What is a Sensor Network?

A Sensor Network is a communications infrastructure that employs a network of distributed sensors and sensor nodes to acquire data and relay it to a central monitoring location. Advanced networks often employ “ad hoc” mesh networking that allows sensor nodes to self-configure. When sensors are attached to hardware with advanced, embedded data processing abilities, the entire assembly is referred to as a Smart Sensor Node. Depending upon the desired application, a sensor network may be wired, wireless or a combination of both, depending on the need of the application and the environment where it is deployed. Sensor networks are frequently used to create “smart” environments that allow for the monitoring and control of everything from in home lighting to structural health monitoring to covert security monitoring.

A sensor network is generally composed of two primary networks:

1. Data Acquisition Network: smart sensor nodes collect data, process data and deliver it to base stations (or other gateway devices), where various stages of data processing occur. Ultimately, the data is transmitted to a management center for storage, analysis and further processing.
2. Data Distribution Network Information may be transmitted to a variety of devices including laptops, PC’s, PDA’s, cell phones and internet cloud servers. If the network is designed accordingly, sensors may be remotely controlled, allowing for re-calibration and measurement updates.
   
   

Functions Employed in a Sensor Network:

Smart Sensor Nodes: Collects measured data from one or more sensors. Processes, stores and transmits data. Receives incoming messages and commands.

Sensor Control Module (SCM): Aggregates and synchronizes sensor data from multiple nodes. Performs processing, stores, and transmits data.

Base Control Modules (BCM): Forwards the information provided by the sensor control module to a server. Also allows for some further processing and filtering of sensor data; as well as additional routing functions.

Embedded Processing: Distributed at multiple levels throughout the sensor network modules.

Power Management Control Module: Manages the overall energy efficiency of a sensor network: may include standard hard-wired or battery power, as well as energy harvested power generation options.

Multi Sensor Data Fusion (MSDF): Provides the ability to fuse data from a variety of sensors, as well as fuse multiple sets of data together using a time stamp to sequence/synchronize. The standard digital tool for combining the information from many sensors is the Kalman Filter.

Application Software: Allows the end user statically analyze, manipulate and visualize the sensor data, as well as remotely control the network.

Function Employed in a Sensor Network

Sensor Types and Technologies:

The basis for all sensor measurements is the transduction principle - this requires that the physical property being measured can be distinguished from the background noise and translated into a usable signal. Good quality sensors are manufactured so that the physical change being measured is extremely predictable over the desired range. Sensor manufacturing techniques employ specialized materials ranging from piezoelectric and photoconductive solar cells, to MEMS silicon etched micro-devices and thin-film coated biochemical materials.

• Sensor Types:
• Pressure
• Temperature
• Velocity, Acceleration
• Wind Speed and Direction
• Relative and Absolute Position
• Strain, Force, Torque
• Vibration
• Motion
• Chemical Concentrations

Sensor Technologies:

MEMS (Microelectromechanical systems):

MEMS are silicon semiconductor based microdevices and range in size from 0.02 to 1.0 mm. Microsensors have been designed that mimic all types of larger scale sensor technologies. MEMS sensors often have built in signal conditioning and temperature compensation to boost the signal-to-noise ratio and provide an accurate sensor measurement.

Network Communications Topologies:

Communications topology refers to the manner in which a network’s nodes are connected, in order to transmit, receive and route messages. The topology is determined by the communication requirements, and constraints of a given network. Several different topologies (subnets) may be combined within a single sensor network. Examples include: Mesh networking, star, bus, ring and tree, etc.

Sensor Network Architecture:

The architecture of a sensor network may be organized into layers that control various functions. The ISO, open standard OSI/RM, is based on the use of seven architectural layers, as follows: Physical, Link, Network, Transport, Session, Presentation and Application. For example, the lowest layer (physical layer) controls the transmission device, while the next layer (Link Layer) determines when the channel is clear for transmission. Many of the IEEE networking standards support this layered architecture approach.

Routing Protocols:

To ensure maximal network connectivity, various routing schemes, and self-healing algorithms, are employed to tackle the issues of blocked connections and data recovery. Examples include: adaptive routing and multi-hop

Sensor Network Standards

Many different standards and protocols have been developed to encourage standardization within the Sensor Network Industry. A few of the most common standards are:

• IEEE 1451 (Standard for Smart Sensor Network)
• IEEE 802.15.4 (ZigBee)
• IEEE 802.11 (WiFi)
• IEEE 802.15 (Bluetooth)
• CAN bus
• ISA100.11a
• BACNet
  

Mobile Sensor Networks

Mobile networks are specialized types of wireless sensor network (WSN) that lend locomotion and GPS tracking to sensor nodes and other network components to expand the application abilities of a sensor network.