied to any stress workload generator for network traffic simulation. Here, as the system to be tested, we use the UMTS/GPRS backbone including SGSN and GGSN, which utilizes GPRS Tunnelling Protocol (GTP-U) user plane messages to carry user data packets. The proposed workload generator system is characterized by high, real traffic load, economical standard hardware, scalability, and flexible extensibility. A large number of independent participants, such as mobile users and Internet servers, are modelled. The realism of traffic is achieved by using a layered modelling approach starting from the user/application level and ending at the network layer. High system throughput is obtained by exploiting preconstructed packet buffers (templates), packet filters, network interface polling, and an efficient, adjustable time resolution scheduler.