Zig Bee：低功耗传感器网络的无线技术外文翻译资料

 2022-08-17 03:08

Zig Bee: 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 industrial 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 have 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 Zig Bee is rewriting the wireless sensor equation, however. A secure network technology that rides on top of the recently ratified IEEE 802.15.4 radio standard (Figure 1), Zig Bee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. In conjunction with 802.15.4, Zig Bee offers battery life of up to several years for common small batteries. Zig Bee 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: Zig Bee 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 Zig Bee yet exists (approval by the Zig Bee Alliance, a trade group, should come late this year), the outlook for Zig Bee appears 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 Zig Bee, 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 Zig Bee-related.

In a sense, Zig Bees bright future is largely due to its low data rates—20k bps to 250k bps, depending on the frequency band used (Figure 2)—compared to a nominal 1 M bps for Bluetooth and 54 M bps for Wifis 802.11g technology. But Zig Bee wont be sending email and large documents, as Wifi 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 Zig Bees low bandwidth helps it fulfill its goals of low power, low cost, and robustness.


Figure 2: Zig Bees data rates range from 20k bps to 250k bps, depending on the frequency used

Because of Zig Bee applications low bandwidth requirements, a Zig Bee node can sleep most of the time, thus saving battery power, and then wake up, send data quickly, and go back to sleep. And, because Zig Bee can transition from sleep mode to active mode in 15 m sec or less, even a sleeping node can achieve suitably low latency. Someone flipping a Zig Bee-enabled wireless light switch, for example, would not be aware of a wake-up delay before the light turns on. In contrast, wake-up delays for Bluetooth are typically around three seconds.

A big part of Zig Bees 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, because the alternative FHSS (frequency-hopping spread spectrum) would have used too much power just in keeping its frequency hops synchronized.

Zig Bee nodes, using 802.15.4, can communicate in any of several different ways, however, and some ways use more power than others. Consequently, Zig Bee users cant necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is Zig Bees hallmark. In fact, some technologists who are planning very large networks of very small wireless sensors say that even Zig Bee is too power hungry for their uses.

A Zig Bee 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 Zig Bee implements CSMA/CA (carrier sense multiple access collision avoidance) technology, and a Zig Bee 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 Ven kat Bah l, marketing vice president for sensor company Ember Corp. and vice chairman of the Zig Bee Alliance, thats not a preferred approach. 'Having to listen burns power,' says Bah l, 'and we dont like to do that.'

Another Zig Bee 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 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, Zig Bee employs a talk-when-ready communication strategy, simply sending data when it has data ready to send and then waiting for an automatic acknowledge. According to Bob Heil, who is chairman of both the Zig Bee 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

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Zig Bee: 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 industrial 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 have 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 Zig Bee is rewriting the wireless sensor equation, however. A secure network technology that rides on top of the recently ratified IEEE 802.15.4 radio standard (Figure 1), Zig Bee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. In conjunction with 802.15.4, Zig Bee offers battery life of up to several years for common small batteries. Zig Bee 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: Zig Bee 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 Zig Bee yet exists (approval by the Zig Bee Alliance, a trade group, should come late this year), the outlook for Zig Bee appears 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 Zig Bee, 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 Zig Bee-related.

In a sense, Zig Bees bright future is largely due to its low data rates—20k bps to 250k bps, depending on the frequency band used (Figure 2)—compared to a nominal 1 M bps for Bluetooth and 54 M bps for Wifis 802.11g technology. But Zig Bee wont be sending email and large documents, as Wifi 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 Zig Bees low bandwidth helps it fulfill its goals of low power, low cost, and robustness.


Figure 2: Zig Bees data rates range from 20k bps to 250k bps, depending on the frequency used

Because of Zig Bee applications low bandwidth requirements, a Zig Bee node can sleep most of the time, thus saving battery power, and then wake up, send data quickly, and go back to sleep. And, because Zig Bee can transition from sleep mode to active mode in 15 m sec or less, even a sleeping node can achieve suitably low latency. Someone flipping a Zig Bee-enabled wireless light switch, for example, would not be aware of a wake-up delay before the light turns on. In contrast, wake-up delays for Bluetooth are typically around three seconds.

A big part of Zig Bees 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, because the alternative FHSS (frequency-hopping spread spectrum) would have used too much power just in keeping its frequency hops synchronized.

Zig Bee nodes, using 802.15.4, can communicate in any of several different ways, however, and some ways use more power than others. Consequently, Zig Bee users cant necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is Zig Bees hallmark. In fact, some technologists who are planning very large networks of very small wireless sensors say that even Zig Bee is too power hungry for their uses.

A Zig Bee 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 Zig Bee implements CSMA/CA (carrier sense multiple access collision avoidance) technology, and a Zig Bee 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 Ven kat Bah l, marketing vice president for sensor company Ember Corp. and vice chairman of the Zig Bee Alliance, thats not a preferred approach. 'Having to listen burns power,' says Bah l, 'and we dont like to do that.'

Another Zig Bee 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 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, Zig Bee employs a talk-when-ready communication strategy, simply sending data when it has data ready to send and then waiting for an automatic acknowledge. According to Bob Heil, who is chairman of both the Zig Bee 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

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