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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. About This Paper. Strawman Contention resolution mechanism

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strawman resolving collisions in bursty low power wireless networks

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
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)
background
Background
  • Radio duty cycling
    • nodes wake up regularly
  • Receiver-initiated radio
  • Traffic Peaks
    • Event detection, network code update, bulk download
background cont
Background Cont.
  • Collisions in duty-cycled networks
  • Hidden Terminals
    • RTS/CTS schemes have high overhead
receiver initiate radio
Receiver Initiate Radio
  • Receiver Probe
  • Sender Reply
  • Collision occur
    • channel activity without successfully receiving a packet

S2

Probe

S1

R

Reply

Collision

S3

starwman
Starwman

Reply longest length

Another request

Send Collision request

  • Multi-channel operation
    • Initial probe at pre-determined channel
    • Rest of communication at the other channel

Until every sender sent its data

Random length Packet

7 bytes granularity (224us)

Winner send data

implementation
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
alleviating channel n oise
Alleviating Channel Noise
  • Transmissions of COLLISION packets are synchronized
    • receiver knows exactly when they occur
  • Max COLLISION packets length is fixed
  • Methods
    • Sample right before transmission
      • If busy  abort
    • If > Max length, abort
    • Two consecutive Strawman rounds abort
      • Go to sleep, use another channel next time
evaluation
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
collision lengths
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

collision signal strengths
Collision Signal Strengths
  • Vary the receiver-contender distance
interference from external noise
Interference from External Noise
  • Two TMote Sky: 3m apart
  • Third TMote Sky node as interferer
    • Control interference
      • change distance between interferer-receiver
interference from out of range contenders
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
energy cost of resolving collisions
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
different traffic loads
Different Traffic Loads
  • TWIST: a testbed with 100 Tmote Sky
  • Areceiver node probingfor data once per second
  • All other nodesare contenders
  • Data generation rate: 1 pkt/m to 2 pkt/s
clear channel assessment sensitivity
Clear Channel Assessment Sensitivity
  • 15 DATA packets per minute
  • Vary the CCA threshold
reacting to sudden traffic bursts
Reacting to Sudden Traffic Bursts
  • 1-hop network with 8 nodes
    • Measuring the resulting goodput
    • Always contend
  • Vary number of active contenders every 10s
coping with hidden terminals
Coping with Hidden Terminals
  • Black Burst protocol

S2

R

S1

coping with hidden terminals1
Coping with Hidden Terminals
  • RI-Strawmanvs RI-Black Burst
multi hop data collection
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
conclusions
Conclusions
  • Leverages synchronized packet collisions to implement efficient and fair contention resolution among hidden terminals
  • Implementation on real testbed
  • Potential weakness in noisy environment
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