Addressing Deafness and Hidden Terminal Problem in Directional Antenna Based Wireless Multi-hop Networks

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Addressing Deafness and Hidden Terminal Problem
in Directional Antenna Based Wireless Multi-hop
Networks

• Anand Prabhu Subramanian and Samir R. Das
• anandps, samir_at_cs.sunysb.edu
• Computer Science Department
• Stony Brook University, NY, USA

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Outline

• Motivation Why Directional Communication?
• Deafness
• Directional Hidden Terminal Problem
• Antenna Model
• CW-DMAC Design
• Performance Evaluation
• Summary

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Motivation - Capacity Problem in Multi-hop
Wireless Networks

• Wireless Multi-hop Networks Ad hoc Networks,
  Mesh Networks
• Data packets forwarded over multiple hops
• Wireless channel is a shared medium
• Capacity of multi-hop networks limited by
  wireless interference
• Directional communication can reduce interference

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Directional Antenna - Basics

• A Directional/Beam forming antenna has certain
  preferred transmit and receive directions
• Steerable beam vs switched beam

Omni-directional Beam
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Directional Antenna - Benefits

• Directional Communication
• Less energy in the undesired directions
• Better spatial reuse
• More energy in the desired direction
• Longer Ranges
• More robust links
• Both higher spatial reuse and longer range can be
  simultaneously obtained

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Directional Communication
Omni- Directional Communication
Directional Communication
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Directional Communication - Challenges

• Just adding a directional antenna is not enough
• New Challenges
• Direction to transmit
• Deafness
• Directional Hidden Terminal Problem
• Broadcasting
• And more

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Deafness Type I

• Destination engaged in communication

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Deafness Type II

• Precautionary Deafness at the Receiver

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Directional Hidden Terminal Problem

• - Due to unheard RTS/CTS packets

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Current Literature

• Many proposals in the past to solve deafness
• ko00infocom, choudhury02mobicom, choudhury04icnp,
  elbatt03wcnc, gossain04globecom, nasipuri00wcnc,
  sundaresan03mobihoc, takai02mobihoc
• These approaches use additional resources such as
  additional channels, radios or busy tones
• Directional hidden terminal problem not addressed

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Our Goal

• Solve both deafness and directional hidden
  terminal problem using
• Single Channel
• Single Radio Interface

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Antenna Model

• Switched beam antenna with N beams covering the
  entire 360 degrees
• Two modes of operation
• Omni mode
• Directional Mode
• Directional Gain Omni Gain

A
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Antenna Model

• 8 phased-array antenna elements
• Can form both omni-directional and directional
  beams
• Customizable beam pattern
• Beam switch time around 150 µs
• Directional gain 15dBi

Commercially available Directional Antenna
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Antenna Model

• Packet Transmission either in omni or
  directional mode
• Packet Reception
• When Idle omni mode
• When it detects a packet, does an azimuthal scan
  and goes to directional mode

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Assumptions

• Nodes are fairly static (e.g. routers in
  Wireless Mesh Networks)
• Each node knows the direction (beam index) to its
  neighbor Simple neighbor discovery protocol
• Nodes need not have an aligned axis

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CW-DMAC Design

• Type I deafness can be solved if
  transmitter/receiver can inform neighbors about
  their impending transmission.
• Type II deafness can be solved if the blocked
  receiver can somehow inform the transmitter that
  their transmission cannot take place without
  disturbing an ongoing transmission.
• Directional hidden terminal problem can be solved
  if the nodes do not miss any RTS/CTS packets in
  the neighborhood

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CW-DMAC Design

• RTS/CTS packets sent omni-directionally
• DATA/ACK packets sent directionally
• RTS/CTS packets are overloaded with the beam
  index of the intended DATA/ACK transmission
• Neighboring nodes set their DNAV tables
  appropriately depending on the beam index in the
  RTS/CTS packets
• Each neighboring node record this transmission in
  their neighborhood transmission table

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CW-DMAC Design
Aware of As transmission
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CW-DMAC Design

• There is a possibility of collision between
  omni-directionally sent RTS/CTS packets and
  DATA/ACK packets.
• We separate transmission of data packets and
  control packets in time
• Control window added in RTS/CTS packets
• Prevents collision between data and control
  packets
• Allows multiple simultaneous transmissions in the
  neighborhood in different directions

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CW-DMAC Design

• Adjustable control window
• Small control window less parallelism
• Large control window poor channel utilization
• To solve deafness of type II
• Negative CTS sent by blocked receivers
• So the transmitter can cancel its transmission

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Performance Evaluation

• Simulation Qualnet 3.7
• 8 beam directional antenna (45 degrees)
• 802.11b physical layer
• 11 Mbps data rate
• Comparison between CW-DMAC and DMAC
• 30 nodes in an area of 1500m x 1500m

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Performance Evaluation - Deafness
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Deafness Ripple Effect
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Directional Hidden Terminal Problem
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Random Network

• - 30 nodes in 1500m x 1500m
• 5 simultaneous flows
• Deafness and directional hidden terminal problem
  cause performance degradation

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Summary

• Directional communication can reduce interference
  in multi-hop networks and improve capacity
• Studied various scenarios in which deafness and
  directional hidden terminal problem could occur
• Proposed a directional MAC protocol that solves
  both the problems using a single radio and single
  channel
• Simulations show the improvement when the both
  the problems are solved

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Thank you
Questions ???