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RSSI is Under-Appreciated Kannan Srinivasan & Philip Levis Stanford Information Networking Group (SING)
RSSI is Under-AppreciatedRSSI is Under-Appreciated
Kannan Srinivasan & Philip LevisStanford Information Networking Group (SING)
EmNets 2006 2
What is RSSI?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
The Buzz about RSSIThe Buzz about RSSI
• 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)
EmNets 2006 4
Plot of Aguayo et al.Plot of Aguayo et al.
SNR averaged over 1 sec
(Aguayo et al. SIGCOMM 2005)
Little correlation with PRR
SNR (dB)
PR
R
EmNets 2006 5
A New Parameter By Newer RadiosA 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?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 & OQPSKOOK, FSK & OQPSK
1 1 10
Data
OOK/ASK
FSK
QPSK
mica1
mica2
telos
EmNets 2006 8
DSSS in CC2420DSSS in CC2420
Several chip errors can still result in a successful decoding of a symbol
EmNets 2006 9
Frame Control
Seq No. Addressing Fields Frame Payload FCS
2 1 0-20 variable 2
Preamble SFD Frame Length/Rsrvd
PHY Payload
4 1 1 variable
LQI (~ CER) between 50 & 110 (8 Symbols)
RSSI (8 symbols)
CC2420 RSSI & LQICC2420 RSSI & LQI
EmNets 2006 10
Experimental MethodologyExperimental Methodology1 4
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All nodes sent packets to all other nodes.Transmit power level was changed.
Repeated experiment.
Mirage, Intel Research, Berkeley
EmNets 2006 11
Results: RSSIResults: RSSI
Transmit Power Level: 0 dBm
OutliersNarrow cliff => Difference in noise floor
Distribution of RSSI for a link
EmNets 2006 12
Noise Floor at NodesNoise Floor at Nodes
Noise
(dBm)
-98 -97 -96 -95 -94 -93 -92
# of Nodes
5 8 4 3 2 3 1
EmNets 2006 13
Results: LQIResults: LQI
Transmit Power Level: 0 dBm
Large variation over time
Single LQI could mean many things
EmNets 2006 14
Results: Average LQIResults: Average LQI
Transmit Power Level: 0 dBm
EmNets 2006 15
What Window Size To Use?What Window Size To Use?
EmNets 2006 16
Results: Average LQIResults: Average LQI
LQI > 100: Single value is fine
Single LQI is not fine
EmNets 2006 17
Results: Average LQIResults: Average LQI
Smaller Mean PRR Error due to averaging
EmNets 2006 18
Going back to Aguayo et alGoing back to Aguayo et al
(Aguayo et al Data)
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.
Averaging SNR maps to different PRR
EmNets 2006 19
ConclusionConclusion
• 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 QuestionsSome 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 22
EmNets 2006 23
The Buzz about RSSIThe 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 RSSIThe 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: RSSIResults: RSSI
Transmit Power Level: 0 dBm
EmNets 2006 26
Results: RSSIResults: RSSI
Transmit Power Level: -7 dBm
EmNets 2006 27
Results: RSSIResults: RSSI
Transmit Power Level: 0 dBm
Outliers
EmNets 2006 28
Results: RSSIResults: RSSI
Transmit Power Level: 0 dBm
Narrow cliff => Difference in noise floor
EmNets 2006 29
Results: RSSIResults: RSSI
Wide cliff due to miscalibration
EmNets 2006 30
Results: LQIResults: LQI
Transmit Power Level: -7 dBm
EmNets 2006 31
Results: Average LQIResults: Average LQI
Transmit Power Level: -7 dBm
Nicer curve fit may be due to difference in time
EmNets 2006 32
CC2420 RSSI and LQICC2420 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
Plot for Son et alPlot for Son et al
Difference in distortion of signal from different interfering nodes
EmNets 2006 34
Results: Average LQIResults: Average LQI
Max
imu
m A
bso
lute
EmNets 2006 35
Results: Average LQIResults: Average LQI
Average Window Size = 5
LQI > 100: Not hard to find
Still a mess
EmNets 2006 36
Results: Average LQIResults: Average LQI
Average Window Size = 10
EmNets 2006 37
Results from another evaluationResults from another evaluation
LQI
EmNets 2006 38
Results: Average LQIResults: Average LQI
Average Window Size = 1
LQI > 100: Single value is fine
Due to outliers or change in “quality”??
Single LQI is not fine
EmNets 2006 39
Results: Average LQIResults: Average LQI
Average Window Size = 20
