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Implementing Energy Efficient Wireless Sensor Networks

Wired networks of sensors have been widely used in many environments for monitoring and control applications, and have contributed to failure reduction, improved productivity, and enhanced security. However, there are significant hurdles to the continued ubiquitous use of wired sensor networks. High installation and maintenance costs result in the use of fewer nodes, compromising the overall quality of the data reported.

Wireless sensing networks (WSN) can significantly reduce these costs by easing installation and eliminating cabling and connectors. A typical WSN is scalable, consumes less power, and has a lower long-term cost of ownership, including lower upfront installation costs and maintenance costs.

Applications that were not possible with wired networks are now being enabled using wireless sensor networks including mobile sensor nodes. With advances in low power systems, we will start to see widespread deployment of wireless sensor networks.

Wireless Sensor Networks

System Description
A wireless sensor network is a wirelessly operating network consisting of spatially distributed devices that use sensors to monitor physical or environmental conditions. The devices are called the "nodes" of the network.

Advantages of wireless sensor nodes include ease of installation, self-identification, self-diagnosis, reliability and time awareness for coordination with other nodes. These wireless sensor nodes communicate between themselves and to a central coordinator, which provides a connection to the wired world where we can collect, process, analyze the measurement data. Together this constitutes a typical WSN system.

L = Wireless sensor Nodes
C = Coordinator of the Wireless Sensor Network

Data is collected at the wireless sensor node and transmitted to the gateway directly or, if required, forwarded to the gateway through other wireless sensor nodes. The transmitted data is then presented to the system by the gateway connection.

The data is then processed for real-time monitoring and control. It could also be archived for later analysis. A client anywhere on the internet can access the data as well

WMSN System

Power Considerations
A wireless device is not really wire-free if it has to have power delivered to it. Routing power wires complicates deployment and dramatically increases installation costs.

Battery operated devices are not typically maintenance free. It is inconvenient and costly to replace batteries on any periodic basis. Disposal of used batteries is not a "green" proposition. Neither is wasting power on the RF link.

In order to reap the benefits of low-cost, wireless, green devices, we need to either eliminate the use of batteries by converting to energy harvesters, or get extremely long life using inexpensive batteries.

Physical Layer Standards
With many sensor manufacturers on the market today, it is not cost-effective to make special transducers for every network on the market. With the introduction of the IEEE 1451 standard, it is easier for sensor manufacturers to interface their devices to any network. Wireless communication protocol standards such as 802.11 (WiFi), 802.15.1 (Bluetooth), 802.15.4 (ZigBee) are being considered as some of the physical interfaces for IEEE P1451.5.

Several network standards based on IEEE 802.15.4-2006 are currently in use. ZigBee is a mesh-networking standard used in consumer devices, home automation and medical data sensing. ZigBee is also being promoted by industrial players.

WirelessHART is an extension of the HART protocol, and is specifically designed for industrial applications. ISA100 and 6lowpan are two additional standards.

Wireless Sensor Network Topologies
Star network (single point to multipoint): In this topology, a single base station can send a message to, or receive a message from, a number of remote nodes. The remote nodes can only send or receive a message from the single base station; they are not permitted to send messages to each other. The advantages of this topology are simplicity and low power consumption in the nodes. The disadvantage of this is the fact that it is not robust because of its dependence on a single node for operation.

Mesh networks: A mesh network allows for multi-hop communication, where a single node can talk to any other node that is within its radio transmission range. In this type of network, if a node wants to send a message to another node that is out of its radio range, it can use an intermediate node to forward it to the desired node.

Scalability and redundancy are the advantages of this topology. The main drawback of this topology is the unequal power dissipation across nodes that implement multihop versus the nodes that do not.

WSNs using the mesh network topology are called WMSNs (Wireless Mesh Sensor Networks). Due to their advantages WMSNs are more prevalent in sensor networking.

Given below are the salient features of WMSNs:
  • Ad-hoc self forming and self healing networks
  • Reliable broadcast messaging
  • Guaranteed message delivery applications
  • Device and service discovery
  • Optional acknowledged service
  • Messaging with optional responses
  • Security - symmetric key with AES-128 authentication and encryption
Differentiating factors of WMSNs:
  • Scalability (number of nodes)
  • Distances (Node to node, network total coverage)
  • Price
  • Bandwidth
  • Power
  • Low interference
  • Standards compliance
Hybrid star mesh networks: A hybrid between the star and mesh networks provides versatility and low power consumption. In this network topology, the lowest power sensor nodes are not enabled with the ability to forward messages. This allows for minimal power consumption to be maintained. However, other nodes on the network are enabled with multihop capability, allowing them to forward messages from the low power nodes to other nodes on the network.

Several electric, water and other utilities are investigating their options to deploy WSNs as part of their Smart Grid and Smart Environment applications.

A typical Smart Environment installation uses sensors to monitor the environment as well as to provide feedback on critical controlled processes, to form a closed loop. Other applications include:
  • Safety and security
    • Emergency Sensing
    • Access & Security
    • Remote Monitoring
    • Theft protection
  • Healthcare applications
    • Patient Monitoring
    • Remote Diagnostics
  • Asset management and Logistics
    • Active RF-ID
    • Electronic Shelf Labeling
    • Asset Tracking
    • Location Tracking
    • Theft protection
  • Building & home automation
    • Building Management
    • Energy Control
    • Occupancy Control
    • Fire Protection
    • Security
    • Current protection
    • Measuring
    • Home Control
    • Remote Control
    • Light Switches
    • Temperature Control
  • Industrial automation
  • Automated Meter Reading
  • Measuring
  • Detection
  • Alarm
  • Quality Control

  • Comit's WMSN Solutions
    Using its nearly two decades of design methodology experience, Comit provides unique green WMSN solutions to the industry that features:
    • ultra low power wireless communication technology
    • affordable and differentiating wireless sensor nodes
    • customized control software for a wide range of applications
    If you wish to contact Comit to see how Comit can help with your organization's needs in this area, please click here
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