Wireless Sensor Network

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21 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Wireless Sensor Network
Deployment Lessons Learned

Steven Lanzisera

Environmental Energy Technologies Division, LBNL

21 January 2011

2


Wireless Sensor Networks in 2002

3


Project Overview


Commercial Buildings


LBNL Building 90


90,000
s.f.

office


Plug
-
in device metering network


6 months of data collection (on going)


4


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

5


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


6


Overview


Zigbee

& Standards Context


Why Wireless Networks Fail


Communication Issues


Other Issues


Final Thoughts

7


Overview


Zigbee

& Standards Context


Wireless Network Characteristics


Final Thoughts

8


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



9


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


10


Mischa Dohler & Thomas
Watteyne @ ICC 2009

IEEE 802.15.4


MAC Layer


Super
-
Frame Structure for Beacon
-
Enabled Mode:


11


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


12


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)



13


Full function device

Reduced function device

Communications flow

Mesh

for full function


Listen all the time

Star
for reduced function


Sleep between transmissions

Zigbee

General Topology

14


Overview


Zigbee

& Standards Context


Why Wireless Networks Fail


Communication Issues


Other Issues


Final Thoughts

15


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

16


Single Channel Solutions


The quality of a link varies

with frequency

with time

t
here is no “best

channel”!

17


Channel Success Probability vs. RSSI

18


Second Challenge: Multipath Fading

19


Second Challenge: Multipath Fading

ch.11

20


Second Challenge: Multipath Fading

ch.11

ch.12

0% reliability

100% reliability

21


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

22


Impact of Interference


Noise






Interference

2.4 GHz

Channels 11
-
26

2.4835 GHz

5 MHz

2.4 GHz

PHY

Relative Noise Power

23


Mischa Dohler & Thomas
Watteyne @ ICC 2009

Interference continued

Spectrum & Interference

25


BT & WLAN interfere with ZigBee


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

26


Mischa Dohler & Thomas
Watteyne @ ICC 2009

Reservation vs. Contention MAC


Example of throughput versus offered load:

Offered Load

Normalized Throughput

reservation based

27


Data Collection Network Reliability

% of Possible Packets

% of Possible Packets

28


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)

29


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)

30


Overview


Zigbee

& Standards Context


Why Wireless Networks Fail


Final Thoughts

31


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

32


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


33


Recommendations


Early study of
Zigbee

in intended environment


Multihop

network


Test latency, reliability,
etc


Consider draft SEP 2.0 (available on the web)



34


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