The Heathland Experiment Results And Experiences · Depth-First Algorithm • Sink started...
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TUHH - TELEMATIK
The Heathland ExperimentResults And Experiences
V. Turau, Ch. Renner, M. Venzke, S. Waschik, Ch.
Weyer, M. Witt
Hamburg University of Technology, Germany
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Introduction
• Wireless sensor networks very active field
• But:
– Number of theoretical publications >> number of
deployments
– Vast majority algorithms only simulated & not
implemented on real sensor networks
– Current simulation tools are not capable to
completely represent sensor networks in harsh
environment over long period of time
• Real deployments are indispensable!
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Heathland Experiment
• Real deployment of a sensor network
• 24 nodes running for 2 weeks in March 2005
• Location: Heathlands of Northern Germany
• First outdoor long term usage of Embedded Sensor Board (ESB)
• Application ran without any human attention
• Objective: Reveal problems that can only be discovered through experience
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Goals
• General radio performance (packet loss, packet errors)
• Constancy of transmission ranges
• Performance of our neighborhood protocol
• Communication in a multi-hop environment
• Dynamic adaption to changes of transmission ranges
• Test depth-first search implementation
• Increased expectation of life was not a goal
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Platform: ESB Nodes
Source: www.scatterweb.net
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Platform: ESB Nodes
• Texas Instruments micro controller MSP 430
• Transceiver TR1001, 868 MHz, 19.2 kbit/s
• RS232 serial interface
• Sensors: light, passive infrared, temperature, vibration, microphone
• Tunable radio transmit power
• 2KB RAM, 8 KB EEPROM
• Powered by 3 AA batteries
• Power consumption: 8 µA (sleep mode) up to 12 mA (all sensors on)
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Packaging
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Packaging
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Packaging
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Weather
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Recorded Temperature
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Application
• Sense & send: Sensors deployed in wide-ranging area take periodic readings & report to central repository
• Nodes periodically send readings from their 5 sensors to sink
• To account for topology changes, routing trees and neighbor relationships were recomputed regularly
• Scalable application
• Application could cope with node failures, deployment of new nodes, decreasing communication radii due to fading battery energy
• Multi-hop network
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Application
Radio Transmission Protocol
Firmware
Sense & Send Application
Wireless Neighborhood Exploration
WNX
Depth-first Algorithm
Routing
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WNX
• Modified implementation of TND, the proactive
neighbor discovery protocol of TBRPF (RFC 3684)
• Simple, agile yet stable
• Uni- & bidirectional links
• Each node records last 32 expected hellos from
each neighbor
• Quality descriptor: weighted moving average
• Hellos associated with weights � Link-quality
between 0 and 255
• Radio signal strength indicator, not included
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WNX
• At most 8 neighbors per node
• It takes 30 sec to discover new bidirectional link / to purge lost link
• Small memory footprint : 152 Bytes per node
• Currently protocol is extended
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Application
• Uncertainty in radio communication � time
triggered scheme
• Activities are initiated by progression of globally
synchronized time-base
• Due to clock drift between individual nodes, sink
sends periodically its local time into network
• All nodes including the sink have the same code
• Simplified deployment process
• Sink node sent data over a standard serial port
interface to a PC (about 6MB per day)
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At start of every clock hour:
List of bidirectional links with link qualitySuspend WNX9
Leaf nodes turn off radio, inner nodes turn off sensors
In intervals of 10 minutes:
- leaf nodes turn on radio
- all nodes send measured data & link states to sink
- the sink sends its local time to all nodes in tree
- leaf nodes turn off radio
Start measurements
14
Depth-first search treeStart DFS12
Nodes send Hello packets, compute link qualities and determine bidirectional links
Start WNX1
Resets state of nodeReset0
ResultActionTime
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Application
• To reduce interferences, send time of leaf nodes randomly distributed in interval
• Every packet includes:
– time-stamp with respect to local clock
– remaining battery energy
– number of packets received and sent
– number of packet transmission retries
– information about clock drift
• Packet size between 70 and 80 Bytes
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Deployment
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Territory
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Deployment
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Deployment
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Deployment
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Deployment
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Deployment
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Configuration
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Radio Link Features
• Asymmetrical links: Connectivity from node A to node B might differ significantly from B to A
• Non-isotropical connectivity: connectivity need not be same in all directions (at same distance)
• Non-monotonic distance decay: Nodes geographically far away from source may get better connectivity than geographically closer nodes
• Link asymmetries may be caused by differences inhardware calibration
• Communication gray zones: Data messages cannot be exchanged although the HELLO messages indicate neighbor reachability
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Analysis
• Altogether 24 nodes were used
• 3 nodes did not send significant amount of data after the first day (reasons are unknown)
• Remaining 21 nodes
• Out of the 210 different pairs of nodes, 45 appeared as links in a search tree
• Quality of these links varied considerably
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2 Indoor Nodes
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2 Outdoor Nodes
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2 Outdoor Nodes
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Retries in Unicast Transmission Scheme
• 15 retransmissions
• Maximal latency: 1143 ms, not including waiting time for channel access
• Observed latencies up to 3 s
• ~ 50% of all unicasts were successful
• Average number of retries: 3.89 � limit was too high
• Gnawali et al. max. 3 retries
Lower value � less congestion & better overall success rate?
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Depth-First Algorithm
• Sink started distributed depth-first search 317 times
in 2 weeks (~22.5 per day)
• Links were chosen in descending quality order
• Search successfully terminated in 205 of these
cases
• Successful depth-first does not visit all nodes of
network
• Largest tree: 19 nodes (~ 90% of active nodes)
• More than 50% of all successfully built trees
included more than 50% of nodes
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Depth-First Algorithm
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Occurrences of Nodes in DFS Trees
0
50
100
150
200
250
7 28
22
24
20
12
17
26
19
15
10
18
16
25
11
2 3 1 8 30
23
13
9 29
Fre
quency o
f O
ccurr
ence in D
FS
Tre
e
Node
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Changing Link Quality
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Changing Link Quality
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Changing Link Quality
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Changing Link Quality
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Changing Link Quality
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Depth First Trees
7182264110
3737158620
7275526
98708822
301619101822201226
List of each node‘s successors is relatively stable over time
Node
Nbs.
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Routing
• DFS tree using for routing towards sink
• Success rate drops approximately exponentially with the depth d
• Transmission success of neighborhood packets exhibits a different run of the curve
• Size of a measurement packet ~ 70 bytes including packet header
• Neighborhood list packets 10% larger
• But significantly lower success rate
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Success Rate
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Success Rate
f(n) =100 * 0.8n
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Success Rate
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Reason
• Main reason:– probably not larger packet size
– but is related to congestion
• Neighborhood packets were sent t seconds after measurement packets, t randomly chosen between 0 and15
• Due to retransmission of packets this time difference became very small after a few hops
• Reason for lower success rates of neighborhood packets for higher hop counts
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Conclusion
• Lessons learned– Timing of actions very important to avoid congestion
– Retransmission policy
– Application has to deal with duplicate packets (lost ACKs)
– High energy consumption for real time clock access
• We faced problems with the firmware implementation (e.g. reset of clock)
• Software installation procedure does not scale
• Unit disc graph model not appropriate
• Depth-first search implementation is robust and fault-tolerant
• Real deployments are a must to evaluate algorithms
• Careful planning: What data is needed, where and when to store it