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
download
Background Cont.
• Collisions in duty-cycled networks
• Hidden Terminals
– RTS/CTS schemes have high overhead
Mechanism and Implementation
Receiver Initiate Radio
1. Receiver Probe
S1
2. Sender Reply
S2
Probe
R
Collision
Reply
S3
3. Collision occur
– channel activity without successfully receiving a
packet
Starwman
Send Collision
request
Reply longest
length
Random length Packet
7 bytes granularity
(224us)
Another
request
Winner
send data
• Multi-channel operation
– Initial probe at pre-determined channel
– Rest of communication at the other channel
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, abort
3. 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
S1
R
S2
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