1、ZigBee: Wireless Technology for Low-Power Sensor Networks Technologists have never had trouble coming up with potential applications for wireless sensors. In a home security system, for example, wireless sensors would be much easier to install than sensors that need wiring. The same is true in indus
2、trial environments, where wiring typically accounts for 80% of the cost of sensor installations. And then there are applications for sensors where wiring isnt practical or even possible. The problem, though, is that most wireless sensors use too much power, which means that their batteries either ha
3、ve to be very large or get changed far too often. Add to that some skepticism about the reliability of sensor data thats sent through the air, and wireless sensors simply havent looked very appealing. A low-power wireless technology called ZigBee is rewriting the wireless sensor equation, however. A
4、 secure network technology that rides on top of the recently ratified IEEE 802.15.4 radio standard (Figure 1), ZigBee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. In conjunction with 802.15.4, ZigBee offers battery l
5、ife of up to several years 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 low, they should be a natural fit even in household products like wireless light switches, wireless thermostats, and s
6、moke detectors. Figure 1: ZigBee adds network, security, and application-services layers to the PHY and MAC layers of the IEEE 811.15.4 radio 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 ap
7、pears bright. Technology research firm In-Stat/MDR, in what it calls a cautious aggressive forecast, predicts that sales of 802.15.4 nodes and chipsets will increase from essentially zero today to 165 million units by 2010. Not all of these units will be coupled with ZigBee, but most probably will b
8、e. Research firm ON World predicts shipments of 465 million wireless sensor RF modules by 2010, with 77% of them being ZigBee-related. In a sense, ZigBees bright future is largely due to its low data rates20 kbps to 250 kbps, depending on the frequency band used (Figure 2)compared to a nominal 1 Mbp
9、s for Bluetooth and 54 Mbps for Wi-Fis 802.11g technology. But ZigBee wont 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 bytes, high bandwidth isnt necessary, and ZigBees low bandwidth he
10、lps it fulfill its goals of low power, low cost, and robustness. Figure 2: ZigBees data rates range from 20 kbps to 250 kbps, depending on the frequency used Because of ZigBee applications low bandwidth requirements, a ZigBee node can sleep most of the time, thus saving battery power, and then wake
11、up, send data quickly, and go back to sleep. And, because ZigBee can transition from sleep mode to active mode in 15 msec or less, even a sleeping node can achieve suitably low latency. Someone flipping a ZigBee-enabled wireless light switch, for example, would not be aware of a wake-up delay before
12、 the light turns on. In contrast, wake-up delays for Bluetooth are typically around three seconds. A big part of ZigBees 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 spread spectrum) technology, for example, be
13、cause the alternative FHSS (frequency-hopping spread spectrum) would have used too much power just in keeping its frequency hops synchronized. 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 users
14、 cant necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBees hallmark. In fact, some technologists who are planning very large networks of very small wireless sensors say that even ZigBee is too power hungry for their uses. A ZigBee
15、 network node can consume extra power, for example, if it tries to keep its transmissions from overlapping with other nodes transmissions or with transmissions from other radio sources. The 802.15.4 radio used by ZigBee implements CSMA/CA (carrier sense multiple access collision avoidance) technolog
16、y, and a ZigBee node that uses CSMA/CA is essentially taking a listen-before-talk approach to see if any radio traffic is already underway. But, as noted by Venkat Bahl, marketing vice president for sensor company Ember Corp. and vice chairman of the ZigBee Alliance, thats not a preferred approach.
17、Having to listen burns power, says Bahl, and we dont like to do that. Another ZigBee and 802.15.4 communications option is the beacon mode, in which normally sleeping network slave nodes wake up periodically to receive a synchronizing beacon from the networks control node. But listening for a beacon
18、 wastes power, too, particularly because timing uncertainties force nodes to turn on early to avoid missing a beacon. In-Your-Face Communication To save as much power as possible, ZigBee employs a talk-when-ready communication strategy, simply sending data when it has data ready to send and then wai
19、ting for an automatic acknowledgement. According to Bob Heile, who is chairman of both the ZigBee Alliance and IEEE 802.15, talk-when-ready is an in-your-face scheme, but one thats very power efficient. We did an extensive analysis that led to the best power-saving strategy in various kinds of envir
20、onments from quiet to noisy, Heile says. We discovered that, hands down, we were better off just sending the packet and acknowledging it. If you dont get an ack, it just means you got clobbered, so send it again. You wind up having much better power management than if you listen and determine if its
21、 quiet before you talk. Fortunately, this in-your-face strategy leads to very little RF interference. Thats largely because ZigBee nodes have very low duty cycles, transmitting only occasionally and sending only small amounts of data. Other ZigBee nodes, as well as Wi-Fi and Bluetooth modules, can e
22、asily deal with such small, infrequent bursts. ZigBees talk-when-ready scheme doesnt suit all purposes, however. For example, in a network of thousands of tiny sensors dropped into a war zone to monitor enemy troop movements, the power savings provided still might not be enough. With each network no
23、de sending data periodicallyand with transmissions repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a network controllerlarge numbers of packet collisions and retransmissions could waste power and significantly shorten sensor node battery life. If
24、the sensor batteries are very small and power-limited, thats especially problematic. Although contention for airwave access isnt generally a problem for ZigBee, it can be. Sensor-network company Dust Networks, in fact, says contention issues are keeping the company from turning to ZigBeefor now, at
25、leasteven though Dust remains a member of the ZigBee Alliance. Each ZigBee device needs to contend for airspace with its neighbors, says Dust director of product management Robert Shear, so theres inevitably some contention and some inefficiency. To avoid ZigBees access contention, Dust uses content
26、ion-free TDMA (time division multiple access) technology. ZigBee, through 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 thats more complex and less power-efficient than TDMA. ZigBee has still more power-sav
27、ing 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 network nodes can be RFDs (Figure 3). RFDs can talk only with FFDs, not to other RFDs, but they contain less circuitry than FFDs, and little or no power-consuming memory.
