Wireless Sensor Network Aggregation Using Overlay Protocol Pedro Barbosa1 and Carlos Baquero2 1
IPCA - Polytechnic Institute, Arcozelo, 4750-117 Barcelos, Portugal
[email protected] 2 Minho University, Campus de Gualtar, 4710-057 Braga, Portugal
[email protected]
Introduction Wireless Sensor Networks (WSNs) are a recent creation of an old idea: spread sensors randomly and expect them to communicate with each other seamlessly, creating an infrastructure for wireless transmission across the network. The networks are basically built by sensor nodes with processor, memory for data storage, sensors, analogue-to-digital converters, data transceivers, controller, and power supply [1, 2]. The overall objective of a WSN is to provide a low-cost solution to gather physical data from the environment, such as noise, light, vibration or temperature, and spread it across the network, possibly directing to a modified node, usually called sink, which serves as a gateway to different networks, such as Ethernet. WSNs are based on distributed systems paradigm, taking it to extreme conditions: as they rely on limited power sources and must be extremely small, constrains are much higher. Therefore, processing power, memory and wireless communication abilities are very limited to reduce power consumption. This may lead to cross-layered approaches, where communication and application are programmed together. There are currently several network classes: with or without infrastructure, with or without positioning ability, static or dynamic, among others. Standards such as ZigBee are being created and evolved allowing manufacturers to design new devices that can inter-operate, increasing its usability and overall performance. Moreover, these standards take in consideration relevant aspects such as energy constrains and network reliability. The main differences between WSN and ad-hoc networks are, as stated by Akyildiz et al. [3]: – The number of sensor nodes in a sensor network can be several orders of magnitude higher than the nodes in an ad hoc network. – Sensor nodes are densely deployed. – Sensor nodes are prone to failures. – The topology of a sensor network changes very frequently. – Sensor nodes mainly use a broadcast communication paradigm, whereas most ad hoc networks are based on point-to-point communications. – Sensor nodes are limited in power, computational capacities, and memory.
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– Sensor nodes may not have global identification (ID) because of the large amount of overhead and large number of sensors. Communication protocols are based on existing distributed protocols, sometimes slightly modified to reduce power consumption. First protocols were based on flooding algorithms. As these algorithms proven to be very inefficient, optimized versions appeared, such as Gossiping, Rumor Routing [4], Direct Diffusion [5, 6], GPSR [7], among others.
Using Overlay Protocol for Network Aggregation The main concern when creating a new WSN is the number of nodes and its unique identifier size. IPv4 proven to be very effective in computer networks, but is not recommended to WSN for two reasons: network size is limited by the maximum number of different addresses and the overhead is too large to be reasonable in such energy-aware environment. This lead to the creation of a new address scheme, based on an overlay protocol. The purpose of using an overlay protocol is to create a connection route between all sensors present in a WSN in a static manner. This is done by creating an unique identifier in every node aggregated to a network. The unique identifier is assigned to each node, and as the network grows in size, the unique identifier grows as well. This is done to reduce message size to the minimum, since the power required to send a message is greater than the power required to process it. At the same time, each node builds its own overlay routing table. This table allows forwarding each message to a specific node through a known path, just by knowing the overlay address. As so, it will direct each message to a node in it vicinity, so it can be forwarded to the destination. By doing this, the message is forwarded through one path only, improving energy conservation.
Ongoing Research The main objective of study is to evaluate the how a fault-free, structured overlay network performs in power consumption, compared to existing protocols. It is expected a large amount of power spent during aggregation, which should be compensated during normal message generation/routing. Current research is being focused on sensor aggregation in a large network. New issues arise as a sub-network size grows, since the number of messages passing may increase exponentially. This issue is being addressed and the protocol is being optimized to reduce messages to a minimum possible. Another major concern is given to central nodes, as they will route more messages than peripheral nodes. This will lead to superior energy drain and consequent shorter life.
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References 1. David Culler, Deborah Estrin, and Mani Srivastava. Overview of wireless sensor networks. IEEE Computer, August 2004. 2. I.F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci. Wireless sensor networks: a survey. Computer Networks, Vol. 38, pp.393-422, March 2002. 3. I. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci. A survey on sensor networks, 2002. 4. D. Braginsky and G. Estrin. Rumor routing algorithm for sensor networks. 5. Deepak Ganesan, Alberto Cerpa, Wan Wu, Deborah Estrin, Wei Ye, and Jerry Zhao. Networking issues in wireless sensor networks, 2003. 6. C. Intanagonwiwat, R. Govindan, D. Estrin, J. Heidemann, and F. Silva. Directed diffusion for wireless sensor networking, 2003.
