On the Lifetime of Wireless Sensor Networks Isabel Dietrich Falko Dressler ACM Transaction on Sensor Networks, 2009 Presented by Group 6 Tianyi Pan

Why Limited Lifetime for WSNs? Various application domains Wildlife monitoring Precision agriculture Logistics applications Environmental conditions

Outline of the Paper Analysis of existing lifetime definitions Overview of the parameters influencing network lifetime Concise redefinition of network lifetime Application of the definition

Existing Lifetime Definitions Based on the number of alive nodes. Based on sensor coverage. Based on connectivity. Based on application quality of services requirements Based on combined measures

Lifetime Based on the Number of Alive Nodes n-of-n lifetime 𝑇 𝑛 𝑛 = min 𝑣∈𝑉 𝑇 𝑣 Normally too short and neglects redundancy Not capable with hardware failures k-of-n lifetime 𝑇 𝑛 𝑘 : at least k out of n nodes are alive Cannot cope with critical nodes m-in-k-of-n: at least k out of n nodes alive and none of the m critical nodes fail The time till the first cluster head fail Require static clusters, which is unrealistic Time till all nodes fail Too optimistic

Lifetime Based on Sensor Coverage Time till certain coverage requirement cannot be met Area/volume coverage Target coverage Barrier coverage Detect all trespassing activities in a region 𝛼-coverage Only 𝛼 percent of the region of interest must be covered (by at least 1 sensor) k-coverage Each point of interest in the region must be covered by at least k sensors.

Lifetime Based on Connectivity The minimum time that the size of the largest connected component drops to a certain size. The minimum time that the percentage of nodes having paths to the sink drops to a certain value. Total number of packets transmitted to the sink. Dependent on algorithms (variation in # packets) Cannot clearly indicate the lifetime Number of successful data gathering trips

Lifetime Based on Application Quality of Services Requirements The time period during which the WSN continuously satisfies the application requirement Too abstract, application characteristics unspecified The time until the network no longer provides an acceptable event detection ratio A specific application requirement yet still vague

Lifetime Based on Combined Measures Coverage & Connectivity The time when either drops below a threshold Cardinality of the largest connected component, number of alive nodes, 𝛼-coverage The time when any of the three is unsatisfied Share the issues with single aspect Can omit some aspects

Overview of the Parameters Node mobility and topology changes Heterogeneity Application characteristics Quality of service

Node Mobility and Topology Changes Mobility happens in all WSNs Node failure in static WSN (similar to mobility) Sensors moved by external forces Mobility can complicate analysis of lifetime Connection can be lost and then recover System “die” and resurrect Mobility can improve sensor coverage Act as relay Mobile sinks

Heterogeneity Varying battery power and power consumption Cluster head may have much larger energy reservoir Some nodes may need to send more data Some location may need to be covered by different types of sensor Transmission ranges can vary Different mobility patterns

Application Characteristics Sensors carry different tasks of an application Need to work in synergy Destination for data packets can affect communication patterns (with multiple sinks) Node activity level Activity can be triggered by: Events Regular time intervals Other nodes

Quality of Service Coverage Event detection ratio Exposure Connectivity (availability, latency, loss) Requirements on continuous service Observation accuracy Main QoS parameters for WSNs

The Concise Definition of Lifetime Basic notations Set of sensor types Y Set of all sensor nodes 𝑆 𝑌 , 𝑆 𝑌 =𝑛 Set of alive nodes Set of active nodes Set of active nodes (relaxed) Set of sink nodes 𝐵 𝑡 ⊂ 𝑆 𝑌

Basic Notations Communication graph 𝐺 𝑡 =(𝑉 𝑡 ,𝐸(𝑡)) Ability of two nodes to communicate in the time interval [𝑡−Δ𝑡,𝑡] Set of target points Sensing area for type 𝑦

Relax the Hard Constraints Graceful degradation Quality drops when hit a soft limit, before hitting the hard limit No “sudden death” 𝜁≥1 means perfect fulfilment 𝜁<0 means no fulfilment In between indicates the service level Time-integrated criteria Requirement can be fulfilled (at least once) within time intervals, not at every time point.

The Criteria Alive nodes: the portion of alive nodes in all existing nodes 𝜓 𝑙𝑛 𝑡 = 𝑢 𝑡 𝑛 Latency: the portion of packets having shorter delay that pre-specified max latency 𝑙 Delivery ratio: portion of packets received correctly

The Criteria Connectivity Indicator for node 𝑣 having connection to any active sink node in 𝐵(𝑡) The portion of active nodes having a connection to a sink

The Criteria Area coverage: fraction of region covered by type-y sensors Target coverage: fraction of targets covered by type-y sensors 𝒌-coverage: single read may be inaccurate

The Criteria Barrier coverage Detect and track all intruder in the region Sensor must cover all possible paths the intruder may take

The Criteria Connected coverage Required in need of transmitting the data of any point of interest to a sink at any time Coverage only possible via the sensor nodes that are connected to a sink

Network Lifetime System up/down times Service disruption Each working interval Accumulated network lifetime 𝒁 𝒂 = 𝒊=𝟎 𝒆 𝒕 𝒊 𝒂 Total network lifetime 𝒁 𝒕 = 𝒕 𝒆 Allowed disruption

Mapping of Existing Definitions

Mapping of Applications

Mapping of Applications Habitat monitoring Sensors need to have connectivity to the sink at least once per time interval (a day) Allow outages for some time (a day) Not critical, not coverage

Mapping of Applications Intrusion detection Connected barrier coverage Short delay and limited packet loss No service disruption Critical and coverage

Mapping of Applications Smart buildings Intermittent coverage of the whole area Send data to sink once every time interval Short disruption allowed Coverage, not critical

Mapping of Applications Human physiological data Need continuous connection to the sink Short delay and limited packet loss’ No service disruption Critical but not coverage

Questions?