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Sensor Networks


Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret ... – PowerPoint PPT presentation

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Title: Sensor Networks

Sensor Networks
  • Introduction and Issues
  • Architecture and Applications
  • Localization
  • Routing and Intercommunication
  • MAC Layer Issues
  • Security Issues

What are Sensor Networks?
  • A group of wireless nodes or embedded devices
  • Collectively the nodes sense, collect, and
    analyze data
  • The sensor nodes are small, low power,
    inexpensive, and have high SNR.
  • The network usually consists of a large number of
    dense nodes distributed at random
  • They can be deployed in any kind of terrain with
    hostile environment places where traditional
    wired network cannot be deployed

Sensor Network Architecture
Internet/ Satellite
Sensor Node Architecture
  • Sensor nodes include a combination of
  • Microelectromechanical systems (MEMS) such as
    sensing devices, actuators, RF components, and
    CMOS building blocks
  • Low power computing and wireless networking

Architecture of a Sensor Node
Trans- ceiver
Sensing Unit
Battery Power
Applications of Sensor Networks
  • Surveillance and security
  • Environmental monitoring
  • Transport monitoring
  • Precision agriculture
  • Smart spaces
  • Manufacturing and inventory control
  • Other specialized tasks

Different from cellular networks and MANETS?
  • Cellular networks and MANETs are designed to
    provide good throughput/delay characteristics
  • Cellular networks and MANETs are supposed to
    facilitate high-bandwidth QoS-sensitive
  • Mobility management is a big concern in cellular
    networks and MANETs.
  • Nodes can be identified by their assigning IP

Differences - continued
  • Sensor networks consists of thousands of nodes
    designed for unattended operations
  • Traffic in sensor networks in statistical in
    nature requires low data rate
  • Addressing may be attribute based
  • The flow of data in sensor networks is
    predominantly unidirectional
  • Prolonging the battery life is a prime goal the
    batteries are usually not rechargeable

Operational Challenges
  • Ad hoc deployment should be able to discover the
    topology and self-configure for
  • Dynamically adapt to changes in topology due to
    node failures and environmental conditions
  • Automatic configuration/reconfiguration
  • Untethered for energy and communication

Localization Issues
  • What is Localization?
  • Why is it important?
  • Categorization
  • Some Localization Mechanisms
  • GPS
  • Beacon based ranging
  • Range free methods

What is Localization?
  • A mechanism for discovering spatial relationships
    between objects

Why is Localization Important?
  • Sensor Network Data is typically interpreted
    based on a sensors location
  • report event origins
  • giving raw sensor readings a physical context
  • Temperature readings ? temperature map
  • objects tracking
  • Enables data-centric network design
  • assist with routing
  • evaluate network coverage

Categorization Bulusu00
  • Coarse-grained Localization
  • Proximity to a given reference point
  • E.g., Active Badge
  • Fine-grained Localization
  • Coordinates estimation
  • E.g., Distance to a given reference point

Fine-Grained Localization
  • Ranging based methods
  • Timing
  • Signal Strength
  • Directionality Based
  • Ranging free methods
  • E.g. Centroid based, DV-hop, APIT

Ranging (Distance Measuring) Techniques
  • Time based methods
  • Time of Arrival (ToA), TDoA
  • Used with radio, IR, acoustic, ultrasound
  • Signal Strength
  • Uses received signal strength indicator (RSSI)
    readings and wireless propagation model
  • Directionality based
  • Angle of Arrival (AoA) measured with directional
    antennas or arrays

  • Time of flight of communication signal
  • Signal Pattern
  • Global Positioning System
  • Local Positioning System
  • Pinpoints 3D-iD
  • Different modalities of communication
  • Active Bat

Signal Strength
  • Attenuation of radio signal increases with
    increasing distance
  • Wall Attenuation Factor based Signal Propagation
  • RF mapping

Directionality Based Fine-Grained Localization
  • Small Aperture Direction Finding
  • Used in cellular networks
  • Requires complex antenna array
  • Disadvantages
  • Costly
  • Not a receiver based approach

Basic Concepts in Ranging
  • Trilateration
  • Triangulation
  • Multi-lateration
  • Considers all available beacons

Sines Rule
Cosines Rule
Localization Mechanisms
  • GPS
  • Beacon based ranging
  • Range free methods

Global Positioning System (GPS) Getting93
  • Started in 1973, built in 1993
  • Wide-area radio positioning system
  • Ranging-based method
  • Using Timing of Arrival (ToA)

GPS System Architecture
  • Constellation of 24 NAVSTAR satellites made by
  • Altitude 10,900 nautical miles
  • Orbital Period 12 hours
  • At least five satellites in view from every point
    in the Globe

How GPS Works
  • The basis of GPS is trilateration" from
  • Distance measuring based on ToA
  • accurate timing is important
  • Along with distance, you need to know exactly
    where the satellites are in space
  • High orbits and careful monitoring are the secret
  • Finally you must correct for any delays the
    signal experiences as it travels through the
  • A Fourth satellite used for correction purpose

Differential GPS
  • Ground-based Station with known location
    information can estimate the GPS measure errors
  • These error estimations are made available to
    other GPS users in the area
  • allow them to mitigate errors in their
  • such differential corrections are transmitted
    in real time over a FM radio link

GPS Not Always Applicable
  • Many contexts you cannot have GPS on every node
  • form factor
  • energy
  • cost
  • obstructions
  • Beacon based approaches for sensor networks
  • Ranging based v.s. ranging free

Beacon Based Location Discovery Savvides01
Beacon Based Location Discovery
  • No need of GPS
  • No infrastructure support
  • Ad hoc deployable
  • Use RSSI for measuring node separation
  • But how should the beacons be placed?
  • Distributed Localization
  • Iterative multilateration

Localization Approach
  • Single hop beaconing
  • Iterative multilateration
  • Dynamic estimate the wireless channel parameters
  • Can be done in conjunction with routing

Iterative Multilateration
  • Start with a small number of beacons
  • Number of beacons increases as more nodes
    estimate their positions

  • Data packets also act as beacon signals
  • Location discovery is almost free
  • Distributed
  • relies on neighborhood information
  • Fault tolerant

However, Ranging still requires expensive
Range Free Methods
  • Centroid approach Bulusu00
  • Adaptive beacon placement Bulusu01
  • Self-configuring localization Bulusu03
  • DV-hop Niculescu01
  • AoA approach Niculescu03
  • APIT He03

Centroid Based Approach Bulusu00
  • Multiple nodes serve as reference points
  • Reference points transmit periodic beacon signals
    containing their positions
  • Receiver node finds reference points in its range
    and localizes to the intersection of connectivity
    regions of these points

Centroid Based Localization
  • (Xest, Yest) (avg(Xi1Xik), avg(Yi1Yik))
  • k No. of beacon nodes within connectivity range
  • Xi1Xik Yi1Yik Beacon nodes locations
  • Disadvantages
  • Design using a idealized radio model with perfect
    spherical radio propagation
  • Assume a regular grid of nodes with known
    location information to serve as Beacons

Impact of Beacon Placement
Beacons randomly placed LARGER mean granularity
Impact of Propagation Vagaries
Self-configuring Beacon Systems
  • Idea
  • Measure and adapt to unpredictable environment
  • Exploit spatial diversity and density of
    sensor/actuator nodes
  • Assuming large solution space, not seeking global
  • Questions
  • What to measure?
  • How to adapt?

Self-configuring Beacon Systems Bulusu02
  • Three schemes
  • GRID
  • HEAP

GRID a Centralized Approach
HEAP a localized approach
  • Given
  • S set of all beacons reachable in grid
  • E - An error estimation model
  • Determine C - (x , y)
  • Such that cumulative localization error in the
  • grid is minimized by adding beacon at C

HEAP Illustration
STROBE Adaptive Density
  • STROBE Selectively TuRning Off BEacons
  • Goals
  • Conserve energy to extend system lifetime without
    diminishing localization granularity
  • Design Goals
  • Localized algorithms
  • Responsive but low adaptation overhead

STROBE Illustration
DV-Hop Niculescu01
  • Standard DV propagation
  • Never measures node distance
  • Insensitive to signal strength errors
  • Basic idea
  • Range hop_count hop_size

DV-Hop How It Works
  • Each node maintains a table Xi, Yi, hi by
    running classic DV
  • Each Landmark Xi, Yi
  • Compute a correction Ci and flood into the
  • Each node
  • Use the correction from the closest landmark
  • Multiply its hop distance by the correction

DV-Hop Example
DV-Hop Example (contd.)
  • Landmarks compute corrections
  • Assume A gets its correction from L2
  • A estimates its ranges to the landmarks
  • L1 316.42, L2 216.42, L3 316.42
  • A performs trilateration with the above ranges

  • Basic idea
  • Point-In-Triangle Test (PIT)
  • three anchors determine a triangle
  • Repeat PIT tests with different anchor
    combination, until accuracy requirement is
  • Calculate center of gravity (COG)

APIT Overview
  • Area-based APIT narrow down the area

Localization Wrap up
  • Localization is important in sensor networks
  • GPS is useful, but not always applicable
  • Beacons (aka, anchors, landmarks) can help
  • Range based methods
  • Range free methods

Routing in Sensor Networks
  • Multihop Routing with the following constraints
    and features
  • Power efficiency
  • Attribute-based addressing
  • Location awareness
  • Data-centric (communication is for named data)

Routing Protocols
  • Flooding
  • Directed Diffusion
  • SPIN
  • Low Energy Adaptive Clustering Hierarchy (LEACH)
  • Rumor Routing

  • Flooding is the simplest form of routing
  • Each node broadcast the packets to all its
    neighbors and the process repeat until a maximum
    number of hops or the packet reaches its
  • Problems
  • Implosion (multiple copies of messages are sent
    to the same node)
  • Overlap (Neighbor nodes receive duplicate
    messages because of overlap in observing region)
  • Resource Blindness (Unaware of resources, energy)

Directed Diffusion Intanagonwiwat00
  • Data-centric routing where sink broadcasts the
  • The sink sends out requirements in terms of
    attribute-value pairs called as interest
  • This dissemination sets up gradients within the
    network designed to draw events
  • Events start flowing towards the originators of
    interests along multiple paths
  • The sensor network reinforces one or a small
    number of these paths.

Directed Diffusion
a) Interest Propagation
c)Data delivery
b) Gradients setup
SPIN Heinzelman99
  • Sensor Protocols for Information via Negotiation
    (SPIN) uses negotiation and resource adaptation
    to address the deficiencies of flooding
  • Propose a family of routing protocols
  • Conserves energy by exchanging metadata during
  • Nodes monitor and adapt to changes in their own
    energy resources to extend the operating lifetime
    of the system

SPIN Protocol
LEACH Heinzelman00
  • LEACH is self-organizing, adaptive clustering
  • Randomly selects nodes as cluster-heads to
    distribute the energy load evenly
  • High-energy dissipation in communicating with the
    base station is distributed among the sensor
  • LEACH performs local data fusion to compress
    the amount of data being sent from the
    cluster-heads to the base station

Dynamic Clusters in LEACH
Rumor Routing Braginsky02
  • A logical compromise between flooding queries and
    flooding event notifications
  • Upon witnessing an event, a node
    probabilistically generates an agent, which
    travels the network, propagating information
    about local events to distant nodes.
  • A query generated by a node traverses in a random
    direction until a TTL value or when it finds a
    node that has the path to the event
  • A query can be retransmitted or flooding can be
    adopted as a last resort.

Medium Access Control
  • Goals
  • Establish communication links
  • Fair and efficient sharing of communication links
  • Should be energy efficient

MAC Layer Issues
  • MAC protocols used for cellular networks or
    MANETs are not suitable
  • Conventional Types
  • Contention-based Channel Access
  • Requires the radio transceivers to monitor the
    channels at all times expensive for low radio
  • Organized Channel Access
  • Requires neighbor discovery and synchronization
    among the nodes expensive for sensor networks

MAC Protocols
  • Self Organized MAC for Sensor Networks (SMACS)
    and Eavesdrop-And-Register (EAR)
  • CSMA-based MAC
  • Hybrid TDMA/FDMA
  • S-MAC

SMACS Protocol Sohrabi00
  • SMACS is a flat and distributed
    infrastructure-building protocol which enables
    nodes to discover neighbors and establish
    transmission/reception schedules for
    communication without any local or global master
  • Neighbor discovery and channel assignment phases
    are combined
  • Nonsynchronous slots in the network and randomly
    chosen frequencies are used for establishing the
    communication links.

EAR Algorithm
  • EAR algorithm provides continuous service to
    mobile nodes
  • Mobile nodes assume full control of the
    connection process
  • The stationary nodes transmit a pilot signal
    periodically. Mobile nodes eavesdrop and records
    information about the connectivity
  • EAR is transparent to SMACS

CSMA-based MAC Woo01
  • The protocol should support highly correlated and
    dominantly periodic traffic
  • Random delays for transmission and phase shifts
    in the backoffs help in robustness and energy
    conservation in sensor networks
  • An adaptive transmission rate control (ARC) is
  • Medium access fairness is achieved by balancing
    the rates of originating and route-thru traffic
  • A progressive signaling scheme is used to adopt a
    linear increase and multiplicative decrease
  • ARC is also extensible for multi-hop environments

Hybrid TDMA/FDMA Shih01
  • Centrally controlled
  • Pure TDMA dedicates full bandwidth to a single
    sensor node pure FDMA allocates minimum signal
    bandwidth per node
  • The Hybrid TDMA/FDMA scheme optimizes the power
    consumption of the transmitter (TDMA) and the
    receivers (FDMA)
  • The hybrid approach results in lowering the
    overall power consumption of the system

S-MAC Ye02
  • Sensor-MAC Inspired by PAMAS, but uses in-band
  • Attempt to address all the following sources of
    power inefficiency
  • Collision - due to follow-on transmissions on
  • Overhearing - node listens to packets meant for
    another destination
  • Control packet overhead
  • Idle listening - listening to receive possible
    traffic that is not sent (consumes 50- 100 of
    energy spent for receiving)

S-MAC overview
  • Avoid collisions
  • uses CSMA/CA (basic 802.11)
  • Avoid overhearing
  • puts a node to sleep when the neighboring nodes
    are transmitting
  • Control overhead
  • Applies message passing
  • Avoid idle listening
  • periodic listen and sleep (scheduled pattern)

Constraints (SmartDust Node link-1)
  • Limited Computational Power
  • 8 bit, 4 MHz
  • Limited Memory
  • 8 KB Instruction Flash (Tiny OS 3500 bytes)
  • 512 bytes RAM, 512 bytes EEPROM
  • Limited Bandwidth and Range
  • 10 Kbps
  • Hence, Public Key and Key Exchange protocols are
    not suitable

SPINS Perrig02
  • Components
  • SNEP Secure Network Encryption Protocol
  • Data Confidentiality
  • Two party data authentication
  • Integrity
  • Freshness
  • µTESLA
  • Authenticated broadcast

SPINS Architecture
  • Base Stations (BS)
  • Trusted
  • Longer lifetime
  • Larger memory
  • Secret key between node and BS
  • Single block cipher implements all cryptographic
  • RC5 algorithm used
  • Loose time synchronization

  • Endpoints(A,B) share a secret symmetric key ?AB
  • Pseudo random function F used to generate
  • Two keys for encryption (KAB and KBA)
  • Two keys for message authentication (KAB and
  • Counters used at both endpoints to ensure data

SNEP Encryption
  • Encryption done in counter mode
  • M(K,ctr)

  • MAC implemented using Cipher Block Chaining
  • MAC(K,x)

  • Authenticated broadcast by BS using MAC
  • Loose time synchronization required
  • Base station sends messages authenticated by key
    that is secret at that time
  • All messages in a single time slot use same key
  • Key revealed d time slots after use
  • Node stores message until key is disclosed

µTESLA (contd.)
  • One way key chain used by sender
  • Last key Kn chose randomly
  • Keys generated by Ki F(Ki1)
  • F one way function
  • Sender can authenticate key by verifiying Ki

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