Co-existence of WiFi and ZigBee

Transcription

1 Co-existence of WiFi and ZigBee Kang G. Shin The University of Michigan Joint work with Xinyu Zhang ACM MobiHoc 2011 Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 1

2 Coexistence of ZigBee and WiFi Spatial coexistence ZigBee (monitoring & control) WiFi (Internet access) Frequency-domain coexistence (spectrum sharing) 20MHz Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 2 2

3 State-of-art for coexistence management Both ZigBee and WiFi rely on built-in MAC mechanism CSMA/CA, i.e., Listen-Before-Talk Designed for same type of devices Is the built-in CSMA/CA effective? Some small-scale measurement studies show: severe collisions occur under moderate-to-high WiFi traffic Evidence from the real-world: In a 90-ZigBee-node network for building energy monitoring: 50+% ZigBee nodes suffer connection loss during WiFi peak hours. [C-J. M. Liang, N. B. Priyantha, J. Liu, and A. Terzis, Surviving Wi-Fi Interferenc e in Low Power ZigBee Networks, in Proc. of ACM SenSys 2010] Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 3 3

4 Why CSMA fails? Heterogeneous MAC/PHY properties => coexistence difficulty Scheduling conflict ZigBee has TDMA which is incompatible with WiFi s CSMA direct collision Disparate transmit power levels WiFi : around15 dbm; ZigBee: below 0 dbm asymmetric interference WiFi s transmissi on range Asymmetric inter ference region ZigBee s transmi ssion range Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 4 4

5 Collision occurs even if both use CSMA and can hear Disparate time resolution: MAC-layer time slots: 9us for WiFi, but 320us for ZigBee WiFi s preemption of ZigBee Different communication mechanisms OFDM for WiFi; DSSS for ZigBee No negotiation mechanism Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 5 5

6 New solution: Cooperative Busy Tone (CBT) Principles of CBT: Make ZigBee visible to WiFi, without interfering with ZigBee s transmission Allow ZigBee to coexist and contend with WiFi in frequency, spatial, and temporal domains Preserve carrier-sensing-based spectrum etiquette Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 6 6

7 CBT Overview WiFi client signaler ZigBee TX ZigBee signaler DATA switching time ACK WiFi AP ZigBee TX WiFi TX CCA, backoff DATA ACK A separate node (signaler) emits a busy-tone to make WiFi aware of ZigBee The busy tone harbingers the data pkt, and continues throughout the DATA-ACK transmission, so as to prevent WiFi s preemption. Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 7 7

8 Challenges and solutions How to prevent the busy-tone from interfering with the ZigBee data packet? => Frequency flip When should the signaler send the busy-tone? => CBT scheduler Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 8 8

9 Signaler frequency flip Avoids signaler interfering with ZigBee data packet: Busy tone Transmitter sends data packet on some channel Signaler sends busy-tone on an adjacent channel Returns to the original channel after sending the busy-tone Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 9 9

10 Busy-tone scheduler objectives Schedule the signaler s busy-tone to: Reduce WiFi s preemptions of ZigBee transmissions Minimize negative influence on WiFi performance Protect both TDMA and CSMA modes of ZigBee Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 10 10

11 Busy-tone scheduler: TDMA mode CCA attempts frequency flip Harbinger time How early can signaler start the first CCA, harbinger time? H s H s too large: busy-tone wastes channel time too small: no idle slot may be sensed, aborting busy-tone H s Analytical framework: relates H s to network performance an d optimizes it. Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 11 11

12 Busy-tone scheduler: CSMA mode backoff CCA switching frequency flip Key parameter: busy-tone duration T b T b T b too large: busy-tone wastes channel time too small: data/ack may not be protected Analytical framework: relates to network performance and makes a tradeoff between channel utilization and the effectiveness of CBT Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 12 T b 12

13 Performance analysis and parameter optimization Network model: Topology: ZigBee and WiFi networks co-located with signal er within WiFi xmitter s sensing range ( ) Traffic: Poisson with arrival rates and Parameters: λ Traffic intensity, z Λ λ Λ Transmit power, z w Use legacy ZigBee or CBT w λ z S z W t λw Z Topology: ZigBee xmitter within range of WiFi xmitter ( ) t W t or not ( ) Z t W t Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 13 13

14 Performance analysis and parameter optimization, cont d Performance metrics: Normalized throughput: and Approach: Γ z Γw Assume ZigBee does not affect WiFi traffic (low power and low duty-cycle) Analyze collision probability under each parameter setting Analyze throughput based on collision probability: Focus primarily on temporal collision probability Incorporate spatial collision probability (capturing node locations and capture effect) Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 14 14

15 ZigBee s TDMA with WiFi Collision probability of legacy ZigBee Tag an arbitrary packet from ZigBee xmitter, and calculate its collision probability with randomly arriving W t packets (assuming ZigBee does not affect WiFi s traffic) Z t Collision probability of CBT Relate CCA failure rate to harbinger time H s Relate collision probability to CCA failure rate Collision probability as a function of,,, etc. Γ w Γ z H s Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 15 15

16 ZigBee TDMA mode with WiFi, cont d Network performance: Z t Model transmission attempts of as a renewal reward process ZigBee throughput = mean reward rate = Prob.[no collision] data packet size Average amount of data sent within an attempt Mean service time of a data packet Includes retransmission, ACK, and switching time WiFi throughput approximated using a simpler model: Depends on whether or not Z W Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 16 t t 16

17 ZigBee s CSMA with WiFi Performance of legacy ZigBee Derive mean service time, based on a Markov chain model BS i : i-th backoff & CCA stage P tx: Transmission probability (after CCA) P d : Data packets collision probability P : ACK packets collision probability a Depends on WiFi s traffic intensity Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 17 17

18 ZigBee CSMA mode with WiFi, cont d Performance of CBT : Similar Markov chain model Also depends on key parameter: busy tone duration Tb T b If = data packet duration + max backoff&cca duration, then collision probability 0 Otherwise, the collision probability is bounded: Bound depends on T b Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 18 18

19 Spatial collision probability I e Probability that a packet cannot be decoded, given that temporal collision already occurred (Accounts for capture effect and random node locations) Approximate I e in a random topology: Result: Depends on transmit power Λ z, Λ w and capture threshold 19 Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 19

20 Evaluation results Simulation Implementation of CBT (TDMA & CSMA modes) in ns-2 Real experiments Legacy ZigBee: based on openzb in TinyOS, running on micaz motes CBT: implementation of signaler in GNURadio, running on USRP2 software radio Synchronize USRP signaler to micaz coordinator for CBT s TDMA, using short notification messages Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 20 20

21 Temporal collision probability ZigBee= 250Kbps, 8pkts/s, 63B/pkt Markers = simulation results; lines = analytical results Analysis matches simulation CBT significantly reduces collision rate of data/ack pkts Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 21 21

22 Spatial collision probability Prob. that ZigBee cannot decode collided pkt (accounting for capture effect and random node locations) d t Zt W t Zt W t Out of interference range Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 22 22

23 Normalized throughput: TDMA mode Sweet spot CBT makes 2 throughput improvement under moderate to high WiFi traffic Negligible degradation of WiFi throughput, compared to legacy ZigBee CBT may have lower throughput than legacy under light WiFi traffic (a sweet spot exists) Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 23 23

24 Impact of harbinger time in TDMA mode Larger K larger more overhead, higher ZigBee m H s throughput under high WiFi interference Under low duty-cycle ZigBee traffic (below 0.05), WiFi throughput is virtually unaffected by harbinger time Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 24 24

25 Experimental testbed configuration Node locations: Nodes A and B are WiFi All other nodes are ZigBee (MICAz motes) Only TDMA mode implemented CBT signaler implemented in GNURadio on USRP2 software radio 25 Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 25

26 Testbed results: Collision probability (TDMA mode) For randomly selected links: CBT reduces collision rate by 60+% for most links Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 26 26

27 Testbed results: Impact on WiFi (TDMA mode) WiFi packet delay: CBT and legacy ZigBee have similar effects on WiFi p erformance WiFi performance essentially unaffected when ZigBee traffic load < 2% Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 27 27

28 Conclusions Traditional CSMA fails in heterogeneous networks Due to disparate MAC/PHY properties CBT resolves collision between ZigBee/WiFi Busy-tone scheduler: ensure busy-tones protect data pkts Frequency flip: preventing signaler/transmitter interference Stochastic models for performance analysis and optimization Extensions to other heterogeneous networks: WiFi/Bluetooth ( ), WiFi/WiMax (802.11y), whitespace networks, Macro- and femto-cells (MobiCom 10),. Applying Autonomics to Create an Intelligent, Ubiquitous Environment Slide 28 28