LINK LAYER. Murat Demirbas SUNY Buffalo

Transcription

1 LINK LAYER Murat Demirbas SUNY Buffalo

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: Significant change in link quality between ground-level and waist-level nodes

4 Circular signal coverage area Signal coverage area is neither circular nor convex; often noncontiguous Among other factors, angles between senderto-receiver & receiversender affects reception strongly

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

8 WSN-side of things 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 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 , 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%)

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 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

12 Non-isotropic connectivity

13 Probability of reception

14 Spatial characteristics Great variability over distance Reception rate not normally distributed around the mean and std. dev. Real communication channel not isotropic 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 Gray area

16 Gray area

17 Gray area

18 Gray area 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 Asymmetric links

21 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

22 Temporal characteristics Time variability is correlated with mean reception rate

23 Packet loss (link layer)

24 Packet loss (MAC layer) indoor outdoor habitat

25 Coding schemes Redundant coding can increase reception rate 4B6B SECDED Manchester

26 Summary Great variability over distance: Reception rate is not normally distributed around the mean and std. dev. ; Real communication channel is not isotropic Found 5 to 30% of asymmetric links: Not correlated with distance 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

27 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

28 Complex behavior at scale

29 Long links