Wireless Sensor Networks

Wireless Sensor Networks

A Wireless sensor network is a mesh of wirelessly interconnected sensor nodes spanning a geographical area. Wireless sensor networks are deployed to sense the physical or environmental conditions such as temperature, humidity or for monitoring purpose in applications such as wildlife habitat monitoring, health monitoring, etc. The sensor nodes work in unison to gather and route information amongst each other or to a base station. Recent advancements have led to novel applications of WSNs such as for disaster management (volcano studies, real-time ood control etc.,), deployed in smart grids to manage energy usage, precision agriculture and industrial process monitoring. Wireless sensor network may consist of hundreds or thousands of nodes spread across a wide area. The future of WSN could possibly involve even millions of nodes. Such a future demands inexpensive nodes with low hardware complexity. The major challenge is to select sensor nodes in such a way that it ensures a long, stable and operational WSN. A node in a WSN is limited by its transmission range capability, power and compute capability.
Although, the nodes are distributed and networked together, increase in computing and communication capability will tend to make the sensor node more expensive.
Localization and positioning is a very signicant problem in WSN’s for algorithms related to routing, topology management and self-organization. While popular positioning technology, Global Positioning System (GPS) which uses Geographic Coordinate System is a candidate, owing to the drawbacks such as high energy consumption and high cost, is not a practical choice for many applications. Even when the geographic coordinates are available, routing is prone to be aected by physical voids and boundaries. This causes degradation in performance of geographic coordinate based routing protocols. An alternative to the Geographic Coordinate System is Virtual Coordinate System (VCS). Virtual Coordinate Systems provides a very ecient solution to localize a sensor without geographic coordinates. In a VCS, a subset of M sensor nodes is selected as landmarks or anchor nodes. VCS characterizes each node with a coordinate vector consisting of shortest path hop distances to those pre-selected anchors. Thus each node maintains a vector of size M, and thus the dimensionality of the coordinate system depends on the number of anchors. Thus VCS is an attractive alternative because it is easy to generate and also insensitive to physical voids.
The virtual coordinates in a multi-dimensional virtual domain are an attractive alternative to geographic domain coordinate. Thus, VCS is free of localization equipment such as GPS and localization algorithms such as RSSI. Also, VCS is transparent to physical voids and VC generation based on hop-count makes it easy to extend the system to 2D as well as 3D sensor network. Due to these advantages, extensive research work is being done on VCS based sensor networks. However there are a few disadvantages namely, 1) A cube with hourglass void network.Number of anchors required and positioning them plays an important role in VC-based routing algorithms. With under-deployment of anchors WSNs suer from identical node coordinates and improper placement leads to local minima problem.
2) The VCS loses directional information with respect to the nodes.
3) The Virtual coordinates are not orthogonal resulting in errors in distance estimation.
Topology Preserving Map introduced in is a novel technique that can overcome the disadvantages of VCs by generating topology maps of 2D and 3D networks that are homeomorphic to the corresponding physical maps. Singular Value Decomposition is used to obtain the topology coordinates of the sensor nodes. The TPM thus generated preserves the external and internal boundaries and basic shape of 2D and 3D WSN is obtained.
So far, the routing algorithms, TPMs have been requiring the complete set of VCs. Owing to node deaths and other unforeseen circumstances, the life of a node is at risk of being disconnected from the WSN. In such cases, there is lack of complete information about the WSN due to inaccessibility or improper routing.