W. Steven Conner

Adapting physical carrier sensing to maximize spatial reuse in 802.11 mesh networks

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.11b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes. Copyright

Making Everyday Life Easier Using Dense Sensor Networks

Advances in hardware are enabling the creation of small, inexpensive devices and sensors. Hundreds or thousands of these devices can be connected using low-power multi-hop wireless networks. These networks foster a new class of ubiquitous computing applications called proactive computing. In proactive applications, computing occurs in the background without requiring human interaction; humans participate to access information or to modify control policies. This paper provides an overview of the application of a large wireless network of sensors to solve everyday problems in the workplace. It describes the implementation of one application that allows people in the workplace to easily find empty conference rooms (e.g., for impromptu meetings). Drawing on this experience, we identify technical challenges and possible directions for building dense networks of sensors that enable proactive computing.

Experimental evaluation of synchronization and topology control for in-building sensor network applications

While multi-hop networks consisting of 100s or 1000s of inexpensive embedded sensors are emerging as a means of mining data from the environment, inadequate network lifetime remains a major impediment to real-world deployment. This paper describes several applications deployed throughout our building that monitor conference room occupancy and environmental statistics and provide access to room reservation status. Because it is often infeasible to locate sensors and display devices near power outlets, we designed two protocols that allow energy conservation in a large class of sensor network applications. The first protocol, Relay Organization (ReOrg), is a topology control protocol which systematically shifts the networks routing burden to energy-rich nodes, exploiting heterogeneity. The second protocol, Relay Synchronization (ReSync), is a MAC protocol that extends network lifetime by allowing nodes to sleep most of the time, yet wake to receive packets. When combined, ReOrg and ReSync lower the duty cycle of the nodes, extending network lifetime. To our knowledge, this paper presents the first experimental testbed evaluation of energy-aware topology control integrated with energy-saving synchronization. Using a 54-node testbed, we demonstrate an 82-92% reduction in energy consumption, depending on traffic load. By rotating the burden of routing, our protocols can extend network lifetime by 5-10 times. Finally, we demonstrate that a small number of wall-powered nodes can significantly improve the lifetime of a battery-powered network.

Guaranteed on-demand discovery of node-disjoint paths in ad hoc networks

In recent years, the low cost and abundance of WLAN products has led to the deployment of self-configuring multihop ad hoc networks. Multipath routing has been increasingly studied to improve network reliability and throughput. However, no existing work guarantees discovery of node-disjoint paths when they exist, which limits their applicability in real networks. This paper presents a theoretical framework that establishes the equivalence between multipath discovery and flow network assignment. This equivalence is used to guarantee the on-demand discovery of an arbitrary number of node-disjoint paths between a pair of nodes as long as they exist. We also present an example protocol that integrates the theoretical framework with the Dynamic Source Routing (DSR) protocol to find two node-disjoint paths, which can be easily extended to finding k node-disjoint paths. Analysis of the example protocol demonstrates a good tradeoff between complexity and capability, particularly when compared with existing on-demand multipath routing protocols. Our simulation data shows the effectiveness of the discovered paths.

Experimental evaluation of topology control and synchronization for in-building sensor network applications

While multi-hop networks consisting of 100s or 1000s of inexpensive embedded sensors are emerging as a means of mining data from the environment, inadequate network lifetime remains a major impediment to real-world deployment. This paper describes several applications deployed throughout our building that monitor conference room occupancy and environmental statistics and provide access to room reservation status. Because it is often infeasible to locate sensors and display devices near power outlets, we designed two protocols that allow energy conservation in a large class of sensor network applications. The first protocol, Relay Organization (ReOrg), is a topology control protocol which systematically shifts the network’s routing burden to energy-rich nodes, exploiting heterogeneity. The second protocol, Relay Synchronization (ReSync), is a MAC protocol that extends network lifetime by allowing nodes to sleep most of the time, yet wake to receive packets. When combined, ReOrg and ReSync lower the duty cycle of the nodes, extending network lifetime. To our knowledge, this research provides the first experimental testbed evaluation of energy-aware topology control integrated with energy-saving synchronization. Using a 54-node testbed, we demonstrate an 82–92% reduction in energy consumption, depending on traffic load. By rotating the burden of routing, our protocols can extend network lifetime by 5–10 times. Finally, we demonstrate that a small number of wall-powered nodes can significantly improve the lifetime of a battery-powered network.

Maximizing Aggregate Throughput in 802.11 Mesh Networks with Physical Carrier Sensing and Two-Radio Multi-Channel Clustering

Spatial reuse in a mesh network allows multiple communications to proceed simultaneously, hence proportionally improving the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous co-channel transmitters to maintain a separation distance that is sufficient to avoid interference. Within that distance, a set of orthogonal channels is employed by different links. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding co-channel interference in 802.11 mesh (static + multi-hop) networks. Moreover, for multi-channel mesh networks, an architecture for channel clustering based on two-radio nodes is proposed. Distributed clustering is achieved using the Highest-Connectivity Cluster (HCC) algorithm. All inter-cluster communications are performed on a common channel using the default radio, while intra-cluster communications use the secondary radio with channel selection based on a new Minimum Interference Channel Selection (MIX) algorithm that minimizes the co-channel interference (CCI). Backward compatibility is guaranteed by allowing legacy single-channel devices to connect to the new two-radio devices through the common default radio. Simulation results for large-scale 802.11b and 802.11a networks demonstrate the significant improvement in one-hop aggregate throughput. Specifically, the new two-radio multi-channel mesh solution increases the aggregate through-put by more than twice w.r.t. the traditional single-radio single-channel mesh.