WIRELESS SENSOR
NETWORKS
Speaker :
Advisor :
Date:
Hsuan-Ling Weng
Dr. Kai-Wei Ke
2015/03/11
Outline
 Introduction
 Sensor Networks Communication Architecture
 Design Factors
 Routing Protocols in WSN
 Conclusion
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Introduction
 Recent advances in wireless communications and electronics have enabled the development of
low-cost, low-power, multifunctional sensor nodes that are small in size and communicate
untethered in short distances.
 These tiny sensor nodes, which consist of sensing, data processing, and communicating
components, leverage the idea of sensor networks. Sensor networks represent a significant
improvement over traditional sensors.
 The sensor networks can be used for various application areas (e.g., health, military, home).
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Sensor Networks Communication Architecture
 The sensor nodes are usually scattered in a sensor field. Each of these scattered sensor nodes
has the capabilities to collect data and route data back to the sink. Data are routed back to the
sink by a multihop infrastructureless architecture through the sink. The sink may communicate
with the task manager node via Internet or satellite.
 The design of the sensor network is influenced by many factors, including fault tolerance,
scalability, production costs, operating environment, sensor network topology, hardware
constraints, transmission media, and power consumption.
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Design Factors - Fault Tolerance & Scalability
 Fault Tolerance — Some sensor nodes may fail or be blocked due to lack of power, or have
physical damage or environmental interference. The failure of sensor nodes should not affect
the overall task of the sensor network. This is the reliability or fault tolerance issue. Fault
tolerance is the ability to sustain sensor network functionalities without any interruption due to
sensor node failures.
 Scalability — The number of sensor nodes deployed in studying a phenomenon may be on the
order of hundreds or thousands. Depending on the application, the number may reach an
extreme value of millions. New schemes must be able to work with this number of nodes. They
must also utilize the high density of the sensor networks. The density can range from few
sensor nodes to few hundred sensor nodes in a region, which can be less than 10 m in diameter.
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Design Factors - Production Costs
 Production Costs — Since sensor networks consist of a large number of sensor nodes, the cost
of a single node is very important to justify the overall cost of the network. If the cost of the
network is more expensive than deploying traditional sensors, the sensor network is not costjustified. As a result, the cost of each sensor node has to be kept low.
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Design Factors - Hardware Constraints
 A sensor node is made up of four basic components,: a sensing unit, a processing unit, a
transceiver unit, and a power unit.
 Sensing units are usually composed of two subunits: sensors and analog-to-digital
converters(ADCs). The analog signals produced by the sensors based on the observed
phenomenon are converted to digital signals by the ADC, and then fed into the processing unit.
 The processing unit, which is generally associated with a small storage unit, manages the
procedures that make the sensor node collaborate with the other nodes to carry out the assigned
sensing tasks.
 A transceiver unit connects the node to the network.
 One of the most important components of a sensor node is the power unit. Power units may be
supported by power scavenging units such as solar cells.
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Design Factors - Hardware Constraints
 They may also have additional application-dependent components such as a location finding
system, power generator, and mobilizer.
 Most of the sensor network outing techniques and sensing tasks require knowledge of location
with high accuracy. Thus, it is common that a sensor node has a location finding system.
 A mobilizer may sometimes be needed to move sensor nodes when it is required to carry out
the assigned tasks.
 All of these subunits may need to fit into a matchbox-sized module. The required size may be
smaller than even a cubic centimeter, which is light enough to remain suspended in the air.
Apart from size, there are some other stringent constraints for sensor nodes. These nodes must
consume extremely low power, operate in high volumetric densities, have low production cost,
be dispensable and autonomous, operate unattended, and be adaptive to the environment.
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Design Factors - Sensor Network Topology
 We examine issues related to topology maintenance and change in three phases:
• Predeployment and deployment phase : Sensor nodes can be either thrown in as a mass or placed
one by one in the sensor field. They can be deployed by dropping from a plane, delivered in an
artillery shell, rocket, or missile, and placed one by one by either a human or a robot.
• Post-deployment phase : After deployment, topology changes are due to change in sensor nodes’
position, reachability (due to jamming, noise, moving obstacles, etc.), available energy,
malfunctioning, and task details.
• Redeployment of additional nodes phase : Additional sensor nodes can be redeployed at any
time to replace malfunctioning nodes or due to changes in task dynamics.
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Design Factors - Environment
 Environment — Sensor nodes are densely deployed either very close or directly inside the
phenomenon to be observed. Therefore, they usually work unattended in remote geographic
areas. They may be working in the interior of large machinery, at the bottom of an ocean, in a
biologically or chemically contaminated field, in a battlefield beyond the enemy lines, and in a
home or large building.
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Design Factors - Transmission Media
 Transmission Media — In a multihop sensor network, communicating nodes are linked by a
wireless medium. These links can be formed by radio, infrared, or optical media. To enable
global operation of these networks, the chosen transmission medium must be available
worldwide.
 Much of the current hardware for sensor nodes is based on RF circuit design. The μAMPS
wireless sensor node uses a Bluetooth-compatible 2.4 GHz transceiver with an integrated
frequency synthesizer. The low-power sensor device uses a single-channel RF transceiver
operating at 916 MHz. The Wireless Integrated Network Sensors (WINS) architecture also uses
radio links for communication.
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Design Factors - Transmission Media
 Another possible mode of internode communication in sensor networks is by infrared. Infrared
communication is license-free and robust to interference from electrical devices. Infrared-based
transceivers are cheaper and easier to build. Another interesting development is that of the
Smart Dust mote, which is an autonomous sensing, computing, and communication system that
uses the optical medium for transmission. Both infrared and optical require a line of sight
between the sender and receiver.
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Design Factors - Power Consumption
 Power Consumption — The wireless sensor node, being a microelectronic device, can only be
equipped with a limited power source (< 0.5 Ah, 1.2 V). In some application scenarios,
replenishment of power resources might be impossible. Sensor node lifetime, therefore, shows a
strong dependence on battery lifetime. In a multihop ad hoc sensor network, each node plays
the dual role of data originator and data router. The malfunctioning of a few nodes can cause
significant topological changes and might require rerouting of packets and reorganization of the
network. Hence, power conservation and power management take on additional importance. It
is for these reasons that researchers are currently focusing on the design of power-aware
protocols and algorithms for sensor networks.
 The main task of a sensor node in a sensor field is to detect events, perform quick local data
processing, and then transmit the data. Power consumption can hence be divided into three
domains: sensing, communication, and data processing.
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Routing Protocols in WSN
 Location-based Protocols:-MECN, SMECN, GAF, GEAR, Span, TBF, BVGF, GeRaF
 Data-centric Protocols: - SPIN, Directed Diffusion, Rumor Routing, COUGAR, ACQUIRE,
EAD
 Hierarchical Protocols:-LEACH, PEGASIS, HEED, TEEN, APTEEN
 Multipath-based Protocols: - Disjoint Paths, Braided paths, N-to-1 Multipath Discovery
 QoS-based protocols: - SAR, SPEED, Energy-aware routing
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Location-based Protocols
 Sensor nodes are addressed by means of their locations. Location information for sensor nodes
is required for sensor networks by most of the routing protocols to calculate the distance
between two particular nodes so that energy consumption can be estimated.
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Location-based Protocols - MECN
 Minimum energy communication network (MECN) maintains a minimum energy network for
wireless networks by utilizing low power GPS.
 The main idea of MECN is to find a sub-network, which will have less number of nodes and
require less power for transmission between any two particular nodes. In this way, global
minimum power paths are found without considering all the nodes in the network. This is
performed using a localized search for each node considering its relay region.
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Data Centric Protocols
 When the source sensors send their data to the sink, intermediate sensors can perform some
form of aggregation on the data originating from multiple source sensors and send the
aggregated data toward the sink. This process can result in energy savings because of less
transmission required to send the data from the sources to the sink.
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Data Centric Protocols - SPIN
 SPIN(Sensor Protocols for Information via Negotiation) is a three-stage protocol as sensor
nodes use three types of messages, ADV, REQ, and DATA, to communicate. ADV is used to
advertise new data, REQ to request data, and DATA is the actual message itself.
 The protocol starts when a SPIN node obtains new data it is willing to share. It does so by
broadcasting an ADV message containing metadata. If a neighbor is interested in the data, it
sends a REQ message for the DATA and the DATA is sent to this neighbor node. The neighbor
sensor node then repeats this process with its neighbors. As a result, the entire sensor area will
receive a copy of the data.
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Hierarchical Protocols
 Clustering is an energy-efficient communication protocol that can be used by the sensors to
report their sensed data to the sink.
 The main aim of hierarchical routing is to efficiently maintain the energy consumption of
sensor nodes by involving them in multi-hop communication within a particular cluster and by
performing data aggregation and fusion in order to decrease the number of transmitted
messages to the sink.
 Cluster formation is typically based on the energy reserve of sensors and sensor’s proximity to
the cluster head.
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Hierarchical Protocols - LEACH
 Low-energy adaptive clustering hierarchy (LEACH) is one of the most popular hierarchical
routing protocol for wireless sensor networks. In LEACH , formation of clusters of the sensor
nodes are done on the received signal strength.
 LEACH uses local cluster heads as routers to the sink. The transmission of data is done only
through these cluster heads rather than all the sensor nodes in the network. This will save
energy as only cluster heads are responsible for transmission of data towards sink.
 These cluster heads change randomly over time depending on energy dissipation of the sensor
nodes. This decision is made by the node choosing a random number between 0 and 1. The
node becomes a cluster head for the current round if the number is less than the threshold given
below:
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Hierarchical Protocols - LEACH
 Where p is the desired percentage of cluster heads (e.g. 0.05), r is the current round, and G is
the set of nodes that have not been cluster heads in the last 1/p rounds.
 LEACH is completely distributed and it does not require global knowledge of network.
LEACH uses single-hop routing where each node can transmit directly to the cluster-head and
the sink. Therefore, it is not applicable networks which are deployed in large regions.
Furthermore, the idea of dynamic clustering brings extra overhead, e.g. cluster head changes,
advertisements etc., which may cause the increase in energy consumption.
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Multipath-based Protocols
 Considering data transmission between source sensors and the sink, there are two routing
paradigms: single-path routing and multipath routing. In single-path routing, each source sensor
sends its data to the sink via the shortest path. In multipath routing, each source sensor finds the
first k shortest paths to the sink and divides its load evenly among these paths.
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Multipath-based Protocols - Disjoint Paths
 Protocol that helps finds a small number of alternate paths that have no sensor in common with
each other and with the primary path. In sensor disjoint path routing, the primary path is best
available whereas the alternate paths are less desirable as they have longer latency. The disjoint
makes those alternate paths independent of the primary path.
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Network flow and QoS-aware protocols
 Although most of the routing protocols proposed for sensor networks fit our classification,
some pursue somewhat different approach such as network flow and QoS. In some approaches,
route setup is modeled and solved as a network flow problem. QoS-aware protocols consider
end to end delay requirements while setting up the paths in the sensor network.
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Network flow and QoS-aware protocols - SAR
 Sequential Assignment Routing (SAR) is the first routing protocol which concentrates more on
the energy efficiency and QOS factors. It creates multiple paths from the nodes to the sink to
help in achieving a more energy efficient structure. It also maximizes the fault tolerance of the
network.
 The SAR protocol creates trees rooted at one-hop neighbors of the sink while considering QoS
metric, energy resource on each path and priority level of each packet. These created trees are
used to find multiple paths from sink to sensors. While selecting one of the paths among these
multiple paths, energy resources and QoS on the path is considered. Routing table consistency
between downstream and upstream on each path is enforced for failure recovery.
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Network flow and QoS-aware protocols - SAR
 Any local failure causes an automatic path restoration procedure locally. SAR offers less power
consumption than the minimum-energy metric algorithm, which focuses only the energy
consumption of each packet without considering its priority. SAR ensures fault-tolerance and
easy recovery but the protocol suffers from the overhead of maintaining the tables and states at
each sensor node especially when the number of nodes is huge.
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Conclusion
Routing Protocols
Advantage
Drawback
Location-based Protocols
◆可隨時掌握每個節點的地理位 ◆GPS 定位系統容易消耗節點
置
大量的電力
Data-centric Protocols
◆繞徑較為簡單
◆容易發生廣播風暴的問題
◆不需要維護節點狀態資訊，所 ◆容易發生Overlap
以較少複雜的繞徑計算
◆容易造成特定節點快速消耗電
力
Hierarchical Protocols
◆平均分攤節點的繞徑工作，減
少特定節點電力大量消耗
◆降低發生廣播風暴的機會
◆當節點數量多時，涵蓋範圍較
大
Network flow and QoSaware protocols
◆可達到點對點傳輸的品質保證 ◆沒有考慮多條路徑傳輸壽命
◆可以防止網路壅塞
◆計算繞徑複雜度較高
◆減少點對點傳輸的延遲時間
◆容易造成叢集頭負載過重
◆當階層愈多時，訊息傳遞可能
發生延遲
◆容易造成負載不平衡
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References
 [1] I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cyirci,“A Survey on Sensor
Networks”, IEEE Communications Magazine, vol. 40, no. 8, Aug. 2002, pp.102 -114
 [2] Deepak Goyal, and Malay Ranjan Tripathy, “Routing Protocols in Wireless Sensor
Networks: A Survey”, Advanced Computing & Communication Technologies (ACCT), 2012
Second International Conference on, pp.474-480, 7-8 Jan. 2012.
 [3] Savita Lonare, and Gayatri Wahane “A Survey on Energy Efficient Routing Protocols in
Wireless Sensor Network”, Computing, Communications and Networking Technologies
(ICCCNT),2013 Fourth International Conference on, pp.1-5, 4-6 July 2013.
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