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EE360: Lecture 16 Ad Hoc Networks. Announcements Introduction to ad-hoc networks Applications Design issues: “Towards self-organized mobile ad-hoc networks,” Mung Chiang Link layer Multiple access “Power controlled multiple access,” Xiangheng Liu Routing and mobility management

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ee360 lecture 16 ad hoc networks
EE360: Lecture 16Ad Hoc Networks
  • Announcements
  • Introduction to ad-hoc networks
  • Applications
  • Design issues:
    • “Towards self-organized mobile ad-hoc networks,” Mung Chiang
    • Link layer
    • Multiple access
      • “Power controlled multiple access,” Xiangheng Liu
    • Routing and mobility management
    • Network issues
    • Power/energy management
  • Homework due today
  • Progress reports due next Wednesday.
  • Last class: next-generation standards debate
    • What will be the baseline technology
    • What data rates must be supported and how
    • What features must be supported and how (voice, data, video, etc.)
    • Economic issues: pricing, spectral auctions, market penetration, etc.
  • Detailed course evaluations (hard copy/web/email)
    • 10 bonus points if turned in by June 8
  • tbp forms will be done in class June 6
    • Can also pick up from Joice any time
    • 10 bonus points if turned in by June 8
ad hoc networks
Ad-Hoc Networks
  • Each node generates independent data.
  • Source-destination pairs are chosen at random.
  • Routing can be multihop.
  • Topology is dynamic (links change, nodes enter and leave)
  • Fully connected network but with different link SNRs
  • Can allocate resources dynamically (rate, power, BW, routes,…)
  • Battlefield communications
  • Wireless LANs
  • Emergency infrastructures
  • Short-term networks (e.g. convention)
  • Sensor networks
    • Medical applications (on-body)
    • Buildings
    • Wide area
  • Cellular phone evolution
  • Communication infrastructure for automated vehicles
    • Automobiles, airplanes, UAVs, robots, etc.

Different channel characteristics, distances, mobility, and rate requirements.

design issues
Design Issues
  • Link layer design
  • Channel access and frequency reuse
  • Reliability
  • Routing
  • Network issues
  • Power/energy management

Must exploit synergies between design layers

link layer design
Link Layer Design
  • Modulation and Coding
    • Robustness
    • Rate requirements
    • Performance
    • Adaptive techniques: rate, power, BER, code, framing, etc.
  • Bandwidth requirements
  • Power control
    • Typically distributed
  • Antenna design
    • Smart antennas
    • Multipath mitigation
    • Multiuser detection
channel access and reuse
Channel Access and Reuse
    • Collision detection or avoidance
    • Power control in multiple access (Xiangheng)
  • ALOHA with DS/FH Spread Spectrum
  • Frequency reuse
    • Bandwidth efficient
    • Distributed allocation
    • Dynamic channel allocation hard for packet data
  • Poor efficiency
  • Poor capture
  • Hidden terminal problem
  • Carrier sensing, collision detection/avoidance
    • Hidden nodes degrade performance
    • Busy tone may interfere with transmission to other nodes (exposed terminal).
  • Power control

Busy Tone

ds spread spectrum code assignment
DS Spread Spectrum:Code Assignment
  • Common spreading code for all nodes
    • Collisions occur whenever receiver can “hear” two or more transmissions.
    • Near-far effect improves capture.
    • Broadcasting easy
  • Receiver-oriented
    • Each receiver assigned a spreading sequence.
    • All transmissions to that receiver use the sequence.
    • Collisions occur if 2 signals destined for same receiver arrive at same time (can randomize transmission time.)
    • Little time needed to synchronize.
    • Transmitters must know code of destination receiver
      • Complicates route discovery.
      • Multiple transmissions for broadcasting.
    • Each transmitter uses a unique spreading sequence
    • No collisions
    • Receiver must determine sequence of incoming packet
      • Complicates route discovery.
      • Good broadcasting properties
    • Poor acquisition performance
  • Preamble vs. Data assignment
    • Preamble may use common code that contains information about data code
    • Data may use specific code
    • Advantages of common and specific codes:
      • Easy acquisition of preamble
      • Few collisions on short preamble
      • New transmissions don’t interfere with the data block
  • Packet acknowledgements needed
    • May be lost on reverse link
    • Should negative ACKs be used.
  • Combined ARQ and coding
    • Retransmissions cause delay
    • Coding may reduce data rate
    • Balance may be adaptive
  • Hop-by-hop acknowledgements
    • Explicit acknowledgements
    • Echo acknowledgements
      • Transmitter listens for forwarded packet
      • More likely to experience collisions than a short acknowledgement.
    • Hop-by-hop or end-to-end or both.
routing 1987
Routing (1987)
  • Flooding
    • Broadcast packet to all neighbors
    • Inefficient
    • Robust for fast changing topologies.
    • Little explicit overhead
  • Point-to-point routing
    • Routes follow a sequence of links
    • Connection-oriented
      • Explicit end-to-end connection
      • Less overhead/less randomness
      • Hard to maintain under rapid dynamics.
    • Connectionless
      • Packets forwarded towards destination
      • Local adaptation
route dessemination
Route dessemination
  • Route computed at centralized node
    • Most efficient route computation.
    • Can’t adapt to fast topology changes.
    • BW required to collect and desseminate information
  • Distributed route computation
    • Nodes send connectivity information to local nodes.
    • Nodes determine routes based on this local information.
    • Adapts locally but not globally.
  • Nodes exchange local routing tables
    • Node determines next hop based on some metric.
    • Deals well with connectivity dynamics.
    • Routing loops common.
routing 1999
Routing (1999*)
  • Table-driven
    • Destination-sequenced distance-vector
    • Clusterhead gateway switch routing
    • Wireless routing protocol
  • On-Demand Routing
    • On-demand distance vector routing
    • Dynamic source routing
    • Temporally ordered routing
    • Associativity-based routing
    • Signal stability routing

*”A review of current routing protocols for ad hoc mobile wireless networks,”

Royer and Toh, IEEE Personal Communications Magzine, April 1999.

other network issues
Other Network Issues
  • Network Capacity
  • Admission Control
  • Interface with wired networks
  • Security
  • Upgrades
    • Software changes
    • Software radios
energy constraints
Energy Constraints
  • Non-renewable batteries impose a hard energy constraint on link and network design
  • Channel capacity must be redefined for energy-constrained nodes
    • Not possible to send a finite number of bits with finite energy and Pe arbitrarily small
    • Capacity per unit cost (Gallager’87, Verdu’90) defines the number of bits transmitted per unit
  • Dynamic resource allocation must take into account a finite battery life
  • Routing optimization must take into account nodes dying away due to battery drainage
what has changed since 1985
What has changed since 1985?
  • Batteries are not much better
  • DSPs are better, cheaper, and use less power.
  • Better coding and modulation.
  • Multiuser detection and smart antennas.
  • Adaptive techniques available
  • How would we leverage these developments to make better ad-hoc networks?
  • Ad-hoc networks provide a flexible network infrastructure for many emerging applications
  • Recent advances in communication techniques should be incorporated into ad-hoc network design
  • Design issues traverse all layers of the protocol stack, and cross layer designs are needed
  • Energy constraints impose an interesting challenge for link design, resource allocation, and routing