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Paving the way to strong sensor, control nets

Posted: 20 Nov 2009 ?? ?Print Version ?Bookmark and Share

Keywords:wireless sensor network? control network? network design?

Coming in the wake of their voice and data counterparts, wireless networks for sensing and control represent the exciting third wave of wireless networking. The combination of wireless communications with emerging energy-harvesting and low-power sensor networks will deliver entirely new classes of electronic devices that could revolutionize how we play, work and conduct the business of day-to-day life.

While connected "smart" sensors present a unique market opportunity, they also pose unique engineering challenges, particularly with respect to cost; global regulatory compliance; interoperability across brands; second sourcing; and of course, powering and maintaining reliable, always-on connectivity. That connectivity must be maintained over many years in home or harsh environments, as well as in "green" buildings with environmental controls, health and safety devices, and automotive and security applications.

By using a combination of standards for the radio and software stacks, developers can avail themselves of a strong, clear roadmap to the future of low-power wireless sensor networks.

 The IEEE 802.15.4 standard defines the physical (PHY) and media access control (MAC) layers.
The IEEE 802.15.4 standard defines the physical (PHY) and media access control (MAC) layers.

Need for standards
It is not enough to remove cabling for connectivity; a truly wireless device also eliminates the power wires. Installing and routing power cables can greatly complicate deployment and increase installation costs. Further, requiring "wireless" devices to use power cabling tethers them to a location and inhibits network efficiency and flexibility.

Wireless power exists today in the form of batteries. But battery-operated devices typically are not maintenance-free. Periodic battery replacement is inconvenient and costly, and disposal of used batteries is not a green proposition.

To reap the benefits of low-cost, wireless, green devices, we must either eliminate the batteries (by converting to energy harvesters) or at least get extremely long life (more than eight years) out of inexpensive batteries. Only power-efficient devices, communicating via specialized, low-power protocols can make that possible.

Several proprietary protocols are available today. But to accelerate the acceptance of the third wave of wireless, we need industry standards for several important reasons:
? Market acceptance!Consumers have grown accustomed to buying devices that conform to industry standards.
? Interoperability!Devices from Manufacturer X should work with those from Manufacturers Y and Z.
? Lower cost!Multiple suppliers must compete with each other on price. Economies of scale lower the prices of common components.
? Silicon business model!High volumes, enabled by global markets, are required to justify the high investment costs for silicon chips.

The wireless networking industry is moving toward adoption of standards governing extremely low-power wireless networks for industrial, home, enterprise, and sense and control applications. Powered by batteries and energy-harvesting devices, truly wireless products and technologies are becoming a reality.

The development and acceptance of technology standards are important factors for the adoption of new technologies. Product integrators require technology standards because standards provide product interoperability, a large body of knowledge and development sources, and second-sourcing flexibility.

Which standard?
The requirements for sensor applications are completely different from those for wireless voice and data networks. The availability of power is the most conspicuous difference; sensors often have to work for years on a non-rechargeable coin cell battery or on energy harvested from the environment through a solar panel or a vibration harvester. Other sensor-specific application requirements are related to automatic network organization, reliability, communications range and the large number of nodes to be supported in a single network.

This overview of the IEEE 802 LAN/MAN group of wireless standards shows the positioning of IEEE 802.15.4, the low-power wireless network standard, also known as the low-rate personal area network.
This overview of the IEEE 802 LAN/MAN group of wireless standards shows the positioning of IEEE 802.15.4, the low-power wireless network standard, also known as the low-rate personal area network.

For wireless sensor transceivers, the dominant standard!indeed, probably the only real standard!is IEEE 802.15.4, for which several vendors offer compliant transceiver chips. Some are minimal implementations of the standard; others offer add-ons that are useful in some application segments, such as power-reducing features targeted toward coin cell and batteryless applications.

One key aspect when choosing any wireless standard is the network stack. The stack has two responsibilities, the first of which is to form and maintain the network.

An important consideration in wireless network stack design is the ability to cope with the constantly varying quality of the wireless links between nodes. For example, in a building automation application the movement of people about the building can compromise the link quality; a link might "disappear" at any moment, possibly isolating a node or even a whole branch of the network. To provide uninterrupted connectivity to all parts of the network, the stack must be able to reorganize the communication routes by establishing new links.

The other responsibility of the network stack is to ensure that messages can travel from a source node to a destination node in a reliable and efficient way. Efficiency here means that latency requirements must be met and message routing bottlenecks avoided.

The underlying technology driving the third generation of low-power networks is IEEE 802.15.4, part of the IEEE 802.x family of networking standards.

In addition to 802.15, there are four other wireless subfamilies that are based on 802: IEEE 802.11, also known commercially as Wi-Fi; IEEE 802.16, for wireless metropolitan area networks (WMANs) such as WiMAX; IEEE 802.20, for mobile WMANs such as WiMAX Mobile; and 802.22, governing wireless radio area network applications such as TV band reuse and cognitive radio.

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