Time Synchronization Issues In Wireless Sensor Network

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Time Synchronization Issues In Wireless Sensor
Network
Meeyoung Cha (mycha_at_cosmos)Advanced Network
LabCS Dept, KAIST
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Outline

• What is WSN?
• Currently-deployed Motes
• Characteristics of WSN
• Tiered Architecture
• Routing In-network Processing
• Automatic Localization
• Storage, Search, Retrieval
• Actuation
• Time Synchronization
• Summary
• References

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A glimpse of the sensor-networked world

• Imagine
• high-rise buildings self-detect structural faults
  (e.g., weld cracks)
• schools detect airborn toxins at low
  concentrations, trace contaminant transport to
  source
• alert swimmers to dangerous bacterial levels
• earthquake-rubbled building infiltrated with
  robots and sensors locate survivors, evaluate
  structural damage
• ecosystems infused with chemical, physical,
  acoustic, image sensors to track global change
  parameters
• battlefield sprinkled with sensors that identify
  track friendly/foe air, ground vehicles,
  personnel

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Characteristics of Sensor Network

• low-cost, rapid deployment, self-organizing, and
  fault tolerance
• Application areas heath, military, and home.
• Large number of sensor nodes that are densely
  deployed.
• The links can be formed by radio, infrared, or
  optical media.
• Nodes use their processing abilities to locally
  carry out simple computations and transmit the
  required and partially processed data.
• Ad hoc networks are not suitable for the sensor
  networks because of their unique features and
  application requirements.

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Proposed Algorithm
encoding bit size , resource constraint
initialization
transition probability graph, codeword graph
greedy mapping
reduction
satisfy resource?
mapping
mapping table
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Currently-deployed Motes

• COTS (Commercial-Off-The-Shelf) Specs.
• 4 MHz, 8 bit MCU (Amtel), 512 bytes RAM, 8K ROM
• 900 MHz radio (RF Monolithics) 10-100 ft. range
• DOT25fx6mm, MICA55x32x25 mm
• DC 3V, 315 MHz, TinyOS, 1.8mA

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Tiered Network
Research Directions

• Many densely distributed micro nodes
• Fewer sparsely distributed macro nodes
• Each micro node clusters with the closest macro
  node.

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Routing In-network Processing
Research Directions

• One example parameter visiting pattern

Directed-diffusion
Source node
Intermediate node with data
Sink node
Intermediate node without data
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Routing In-network Processing
Research Directions

• Internet, ad-hoc wireless network
• Focus on building the network a transport
  mechanism
• (A way to route packets to a particular
  endpoint)
• Above VS. Wireless sensor network
• For efficiency, do as much in-network processing
  as possible
• (Aggregating similar data, filtering redundant
  information, ..)
• Ex) report the location where the following bird
  call is heard
• -gt report only time and location of a match

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Directed Diffusion
Research Directions

• Basic idea
• Data-centric routing
• name data (not nodes) with externally relevant
  attributes data type, time, location of node,
  SNR.
• Nodes (sinks) request data, sends interests
  to the network, data gathered at source node
  matching interests flow toward sinks.
• diffuse requests and responses across network
  using application driven routing (e.g., geo
  sensitive or not)
• support in-network aggregation and processing
• Data sources publish data, data clients subscribe
  to data
• however, all nodes may play both roles
• node that aggregates/combines/processes incoming
  sensor node data becomes a source of new data
• true peer to peer system?

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Automatic Localization
Research Directions

• Goal Detect a stationary phenomenon P
• (P water table that has been polluted within a
  field of chemical sensors)
• Time Location
• Size, shape, speed,
• Capability of localizing themselves after being
  deployed.
• GPS (Global Positioning System) provides solution
  where available (with differential GPS providing
  finer granularity)
• GPS not always available, too costly, too bulky
• other approaches under stud

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Storage Wired Sensor Network
Research Directions

• Warehouse
• Data extracted from sensors, stored on server
• Query processing takes place on server later

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Storage Wireless Sensor Network
Research Directions

• Sensor Database System
• Standard database assumptions no longer hold
• Resource constraints
• Store data locally and query on-demand from users
• Storage space -gt in-network processing
• supports distributed query processing over sensor
  network

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Actuation
Research Directions

• No longer a passive system
• 1. Enhance the sensing task pointing cameras,
  repositioning
• 2. Affect the environment open valves, emit
  sound
• Sensor Mobility
• Contaminated soil region is determined with more
  precision

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Time Synchronization
Research Directions

• Please wait.

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An Autonomic Routing Framework for Sensor Networks
TTDD
Diffusion
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Whats so different with ad-hoc?

• The number of sensor nodes
• Sensor nodes are densely deployed
• Sensor nodes are prone to failures
• The topology of a sensor network changes very
  frequently
• Broadcast communication vs. point-to-point
  communication
• Sensor nodes are limited in power, computational
  capacities, and memory
• Sensor nodes may not have global identification
  (ID)

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Why Cant Adopt Internet Protocols?

• Internet routes data using IP Addresses in
  Packets and Lookup tables in routers
• humans get data by naming data to a search
  engine
• many levels of indirection between name and IP
  address
• embedded, energy-constrained (un-tethered,
  small-form-factor), unattended systems cant
  tolerate communication overhead of indirection
• special purpose system function(s) dont need
  want Internet general purpose functionality
  designed for elastic applications.