1 Link Characteristics in Sensor Networks

2 Why Such a Study? (in)validate whether the basic model used in design is accurate or not  Remember you have to assume a model in any protocol design  Over time, people tend to forget or take for granted what they have assumed It is also about how to design a good set of experiments  Your product needs those numbers to later claim how good your product is!  Know the strength, weakness, or ignorance of your design

3 The Case for Link Study Wireless Link is the foundation of sensor networks!  Any finding will carry big implications on protocol design Protocol design has been using a DIFFERENT set of assumptions  The initial goal is to simplify everything It shows you can almost redesign everything (at different layers of the protocol stack) based on the new findings

4 Link Experiments Link Characterization  results  summary Why?  reality guides algorithm development & protocol parameter tuning  data for better propagation models used in simulations

5 Radio Channel Features* Asymmetrical links: connectivity from node a to node b might differ significantly from b to a 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 *Ganesan et. al. 02; Woo et. al. 03; Zhao et. al. 03; Cerpa et. al. 03; Zhou et. al. 04

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

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

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9 *Cerpa et. al. 03

10 *Cerpa et. al. 03

11 *Cerpa et. al. 03

12 Antenna height *CS 213 – Boelter Hall court yard measurements - 04

13 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 not isotropic Low degree of correlation between distance and reception probability; lack of monotonicity and isotropy Region of highly variable reception rates is 50% or more of the radio range, and not confined to limit of radio range

14 *Cerpa et. al. 03

15 *Cerpa et. al. 03

16 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

17 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

18 *Cerpa et. al. 03

19 *Cerpa et. al. 03

20 *Cerpa et. al. 03

21 Temporal Characteristics Time variability is correlated with mean reception rate Time variability is not correlated with distance from the transmitter

22 *Zhao et. al. 03 4B6BSECDED Manchester

23 *Zhao et. al. 03

24 *Cerpa et. al. 03

25 Optimal Packet Size? Larger packets produce a slight decrease in reception rate… …BUT, larger packets reduce start symbol and header overhead Efficiency:

26 *Cerpa et. al. 03

27 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 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