Fall 2006
Introduction to Introduction to Wireless Sensor NetworkWireless Sensor Network
Part 2Part 2
Choong Seon HongChoong Seon Hong
Kyung Hee UniversityKyung Hee University [email protected]@khu.ac.kr
2Fall 2006
Sensor Networks ArchitectureSensor Networks Architecture
Sensor node Made up of four basic components
• Sensing unit, Processing unit, Transceiver unit, and Power unit
Additional application-dependent components• Location finding system, power generator, and mobilizer
Scattered in a sensor field Collect data and route data back to the sink
Sink Communicate with the task manager node (user) v
ia Internet or satellite
3Fall 2006
Hardware ConstraintsHardware Constraints
Location finding system Mobilizer
Power Unit
Sensor ADCProcessorStorage
Transceiver
Sensing Unit
Processing Unit
Power generator
Optional part
Components of a Sensor Node
4Fall 2006
Energy Consumption in Each Sensor NodeEnergy Consumption in Each Sensor Node
Each sensor node has limited energy supplyNodes may not be rechargeableEnergy consumption in
Sensing Data processing Communication (most energy intensive)
5Fall 2006
Sensor Network Protocol StackSensor Network Protocol Stack
Transport
Data Link
Physical
Network
Pow
er
Managem
ent
Application
Mobility
Managem
ent
Task M
anagem
ent
Power Management – How the sensor uses its power, e.g. turns off its circuitry after receiving a message.
Mobility Management – Detects and register the movements of the sensor nodes
Task Management – Balances and schedules the sensing tasks given to a specific region
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Physical LayerPhysical Layer
Physical
Data Link
Network
Transport
Application
Frequency selection – The use of the industrial, scientific, and medical (ISM) bands has been often proposed
Carrier frequency generation and Signal detection – Depend on the transceiver and hardware design constraints which aim for simplicity, low power consumption, and low cost per unit
Modulation Binary and M-ary modulation schemes can transmit multiple bits per
symbol at the expense of complex circuitry Binary modulation schemes are simpler to implement and thus deemed
to be more energy-efficient for WSN applications
Low transmission power and simple transceiver circuitry make Ultra Wideband (UWB) an attractive candidate Baseband transmission, i.e. no intermediate or carrier frequencies Generally uses pulse position modulation Resilient to multipath Low transmission power and simple transceiver circuitry
7Fall 2006
Physical LayerPhysical Layer
Radio Model – Energy Consumption
Energy consumption minimization is of paramount importance when designing the physical layer for WSN in addition to the usual effects such as scattering, shadowing, reflection, diffraction, multipath, and fading.
)(),(),( mEdmEdmE RTL
),()(),( dmEmEdmE TATCT ETC = energy used by the transmitter circuitryETA = energy required by the transmitter amplifier to achieve an acceptable
signal to noise ratio or at the receiver
8Fall 2006
Physical LayerPhysical Layer
Assuming a linear relationship for the energy spent per bit by the transmitter
and receiver circuitry
deemdmE TATCT ),( RCR memE )(
eTC, eTA, and eRC are hardware dependent parameters
An explicit expression for energy consumption in the AMP can be derived as,
))()((
4))()(( 0
bitampant
Rxr
TA RG
BWNNFN
S
e
9Fall 2006
Physical LayerPhysical Layer
(S/N)r = minimum required signal to noise ratio at the receiver’s demodulator for an acceptable Eb/N0
NFrx = receiver noise figureN0 = thermal noise floor in a 1 Hertz bandwidth (Watts/Hz)BW = channel noise bandwidthλ = wavelength in metersα = path loss exponent whose value varies from 2 (for free space)
to 4 (for multipath channel models)Gant = antenna gainηamp = transmitter power efficiencyRbit = raw bit rate in bits per second
rTA N
Se
))()((
4))()(( 0
bitampant
Rx
RG
BWNNF
10Fall 2006
Data Link LayerData Link Layer
Medium Access Control (MAC) – Let multiple radios share the same communication media
Functions: Local Topology Discovery and Management Media Partition by Allocation or Contention Provide Logical Channels to Upper Layers
Physical
Data Link
Network
Transport
ApplicationThe data link layer is responsible for
•Multiplexing of data stream•Data frame detection•Medium access and error control•Ensures reliable point-to-point and point-to-multipoint connections in a communication network
MAC protocol for sensor network must have built-in power conservation mechanisms, and strategies for the proper management of node mobility or failure
11Fall 2006
Wireless MAC ProtocolsWireless MAC ProtocolsWireless MAC protocols can be classified into two categories, distributed and centralized, according of the type of network architecture for which they have been designed. Protocols can be further classified, based on the mode of operation, into random access protocols, guaranteed access protocols, and hybrid access protocols
Wireless MAC protocols
DistributedMAC protocols
CentralizedMAC protocols
Randomaccess
Randomaccess
Guaranteedaccess
Hybridaccess
Since it is desirable to turn off the radio as much as possible in order to conserve energy some type of TDMA mechanism is often suggested for WSN applications. Constant listening times and adaptive rate control schemes have also been proposed.
12Fall 2006
Network LayerNetwork Layer
Physical
Data Link
Network
Transport
ApplicationBasic issues to take into account when designing the network layer for a WSN are:
Power efficiency Data centric – The nature of the data (interest requests
and advertisement of sensed data) determines the traffic flow
Data aggregation is useful to manage the potential implosion of traffic because of the data centric routing
Rather than conventional node addresses an ideal sensor network uses attribute-based addressing, e.g. “region where humidity is below 5%”
Locationing systems, i.e. ability for the nodes to establish position information
Internetworking with external networks via gateway or proxy nodes
13Fall 2006
RoutingRouting
Multihop routing common due to limited transmission range
Phenomenonbeing sensed
Sink
Low node mobility Power aware Irregular topology MAC aware Limited buffer space
Some routing issues in WSNs
Data aggregationtakes place here
14Fall 2006
Data AggregationData Aggregation
It is a technique used to solve the problem of implosion in WSNs.
This problem arises when packets carrying the same information arrive a node.
This situation can happen when more than one node sense the same phenomenon.
This is different than the problem of “duplicate packets” in conventional ad hoc networks.
Here it is the high level interpretation of the data in the packets is what determines if the packets are the “same.”
Even for the case when the packets are deemed to be different they could still be aggregated into a single packet before the relaying process continues.
Data coming from multiple sensor nodes are aggregated, if they have about the same attributes of the phenomenon being sensed, when they reach a common routing or relaying node on their way to the sink. In this view the routing mechanism in a sensor network can be considered as a form of reverse multicast tree.
Phenomenon being sensed
15Fall 2006
Data CentralityData Centrality
In data-centric routing, an “interest ” dissemination is performed in order to assign the sensing tasks to the sensor nodes. This dissemination can take different forms such as:
The sink or controlling nodes broadcast the nature of the interest, e.g. “four legged animals of at least 50 Kg in weight”
Sink
Four-legged animal of at least 50 Kg
Flow of the request
16Fall 2006
Data CentralityData Centrality Sensor nodes broadcast an advertisement of available sensed data and wait for a
request from the interested sinks
Sink
Flow of the advertisement
Tiger, tiger, burning bright,In the forest of the night,
What immortal hand or eyeCould frame thy fearful symmetry?
17Fall 2006
Transport LayerTransport Layer
Physical
Data Link
Network
Transport
ApplicationTCP variants developed for the traditional wireless networks are not suitable for WSNs where the notion of end-to-end reliability has to be reinterpreted due to the “sensor” nature of the network which comes with features such as:
Multiple senders, the sensors, and one destination, the sink, which creates a reverse multicast type of data flow
For the same event there is high level of the redundancy or correlation in the data collected by the sensors and thus there is no need for end-to-end reliability between individual sensors and the sink but instead between the event and the sinkOn the other hand there is need of end-to-end reliability between the sink and individual nodes for situations such as re-tasking or reprogrammingThe protocols developed should be energy aware and simple enough to be implemented in the low-end type of hardware and software of many WSN applications
18Fall 2006
Application LayerApplication Layer
Physical
Data Link
Network
Transport
ApplicationThere has not been a lot of development on this layer for WSNs. Some potential applications have been suggested as listed below but little work of substance has been reported on any particular area.
Authentication, key distribution, and other security tasks Sensor movement management
Sensor Management Protocol (SMP) – Carries out tasks such as: Turning sensors on and off Exchanging data related to the location finding algorithms
Interest Dissemination – Interest is send to a sensor or a group of sensors. The interest is expressed in terms of an attribute or a triggering event.
Advertisement of Sensed Data – Sensor nodes advertise sensed data in a concise and descriptive way and users reply with requests of data they are interested in receiving
19Fall 2006
Technical ChallengesTechnical Challenges
Performance metrics Energy efficiency/system lifetime: The sensors are battery
operated, sensor energy must be wisely managed in order to extend the lifetime of the network
Latency: Many sensor applications require delay-guaranteed service. Protocols must ensure that sensed data will be delivered to the users within a certain delay.
Accuracy: Obtaining accurate information is the primary objectives; accuracy can be improved though joint detection and estimation
Fault tolerance: Robustness to sensor and link failures must be achieved through redundancy and collaborative processing and communication
20Fall 2006
Technical Challenges (cont’d)Technical Challenges (cont’d)
Scalability: Because a sensor network may contain thousand of nodes, scalability is a critical factor that guarantees that the network performance does not significantly degrade as the network size (or node density) increases
Transport capacity/throughput: Because most sensor data must be delivered to a single base station or fusion center, a critical area in the sensor network exists, whose sensor nodes must relay the data generated by virtually all nodes in the network
BSBSCritical RegionCritical Region
21Fall 2006
Technical Challenges (Cont’d)Technical Challenges (Cont’d)
Power Supply The most difficult constraints in the design of WSN When miniaturizing the node, the energy density of the
power supply is the primary issue Current technology yields batteries with approximately 1
J/mm3, while capacitors can achieve 1mJ/mm3
Sensor acquisition can be achieved at 1nJ/sample and modern processors can perform computations as low as 1nJ/instruction
In WSN, where sensor sampling, processing, data transmission and possibly actuation are involved, the trade-off between these tasks plays important role in power usage. Balancing these parameters will be the focus of the design process of WSN
22Fall 2006
Technical Challenges (cont’d)Technical Challenges (cont’d)
Design of energy-efficient protocols: Clustering is an efficient way to save energy for
static sensor networks• Data compression can be applied to reduce the number
of packet• Data-centric property makes and identity• Randomize rotation of cluster heads helps ensure a
balanced energy consumption Broadcast and multicast trees can be used to
increase energy efficiency• Communication pattern many to one (when data from
sources to sink) and one to many (when query from sink to sources). So, multicast advantages offers less benefit
The exploration of sleep modes
23Fall 2006
Technical Challenges (cont’d)Technical Challenges (cont’d)
Capacity/Throughput Throughput is a traditional measure of how much
traffic can be delivered by the network In packet network, the throughput is defined as the
expected number of successful packet transmissions of a given node per time slot
Throughput is related to transmission rate of each transmitter, which in turn, is upper bounded by the channel capacity
Considering node density, congestion, channel condition achieving good throughput is necessary for energy constrained sensor network
24Fall 2006
Technical Challenges (Cont’d)Technical Challenges (Cont’d)
Channel Accessing and Scheduling Scheduling is studied at two levels
• System level: At system level, a scheme determines which nodes will be transmitting.
– System level scheduling essentially a medium access (MAC) problem, with the goal of minimum collisions and maximum spatial reuse
• Node level: At the node level, a scheduler determines which flow among all multiplexing flows will be eligible to transmit next
Current scheduling algorithms aim at improved fairness, delay, robustness (with respect to network topology changes) and energy efficiency
The scheduling algorithm and routing protocols must aim at energy and delay balancing, ensuring
• Packets originating close and far away from the base station experience a comparable delay and
• Critical nodes do not die prematurely due to the heavy traffic
25Fall 2006
Technical Challenges (Cont’d)Technical Challenges (Cont’d)
Energy Consumption Considering energy consumption, four basic states
can be defined: Transmission, reception, listening and sleeping. They consists of the following tasks:
• Acquisition: A/D conversion and preprocessing • Transmission: Address determination, packetization,
encoding, framing and queuing• Reception: similar activities like transmission• Listening: same as reception except that the signal
processing chain stops at the detection• Sleeping: power supply to stay alive
26Fall 2006
Technical Challenges (cont’d)Technical Challenges (cont’d)
Connectivity: Network connectivity is an important issue
because it is crucial for most applications that the network is not partitioned into disjoint parts
Node redundancy should be in such a rate that frequent node failure must keep the network connected
Protocols practicing periodic sleep must ensure connectivity among active node for reliable data transfer from sources to sink
27Fall 2006
Technical Challenges (Cont’d)Technical Challenges (Cont’d)
Node Distribution and MobilitySecurityDistributed Signal ProcessingSynchronization and LocalizationWireless ProgrammingWireless Link Modeling
28Fall 2006
Cross Layer DesignCross Layer Design
Avoid Conflicting Behavior – For example a routing protocol that favors smaller hops to save transmission energy consumption does require a proper MAC protocol to coordinate the transmissions along the data flow that minimizes contention and keeps the transceivers off as much as possible
Remove Unnecessary Layers – Some applications do not require all layers
New Paradigm – WSNs does not have many of the feature of the conventional networks for which the OSI protocol layer stack model has proven to be successful. Therefore it is quite possible that a different mix of layers might prove to be more efficient for many WSN applications
Motivations:
29Fall 2006
Sensor Network Management IssuesSensor Network Management Issues
WSN is a tool for distributed sensing of one or more phenomenon that reports the sensed data to one or more observers
Wireless network services for observers as well as for itself Sensor nodes execute a common goal in a collaborative way A managed WSN is responsible for configuring and
reconfiguring under varying conditions Energy is a critical resource in WSNs. Thus all operations
performed in the network should be energy efficient Topology is dynamic because sensor nodes become out of
service temporarily or permanently. In this scenario failure in sensor network is a common fact and needed to be managed efficiently
WSN must detect, identify and protect itself against various types of attacks to maintain overall systems security and integrity
30Fall 2006
Sensor Network Management Issues (cont’d)Sensor Network Management Issues (cont’d)
An well managed sensor network must know its environment and the context surrounding its activities and act accordingly
WSN must be autonomic, i.e., self managed and robust to changes in network states while maintaining the quality of service
It must be capable of self-configuration, self-organization, self-healing and self-optimized
Finally management of WSN is concerned with how the management can promote plant and resource productivity, and
How it integrates functions of configuration, operation, administration and maintenance of all elements and services in an efficient way
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Dimensions for WSN ManagementDimensions for WSN Management WSN Functionalities
Configuration Maintenance Sensing Processing Communication
Functional Areas Configuration
Management Fault Management Performance
Management Security
Management Accounting
Management
Management Levels Business
Management Service Management Network Management Network Element
Management Network Element
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Management LevelsManagement Levels
Business Management Development and determination of cost functions Cost function is associated with network setup,
sensing, processing, communication and maintenance
WSN applications have enormous potential benefits for society as a whole and represent new business opportunities
In the future, it is expected to have Internet end-points equipped with a variety of sensors to monitor the network and their own state
33Fall 2006
Management Levels (cont’d)Management Levels (cont’d)
Service Management Quality of service: QoS support to WSN includes
• QoS model: It specifies the architecture in which some of the services can be provided in WSN
• QoS sensing: QoS sensing considers the sensor device calibration, environment interference monitoring and exposure (time, distance, and angle between sensor device and phenomenon)
• QoS dissemination: Handled in two layers– QoS routing
– QoS medium access control
• QoS processing: Processing quality depends on the robustness and complexity of the algorithms used, as well as processor and memory capacities
34Fall 2006
Management Levels (Cont’d)Management Levels (Cont’d)
Network Management Network Element Management
• Power Management• Mobility Management (in case of mobile sink)• State Management (States: operational, administrative and
usage)• Task Management (Schedules the sensing, processing and
dissemination) Network Element
• Power Supply– Linear Model– Dependent Model– Relaxation Model
• Computational Model• Sensor Element• Transceiver• Software
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WSN FunctionalitiesWSN Functionalities
Configuration: The configuration functionality is related to the following: Definition of WSN application requirements Determination of the monitoring area (shape and
dimension) Characteristics of environment Choice of nodes Definition of the WSN type Service provided
36Fall 2006
WSN Functionalities (Cont’d)WSN Functionalities (Cont’d)
Sensing: Lowest level of sensing application is provided by the
autonomous sensor nodes Sensing functionality depends on the type of the
phenomenon Sensing can be classified into
• Continuous (when sensors collect data continuously along the time)
• Reactive (when they answer to an observer’s query)
• Periodic (when nodes collects data according to conditions defined by the application)
Redundancy (overlapping sensing coverage) should be utilized in such a way that fault tolerance in the communication network is avoided and better accuracy can be found
37Fall 2006
WSN Functionalities (Cont’d)WSN Functionalities (Cont’d)
Processing Memory and processor of a sensor node form the
computational module It is a programmable unit that provides
computation and storage for other nodes in the system
The computational module performs • Basic signal processing• Dispatches the data according to the application
Processing also involves data aggregation Other tasks are security processing and data
compression
38Fall 2006
WSN Functionalities (Cont’d)WSN Functionalities (Cont’d)
Communication Two types of communication cost
• Infrastructure: refers to the communication needed to configure, maintain and optimize operation
• Application: relates to the transfer of sensed data
The communication approach can be classified as:• Flooding • Gossiping• Bargaining• Unicast• Multicast
39Fall 2006
MANNA – An Integrating ArchitectureMANNA – An Integrating Architecture
MANNA architecture was proposed to provide a management solution to different WSN application
It provides a separation between both sets of functionalities, i.e. Application and management Making integration of organizational, administrative and
maintenance activities possible for this kind of network
The architecture also provides some automatic services, which feature Self-managing self-sustaining self-diagnostic, with a minimum of human interference
40Fall 2006
MANNA (Cont’d)MANNA (Cont’d)
Figure represents scheme to construct management
To determine management service the scheme uses different WSN model
A management service can use one or more management functions
Different services can use common functions that use models to retrieve a network state concerning a given aspect
WSN model WSN model
Function 1 Function 2
Service Y
Function 3
Uses UsesUses
Service X
Uses Uses Uses Uses
41Fall 2006
MANNA (Cont’d)MANNA (Cont’d)
Partial List of Management Functions Environmental monitoring functions Monitored area definition function Coverage area supervision function Node deployment definition function Environmental requirement acquisition function Network operating parameters configuration function Topology map discovery function Network connectivity discovery function Aggregation function Data fusion function Node density control function Management operation schedule function Energy map generation function Energy level discovery function Node localization discovery function Node mobile function
42Fall 2006
MANNA (Cont’d)MANNA (Cont’d)
Functional architecture WSN manager
• Depends on type or network• In centrally managed network, a single manager collects
information from all agents and control the entire network• Distributed managed network has several managers,
each responsible for a sub-network and communicating with other managers
• In hierarchically managed network, intermediate managers distribute the management tasks
WSN Agent
43Fall 2006
MANNA (Cont’d)MANNA (Cont’d)
Manager and agent location in flat WSN Agents inside the
network and external manager
Agent in the sink node Agents and manager
in the network• Hierarchical
organization
• Distributed organization
Manager Sink
Sensor report responses notifications
Manager Sink
Sensor report responses notifications
Manager Sink
Manager Sink Agent Ordinary node
Sensor report responses notifications
Manager Sink
Sensor report responses notifications
44Fall 2006
MANNA (Cont’d)MANNA (Cont’d)
Manager and agent location in hierarchical WSN Agents in the cluster head and external manager Agent in the base station
ManagerBase
Station
Sensor report responses notifications
ManagerBase
Station
Sensor report responses notifications
ManagerAgent Cluster HeadBase
StationOrdinary node
45Fall 2006
MANNA (Cont’d)MANNA (Cont’d)
Manager and agent location in hierarchical WSN Agents in the network and intermediate manager Agents and distributed managers in the network
ManagerBase
Station
Sensor report responses notifications
ManagerAgent Cluster HeadBase
StationOrdinary node
ManagerBase
Station
Sensor report responses notifications
46Fall 2006
Concluding WordsConcluding Words
Wireless Sensor Networks provide a fundamentally new set of research and application challenges
WSNs are a rich source of problems in communication protocols, sensor tasking and control, sensor fusion, distributed data bases, probabilistic reasoning, and algorithmic design
Few more issues in sensor network are: Deployment strategies Node localization Network capacity Fault tolerance Security
47Fall 2006
Example of Some Sensor NodesExample of Some Sensor Nodes
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ReferencesReferences
“Handbook of Sensor Networks: Compact Wireless and Wired Sensing Systems, CRC PRESS, 2005
“I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci, “A survey on sensor networks,” IEEE Communications Magazine, 2002.
Tilak, S., Abu-Ghazaleh, N.B., Heinzelman, W.: A taxonomy of wireless micro-sensor network models. ACM Mobile Computing and Communications Review (MC2R) 6 (2002) 28–36
L.B. Ruiz, J.M.S. Nogueria and A.A.F Loureiro, “MANNA: a management architecture for wireless sensor network”, IEEE communications magazine, 41(2) 116-125, February 2003
www.google.com
49Fall 2006
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