Strawman resolving collisions in bursty low power wireless networks
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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


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)


  • 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









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


  • 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


  • 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

Goodput and fairness
Goodput and Fairness

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




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


  • Leverages synchronized packet collisions to implement efficient and fair contention resolution among hidden terminals

  • Implementation on real testbed

  • Potential weakness in noisy environment