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Woo-Yong ChoiElectronics and Telecommunications Research Institute

wychoi53@etri.re.kr ETRI

Wireless LAN Services Wireless LAN Services for Hotfor Hot--SpotSpot

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Contents

? Overview? Wireless LAN Services? Current IEEE 802.11 MAC Protocol

? Distributed Coordination Function (DCF)? Point Coordination Function (PCF)

? MAC Enhancements? Dynamic RTS-CTS Threshold Control Method

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Overview

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Overview

? IEEE 802.11 WLAN is currently dominant technology? Next generation WLAN technology will be developed

based on the current IEEE 802.11 WLAN? Most recent next generation WLAN will be IEEE

802.11e WLAN? At the end of next year, IEEE 802.11n standard will

emerge to support 100Mbps at MAC SAP? Current and future MAC technologies will be

addressed in this presentation

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Wireless LAN Services

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VoIP + Wireless LAN

? Wireless extension of current VoIP service? Initial stage of home or office networking that is

possible in a near future

Router

Internet

Gateway

SS7

SS7

IP/PBXConverter

IP/PBXConverter

IP PBX

T1/E1

IP/PBXConverter

LegacyPBX

VoIPGateway Router/

Switch

Router/SwitchRouter

Router/SwitchRouter

IPSwitch/Router

AP

AP

AP

VoIPPhone

VoIPPhone

VoIPPhone

PSTN

SS7

Source: TI

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Home & Office Networks

? Realization of networks of home or office equipments? Wireless transmission service for game, HDTV,

medical examination, etc.

Consumers want theirdevices to work together

and share content

MEDIAMEDIAPre-Recorded Content

Personal Media

MOBILE MULTIMEDIAMOBILE MULTIMEDIAEntertainment,

Personal Pictures and Video,Services

BROADCASTBROADCASTServices,

Entertainment

BROADBANDBROADBANDEntertainment,

E-Business, Services

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Info-Station

? Multimedia service in gas station, shopping mall, etc.? Quick download of music and cinema

GAS STATION

GAS

GAS

GAS200Mbps

~ 1Gbps200Mbps~ 1Gbps

200Mbps~ 1Gbps

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Current IEEE 802.11 MAC Protocol

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IEEE 802.11 System

? Infrastructure Mode? Infrastructure Basic Service Set (BSS)? One Access Point (AP) and multiple stations (STAs)

? Ad hoc Mode? Independent Basic Service Set (IBSS)? No AP and multiple STAs

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DCF

? Distributed Coordination Function (DCF)? 802.11 MAC protocol is based on CSMA-CA scheme

( Ethernet MAC protocol: CSMA -CD scheme )

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DCF

? Hidden Node Problem

STA 1AP

STA 2

STA 1: currently transmittingSTA 2: attempting to transmit

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DCF

? Collision Avoidance (CA)? Solves the hidden node problem

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PCF

? Point Coordination Function (PCF)? Polling based transmission

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Limitations

? Signaling Overhead? RTS-CTS or polling is needed for every MSDU transmission? Ack is needed for every MPDU (a fragment of MSDU)

transmission? This is inefficient especially for real time service

? Not Sufficient Support for QoS Service? No specification for the service with various service

requirements? No policy for the general QoS service

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MAC Enhancements(Based on IEEE 802.11n Contributions, IEEE

802.11-03/0509r0 & 04/0312r0)

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Two Factors for Efficiency of RTS-CTS Method

? Data Frame Length? RTS-CTS method is efficient in the case of

transmissions of long data frames? But, inefficient with short data frames

? Numbers of STAs and Transmission Attempts? Related with the number of retransmissions? Efficient in the case of many STAs and transmission

attempts ? But, inefficient with small numbers of STAs and

transmission attempts

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Dynamic RTS-CTS Threshold Control

? Infrastructure BSS? AP periodically monitors the number of STAs

connected to itself, the number of transmission attempts and the data frame size distribution

? AP calculates the optimal dot11RTSThreshold? AP broadcasts the updated threshold value to be

used by STAs? Detailed algorithm is for further research

? Independent BSS? For further research

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Simulation Condition

? n greedy STAs attempt to transmit data frames continuously using DCF protocol

? The length of data frames is variable based on the experimental statistics from NLANR (National Lab. for Applied Network Research) (www.nlanr.net/NA/Learn/paketsizes.html)

? p: the probability that a transmission attempt fails due to the hidden node problem (p = 0, 0.25, 0.5)

? Optimal RTS-CTS threshold was obtained using computer simulations for maximizing MAC throughput

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Throughput Analysis (p=0)

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Throughput Analysis (p=0.25)

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Throughput Analysis (p=0.5)

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Optimal RTS-CTS Threshold (p=0)

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Optimal RTS-CTS Threshold (p=0.25)

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Optimal RTS-CTS Threshold (p=0.5)

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Performance Improvements

? Average 25 % throughput improvement in IEEE 802.11a

? Average 28 % throughput improvement in IEEE 802.11b