Ricardo Campanha Carrano

IEEE 802.11s Multihop MAC: A Tutorial

Recently, IEEE started a task group to investigate adding wireless mesh capabilities to its ubiquitous IEEE 802.11 wireless local area networks standard. The proposal is specified as the IEEE 802.11s amendment. Although the IEEE802.11s amendment is still a draft, some implementations based on it may already be found. The first and most widespread of these implementations is the one developed by One Laptop per Child (OLPC) for its educational laptop - the XO. One notable feature of IEEE 802.11s is the fact that the mesh network is implemented at the link layer, relying on MAC addresses rather than IP addresses for its mechanisms. This feature enables the design and development of new CPU-free network devices that provide layer-2 multihop communication. This tutorial describes the main characteristics of the IEEE 802.11s proposal illustrating the advantages and disadvantages of the MAC layer approach in comparison to the traditional layer three paradigm to multihop wireless networks. To achieve this, this work provides a detailed analysis of 802.11s traffic captured in a real testbed, with special attention to the path discovery mechanism. The step by step explanation of the mesh mechanisms highlights how some of the design choices may impact on the scalability and reliability of such networks.

A comprehensive analysis on the use of schedule-based asynchronous duty cycling in wireless sensor networks

Duty cycling is a fundamental mechanism for battery-operated ad hoc networks, such as Wireless Sensor Networks, Delay Tolerant Networks, and solar-powered Wireless Mesh Networks. Because of its utter importance, it has been proposed in a wide variety of flavors, one of the most prominent being that of the asynchronous mechanisms. In particular, schedule-based duty cycling has earned attention due to its low requirements and simplicity of implementation. Despite its potential, a comprehensive and realistic study on the neighbor discovery latency that results from schedule-based asynchronous duty cycling is still missing. This paper fills in this gap, by providing accurate models for major schedule-based mechanisms: Block Designs, Quorum systems and Disco. The provided models consider message loss probability and yield more precise estimations than traditional models. Based on this improved accuracy, the relative latency, a new metric for studying the trade off between latency and power, is proposed as a substitute to the power-latency product. Finally, a practical mapping of which schedule is more adequate for given requirements of latency, energy savings and link reliability is presented.

Centralized channel allocation algorithm for IEEE 802.11 networks

The sharing of the wireless spectrum is a major concern of network administrators. Access points in the same network interfere with each other, degrading the aggregate performance of stations. Moreover, wireless networks usually coexist with others applications that share the same spectrum and negatively impact the packet transmission. To overcome these issues, we propose the channel allocation algorithm designed for central controllers of infra-structured IEEE 802.11 networks. Our algorithm reduces the interference in controlled access points through the dynamic choice of their operating channels and, unlike other proposals, was designed to operate in a network composed of low cost devices from different brands, and open source software. Furthermore, we also consider the interference caused by unmanaged networks, adjusting the settings of the managed access points according to the wireless environment. The proposal was implemented and evaluated in an open testbed, and the results show that our controller efficiently manages the spectrum with low cost equipment and a low complexity algorithm.

Nested block designs

In schedule-based asynchronous duty cycling, nodes activate and deactivate their radio interfaces according to a specially designed wakeup schedule, which guarantees overlapping active time between nodes, irrespective of their synchronization offsets. When compared to synchronous duty cycling, such an approach has the advantage of being simple to implement, eliminating the need for synchronization protocols, complex computations or extra hardware. However, among published proposals, there is no single schedule-based mechanism that provides the lowest latency in all scenarios, when considering duty cycling symmetry, frame delivery probability and duty cycling rate. This paper introduces nested block designs, a new schedule that extends the use of block designs to application scenarios for which they were previously not possible or not as efficient as other schedules. Nested block designs provide the lowest latency among known schedule-based asynchronous duty cycling mechanisms for a wide range of applications, as confirmed by analytical models and real implementations on WSN motes.

Link quality estimation for AMI

An important application in Smart Grids is the Advanced Metering Infrastructure (AMI). Under AMI, utilities use smart meters and aggregators to collect consumer data that support better-informed and automated decisions. AMIs are typically formed by wireless links, reducing the investments in infrastructure. Although beneficial, the use of wireless raise issues of reliability and coverage, and requires the estimation of the success rate in delivering packages. This paper discusses the application of the Extended Hata-SRD propagation model in urban, suburban and rural scenarios, for IEEEs 802.11g and 802.15.4, to determine the success rate of packets exchanged between meters and aggregators.

On the decrease in frame reception probability under heavy transmission loads in IEEE 802.11 networks

It is widely accepted that the reception probability of a wireless link tends to decrease with the increase of network load. This is commonly attributed to factors such as the increase in frame collision probability under heavier medium usage. We present experimental results regarding this probability decrease in IEEE 802.11 networks in order to investigate the causes of this phenomenon. Surprisingly, our experiments indicate that when a wireless interface is under heavy transmission load, its capacity of receiving frames from other stations diminishes, leading to a noticeable drop in its frame reception probability, even when collisions are not an issue.

Neighbor discovery time in schedule-based asynchronous duty cycling

In schedule-based asynchronous duty cycling, nodes alternate between active and inactive time slots in cycles that guarantee the occurrence of overlapping active time, therefore ensuring that neighbors will have opportunities to communicate. Among these schedules, Block Designs provide the minimal duty cycle for a given number of time slots [1]. However, there exists no precise estimation model for the resulting NDT when such schedules are employed. This paper provides an accurate model for nodes operating under Block Design schedules.

Multi-Channel Continuous Rendezvous in Cognitive Networks

The rapid growth of wireless networking technologies, the emergence of several new devices that offer or need Internet interconnection, and a pent-up demand for wide band access, especially away from the big cities, are hampered by the problem of the frequency spectrum exhaustion for telecommunications services. A more efficient use of the spectrum passes through solutions, such as the improvement and deployment of radios with cognitive ability. In this context, the problem of neighbor discovery extends not only for the initial blind rendezvous, but also for the maintenance of periodical encounters of neighbors after such initial encounter. At this stage, it will be necessary for a node that has already found a peer to interrupt its data communication, so that nodes can become aware of changes in their surroundings and the network can support the addition of new nodes. The contribution of this paper is the creation of asynchronous, distributed and robust schedules to guarantee multiple continuous rendezvous and communication opportunities between two or more cognitive radios using control channels, employing frequency hopping with new sequences and mappings based on combinatorial design theory.

Fault detection and diagnosis for solar-powered Wireless Mesh Networks using machine learning

The inherent complexity of Wireless Mesh Networks (WMNs) makes management and configuration tasks difficult, specially for fault detection and diagnosis. In addition, manual inspections are extremely costly and require a highly skilled workforce, thus becoming impractical as the problem scales. To address this issue, this paper proposes a solution that makes use of machine learning techniques for automated fault detection and diagnosis (FDD) on solar-powered Wireless Mesh Networks (WMNs). We have used the Knowledge Discovery in Databases (KDD) methodology and a pre-defined dictionary of failures based on our previous experience with the deployment of WMNs. Thereafter, the problem was solved as a pattern classification problem. Several classification algorithms were evaluated, such as Naive Bayes, Support Vector Machine (SVM), Decision Table, k-Nearest Neighbors (k-NN) and C4.5. The SVM presented the best results, achieving a 90.59% overall accuracy during training and over 85% in validation tests.

An Exact Model of the Neighbor Discovery Time for Schedule-Based Asynchronous Duty Cycling

In Wireless Sensor Networks, if two neighbor nodes operate under schedule-based asynchronous duty cycles, it is nontrivial to determine when they will discover each other, since the discovery opportunities may be distributed irregularly throughout the cycle, and also because nodes present random time offsets to each other. Moreover, the probability of message loss must be considered for useful estimations. This paper presents a method to find the expected neighbor discovery time when opportunities are distributed in arbitrary time slots, and subject to message loss. As a case study, the method was employed to solve a real problem in WSN design.