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Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs. Byung-Seo Kim, Yuguang Fang, Tan F. Wong, and Younggoo Kwon IEEE Transactions on Vehicular Technology Vol. 54, No. 4, July 2005. Outline. Introduction Proposed protocol Fragmentation Scheme Fragment burst NAV update

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throughput enhancement through dynamic fragmentation in wireless lans

Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs

Byung-Seo Kim, Yuguang Fang, Tan F. Wong, and Younggoo Kwon

IEEE Transactions on Vehicular Technology

Vol. 54, No. 4, July 2005

outline
Outline
  • Introduction
  • Proposed protocol
    • Fragmentation Scheme
    • Fragment burst
    • NAV update
  • Performance evaluation
  • Conclusion
introduction
Introduction
  • Wireless communication link in a WLAN is time varying
  • Rate-adaptive MAC protocols have been proposed in the past for WLANs
    • Channel condition
    • RTS/CTS handshake
introduction cont
Introduction (cont.)
  • Receiver based rate determination
    • RTS
    • CTS (selected rate information)
    • DATA
      • Overhearing nodes update their NAV
    • ACK
introduction cont1
Introduction (cont.)
  • Fragmentation in IEEE 802.11 MAC with rate adaption scheme

> aFragmentThreshold

Add

PLCP Header

and Preamble

introduction cont2
Introduction (cont.)
  • These proposed protocols only allowed static size of fragments
    • Higher overhead of transmitting each fragment
    • Channel could not be used effectively
proposed protocol
Proposed protocol
  • A dynamic fragmentation scheme to enhance throughput
    • Durations of all fragments, except the last one, should be set the same in any data rate
    • A Rate-Based fragmentation thresholding scheme is employed
    • A new fragment is generated only when the rate is decided for the next fragment transmission
      • Dynamic Fragmentation
fragmentation scheme
Fragmentation Scheme
  • In order to generate fragments with the same time duration, the different aFragmentationThresholds should be used in different data rate R
fragmentation scheme cont
Fragmentation Scheme (cont.)
  • The additional overhead of ThresholdB is
fragment burst
Fragment Burst

Modified format of DSSS

PLCP Header

Current

Rate (4)

Next

Rate (4)

fragment burst cont
Fragment Burst (cont.)

RTS (base rate)

S

R

CTS (base rate)

next rate is included

DATA is transmitted with selected rate

In aFragmentationThreshold size

ACK (selected rate)

next rate is included

DATA is transmitted with selected rate

In aFragmentationThreshold size

˙

˙

˙

RTS

Fragment 1

Fragment 2

˙˙˙

CTS

ACK

Next rate

Next rate

nav update
NAV update
  • Because the durations of all the fragments are the same, except the last fragment
    • MoreFragments = 1
  • NAV update
failure policy
Failure policy
  • When the transmission of a fragment fails
    • The size of the retransmitted fragment may not be the same as before
      • Channel condition may have changed
    • Sender only decreases the remaining MSDU size when receives the ACK from the receiver
performance evaluation
Performance Evaluation
  • Single-hop environment
    • No hidden node
  • Transmission rage : 300 m
  • Rate selection
performance evaluation cont
Performance Evaluation (cont.)
  • 3 different configuration
    • RFT-DF
      • Rate-based
      • Dynamic
    • RFT-CF
      • Rate-based
      • Conventional
    • SFT-CF
      • Single fragmentation threshold
      • Conventional
conclusion
Conclusion
  • This paper proposes a new rate-adaptive MAC protocol with dynamic fragmentation
  • Nodes with good channels can transmit more data
  • Constant duration in physical layer simplifies the process of NAV update
  • Simulations show the throughput gain from the conventional scheme
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