Wireless LANIEEE802.11

Jun Murai/Larry Landweber

For

KEIO Univ., Univ. Wisconsin Madison, and NAIST

Overview

• Wireless Technologies– Air channels controls– Access methods

• CSMA/CA

• IEEE802.11 Standards

• Future

Characteristic of the wireless air channel

• Ad-hoc and Base station model• Fading, interference and noise• Efficient use of the available spectrum• Contention to take an air channel• Contention detection, multiple access and the ‘hidden

node problem’• Multiple International regulatory requirements• Mobility• Battery usage and power management • Security

Basics on Highs Speed Wireless

• Transmit and Receive– Difference of power– Make it difficult to do them together

• Diversity technologies– Select from multiple path signals– Integrated multiple path signals

• Modulation technologies– Non-Linear

• MSK(minimum shift keying), GMSK(Gaussian Filtered)

– Linear• BPSK(Binary Phase) QPSK(Quadrature Phase)

• Spread Spectrum– Direct sequence Modulation(DS)– Frequency Hopping Modulation(FH)– Chirp Modulation

Access Technologies

• Multiple Access technology– Multiple terminals to share the air channel

• Multiplex technology– A transmitting station to merge data

• Duplex technology– Sharing between uplink and downlink

• Access Algorithm– Control method for access

A

B

C

channel

1. Multiple Access

2. Multiplex

3. Duplex

4.Access Algorithm

Base Station

Multiple Access

• For multiple terminals to share the air channels• Three dimensions to consider

– Coding, Frequency, and Time

• TDMA– Time Division Multiple Access

• FDMA– Frequency Division Multiple Access

• CDMA– Code Division Multiple Access

Code

Time

Frequency

(1)TDMATime Division Multiple Access

Code

Time

Frequency

(2) FDMAFrequency Division Multiple Access

Code

Time

Frequency

(3) CDMACode Division Multiple Access

Code

Time

Frequency

(4) Complex Combination

TDMA

• Frame and Slot– Frame

• A cycle of signal

– Slot• Divided block of tie in a frame

A B C

sss

… A B C

sss

… A B C

sss

…

Frame Frame Frame

Frame and Slot

A B C

データ

Multiplex Technology

• For a station to merge signals• TDM

– Time Division Multiplex

• FDM– Frequency Division Multiplex

• CDM– Code Division Multiplex

• A frame to send, a slot to receive

Duplex Technology

• For uplink and downlink to share a space

• FDD– Frequency Division Duplex

• Static allocation

• TDD– Time Division Duplex

• Flexible allocation

• Portable phones

Access Algorithm

• Who take a control and how?

• Pre-assign– Static assignment

• Demand assign– Dynamic assignment– Slot allocation on TDMA

• Random assign

Summary of Access Method• Complex combination of four methods

– Multiple Access, Multiplex, Duplex, and Access algorithm

• Example of standard cell phone:– Uplink: TDMA (or CDMA+TDMA)– Downlink: TDM (or FDM)– Duplex: TDD (or FDD)– Access: Demand assign

• Example of LAN– Multiple access: CDMA (or TDMA)

• CSMA/CA (a complex combination of above)

– Duplex: none– Access algorithm: complex Random access

CSMA/CD

• CSMA/CD– Carrier Sense, Multiple Access/Collision Detect

ion

• For wire communication• No control BEFORE transmission

– Generates collisions

• Collision Detection– How?

CSMA/CA

• CSMA/CA– Carrier Sense, Multiple Access/Collision

Avoidance

• For wireless communication

• Collision avoidance BEFORE transmission– Pre-avoidance of collision

• Why avoidance on wireless?

Collision detection on wireless

• Difference on energy/power for transmit and receive– At maximum, transmission power is a million

times larger than receiving– Very hard to detect because of this difference

• Energy often matter on wireless environment– Portable devices/terminals with batteries

Backoff

• Decrease the possibility of contention/collision

• Backoff window– Time to wait ot avoid collision

• Random backoff– Use random length of time to wait

IFS – Inter Frame Spacing

• Defined length of time for control

• To assure the control of multiple access– DIFS – Distributed Inter Frame Spacing– PIFS – Point Inter Frame Spacing– SIFS – Short Inter Frame Spacing

• DIFS (MAX) > PIFS > SIFS (MIN)

Transmitter

Receiver

Other

DataRTS

ACKCTS

DIFS

DIFS

SIFS SIFS SIFS

NAV(RTS)NAV(CTS)

Defer Access Backoff process

Contention Window

DIFS: Distributed IFSRTS: Request To SendSIFS: Short IFSCTS: Clear To Send

ACK: AcknowledgementNAV: Network Allocation VectorDCF: Distributed Coordination Function

RTS-CTS-DATA-ACK

Transmitter

Receiver

Other

Data

DIFS

DIFS

SIFS

Defer Access Backoff process

Contention Window

DIFS: Distributed IFSSIFS: Short IFS

RTS: Request To SendCTS: Clear To SendACK: Acknowledgement

DATA-ACK

ACK

Transmitter

Receiver

Other

NAV: Network Allocation VectorRTS: Request To SendCTS: Clear To SendACK: Acknowledgement

DIFS: Distributed IFSPIFS: Point IFSSIFS: Short IFS

Fragmentation

RTS

SIFS

CTS

SIFS

SIFS

ACK0

SIFS

SIFS

ACK1

SIFS

SIFS

ACK2

SIFS

NAV(CTS)NAV(RTS) NAV(Fragment0)NAV(Fragment1)

NAV(ACK0) NAV(ACK1)

Fragment0 Fragment1 Fragment2

SIFS

PIFS

DIFSBack off Window

How to handle BE and Priority

• DCF and PCF

NAV: Network Allocation VectorPCF: Point Coordination FunctionDCF: Distributed Coordination FunctionB: Beacon Frame

DCF and PCF

NAV NAV

B PCF B PCFBusyDCF DCF

Distribution System

• Used to interconnect wireless cells– multiple BSS, Basic Service Set, connected

together from an ESS, Extended Service Set– Allow mobile stations to access fixed resources

• Not part of 802.11 standard– could be bridged IEEE LANs, wireless, other

networks…– Distribution System Services are defined

802.11 wireless LAN StandardIEEE 802.11 Wireless LAN Standard

Frequency 2.4GHz 2.4GHz Infra-red(850mm~950mm)

Spread Spectrum DSSS FHSS

Baseband Modulation DBPSK DQPSK 2GFSK 1GFSK 16-PPM 4-PPM

Max bandwidth 1Mbps 2Mbps 1Mbps 2Mbps 1Mbps 2Mbps

Access Control CSMA/CA, RTS/CTS

Area(open air/office) 100~300m/20~100m 20~30m/about 5m

IEEE 802.11 Technical Term

• DBPSK: Differential Binary Phase Shift Keying• DQPSK: Differential Quadrature Phase Shift Keying• 2GFSK: 2Level Gaussian-filltered Frequency Shift Keying• 4GFSK: 4Level Gaussian-filltered Frequency Shift Keying• 16-PPM: 16 Pulse Position Modulation• 4-PPM: 4 Pulse Position Modulation• RTS/CTS: Request To Send / Clear To Send

FHSS - hopping frequency

Country/Area LowestFrequency

Highest Frequency

Regulated Frequency range

North America 2,402 2,480 2,400~2,4835

Europe 2,402 2,480 2,400~4,2835

Japan 2,473 2,495 2,471~2,497

Spain 2,417 2,473 2,445~2,475

France 2,448 2,482 2,4465~2,4835

FHSS - # of Channel

Country/Area # of ChannelsHopping Frequency

Minimum number of Channels

North America 79 75

Europe 79 20

Japan 23 --

Spain 27 20

France 35 20

EHF(m

illi-wave)SHF

(micro-wave)

VHFUHF

UHFW

ired LAN

Middle Speed 2.4GHz Range LAN

High Speed5GHz Range LAN

Very High Speed60 GHz Range LAN

4Mbps/16MbpsToken RingIEEE 802.5

10MbpsEthernet

IEEE 802.3

25/52/100MbpsATM-LAN

(ATM Forum)

100MbpsFast EthernetIEEE 802.3u

156/622 MbpsATM-LAN

(ATM Forum)

1000MbpsGigabit Ethernet

IEEE 802.3z, 802.3ab

Bandwidth

Frequency

IEEE802.111Mbps/2Mbps

IEEE802.11b5.5Mbps/11Mbps

(1)IEEE 802.11a 6/12/24Mbps

(2)HIPERLAN (ETSI BRAN)Type ½: 23.5/25Mbps

(3)WATM (ATM Forum) 25Mbps

19GHz range LAN10Mbps (ARIB)

(1) 156MbpsMMAC(Japan)

(2) 156MbpsMEDIAN (German)

300GHz

30GHz

3GHz

300MHz

1GHz

EHF: Extremely High FrequencySHF: Super High FrequencyUHF: Ultra High FrequencyVHF: Very High FrequencyWLAN : Wireless LANIEEE: Institute of Electrical EngineeringARIB: Association of Radio Industries and BusinessCCK: Complementary Code KeyingOFDM: Orthogonal Frequency Division MultiplexingHIPERLAN: High Performance Radio LANETSI: Europe Telecommunications Standards InstituteBRAN: Broadband Radio Access NetworkWATM: Wireless ATMMMAC: Multimedia Mobile Access Communication SystemsMEDIAN: Wireless Broadband CPN (Computer Premises Network)/ LAN for

Professional and Residential Multimedia ApplicationsFWA: Fixed Wireless Access

WirelessTechnical Term