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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 Wireless LANs

  • Introduction

  • Proposed protocol

    • Fragmentation Scheme

    • Fragment burst

    • NAV update

  • Performance evaluation

  • Conclusion


Introduction
Introduction Wireless LANs

  • 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 Wireless LANs(cont.)

  • Receiver based rate determination

    • RTS

    • CTS (selected rate information)

    • DATA

      • Overhearing nodes update their NAV

    • ACK


Introduction cont1
Introduction Wireless LANs(cont.)

  • Fragmentation in IEEE 802.11 MAC with rate adaption scheme

> aFragmentThreshold

Add

PLCP Header

and Preamble


Introduction cont2
Introduction Wireless LANs(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 Wireless LANs

  • 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 Wireless LANs

  • 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 Wireless LANs(cont.)

  • The additional overhead of ThresholdB is


Fragmentation scheme cont1
Fragmentation Scheme Wireless LANs(cont.)


Fragment burst
Fragment Burst Wireless LANs

Modified format of DSSS

PLCP Header

Current

Rate (4)

Next

Rate (4)


Fragment burst cont
Fragment Burst Wireless LANs(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 Wireless LANs

  • Because the durations of all the fragments are the same, except the last fragment

    • MoreFragments = 1

  • NAV update


Failure policy
Failure policy Wireless LANs

  • 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 Wireless LANs

  • Single-hop environment

    • No hidden node

  • Transmission rage : 300 m

  • Rate selection


Performance evaluation cont
Performance Evaluation Wireless LANs(cont.)

  • 3 different configuration

    • RFT-DF

      • Rate-based

      • Dynamic

    • RFT-CF

      • Rate-based

      • Conventional

    • SFT-CF

      • Single fragmentation threshold

      • Conventional


Throughput vs number of nodes
Throughput vs. number of nodes Wireless LANs

4 m/s


Packets per msdu vs number of nodes time overhead vs number of nodes
Packets per MSDU vs. number of nodes Wireless LANsTime overhead vs. number of nodes


Per vs number of nodes mac service time vs number of nodes
PER vs. number of nodes Wireless LANsMAC Service Time vs. number of nodes



Throughput vs node speed
Throughput vs. Node speed Wireless LANs

40 nodes


Throughput vs max msdu size
Throughput vs. Max. MSDU Size Wireless LANs

4 m/s

40 nodes


Throughput vs predictor efficiency
Throughput vs. Predictor Efficiency Wireless LANs

4 m/s

40 nodes


Conclusion
Conclusion Wireless LANs

  • 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