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Wireless products and technology for sensing & control applications are quickly becoming a reality. Numerous analysts, technology providers and product integrators agree that widespread adoption of wireless technology is only a matter of time.
Still, standardization organizations and technology providers have not done a very good job addressing the bewildering assortment of competing solutions and technologies. If anything, many have contributed to the general frustration by being vague about their application scope.
End users and system developers need standardization for a variety of reasons: compliance with global regulations, interoperability across brands, second sourcing availability, competition to drive prices down, and the opportunity to tap into a large body of knowledge. But there is more.
Some technology components are so expensive to develop that they can only generate an economic return through very high volumes. And when volumes need to be large, the presence of a global market is paramount. Standards are an excellent vehicle to generate global awareness and prepare for a global market for ramp up.
Wireless sensor system architecture
The basic architecture of a wireless sensor system consists of three layers, as depicted in Figure 1.
Click here for Figure 1.
Figure 1: Basic architecture of a wireless sensor device.
The Wireless Transceiver, at the bottom of the stack, translates digital information into a wireless electromagnetic signal that can be broadcast by the transmitter and reconstructed at the receiver end.
In previous generations of wireless technology, you either had a transmitter for transmission only, or a receiver that was only capable of reception. Nowadays, to improve reliability and performance, technology has shifted to combine receive and transmit devices.
Wi-Fi vs Bluetooth vs IEEE 802.15.4
Chip manufacturers need high volume sales to generate meaningful return, and high volumes require global markets. For a global technology market to take off, history has demonstrated that standards are essential.
This was true for both Wi-Fi wireless internet, technically termed IEEE 802.11 a/b/g/n/, and Bluetooth, which is based on a standard defined in the IEEE 802.15.1 specification. It will also be true for sensor networks, which are governed by the IEEE 802.15.4 (a/b) standard (Working Group for Wireless Personal Area) set up in 2003.
All three of these technologies target different applications. Wi-Fi was conceived as an alternative to wired Ethernet PC communication: high data rate networks with a base station at the center and PCs nearby (i.e. a star-network topology). In order to achieve high data rates in a local area, WI-FI consumes a fair amount of power, usually sourced from a laptop battery.
Data rates degrade quickly as distance to the base station increases. Bluetooth was conceived with the mobile phone as the center of the universe: it connects the phone to an earpiece, to a GPS device and to a laptop.
The Bluetooth data rate of one Mbps is high enough to carry voice, but is at least one order of magnitude smaller than that of Wi-Fi. In return, the power consumption is lower, most often sourced from a mobile phone battery.
In general, the communication range is also smaller than that of Wi-Fi, which reflects the fact that the phone is usually in the vicinity of the earpiece, the laptop and the GPS device.
Sensor applications have totally different requirements, particularly with regard to power consumption: sensors often have to work for years on a coin cell battery, or on energy harvested from the environment through a solar panel or vibration harvester. The battery cannot be recharged like a laptop or a phone battery.
Other sensor-specific requirements are determined by factors such as reliability, communication range, the large number of nodes that may need to be supported in a single network, and the need for automatic network organization. In return, a lower data rate is generally acceptable, as most sensors generate fairly small amounts of data, and generally not on a continuous basis.
For wireless sensor transceivers, the dominant and probably only real standard is the IEEE 802.15.4 specification. The first version was ratified in 2003, with an update in 2006. Several vendors offer transceiver chips.
Some of them are a minimal implementation of the standard. Others offer add-ons which are useful in some application segments. GreenPeak's own GP-2000 transceiver, for example, has many power reducing features optimized for coin-cell and battery-less applications.
Table 1 lists the main parameters of the IEEE 802.15.4 standard and compares them to Bluetooth.
Click here for Table 1.
Table 1.
There have been efforts to use Bluetooth and Wi-Fi for sensor applications. In both cases, Bluetooth and WI-FI were used in a non-standard way, weaving the principles of IEEE 802.15.4 into their native implementation. Nowadays, it is widely accepted that IEEE 802.15.4 offers the best solution for wireless sensor applications.
Not all technology suppliers adhere to the IEEE 802.15.4 standard. Some have chosen to build proprietary transceivers, with the goal of reducing complexity and cost. It remains to be seen if these proprietary solutions will achieve the volume needed to actually reduce cost. Moreover, reducing complexity generally goes hand in hand with sacrificing performance, thereby and limiting the range of applications for these solutions.
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