A Simple and Effective Cross Layer Networking System for Mobile Ad Hoc Networks

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A Simple and Effective Cross Layer Networking
System for Mobile Ad Hoc Networks

• Wing Ho Yuen, Heung-no Lee and Timothy Andersen

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Outline

• Introduction to cross layer design
• Proposed channel model
• Rate adaptation scheme
• Routing metrics utilizing MAC info
• Simulation setup and results

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Why Cross-layer?

• Nature of wireless ad hoc networks
• Limited capacity, constrained energy, mobility
• Application requirements
• Real time, mission critical
• Current layered design paradigm is inflexible and
  sub-optimal for wireless networks
• Cross-layer design requires info exchanged across
  layers, thus allows protocols to adapt in a
  global manner, eventually achieves optimal
  network performance

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Cross Layer Example
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Channel Model

• Three signal strength attenuation factors are
  considered, namely, pass loss, shadowing and
  multipath fading
• For channel-adaptive protocols, A good
  time-varying channel model is needed for
  simulation
• A correlated shadowing channel model is proposed

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Correlated Shadowing Model
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Correlated Shadowing Model

• Shadowing attenuation Aj of a node j does not
  change until node j moves out of a disc of radius
  d from previously reference position
• Suppose node j moves out of the disc, new
  attenuation is
• Suppose both node i and j move out of the disc
• Where
• W is a zero mean Gaussian random variable

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Rate Adaptation Scheme

• Modification on IEEE 802.11 protocol
• RTS,CTS and ACK packets sent at nominal rate
• SNR is estimated at when node receives RTS
• Transmission rate is mapped from the estimated
  SNR, and appended to the CTS
• The sender transmits data at the adapted rate

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Rate Adaptation Scheme

• An M-QAM scheme is used in which the
  constellation size changed with SNR
• Constellation size is decided by
  where is a constant determined based on the
  power constraints, is SNR.
• Threshold rule is used, if , assign
  to

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NAV modification
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Routing Metrics

• Bandwidth awareness , represents the
  rate of link between node i and j
• Interferences awareness , Where is
  the interval from the when the RTS packet is sent
  to when the data packet is received
• Congestion awareness , where is the
  queuing delay in the buffer of transmit node
• Interference awareness is implemented

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Implementation in DSR

• DSR route maintenance unmodified since only low
  mobility scenarios are considered
• Received SNR information are appended in RREQ,
  since RTS and CTS are not used when broadcasting
  RREQ, no rate adaptation is used in RREQ packets
• RREP packets are unicast packets to the source
  node using rate adaptation based on the SNR
  information along the route
• Source node compute the MAC delay of every RREP
  packets and choose the route with min delay

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Simulation Setup

• Ns-2 with wireless extensions by the Monarch
  Project, CMU
• Channel Model Correlated shadowing, implemented
  in C
• MAC layer 802.11b at 914MHz, 2MHz bandwidth
• Network layer modified (dynamic source routing)
  DSR algorithm
• Transport layer UDP agent
• Application layer CBR application

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Simulation Environment

• 50 nodes
• Transmission range 250m
• Scenario size 1500X300m
• Channel model
• Path loss model 2 ray ground reflection model
• shadowing variance s12 (severe shadowing)
• Correlated fading (slow fading at low mobility)
• Mobility model
• Random waypoint model
• 2 values of node mobility s0m/s and 1m/s
• corresponding to stationary and pedestrian
  scenarios

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Simulation Environment

• Each scenario has 20 flows (source destination
  pairs)
• Packet rate varies from 10 to 60 packet/s
• Each traffic flow starts at staggered time
  between 0s and 100s
• Performance metrics throughput, delay, packet
  delivery ratio
• Three schemes are investigated
• Plain DSR
• RA rate adaptation
• IARA interference aware rate adaptation

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Results Stationary Scenario
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Results Pedestrian Scenario
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Conclusions

• Spectrally efficient rate adaptation scheme leads
  to drastic improvement in throughput
• Simple routing metric incorporated to the DSR
  protocol, leading to modest decrease in packet
  delay