Francesca Cuomo

Routing in cognitive radio networks: Challenges and solutions

Cognitive radio networks (CRNs) are composed of cognitive, spectrum-agile devices capable of changing their configurations on the fly based on the spectral environment. This capability opens up the possibility of designing flexible and dynamic spectrum access strategies with the purpose of opportunistically reusing portions of the spectrum temporarily vacated by licensed primary users. On the other hand, the flexibility in the spectrum access phase comes with an increased complexity in the design of communication protocols at different layers. This work focuses on the problem of designing effective routing solutions for multi-hop CRNs, which is a focal issue to fully unleash the potentials of the cognitive networking paradigm. We provide an extensive overview of the research in the field of routing for CRNs, clearly differentiating two main categories: approaches based on a full spectrum knowledge, and approaches that consider only local spectrum knowledge obtained via distributed procedures and protocols. In each category we describe and comment on proposed design methodologies, routing metrics and practical implementation issues. Finally, possible future research directions are also proposed.

Radio resource sharing for ad hoc networking with UWB

Ultra-wideband (UWB) radio is becoming a promising field for new generations digital communication systems. This technique, based mainly on the impulse radio paradigm, offers great flexibility and shows enormous potential in view of a future broadband wireless access. We present the main principles to design a multiaccess scheme based on UWB. The potential of UWB is exploited within a distributed ad hoc wireless system, where we describe the principles for the definition of a medium-access control (MAC) for mobile computing applications and we analyze the main performance results derived from simulations. A general framework for radio resource sharing is outlined for classes of traffic requiring both elastic-dynamic and guaranteed-reserved bandwidth. Then, we discuss the issue of supporting the proposed radio resource sharing scheme by means of a distributed MAC protocol.

Routing in ZigBee: Benefits from Exploiting the IEEE 802.15.4 Association Tree

An IEEE 802.15.4-based wireless sensor network is considered, and the relationship between the IEEE 802.15.4 topology formation mechanism and possible routing strategies at the network layer is studied. Two alternative routing schemes proposed in the framework of the ZigBee alliance are analyzed. The first is the well-known ad-hoc on demand distance vector (AODV) routing protocol, which was designed for highly dynamic application scenarios in wireless ad-hoc networks. The second is a tree-based routing scheme based on a hierarchical structure established among nodes during the network formation phase. This latter approach, referred to as HERA (hierarchical routing algorithm) in the paper, routes packets from sensors to sink based on the parent-child relationships established by the IEEE 802.15.4 topology formation procedure. An extensive simulation analysis is carried out to compare HERA and AODV. It is shown that a hierarchical routing scheme based on the MAC association procedures offers several benefits with respect to reactive routing in typical sensor network applications. Moreover, it is to be noted that most sensor network scenarios are concerned with delivery of packets from a series of static sensors to a single, static, sink.

Gymkhana: A Connectivity-Based Routing Scheme for Cognitive Radio Ad Hoc Networks

The topology of a cognitive radio Ad Hoc network is highly influenced by the behavior of both licensed (primary) and unlicensed (secondary) users. In fact, the network connectivity could be impaired by the activity of primary users. This aspect has a significant impact on the design of routing protocols. We design a routing scheme for cognitive radio Ad Hoc networks, named Gymkhana, which is aware of the degree of connectivity of possible paths towards the destination. Gymkhana routes the information across paths that avoid network zones that do not guarantee stable and high connectivity. To this aim we use a mathematical framework, based on the Laplacian spectrum of graphs, that allows a comprehensive evaluation of the different routing paths of the cognitive radio network. Laplacian matrixes are used to compute the connectivity of the different network paths. Gymkhana uses a distributed protocol to collect some key parameters related to candidate paths from an origin to a destination. The parameters are fed into a basic mathematical structure which is used to compute efficient routing paths. Besides the basic idea of Gymkhana, the use of Laplacian matrixes to derive a closed formula to measure the path connectivity is another contribution of ours.

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.

Network pruning for energy saving in the Internet

Many scientific works propose methods of reducing the amount of energy consumed by the Internet. Although the structure of the Internet was not developed with specific attention to energy consumption, there are various components on which it is possible to act. In our work, we analyze the possibility of affecting the topology of the network. We propose a heuristic called Energy Saving based on TOPology control (ESTOP), which identifies poorly used router line cards by leveraging certain topological properties of the graph modeling an Internet Service Provider (ISP). By acting on these line cards - for example, by putting them into sleep mode - we prune the Internet topology and achieve significant energy savings while preserving the primary topological characteristics of the pruned network. Although ESTOP is traffic-unaware, we assess its behavior under real traffic loads, demonstrating that its performance is comparable to the more complex traffic-aware solutions proposed in the literature.

Keeping the connectivity and saving the energy in the internet

Nowadays a big effort is spent to reduce the Internet energy consumption. Actual Internet topologies have space to power off some links and devices to reduce the energy consumed in off-peak periods still guaranteeing connectivity among terminals. In this work we leverage the algebraic connectivity of the graph modeling an ISP network in order to define the ESACON (Energy Saving based on Algebraic CONnectivity) algorithm. We then consider the network connectivity as a first target performance to be assured. To this aim we identify a metric based on the algebraic connectivity that, on one side, allows to switch off several links with the consequent significant energy saving and, on the other side, still preserves network connectivity and network performance for efficiently supporting the Internet traffic. We find that ESACON achieves better performance with respect to similar topology-aware approaches; moreover ESACON performance are comparable with ones of a complex traffic-aware solution.

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.

A distributed beaconless routing protocol for real-time video dissemination in multimedia VANETs

We design a framework for video (and in general high data rate applications) transmission over a VANET.Our framework includes both application and routing layer design.We analyze MAC layer behavior and improve its utilization by solving the Spurious Forwarding problem.We tested our protocols on a real scenario, and considered both QoS and QoE, including MOS experiments in a real car.We grant a high data rate in ad-hoc mode, also for long distance, through our backbone beaconless approach. Vehicular Ad-Hoc Networks (VANETs) will play an important role in Smart Cities and will support the development of not only safety applications, but also car smart video surveillance services. Recent improvements in multimedia over VANETs allow drivers, passengers, and rescue teams to capture, share, and access on-road multimedia services. Vehicles can cooperate with each other to transmit live flows of traffic accidents or disasters and provide drivers, passengers, and rescue teams rich visual information about a monitored area. Since humans will watch the videos, their distribution must be done by considering the provided Quality of Experience (QoE) even in multi-hop, multi-path, and dynamic environments. This article introduces an application framework to handle this kind of services and a routing protocol, the DBD (Distributed Beaconless Dissemination), that enhances the dissemination of live video flows on multimedia highway VANETs. DBD uses a backbone-based approach to create and maintain persistent and high quality routes during the video delivery in opportunistic Vehicle to Vehicle (V2V) scenarios. It also improves the performance of the IEEE 802.11p MAC layer, by solving the Spurious Forwarding (SF) problem, while increasing the packet delivery ratio and reducing the forwarding delay. Performance evaluation results show the benefits of DBD compared to existing works in forwarding videos over VANETs, where main objective and subjective QoE results are measured.