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DAZED AND CONFUSED

by Tom Cantrell

Start ı BalkLANization ı Pain in the Pan ı Whatıs All the Buzz About? ı Rock Around the Clock ı Sources and PDF

WHATıS ALL THE BUZZ ABOUT?

Apparently Iım not alone in my speculation that thereıs an opening for something between Bluetooth and garage door openers. Enter ZigBee. Huh? That was my reaction when I stumbled across it, too.

Initially, I was inclined to dismiss ZigBee as more standards chicanery. Itıs got its roots with the HomeRF gang, whoıve come under fire recently for the bit of ham-fisted arm twisting they applied to the FCC in a last-ditch effort to fight off the advance of 802.11. Their push to change the standards in a way that favored their protocol was ultimately approved, although most observers seem to feel itıs too little too late.

Then, I noticed that the main focus of the ZigBee initiative seemed to be more about picking a name than actually doing anything. First, there was HomeRF-Lite, then RF-Lite, Firefly, and RF-Easylink when they finally settled on ZigBee. My confidence was uninspired.

Meanwhile, as I was poking around the IEEE 802.11 web pages, I noticed a passing reference to yet another standard, 802.15 Wireless Personal Area Networks (WPAN). Make that standards, as in 802.15.1, .2, .3, and .4.

802.15.1 brings Bluetooth under the umbrella of IEEE standardization, although in this case, I think itıs safe to say the cart of commercial Bluetooth interests is coming before the horse of IEEE standardization.

With everybody and their brother crowding the spectrum, 802.15.2 is trying to get ahead of the coexistence curve by defining standard and recommended practices to, for example, prevent Bluetooth and 802.11 gear from stepping all over each other.

802.15.3 targets whatıs being called a high-rate (i.e., faster) WPAN. At a superficial glance, Iım not sure if theyıre heading towards a Bluetooth-II, an 802.11-Lite, or some combination of the two.

Finally, thereıs 802.15.4, which is being called the low-rate WPAN. As I dropped in and cut back from link to link, I started noticing a striking similarity between 802.15.4 and ZigBee. Sure enough, it turns out the two groups are working closely together. If thereıs a distinction, itıs that the IEEE is focusing more on the lower layers (i.e., MAC and PHY) while the ZigBee folks are also paying attention to some of the higher layers.

Anyway, as I write this, the 802.15.4 spec is nearing its first ballot and is not yet available to the public. However, Iıve seen enough to know that this is something thatıs going to be of great interest to embedded types. Compared to all the other wireless options, 802.15.4 is less about performance and fancy features than simplicity, low power, and low cost.

Like everything else, 802.15.4 hops on the 2.4-GHz spectrum bandwagon. Yes itıs crowded, but keep in mind that applications like a thermostat call for a minuscule duty cycle, minimizing both interferer and interferee concerns.

The 2.4-GHz spectrum is renowned for its nearly worldwide harmonization. But, how many folks take a thermostat on their overseas vacation? With global capability less of a concern, 802.15.4 also defines some popular localized options, including 900 MHz in the U.S. (see Figure 2). The data rate is specified at 250 kbps for 2.4 GHz and 20 kbps for 900 MHz. Leave your streaming A/V aspirations at home, but thatıs adequate for many simple embedded apps.

Figure 2ıThe low-rate Wireless Personal Area Network proposed by the ZigBee and IEEE 802.15.4 alliance includes both global (2.4 GHz) and localized (900 MHz in the U.S.) spectrum options. [3]

Personally, Iıve always wondered why somebody didnıt just take an inexpensive 900-MHz portable phone and hijack it for data. Indeed, the fact these phones are already digitizing the voice means a 900-MHz data modem (i.e., a 900-MHz phone without the CODEC) is trivial (see Photo 1).

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