Computer Networking Lecture Notes

2014 December 1 • Sensor Networks

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Outline

• Sensor networks
• Communication architecture
• Physical layer
• The data-link layer
• Network layer
• Transport layer
• The application layer

Introduction

• low cost, low power, sensor nodes
• Scatter sensor nodes over the terrain
• Ad hoc sensing and monitoring networks

Characteristics

• orders of magnitude more nodes
• dense deployment
• failure prone nodes
• frequent topology changes
• broadcast transmission
• resource limited (power, cpu, store)
• anonymous nodes

Communication Architecture

• a sensor field channeling data back to a sink
• Design Factors
  • influencing factors
  • Fault tolerance
    • high node failure rate, random and correlated
  • Scalability
    • 10^3 to 10^6 nodes in a small area with no overall organization
  • Production costs
    • ridiculously cheap: < $1/node
  • Hardware constraints
    • sensing, transceiver, computing, power
    • cheap, low power, autonomous, adaptive
  • Sensor network topology
    • dense (20 nodes/m^3) uneven deployment
    • predeployment and deployment - what you want
    • post-deployment - what you got
    • redeployment - adjusting old and adding new nodes
  • Environment
    • generally hostile environment
  • Transmission media
    • wireless, radio, infrared, optical
    • radio, bluetooth (2 GHz) to microwave (high 100 MHz)
    • infrared, optical: cheap, line-of-sight,
  • Power consumption
    • limited power budget (0.5 Ah @ 1.2 V) force power-aware protocols.
    • sense, compute, and transmit draw downs
• Protocol stack
  • physical, data-link, network, transport, application
  • power, mobility (sensor location), and task services

The Physical Layer

• wireless transmission (915 MHz ISM)
• power proportional to d^n 2 <= n <= 4
• simplicity is cheap and ineffective, complexity is effective and expensive

The Data Link Layer

• data stream multiplexing, framing, medium access, error control, point-to-point and point-to-multipoint communication.
• Medium Access Control
  • first build the network, then use it efficiently (and maintain it)
  • Reasons Existing MAC Protocols can't be used
    • other wireless mac
    • cell-phone networks: one-hop, non-redundant, qos obsessed (really)
    • bluetooth, mobile ad hoc networks: one-hop, small, not power limited, semi-mobile, temporal scope
    • larger, more dynamic, power conservative
  • Sensor network MACs
    • fixed allocation, random access, not consensus
  • Self-Organizing Medium Access Control for Sensor Networks (SMACS) and the Eavesdrop-and-Register (EAR) algorithm
    • discover and build a network.
    • continuous service in mobile and stable circumstances
  • CSMA-based medium access
    • listen and back-off
    • adaptive transmission rate control: flow-through vs originating
    • coordinate in the cpu, not over the air
• Hybrid TDMA/FDMA
  • sink relative; use fdma then tdma
  • an overview
• Power saving operating modes
  • going to and from the modes costs energy
• Error control
  • ARQ is useless (retransmissions cost); FEC is computationally expensive
  • Forward error correction
    • BER and link reliability
    • talk louder or say more
    • simple schemes with simple decoding, possibly by special-purpose processors

Network Layer

• organize individual clusters into a complete network
• power efficient, data-centric, attribute-based addressing
• Maximum PA route
  • Route along power-available nodes (not efficient)
• Minimum energy (ME) route
  • cheapest to send
• Minimum hop (MH) route
  • the classic, a proxy for efficiency
• Maximum minimum PA node route
  • break ties using capable nodes
• node control: push or broadcast-response
• addressing nodes with attributes: hot nodes, or shaking nodes
• aggregate data as it moves toward the sink
• interconnect with other networks
• Small minimum energy communication network
  • minimum energy routes
• Flooding
  • our old friend, but expensive, redundant and resource blind
• Gossiping
  • a.k.a. hot-potato routing
• Sensor protocols for information via negotiation
  • adaptive negotiated protocols
  • a source offers data, a sink requests it, repeat
• Sequential assignment routing
  • discover neighbors, then form trees rooted at the sink
    • energy resources along the path
    • path qos
  • nodes select among paths based on data, energy, qos
• Low-energy adaptive clustering hierarchy
  • diffuse hot-spots via random assignment
  • a random set of leaders broadcast, followers select, leaders schedule
  • followers sense and report, leaders aggregate and forward
• Directed diffusion
  • broadcast request for data, nodes map and respond appropriately

Transport Layer

• not ip
• specialization within the sensor network

The Application Layer

• services offered
• Sensor management protocol
  • managing the network
  • node addressing by attributes or location, not a global id
    • new procedures, location finding, time synchronization, repositioning, enabling and disabling, status, security
• Task assignment and data advertisement protocol
• what the sink wants, what the nodes have
• Sensor query and data dissemination protocol
  • data request and gather
  • higher level task descriptions and active messages

Summary

References

• Survey on Sensor Networks by Ian Akyildiz, Weilian Su, Yogesh Sankarasubramaniam and Erdal Cayirci in IEEE Communications Magazine, August 2002.
• Introduction, WSN Applications, and Factors Influencing WSN Design (chapters 1–3) in Wireless Sensor Networks by Ian Akyildiz and Mahamet Vuran, Wiley, 2010.

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This page last modified on 2011 October 9.