RushNet: Practical Traffic Prioritization for Saturated Wireless Sensor Networks Chieh-Jan Mike Liang †, Kaifei Chen ‡, Nissanka Bodhi Priyantha †, Jie Liu †, Feng Zhao † † Microsoft Research, ‡ University of California, Berkeley 1

Motivation  How to provide traffic prioritization over saturated radio medium? 2

Possible Solutions  Time Division Multiple Access (TDMA)  Synchronization overhead  Waste of resource for sporadic and unpredictable events  Frequency Division Multiple Access (FDMA)  Waste of resource for sporadic and unpredictable events  Cannot avoid external interference  Carrier Sense Multiple Access  Does not work for busy networks  Resource Reservation like RSVP  Setup overhead and delay 3

Our Solution: RushNet  a schedule-free and coordination-free wireless network stack on COTS transceivers that enables packet prioritization  Reserve the highest transmission power for high priority packets  Send high priority packets to preempt on-going normal packets  Cache normal packets by predicting whether they were preempted 4

Outline  Motivation and Approach  Naïve Preemption  Preemptive Packet Train  Interference Recovery Caching  Deployment and Evaluation 5

Naïve Preemption Experiment Lower-Power Packets Higher-Power Packets 100cm  COTS transceivers: Atmel RF231 and TI CC2420  Vary higher power levels  Measure packet reception ratio (PRR) of higher power packets at receiver 6 Lower Power Transmitter Higher Power Transmitter Receiver

Naïve Preemption Results Atmel RF231 Lower Power RSS TI CC2420 Lower Power RSS 7

Why Doesn’t Naïve Preemption Work?  Bit Spreading  Atmel RF231 and TI CC2420 have reception state machine 8

Bit Spreading PN n PN 1 SYNC Bit Spreading Low-Power Packets High-Power Packets Received Packets (Corrupted) pseudo-random noise (PN) sequences PN n PN 1 SYNC Bit Spreading Invalid PN sequence 9

Chip State Machine Low-Power Packets High-Power Packets SYNC Radio Chip States SYNC Listening Sync (Lock on the highest power packet) Reception (Not preempt-able) SYNC 10

Outline  Motivation and Approach  Naïve Preemption  Preemptive Packet Train  Interference Recovery Caching  Deployment and Evaluation 11

RushNet Solution  Naïve preemption doesn’t work  RushNet repeats the high priority packet and send back- to-back, which we call a preemptive packet train 12

RushNet Preemptive Packet Train SYNC Corrupted Noise SYNC Low-Power Packets A Repeatedly 2-packet High-Power Packet Train Received Packets (1 High Power Packet) Repeated packet 13

Preemptive Packet Train Experiment Setup  Same setup as naïve approach EXCEPT  Only use TI CC2420  Vary high power packet train length from 2 to 7 14 Lower-Power Packets Higher-Power Packets 100cm Lower Power Transmitter Higher Power Transmitter Receiver

Preemptive Packet Train Performance 15

What Happens to Normal Packets  Higher power packet train can preempt normal packets  What happens to these normal packets? 16

Outline  Motivation and Approach  Naïve Preemption  Preemptive Packet Train  Interference Recovery Caching  Deployment and Evaluation 17

Interference Recovery Caching  Memory size is limited  RushNet predicts most possibly destroyed normal packets using tail channel signal strength 18 Lower-Power Packets Higher-Power Packets Memory Lower Power Transmitter Higher Power Transmitter Receiver

Predict Packet Loss After Transmission Category 1Category 3 Channel SS Category 2 Channel SS Category 3 Channel SS Category 1: Very likely to be lost Category 2: Less likely to be lost Category 3: Unlikely to be lost Channel SS > threshold 19 CSS

Packet Caching Experiment Setup Lower-Power Packets Higher-Power Packets 50cm 20 Lower Power Transmitter Higher Power Transmitter Receiver 1700 Packets 1700 tail Channel Signal Strengths Reception Prediction Algorithm Compare output with Ground Truth at receivers

Packet Reception Prediction Correct Prediction: ~90% 21 Predict All Lost Predict All Received

Outline  Motivation and Approach  Naïve Preemption  Preemptive Packet Train  Interference Recovery Caching  Deployment and Evaluation 22

Office Deployment  41 TelosB motes (TI CC2420)  Run our WRAP protocol in SenSys’09 paper 1, which uses a tree topology and token-based multiple access  Each mote generates one normal packet per 15 seconds  We randomly send 100 higher power packets with train length 4 on a 5- hop branch over 10 minutes, and measure their latencies and PRRs Liang et al. RACNet: a high-fidelity data center sensing network. SenSys’09.

High Power Packet Latency and PRR 24

Limitations and Future Work  Power Consumption  Atmel RFR2 supports payload preemption/ receiver override 25

Conclusions  RushNet enables practical packet prioritization on wireless sensor network  It uses a back-to-back train of preemptive repeated high power packet to preempt normal packet, we can achieve 90% PRR with 4-packet train  It uses tail channel signal strength sampling to predict packet collision for better caching efficiency, we can achieve ~90% prediction accuracy 26

Thank you! 27

Motivation  Gateway mote connects to a station  Sensor motes form routing topology 28

Motivation  Gateway mote connects to a station  Sensor motes using any data collection protocol, e.g., CTP, RPL.  Only consider the scenario when motes saturate bandwidth on available channels 29

Motivation  Gateway mote connects to a station  Sensor motes form routing topology  They have mechanism to avoid collisions. For example example, a token. 30

Design Requirements  Minimize resource usage  Minimize control overhead 31

Capture Effect  FM radio transceiver, which can decode a stronger signal from one transmitter in the presence of a weaker signal from another transmitter.  TODO: show diagram for capture effect 32

Preemption on Chips  Hardware state machine  bit spreading  TODO: diagram for this two problems 33

Sufficient Power Difference 34

Preemption on Chips Low-Power Packets High-Power Packets SYNC 100cm SYNC 35

Preemption on Chips SYNC Bit Spreading SYNC Bit Spreading 36

Implementation  Maybe skip it or quickly go thru, unless we have open sourced platform  Do we talk about the tree protocol? Diff RPL  TODO: Need more understanding on Figure 5. what is a transport service (diff from transport layer), why LP has E2E reliability, why ack is in HP? Etc. 37

Outline  Motivation and Approach  Capture Effect  Preemptive Packet Train  Interference Discussion  Implementation  Evaluation 38

Evaluation  What should be the transmission power difference?  How long should HP train be?  What is the caching hit rate. 39

Preemption Support in Chips  TI CC2420 and Atmel AT86RF231 hides bit spreading information  ATmega2564/1284/644RFR2 supports payload preemption/ receiver override 40

Packet Caching Miss Rate 41