Gertjan P. Halkes

Crankshaft: an energy-efficient MAC-protocol for dense wireless sensor networks

This paper introduces Crankshaft, a MAC protocol specifically targeted at dense wireless sensor networks. Crankshaft employs node synchronisation and offset wake-up schedules to combat the main cause of inefficiency in dense networks: overhearing by neighbouring nodes. Further energy savings are gained by using efficient channel polling and contention resolution techniques. Simulations show that Crankshaft achieves high delivery ratios at low power consumption under the common convergecast traffic pattern in dense networks. This performance is achieved by trading broadcast bandwidth for energy efficiency. Finally, tests with a TinyOS implementation demonstrate the real-world feasibility of the protocol.

Comparing energy-saving MAC protocols for wireless sensor networks

Applications for wireless sensor networks have notably different characteristics and requirements from standard WLAN applications. Low energy consumption is the most important consideration. The low message rate that is typical for sensor network applications and the relaxed latency requirements allow for significant reductions in energy consumption of the radio. In this article we study the energy saved by two MAC protocols optimized for wireless sensor networks, S-MAC and T-MAC, in comparison to standard CSMA/CA, We also report on the effects of low-power listening, a physical layer optimization, in combination with these MAC protocols. The comparison is based on extensive simulation driven by traffic that varies over time and location; sensor nodes are inactive unless they observe some physical event, or send status updates to the sink node providing the connection to the wired world. T-MAC} in combination with low-power listening saves most energy, but can not handle the same peak loads as CSMA/CA and S-MAC.

Link layer measurements in sensor networks

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. 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.

Experimental evaluation of simulation abstractions for wireless sensor network MAC protocols

The evaluation of MAC protocols for Wireless Sensor Networks (WSNs) is often performed through simulation. These simulations necessarily abstract away from reality in many ways. However, the impact of these abstractions on the results of the simulations has received only limited attention. Moreover, many studies on the accuracy of simulation have studied either the physical layer and per link effects or routing protocol effects. To the best of our knowledge, no other work has focused on the study of the simulation abstractions with respect to MAC protocol performance. In this paper, we present the results of an experimental study of two often used abstractions in the simulation of WSN MAC protocols. We show that a simple SNR-based reception model can provide quite accurate results for metrics commonly used to evaluate MAC protocols. Furthermore, we provide an analysis of what the main sources of deviation are and thereby how the simulations can be improved to provide even better results.

Experimental Evaluation of Simulation Abstractions for Wireless Sensor Network MAC Protocols

The evaluation of MAC protocols for Wireless Sensor Networks (WSNs) is often performed through simulation. These simulations necessarily abstract away from reality in many ways. However, the impact of these abstractions on the results of the simulations has received only limited attention. Moreover, many studies on the accuracy of simulation have studied either the physical layer and per link effects or routing protocol effects. To the best of our knowledge, no other work has focused on the study of the simulation abstractions with respect to MAC protocol performance. In this paper we present the results of an experimental study of two often used abstractions in the simulation of WSN MAC protocols. We show that a simple SNR-based reception model can provide quite accurate results for metrics commonly used to evaluate MAC protocols. Furthermore we provide an analysis of what the main sources of deviation are and thereby how the simulations can be improved to provide even better results.

UDP NAT and firewall puncturing in the wild

Peer-to-Peer (P2P) networks work on the presumption that all nodes in the network are connectable. However, NAT boxes and firewalls prevent connections to many nodes on the Internet. For UDP based protocols, the UDP hole-punching technique has been proposed to mitigate this problem. This paper presents a study of the efficacy of UDP hole punching on the Internet in the context of an actual P2P network. To the best of our knowledge, no previous study has provided similar measurements. Our results show that UDP hole punching is an effective method to increase the connectability of peers on the Internet: approximately 64% of all peers are behind a NAT box or firewall which should allow hole punching to work, and more than 80% of hole punching attempts between these peers succeed.

A simulation study of integrated service discovery

The research in the field of service discovery in mobile ad-hoc networks is characterised by a lack of quantitative research. Many ideas have been put forward but few have been tested, either in simulation or real life. This paper fills part of that void, by comparing through simulation a simple broadcast-flood protocol, an integrated routing and service-discovery approach, and a global-knowledge based approach. The results show that using an integrated approach can achieve a similar level of performance as a global-knowledge based approach.

Verifiable Encryption for P2P Block Exchange

Free-riding is an important problem in Peer-to-Peer (P2P) file-sharing networks. When peers refuse to contribute upload bandwidth, the whole network can collapse. A relatively new free-riding vulnerability in BitTorrent is the Large View Exploit, in which a peer connects to as many other peers as possible to increase the chance to get free data. This exploit can not be thwarted by tit-for-tat-like mechanisms which have traditionally been used to ban free-riding. Several approaches have been proposed to combat the Large View Exploit in fully decentralized systems, most of which rely on encryption. However, the use of regular encryption makes it impossible to verify the correctness of received data. In this paper we propose a novel encryption method which does allow verification of the plaintext data without decryption, at the expense of encryption strength. We show that a colluding peer still has to send data that is at least 40% of the size of the original data to allow decryption.

The λMAC framework: redefining MAC protocols for wireless sensor networks

Most current WSN MAC protocol implementations have multiple tasks to perform—deciding on correct timing, sending of packets, sending of acknowledgements, etc. However, as much of this is common to all MAC protocols, there is duplication of functionality, which leads to larger MAC protocol code size and therefore increasing numbers of bugs. Additionally, extensions to the basic functionality must be separately implemented in each MAC protocol. In this paper, we look at a different way to design a MAC protocol, focusing on the providing of interfaces which can be used to implement the common functionality separately. This leaves the core of the MAC protocol, determining only when to send, which is substantially different for each protocol. We also look at some examples of MAC extensions that this approach enables. We demonstrate a working implementation of these principles as an implementation of B-MAC for TinyOS, and compare it with the standard TinyOS B-MAC implementation. We show a 35% smaller code size, with the same overall functionality but increased extensibility, and while maintaining similar performance. We also present results and experiences from using the same framework to implement T-MAC, LMAC, and Crankshaft. All are demonstrated with data from real-world experience using our 24 node testbed.

Practical Considerations for Wireless Sensor Network Algorithms

Many researchers from different backgrounds have found interesting research challenges that arise from the physical constraints and envisaged applications in Wireless Sensor Networks (WSNs). The WSN community that has formed over the years is divided into two sub-communities: the systems sub-community and the theory sub-community. However, there seems to be no connection between the two. Algorithms developed from a theoretic perspective are rarely implemented on real hardwares. In this paper we identify the most important reasons why these algorithms are disregarded by the systems sub-community, and provide pointers to remedy the lack of connection.