Understanding Packet Delivery Performance in Dense Wireless Sensor Networks

1. Understanding Packet Delivery Performance in Dense Wireless Sensor Networks Jerry Zhao & Ramesh Govindan SenSys ‘03

2. Motivation • WSNs can be deployed in harsh environment • Measure packet delivery performance • Spatio-temporal charasteristics of packet loss • Environmental dependence • Medium scale (up to 60 Mica motes) indoor, habitat with moderate foliage, and open parking lot -> Implications for the design & evaluation of routing & MAC protocols

3. Why packet delivery performance is important? • Determines energy efficiency & network lifetime • Poor packet delivery may degrade application performance & consume a lot of energy • Important for evaluating communication protocols • Experimentally verify WSN design principles, for example, low-power RF transceivers for multiple short hops • More energy efficient than a single hop over a long range • Spatial multiplexing

4. Backgrounds: Some Wireless Communication Vagaries • Hidden node problem: Node A transmits to B, Node C cannot hear it and transmits to B -> Collision at B C A B

5. Backgrounds: Some Wireless Communication Vagaries • Exposed node problem: Node B is transmitting to A, Node C has a packet intended for node D -> C cannot transmit, although it’s OK C D A B

6. Backgrounds: Some Wireless Communication Vagaries • Multipath problem • A radio signal is reflceted by obstacles • Parts of the signal may take different paths to the sink, confusing the receiver Source: Wireless Lan, Multipath and Diversity http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml

7. Backgrounds: Some Wireless Communication Vagaries • Signal attenuation • Attenuation = (10/L) log10 (Pi/Po) where L is the distance, e.g., meter or km • dB/m or dB/km • Signal strength drops exponentially • Signal strength is proportional to 1/ra where r is the distance and 2 ≤ a ≤ 5

8. Packet delivery performance • Physical layer • If there’s no interfering transmission, delivery perf is largely determined by a function of environment, physical layer coding scheme, individual receiver charasteristics (not a major factor) • MAC layer • Interfering transmissions contribute to poor perf. • Evaluate the efficacy of carrier sense and link layer retransmission

9. Contributions • Experiments & observations • No new protocols or algorithms • Lack of the related work on delivery performance measurement in a medium scale WSNs (when the paper was published) • Although the results do not necessarily mean radio communications in WSNs are always like this, they provide important insights

10. Key Results • Heavy-tailed distributions of packet losses • For example, in an indoor setting, half of the links experience more than 10% packet loss, and a third suffer more than 30% loss • Physical layer: Gray area within the communication range • Receivers suffer choppy packet reception • In some case, gray area is 1/3 of the comm. range • MAC layer: Packet loss is heavy-tailed • 50% - 80% comm. energy is wasted to overcome packet collisions & environmental effects • About 10% of links exhibit asymmetric packet loss

11. Authors suggest • Topology control, via actual measurement of actual perf, needs to carefully discard poorly performing links or neighbors to whom asymmetric links exist • Packet level mechanisms, e.g., RTS/CTS, are not enough • Make decisions at the granularity of links to neighbors

12. I. Packet delivery at the physical layer • Disable TinyOS MAC to measure pure packet delivery at physical layer • Vary three factors • Environments • Physical layer coding schemes • Transmit power settings

13. Environment 1 • I:Indoor office building • 2m * 40m hallway • 60 motes placed in a line • 0.5m apart • 025m apart near the edge of the comm range • Removed some node from near the transmitter • Harsh due to significant multipaht reflection effects

14. Environment 2 • H: 150m * 150m segment of a state park • Downhill slope with foliage and rocks • Multi-path problems due to foliage & rocks

15. Environment 3 • O: 150m * 150m open parking lot • No obstacles • Multipath only due to ground reflections • Not much to sense

16. Physical layer encoding scheme • SECDED (Single Error Correction and Double Error Detection) • TinyOS default • Convert each byte into 24 bits • Can detect 2 bit errors & correct one bit error • Manchester encoding • Convert a byte into 16 bits • Detect an error out of 2 bits • 4-bit/6-bit scheme (4bsb) • Encode one byte into 12 bits • Detect 1 bit error out of 6 bits

17. Discrete control of transmit power in a mote • Three settings are considered • High (potentiometer 0) • Medium (potentiometer 50) • Low (potentiometer 90) • Potentiometer is an electric device with user-adjustable resistance

18. Aggregate packet delivery performance • Pacekt loss with 4b6b coding, high Tx power -> Worst case pkt delivery perf. I H O

19. Aggregate packet delivery performance • Packet loss vs Tx power in I, 4b6b coding • Observelower power improves dilivery perf considerably possibly due to the reduced comm range and multi-path problems H M L

20. Aggregate packet delivery performance • Pkt loss vs coding schemes in I, high Tx Power • SECDED is much better for the cost of consuming more bandwidth than 4BSB and Manchester • Not much difference btwn 4BSB and Manchester

21. Spatial Characteristics of Packet Delivery • How does reception rate vary with distance from the transmitter? • Gray area due to multipath problems Spatial profile of packet delivery: 4B6B, High Tx Power I H O

22. Why servere multipath problem? • No frequency diversity • Motes use a single, narrow frequency band • How about emerging UWB (Ultra Wide Band) technology? • 3.1 – 10.6 GHz • Bandwidth > 500MHz • Data rate > 54Mbps • Low power

23. Lessons • Selecting a shortest path simply based on the geographic distance or hop count is not sufficient! • Nodes need to carefully select neighbors based on the measured packet delivery perf!

24. Signal strength & packet delivery • Try to answer a question: “Can signal strength by itself estimate link quality?” • Unfortunately, the answer is “NO” High Tx Power, I

25. Coding Schemes • “Can sophisticated physical layer coding schemes mask the gray area?” • Not necessarily, SECDED has the lowest effective bandwidth -> Topology control to avoid pathological links in the gray area together with bandwith efficient coding scheme

26. Spatial Correlation • “Are two receivers in their linear topology likely to see similar loss patterns?” • Significantly different correlation characteristics for different environments: I & O show noticeably higher correlated packet loss than H • At the physical layer, independent losses are a reasonable assumption I O H

27. Temporal characteristics of packet delivery • Large variations in average reception rate and big standard deviations imply time varying packet losses

28. II. Packet Delivery at the Medium Access Layer • TinyOS • CSMA/CA: Random back off upon carrier sense • Link layer ACK: Send 4 byte ACK to the sender • Authors added retransmission scheme • When there’s no ACK, retransmit up to 3 times

29. Packet loss distribution under the Retransmission Scheme • Too many packet loss • 50% - 80% communication energy is wasted on repairing lost transmissions • Better MAC, e.g., S-MAC, B-MAC, Z-MAC, is required

30. Asymmetry in packet delivery • Asymmetry in wireless communication is well known, but the extent is not • Topology control should control pathological links

31. Conclusions • Performed experiments to understand packet delivery perf in dense sensor network deployments • Quantify the prevalence of gray area • Mostly “observations” • “Causes” for phenomena are not for sure • Most of them are conjectures, guesses, etc. • Still an open issue 

32. Questions?