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FlockLab: A Testbed for Distributed, SynchronizedTracing and Profiling of Wireless Embedded Systems

IPSN 2013

NSLab study group 2013/04/08Presented by: Yu-Ting

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Outline

• Introduction• Architecture• Benchmark• Application

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Introduction

• A testbed with 30 observers (4 at outdoors) and a set of servers

• Support 4 different targets (nodes)• Other than serial port service,

these nodes can synchronously:– Trace GPIO– Actuate GPIO– Power profiling– Adjust supply voltage

• Much better than multiple logic analyzers, mixed-signal oscilloscopes, and power analyzers

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Outline

• Introduction• Architecture• Benchmark• Application

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Hardware

CPU: 624MHzRAM: 128MBFlash: 32MB

8GB

For outdoor nodes without Ethernet

To profile power

Can also controlUSB fan foroutdoor nodes

5V for embedded Com3.3V for others

Step: 0.1V

9 LEDsBy GPIO or UART

This selection is doneby two 8-bitsignal translator

1000 USD

Shunt resistor Amplify the volt across shuntby a gain of 100

Map slot withtarget boardsby serial ID chiip

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More about measuring power

• Resolution in power: 10nA• Limit: 160mA (enough)• ADC sample at 56kHz in high-speed mode and

28kHz in high-resolution (SNR = 109dB) mode with 14.3MHz clock source=> resolution in time: 35.7us or 17.85us

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Software

• OS: OpenEmbedded Linux• NTP client: Chrony, synchronize every 1-2min

with NTP server of FlockLab

Efficient data handling(caching)

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Backend Infrastructure

• Time Synchronization server (NTP server)=> sync by another server on campus and PPS by GPS receiver

• Web server• Test management server (also stores data)• Database server• Monitoring server

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Deployment

• Indoor * 26 (in same LAN segment)• Outdoor * 4• Yellow level: RSSI value when idle

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Outline

• Introduction• Architecture• Benchmark• Application

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Time Accuracy - GPIO

• Use GPIO to trace PPS of same GPS clock• Use mixed-signal oscilloscope to actuate GPIO• Pairwise timing error

• GPIO tracing VS PPS: normal distribution

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Time Accuracy – Power Profiling

• SFD rise -> toggle GPIO & turn on LEDSFD fall -> turn off LED

• SFD events happen at the same time (< 1us)• Most error of same observer comes from random delay of

GPIO event and the following power sample• Error of different observer is comparable to

pairwise timing error of GPIO events

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Power Accuracy

• Test by high-precision power analyzer• Target resistance: 259mΩ

AA batteries: 947mΩ• Calibration: Use linear regression to estimate more

accurate constant of offset and gain of current-sense amplifier, and shunt resistor

Relative error: observer VS power analyzer

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Limits in Capturing GPIO Events

• No new events can be captured until the respective flag is cleared

• Minimum required interval between consecutive GPIO events to be captured=> depends on the interrupt delay and ISR execution time

• SFD events may still loss for small packet(<9bytes)

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Outline

• Introduction• Architecture• Benchmark• Application

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Analysis of Tasks Without FlockLab• Generally speaking, they are:

– More intrusive and less accurate– Need to recompile due to code changes– Need to change existing codes for different platform

• Power profiling– Energest in Contiki, but also intrusive– Customize yourselves in TinyOS…

• End-to-end delay– Timestamps in serial logging, but it’s inaccurate– Run time synchronization protocol

=> Multiple protocols may cause performance losses or even failures• Actuate events (controlling) for multiple nodes

– Also need time synchronization• Using network simulators like Cooja

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Comparative Multi-Platform Analysis

• Run CTP on top of LPL: how each platform affects the trade-offs between metrics

• Observation– 200ms is best for such topology & traffic load– Tmote Sky is most sensitive, while IRIS is best– The trades are the same

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Finding and Fixing Bugs

• Bug: the initial results with LPL wake-up intervals of 500 ms and 1s from Tinynode and Opal nodes were significantly worse

• Reason: children were transmitting at most one packet during an LPL wake-up interval, although they had multiple packets ready to be sent

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Controlling and Profiling Applications

• Purpose– One node to generate a packet every 2s for 260s, from t =

30s to t = 290s– In the mean time, measure the energy consumption

• Task 1 is easily done by GPIO actuation• Comparison of task 2 between different methods– FlockLab is non-intrusive and highly accurate

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Measuring Clock Drift

• Enable FTSP every 3s and get the clock drifts of each node compared with the root

• Toggle a GPIO pin every 0.5s• Compare the difference between GPIO

timestamp interval and 0.5s with clock drifts– Average over 5min to limit GPIO timing errors

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Multi-Modal Monitoringat Network Scale

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Q&A