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– 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

• 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

Mechanism and Implementation

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

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

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 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

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

• 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

• Vary the receiver-contender distance

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

• 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

• 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

• 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

Goodput and Fairness

Clear Channel Assessment Sensitivity

• 15 DATA packets per minute• Vary the CCA threshold

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

• Black Burst protocol

R S2S1

Coping with Hidden Terminals

• RI-Strawman vs RI-Black Burst

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

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

• Implementation on real testbed• Potential weakness in noisy environment