strawman : resolving collisions in bursty low-power wireless networks
DESCRIPTION
Strawman : Resolving Collisions in Bursty Low-Power Wireless Networks. Fredrik Österlind , Luca Mottola , Thiemo Voigt, Nicolas Tsiftes , Adam Dunkels Swedish Institute of Computer Science Presenter:SY. About This Paper. Strawman Contention resolution mechanism - PowerPoint PPT PresentationTRANSCRIPT
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Strawman: Resolving Collisions in Bursty Low-Power Wireless Networks
Fredrik Österlind, Luca Mottola, Thiemo Voigt, Nicolas Tsiftes, Adam Dunkels
Swedish Institute of Computer Science
Presenter:SY
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About This Paper
• Strawman– Contention resolution mechanism– Resolve collision in low-power duty-cycled networks
that experience traffic bursts– Copes with hidden terminals and is designed for
receiver-initiated duty-cycled protocols• Contribution– Builds upon two previous papers– Improve Strawman along several dimensions– Embed it within RI-MAC (real implementation)
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Background
• Radio duty cycling– nodes wake up regularly
• Receiver-initiated radio
• Traffic Peaks– Event detection, network code update, bulk
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Background Cont.
• Collisions in duty-cycled networks
• Hidden Terminals– RTS/CTS schemes have high overhead
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Mechanism and Implementation
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Receiver Initiate Radio
1. Receiver Probe
2. Sender Reply
3. Collision occur– channel activity without successfully receiving a
packet
R
S2S1
S3
Probe
Reply
Collision
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Starwman
• Multi-channel operation– Initial probe at pre-determined channel– Rest of communication at the other channel
Send Collision request
Random length Packet7 bytes granularity (224us)
Reply longest length
Winner send data
Another request
Until every sender sent its data
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Implementation• Contiki + Tmote Sky • RI-MAC– Version 1: Strawman + multi-channel operation– Version 2: random backoff (geometric distribution)
• Collision length estimation– Clear Channel Assessment (CCA)– Default threshold: -77 dBm
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Alleviating Channel Noise
• Transmissions of COLLISION packets are synchronized– receiver knows exactly when they occur
• Max COLLISION packets length is fixed• Methods
1. Sample right before transmission• If busy abort
2. If > Max length, abort3. Two consecutive Strawman rounds abort• Go to sleep, use another channel next time
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Evaluation
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Evaluation
• Key findings– Collision packet length estimation is accurate– No overhead when no collisions, limited energy
cost when resolving collisions– Sustain a range of different traffic loads– Able to cope with hidden terminals efficiently– Increase robustness in standard tree routing
protocols
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Collision Lengths
• Two TMote Sky: sender + receiver– COLLISION packet different length– Vary distance: 0.5m (nearby), 10m (distant,
decreased TX power)
Within the 7-byte granularity
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Collision Signal Strengths
• Vary the receiver-contender distance
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Interference from External Noise
• Two TMote Sky: 3m apart• Third TMote Sky node as interferer– Control interference• change distance between interferer-receiver
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Interference from Out-of-range Contenders
• 3 nodes: 1 receiver and 2 contenders– One receiver kept at 0.5 m • 0 bytes payload
– Another vary the distance: 0.5 to 20 m• 112 bytes payload
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Energy Cost of Resolving Collisions
• simulate a single receiver and four contenders in Cooja– Contenders hidden to each other– 1 data packet every 4 seconds– vary the nodes’ wakeup intervals
• four times per second to once every 32 seconds
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Different Traffic Loads
• TWIST: a testbed with 100 Tmote Sky• A receiver node probing for data once per
second• All other nodes are contenders• Data generation rate: 1 pkt/m to 2 pkt/s
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Goodput and Fairness
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Clear Channel Assessment Sensitivity
• 15 DATA packets per minute• Vary the CCA threshold
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Reacting to Sudden Traffic Bursts• 1-hop network with 8 nodes
– Measuring the resulting goodput– Always contend
• Vary number of active contenders every 10s
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Coping with Hidden Terminals
• Black Burst protocol
R S2S1
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Coping with Hidden Terminals
• RI-Strawman vs RI-Black Burst
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Multi-hop Data Collection• 82 nodes in the TWIST testbed
– Multi-hop topologies (at least 4 hops)– Contiki Collect protocol
• Traffic patterns– No traffic (NT)– Periodic traffic (PT): 1 pkt every 5 minutes– Bursty traffic (BT):
• Instantaneously generate 1 pkt on 8 randomly-selected nodes
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Conclusions
• Leverages synchronized packet collisions to implement efficient and fair contention resolution among hidden terminals
• Implementation on real testbed• Potential weakness in noisy environment