Throughput Enhancement Through Dynamic Fragmentation in Wireless LANsByung-Seo Kim, Yuguang Fang, Tan F. Wong, and Younggoo Kwon

IEEE Transactions on Vehicular TechnologyVol. 54, No. 4, July 2005

Outline Introduction Proposed protocol

Fragmentation SchemeFragment burstNAV update

Performance evaluation Conclusion

Introduction

Wireless communication link in a WLAN is time varying

Rate-adaptive MAC protocols have been proposed in the past for WLANsChannel conditionRTS/CTS handshake

Introduction (cont.)

Receiver based rate determinationRTSCTS (selected rate information)DATA

Overhearing nodes update their NAV

ACK

Introduction (cont.) Fragmentation in IEEE 802.11 MAC with r

ate adaption scheme

> aFragmentThreshold

AddPLCP Headerand Preamble

Introduction (cont.)

These proposed protocols only allowed static size of fragmentsHigher overhead of transmitting each

fragmentChannel could not be used effectively

Proposed protocol A dynamic fragmentation scheme to enhanc

e throughputDurations of all fragments, except the last one,

should be set the same in any data rateA Rate-Based fragmentation thresholding sche

me is employedA new fragment is generated only when the rate

is decided for the next fragment transmission Dynamic Fragmentation

Fragmentation Scheme In order to generate fragments with the sa

me time duration, the different aFragmentationThresholds should be used in different data rate R

Fragmentation Scheme (cont.)

The additional overhead of ThresholdB is

Fragmentation Scheme (cont.)

Fragment Burst

CurrentRate (4)

NextRate (4)

Modified format of DSSSPLCP Header

Fragment Burst (cont.)S R

RTS (base rate)

CTS (base rate)next rate is included

DATA is transmitted with selected rateIn aFragmentationThreshold size

ACK (selected rate)next rate is included

DATA is transmitted with selected rateIn aFragmentationThreshold size

˙˙˙

RTS

CTS

Fragment 1

ACK

Fragment 2 ˙˙˙

Next rate Next rate

NAV update Because the durations of all the fragments

are the same, except the last fragmentMoreFragments = 1

NAV update

Failure policy

When the transmission of a fragment failsThe 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

Single-hop environment No hidden node

Transmission rage : 300 m Rate selection

Performance Evaluation (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

4 m/s

Packets per MSDU vs. number of nodesTime overhead vs. number of nodes

PER vs. number of nodesMAC Service Time vs. number of nodes

MSDU dropping rate vs. number of nodes

Throughput vs. Node speed

40 nodes

Throughput vs. Max. MSDU Size

4 m/s

40 nodes

Throughput vs. Predictor Efficiency

4 m/s

40 nodes

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