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RSSI is Under-Appreciated

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  1. RSSI is Under-Appreciated Kannan Srinivasan & Philip Levis Stanford Information Networking Group (SING)

  2. What is RSSI? • Before going anywhere: what is RSSI? • RSSI is Received Signal Strength Indicator • It’s an estimate of average received signal power EmNets 2006

  3. RSSI is a bad indicator of link quality Why is it believed so? Many studies on wireless platforms Zhao et al. Ganesan et al. Son et al. Aguayo et al. (802.11 Roofnet nodes) The Buzz about RSSI EmNets 2006

  4. Little correlation with PRR SNR averaged over 1 sec Plot of Aguayo et al. PRR SNR (dB) (Aguayo et al. SIGCOMM 2005) EmNets 2006

  5. A New Parameter By Newer Radios • CC2420 provides a new parameter, LQI for every successful packet • LQI from a single (previous) packet is believed to be a good indicator • Many protocols have adopted single packet LQI • No extensive evaluation to support this claim EmNets 2006

  6. Why Evaluate CC2420? • Several motes are based on it: Micaz, Telos and Intel2 • Based on IEEE 802.15.4 standard • Operates in 2.4 GHz ISM band • High data rate ~ 250 Kbps • Different modulation – OQPSK • Older WSN radios used OOK (mica1) and FSK (mica2) • Uses Direct Sequence Spread Spectrum EmNets 2006

  7. OOK, FSK & OQPSK 1 0 1 1 Data mica1 OOK/ASK mica2 FSK telos QPSK EmNets 2006

  8. DSSS in CC2420 Several chip errors can still result in a successful decoding of a symbol EmNets 2006

  9. CC2420 RSSI & LQI LQI (~ CER) between 50 & 110 (8 Symbols) Preamble SFD Frame Length/Rsrvd PHY Payload 4 1 1 variable RSSI(8 symbols) Frame Control Seq No. Addressing Fields Frame Payload FCS 2 1 0-20 variable 2 EmNets 2006

  10. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 A 25 12 19 B C 14 4 D 7 15 20 23 11 E 24 3 6 18 28 F G H 8 10 21 26 I 13 17 27 29 2 5 J 22 K L M N O P 1 9 16 30 Experimental Methodology All nodes sent packets to all other nodes. Transmit power level was changed. Repeated experiment. Mirage, Intel Research, Berkeley EmNets 2006

  11. Distribution of RSSI for a link Results: RSSI Transmit Power Level: 0 dBm Outliers Narrow cliff => Difference in noise floor EmNets 2006

  12. Noise Floor at Nodes EmNets 2006

  13. Large variation over time Single LQI could mean many things Results: LQI Transmit Power Level: 0 dBm EmNets 2006

  14. Results: Average LQI Transmit Power Level: 0 dBm EmNets 2006

  15. What Window Size To Use? EmNets 2006

  16. Single LQI is not fine LQI > 100: Single value is fine Results: Average LQI EmNets 2006

  17. Results: Average LQI Smaller Mean PRR Error due to averaging EmNets 2006

  18. Averaging SNR maps to different PRR Going back to Aguayo et al Look at the SNR variation If SNR varies so does the PRR What matters is how signal and noise look like when you receive a packet: not before not after. (Aguayo et al Data) EmNets 2006

  19. Conclusion • RSSI is NOT a bad indicator • Above -87 dBm and stable => good link • Below -87 dBm or unstable => don’t know • Single packet LQI is NOT a good indicator of intermediate links • LQI, when averaged, has potential EmNets 2006

  20. Some Open Questions • What is going on with the outliers in RSSI and LQI plots? • Can combined knowledge of RSSI, average LQI and noise estimate PRR accurately? EmNets 2006

  21. Thank You! Questions? Kannan Srinivasan (srikank@stanford.edu) Philip Levis (pal@cs.stanford.edu) EmNets 2006

  22. Backup Slides EmNets 2006

  23. The Buzz about RSSI • RSSI is a bad link quality indicator • Why is it believed so? • After many evaluations on older radios • Zhao et al SS > 550 PLR ~ 80% PLR < 5% had SS > 550 but converse not true (Zhao et al ENSS 2003) EmNets 2006

  24. The Buzz about RSSI • RSSI is a bad link quality indicator • Why is it believed so? • After many evaluations on older radios • Zhao et al • (PLR < 5%) => (SS > 550) but converse not true • Son et al (evaluated concurrent transmissions) SINR threshold: SINR with PRR > 0.9 (Son et al ISI-TR-2005) EmNets 2006

  25. Results: RSSI Transmit Power Level: 0 dBm EmNets 2006

  26. Results: RSSI Transmit Power Level: -7 dBm EmNets 2006

  27. Results: RSSI Transmit Power Level: 0 dBm Outliers EmNets 2006

  28. Results: RSSI Transmit Power Level: 0 dBm Narrow cliff => Difference in noise floor EmNets 2006

  29. Wide cliff due to miscalibration Results: RSSI EmNets 2006

  30. Results: LQI Transmit Power Level: -7 dBm EmNets 2006

  31. Results: Average LQI Nicer curve fit may be due to difference in time Transmit Power Level: -7 dBm EmNets 2006

  32. CC2420 RSSI and LQI • RSSI calculated over 8 symbols • From analog signal • LQI (Link Quality Indicator): • 2 ways to calculate (Chipcon) • From RSSI • Chip correlation in a byte (can be looked at as Chip Error Rate) – provided by CC2420 • Statistical in nature • Remember: many chip errors can still lead to a correct symbol decoding EmNets 2006

  33. Difference in distortion of signal from different interfering nodes Plot for Son et al EmNets 2006

  34. Results: Average LQI Maximum Absolute EmNets 2006

  35. Still a mess LQI > 100: Not hard to find Results: Average LQI Average Window Size = 5 EmNets 2006

  36. Results: Average LQI Average Window Size = 10 EmNets 2006

  37. LQI Results from another evaluation EmNets 2006

  38. Single LQI is not fine Due to outliers or change in “quality”?? LQI > 100: Single value is fine Results: Average LQI Average Window Size = 1 EmNets 2006

  39. Results: Average LQI Average Window Size = 20 EmNets 2006