wlan material

7/31/2019 Wlan Material

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Wireless LAN

TLI348 Wireless systems Wireless LAN p.1/41

Outline

History & General information

General architecture

Link level

Physical level

Summary

Related resources



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Why WLAN ?

GSM/HSCSD/GPRS/EDGE/. . . complex infrastructure licensed radio resources may be too expensive

IrDA short distance point-to-point connection

BlueTooth short distance no proper roaming


WLAN network

unlicensed radio resources

bandwidth up to 54 Mbps

distance up to 400 m

up to 50 nominal users

carrier sense multiple access with collision avoidance (CSMA/CA)



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IEEE standards

802.2 Logical link control

802.3 Ethernet (CSMA/CD)

802.4 Token Ring

802.10 Security

802.11 Wireless LAN (CSMA/CA)


802.11 standards

802.11 (developed in 1997)

802.11b (ratified in 1999)

802.11a (ratified in 1999, production starts in 2001)

802.11g (ratified in 2003)

802.11e (draft, Quality-of-Service)



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Scope of the 802.11 standards

applicationpresentation

sessiontransport

networklink MAC

physical PHY

WLAN only specifies the link and the physical level, thus enabling high-levelprotocol to function in the same way.


General structure



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Ad-hoc network

communication is established between multiple stations (STAs)

all stations have a fair access to the network (BSS)

BSS

STA

STA

STA


Infrastructure network

the access point(AP) connects the WLAN network to the backbone(DS)

set of APs and DS forms the extended service network (ESS)

DS

BSS 2

AP

STA

STA

BSS 1

AP

STASTA

AP



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Mobility types

no transition: STA moves within a cell

BSS transition: STA moves from one cell to another within the sameESS

ESS transition: STA leaves the ESS

BSS 2

AP

STA

BSS 1

AP

STA

STAno

transiti

on

BSS

tran

sitio

n

ESS

tran

sitio

n


Roaming procedure

roaming type nomadic roaming seamless roaming

roaming decision

signal/noise level packet loss . . .

roaming direction preemptive AP discovery roam-time discovery active/passive scanning

roaming

deassociation from the old AP association with a new AP



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Operation modes

Distributed coordination function (DCF) used in ad-hoc and infrastructure networks all stations have equal rights to access the medium

advantages: simplicity, cheap solutions disadvantages: collisions, degraded Quality-of-Service (QoS)

Point coordination function (PCF) used in infrastructure networks only behaviour of all stations is coordinated by PC

advantages: coordinated access to the medium, no collisions,better functioning

disadvantages: increased complexity, ineffective bandwidthallocation


Link level



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Frame format

2 2 6 6 6 2 6 0-2312 4

Frame

Control

Duration/

IDAddress 1 Address 2 Address 3

Sequence

ControlAddress 4

Frame

BodyFCS

Frame Control

Duration/ID Time necessary to transmit data Association Identity (AID)

Address X BSSID - identifier of the cell DA - destination address SA - source address RA - receiving station address TA - transmitting station address

Sequence Control consists of the Sequence and Fragment NumberFrame Body

FCS - frame control sequence calculated over all fields


Frame control field

2 2 4 1 1 1 1 1 1 1 1

Protocol

versionType Subtype

To

DS

From

DS

More

FragRetry

Pwr

Mgmt

More

DataWEP Order

protocol version determines the version

type and subtype fields identify the function of the frame

To DS is set if a frame is destined for the DS

From DS is set if a frame exits the DSMore Frag is set if there is another frame to follow

Retry is set if this frame is a retransmission

Pwr Mgmt indicates the power management mode

More Data indicates that there are more dataWEP indicates that data in the frame body has been processed by WEP

Order indicates the Strictly Ordered class



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Types of frames

Management

authentication/deauthentication association/reassociaton/deassociation probe beacon

Control power-save polls acknowledgments

Data user data contention-free polls/acknowledgements


Distributed coordination functioning

Backoff time

as the medium becomes idle, all STAs wait for DIFS seconds

after, all STAs generate randomly the backoff time

STA with the elapsed backoff timer begins to transmit

STA1 busy medium DIFS -

backoff - data

STA2 busy medium DIFS -

backoff -



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Acknowledgments

The SIFS interval, which is shorter than DIFS, is used to send the controldata.

busy medium DIFS -

backoff -

busy medium SIFS-

control data



Simple interaction

STA1 sends data to STA2

STA2 acknowledges the received data

DIFS -

STA1 data busy medium DIFS -

STA2 busy mediumSIFS

- ACKDIFS

-

Other busy medium busy medium DIFS -



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Point coordination functioning

PC sends data to STA1 and asks whether it has data to send

STA1 receives data, acknowledges the CF-Poll and sends data

PC acknowledges data from STA1, sends data to STA2 and asks

whether it has dataSTA2 receives data, acknowledges the CF-Poll and sends data

PC acknowledges data and end the CF period

PC PIFS-

B SIFS-

dataCF-Poll

SIFS-

dataCF-ACK+CF-Poll

SIFS-

SIFS-

CF-ACKCF-End

STA1 SIFS-

dataCF-ACK

SIFS-

STA2 SIFS-

dataCF-ACK

SIFS-


Physical level



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Structure of the physical level

The physical layer consists of two sublayers:

Physical Layer Convergence Procedure (PLCP)

Physical Medium Dependent (PMD)

modulation frequency band

MAC

PHYPLCP

PMD


PLCP frame (802.11)

PLCP preamble and PLCP header are always encoded at the rate of1Mbit/s so that these data can always be decoded. 1Mbit/s -

128 16 8 8 16 16

Sync SFD Signal Service Length CRC MPDU

PLCP Preamble - PLCP Header -

PPDU -

Sync: synchronization field

SFD: start frame delimiterSignal: PHY modulation

Service: reservedLength: number of microseconds required to transmit the MPDU

CRC: protects Signal, Service, and Length fieldsTLI348 Wireless systems Wireless LAN p.26/41


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Short PLCP frame (802.11b)

Sync field is shorter (56 bits)

PLCP header is encoded at the rate of 2Mbit/s

1Mbit/s - 2Mbit/s -

56 16 8 8 16 16

Sync SFD Signal Service Length CRC MPDU

PLCP Preamble - PLCP Header -

PPDU -

Short PLCP frame is optional in 802.11b and is mandatory in 802.11g.


PLCP frame (802.11a)

6 Mbit/s -

16s 24 16 6

Sync Signal Service MPDU Tail

4 1 12 1 6

Rate X Length Parity Tail

PLCP header -

Sync: synchronization field

Signal: describes the way data are encoded Rate: rate of data Length: length of PSDU Parity: parity bit for bits 0 16 Tail: additional time for decoder to switch to the necessary mode

Service: descrambler initialization



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Long & short hybrid PLCP frame (802.11g)

DSSS is used to encode preamble and header (802.11b)

OFDM is used to encode data (802.11a)

often referred to as the DSSS-OFDM modulation

1Mbit/s -

128 16 8 8 16 16

Sync SFD Signal Service Length CRC Sync Signal MPDU Tail

OFDM PSDU -

1Mbit/s - 2Mbit/s -

56 16 8 8 16 16

Sync SFD Signal Service Length CRC Sync Signal MPDU Tail

OFDM PSDU -


Modulations

Frequency Hopping Spread Spectrum (FHSS) defined in 802.11

Direct Sequence Spread Spectrum (DSSS) defined in 802.11

High-Rate Direct Sequence Spread Spectrum (HR/DSSS) defined in 802.11b extended in 802.11g

Orthogonal Frequency Division Multiplexing (OFDM) defined in 802.11a



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DSSS

data are sent in the form of chips (11Mchips/s)

11 consecutive chips constitute one symbol (1Msymbol/s)

the chipping sequence 10110111000 is used to encode one bit

Differential Binary Phase Shift Keying (DBPSK) (1 bit/symbol)

Input Phase change

0 0

1

Differential Quadrature Phase Shift Keying (DQPSK) (2 bits/symbol)

Input Phase change

00 001 /2

11

10 3/2 or (/2)


HR/DSSS

The Complementary Code Keying (CCK) modulation is used to encode thesequence of chips.

data are sent in the form of chips (11Mchips/s)

8 consecutive chips form one symbol (11/8 Msymbol/s)

c = {ej(1+2+3+4), ej(1+3+4), ej(1+2+4),ej(1+4),

ej(1+2+3), ej(1+3),ej(1+2), ej1}

CCK modulation at 5.5 Mbit/s (4 bits/symbol) bits (0,1) define the phase change using DQPSK bits (2,3) encode the basic symbol. i.e. the sequence of chips

CCK modulation at 11 Mbit/s (8 bits/symbol) bits (0,1) define the phase change using DQPSK bits (2,3) (4,5) (6,7) encode the basic symbol using QPSK



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Extended Rate PHY (ERP)

A single-carrier modulation scheme that encodes the payload using thepacket binary convolutional code (PBCC).

ERP-PBCC-22 clock works at 11MHz symbol rate is 11Msymbol/s 2 bits are transferred per symbol (22 Mbit/s)

ERP-PBCC-33 clock works at 16.5MHz symbol rate is 16.5Msymbol/s 2 bits are transferred per symbol (33 Mbit/s)

These data rates are optional and may be not supported by an STA.


OFDM

each OFDM symbol consists of 52 subcarriers (NST) 48 data subcarriers (NSD) 4 pilot subcarriers (NSP)

each subcarrier can encode 1 or more bits (NBPSC) BPSK 1 bit per subcarrier QPSK 2 bits per subcarrier 16-QAM 4 bits per subcarrier 64-QAM 6 bits per subcarrier

convolutional code produce data bits at a certain rate (R) 1/2 2/3 3/4



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Frequency band 5GHz (802.11a)

Frequency band 5.15 5.825 GHz is used lower subband (5.15 5.25) middle subband (5.25 5.35)

upper subband (5.725 5.825)Each subband accommodates 4 non-overlapping channels


Summary



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WLAN specifications

802.11b 802.11g 802.11a

Year 1999 2003 1999

Number of channels 3 3 8(4)

Data rates (Mbps) 33, 22, 11, 5.5, 2,

1

54, 48, 36, 24,

18, 12, 11, 9, 6,

5.5, 2, 1

54, 48, 36, 24,

18, 12, 9, 6

Indoor range 30m (11Mbps)

91m (1Mbps)

30m (54Mbps)

91m (1Mbps)

12m (54Mbps)

91m (6 Mbps)

Outdoor range (LOS) 120m (11Mbps)

460m (1Mbps)

120m (54Mbps)

460m (1Mbps)

30m (54 Mbps)

305m (6Mbps)

Wireless band (GHz) 2.4 2.497 2.4 2.497 5.15 5.875

Modulation DSSS

HR/DSSS (CCK)

ERP-PBCC

DSSS

DSSS-OFDM

OFDM


Hardware vendors

Abocom

Accton

Acer

Acrowave

Actiontec

Adaptec

Airvast

Alfa

Allied Telesyn

Allnet

Alloy

Askey

Asus

Buffalo

Cameo

CC&C

Belkin

Cisco/Aironet

Compaq

CompuShack

D-Link

Dell

Delta Networks

Eusso

IST Wave

Fujitsu Siemens

GemTek

Global Sun

HP

IBM

Intel

LG

LinkPro

LinkSys

Microsoft

Motorola

Netgear

Nokia

Nortel

Proxim

Samsung

Sitecom

SMC

Sparklan

3Com

Tellus

W-Link

Z-Com

Zyxel



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Useful links & resources

William Stallings. Data and Computer Communication

Pejman Roshan, Jonathan Leary. 802.11 Wireless LAN Fundamentals.Cisco Press

Bruce Alexander. 802.11 Wireless Network Site Surveying andInstallation. Cisco Press

IEEE Wireless Standardshttp://standards.ieee.org/getieee802/802.11.html

WLAN forumhttp://www.wlanforum.com

Linux WLAN pagehttp://www.linux-wlan.org

NS-2 simulatorhttp://www.isi.edu/nsnam


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