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

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

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

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 A B C

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 AB CD

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

Backgrounds: Some Wireless Communication Vagaries Signal attenuation –Attenuation = (10/L) log 10 (P i /P o ) 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/r a where r is the distance and 2 ≤ a ≤ 5

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

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

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

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

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

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

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

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

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

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

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

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

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

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 O H

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

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!

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

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

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 IOH

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

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

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

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

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

Questions?