Title: Wireless Sensor and Actor Networks
1- Chapter 14
- Wireless Sensor and Actor Networks
2Wireless Sensor and Actor Networks I.F. Akyildiz
and I. H. Kasimoglu,Wireless Sensor and Actor
Networks Research Challenges Ad Hoc Networks
Journal (Elsevier), pp.351-367, Oct. 2004.
Task Manager Node
Sink
Sensor/Actor Field
Actors
Sensors
3Actuators vs. ActorsWhy do we call them actors?
- Actuator (Texas Instruments Technical Glossary)
- An actuator is a device to convert an electrical
control signal to a physical action. Actuators
may be used for flow-control valves, pumps,
positioning drives, motors, switches, relays and
meters. - The mobility of a robot may be enabled by several
actuators (motors, servo-mechanisms, etc) - However, the robot represents one single network
entity which we refer to as actor - Hence, one actor can be endowed with multiple
actuators
4Wireless Sensor and Actor Networks
- Sensors
- Passive elements sensing from the environment
- Limited energy, processing and communication
capabilities - Actors
- Active elements acting on the environment
- Higher processing and communication capabilities
- Less constrained energy resources (Longer battery
life or constant power source)
5 Sub-Kilogram Intelligent Tele-robots (SKITs)
Networked Robots having Coordination Wireless
Communication Capabilities
6Robotic Mule Autonomous Battlefield Robot
designed for the Army
7Mini-Robot (developed at Sandia National
Laboratories)
8Helicopter Platform
9Components of Sensor Actor Nodes
Sensing Unit
Transceiver
Processor Storage
ADC
Sensor Node
Power Unit
Controller (Decision Unit)
Processor Storage
Actuation Unit
Transceiver
DAC
Actor Node
Power Unit
10Integrated Sensor Actor Nodes
11WSAN Applications
- Environmental Applications
- Detecting and extinguishing forest fire.
- Microclimate control in buildings
- In case of very high or low temperature values,
trigger the audio alarm actors in that area. - Distributed Robotics Sensor Network
- (Mobile) robots dispersed throughout a sensor
network alarm actors in that area.
12WSAN Applications
- Parking
- Airport Safety
- City Maintenance
- Sewage and Contamination Control
- Battlefield Applications
- Sensors detect mines or explosive substances
- Actors annihilate them or function as tanks
13WSANs vs. Wireless Sensor Networks
- Real-Time Requirements for Timely Actions
- Rapidly respond to sensor input (e.g., fire
application) - To perform right actions, sensor data must be
valid at the time of acting - Heterogeneous Node Deployment
- Sensors
- Actors
Densely deployed
Loosely deployed due to the different coverage
requirements and physical interaction methods of
acting task
14WSANs vs. Wireless Sensor Networks
- Coordination Requirements
- Sensor-Actor Coordination
- Actor-Actor Coordination
15WSAN Communication Architecture Semi-Automated
Architecture
Sink
- Sensors ? Sink ? Actors
- Requires manual intervention at sink
- No sensor-actor and actor-actor coordination
needed - Similar to the conventional WSN architecture
Event Area
16WSAN Communication Architecture Automated
Architecture
Sink
Event Area
- Sensors ? Actors
- No intervention from sink is necessary
- Localized information exchange
- Low latency
- Distributed sensor-actor and actor-actor
coordination required
17SENSOR-ACTOR COORDINATION
- Challenges
- Which sensor(s) communicate with which actor(s)
(Single or Multiple Actors) - How should the communication occur? (i.e.,
single-hop or multi-hop) - What are the requirements of the communication
(i.e., real-time, energy efficiency)
18Sensor-Actor Coordination
- Which sensor(s) communicate with which actor(s)?
- CASE 1
- Minimum number of sensors to report the sensed
event - CASE 2
- Minimum set of actors to cover the event region
- Both cases above
- The entire set of sensors and actors in the
vicinity of the region - The set of actors whose acting regions do not
overlap
19Sensor-Actor CoordinationSINGLE ACTOR
Sensor
Actor
Event Area
- Selection of the most appropriate actor
- To select, sensors need to coordinate with each
other
20 Sensor-Actor Coordination SINGLE ACTOR
- Selecting a single actor node may be based on
- The distance between the event area and the actor
- The energy consumption of the path from sensors
to the actor - The action range of the actor
21Sensor-Actor CoordinationMULTI ACTORS
Sensor
Actor
Event Area
- Sensors only need to coordinate with sensors
within some neighborhood to form clusters or
groups
22Sensor-Actor Coordination MULTI ACTORS
- Clusters may be formed such a way that
- The event transmission time from sensors to
actors is minimized - The events from sensors to actors are transmitted
through the minimum energy paths - The action regions can cover the entire event area
23ACTOR-ACTOR COORDINATION
- Challenges
- Which actor(s) should execute which action(s)?
- How should multi-actor task allocation be done?
24Actor-Actor Coordination
- Single-Actor Task vs. Multi-Actor Task
- Single-Actor Task
- How is the single actor selected?
- Multi-Actor Task
- What is the optimum number of actors performing
actions? - Selection of most fit actors among the capable
actors for that task - Only a subset of actors covering the entire event
region may perform the task to save action energy
25A Distributed Coordination Framework for WSANsT.
Melodia, D. Pompili, V. C. Gungor, I. F.
Akyildiz, ACM MOBIHOC05, May 2005. Also in IEEE
Transactions on Mobile Computing, 2007.
- Comprehensive framework for coordination problems
- SENSOR-ACTOR COORDINATION
- Optimal Event-driven Clustering
- A Distributed Scalable Protocol
- ACTOR-ACTOR COORDINATION
- Optimal Solution
- Real-time Localized Auction
26Coordination Requirements
- Sensor-Actor Coordination
- Establish data paths between sensors and actors
- Meet energy efficiency and real time requirements
- Actor-Actor Coordination
- Decision Does an action need to be performed?
- Which action should be performed?
- How to share the workload among actors?
27Sensor-Actor Coordination
- Objectives
- Establish data paths between sensors and actors
- Meet energy efficiency and real-time requirements
- Question
- To which actor does each sensor send its data?
- Solution
- Event Driven Clustering with Multiple Actors
28Event-Driven Clustering with Multiple Actors
Event Area
1. Event Occurs 2. Sensor-Actor Coordination
Event-Driven Clustering
What is the optimal clustering
strategy?Distributed algorithm?
29Reliability
- Definition 1.
- The latency bound B is the maximum allowed time
between sampling of the physical features of the
event and the moment when the actor receives a
data packet describing these event features
30Reliability
- Definition 2
- A data packet is EXPIRED (UNRELIABLE), if it does
not meet the latency bound B - Definition 3
- A data packet is UNEXPIRED (RELIABLE), if it is
received within the latency bound B
31Reliability
- Definition 4
- The event reliability r is the ratio of reliable
data packets over all packets received in a
decision interval - Definition 5
- The event reliability threshold rth is the
minimum event reliability required by the
application - OBJECTIVE
- Comply with the event reliability threshold
(rgtrth) with minimum energy expenditure!
32Event-Driven Clustering with Multiple Actors
- Objective
- Find the optimal strategy for event-driven
clustering (To which actors is data sent? Which
paths are used?) - ? a joint Clustering and Routing problem
33Event-Driven Clustering with Multiple Actors
- Requirements of the Optimal Solution
- Provide reliability above the event reliability
threshold (rgtrth) - Minimize overall Energy Consumption
- Optimal solution obtained by ? Integer Linear
Programming formulation
34Event-Driven Clustering with Multiple Actors
- ILP Formulation is provided -gt allows finding the
optimal solution - BUT NP-Complete problem
- Not scalable (lt100 nodes)
- Centralized solution
- Helps gaining insight in the properties of the
optimal solution - Performance benchmark for distributed, more
scalable solutions
35A Distributed Protocol
- Find the optimal working point of the network,
i.e. - rgtrth (? reliability over the threshold)
- Minimum energy consumption
- Based on the feedbacks from actors
- Actor calculates reliability r and broadcasts its
value to the sensors
36A Distributed Protocol
- If the reliability r is complied with (rgtrth), a
certain portion of the sensors switch in the
aggregation state to save energy (lower energy
consumption, higher delay) - Equilibrium is reached when reliability threshold
is met (r rth) with minimum energy consumption.
37A Distributed Protocol
- BASIC IDEA
- When the event is first sensed, sensors all begin
in the start-up state and establish data paths to
the actors - If reliability is advertised to be low (rltrth)
- Certain portion of the sensors switch to speed-up
state, which shortens the end-to-end paths (lower
delay, higher energy consumption)
38A Distributed Protocol
- Sensors probabilistically switch among three
different states according to feedback from the
actors - Start-up State
- Quickly establish a data path from each source to
one actor - Compromise between energy consumption and latency
39A Distributed Protocol
- Speed-up State
- Reduce the number of hops in sensor-actor paths
so as to reduce the end-to-end delay - Obtained by sending packets to far neighbors
(closer to the destination actor)
40A Distributed Protocol
- Aggregation state
- Reduce the overall energy consumption when
compliant with event reliability - Send packets to closer neighbors (higher number
of hops)
41Example Path Establishment
nodes establish paths (start-up state)
idlestart-up state
an event occurs
Another actor is too far away and thus not
energy efficient for any of the nodes in the
event area
42Example Low Reliability
Some sensors switch to the speed-up state
(probabilistically) and select as next hop the
closest node to the actor ? reduce latency
The actor advertises low reliability (rltrth)
idlestart-up statespeed-up state
43Example High Reliability
Some sensors switch to the aggregation state
(probabilistically) and select as next hop the
closest node already in the da-tree ? reduce
energy consumption
The actor advertises high reliability (rgtrth)
idlestart-up statespeed-up
stateaggregation state
44Actor-Actor Coordination
- Objective
- Selecting the best actor(s) in terms of action
completion time and energy consumption so as to
perform the action! - Challenges
- Which actor(s) should execute which action(s)?
- How should multi-actor task allocation be done?
45Actor-Actor Coordination Model
- DEFINITIONs
- Overlapping Area
- Area can be acted upon by multiple actors
- Non-Overlapping Area
- Area can be acted upon only by one actor
46Actor-Actor Coordination Model
- Action Completion Time Bound
- The maximum allowed time from the moment when the
event is sensed to the moment when the action is
completed - Power Levels
- Discrete levels of power for performing the
action ? A higher power level corresponds to a
lower action completion time!
47Actor-Actor Coordination Problems
- For an Overlapping Area, actor-actor coordination
problem - Selecting a subset of actors
- Adjusting action power levels ? Maximize the
residual energy and complete the action within
the action completion bound
48Actor-Actor Coordination Problems
- For a Non-Overlapping Area, actor-actor
coordination problem - Adjusting action power levels ? Maximize the
residual energy
49Actor-Actor Coordination
- Optimal Solution
- Actor-actor coordination problem formulated as a
Residual Energy Maximization Problem using Mixed
Integer Non-Linear Programming (MINLP) - Distributed Solution
- Real-Time Localized Auction-Based Mechanism
50Real-Time Localized Auction-Based Mechanism
- Inspired by the behaviors of agents in a Market
Economy ? Interactions between buyers and sellers - Possible Roles of the Actors
- Seller Actor receiving the event features
- Auctioneer Actor in charge of conducting the
auction - Buyer Actor(s) that can act on a particular
overlapping area
51Real-Time Localized Auction-Based Mechanism
- For overlapping areas
- Seller selects one auctioneer for each
overlapping area, i.e., the closest actor to the
center of the overlapping area ? Energy spent for
auction and auction time reduced! - Seller informs each auctioneer about the auction
area and the action time bound
52Real-Time Localized Auction-Based Mechanism
- Auctioneer determines the winners of the auction
based on the bids received from the buyers. - Bids consists of available energy, power level
and action completion time
53Real-Time Localized Auction-Based Mechanism
- The auctioneer finds the winners by calculating
the optimal solution of the Residual Energy
Maximization Problem - For Non-Overlapping areas
- The corresponding actor is directly assigned the
action task
54Sensor-Actor Coordination
Start-up (speed-up) configuration all nodes are
in the start-up (speed-up) state
Comparison between the optimal solution of the
event-driven clustering problem and the energy
consumption of start-up, speed-up, aggregation
configuration with varying event ranges (60
sensors 4 actors)
55Sensor-Actor Coordination
Comparison of the energy consumption of
different configurations The energy
consumption in the aggregation configuration is
much lower that in the start-up and speed-up
configuration
56Sensor-Actor Coordination
Comparison of average number of hops for
start-up and speed-up configuration. The
speed-up configuration shows paths with lower
delay (less hops)
57Cyber Physical Systems
- Integration of computation with physical
processes. Embedded computers and networks
monitor and control physical processes in
feedback loops where physical processes affect
computations and vice versa. - CPS will blend sensing, actuation,
computation, networking, and physical processes
as action networks. - "Networked Information Technology Systems
Connected With The Physical World", also referred
to as cyber-physical systems, are cited as the
top technical priority for networking and IT
research and development. - President's Council of Advisors on Science and
Technology