Multi-Channel Reliability and Spectrum Usage in Real Homes Empirical Studies for Home-Area Sensor Networks Experimental methodology Empirical study in homes Spectrum study of existing wireless signals 802.15.4 link reliability in all 16 channels Mo Sha, Gregory Hackmann, Chenyang Lu Department of Computer Science and Engineering 2 Smart Grid Home Area Network Power meters, smart thermostats, home appliances. Enables both wired and wireless communication between utility companies and household devices Wireless Sensor Networks Advantage Do not require wired infrastructure. Easily and inexpensively retrofit existing homes. Energy efficiency Reliability challenges Crowded 2.4 GHz ISM band Unpredictable environment 3 4 Experimental methodology Empirical study in homes Spectrum study of existing wireless signals 802.15.4 link reliability in all 16 channels Methodology Spectrum usage between 2.400 GHz and 2.495 GHz Wi-Spy 2.4x spectrum analyzer Sweep across the 2.4 GHz spectrum Sampling period: 40 ms Signal strength reading on each of 254 discrete frequencies Traces over 7 days in 6 apartments and Bryan Hall Normal daily activities 15,120,000 readings for each of the 254 frequencies 2.5 GB of data per location Convert signal strength readings to binary values based on a threshold Communication theory: 0: idle channel 1: busy channel 5 6
Spectrum Usage Traces Collected from the 2.4 GHz spectrum in six apartments and an office Questions 1. Is there a channel free in all apartments? No. There is no golden channel. 2. Do homes have similar spectrum usage patterns as offices? No. Test in lab is not enough. 3. Does spectrum usage change over time? Yes. Channel configuration won t work. 4. Is 802.11 the dominant interferer in homes? 7 8 9 10 While Wi-Fi is a major source of interference, others can be non-negligible contributors to spectrum occupancy. While Wi-Fi is a major source of interference, others can be non-negligible contributors to spectrum occupancy. Channels overlapping with Wi-Fi channel Channels not overlapping with Wi-Fi channel Channels overlapping with Wi-Fi channel Channels not overlapping with Wi-Fi channel 11 12
Experimental methodology Empirical study in homes Spectrum study of existing wireless signals 802.15.4 link reliability in all 16 channels Methodology Platform Tmote Sky and TelosB motes IEEE 802.15.4 compliant Chipcon CC2420 radio 16 channels (11 26) in 5 MHz steps TinyOS 2.1 using default CSMA/CA MAC layer Packet Reception Rate (PRR) of all 802.15.4 channels 10 apartments, 24 hours per apartment A node broadcast 100 packets per channel to multiple receivers, cycling through all 16 channels in 5 minutes Receivers recorded the PRRs in onboard Flash 13 14 Questions 1. Is there a persistently reliable channel? 2. If a good channel cannot be found, are retransmissions sufficient to deal with packet loss? 3. If no single channel can be used for reliable operation, can we exploit channel diversity to achieve reliability? 4. Do channel conditions exhibit cyclic behavior over time? 5. Is reliability strongly correlated among different channels? Is there a persistently reliable channel? Link reliability varies among apartments and links. Different apartments Different links within a same apartment 15 16 Is there a persistently reliable channel? Link reliability varies among apartments and links. Is there a persistently reliable channel? Link reliability varies among apartments and links. Different apartments Different apartments Different links within a same apartment Different links within a same apartment 17 18
Is retransmission sufficient? No, due to burstiness of transmission failures. Is retransmission sufficient? No, due to burstiness of transmission failures. 10 % of time, consecutive drops larger than 60 19 20 Is channel hopping effective? Yes! How often needs a link switch channel? Only a small number of channel hops per day. Optimal channel hopping schedule Single best channel Number of channel hops required under an optimal schedule (one link selected randomly per apartment) 21 22 Questions 1. Is there a persistently reliable channel? 2. If a good channel cannot be found, are retransmissions sufficient to deal with packet loss? 3. If no single channel can be used for reliable operation, can we exploit channel diversity to achieve reliability? 4. Do channel conditions exhibit cyclic behavior over time? No. The channel behavior is not cyclic. 5. Is reliability strongly correlated among different channels? Yes. Avoid using adjacent channel. Findings Home environments are much more complicated than offices numerous and diverse Wi-Fi APs and others. There is no channel that works for all homes no channel works for all. Channel reliability changes dynamically cannot pre-select channels. Channel hopping is effective enhance reliability with a few channel switches per day. 23
ARCH: Practical Channel Hopping for Reliable Home-Area Sensor Networks Mo Sha, Gregory Hackmann, Chenyang Lu Department of Computer Science and Engineering Related work Protocol design Design Insights ARCH Protocol Coordinated Channel Hopping Handling Channel Desynchronization Evaluation 26 ARCH Protocol Receiver-oriented protocol Monitor channel condition Maintain a sliding window of ETX values of incoming links Mark channel unreliable if ETX values exceed threshold Blacklist current channel when channel condition degraded Switch to a new picked channel There is strong correlation among adjacent channels Uses a probabilistic scheme Generate a random number If q falls into the range then, switch to channel i Coordinated Channel Hopping Upon selecting a new channel, nodes notify their neighbors of this change. Neighbors update their neighbor tables. Multi-hop problem Multi-sender problem 27 28 Handling Channel Desynchronization Default channel No data transmission, only for resynchronization Senders use when reach maximum retransmissions Receivers use when reach maximum waiting time False detection Channel is too noisy Exchange previous channels when resynchronizing Blacklist this channel and pick a new one Related work Protocol design Evaluation * Channel Selection Scheme * Channel Quality Estimator * Single-Hop Data Collection * Multi-Hop Data Collection 29 30
Channel Selection Scheme Channel Selection Scheme 31 32 Channel Quality Estimator Channel Quality Estimator 33 34 Single-Hop Data Collection Single-Hop Data Collection 35 36
37 38 39 40 Conclusion ARCH has the following salient features that distinguish it from existing channel diversity schemes: Adaptively channel selection Distributed approach Lightweight and robust Minimal communication overhead Results 42.3% decrease in packet retransmissions 17% increase in the proportion of links with perfect delivery rates 2.2X increasing in the minimum delivery rate for the most challenging of links 31.6% average reduction in radio usage