Beyond Co-existence: Exploiting WiFi White Space for ZigBee Performance Assurance

1. Beyond Co-existence: Exploiting WiFi White Spacefor ZigBee Performance Assurance Jun Huang 1,  Guoliang Xing 1,  Gang Zhou 2, Ruogu Zhou 1 1 Michigan State University,  2 College of William and Mary

2. ZigBee Networks • Low communication power (10~50 mw) • Application domains • Smart energy, healthcare IT, Industrial/home automation, remote controls, game consoles…. • Ex: 10 million smart meters installed in the US by 2010 Industrial sensor networks (Intel fabrication plant) Smart thermostat (HAI ) Smart electricity meter (Elster)

3. Challenge & State of the Art • Interference in open radio spectrum • Numerous devices in 2.4 GHz band: WiFi, bluetooth… • AT&T public WiFi usage: 300% up Q1/09~Q1/10 [1] • Multi-channel assignment • WiFi interferes with 12 of total 16 ZigBee channels • Co-existence on same/overlapping channels • Carrier sense multiple access (CSMA) [1] http://attpublicpolicy.com/wireless/the-summer%E2%80%99s-hottest-hotspot/

4. Empirical Study of Coexistence WiFi interferer: 802.11g • Change WiFi node location • Measure ZigBeesending rate • WiFi interference on sender • Measure ZigBeepacket delivery ratio • WiFi interference on receiver Interference link Data link ZigBee sender and recver TelosB with CC2420 WiFi Interferer Position

5. WiFi Hidden Terminals • Don’t trigger backoff at ZigBee sender • Corrupt packets at ZigBee receiver WiFi Interferer Position

6. WiFi Exposed Terminals • Defer ZigBee sender’s transmissions • Not strong enough to corrupt ZigBeepackets WiFi Interferer Position

7. WiFi Blind Terminals • Interfere both ZigBee sender and receivers • Severe packet loss on ZigBeelink • WiFi sending rate not affected

8. Why Blind Terminals ? ZigBee tx range ZigBee sender ZigBee recver WiFi interferer WiFi tx range • Power asymmetry • Heterogeneous PHY layers • WiFi only senses de-modulatable signals • Energy-based sensing? 8

9. White Space in Real-life WiFi Traffic • Large amount of channel idle time • WiFi frames are clustered white space: cluster gaps that can be utilized by ZigBee

10. Self-Similarity of Cluster Arrivals • Variance is similar at different time scales • Rigorously tested via rescaled range statisticsand periodogram-based analysis # clusters/5s # clusters/s

11. Modeling WiFi White Space • Length of white space follows iid Pareto distri. • Implementation • Collect white space samples in a moving time window • Generate model by Maximum Likelihood Estimation α = 1ms shorter intervals are not usable for ZigBee

12. Pareto Model: Goodness of Fit OSDI ’06 traces SigCOMM’08 traces Pareto model is accurate when modeling window < 100ms Sampling frequency is about 200Hz  20 samples are enough!

13. Outline Motivation Blind Terminal Problem WiFi White Space Modeling WISE: WhIte Space-aware framE adaptation Experimental Results 13

14. Basic Idea of WISE • Sender splits ZigBee frame into sub-frames • Fill the white space with sub-frames • Receiver assembles sub-frames into frame WiFiframe cluster ZigBee sub-frames ZigBee Time sampling window ZigBeeframe pending

15. Frame Adaptation • Collision probability • Sub-frame size optimization Sub-Frame size White space age ZigBee data rate 250Kbps Collision Threshold Maximum ZigBee frame size 15

16. Experiment Setting • ZigBeeconfiguration • TelosB with ZigBee-compliant CC2420 radios • Good link performance without WiFi interference • WiFi configuration • 802.11g netbooks with Atheros AR9285 chipset • D-ITG for realistic traffic generation • Baseline protocols • B-MAC and Opportunistic transmission (OppTx) • Evaluation metrics • Modeling accuracy, sampling frequency, delivery ratio, throughput, overhead 16

17. Frame Delivery Ratio Broadcast Unicast with 3 retx 17

18. Conclusions • Empirical study of WiFi and ZigBee coexistence • Blind terminal problem • WiFiwhite space modeling • Rigorous statistic analysis on real WiFi traffic • WISE: White space aware frame adaptation • Implemented in TinyOS 2.x on TelosB • Significant performance gains over B-MAC and OppTx 18

19. Throughput Overhead 19

20. Throughput

21. WiFi Interference Summary Design flaw of CSMA CSMA supposed to work. Why blind terminals? 21

22. Self-Similarity of WiFi Frame Clusters • Arrival process of frame cluster is self-similar • Variance is similar at different time scales

23. WISE Protocol Design • Original ZigBee frame • Sub-frame layout • WISE treat each MAC layer frame as a session • MAC protocol independent • Protocol overhead? • Small sub-frames have low collision probability • Large sub-frames are transmission efficient PHY Hdr MAC Hdr Payload CRC PHY Hdr MAC Hdr ID PHY Hdr ID Payload PHY Hdr ID Payload CRC 23

24. Frame Adaptation • Optimal sub-frame size Average white space lifetime λ and ρ are measured on-line 24

25. Measure the White Space Model • WiFi white space sampling • Sampling the interrupt on CCA pin of CC2420: sampling frequency 4K~8KHz • Record white space sample if • Signal cannot be decoded • Interval between signals is longer than 1ms • Impact of ZigBee interference 25

26. Effect of Sampling Frequency 26

27. CSMA is NOT White Space Aware Collisions CCA Transmission ZigBee WiFi channel trace Time

28. ZigBee Link Performance Analysis • What’s the prob. of colliding w/ WiFi packets? • Analytical collision probability model • ZigBee carrier sensing model • White space model

29. Why Blind Terminals ? No 802.11 modulated packet in channel No Choose random waiting time T between [1, CW] Carrier Sense T=0? Count down T 802.11 modulated packet detected Yes Increase T by the packet duration Data ready Send ZigBee In-friendly • Heterogeneous PHY layer • 802.11 backoff algorithm 29