Transcript Contention-Free Periodic Message Scheduler MAC in WSNs

Lightning:
A fast and lightweight acoustic localization protocol using low-end wireless
micro-sensors.
Qixin Wang, Rong Zheng, Ajay Tirumala, Xue
Liu and Lui Sha.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Outline of the presentation
•
•
•
•
•
•
Demand
Observations and Solution Heuristics
Protocol Details
Theorems and Experiment Results
Demo Video
Conclusion
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Demand
• Want a fast, deterministic (i.e.O(1)
response time) acoustic event localization
scheme.
• Fits low-end wireless micro-sensor
networking.
• Proximity localization, i.e. electing the
closest sensor, is good enough.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Observations
• Sound sources are often directional and of
unknown intensities. This implies:
– Intensity-based localization is not desirable.
– Time-Of-Arrival (TOA) is a more reliable metric. When
sensors are densely deployed, Line-Of-Sight to the
closest sensor can usually be guaranteed, therefore
earliest TOA  closest sensor.
• Radio (RF) wave travels much faster than
acoustic wave.
– When a lightning strikes, people see the lightning
before hearing the rumbling of the thunder.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Solution Heuristic I
• When sound reaches the closest sensor,
the closest sensor should immediately
announce the event (via RF broadcast) to
all other sensors and suppress them even
before they hear the sound.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Observation
• Immediate Data Packet (DP) RF
broadcast is not practical, because of
collisions.
– To think several sensors, all almost the same
distance to the sound source, try to broadcast
data packets at almost the same time.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Solution Heuristic II
• Do NOT use data packet broadcast,
broadcast RF burst instead.
RF burst is not susceptible to overlapping.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Observation
• If there are multiple closest sensors, there
can be multiple election winners.
• How to guarantee every time there is only
one winner?
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Solution Heuristics III
• Color the sensors, to differentiate RF burst
duration, to break ties.
– It is proven, with regular sensor layout and
proper coloring, it is guaranteed to always
elect one winner sensor.
– (To be included in our upcoming publications)
Empirically, even with random sensor layout
and without coloring, the number of winners is
still well limited.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Remark
• Heuristics I, II and III lead to the design of
Basic Lightning Protocol.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Observation
• Energy cost is a concern.
– Currently, a sensor has to have RF on all the
time to listen to possible RF bursts.
Remember a sensor to be suppressed
receives RF burst before hears the sound.
• How to have RF module sleep during most
of the time and only be turned on when
there is an acoustic event?
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Solution Heuristics IV
• RF Sleep during usual time.
• When hears a sound, turn on RF and RF listen
for Δdefer sec, to make sure all other sensors that
can hear the sound have turned on their radios.
Then carry out the same procedure as Basic
Lightning Protocol.
• Equivalent to the sound takes place Δdefer sec
later in real-world, and Basic Lightning Protocol
is deployed.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Remark
• Heuristics IV leads to the design of
Energy-Efficient Lightning Protocol.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Basic Lightning Protocol
• Regular sensor layout with
certain coloring
• All sensors are initially in RFlisten mode
• Beep recognized:
– broadcast RF burst without
backoff for iTburst.
– Listen for RF bursts for Tb.
• No other burst recognized
“elected”
• Other burst recognized –
”supressed”
Beep recognized
RF Listen
for i.Tburst
Timer expires
Burst
recognized
Supressed Burst recognized
Set reset timer
Post burst listen
• During RF listen
– RF burst recognized
RF burst
Timer expires
• Enter supressed mode
• Reenter RF listen mode after
basic timer expires.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
No burst recognized
Elected
set reset timer
Power Point created by Qixin Wang and Ajay Tirumala
Energy-efficient lightning protocol
• All sensors initially in RF
sleeping
• When beep is recognized
Beep recognized
– Listen for RF bursts but defer
bursting for Δdefer
– If no burst is recognized,
transmit RF burst without
backoff，for iTburst sec.
– Post burst RF listen
• No other burst recognized
“elected”
• Other burst recognized –
”suppressed”
– If burst is recognized in RF
listen – enter suppressed mode
• When reset timer expires,
return to RF Sleeping mode
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
RF listen for
defer
RF Sleeping
Timer expires
Burst
recognized
Supressed Burst recognized
Set reset timer
RF burst
for i.Tburst
Timer expires
No burst recognized
Elected
set reset timer
Post burst listen
Power Point created by Qixin Wang and Ajay Tirumala
Key Properties
• Theorem 1,2: Both Basic and Energy-Efficient
Lightning Protocol elect a unique winner with
deterministic localization distance error.
• Corollary 1,2: Both have a short and O(1) time
bound for the election.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Experiment results
• Implemented on U.C. Berkeley MICA Motes
• Directional Sound Source
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Experiment results
• Comparable, or even
better accuracy than
ideal (no pkt loss)
data packet based
localization.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Experiment Results
60
50
Lightning
Color 4 (ms)
DP - Closest
First (ms)
DP - Any first
(ms)
40
30
20
10
0
Max
Min
Mean
• Fast: in the sense that the Upper Bound of
Lightning Protocol Election Delay is shorter than
the Lower Bound of Data Packet Election
Protocol
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Experiment Results
• Deterministic Election Delay, while data packet
localization’s election delay are random due to
MAC contention.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Experiment Results
Number of bursts in
lightning protocol
1
2
>2
% of localization trials
81.4
18.6
0
• O(1) broadcasts
– Theoretical: Guaranteed to be <= 4.
– Experiment: Never more than 2.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Demo video (Qixin Wang)
URL: http://www-rtsl.cs.uiuc.edu/papers/LightningDemo.html
See reference [13]
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Conclusion
• Fast and Deterministic:
– O(1) election delay.
– O(1) number of broadcasts.
• Lightweight:
– only involves simple comparison (<>) operation;
– no clock synchronization needed;
– simple network stack.
• Comparable, or even better localization accuracy than ideal data
packet scheme.
• Robust:
– Immune to RF broadcast overlapping;
– Handles directional sound source
• Energy Efficient:
– only turns on RF module when there is an acoustic event.
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala
Thank you!
Dec 8th, RTSS 2004, Presented by Ajay Tirumala
Power Point created by Qixin Wang and Ajay Tirumala