Emanuele Cipollone

Performance analysis of IEEE 802.15.4 wireless sensor networks: An insight into the topology formation process

Topology formation is an important issue in a wireless sensor network. Performance parameters such as energy consumption, network lifetime, data delivery delay, sensor field coverage depend on the network topology. In this paper, we analyze the process of formation of a wireless sensor network according to the IEEE 802.15.4/ZigBee standards. We focus on both single-sink scenarios and multi-sink ones where: (i) we characterize the topology in terms of network depth and nodes distribution at different network levels; (ii) we analyze some network performance as a function of the number of sinks; (iii) we investigate the effects of some topology constraints on network performance; (iv) we study the effects of nodes mobility on the network formation. The whole study is performed by taking into account the specific features and recommendations of the IEEE 802.15.4/ZigBee standards; thus, our results can be used to configure IEEE 802.15.4/ZigBee procedures and set the related parameters, as a function of the desired application requirements.

Topology Formation in IEEE 802.15.4: Cluster-Tree Characterization

The IEEE 802.15.4 standard defines a set of procedures to set-up a low-rate wireless personal area network where nodes self-organize into a logical communication structure through which data can be routed, hop by hop, from sources to destinations. The network formation of the IEEE 802.15.4 does not impose constraints on the topology. The Zigbee alliance uses the IEEE 802.15.4 layers to build a complete protocol stack for the implementation of wireless sensor networks. ZigBee specifies the network layer for star, tree and peer-to-peer topologies. Starting from these, more complex cluster-tree topologies can be formed. To control the network topology ZigBee fixes the maximum number of routers and end-devices that each router may have as children and also fixes the maximum depth of the tree. To better understand the importance of these constraints we simulate and analyze the IEEE 802.15.4 formation procedure in different network settings (single-sink and multi- sink scenarios). The goal is to provide guidelines for the practical implementation of ZigBee network formation with the aforementioned constraints.

Cross-layer network formation for energy-efficient IEEE 802.15.4/ZigBee Wireless Sensor Networks

In IEEE 802.15.4/ZigBee Wireless Sensor Networks (WSNs) a specific node (called the PAN coordinator or sink) controls the whole network. When the network operates in a multi-hop fashion, the position of the PAN coordinator has a significant impact on the performance: it strongly affects network energy consumption for both topology formation and data routing. The development of efficient self-managing, self-configuring and self-regulating protocols for the election of the node that coordinates and manages the IEEE 802.15.4/ZigBee WSN is still an open research issue. In this paper we present a cross-layer approach to address the problem of PAN coordinator election on topologies formed in accordance with the IEEE 802.15.4. Our solution combines the network formation procedure defined at the MAC layer by the IEEE 802.15.4 standard with a topology reconfiguration algorithm operating at the network layer. We propose a standard-compliant procedure (named PAN coordinator ELection -PANEL) to self-configure a IEEE 802.15.4/ZigBee WSN by electing, in a distributed way, a suitable PAN coordinator. A protocol implementing this solution in IEEE 802.15.4 is also provided. Performance results show that our cross-layer approach minimizes the average number of hops between the nodes of the network and the PAN coordinator allowing to reduce the data transfer delay and determining significant energy savings compared with the performance of the IEEE 802.15.4 standard.

Measuring the connectivity of a cognitive radio ad-hoc network

In Cognitive Radio Ad-Hoc Networks (CRAHN) the behavior of the primary users influences the secondary network connectivity and the relevant performance. The methodologies used to evaluate network connectivity have to be worked up to take into account this aspect. In this letter we propose the use of Laplacian matrix and its second smallest eigenvalue to measure the network algebraic connectivity of a CRAHN. We re-elaborate the Laplacian matrix in order to have in its second smallest eigenvalue a function of the primary users behavior expressed as an activity factor. In this way we are able to monitor the algebraic connectivity of CRAHNs. This metric can be a useful instrument for network planning, data routing and network maintenance. Performance results show how this methodology can be efficiently applied in this kind of networks.

Analysis of k-connectivity of a cognitive radio ad-hoc network

The cognitive radio paradigm will allow the formation of spontaneous wireless networks able to communicate in sectrum bands temporarily left free by licensed primary users. The behavior of the primary users has a high influence on the network connectivity and impacts the performance of the Cognitive Radio Network (CRN). When an ad-hoc network is established among cognitive radio nodes, not only the position of nodes but also the activity of the primary users impacts the network connectivity. The aim of this paper is twofold: on one side it provides a methodology to evaluate the

Constraining the network topology in IEEE 802.15.4

k

A Distributed Procedure for IEEE 802.15.4 PAN Coordinator Election in Emergency Scenarios

-connectivity of a cognitive radio ad-hoc network. The Laplacian matrix is here extended in order to include the primary users behavior and to evaluate the degree of connectivity of a CRN. On the other side this study allows a deep analysis of the network connectivity and of the impact that different parameters can have on it. In this work some representative case studies are analyzed. The results are important for evaluating the proposed methodology and for understanding the behavior of the network connectivity in a wireless multi-hop cognitive radio network.

Topology reconfiguration in IEEE 802.15.4 WPANs for emergency management

The IEEE 802.15.4 standard defines a MAC association procedure that allows devices of a Wireless Personal Area Network (WPAN) to associate one each other in accordance to parent-child relationships. This standard can be adopted in Wireless Sensor Networks (WSNs) used to monitor environmental phenomena and to collect data in specific nodes named sinks. The ZigBee Alliance, with the Zig-Bee Specification, defines upper layers of a WSN architecture based on the IEEE 802.15.4. Tree shaped multi-sink networks can be formed by adopting in conjunction these two standards. A key aspect to be evaluated for these kind of networks is how their performance are affected by constraining some topological characteristics. In this paper, in accordance to the ZigBee Specification, we constraint the tree depth of a WSN and evaluate some performance metrics that have a remarkable relevance for WSNs. Moreover, we propose a new strategy to join an existing WPAN that allows a node to select a suitable coordinator to connect with. Performance results show the trade-off in the selection of the maximum tree depth. While benefits of having reduced tree depths can be measured during traffic delivery, due to a reduction of the energy consumption, a counter-party is the energy spent in the network formation that deeply depends on this topological parameter. Moreover, the maximum tree depth has also a great impact on the percentage of nodes that are able to join a network.

Autonomic Network Configuration in IEEE 802.15.4: A Standard-Compliant Solution

In an emergency scenario, the presence of a reliable communication infrastructure is fundamental. In response to massive catastrophes, such as earthquakes, floods, fires, etc., public authorities are expected to undertake actions to control and limit damages for people and for buildings. To this end, security agents are dispatched to the emergency area and they need to communicate to the people from a remote centre and they are responsible for emergency management. In this context, the use of IEEE 802.15.4 wireless personal area networks (WPANs) to allow communications among security agents seems particularly appropriate, thanks to their characteristics of self-configurability, adaptability, scalability, and low cost. Moreover, this kind of network can be used, if necessary, in a pervasive mode for data collection in the emergency area. In this work, we present an algorithm to self-configure an IEEE 802.15.4 WPAN by electing, in a distributed manner, a suitable node for the WPAN coordination. Our approach achieves a reduction of the network depth and a better distribution of nodes at different levels of the network.

A Case Study for Evaluating IEEE 802.15.4 Wireless Sensor Network Formation with Mobile Sinks

Communication networks play a fundamental role in the response to massive catastrophes. In case of emergency, the main purpose of public authorities is to avoid damages for people and for buildings and the presence of a reliable communication infrastructure is vital. In this context, the use of IEEE 802.15.4 Wireless Personal Area Networks seems particularly appropriate to allow communications among security agents and for data collection, thanks to their characteristics of self-configurability, flexibility and low cost. In this work we present a centralized algorithm that selects the best node to perform the PAN coordination in accordance to rules related to the concept of network depth and we extend the proposal with a distributed heuristic in order to have it working in an autonomic way. Performance results show that our approach achieves a better distribution of nodes in the network topology, with a well-known impact on key performance related to energy consumption and data delivery delay.