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ASCENT: Adaptive SelfConfiguring sEnsor Networks
Topologies
Authors: Alberto Cerpa, Deborah Estrin
Presented by Suganthie Shanmugam
11/15/2005
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Presentation Topics
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Introduction
Assumptions and Contributions
ASCENT Design
Analytical Performance Analysis
Experimental Simulation
Simulation Results
Related Work
Conclusion
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Introduction
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Advances in micro-sensor and radio technology
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Nodes perform local processing
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Reduce communications and energy costs
Low per-node cost → densely distributed network
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Smart sensors deployed in wireless network
Results in non-uniform communication density
ASCENT
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Only a subset of nodes necessary to establish routing as
node density increases
Each node assesses its connectivity and adaptively selfconfigures to underlying topology
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ASCENT Introduction
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How It works
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A node signals when it detects high packet loss
Requests other nodes to join the network
Reduces its load and does not join network till it is “helpful”
to do so
Adaptive configuration cannot be done from a
central node
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Single node cannot sense conditions of nodes distributed in
space
Other nodes will be required to communicate detailed
information to central node
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Assumptions and Contributions
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Distributed Sensor Network Scenario
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Ex: A habitat monitoring sensor network
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Sensors hand-placed or dropped from a plane
Conditions
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Ad-hoc deployment
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Energy Constraints
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Sensor network cannot be deployed in regular fashion
Uniform deployment does not correspond to uniform
connectivity
Expend minimal energy to maximize network lifetime
Unattended operation under dynamics
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Preclude manual configuration and design-time preconfiguration
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Assumptions and Contributions
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Easier to deploy large number of nodes initially
Too few nodes used
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All nodes used
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Distance between neighboring nodes – large
Packet loss rate increases
Energy required to transmit – prohibitive
Unnecessary energy expended
Nodes interfere with each other – channel congestion
Perfect platform for ASCENT design
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Assumptions and Contributions
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Assumption – CSMA MAC protocol used in network
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Resource contention when many nodes involved in routing
ASCENT
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Does not detect or repair network partitions
Is not suitable when node density is low
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All nodes required to form effective network
Two primary contributions
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Use of adaptive techniques to configure the underlying
network
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Saves Energy, Extends Network lifetime
Use of self-configuring techniques
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Reacts to operating conditions locally
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ASCENT Design
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ASCENT adaptively elects “Active” nodes
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Awake all the time and perform multi-hop packet routing
Passive nodes
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Periodically check if they should become active
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ASCENT Design - State Transitions
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ASCENT Design - Parameters Tuning
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NT (Neighbor Threshold)
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LT (Loss Threshold)
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Max. amount of data loss that an application can tolerate
Application dependent – Set to 20%
Tt, Tp – Test Timer, Passive Timer
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Average degree of connectivity in the network - Set to 4
Max. time a node remains in test and passive states
Tt = 2 minutes ; Tp = 4 minutes
Ts – Sleep Timer
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Amount of time a node sleeps to conserve energy
Large Ts – Large energy savings but doesn’t react to
dynamics
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Neighbor and Data Loss Determination
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Number of active neighbors, Avg. data loss rate
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Definitions
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Values measured locally by each node while in passive and
test states
Neighbor node - From which certain % of packets received
History Window CW – Keep track of packets received from
each node
Each node increases the sequence number when
each packet is transmitted
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When a sequence number is skipped, loss is detected
Final packet loss:
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Filter constant ρ set to 0.3
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Neighbor and Data Loss Determination
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The number of active neighbors (N)
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Number of neighbors with link packet loss smaller than the
neighbor loss threshold (NLS)
NLS = 1- (1/N)
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N : the number of neighbors calculated in the previous cycle
If neighbor packet loss > NLS, node deleted from list
As number of neighbors increase, NLS should be increased
Average data loss rate (DL)
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Calculated based on application data packets
Detected using data sequence numbers
If message not received from any neighbor - data loss
Control messages are not considered
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Help, neighbor announcement and routing control
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Interactions with Routing
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ASCENT
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runs above link and MAC layer below routing layer
is not a routing or data dissemination protocol
decides which nodes should join the routing infrastructure
Nodes become active or passive independent of routing
protocol
Does not use state gathered by the routing protocol
Does not require changing the routing state
Test state (actively routing packets)  passive state
(listen-only)
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Cause some packet loss
Improvement : Traffic could be rerouted in advance by
informing the routing protocol of ASCENT’s state changes
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Performance Analysis – Goals and Metrics
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One-Hop Delivery Rate
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Measures % of packets received by any node in network
Indicates effective one-hop bandwidth available to nodes
When all nodes are turned on –Active case – packet
reception includes all nodes.
ASCENT case - includes all except nodes in sleep state.
End-to-End Delivery Rate
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Ratio of Number of distinct packets received by destination
to the Number originally sent by source
Provides an idea of quality of paths in the network and the
effective multi-hop bandwidth
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Performance Analysis – Goals and Metrics
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Energy Savings
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Ratio of energy consumed by Active case to Energy
consumed by the ASCENT case
Average Per-Hop Latency
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Measures average delay in packet forwarding in a multihop network
Provides estimate of end-to-end delay in packet forwarding
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Analytical Performance Analysis
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Assumptions
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Nodes randomly distributed in an area A
Average degree of connectivity (n)
Packets propagated using flooding with random back-off
Probability of successfully transmitting a packet
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P (success) = [(S – 1)/S]T
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Node density increase → P (success) decreases
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When all nodes can transmit and receive, T = n
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Since every node in vicinity can transmit
Node density increase → P (collisions) increases
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Analytical Performance Analysis
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Average latency per
hop related to S and T
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S = No. of slots
T = No. of active nodes
Each T node picks a
random slot
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S1, S2…ST
Mean = S / 2
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Uniform probability
distribution
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Analytical Performance Analysis
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Analytical Performance Analysis
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P(δ) distribution for
different T and S =20
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T=n
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When all nodes can
transmit and receive
As n ↑, P(δ) ↓
In ASCENT case
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T = NT
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Independent of n
P(δ) remains constant
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Analytical Performance Analysis
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Energy Savings
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Numerator – Power consumed by all nodes without
ASCENT
Denominator – Power consumed by all nodes running
ASCENT
1: Power consumed by NT nodes selected by ASCENT to
have their radios on
2: Energy of non-active nodes in passive state
3: Energy consumed in sleep state
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3
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Analytical Performance Analysis
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Energy Savings
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α = Ratio of passive timer to sleep timer
β = Ratio of sleep mode to idle mode power consumption
NT = fixed, β = small, as density ↑ power consumption is
dominated by passive nodes
When α = small and Ts >>Tp, large energy savings
Large Ts → slow reaction of passive nodes
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Analytical Performance Analysis
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Energy savings of
ASCENT with Adaptive
timers
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No asymptotic behavior
Energy savings increase
linearly with density
Slope of line primarily
determined by Probability
Threshold Pt
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Simulation & Experimental Methodology
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Implementation
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LinkStats module
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Neighbor Discovery module
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Adds increasing sequence
number to each packet
Monitors packets
Maintains packets statistics
Sends and receives
Heartbeat messages
Maintains list of active
neighbors
Energy Manager module
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Evaluate Energy Usage
Acts as simulated battery
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Simulation & Experimental Methodology
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Simulator
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Experimental Test bed
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Built-in simulator (emsim) of EmStar used
Provides channel simulator to model environment behavior
Statistical model
Total of 55 nodes used, All nodes wall-powered
Routing
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Flooding used as routing protocol for simplicity
On receiving a packet, flood module waits for a random
time
Randomization interval = 5 seconds
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Simulation & Experimental Methodology
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Scenarios and Environment
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Experiments conducted with different densities ranging
from 5 to 40 nodes
Density defined topologically
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Defined by average degree of connectivity between all nodes
not by physical location
Achieved by adjusting transmit power of the RF transceiver
Average number of hops = 3
Traffic
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One source sends approximately 200 messages
Data Rate = 3 messages / minute
Nodes do not experience congestion
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Simulation Results – Network Capacity
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No major difference
between analytical and
simulated performance
Active case
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All nodes join network and
forward packets
Low delivery rate
As node density increases,
P (collisions) increases
ASCENT case
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Limits active nodes
Channel contention does
not increase
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Simulation Results – Network Capacity
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No. of hops = 3
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No. of hops = 6
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Experiments
Simulations
Increase in density
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ASCENT performs better
than ACTIVE case
Remains stable
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Simulation Results – Energy Savings
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ASCENT provides
significant Energy
savings
As density increases
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Fixed State Timers
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Energy savings do not
increase proportionally
Number of Active nodes
remains stable
Adaptive State Timers
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Energy savings increase
proportionally
Passive nodes
aggressive
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Simulation Results – Latency
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ACTIVE case
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As density ↑, average perhop latency is reduced
Larger probability of a node
picking a smaller random
interval to forward the
packet
ASCENT
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As density ↑, average perhop latency remains stable
Number of nodes able to
forward packets remains
constant
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Results – Reaction to Dynamics
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Evaluate how ASCENT
reacts to node failures
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Let system run till stable
topology reached
Manually kill set of active
nodes
At high density, end-to-end
delivery rate does not
decrease
High probability of a passive
node to fix communication
hole
ASCENT with adaptive
state timers – more stable
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Results – Sensitivity to Parameters
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Larger randomization
interval
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average one-hop delivery
rate increases
Increases end-to-end
latency
ASCENT outperforms
ACTIVE case
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Conclusions and Future Work
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Paper describes design, implementation, analysis,
simulation and experimental evaluation of ASCENT
ASCENT
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Has potential to significantly reduce packet loss
Increases Energy efficiency
Was responsive & stable under varied conditions
Future Work
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Evaluate interactions of ASCENT with MAC
Investigate use of load balancing techniques
Understand relationships between ASCENT and other
routing strategies
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