Wireless Mesh Networks in Industry: How Radios Really PerformJanuary 1, 2005 By: Daniel Sexton, Jay Werb Sensors
It is now possible to embed radio technology in a wide variety of inexpensive battery-powered devices. In the industrial world, many new products are being developed to place inexpensive sensors wherever there are interesting data. This emerging technology, unconstrained by expensive wiring, has the potential to save industry a lot of lost productivity.
Two new and closely related standards are fueling interest in wireless sensor networking: IEEE 802.15.4 and ZigBee. IEEE 802.15.4 is a physical and MAC layer standard for low-rate RF networks, whereas ZigBee is a standard proposed by the ZigBee Alliance, which provides a recommendation for the network and application interfaces and operation of low-rate wireless networks. Market forecasts for IEEE 802.15.4 transceivers project hundreds of millions of units over the next five years. Before it is used in the kind of applications where millions of dollars are at stake, the technology must be proven and well tested. As a first step toward this goal, we've been carefully validating the real-world performance of these early offerings under a Department of Energy (DoE) grant.
Evaluating the Radios
Current standards-body compliance procedures are limited to interoperability testing, rather than testing whether the radios meet their specifications under a reasonable range of operating conditions, such as multipath, temperature, and vibration. Multipath is a phenomenon that occurs when multiple copies of the same information, following different paths at different times, arrive at a single point; it can be both destructive as well as constructive. Temperature can cause issues with receiver sensitivity, such as transmitter-to-receiver clock skew, as well as transmitter output degradation. Vibration can cause random variations in crystal frequency, affecting both radio performance and system timing. With dozens of IEEE 802.15.4 radios coming on the market over the next several months, we need industry-wide benchmarks for radio performance under well-defined conditions. Without benchmarks of this sort, users cannot make informed comparisons.
In early 2004, there were only two IEEE 802.15.4 radio transceivers available, one of which was still in the beta stage. To validate the radios' specifications, we first tested their performance in a wired bench configuration. We found the beta product to have 4.5 dB less sensitivity under ideal conditions and 6.5 dB less sensitivity under controlled multipath conditions, despite similar specifications. According to vendor claims, only about 2 dB of this difference could be attributed to the use of beta-level technology. However, even accounting for beta-level technology, this still leaves the beta units at a 4.5 dB disadvantage under multipath conditions—which are frequently encountered in indoor applications.
Theoretical Performance in the Factory
We then explored performance in factory environments. We characterized three basic factors: channel fading, channel response, and interference.
Channel Fading. Channel fading is the effect of multipath on the received signal. Factory radio environments are far from ideal, with many metal surfaces to block and reflect radio signals. In practice we've observed that small changes of position (a few inches) or small changes in frequency (a few MHz) will radically change the signal level and radio connectivity.
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