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ASCENT: Adaptive SelfConfiguring sEnsor Networks Topologies Authors: Alberto Cerpa, Deborah Estrin Presented by Suganthie Shanmugam 11/15/2005 1 Presentation Topics Introduction Assumptions and Contributions ASCENT Design Analytical Performance Analysis Experimental Simulation Simulation Results Related Work Conclusion 11/15/2005 2 Introduction Advances in micro-sensor and radio technology Nodes perform local processing Reduce communications and energy costs Low per-node cost → densely distributed network Smart sensors deployed in wireless network Results in non-uniform communication density ASCENT Only a subset of nodes necessary to establish routing as node density increases Each node assesses its connectivity and adaptively selfconfigures to underlying topology 11/15/2005 3 ASCENT Introduction How It works 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 Single node cannot sense conditions of nodes distributed in space Other nodes will be required to communicate detailed information to central node 11/15/2005 4 Assumptions and Contributions Distributed Sensor Network Scenario Ex: A habitat monitoring sensor network Sensors hand-placed or dropped from a plane Conditions Ad-hoc deployment Energy Constraints 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 11/15/2005 Preclude manual configuration and design-time preconfiguration 5 Assumptions and Contributions Easier to deploy large number of nodes initially Too few nodes used All nodes used 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 11/15/2005 6 Assumptions and Contributions Assumption – CSMA MAC protocol used in network Resource contention when many nodes involved in routing ASCENT Does not detect or repair network partitions Is not suitable when node density is low All nodes required to form effective network Two primary contributions Use of adaptive techniques to configure the underlying network Saves Energy, Extends Network lifetime Use of self-configuring techniques 11/15/2005 Reacts to operating conditions locally 7 ASCENT Design ASCENT adaptively elects “Active” nodes Awake all the time and perform multi-hop packet routing Passive nodes Periodically check if they should become active 11/15/2005 8 ASCENT Design - State Transitions 11/15/2005 9 ASCENT Design - Parameters Tuning NT (Neighbor Threshold) LT (Loss Threshold) Max. amount of data loss that an application can tolerate Application dependent – Set to 20% Tt, Tp – Test Timer, Passive Timer 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 Amount of time a node sleeps to conserve energy Large Ts – Large energy savings but doesn’t react to dynamics 11/15/2005 10 Neighbor and Data Loss Determination Number of active neighbors, Avg. data loss rate Definitions 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 When a sequence number is skipped, loss is detected Final packet loss: Filter constant ρ set to 0.3 11/15/2005 11 Neighbor and Data Loss Determination The number of active neighbors (N) Number of neighbors with link packet loss smaller than the neighbor loss threshold (NLS) NLS = 1- (1/N) 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) 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 11/15/2005 Help, neighbor announcement and routing control 12 Interactions with Routing ASCENT 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) Cause some packet loss Improvement : Traffic could be rerouted in advance by informing the routing protocol of ASCENT’s state changes 11/15/2005 13 Performance Analysis – Goals and Metrics One-Hop Delivery Rate 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 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 11/15/2005 14 Performance Analysis – Goals and Metrics Energy Savings Ratio of energy consumed by Active case to Energy consumed by the ASCENT case Average Per-Hop Latency Measures average delay in packet forwarding in a multihop network Provides estimate of end-to-end delay in packet forwarding 11/15/2005 15 Analytical Performance Analysis Assumptions 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 P (success) = [(S – 1)/S]T Node density increase → P (success) decreases When all nodes can transmit and receive, T = n Since every node in vicinity can transmit Node density increase → P (collisions) increases 11/15/2005 16 Analytical Performance Analysis Average latency per hop related to S and T S = No. of slots T = No. of active nodes Each T node picks a random slot S1, S2…ST Mean = S / 2 11/15/2005 Uniform probability distribution 17 Analytical Performance Analysis 11/15/2005 18 Analytical Performance Analysis P(δ) distribution for different T and S =20 T=n When all nodes can transmit and receive As n ↑, P(δ) ↓ In ASCENT case T = NT 11/15/2005 Independent of n P(δ) remains constant 19 Analytical Performance Analysis Energy Savings 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 1 2 3 11/15/2005 20 Analytical Performance Analysis Energy Savings 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 11/15/2005 21 Analytical Performance Analysis Energy savings of ASCENT with Adaptive timers No asymptotic behavior Energy savings increase linearly with density Slope of line primarily determined by Probability Threshold Pt 11/15/2005 22 Simulation & Experimental Methodology Implementation LinkStats module Neighbor Discovery module 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 11/15/2005 Evaluate Energy Usage Acts as simulated battery 23 Simulation & Experimental Methodology Simulator Experimental Test bed 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 Flooding used as routing protocol for simplicity On receiving a packet, flood module waits for a random time Randomization interval = 5 seconds 11/15/2005 24 Simulation & Experimental Methodology Scenarios and Environment Experiments conducted with different densities ranging from 5 to 40 nodes Density defined topologically 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 One source sends approximately 200 messages Data Rate = 3 messages / minute Nodes do not experience congestion 11/15/2005 25 Simulation Results – Network Capacity No major difference between analytical and simulated performance Active case All nodes join network and forward packets Low delivery rate As node density increases, P (collisions) increases ASCENT case Limits active nodes Channel contention does not increase 11/15/2005 26 Simulation Results – Network Capacity No. of hops = 3 No. of hops = 6 Experiments Simulations Increase in density ASCENT performs better than ACTIVE case Remains stable 11/15/2005 27 Simulation Results – Energy Savings ASCENT provides significant Energy savings As density increases Fixed State Timers Energy savings do not increase proportionally Number of Active nodes remains stable Adaptive State Timers 11/15/2005 Energy savings increase proportionally Passive nodes aggressive 28 Simulation Results – Latency ACTIVE case As density ↑, average perhop latency is reduced Larger probability of a node picking a smaller random interval to forward the packet ASCENT As density ↑, average perhop latency remains stable Number of nodes able to forward packets remains constant 11/15/2005 29 Results – Reaction to Dynamics Evaluate how ASCENT reacts to node failures 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 11/15/2005 30 Results – Sensitivity to Parameters Larger randomization interval average one-hop delivery rate increases Increases end-to-end latency ASCENT outperforms ACTIVE case 11/15/2005 31 Conclusions and Future Work Paper describes design, implementation, analysis, simulation and experimental evaluation of ASCENT ASCENT Has potential to significantly reduce packet loss Increases Energy efficiency Was responsive & stable under varied conditions Future Work Evaluate interactions of ASCENT with MAC Investigate use of load balancing techniques Understand relationships between ASCENT and other routing strategies 11/15/2005 32