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Sensor Networks

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Title: Sensor Networks


1
Sensor Networks
2
Outline
  • Introduction and Issues
  • Architecture and Applications
  • Localization
  • Routing and Intercommunication
  • MAC Layer Issues
  • Security Issues

3
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

4
Sensor Network Architecture
SINK
Internet/ Satellite
TASK MANAGER
5
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
    support

6
Architecture of a Sensor Node
Software
Processor
Trans- ceiver
Sensing Unit
A/D
Memory
Battery Power
7
Applications of Sensor Networks
  • Surveillance and security
  • Environmental monitoring
  • Transport monitoring
  • Precision agriculture
  • Smart spaces
  • Manufacturing and inventory control
  • Other specialized tasks

8
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
    applications
  • Mobility management is a big concern in cellular
    networks and MANETs.
  • Nodes can be identified by their assigning IP
    addresses.

9
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

10
Operational Challenges
  • Ad hoc deployment should be able to discover the
    topology and self-configure for
    intercommunication
  • Dynamically adapt to changes in topology due to
    node failures and environmental conditions
  • Automatic configuration/reconfiguration
  • Untethered for energy and communication

11
Localization Issues
  • What is Localization?
  • Why is it important?
  • Categorization
  • Some Localization Mechanisms
  • GPS
  • Beacon based ranging
  • Range free methods

12
What is Localization?
  • A mechanism for discovering spatial relationships
    between objects

13
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

14
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

15
Fine-Grained Localization
  • Ranging based methods
  • Timing
  • Signal Strength
  • Directionality Based
  • Ranging free methods
  • E.g. Centroid based, DV-hop, APIT

16
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

17
Timing
  • Time of flight of communication signal
  • Signal Pattern
  • Global Positioning System
  • Local Positioning System
  • Pinpoints 3D-iD
  • Different modalities of communication
  • Active Bat

18
Signal Strength
  • Attenuation of radio signal increases with
    increasing distance
  • RADAR
  • Wall Attenuation Factor based Signal Propagation
    Model
  • RF mapping

19
Directionality Based Fine-Grained Localization
  • Small Aperture Direction Finding
  • Used in cellular networks
  • Requires complex antenna array
  • Disadvantages
  • Costly
  • Not a receiver based approach

20
Basic Concepts in Ranging
  • Trilateration
  • Triangulation
  • Multi-lateration
  • Considers all available beacons

Sines Rule
B
b
A
C
a
c
Cosines Rule
21
Localization Mechanisms
  • GPS
  • Beacon based ranging
  • Range free methods

22
Global Positioning System (GPS) Getting93
  • Started in 1973, built in 1993
  • Wide-area radio positioning system
  • Ranging-based method
  • Using Timing of Arrival (ToA)

23
GPS System Architecture
  • Constellation of 24 NAVSTAR satellites made by
    Rockwell
  • Altitude 10,900 nautical miles
  • Orbital Period 12 hours
  • At least five satellites in view from every point
    in the Globe

24
How GPS Works
  • The basis of GPS is trilateration" from
    satellites
  • 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
    atmosphere
  • A Fourth satellite used for correction purpose

25
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
    measurements
  • such differential corrections are transmitted
    in real time over a FM radio link

26
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

27
Beacon Based Location Discovery Savvides01
28
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

29
Localization Approach
  • Single hop beaconing
  • Iterative multilateration
  • Dynamic estimate the wireless channel parameters
  • Can be done in conjunction with routing

30
Iterative Multilateration
  • Start with a small number of beacons
  • Number of beacons increases as more nodes
    estimate their positions

31
Advantages
  • Data packets also act as beacon signals
  • Location discovery is almost free
  • Distributed
  • relies on neighborhood information
  • Fault tolerant

However, Ranging still requires expensive
circuits!
32
Range Free Methods
  • Centroid approach Bulusu00
  • Adaptive beacon placement Bulusu01
  • Self-configuring localization Bulusu03
  • DV-hop Niculescu01
  • AoA approach Niculescu03
  • APIT He03

33
Centroid Based Approach Bulusu00
  • Multiple nodes serve as reference points
    (Beacons)
  • 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

34
Model
35
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

36
Impact of Beacon Placement
Beacons randomly placed LARGER mean granularity
37
Impact of Propagation Vagaries
38
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
    optimal
  • Questions
  • What to measure?
  • How to adapt?

39
Self-configuring Beacon Systems Bulusu02
  • Three schemes
  • GRID
  • HEAP
  • STROBE

40
GRID a Centralized Approach
41
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

42
HEAP Illustration
43
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

44
STROBE Illustration
SLEEP state VOTING state DESIGNATED state
45
DV-Hop Niculescu01
  • Standard DV propagation
  • Never measures node distance
  • Insensitive to signal strength errors
  • Basic idea
  • Range hop_count hop_size

46
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
    network
  • Each node
  • Use the correction from the closest landmark
  • Multiply its hop distance by the correction

47
DV-Hop Example
48
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

49
APIT He03
  • Basic idea
  • Point-In-Triangle Test (PIT)
  • three anchors determine a triangle
  • Repeat PIT tests with different anchor
    combination, until accuracy requirement is
    satisfied
  • Calculate center of gravity (COG)

50
APIT Overview
  • Area-based APIT narrow down the area

51
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

52
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)

53
Routing Protocols
  • Flooding
  • Directed Diffusion
  • SPIN
  • Low Energy Adaptive Clustering Hierarchy (LEACH)
  • Rumor Routing

54
Flooding
  • 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
    destination
  • 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)

55
Directed Diffusion Intanagonwiwat00
  • Data-centric routing where sink broadcasts the
    request
  • 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.

56
Directed Diffusion
Event
Event
Event
Source
Source
Source
Sink
Sink
Sink
a) Interest Propagation
c)Data delivery
b) Gradients setup
57
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
    negotiation
  • Nodes monitor and adapt to changes in their own
    energy resources to extend the operating lifetime
    of the system

58
SPIN Protocol
ADV
REQ
DATA
ADV
DATA
REQ
59
LEACH Heinzelman00
  • LEACH is self-organizing, adaptive clustering
    protocol
  • 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
    nodes.
  • LEACH performs local data fusion to compress
    the amount of data being sent from the
    cluster-heads to the base station

60
Dynamic Clusters in LEACH
61
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.

62
Medium Access Control
  • Goals
  • Establish communication links
  • Fair and efficient sharing of communication links
  • Should be energy efficient

63
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
    ranges
  • Organized Channel Access
  • Requires neighbor discovery and synchronization
    among the nodes expensive for sensor networks

64
MAC Protocols
  • Self Organized MAC for Sensor Networks (SMACS)
    and Eavesdrop-And-Register (EAR)
  • CSMA-based MAC
  • Hybrid TDMA/FDMA
  • S-MAC

65
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
    nodes.
  • Neighbor discovery and channel assignment phases
    are combined
  • Nonsynchronous slots in the network and randomly
    chosen frequencies are used for establishing the
    communication links.

66
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

67
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
    adopted.
  • 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
    approach
  • ARC is also extensible for multi-hop environments

68
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

69
S-MAC Ye02
  • Sensor-MAC Inspired by PAMAS, but uses in-band
    signaling
  • Attempt to address all the following sources of
    power inefficiency
  • Collision - due to follow-on transmissions on
    collisions
  • 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)

70
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)

71
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

72
SPINS Perrig02
  • Components
  • SNEP Secure Network Encryption Protocol
  • Data Confidentiality
  • Two party data authentication
  • Integrity
  • Freshness
  • µTESLA
  • Authenticated broadcast

73
SPINS Architecture
  • Base Stations (BS)
  • Trusted
  • Longer lifetime
  • Larger memory
  • Secret key between node and BS
  • Single block cipher implements all cryptographic
    functions
  • RC5 algorithm used
  • Loose time synchronization

74
SNEP
  • 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
    KBA)
  • Counters used at both endpoints to ensure data
    freshness

75
SNEP Encryption
  • Encryption done in counter mode
  • M(K,ctr)

76
SNEP MAC
  • MAC implemented using Cipher Block Chaining
  • MAC(K,x)

77
µTESLA
  • 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

78
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
    F(Ki1)

79
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