Papers

Key issues in wireless sensor networks such as data aggregation, localisation, MAC protocols and routing, all have to do with communication at some level. At a low level, these are influenced by the link layer performance between two nodes. The lack of accurate sensor network specific radio models, and the limited experimental data on actual link behaviour, warrant additional investigation in this area.
In this paper we present the results from extensive experiments, exploring several factors that are relevant for the link layer performance. These include (i) the effect of interference from simultaneous transmissions, which has not been looked into before, (ii) the degree of symmetry in the links between nodes, and (iii) the use of calibrated RSSI measurements. Finally, we present some guidelines on how to use the results for effective protocol design.

The need to reprogramme a wireless sensor network may arise from changing application requirements, bug fixes, or during the application development cycle. Once deployed, it will be impractical at best to reach each individual node. Thus, a scheme is required to wirelessly reprogramme the nodes.
We present an energy-efficient code distribution scheme to wirelessly update the code running in a sensor network. Energy is saved by distributing only the changes to the currently running code. The new code image is built using an edit script of commands that are easy to process by the nodes.
A small change to the programme code can cause many changes to the binary code because the addresses of functions and data change. A naive approach to building the edit script string would result in a large script. We describe a number of optimisations and present experimental results showing that these significantly reduce the edit script size.

This paper studies the problem of determining the node locations in ad-hoc sensor networks. We compare three distributed localization algorithms (N-hop multilateration, Ad-hoc positioning, and Robust positioning) on a common simulation platform. The algorithms share a common, three-phase structure: 1) determine node-anchor distances, 2) compute node positions, and 3) optionally refine the positions through an iterative procedure. We present a detailed analysis comparing the various alternatives for each phase, as well as a head-to-head comparison of the complete algorithms. The main conclusion is that no single algorithm performs best; which algorithm is to be preferred depends on the conditions (range errors, connectivity, anchor fraction and placement). In each case, however, there is significant room for improving accuracy and/or increasing coverage.

In this paper we describe T-MAC, a contention-based Medium Access Control protocol for wireless sensor networks. Applications for these networks have some characteristics (low message rate, insensitivity to latency) that can be exploited to reduce energy consumption by introducing an active/sleep duty cycle. To handle load variations in time (responding to real-world events) and location (edge vs. sink) T-MAC introduces an adaptive duty cycle in a novel way: by dynamically ending the active part of it. This reduces the amount of energy wasted on idle listening, in which nodes wait for potentially incoming messages, while still maintaining a reasonable throughput.
Detailed simulations show that, for typical wireless sensor network traffic, radio energy consumption is reduced with more than 85% when compared to traditional (non-sleeping) protocols. Protocols with a fixed duty cycle do not sufficiently reduce energy consumption, because they must be able to handle peak loads.