Download Smart Dust and TinyOS: Hardware and Software for Network

Tiny Networking
David Culler
University of California, Berkeley
Intel Research Berkeley
http://webs.cs.berkeley.edu
Vast Networks of Tiny Devices
• Past 25 years of internet technology built up around
powerful dedicated devices that are carefully configured and
very stable
– local high-power wireless subnets at the edges
– 1-1 communication between named computers
• Here, ...
•
•
•
•
•
•
every little node is potentially a router
work together in application specific ways
collections of data defined by attributes
connectivity is highly variable
must self-organize to manage topology, routing, etc
and for power savings, radios may be off 99% of the time
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NEC Networking
Directed Diffusion Concept
• Nodes express ‘interest’ in data with certain
attributes (sinks)
• Establishes gradient from sources
Estrin, Govindan, Heideman
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NEC Networking
Directed Diffusion Concept
• Nodes express ‘interest’ in data with certain
attributes (sinks)
• Establishes gradient from sources
• Sources generate data
• Useful paths reinforced, others suppressed
• in-network aggregation
• nested queries
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NEC Networking
Huge Design Space: Application
• Traffic
– any-to-any, all-to-one, one-to-all, collection of sub-groups, ...
– steady low-BW steam of readings/findings, bursts of data, periodic logs,
...
• Duration
– years (hazard alarm), months (field season), when the big red button is
pressed
• Available infrastructure
– power, base-stations, location
• Mobility, Stationary
– all fixed, all mobile, mixture, changing environment
• Placement / Physical Topology / Scale
– arbitrary, controlled, unknown
• Network Services
– time synchronization, localization, proximity, ...
=> Best way to get a networking solution now is to define your
application context
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NEC Networking
Design Space: Underlying Technology
• Link (Radio) Technology
– range, control of signal strength, noise tolerance
– channel capacity, coding, error rates
– single channel, multi-channel, tunable
• Device Density, Failures over time
• MAC (media access control)
– channel sensing, back-off, protocols, collision behavior, link-level acks
– time synch, energy aware
• Power management & Energy Constraints
– scheduling, functional allocation
• Transmission Rate Control
• Topology Formation
– hierarchical spine, geographic, static, dynamic
• Routing
– single path, multipath, passive participation
– explicit vs implicit nbhd detection
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NEC Networking
Losing the forest for the trees
• Systems side
– clever fix to one particular aspect, but only looking at it in
sliver of the design space
• Theory side
– assuming cell coverage is a disk of radius r with sharp
boundary
– within radius Pconnect = 1, outside Pconnect = 0
– eg., unit disk-graphs for routing, maximal independent sets,
min. dominating sets, leader election
• Wireless communication between small devices is
inherently noisy, unpredictable, & non-deterministic
–
–
–
–
–
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Radio signal fades with distance
Interference
Multipath (reflections)
Collisions
Mobility or not
NEC Networking
Surge Demo
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NEC Networking
Local Operations => Global Behavior
• Nodes ‘sense’ network environment
– uncertain, partial information
• Packets directed to a “parent” neighbor
– all other neighbors “hear” too
– carry additional organizational information
• Each nodes builds estimate of neighborhood
– adjusted with every packet and with time
• Interactively selects parent
• Routes traffic upward
 Collectively they build and maintain a stable spanning tree
 takes energy to maintain structure
Predictable global behavior built from
local operations on uncertain data
node #
dept
h
17
1
6
3
...
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NEC Networking
child?
yes
parent
?
%
link
goodnes
s
yes
90
.7
75
.6
Wireless “Connectivity”
• Controlled study on
13x13 array of Rene
nodes
• Single transmitter
• Record fraction of
packets received at each
node
• Many packets at each of
several transmit power
levels
complex fall-off over
substantial range
‘range’ defined by CEP
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Contours of probability of
reception from center node for
range of transmit power strengths
with Ganesan, Woo, Krishnamacheti, Estrin
NEC Networking
Alternative Perspective
• Develop algorithms that are built fundamentally
upon a probabilistic connectivity structure
– Embrace noise, rather than fight it
– the network is really another sensor (and actuator)
• Utilize simple, local rules, rather than complex
protocols
• Challenge: obtain predictable global behavior
• Challenge: interfaces for imperfect operation
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NEC Networking
Simple Epidemic Broadcast Schema
Local Rule:
if (new mcast) then
take local action
retransmit modified request
• Should forms roughly breadth-first spanning tree
• Examples: Network wakeup, command propagation
– Build spanning tree
» record parent
• Naturally adapts to available connectivity
• Minimal state and protocol overhead
=> surprising complexity in this simple mechanism
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NEC Networking
Network Discovery: Radio Cells
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NEC Networking
Network Discovery
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NEC Networking
Behavior at Scale
• Variations in
connectivity
• With many nodes,
likely that one far
away will hear
• Long links tend to be
asymmetric
• Structure dominated
by contention
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NEC Networking
Final Tree
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NEC Networking
Power Laws ?
1000
100
100
Links
Count
1000
10
10
1
1
1
10
100
1
10
Cluster Size
Cluster Size (1 + # children)
• Most nodes have very small degree (ave = .92)
• Some have degree = 15% of the population
• Few large clusters account for most of the edges
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NEC Networking
100
Open Territory => Many Children
• Example: Level 1
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NEC Networking
Open Territory => Many Children
• Example: Level 2 – variation in distance
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NEC Networking
Open Territory => Many Children
• Example: Level 3 – long links
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NEC Networking
Importance of Asymmetric Links
• Asymmetric Link:
– >65% successful reception in one direction
– <25% successful reception in the other direction
• 10%-25% of links are asymmetric
• Many long links are asymmetric
– in large field it is likely that someone far away can hear you
– what does this mean for protocol design?
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NEC Networking
Collisions are primary factor
• Nodes out of range may have overlapping cells
– hidden terminal effect
• Collisions => these nodes hear neither ‘parent’
– become stragglers
• As the tree propagates
– folds back on itself
– rebounds from the edge
– picking up these stragglers.
• Seen in many experiments
• Mathematically complex because behavior is not
independent beyond singe cell
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NEC Networking
Probabilistic Connectivity Model
• Radio signal fades with distance in complex manner
depending on environment
• Radio receiver has complex behavior to extract signal
• What matters to algorithms is whether packets are
delivered or not
 work directly with probabilistic communication model
 but which one?
• Calculate comm. rates for numerous transmit/receive pairs
at range of distances
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NEC Networking
Fall-off of Probability of Comm.
Low Power
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High Power
NEC Networking
Drive Simulation from Empirical
Stochastic behavior
clear
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transition
silent
NEC Networking
Example Cell Coverage from Model
feet
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feet
NEC Networking
Reception Model with Collisions
• Second experiment with two nodes sending at once, record
which nodes hear which one
– follows P success closely
• When does a second sender collide?
– Clear comm. region => YES
– silent region => NO
– transition region => collides if would have communicated
• Reception Model
– Assume pij is the probability of success (i->j), Probability for B to
receive A’s message
= pab*
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i=collider(1-pib)
NEC Networking
Common Special Case: Data Gathering
• Collection of nodes take periodic samples
• Stream data towards a root node
• Root announces interest
– depth = 0
• Nodes listen to neighbors
• When hear neighbor with smaller depth
– start transmitting data to good neighbor with smallest depth
– set own depth to one greater and include with data
• Data transmission continuously reinforces /
adjusts routes
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NEC Networking
Use Case Assumptions
• Application
– N-to-1 all data, no aggregation
– Each node generates small packets at regular interval
» Appln phase shift on collision
– Routed to a specific node, e.g, base station or root of request
– data rate below saturation
– 7x7 grid, 10 ft spacing => several hops
• Underlying
– link-level acknowledgement (may be used for retransmission)
– CSMA with simple channel sensing, fixed backoff, initial random
delay
– Receivers always attentive
» may use sampled listening
• Broadcast: choose parent = first contact
• Shortest Path: choose parent = closer (which one)
• Min path loss: choose parent = next step on min loss path
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NEC Networking
Max-Path-Reliability routing
• each node maintains estimate of loss rate over
entire path to root
• select nbr on the minimum loss path as parent
– Pi[to root] through j = P[link i,j] * Pj[to root]
– assuming loss rate along path is
independent of how packets enter the path
• Subtleties
–
–
–
–
–
–
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P=0.86
95
65
estimating link rates
transient error in link rate may lead to cycles
rate of updates
stability vs responsiveness
60
congestion
warm-up phase
NEC Networking
95
70
95
99
Broadcast
• overall success rate: 18.9%
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NEC Networking
SP50 Path reliability
• overall success rate: 44.8%
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NEC Networking
MPR path reliability
• overall success rate: 54.7%
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NEC Networking
Routing Distance Distribution
SD50
MPR
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NEC Networking
Distribution of success rates
MPR
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SP50
NEC Networking
Observations
• Better success rate with more, better hops
• Partial information causes temporary loss
– cycles introduced when link error estimates are poor
– tracks std. dev. of link error rates
– if difference between candidates not statistically significant,
choose based on hop count
– especially important during warm-up phase
» also after topology changes
• Shortest Path will tend to use most marginal
neighbors
– although range is highly variable, for particular pairs of nodes
at particular distances connectivity is very bimodal
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NEC Networking
SP75 Path Reliability
• overall success rate: 52.7% (vs 54.7% for MPR)
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NEC Networking
Multihop Path-Rate Contours
Min Path Loss
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Min Hop (75% nbr)
NEC Networking
Hop-by-hop retransmission
• Decent neighbors become good neighbors if you
are willing to chat for a moment
• may choose parent that is on “path of least
transmissions”
– expected number of transmissions = 1 / P(success)
• Distributed computation is estimate of SUM of
retransmissions
• A few retransmissions make large difference in
success rate
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NEC Networking
Overall Results
Success
#
Rate to BS Retrans /
App
packet
Ave.
Hop
Count
MRP
83.94%
1.03
8.36
6.12
Shortest
Path
85.79%
0.82
7.13
6.18
5.76
13.13
4.92
Broadcast
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Total Msg.
Overhead/
pack. Recv.
By BS
42.96%
NEC Networking
Fairness
SP75
MRP
Broadcast
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NEC Networking
Stability
• How often does the routing algorithm change?
• # of parent changes per unit time
MRP
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SP75
NEC Networking
Stability (Broadcast)
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NEC Networking
Key Building Block: Link estimator
• Nodes assess quality of link from packets they
hear
– directed to them or snoop
– sequence number => infer losses between packets
• For each new packet (or empty window) revise
estimate of link probability
• classic EWMA
– P i+1 = aP i + (1-a) X i , where X i = 0,1 if loss, success
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NEC Networking
Link Probability Changes Dynamically
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NEC Networking
Revisit Classic Estimator
• Want estimator that is responsive to change, but
stable with low error
• Candidates
– EWMA stable, agile, flip-flop
– Moving Average
– ...
• Best ended up being the own that we eyeballed
– EWMA cascaded with average over a time window
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NEC Networking
Estimator driven from prob. generator
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NEC Networking
Empirical Trace
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NEC Networking
Bottom Line
• Can only estimate link rate to within ~10%
• Takes about 100 packets to settle
• Design distributed algorithms with this kind of
approximation of an inherently noisy world
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NEC Networking
So how about that demo?
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NEC Networking
Higher Level Network Services
• Time Synchronization
– many nodes with drift and offset variation
– classic:
» pairwise timesynch limited to half the variance in RTT
» NTP hierarchy to establish rough global time
– local timesynch
» deep integration into network stack (Hill)
» multiple receivers, not send-rcv (Ellison, Girod)
– global propagation: huge open distributed algorithm question
• Ranging & Localization
• Naming
• Data distribution, Aggregation, Storage
• We’ve just begun to tackle the full problem
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NEC Networking
Conclusion
• The networking part of the “future sensor
networks” very different the power-hungry,
sparse IP world
–
–
–
–
severe constraints, noisy, dynamic, partial information
Low power means you hardly ever listen
self-organized and inherently distributed
opportunity to work across layers of abstraction
• Presents a rich set of open problems
– Probabilistic connectivity model is usable and powerful
– On-line distributed approximations to global optimization
– a whole new internet to invent
• Applications define valid assumption sets for
network design
– guide the front of progress in this huge space
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NEC Networking
Engineering Perspective
• Low-hanging fruit
– enough base stations that only need single hop
• Mid-range fruit
– stationary, well-powered multi-hop backbone
• Research centroid
– unstructured, ad hoc, multihop routing in energy starved
environment
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NEC Networking