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ZigBee: Wireless Technology for Low-Power Sensor Networks Gary Legg

Technologists have never had trouble coming up with potential applications for wirsensors. In a home security system, for example, wireless sensors would be much eto install than sensors that need wiring. The same is true in industrial environmentswhere wiring typically accounts for 80% of the cost of sensor installations. And thethere are applications for sensors where wiring isn't practical or even possible.

The problem, though, is that most wireless sensors use too much power, which meathat their batteries either have to be very large or get changed far too often. Add to some skepticism about the reliability of sensor data that's sent through the air, and wireless sensors simply haven't looked very appealing.

A low-power wireless technology called ZigBee is rewriting the wireless sensor equation, however. A secure network technology that rides on top of the recently raIEEE 802.15.4 radio standard (Figure 1), ZigBee promises to put wireless sensors everything from factory automation systems to home security systems to consumerelectronics. In conjunction with 802.15.4, ZigBee offers battery life of up to severayears for common small batteries. ZigBee devices are also expected to be cheap, eventually selling for less than $3 per node by some estimates. With prices that lowshould be a natural fit even in household products like wireless light switches, wirethermostats, and smoke detectors.

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Although no formal specification for ZigBee yet exists (approval by the ZigBee Alliance, a trade group, should come late this year), the outlook for ZigBee appearsbright. Technology research firm In-Stat/MDR, in what it calls a "cautious aggressforecast, predicts that sales of 802.15.4 nodes and chipsets will increase from essenzero today to 165 million units by 2010. Not all of these units will be coupled withZigBee, but most probably will be. Research firm ON World predicts shipments ofmillion wireless sensor RF modules by 2010, with 77% of them being ZigBee-rela

In a sense, ZigBee's bright future is largely due to its low data rates—20 kbps to 25

kbps, depending on the frequency band used (Figure 2)—compared to a nominal 1

for Bluetooth and 54 Mbps for Wi-Fi's 802.11g technology. But ZigBee won't be

sending email and large documents, as Wi-Fi does, or documents and audio, as Bluetooth does. For sending sensor readings, which are typically a few tens of bytehigh bandwidth isn't necessary, and ZigBee's low bandwidth helps it fulfill its goallow power, low cost, and robustness.

Because of ZigBee applications' low bandwidth requirements, a ZigBee node can smost of the time, thus saving battery power, and then wake up, send data quickly, aback to sleep. And, because ZigBee can transition from sleep mode to active modemsec or less, even a sleeping node can achieve suitably low latency. Someone flippZigBee-enabled wireless light switch, for example, would not be aware of a wake-

delay before the light turns on. In contrast, wake-up delays for Bluetooth are typicaround three seconds.

A big part of ZigBee's power savings come from the radio technology of 802.15.4,which itself was designed for low power. 802.15.4 uses DSSS (direct-sequence spr

Figure 1: ZigBee adds network, security, and application-services layers to the PHY and MAC layers of the IEEE 811.15.4 radio

Figure 2: ZigBee's data rates range from 20 kbps to 250 kbps, depending onthe frequency used


8/20/2005file://E:\Books\Zigbee\TechOnLine - ZigBee Wireless Technology for Low-Power Sensor...

spectrum) technology, for example, because the alternative FHSS (frequency-hoppspread spectrum) would have used too much power just in keeping its frequency hosynchronized.

ZigBee nodes, using 802.15.4, can communicate in any of several different ways, however, and some ways use more power than others. Consequently, ZigBee usersnecessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBee's hallmark. In fact, some technologists whplanning very large networks of very small wireless sensors say that even ZigBee ipower hungry for their uses.

A ZigBee network node can consume extra power, for example, if it tries to keep ittransmissions from overlapping with other nodes' transmissions or with transmissiofrom other radio sources. The 802.15.4 radio used by ZigBee implements CSMA/C(carrier sense multiple access collision avoidance) technology, and a ZigBee node tuses CSMA/CA is essentially taking a listen-before-talk approach to see if any radtraffic is already underway. But, as noted by Venkat Bahl, marketing vice presidensensor company Ember Corp. and vice chairman of the ZigBee Alliance, that's not preferred approach. "Having to listen burns power," says Bahl, "and we don't like tthat."

Another ZigBee and 802.15.4 communications option is the beacon mode, in whichnormally sleeping network slave nodes wake up periodically to receive a synchroni"beacon" from the network's control node. But listening for a beacon wastes powerparticularly because timing uncertainties force nodes to turn on early to avoid missbeacon.

In-Your-Face Communication

To save as much power as possible, ZigBee employs a talk-when-ready communistrategy, simply sending data when it has data ready to send and then waiting for anautomatic acknowledgement. According to Bob Heile, who is chairman of both theZigBee Alliance and IEEE 802.15, talk-when-ready is an "in-your-face" scheme,

one that's very power efficient. "We did an extensive analysis that led to the best posaving strategy in various kinds of environments from quiet to noisy," Heile says. "discovered that, hands down, we were better off just sending the packet and acknowledging it. If you don't get an ack, it just means you got clobbered, so send again. You wind up having much better power management than if you listen and determine if it's quiet before you talk."

Fortunately, this in-your-face strategy leads to very little RF interference. That's labecause ZigBee nodes have very low duty cycles, transmitting only occasionally ansending only small amounts of data. Other ZigBee nodes, as well as Wi-Fi and Bluetooth modules, can easily deal with such small, infrequent bursts.

ZigBee's talk-when-ready scheme doesn't suit all purposes, however. For examplenetwork of thousands of tiny sensors dropped into a war zone to monitor enemy tro



movements, the power savings provided still might not be enough. With each netwnode sending data periodically—and with transmissions repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a netcontroller—large numbers of packet collisions and retransmissions could waste powand significantly shorten sensor node battery life. If the sensor batteries are very smand power-limited, that's especially problematic.

Although contention for airwave access isn't generally a problem for ZigBee, it canSensor-network company Dust Networks, in fact, says contention issues are keepin

company from turning to ZigBee—for now, at least—even though Dust remains a member of the ZigBee Alliance. "Each ZigBee device needs to contend for airspacits neighbors," says Dust director of product management Robert Shear, "so there'sinevitably some contention and some inefficiency." To avoid ZigBee's access conteDust uses contention-free TDMA (time division multiple access) technology. ZigBthrough the 802.15.4 MAC layer, provides guaranteed time slots in a scheme that somewhat resembles TDMA, but only as part of an optional "superframe" that's mocomplex and less power-efficient than TDMA.

ZigBee has still more power-saving tricks up its sleeve, however. For example, it reduces power consumption in ZigBee components by providing for power-saving reduced-function devices (RFDs) in addition to more capable full-function devices(FFDs). Each ZigBee network needs at least one FFD as a controller, but most netwnodes can be RFDs (Figure 3). RFDs can talk only with FFDs, not to other RFDs, they contain less circuitry than FFDs, and little or no power-consuming memory.

ZigBee conserves still more power by reducing the need for associated processing.Simple 8-bit processors like an 8051 can handle ZigBee chores easily, and ZigBee protocol stacks occupy very little memory. An FFD stack, for example, needs abou

Figure 3: ZigBee networks can contain as many as 65,536 nodes in a varietyof configurations



kbytes, and an RFD stack needs only about 4 kbytes. Those numbers compare with250 kbytes for the far more complex Bluetooth technology.

From ZigBee's relatively simple implementations, cost savings naturally accrue. RFof course, reduce ZigBee component costs by omitting memory and other circuitrysimple 8-bit processors and small protocol stacks help keep system costs down. Ofapplication's main processor can easily bear the small additional load of ZigBee processing, making a separate processor for ZigBee functions unnecessary.

But the main strategy for keeping ZigBee prices low is to have big markets and higvolumes. The ZigBee Alliance, by making ZigBee an open standard and by vigoroupromoting interoperability among ZigBee devices, expects that ZigBee will be veryin applications such as home and building automation. The alliance is currently woon interoperability procedures for those particular applications, which it expects to complete later this year along with ZigBee Specification 1.0.

One reason for optimism about ZigBee adoption for home automation and securityease of use. ZigBee networks are self-forming, making it easy even for consumers them up. "In the residential space, there's no configuration involved," says the ZigBAlliance's Heile. "You take something out of the box, put the batteries in, and maybsomething as simple as button-press security—bring two devices close together, pubuttons until the green lights come on, and you're done."

ZigBee networks can also self-form in commercial and industrial settings, but professional installers will have tools that provide additional control, particularly fosecurity. ZigBee security is flexible, says Heile, to give both consumer and professusers what they need. "You don't have to have 128-bit public-key encryption for a

smoke detector," he says, "but if I'm in a high-rise office complex, that's exactly th

level of security I'm going to have for my fluorescent light fixtures. If you're in a hrise building on Fifth Avenue, you don't want someone going down the street and turning your lights off."

Proprietary Competition Competition for ZigBee comes almost entirely from proprietary technologies. Senscompany Dust, as noted, is sticking with its own technology, and Ember, although pushing strongly into the ZigBee arena, plans to keep offering its proprietary Embeas well. In addition, Zensys is providing its Z-Wave technology to customers. Sylv

for example, is already using Z-Wave for lighting control, while ZigBee systems rat least several months away.

By offering interoperability, however, ZigBee adds capabilities that proprietary procan't. For example, says Ember's Bahl, interoperability allows the ZigBee nodes oflighting system to work with the ZigBee network of an HVAC system, or vice vers"Philips Lighting is really excited about this," Bahl, says, "because it turns them froballast manufacturer into the infrastructure backbone of a building-automation sys



Needless to say, many of the major semiconductor companies, and especially thoseare big in embedded systems, are eagerly anticipating ZigBee's entry into mass marFreescale Semiconductor (until recently known as Motorola's Semiconductor ProduSector) is already providing ZigBee-ready technology to select customers. Other semiconductor companies, including AMI, Atmel, Microchip, Philips, and Renesasmembers of the ZigBee Alliance.

ZigBee will likely be slow to penetrate the industrial market for wireless sensors, however. According to market research firm ON World, it will take five to seven yto convince industrial customers of the reliability, robustness, and security of wirel

sensor systems. ON World does predict significant long-term growth of ZigBee inindustry, though. By 2010, the company projects, RF modules used in industrial monitoring and control will reach 165 million units, up from 1.9 million in 2004. A75% of those, ON World predicts, will be based on ZigBee and 802.15.4.

Eventually, ZigBee could go into a wide variety of applications. In household appliances, it could help monitor and control energy consumption. In automotive applications, it could provide tire-pressure monitoring and remote keyless entry. Zcould also be used in medical devices or even in computer peripherals, such as wirekeyboards or mice.

Concern is increasing, though, that ZigBee could turn into a one-size-fits-all technology that doesn't fit any application particularly well. Some skeptics, for exaworry that an attempt to make ZigBee all-encompassing could make the ZigBee protocol stack too large for ZigBee's twin goals of very low power consumption anlow cost. If that happens, then ZigBee's low-power, low-data-rate niche—narrow

is—will have proven to be too broad. And then, perhaps, we'll need yet another wirstandard to go with the burgeoning number we already have.

About the Author Gary Legg is a Boston-based freelance writer. He holds a BSEE degree and is a foeditor and executive editor of EDN magazine. He can be reached at gary@garylegg

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