Marcos F. Caetano

IEEE802.11b/g Standard: Theoretical Maximum Throughput

Estimating the throughput of an WiFi connection can be quite complex, even when considering simplified scenarios. Indeed, the varying number of parameters specified in the standards makes it hard to understand their impact in terms of delay and throughput. The main contribution of this work is to present a simple scheme to compute the exact maximum throughput for an IEEE 802.11g network. The proposed scheme incorporates all the timings and settings which allows one to calculate the throughput for different channel spacing and modulation techniques specified in the standard. Numerical and experimental results showing the accuracy of the proposed scheme are also presented.

A Packet Aggregation Mechanism for Real Time Applications over Wireless Networks

This work presents a packet aggregation mechanism tailored for real time applications over wireless network environments. In the core of the proposed mechanism lies an elaborated packet holding time estimation. This estimation is used to allow for packet aggregation along a multihop path. The proposed mechanism was evaluated and compared with another prominent packet aggregation scheme. The simulation results have shown that the proposed mechanism is capable to keep jitter and total delay within application bounds. Furthermore, the proposed scheme allows for substantial reduction on the number of packet transmissions as well as the overall packet overhead. The savings in terms of packet transmissions reached nearly 80% in the evaluated scenarios. These results have shown that the proposed scheme is able to cope with varying network link capacity and strict application timing requirements.

A Cluster Based Collaborative Cache Approach for MANETs

The main contribution of this work is to propose a distributed and collaborative cache solution in which the decisions to cache objects are performed in a collaborative way. In our solution, objects are classified in private and shared objects. Private objects are managed as in an individual cache system whereas the management of shared objects are performed collectively and are stored in a shared area. The simulation results have shown that our solution can expressively reduce the server load by increasing cache diversity and the probability of cache hits. Also, our solution provides significant savings in terms of battery power and congestion on the routes towards the sink node.

Proteus: an architecture for adapting web page on small-screen devices

Reading the contents ofWeb page with a small-screen device, such as a PDA or cell-phone, is still far from being a pleasant experience. Owing to the device limitations, current mobile browsers cannot handle all HTML tags, such as tables, for instance. Thus, most mobile browsers provide a linearized version of the source HTML page, leading to a large amount of scrolling, not to mention the difficulty in finding the desired content. The main contribution of this work is to propose an architecture for adapting web page on small-screen devices. Among the features that our architecture offers, we can cite on-the-fly Web page adaptation and customization according to the user and device characteristics; text summarization; page blocks identification and content mapping to easy the task of locating user interests.

An Energy-Optimum and Communication-Time Efficient Protocol for Allocation, Scheduling and Routing in Wireless Networks

The growing demand for mobile wireless access has stimulated the emergence of new communication technologies. Opportunistic Spectrum Access (OSA) is viewed as a promising alternative to overcome the problems caused by static spectrum assignment. Opportunistic access allows dynamic mapping of the transmission needs and communication opportunities. However, performing this task efficiently is not trivial. Indeed, it has been shown to be NP-complete. In this context, this paper presents an efficient heuristic for solving the problem of channel allocation and routing, according to the opportunities and channels available. The proposed heuristic is optimal in terms of energy consumption, being close to the optimum, about 5% above, in terms of transmission time.

An Opportunistic MAC Protocol Based on Statistical Spectrum Analysis

The current spectrum allocation model aims to avoid interferences in licensed spectrum communications without further concerns regarding spectrum usage. Because the radio spectrum is a finite resource, several approaches have been presented in order to improve the effectiveness of its utilization. The main contribution of this work is to present an opportunistic approach to spectrum sensing and allocation. Compared to other alternatives, our approach presents significant improvements in the spectrum access and utilization, improving network throughput up to 16 times.

An energy optimal technique for multi-channel allocation and data scheduling in wireless networks

The growing demand for mobile wireless access has stimulated the emergence of new communication technologies. Opportunistic Spectrum Access (OSA) is viewed as a promising alternative to overcome the problems caused by static spectrum assignment. Opportunistic access allows dynamic mapping of the transmission needs and communication opportunities. However, performing this task efficiently is not trivial. Indeed, it has been shown to be NP-complete. In this context, this paper presents an efficient heuristic for solving the channel allocation and data assignment problem, according to the opportunities and channels available. The proposed heuristic is optimal in terms of energy consumption, being close to the optimum, about 5% above, in terms of transmission time.

Multiple Biological Sequence Alignment with a Parallel Island Injection Genetic Algorithm

Multiple sequence alignment (MSA) is an important problem in Bioinformatics since it is often used to identify evolutionary relationships and predict secondary/tertiary structure, among others. MSAs are usually scored with the Sum-of-Pairs (SP) function and the exact SP MSA is known to be NP-Hard. Therefore, heuristic methods are used to solve this problem. In this paper, we propose and evaluate a parallel island injection genetic algorithm to solve the MSA problem. Unlike the other strategies, our parallel solution uses two types of interconnected archipelagoes, each with distinct types of individuals. Our results with real protein data sets show that our strategy is able to obtain better results, when compared to the traditional island model. Also, we were able to reduce considerably the execution time, when compared to the sequential version.

A collaborative cache approach for mobile ad hoc networks

The main contribution of this article is to propose a collaborative and distributed cache mechanism tailored for ad hoc networks. Each nodes cache is partitioned into a shared area and a private area. The proposed collaborative cache is formed by each nodes shared area, which is used to store contents relevant to the group, while the private area is used to store information relevant to its owner. Empirical results have shown that our collaborative cache mechanism can significantly reduce the amount of network traffic, latency and energy consumption as well as the servers load.

A Fair Randomized Contention Resolution Protocol for Wireless Nodes without Collision Detection Capabilities

Contention-based protocols are commonly used for providing channel access to the nodes wishing to communicate. The Binary Exponential Back off (BEB) is a well-known contention protocol implemented by the IEEE 802.11 standard. Despite its widespread use, Medium Access Control (MAC) protocols employing BEB struggle to concede channel access when the number of contending nodes increases. The main contribution of this work is to propose a randomized contention protocol to the case where the contending stations have no-collision detection (NCD) capabilities. The proposed protocol, termed RNCD, explores the use of tone signaling to provide fair selection of a transmitter. We show that the task of selecting a single transmitter, among n ≥ 2 NCD-stations, can be accomplished in 48n time slots with probability of at least 1 - 2-1.5n. Furthermore, RNCD works without previous knowledge on the number of contending nodes. For comparison purpose, RNCD and BEB were implemented in OMNeT++ Simulator. For n = 256, the simulation results show that RNCD can deliver twice as much transmissions per second while channel access resolution takes less than 1% of the time needed by the BEB protocol. Different from the exponential growth tendency observed in the channel access time of the BEB implementation, the RNCD has a logarithmic tendency allowing it to better comply with QoS demands of real-time applications.