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					Wireless Sensor Networks
An overview

Student: Loucas Paraskeva
Course Instructor: Andreas Pitsilides

University of Cyprus
Department of Computer Science
EPL 657 Wireless Networks




                                        EPL 657, UCY, March 21, 2006
Outline
•   Introduction
•   Design Factors
•   Sensors Organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions & References

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Introduction
• Sensor Nodes
     Small, inexpensive, low power, distributed devices.
     A sensor node usually consists of four units:
        Sensing unit.
        Data processing unit.
        Communicating unit.
        Power supply unit.
     Are capable of local processing and wireless
      communication.


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Introduction …
                 Nasa - JPL Sensor Nodes

                       Can be deployed from spacecraft to the
                        surface of another planet, providing scientists
                        here on Earth with a map of that planet's
                        environmental factors, such as trace gases
                        possibly created by microorganisms living
                        below the surface. This capability will help
                        investigators evaluate environmental
                        conditions and determine if life is possible, or
                        has ever been possible, on a planet.

                       May also be used on Earth. Nasa`s sensor
                        nodes provide the opportunity for planetary
                        scientists and biologists to have a continual
                        virtual presence in an area, greatly aiding the
                        study and monitoring of ecosystems.
                        Precision environmental measurements in
                        hard to reach environments, such as steep
                        canyon walls or ocean floors will also be
                        possible.




            UCY EPL 657                                              4
Introduction …
• Sensor Networks
     A sensor network can be described as a collection
      of sensor nodes which coordinate to perform some
      specific action.
     Unlike traditional networks, sensor networks
      depend on dense deployment and coordination to
      carry out their tasks.
     The position of sensor nodes needs not be
      engineered or predetermined (Random
      Deployment).

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Introduction …




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Introduction …




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Introduction …
• Applications of Sensor Networks
     Military: surveillance, intelligence and
      targeting systems.
     Health: monitor patients and assist
      disabled patients.
     Environment: air, soil and water
      monitoring, habitat monitoring, seismic
      detection.

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What next
•   Introduction
•   Design Factors
•   Sensors Organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

                     UCY EPL 657   9
Design Factors
• Environment
      Densely deployed either very close or directly inside the
       phenomenon to be observed.
      Battlefield, home or large building, at the bottom of an
       ocean.
• Transmission Media
      To enable global operation of SNs, the chosen transmission
       media must be available worldwide.
      RF Circuit Design.
      Infrared medium (licence free, cheaper and robust to
       interference transceivers).
      Optical medium (Smart Dust node).


                            UCY EPL 657                            10
Design Factors …
• Fault Tolerance
      Nodes may fail due lack of power or physical damage.
      The SN have the ability to sustain the functionalities without
       any interruption due to sensor nodes failures.
• Scalability
      The sensor network must be able to work with a large
       number of nodes. The density can range from few nodes to
       few hundred sensor nodes in a region (20 nodes/m3).
• Power Consumption
      The wireless sensor node can only be equipped with a
       limited power source.



                            UCY EPL 657                           11
Design Factors …
• Production Costs
     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.
• Hardware Constraints
     All the subunits (sensing, processing, transceiver,
      power) may need to fit into a tiny sized module.
     Low power consumption.


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What next
•   Introduction
•   Design Factors
•   Sensors organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

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Sensors Organization
• Hierarchical clustering
      The sensor nodes are organized into a hierarchy, based on
       their power levels and proximity.

• Attribute based naming
      The sensor nodes are named based on their attributes.
      For example, consider a system which is used to measure
       temperature at a particular location.
      The name [type=temperature, location=N-E,
       temperature=103] refers to all the sensors located at the
       northeast quadrant with a temperature reading of 103F.
      They can reply when a query like "which area has a
       temperature more than 100F" is posed.


                            UCY EPL 657                            14
What next
•   Introduction
•   Design Factors
•   Sensors organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

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Power Saving
• Just turn it off
     Turning the transceiver off during idling
      may not always be efficient due to energy
      spent in turning it back on each time.
• A better way
     Operation in a power saving mode is
      energy efficient only if the time spent in
      that mode is greater than a certain
      threshold.

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What next
•   Introduction
•   Design Factors
•   Sensors organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

                     UCY EPL 657   17
Localization
• Localization Problem
     In sensor networks, nodes are deployed into an
      unplanned infrastructure where there is no a priori
      knowledge of location.
     The problem of estimating spatial-coordinates of
      the node is referred to as localization.
     GPS is not a solution.
        Can work only outdoor.
        Expensive and not suitable in the construction of small
         cheap sensor nodes.
     Localization algorithms use some form of
      Trilateration.

                          UCY EPL 657                          18
Localization …
• Localization Techniques
      Fine – Grained
         Timing: The distance between the receiver node and a reference point
          is determined by the time of flight of the communication signal.
         Signal strength: As a signal propagates, attenuation takes place
          proportional to the distance travelled. This fact is made use of to
          calculate the distance.
      Coarse – Grained
         Proximity based localization
                A node, which has to calculate its position, receives signals from a collection
                 of reference points.
                Once the node receives the signal, it calculates its position as the centroid
                 of the positions of all the reference nodes as:
                 (Xest, Yest) = ( (Xi1+…+Xik)/k, (Yi1+…+Yik)/k) )
                 where Xi1, Yi1 gives the position of the first reference point, Xi2, Yi2 gives
                 the position of the second reference point and so on.




                                     UCY EPL 657                                           19
What next
•   Introduction
•   Design Factors
•   Sensors organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

                     UCY EPL 657   20
Routing Protocols
• Principles
     Power efficiency is always an important
      consideration.
     Sensor networks are mostly data centric.
     An ideal SN has attribute based addressing
      and location awareness.




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Routing Protocols …
• Minimum energy (ME) route
     The route that consumes minimum energy to
      transmit the data packets between the sink and
      the sensor node is preferred.
• Maximum power (PA) route
     The route that has the maximum available power
      is preferred.
• Minimum hop (MH) route
     The route that makes the minimum hop to reach
      the sink is preferred.

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Routing Protocols …
• Flooding
      Old technique that can also be used for routing in sensor
       networks.
      In flooding each node receiving a data or management
       packet repeats it by broadcasting.
      When a maximum number of hops for the packet is reached
       or the destination of the packet is the node itself the
       flooding is terminated.
• Gossiping
      Derivation of flooding
      Nodes do not broadcast but send the incoming packets to a
       randomly selected neighbour.


                           UCY EPL 657                        23
Routing Protocols …
•   Directed Diffusion
       Routing technique in which routes are established when requested.
       A sensing task or interest is propagated throughout the network for named
        data by a node. The querying node is the sink node and it broadcasts its
        interest message periodically to all of its neighbours.
       The interest contains a timestamp and several gradients fields.
       A gradient specifies both the data rate as well as the direction in which the
        events are to be sent.
       All nodes have an interest cache in which each item corresponds to a
        different interest.
       When a node receives an interest, it checks its interest cache to check if it
        has entry.
           It creates one if there is no matching interest and a single gradient field is
            created towards the neighbour from which the interest is received.
           If the interest exists, the timestamp and the duration fields are updated in the
            entry.
           A gradient is removed from its interest entry when it expires.
        Data which matches this interest is then sent towards this node through the
         interest's gradient path (chooses the best path to destination).



                                       UCY EPL 657                                           24
Routing Protocols …
• Directed Diffusion example

    Source




                                             Sink

    Interest = Interrogation
    Gradient = Who is interested

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Routing Protocols …
• Directed Diffusion example


        Source




                                             Sink

        Low rate event
        Reinforcement = Increased interest

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Routing Protocols …
• Low Energy Adaptive Clustering Hierarchy (LEACH)
      Is a clustering based protocol that minimizes
       energy dissipation in SNs.
      Randomly selects sensor nodes as clusterheads so
       the high energy dissipation in communicating with
       the base station is spread to all sensor nodes in
       the SN.
      Operation in two phases, setup and steady phase.




                        UCY EPL 657                   27
Routing Protocols …
• Leach Setup Phase
      A sensor node chooses a random number x between 0 and 1.
      If x < T(n) threshold, the sensor node is clusterhead.
      Advertise to all sensor nodes that are the new clusterheads.
      Sensor nodes determine the cluster to which they want to belong
       based on signal strength (advertisment).
      Sensor nodes inform the appropriate clusterheads that they will be
       a member of the cluster.
      Clusterheads assign to their sensor nodes the data send time
       (TDMA approach).
• Leach Steady Phase
      Sensor nodes can begin sensing and transmitting data to the
       clusterheads.
      After a certain period of time spent on the steady phase, the
       network goes into setup phase again.


                              UCY EPL 657                              28
What next
•   Introduction
•   Design Factors
•   Sensors organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

                     UCY EPL 657   29
Overload Response
• A network is overloaded when the incoming traffic is outside the
  feasibility region determined by the network topology.
• In such a situation nodes will be overloaded.
• It is important to maintain a balanced network overload, while
  ensuring that maximum amount of traffic reaches the sink
  nodes.
      A recent paper by Leonidas Georgiadis & Leandros Tassiulas
       showed that Polynomial time algorithms for determining a
       superflow (α generalized notion of flow, where the aggregate
       incoming flow in a node may exceed the outgoing), exist.
      Those algorithms do not lead to distributed adaptive policies which
       are of main concern when ad-hoc and sensor networks are
       considered.




                              UCY EPL 657                              30
What next
•   Introduction
•   Design Factors
•   Sensors organization
•   Power Saving
•   Localization
•   Routing Protocols
•   Overload Response
•   Security
•   Conclusions

                     UCY EPL 657   31
Security
• How can we provide security sensor networks?
• Security is not easy, compared with conventional
  desktop computers.
• Sensors have limited processing power, storage,
  bandwidth, and energy.
• In the future, wireless sensor networks will be used
  for emergency and life-critical systems.
• Serious privacy questions arise if third parties can
  read or tamper with sensor data.



                       UCY EPL 657                   32
Security …
• Requirements for Sensor Network Security
      Data Confidentiality
         A sensor network should not leak sensor readings to
          neighbouring networks.
      Data Authentication
         Message authentication is important for many applications in
          sensor networks.
      Data Integrity
         In communication, data integrity ensures the receiver that the
          received data is not altered in transit by an adversary.
      Data Freshness
         Informally data freshness implies that the data is recent, and it
          ensures that no adversary replayed old messages.


                              UCY EPL 657                               33
Security …
• Proposal
     SPINS Security Building Blocks (University
      of California, Berkeley)
        SNEP
             Provides data confidentiality, two-party data
              authentication, integrity, and freshness.
        TESLA
             Provides authentication for data broadcast.




                           UCY EPL 657                        34
Conclusions & References
• Sensor networks will create many new and exiting application
  areas for remote sensing because of their special
  characteristics.
• In the future this wide range of application areas will make
  sensor networks an integral part of our lives.
• Further work is necessary in the areas of media access control,
  security and privacy.
• A taste from the future

• References:
    [1]   Sensor Networks an Overview
    [2]   A Survey on Sensor Networks
    [3]   Most Balanced Overload Response in Sensor Networks
    [4]   SPINS: Security Protocols for Sensor Networks



                              UCY EPL 657                        35

				
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