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Link layer

Link layer. Murat Demirbas SUNY Buffalo CSE Dept. Mistaken axioms of wireless research. The world is flat A radio’s transmission area is circular If I can hear you at all, I can hear you perfectly All radios have equal range If I can hear you, you can hear me (symmetry)

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Link layer

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  1. Link layer Murat Demirbas SUNY Buffalo CSE Dept.

  2. Mistaken axioms of wireless research • The world is flat • A radio’s transmission area is circular • If I can hear you at all, I can hear you perfectly • All radios have equal range • If I can hear you, you can hear me (symmetry) • Signal strength is a simple function of distance D. Kotz, C. Newport, R. Gray, J. Liu, Y. Yuan, and C. Elliott, Experimental Evaluation of Wireless Simulation Assumptions MSWIM’04

  3. Flat world • Multipath effects: • Hills and buildings present obstacles that dramatically affect wireless signal propagation • Near-ground effects • Gaertner and Cahill noted a significant change in link quality between ground-level and waist-level nodes

  4. Signal coverage area is neither circular nor convex Often non-contiguous Among other factors, angles between sender-to-receiver & receiver-sender affects reception strongly Circular signal coverage area

  5. Perfect reception • No visible threshold under which reception quality is 1 and beyond which reception probability is 0 • Reception quality fades with distance [[roughly!]]

  6. Symmetry • Asymmetric (unidirectional) links are common • The figure shows conditional probability of symmetric beacon reception wrt distance between nodes

  7. Signal strength a function of distance • Average signal strength fades with distance according to a power-law model • BUT, there are great & unpredictable variations!

  8. Wireless sensor networks • Physical layer packet-delivery experiments • J. Zhao and R. Govindan, Understanding Packet Delivery Performance In Dense Wireless Sensor Networks, The First ACM Conference on Embedded Networked Sensor Systems (Sensys'03), November 2003 • Gang Zhou, Tian He, and John A. Stankovic. Impact of Radio Irregularity on Wireless Sensor Networks. In The Second International Conference on Mobile Systems, Applications, and Services (MobiSys), June 2004. • D. Ganesan, B. Krishnamachari, A. Woo, D. Culler, D. Estrin, and S. Wicker, Complex Behavior at Scale: An Experimental Study of Low-Power Wireless Sensor Networks, Technical Report UCLACSD TR 02-0013, July 2002

  9. Radio Channel Features* • Non-isotropical connectivity:  connectivity need not be same in all directions (at same distance from source) • Non-monotonic distance decay:  nodes geographically far away from source may get better connectivity than nodes that are geographically closer • Asymmetrical links:  connectivity from node a to node b might differ significantly from b to a • Packet loss: packet loss is common (may be >50%) in WSN *Ganesan et. al. 02; Woo et. al. 03; Zhao et. al. 03; Cerpa et. al. 03; Zhou et. al. 04

  10. Parameters • Environment type:  e.g., indoors or outdoors, different levels of physical interference (furniture, walls, trees, etc.), and different materials (sand, grass, concrete, etc.) • Transmission gain control:  most WSN low power radios have some form TX gain control • Antenna height: relative distance of antenna wrt reference ground • Radio frequency and modulation type • Data rate: # packets transmitted per second • Packet size:  # bits per packet can affect likelihood of receiving the packet with no errors

  11. Non-isotropic connectivity *Zhou et. al. 04

  12. *Zhou et. al. 04

  13. Probability of reception

  14. Spatial Characteristics • Great variability over distance (50 to 80% of radio range) • Reception rate not normally distributed around the mean and std. dev. • Real communication channel notisotropic • Gray area for >1/3rd of communication range • Low degree of correlation between distance and reception probability; lack of monotonicity and isotropy • Region of highly variable reception rates is 30% or more of the radio range, and not confined to limit of radio range

  15. *Cerpa et. al. 03

  16. *Cerpa et. al. 03

  17. *Cerpa et. al. 03

  18. indoor outdoor habitat

  19. Asymmetric Links • Found 5 to 30% of asymmetric links • Claim: No simple correlation between asymmetric links and distance or TX output power • They tend to appear at multiple distances from the radio range, not at the limit

  20. *Cerpa et. al. 03

  21. *Cerpa et. al. 03

  22. Main cause of asymmetric links? • When swapping asymmetric links node pairs, the asymmetric links are inverted (91.1% ± 8.32) • Claim: Link asymmetries are primarily caused by differences in hardware calibration

  23. Temporal Characteristics • Time variability is correlated with mean reception rate

  24. *Cerpa et. al. 03

  25. Zhao et. al.

  26. Packet loss (link layer)

  27. Packet loss (MAC layer) indoor outdoor habitat

  28. Coding schemes • Redundant coding can increase reception rate

  29. 4B6B SECDED Manchester *Zhao et. al. 03

  30. *Zhao et. al. 03

  31. Summary • Great variability over distance (50 to 80% of radio range) • Reception rate is not normally distributed around the mean and std. dev. • Real communication channel is notisotropic • Found 5 to 30% of asymmetric links • Not correlated withdistance or transmission power • Primary cause: differences in hardware calibration (rx sensitivity, energy levels) • Time variability is correlated with mean reception rate and not correlated with distance from the transmitter • Possible to optimize performance by adjusting the coding schemes and packet sizes to operating conditions

  32. Complex behavior at scale • Large scale (150 nodes) empirical study • Even a simple flooding protocol can exhibit surprising complexity at scale • Link asymmetry, non-isotropic communication, gray area • Contention, collision

  33. Long links

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