Link layer

Murat Demirbas

SUNY Buffalo

CSE Dept.

2

Mistaken axioms of wireless research

1. The world is flat

2. A radio’s transmission area is circular

3. If I can hear you at all, I can hear you perfectly

4. All radios have equal range

5. If I can hear you, you can hear me (symmetry)

6. 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

Circular signal coverage area

• Signal coverage area is neither circular nor convex– Often non-contiguous

• Among other factors, angles between sender-to-receiver & receiver-sender 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 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

*Zhou et. al. 04

Non-isotropic connectivity

*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 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

*Cerpa et. al. 03

*Cerpa et. al. 03

*Cerpa et. al. 03

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

*Cerpa et. al. 03

*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

*Cerpa et. al. 03

Zhao et. al.

26

Packet loss (link layer)

27

Packet loss (MAC layer)in

door

habitat

outd

oor

28

Coding schemes

• Redundant coding can increase reception rate

*Zhao et. al. 03

4B6B SECDED

Manchester

*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 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

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

34

Long links