Wireless Technology for Low-Power Sensor
Networks
无线技术低功耗传感器
.Gary Legg
We have never had trouble coming up with potential applications for wireless sensors.In a home security system,for example,wireless sensors would bu much easier to install than sensors that need wiring.the same is true in industrial environments,where wiring typically accounts for 80% of the cost of sensor installations.And then there are applications for sensors where wiring isnrsquo;t practical or even possible.
The problem, though,is that most wireless sensors use too much power, which means that their batteries either have to be very large or be very large or get changed far too often.Add to that some skepticism about the reliabbility of sensor data thatrsquo;s sent through the air,and wireless sensors simply havenrsquo;t looked very appealing.
A lot-power wireless technology called ZigBee is rewriting the wireless sensor equation,howerer,A secure network techonlogy that rides on top of the recently ratified IEEE802.15.4radio standard.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 life 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 smoke detectors.
Figure 1:ZigBee adds network,security,and application-services layers to the PHY and MAC layers of the IEEE811.15.4.radio
Although no formal specification for ZigBee yet exists,the outlook for ZigBee appears bright.Technology research firm In-Stat/MDR, in what it calls a “cautious aggressive”forcast,predicts that sales of 802.15.4nodes 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 be.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,ZigBeersquo;s bright future is largely due to its low data rates—20kbps to 250 kbps,depending on the frequency band used(Figure 2).But ZigBee wonrsquo;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 bytes,high bandwidth isnrsquo;t necessary, and ZigBeersquo;s low bandwidth helps it fulfill its goals of low power, low cost,and robustness.
Figure 2:ZigBeersquo;s data rates range from 20kbps to 250kbps,depending on the frequency used.
Because of ZigBee applicationsrsquo; low bandwidth requirements,a ZigBee node can sleep most of the time,thus saving battery power.and then wake up ,send data active mode in 15 msec or less,even a sleeping node can achieve suitably low latency.
A big part of ZigBeersquo;s power saving 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,because 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 cant necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBeersquo;s hallmark.
A ZigBee network node can consume extra power,for example,if it trier to keep its transmissions from overlapping with other nodesrsquo; 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)technology,and a ZigBee node that uses CSMA/CD is essentially taking a listen-before-talk approach to see if any radio traffic is already underway.
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 waiting for an automatic acknowledgement.According to Bob Heile,who is chairman of both the ZigBee Alliance and IEEE 802.15.4,talk-when-ready is an lsquo;in-your-facersquo; scheme,but one thatrsquo;s very power efficient,rsquo;We did an extensive analysis that led to the best power-saving strategy in various kinds of environments from quiet to noisy.rsquo;Heile says.
Fortunately, this in-your-face strategy leads to very little PF interference.Thatrsquo;s largely because ZigBee nodes have very low duty cycles,transmitting only occasionally and sending only small amount of data.Other ZigBee nodes,as well as Wi-Fi and Bluetooth modules,can easily deal with such small,infrequent bursts.scheme doesnrsquo;t suit all purposes,ho
ZigBeersquo;s talk-when-ready scheme doesnrsquo;t suit all purposes,however.For example, in a network of thuosands of tiny sensors dropped into a war zone to monitor enemy troop movements.the sending data periodically—and with tranmissions repeated numerous times though other neraby nodes of a mesh network configuration in order to reach a network controler—large numbers of packet collisions and enough.With each network node sending data periodically—and with transmissions repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a network controller—large numbers of packer collisions and retransmissions could waste power and significantly shorten sensor node battery life.If the sensor batteries are very small and power-limited, thatrsquo;s especially problematic.
ZigBee has stillmore power-saving tricks up its sleeve,horever.For example,it reduces power consumption in ZigBee components by providing for power-saving reduced-fuction devices(RFDs)in ad
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Wireless Technology for Low-Power Sensor
Networks
无线技术低功耗传感器
.Gary Legg
We have never had trouble coming up with potential applications for wireless sensors.In a home security system,for example,wireless sensors would bu much easier to install than sensors that need wiring.the same is true in industrial environments,where wiring typically accounts for 80% of the cost of sensor installations.And then there are applications for sensors where wiring isnrsquo;t practical or even possible.
The problem, though,is that most wireless sensors use too much power, which means that their batteries either have to be very large or be very large or get changed far too often.Add to that some skepticism about the reliabbility of sensor data thatrsquo;s sent through the air,and wireless sensors simply havenrsquo;t looked very appealing.
A lot-power wireless technology called ZigBee is rewriting the wireless sensor equation,however,A secure network technology that rides on top of the recently ratified IEEE802.15.4radio standard.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 life 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 smoke detectors.
Figure 1:ZigBee adds network,security,and application-services layers to the PHY and MAC layers of the IEEE811.15.4.radio
Although no formal specification for ZigBee yet exists,the outlook for ZigBee appears bright.Technology research firm In-Stat/MDR, in what it calls a “cautious aggressive”forcast,predicts that sales of 802.15.4nodes 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 be.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,ZigBeersquo;s bright future is largely due to its low data rates—20kbps to 250 kbps,depending on the frequency band used(Figure 2).But ZigBee wonrsquo;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 bytes,high bandwidth isnrsquo;t necessary, and ZigBeersquo;s low bandwidth helps it fulfill its goals of low power, low cost,and robustness.
Figure 2:ZigBeersquo;s data rates range from 20kbps to 250kbps,depending on the frequency used.
Because of ZigBee applicationsrsquo; low bandwidth requirements,a ZigBee node can sleep most of the time,thus saving battery power.and then wake up ,send data active mode in 15 msec or less,even a sleeping node can achieve suitably low latency.
A big part of ZigBeersquo;s power saving 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,because 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 cant necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBeersquo;s hallmark.
A ZigBee network node can consume extra power,for example,if it trier to keep its transmissions from overlapping with other nodesrsquo; 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)technology,and a ZigBee node that uses CSMA/CD is essentially taking a listen-before-talk approach to see if any radio traffic is already underway.
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 waiting for an automatic acknowledgement.According to Bob Heile,who is chairman of both the ZigBee Alliance and IEEE 802.15.4,talk-when-ready is an lsquo;in-your-facersquo; scheme,but one thatrsquo;s very power efficient,rsquo;We did an extensive analysis that led to the best power-saving strategy in various kinds of environments from quiet to noisy.rsquo;Heile says.
Fortunately, this in-your-face strategy leads to very little PF interference.Thatrsquo;s largely because ZigBee nodes have very low duty cycles,transmitting only occasionally and sending only small amount of data.Other ZigBee nodes,as well as Wi-Fi and Bluetooth modules,can easily deal with such small,infrequent bursts.scheme doesnrsquo;t suit all purposes,ho
ZigBeersquo;s talk-when-ready scheme doesnrsquo;t 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 sending data periodically—and with tranmissions repeated numerous times though other neraby nodes of a mesh network configuration in order to reach a network controler—large numbers of packet collisions and enough.With each network node sending data periodically—and with transmissions repeated numerous times through other nearby nodes of a mesh network configuration in order to reach a network controller—large numbers of packer collisions and retransmissions could waste power and significantly shorten sensor node battery life.If the sensor batteries are very small and power-limited, thatrsquo;s especially problematic.
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-fuction devices(RFDs)in a
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