Energy-Aware Synchronization in Wireless Sensor Networks

1
Energy-Aware Synchronization in Wireless Sensor
Networks

• Yanos Saravanos
• Major Advisor Dr. Robert Akl
• Department of Computer Science and Engineering

2
Outline

• Background on wireless sensor networks
• Flooding to create network topology
• Existing synchronization algorithms
• Reference Broadcast Synchronization (RBS)
• Timing-sync Protocol for Sensor Networks (TPSN)
• Hybrid algorithms
• Flooding
• Synchronization
• Root node re-election
• Results
• Conclusions

3
Wireless Sensors

• Physically small sensing unit
• Battery
• Processor
• Slow
• Drift
• Radio/antenna
• Sensor modules
• Covert
• Short battery life

4
Applications

• Temperature
• Fire detection
• Brake usage
• Humidity
• Flood detection
• Pressure
• Object tracking
• Animal movement and migrations
• Vehicle tracking

• Noise levels
• Search and rescue efforts
• Locating a snipers position
• Contamination levels
• Monitoring pollution levels
• Chemical/biological agent detection
• Mechanical stress on supporting structures

5
Wireless Sensor Network (WSN)

• Network using many wireless sensors
• Dropped from a plane to monitor area
• Random placement
• Sensors build hierarchical network once deployed

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Wireless Communication

• Signal strength decays over distance
• PT initial power of transmission
• d distance from transmitter
• c path loss coefficient

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Network Flooding

• Broadcast packet from root node
• If packet received for the first time
• Set Parent on Tree Source of message
• Change Source field to MyId
• Increment HopCount field
• Rebroadcast packet

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Network Flooding
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Motivation for Time Synchronization

• Most applications require some synchronization
  accuracy
• Fire and flood tracking
• Animal movement
• Vehicle movement
• Gunshot detection

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Existing Synchronization Solutions

• Global Positioning System (GPS)
• Power-hungry
• Network Time Protocol (NTP)
• Computationally infeasible for wireless sensors
• Reference Broadcast Synchronization (RBS)
• Receiver-receiver synchronization
• Timing-sync Protocol for Sensor Networks (TPSN)
• Transmitter-receiver synchronization

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Reference Broadcast Synchronization

• Receiver-to-receiver synchronization
• Two stages
• Transmitter broadcasts clock time
• Receivers exchange observations

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RBS Synchronization
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RBS Energy Usage

• Given n receivers
• Transmissions grow as O(n)
• Receptions grow as O(n2)

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Timing-sync Protocol for Sensor Networks

• Traditional handshake approach
• Timestamp at the MAC layer
• Two stages
• Level Discovery Phase (Flooding)
• Synchronization Phase

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TPSN Model Level Discovery Phase

• Assign root (level 0) node
• Broadcast level_discovery packet
• Nodes 1 hop away assigned to level 1
• Ignore all subsequent level_discovery packets
• Broadcast level_discovery packet

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TPSN Model Synchronization Phase

• Each node (A) broadcasts synchronization_pulse
• Timestamped at T1
• Node B receives pulse at T2, broadcasts ack at T3
• Node A receives ack at T4
• ? is clock drift
• d is propagation delay

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TPSN Synchronization
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TPSN Energy Usage

• Given n receivers
• Transmissions and receptions grow as O(n)
• Large energy savings over RBS for large n
• Less efficient for small n

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Sources of Packet Delay

• Send time time to create the packet
• Access time delay until channel is accessible
• Transmission time time each bit takes to get
  onto physical medium
• Reception time time to receive bits off physical
  medium
• Receive time time to reconstruct packet

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Uncertainties

• Sender uncertainty
• RBS removes it completely
• Minimized in TPSN by timestamping at MAC layer
• Propagation/receiver uncertainties, and relative
  local clock drifts
• TPSN outperforms RBS by factor of 2

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Accuracy Comparison
TPSN RBS
Avg error (µs) 16.9 29.1
Worst-case error (µs) 44 93
Best-case error (µs) 0 0
time error lt avg 64 53
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Hybrid Summary

• Complete system for WSN operation
• Three stages
• Build hierarchical tree with flooding
• Transmitters know how many receivers are
  connected
• Periodically synchronize sensors
• Re-elect new root when current one dies

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Hybrid Flooding Algorithm

• Broadcast flood_packet from root node
• If current_node receives flood_packet
• Set parent of current_node to source of broadcast
• Set current_node_level to parents node level 1
• Rebroadcast flood with current_node_ID and
  current_node_level
• Broadcast ack_packet with current_node_ID
• Ignore subsequent flood_packets
• Else If current_node receives ack_packet
• Increment num_receivers

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Hybrid Synchronization

• RBS best for small n, TPSN best for large n
• Calculate optimal cutoff value to choose RBS or
  TPSN algorithm (receiver_threshold)
• Transmissions and receptions draw different
  current
• where a is reception-to-transmission current ratio

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Hybrid Synchronization

• Equate energies of both RBS and TPSN
• Solve equation to find receiver_threshold

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Reception-to-Transmission Ratio

• Mica2DOT architecture
• TX 25 mA
• RX 8 mA
• a0.32
• n4.4

• MicaZ architecture
• TX 14.0 mA
• RX 19.7 mA
• a1.41
• n3.4

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Hybrid Synchronization Algorithm

• If num_receivers lt receiver_threshold
• Transmitter broadcasts sync_request
• For each receiver
• Record local time of reception for sync_request
• Broadcast observation_packet
• Receive observation_packet from other receivers
• Else
• Transmitter broadcasts sync_request
• For each receiver
• Record local time of reception for sync_request
• Broadcast ack_packet to transmitter with local
  time

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Hybrid Synchronization
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Hybrid Root Election Algorithm

• If root nodes power allows 1 more TX
• Broadcast elect_packet with cur_node_ID
• If cur_node_level 2 and receives elect_packet
  from root
• Broadcast elect_packet with cur_node_ID,
  cur_node_power
• If cur_node receives elect_packet and
  elect_packet_power gt cur_node_power
• Set elect_packet_ID to root node

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Simulation Results

• Two sets of simulations
• Change the sensor architecture
• Change the number of sensors in network
• 1000m x 1000m
• Path loss coefficient 3.5
• 20 networks per simulation
• Assume perfect directional antennas
• Minimum number of receptions

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Sensor Synchronization Simulations

• Verify the hybrid synchronization algorithm works
  with several sensor architectures
• Run RBS, TPSN, hybrid using optimal
  receiver_threshold
• Run hybrid using non-optimal receiver_threshold
  values
• Change sensor architecture
• Used 500 sensors per network

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Sensor Synchronization Simulations

• Mica2DOT
• TX 25 mA
• RX 8 mA
• a0.32
• n4.4

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Sensor Synchronization Simulations

• MicaZ
• TX 17.4 mA
• RX 19.7 mA
• a1.41
• n3.4

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Sensor Synchronization Simulations

• Hypothetical
• TX 25 mA
• RX 2.7 mA
• a0.11
• n6.1

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Sensor Synchronization Simulations

• Hypothetical
• TX 25 mA
• RX 0.7 mA
• a0.03
• n10.3

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Synchronization Simulations forVariable Network
Size

• Verify the hybrid synchronization algorithm works
  with various network sizes
• Run RBS, TPSN, hybrid using optimal
  receiver_threshold
• Run hybrid using non-optimal receiver_threshold
  values
• Change number of sensors deployed in network
• Used Mica2DOT architecture

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Synchronization Simulations for 250 Sensors
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Synchronization Simulations for 500 Sensors
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Synchronization Simulations for 750 Sensors
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Synchronization Simulations for 1000 Sensors
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Synchronization Simulations for 1250 Sensors
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Synchronization Simulations for 1500 Sensors
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Network Size Simulations
Sensors 250 500 750 1000 1250 1500
RBS 446 1046 1844 2762 3756 5060
TPSN 511 983 1434 1885 2331 2770
Hybrid 404 828 1253 1672 2095 2514
RBS Savings 9.29 20.79 32.04 39.46 44.22 50.31
TPSN Savings 20.80 15.73 12.65 11.28 10.11 9.23

• Hybrid saves up to 50 over RBS, up to 20 over
  TPSN
• Hybrid is still more efficient in networks
  favoring either RBS or TPSN

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Conclusions

• Synchronization is necessary for most sensor
  networks to operate effectively
• Both TPSN and RBS synchronize sensor clocks
  locate origin of gunshot blast
• Neither TPSN nor RBS are designed for low energy
  usage
• Hybrid algorithm adapts to any size network and
  saves energy over other algorithms

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Future Work

• Physical implementation
• Localized re-flooding
• Non-uniform path loss coefficient
• Dropped packet analysis

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Questions?