Wireless LAN Services
for Hot- Spot
Woo-Yong Choi
Electronics and Telecommunications Research Institute
wychoi53@etri.re.kr
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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
VoIP
Gateway
T1/E1
Router
Router/
Switch
AP
Legacy
PBX
VoIP
Phone
SS7
IP/PBX
Converter
Router
SS7
PSTN
Router/
Switch
AP
IP PBX
VoIP
Phone
IP/PBX
Converter
IP
Switch/
Router
SS7
Router
Router/
Switch
VoIP
Phone
Gateway
Internet
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AP
IP/PBX
Converter
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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.
BROADBAND
MOBILE MULTIMEDIA
Entertainment,
E-Business, Services
MEDIA
Pre-Recorded Content
Personal Media
Entertainment,
Personal Pictures and Video,
Services
Consumers want their
devices to work together
and share content
BROADCAST
Services,
Entertainment
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Info-Station
? Multimedia service in gas station, shopping mall, etc.
? Quick download of music and cinema
GAS STATION
200Mbps
~ 1Gbps
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A
S
G
A
S
G
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200Mbps
~ 1Gbps
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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 1
AP
STA 2
STA 1: currently transmitting
STA 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
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