Wireless Sensor Network Deployment Lessons Learned

Steven LanziseraEnvironmental Energy Technologies Division, LBNL

21 January 2011

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Wireless Sensor Networks in 2002

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Project Overview – Commercial Buildings

• LBNL Building 90– 90,000 s.f. office

• Plug-in device metering network• 6 months of data collection (on going)

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Current Building 90 Deployment• 300+ ACmes installed throughout building• 500 at full build-out

– 0.5 nodes per 100 s.f.

• 802.15.4, CSMA, 6LowPAN, RPL (draft), SMAP (custom)

• Power, apparent power, energy every 10s

245 ft / 75 m

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Residential Deployments

• 5 Houses (4 bay area, 1 Boston area)– ~80 nodes installed per house– 1 gateway– Data reported every 10s– 6 months of data collection (ongoing)

• Gearing up for 70 homes in next year– 15-20 nodes per home

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Overview

• Zigbee & Standards Context• Why Wireless Networks Fail

– Communication Issues– Other Issues

• Final Thoughts

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Overview

• Zigbee & Standards Context• Wireless Network Characteristics• Final Thoughts

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IEEE 802.15.4 – Overview

• Emphasis of IEEE 802.15.4 is:– low-cost, low-speed ubiquitous communication between nearby devices with little to

no underlying infrastructure– Nominal communication at 250 kb/s– 10m communication range assumed – to meet embedded constraints, several PHY layers are available

• Key technology features are: – collision avoidance through CSMA/CA– integrated support for secure communications (128-bit AES encryption)– power management functions such as link quality and energy detection– 16 channels in the 2.4 GHz band– star and mesh topologies can theoretically be built

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IEEE 802.15.4 – MAC Layer

• There are two general channel access methods:

• Non-Beacon Network:– simple, traditional multiple access system used in simple peer networks– standard CSMA conflict resolution– positive acknowledgement for successfully received packets

• Beacon-Enabled Network– can be used in beacon-request mode without superframes– superframe structure - network coordinator transmits beacons at predetermined

intervals– dedicated bandwidth and low latency– low power consumption mode for coordinator

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Mischa Dohler & Thomas Watteyne @ ICC 2009

IEEE 802.15.4 – MAC Layer

• Super-Frame Structure for Beacon-Enabled Mode:

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IEEE 802.15.4 MAC in Practice

• Beacons are rarely used• Contention based networks are common• Zigbee doesn’t require one or the other

– Often implemented without beacons

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ZigBee

• ZigBee in short:– international alliance for wireless control applications; SIG certifies platforms– based on IEEE 802.15.4 PHY & MAC– millions of products today are embedding a chipset of the ZigBee family– Small numbers of ZigBee certified products are available

• Provides network through application layers• Most devices listen all the time (and must be mains powered)

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Full function device

Reduced function device

Communications flow

Mesh for full functionListen all the time

Star for reduced functionSleep between transmissions

Zigbee General Topology

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Overview

• Zigbee & Standards Context• Why Wireless Networks Fail

– Communication Issues– Other Issues

• Final Thoughts

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Assumptions

• Multi-hop network of low power wireless sensors

• Communicating using IEEE802.15.4 radio chips (16 frequency channels in the 2.4GHz band)

2.4 GHz

Channels 11-26

2.4835 GHz

5 MHz

2.4 GHz PHY

A

B

C

D

E

F

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Single Channel Solutions

• The quality of a link varies

with frequency with time

there is no “best channel”!

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Channel Success Probability vs. RSSI

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Second Challenge: Multipath Fading

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Second Challenge: Multipath Fading

ch.11

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Second Challenge: Multipath Fading

ch.11 ch.12

0% reliability 100% reliability

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Second Challenge: Multipath Fading

ch.11

ch.13

ch.15

ch.17

ch.12

ch.14

ch.16

ch.18

ch.19

ch.21

ch.23

ch.25

ch.20

ch.22

ch.24

ch.26

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Impact of Interference

• Noise

• Interference

2.4 GHz

Channels 11-26

2.4835 GHz

5 MHz

2.4 GHz PHY

Relative Noise Power

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Mischa Dohler & Thomas Watteyne @ ICC 2009

Interference continued

Spectrum & Interference

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BT & WLAN interfere with ZigBee

• Theoretical results indicate that interference is an issue [SPC07]:

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Mischa Dohler & Thomas Watteyne @ ICC 2009

Reservation vs. Contention MAC

• Example of throughput versus offered load:

Offered Load

Nor

mal

ized

Thr

ough

put

reservation based

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Data Collection Network Reliability%

of P

ossi

ble

Pac

kets

% o

f Pos

sibl

e P

acke

ts

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Latency

• Multihop latency suffers because of communication failures

• 1-hop latency is < 10ms if it works• Backoff after failure increases latency• Tests w/50 ms backoff & 5 hops

– Average latency ~100ms– 90% of packets arrive by 500ms– MAC time out occurs before 99% (1s)

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Link Length & Routing Stability

In B90 (Office building)• Typical links 30ft• Longest (reliable) links 50ft• 60% of routes didn’t change this week• 20% of routes changed >5 times (Check daily)

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Overview

• Zigbee & Standards Context• Why Wireless Networks Fail• Final Thoughts

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Why Zigbee?

• Zigbee is a “new” protocol• Limited industry experience• Known for interoperability, reliability problems• Latency, packet size are far from ideal• Very few products on the market

• Plus side: could be cheap(er)– Somewhat lower power

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Consider WiFi

• Over 2M WiFi chips shipped every day• Same MAC, but better coexistance• SEP 2.0 is not linked to a PHY• SEP 2.0 and other Zigbee will work on IP• Power difference isn’t large (0.3W vs 0.1W)• Cost difference negligible

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Recommendations

• Early study of Zigbee in intended environment– Multihop network– Test latency, reliability, etc

• Consider draft SEP 2.0 (available on the web)

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Summary

• Lots of Zigbee-like networks deployed• Lots of problems negatively impact the network• Need to study Zigbee performance

– Because it’s not well known like WiFi– Will kill the project if performance is poor