UnderstandingtheReal-WorldPerformanceofCarrierSense

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Understanding the Real-World Performance of Carrier Sense,MIT Computer Science and Artificial Intelligence Laboratory,Networks and Mobile Systems,Kyle Jamieson, Bret Hull, Allen Miu, Hari Balakrishnan,1,Introduction,Carrier sense is a,crucial building block,for many radio networks,Wireless sensor networks,Wireless local area networks,Performance,depends on carrier sense,MAC layer,Physical layer,Application layer,Carrier sense,2,A research direction,Lets,quantify how well carrier sense performs,in real-world radio networks,Lets study,diverse,radio networks and draw high-level conclusions,Modulation type,Network size (number of nodes),Data rates,3,Experimental setup,4,Sensor network testbed,60-node Mica2 sensor network,Six radio hops in diameter,Ethernet backchannel to log packet receptions,100 ft.,16,076 sq. ft.,5,Outline,Introduction,Implementing carrier sense,Benefits of carrier sense,Drawbacks of carrier sense,Conclusion,6,How carrier sense works:energy detection,Signal strength (dBm),Time,Squelch (“noise floor”),Instantaneous signal strength,Energy detect clear,Energy detect busy,7,How carrier sense works: other mechanisms,Preamble detection,Decorrelation amplitude,Unique to spread-spectrum radios,AGC unlock,True when AGC adjusts rapidly,Spreading code,Received data,Spreading code,Transmit data,Packet,Preamble,8,Outline,Introduction,Implementing carrier sense,Benefits of carrier sense,Drawbacks of carrier sense,Conclusion,9,Aggregate load lowers link delivery rate,WSN experiment with all nodes sending, carrier sense,on,360 links 70% at 4 pps,10,Carrier sense improves link delivery rates,Carrier sense avoids collisions under high load,Only 80 links in the network are 70% without CS,11,Carrier sense improves throughput,Large-scale experiment with an offered load of,1 pps/node,12,Outline,Introduction,Implementing carrier sense,Benefits of carrier sense,Drawbacks of carrier sense,Conclusion,13,Sender-side decision;receiver-side collision,R,S,Will any transmissions collide with mine?,Carrier sense is at best a heuristic for predicting transmissions success,14,Exposed terminals fool carrier sense,R,S,S,R,Carrier sense indicates busy, yet the transmission would have succeeded (S, S are exposed terminals),Missed transmission opportunity,15,Carrier sense misses transmit opportunities,Large-scale experiment with CS energy detect, 0.25 pps per node,16,Carrier sense misses transmit opportunities,Large-scale experiment with carrier sense,off, 0.25 pps per node,17,Capture fools carrier sense,R,captures,Bs transmission despite As concurrent transmission,R,A,B,Missed transmission opportunity,18,Capture prevalent at low bit rates,At some low 802.11 bit rates, node B should disable carrier sense,Collision,Capture,19,Hidden terminals fool carrier sense,R,S,S,Carrier sense is free!,Carrier sense indicates free, yet both transmissions fail (S, S are hidden terminals),20,Related work,Capture-aware MAC,Whitehouse et al., Em-Nets 05,Priyantha, PhD thesis 05,Channel sampling to infer congestion,CODA, Wan et al., SenSys 04,Models to pick carrier sense sensitivity,Yang and Vaidya, INFOCOM 05,21,Conclusion and future research,An experimental evaluation of the benefits and drawbacks of carrier sense,Algorithm to track correlation between signal strengths and packet reception,Use a,congestion control,algorithm: CODA or Fusion SenSys and turn off or reduce carrier sense,22,