Redes Inalámbricas y Computación Ubicua.pdf

8/10/2019 Redes Inalmbricas y Computacin Ubicua.pdf

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Redes Inalmbricas y Computacin Ubicua/2006-2007

Tema 3.-

Tecnologas inalmbricas de red Aspectos bsicos de las comunicaciones inalmbricas

o Conceptos bsicoso Modelos de propagacin

Gran escalaPequea escala


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2

Decibelios

Los dB (deciBel) son una unidad adimensional utilizada pararepresentar ratios y ganancias en escala logartmica

o Las ganancias se suman en vez que multiplicarse

Ejemplo: clculo de la atenuacin T-Ro P T=100, P R=10

o [P T/P R]dB = 10 log(P T/P R) = 10 log(10) = 10 dB

Valores tiles:o [1/2] dB -3 dBo [1/1000] dB = -30 dB

Permiten expresar valores absolutos:o [n mW] dBm = [n/mW] dB Ej.: [1mW] dBm = 0 dBmo [n W] dBW = [n/W] dB Ej.: [1 mW] dBW = -30 dBW

De decibelios a potencia: P = 10dB/10

)log(log10log10 121

2101

2 p p p p

p p

dB=

=

dbm watt0 0.001

10 0.01

20 0.130 140 10

dbm watt0 0.001

10 0.01

20 0.130 140 10


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Antenas: conceptos bsicos

Non Isotropic Antennao Irradia la energa que recibe solo

en una seccin del espacio que

la rodea.

Isotropic Antennao Irradia la energa que recibe en

todo el espacio que la rodea.

YAGI Directional Antenna


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Antena Yagi exterior (13,5 dBi)

Alcance: 6 Km a 2 Mb/s2 Km a 11 Mb/s

Antena Parablica exterior (20 dBi)

Alcance: 10 Km a 2 Mb/s4,5 Km a 11 Mb/s

Antenas de largo alcance


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Antena dipolo diversidad paraefectos multipath

Antena de parche para montajeen pared interior o exterior (8,5 dBi)

Alcance: 3 Km a 2 Mb/s1 Km a 11 Mb/s

Horizontal Radiation

Ms ejemplos de antenas


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Bandas sin licencia en USA

Industrial, Scientific, and Medical (ISM)o 902 928 MHz band.

Ahora en disuso para WLAN

o 2400 2483.5 MHz ISM band.

Unlicensed National Information Infrastructure (UNII):o 5.15 5.25 GHz.o 5.25 5.35 GHz.o 5.725 5.850 GHz ISM band.

ExtremelyLow

VeryLow

Low Medium High VeryHigh

UltraHigh

Super High

Inf rar ed Vi si bl eLight

Ultra-violet

X-Rays

Audio AM BroadcastShort Wave Radio FM Broadcast

Television Infrared w ireless LAN

902 - 928 MHz26 MHz

Cellul ar (840MHz)NPCS (1.9GHz)

2.4 - 2.4835 GHz83.5 MHz

(IEEE 802.11)

5 GHz(IEEE 802.11)

HyperLANHyperLAN2


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Bandas sin licencia en Europa

Bandas aprobadas por el CEPT (European Conference of Postaland Telecommunications Administrations)

2400 2483.5 MHz, basada en la ISM.5.15 5.35 GHz.5.470 5.725 GHz.

ExtremelyLow

VeryLow

Low Medium High VeryHigh

UltraHigh

Super High

Inf rar ed Vi si bl eLight

Ultra-violet

X-Rays

Audio AM BroadcastShort Wave Radio FM Broadcast

Television Infrared w ireless LAN

Cellul ar (840MHz)NPCS (1.9GHz)

2.4 - 2.4835 GHz83.5 MHz

(IEEE 802.11)

5 GHz(IEEE 802.11)

HyperLANHyperLAN2


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Europe19 Channels

(*assumes noantenna gain)

1W200mW

Detalles de la banda de 5 GHz

5.15 5.35 5.470 5.725 5.8255 GHzUNII Band

5.25

UNII-1: Indoor Use, antenna must be fixed to the radioUNII-2: Indoor/Outdoor Use, fixed or remote antennaUNII-3: Outdoor Br idging Only (EIRP limit is 52 dBm if PtP)

UNII-140mW

(22 dBm EIRP)

UNII-2200mW

(29 dBm EIRP)

US (FCC)12 Channels(*can use up to

6dBi gain antenna)

UNII-3800mW

(35 dBm EIRP)

4 Channels

*if you use a higher gain antenna, you must reduce the transmit power accordingly

4 Channels 4 Channels11 Channels


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Calculo del Power Budget

PP rxrx == PP txtx+G+G papa --GG txltxl+G+G txatxa --LLpathpath +G+G rxarxa +G+G rara --GG rxlrxlo P tx[dBm]=Power generated by TXo G pa [dB]=Gain of the Power Amplifier

o G txa [dBi]=Gain of TX antennao G txl[dB]=Gain (loss) of transmission lineo Lpth [dB]=Loss of the transmission mediumo G rxa [dBi]=Gain of RX antenna

o G ra [dB]=Gain of the Receive Amplifier o G rxl[dB]=Gain (loss) of receiving lineo P rx[dBm]=Power receivedo S r [dBm]=Sensivity of receiver G txl

Debe valer la condicinDebe valer la condicin PrxPrx >> Sr Sr

EIRP (Effective Isotropically RadiatedPower) = P tx+Gpa+G txa-Gtxl

TX P A

RXR A

P tx G pa G TXA L path G rxa G ra S r

G txl G rxl


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Representacin grafica del power budget


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Caractersticas del canal inalmbrico

Theodore S. Rappaport , Wireless Communications : Principlesand Practice , Prentice Hall, July 1999

El canal inalmbrico sufre de los efectos de los dos siguientesfenmenos:

o Distancia Path attenuation (Large scale fading )

o Multipath (Small scale fading )dispersin en el tiempo debida a los diferentes caminos de la seal

La seal verde recorre 1/2 ms de la lnea enamarillo. El receptor recibe la seal en rojo.

Para f = 2,4 GHz , = c/f = 12.5cm

La seal verde recorre 1/2 ms de la lnea enamarillo. El receptor recibe la seal en rojo.

Para f = 2,4 GHz , = c/f = 12.5cm

T R


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Mecanismos/modelos de propagacin

Propagacin Free Spaceo Solamente ondas directas. Ejemplos: regiones cerca de la fuente,

cmaras anecoicas, antenas muy elevadas.

Propagacin terrestre:o Contacto visual (tierra plana):

Refraccin ( Refraction ): depende del tipo de los materiales conductores odielctricos

o Tierra esfrica:Diffraction: z onas de Fresnel

Propagacin en el espacio:

o Scattering , absorcin, ...


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Propagacin free space

Calculo de la potencia recibida cuando T&R tienen contacto visual(LOS)

o la atenuacin de la seal que resultara eliminando todos los efectos deabsorcin, difraccin, obstruccin, reflexin, dispersin.

Ecuacin de Friis :

Valida solo para d en la regin de Fraunhofer (far-field region ):el rea alrededor de una antena en la cual la radiacin aparecevenir de un solo punto

d > df = 2D 2/;

D es la dimensin ms grande de la antena longitud de onda de la portadora

222

2

)4()(

d P

K Ld

GGPd P t r t t r ==


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Difraccin

La difraccin ocurre cuando una ondagolpea el borde de un obstculo

o El exceso de longitud del recorridoproduce un desplazamiento de fase

o Las zonas de Fresnel meten enrelacin los desplazamientos de fasecon la posicin de los obstculos

Las zonas de Fresnel son elipses con T y Ren los focos

o L1 = L2+ o El campo visual (line of sight, LOS)corresponde a la primera zona

o Si la LOS est parcialmentebloqueada la zona 2 puede interferir

de forma destructiva

T R

1st Fresnel zone

Obstruction

Path 1

Path 2


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Tema 2.-

Tecnologas inalmbricas de red Aspectos bsicos de las comunicaciones inalmbricas

o Conceptos bsicos

o Modelos de propagacinGran escalaPequea escala


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Modelos de propagacin

Gran escala ( large scale ): predicen el comportamiento de una sealpromediado sobre grandes distancias ( )

o Depende de la distancia y de las caractersticas relevantes del entornoo Bsicamente independiente de la frecuencia

Pequea escala ( small scale fading ): describen las variaciones delas seal en escala con

o Domina el efecto del Multipath (phase cancellation ). La atenuacin sepuede considerar constante

o Depende de la frecuencia y del ancho de bandao Fading: representa cambios rpidos de la seal sobre pequeas

distancia o intervalos temporales


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Comparacin

Distancia

P o t e n c i a

10-100 m

(1-10 secs)

0.1 -1 m(10-100 msecs)

Exponencial

Large-scale Fading

Small-scale Fading


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Modelos de propagacin a gran escala: path loss

Se refiere al calculo de la potencia de la seal en el receptor enbase a la distancia d entre transmisor ( T) y receptor ( R)

Calculo del path loss : PL(d) = PL (d 0)+10 nlog(d/d 0) (dB)

PL(d 0) se calcula con la ecuacin de Friis, considerando G t=G r =L=1 :

=

==

d

d PP

d PLr

t )4(

log20)4(log10log10)( 222

0

T Rd

d0

d f


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Modelos de propagacin a gran escala: path loss un ejemplo

Datos: d=10km, f=900MHz, =c/f = 3*10 8/9*10 8 = 1/3mo d 0=1km

PL(d 0) = 20log(4 1000/ ) = 91,5 dB

o free space n=2PL(d) = PL (d 0)+10nlog(d/ d 0)= 91,5 + 10*2*log(10000/1000)= 111,5 dB

o ciudad n=3.5

PL(d) = PL (d 0)+10nlog(d/ d 0)= 91,5 + 10*3.5*log(10000/1000)= 126,5 dB 4-6Indoor no LOS

1.6-1.8Indoor LOS

3-5Shadowed urban area

2.7-3.5Urban area

2Free space

nEnvironment

T Rd

d0d f


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Modelos de propagacin a gran escala: modelos para exteriores

Modelos: 2-Ray Ground Reflectiono para d >> h r h t,

El bajo ngulo de incidencia permite a latierra de actuar como un reflector

La seal reflejada tiene un desfase de 180 P r 1/d 4 (n=4)

Modelos de difraccin para terrenosmontaosos ( hilly terrain )

o La propagacin puede pude ser LOS oresultar en la difraccin en las variascrestas

o Si no hay LOS la difraccin puede ayudar

...

R T

ht hr

Phase shift!


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Modelos de propagacin a gran escala: modelos para interiores

En los modelos para interiores se puede generalizar menos:o El entorno es ms dinmicoo Las caractersticas importantes son mas pequeaso Distancias son ms cercanas al near-fieldo Ms clutters , hay scattering , menos LOS

Tcnicas de modelado:o Log-Normal , n


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Propagacin radio en un entorno cerrado

7


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Otro ejemplo

7

7


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Modelos de propagacin a pequea escala:efectos del Multipath

Pequea escala ( small scale fading ): describen las variaciones delas seal en escala con

o Domina el efecto del Multipath (phase cancellation ). La atenuacin sepuede considerar constante

o Depende de la frecuencia y del ancho de bandao Fading: representa cambios rpidos de la seal sobre pequeas

distancia o intervalos temporales

Rpidas variaciones de la potencia de la seal a causa de la

cancelacin de faseModulacin de la frecuencia producida por Doppler shifts a causadel movimiento de R o T

7

7


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Modelos estadsticos del fading

Los modelos de Fading modelan la probabilidad de fading endeterminadas localizacionesLos modelos ms sencillos estn basados en el principio deWSSUS

o Wide Sense Stationary (WSS)Las estadsticas son independientes de pequeas perturbaciones en eltiempo y posicinEs decir, parmetros estadsticos fijos para nodos estacionarios

o Uncorrelated Scatter (US)

Los diferentes paths no estn correlatos en fase o atenuacinEs decir, Las componentes multipath componentes pueden ser VCindependientes

Distribuciones comuneso Rayleigh fading distribution

Models a flat fading signalUsed for individual multipath components

o Ricean fading distributionUsed when there is a dominant signal component, e.g. LOS + weaker multipaths

parameter K (dB) defines strength of dominant component; for K=- ,equivalent to Rayleigh


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Tema 3.- Tecnologas inalmbricas de red

Redes WLAN: IEEE 802.11o la capa fsica

o la capa MACo calidad de servicio: 802.11eo seguridad: WAP y 802.11i

o MIMO: 802.11no herramientas de gestin


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7

0 7


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El estndar IEEE 802.11

En el 1997 nace el:o IEEE Working Group for WLAN Standards:

http://grouper.ieee.org/groups/802/11/index.html

Se define el MAC y tres diferentes niveles fsicos, que operan a

1Mbps y 2Mbps:o Infrarrojos (IR) en banda baseo Frequency hopping spread spectrum (FHSS), banda de 2,4 GHzo Direct sequence spread spectrum (DSSS), banda de 2,4 GHz

IEEE Std 802.11b (septiembre 1999):o Extensin de DSSS; hasta 11 Mbps

IEEE Std 802.11a (diciembre 1999):o Otro estndar de nivel fsico (OFDM):

Orthogonal frequency domain multiplexingo Hasta 54 Mbps

IEEE Std 802.11g (junio 2003)...

PHYLayer

Infra-Red(IR)

5 GHz (OFDM)Orthogonal Frequency Division Multiplexing

2.4 GHz (DSSS)Direct Sequence Spread Spectrum

2.4 GHz (FHSS)Frequency Hopping Spread Spectrum

802.11 IR1 / 2 Mbit/s

802.11b / TGbHigh Data Rate Extension5.5/11 Mbit/s

802.11b-cor1 / TGb-cor1Corrigendum MIB

802.11g / TGgData Rates >20 Mbit/s

802.11d / TGdRegulatory Domain Update

802.11 FHSS1 / 2 Mbit/s

802.11 DSSS1 / 2 Mbit/s

802.11a / TGaHigh Data Rate Extension

6/12/24 Mbit/sOptional 9/18/36/54 Mbit/s

802.11h / TGhSpectrum Managed

802.11a

WLANs Next Gemeration SCGlobalization &Harmonization

0 7

0 7


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Tcnicas de Spread Spectrum

Beneficioso No se producen interferencias

con otras sealeso Mayor inmunidad al ruido

o Difciles de interceptar Frequency Hopping (FHSS)o Utiliza una de 78 secuencias del

salto. Salto a un nuevo canal del1MHz (sobre un total de 79 canales)por lo menos cada 400milliseconds

o Requiere la adquisicin y lasincronizacin del salto

Direct Sequence (DSSS)o Utiliza uno de los 11 canales

solapados y multiplica la seal para

una secuencia de spreading paraensanchar datos a 1M-symbol/secdata sobre 11MHz

X

=

symbol

Barker sequence

Result of multiplication

Symbol time t s1 0

Chip time t c

TimeTime

f f 33f f 22f f 11

f f 44f f 55f f 66f f 77

F r e q

.

F r e q

.

0 7

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802.11 DSSS Radio Interface

1 Mbpso 1 Msymbol/s BPSK spread by 11 chip Barker code,o (-4 dB Bandwidth = 11 MHz, main lobe = 22 MHz),o IEEE 802.11

2 Mbpso 1 Msymbol/s QPSK spread by 11 chip Barker codeo (-4 dB Bandwidth = 11 MHz, main lobe = 22 MHz),o IEEE 802.11

5.5 Mbpso 2 Msymbol/s QPSK like symbols spread by 8 chip Complementary Code Keying

(CCK).o IEEE 802.11b

11 Mbpso 4 Msymbol/s QPSK like symbols spread by 8 chip Complementary Code Keying

(CCK).o IEEE 802.11b

54 Mbpso OFDM with max. 52 sub-carriers,o IEEE 802.11a / IEEE 802.11g

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802.11 DSSS Frame Format

PPDU: PLCP protocol data unit

o Durante la transmisin las MPDU son precedidas por el prembulo y la cabecera del PLCP para crear la PPDUo El prembulo y la cabecera del PLCP son transmitidas utilizando una modulacin DBPSK a 1 Mbit/so Todos los bits son scrambled para obtener la funcin de data whitener

Synchronization (SYNC) fieldo Field allows receiver to perform necessary SYNC operationso Also provides for:

gain setting, energy detection, antenna selection, frequency offset compensation

Start Frame Delimiter (SFD) fieldo Indicates start of PHY-dependent parameters within PLCP PreambleSIGNAL field

o Indicates to PHY the modulation used for transmission (and reception) of the MPDUo Data rate = signal field value x 100 kbit/s

SERVICE fieldo Reserved for future use. Value of 00 signifies IEEE 802.11 device compliance

LENGTH fieldo Unsigned 16-bit integer. Indicates time (in S) required to transmit the MPDU

Header Error Check (HEC) fieldo SIGNAL, SERVICE, and LENGTH fields protected with a CCITT CRC-16 Frame Check Sequence (FCS)o All FCS calculations made prior to data scrambling

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802.11b High Rate DSSS Frame Format

Extension of IEEE 802.11 DSSS format Additionally provides 5.5 Mbit/s and 11 Mbit/s data ratesUses same PLCP preamble and header as DSSS

o both PHYs can co-exist in the same BSSo can use the rate switching mechanism as provided.

Two different preambles and headers are defined:o Long Preamble & Header

mandatory & supportedinteroperates with the current 1 Mbit/s and 2 Mbit/s DSSS spec

o Short Preamble & Header optionalwhere maximum throughput is desired

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0HR/DSSS: Long Frame Format

Same format as IEEE 802.11 DSSS FrameThe only exceptions are:

o Encoding of the rate in the SIGNAL field. Represent rate in units of 100 kbit/s0A for 1 Mbit/s, 14 for 2 Mbit/s, 37 for 5.5 Mbit/s, 6E for 11 Mbit/s

o SERVICE fieldRightmost bit (bit 7) used to supplement LENGTH field

resolve an ambiguity in PSDU length in octets, when length expressed in S

Bit 3 used to indicate whether modulation method is CCK or PBCC Bit 2 used to indicate that transmit frequency and symbol clocks are derived from same oscillator

An IEEE 802.11-compliant device shall set values of bits b0, b1, b4, b5, and b6 to 0

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0HR/DSSS: Short Frame Format

Short Synch fieldo Consists of 56 scrambled 0 bits.o So receiver can perform necessary SYNC

Start Frame Delimiter (SFD) field

o Time reverse of long PLCP SFD fieldo Not detected by non-compliant receivers

SIGNAL fieldo Only 1 Mbit/s removed

o Remaining fields same as HR DSSS Long Frame format

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0Los canales de 802.11b Overview

El estndar prev 14 canales de 22 MHz de amplitudo el FCC solo utiliza los primeros 11o en Espaa solo el 10 y el 11

3 canales que no solapan (1, 6,11)data rate de 11 MbpsTres puntos de acceso pueden estar cerca para obtener un total de 33 Mbps dethroughput agregado

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0estndar IEEE 802.11a

5 GHz, 54 Mbps, OFDM technologyo Data rates supported: 54, 48, 36, 24, 12, and 6 Mbpso Can downshift to lower data rates for longer range

Worldwide compatibility issues for 5 GHz bando Effort underway to allow 802.11a operation in European countrieso Long-term: Worldwide usage with adoption of Transmit Power Control

(TPC) and Dynamic Frequency Selection (DFS) per 802.11h standard

5 GHz band has more channels than 2.4 GHz bando UNII-1 + UNII-2 = 8 non-overlapping channels (vs. 3 channels for

2.4GHz)

5 GHz band subject to less interference than 2.4 GHz ISM bando However, 2.4GHz interference not a major problem in most business

environments

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0estndar IEEE 802.11g

Ratificado en junio del 2003 por el IEEE Standards Boardo draft standard preliminar presentado en diciembre de 2001;o ultimo draft v8.2

Utiliza la banda de 2.4 GHzo modulacin OFDM y codificacin PBCC

Compatible hacia atrs con IEEE 802.11bo Pueden co-existir en la misma WLAN

Nuevas velocidades de transmisin suportadas:o 6, 9, 12, 18, 24, 36, 48 & 54 Mbps

0 0 7

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Ejemplo de parmetros fsicos de un dispositivo real

DATA SHEET de un Cisco Aironet 802.11a/b/g CardBus WirelessLAN Client Adapter

Link:http://www.grc.upv.es/docencia/cosas/products_data_sheet09186a00801ebc29.html

T 3


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2 0 0 7

2 0 0 7

A it t di ibl


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Independent Basic Service Set (IBSS)o Estructura descentralizadao Flexible:

Redes pequeas y grandes,Redes transitorias y permanentes

o Control del consumo de potencia

infrastructure Basic Service Set (BSS)o Componentes:

Estacin (STA)

Access Point (AP)o Point Coordinator (PC)

o Basic Service Set (BSS)o Extended Service Set (ESS)

2 0 0 7

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El MAC t g d d t fi bl


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CSMA/CA con binaryexponential backoff El protocolo mnimo consiste dedos tramas: los datos y el

correspondiente ACKPointCoordinationFunction (PCF)

Distributed CoordinationFunction (DCF)

M A C

Servicios sincontienda

Servicios concontienda

DIFS DIFS

PIFS

SIFS

ventana de contienda

defer access

busy medium

s l o t

Los 5 valores de timing: Slot time SIFS: short interframe space (< slot

time) PIFS: PCF interframe space (=SIFS+1slot) DIFS: DCF interframe space (=SIFS+2slots) EIFS: extended interframe space

Los 5 valores de timing: Slot time SIFS: short interframe space (< slot

time) PIFS: PCF interframe space (=SIFS+1slot) DIFS: DCF interframe space (=SIFS+2slots) EIFS: extended interframe space

2 0 0 7

2 0 0 7

Estructura de las tramas


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FrameControl

FrameControl

Duration /ID

Duration /ID Addr. 1 Addr. 1 Addr. 2 Addr. 2 Addr. 3 Addr. 3

Seq.Control

Seq.Control Addr. 4 Addr. 4 FCSFCS

2 2 6 6 6 2 6 0-2312 4

management (00) control (01), data (10), reserved (11)

Tipos de direcciones:

Source address (SA)

Destination Address (DA)

Transmitter Address (TA) Receiver Address (RA)

BSS identifier (BSSID)

Tipos de direcciones:

Source address (SA)

Destination Address (DA)

Transmitter Address (TA)

Receiver Address (RA)

BSS identifier (BSSID)

SADATARA11Wireless DS

-DASARA = BSSDI01To the AP

-SABSSDIRA = DA10From the AP

-BSSDISARA = DA00IBSS

Addr. 4 Addr. 3 Addr. 2 Addr. 1FromDSToDSFuncin

FrameBody

FrameBody

prot.vers

prot.vers typetype subtypesubtype

ToDS

ToDS

FromDS

FromDS

Morefrags

Morefrags retryretry

Power mgt

Power mgt

Moredata

Moredata WEPWEP order order

2 2 4 1 1 1 1 1 1 1 1 bits

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2 0 0 7

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Addressing and DS bits

SADATARA11Wireless DS

-DASARA = BSSDI01To the AP

-SABSSDIRA = DA10From the AP

-BSSDISARA = DA00IBSS

Addr. 4Addr. 3Addr. 2Addr. 1FromDS

ToDS

Funcin

Server

DA

DSRA (BSSID)

SA/TA

Client AP

Server

SA AP

AP

TA

Client

RA

DA

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Ejemplo de funcionamiento del DCF


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Los intervalos de backoff son escogidos dentro de la congestionwindow . Es decir en el intervalo [0, CW]La CW puede variar entre 31 slots (CW min) y 1023 slots (CW max )CW se incrementa despus de una transmisin fallida y se re-inicializa despus de una transmisin con xito

data

wait

B1 = 5

B2 = 15

data

wait

B1 = 25

B2 = 20

B1 y B2 son intervalos de backoff en las STA 1 y 2CW = 31

B2 = 10

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Otro ejemplo


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El transmisor utiliza CW = CWmin (15 slots) del 802.11a y ha seleccionado unrandom backoff time de 12 slots.

- 2 0 0 7

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Situaciones problemticas


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Exposed nodeHidden node

A

B

C

A

B C

D

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Hidden nodes


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Obstacle Null

Opposite Side of AP

The receivedsignals cancelone anotherresulting in aNULL signal

The obstacleprevents MU1 andMU2 from hearingone another

MU3 cannot hearMU1 or MU2because of thedistance

Hidden nodes


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6 - 2

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Mecanismo RTS/CTS


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Se basa en el network allocation vector (NAV)

RTSDIFS

CTSSIFS

data

ACKSIFS SIFS

DIFS

NAV (RTS)NAV (CTS)

fuente

destino

otro STA

defer access

ventana de contienda

6 - 2

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6 - 2

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PCF: Point Coordination Function


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Data+Poll

DATA+ACK Beacon

Data+Poll

ACK

Station 2 sets NAV(Network Allocation Vector)

CF-End

PIFS SIFS SIFS SIFS SIFS

SIFS(no response)

PIFS

CP

PC

STA1

Contention Free Period CP

Data+Poll

SIFS

STA2 NAVReset

TimeSTA3 Station 3 is hidden to the PC, it does not set the NAV.It continues to operate in DCF.

Los beacon se utilizan para mantener la sincronizacin de los timersen las estaciones y para enviar informacin de control El AP genera los beacon a intervalos regulares Las estaciones saben cuando llegar el siguiente beacon

los target beacon transmission time (TBTT) son anunciados en elanterior beacon


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6 - 2

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Servicios


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La arquitectura IEEE 802.11 define 9 servicios: para la estacin ypara la distribucinStation services:

o Authentication

o Deauthenticationo Privacy WEPo Data delivery

Distribution services:

o Association genera una conexin entre STA y APo Disassociationo Reassociation como association pero informando del AP anterior o Distribution

o integration conexin de la WLAN con otras LANs;uso de un portal

Parecidos a conectar/desconectarel cable en una red tradicional

6 - 2

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0 0 7

Variables de estado y servicios


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State 1:unauthenticated,

unassociated

State 1:unauthenticated,

unassociated

State 2:authenticated,unassociated

State 2:authenticated ,unassociated

State 3:authenticated,

associated

State 3:authenticated,

associated

Disassociation notification

Successful authentication Deauthentication notification

Successful authenticationor reassociation

Class 1, 2 & 3frames

Class 1 & 2frames

Class 1frames

Deauthentication notification

En una IBSS no hay niauth., ni ass. Sepermite laimplementacin deldata service

Una STA puedeestar autenticadapor varios AP peroasociada con soloun AP

Una STA puedeestar autenticadapor varios AP peroasociada con soloun AP

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Scanning


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Parmetros : BSStype, BSSID, SSID, ScanType, ChannelList,ProbeDelay, Min/MaxChannelDelayScanType : Pasivo

o Las estaciones esperan beacons de los APs

ScanType : Activoo Las estaciones envan probe requests

Se genera un scan reportLa fase siguiente es la fase de joining; esta fase precede lasecuencia de acciones hasta llegar a la asociacin

0 6 - 2

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0 0 7

BSSID y SSID


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BSSID (Basic Service Set Identity )o BSS: direccin MAC del APo Ad-Hoc: numero random de 46 bit

SSID (Service Set ID )o Conocido como el Network Name por que bsicamente es el nombre

que identifica la WLANo Longitud: 0~32 octetos

0: el broadcast SSIDo Gestionado manualmente o de forma automticao identificador nico de 32 caractereso sirve para diferenciar las WLAN entre ellaso Los puntos de acceso y las estaciones que quieran conectarse a una

WLAN nica tienen que utilizar el mismo SSID

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0 0 7

Ahorro de potencia


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Los nodos que estn en la modalidad de ahorro de energa (PS mode) apagan eldispositivo radio durante un cierto intervalo de tiempoo La mayor parte de la potencia de la batera es utilizada por los circuitos de PHY RX

el PHY puede ser apagado cuando no se este transmitiendo auque la estacin sea activao En media la TX en una WLAN es activa menos del 2% del tiempoo los nodos transmisores tienen que bufferizar los frames

o El aumento de la latencia causado por el PS puede ser controlado reduciendo los timeoutsen los protocolos de alto nivelFcil de implementar con PFC. No tan fcil en DCFLos AP generan beacons con un time-stamp y lo transmiten cada intervalo de beaconing(~100 ms)

o la transmisin del beacon es aplazada si el canal est ocupada

o los nodos despiertan antes de que el intervalo de beaconing termina y permanecendespiertos hasta que el beacon es totalmente recibido

o los timestap son utilizados para actualizar los timer localesLos nodos son sincronizados para despertar al mismo tiempo cuando el transmisoranuncia frames buferizados

o los nodos que tienen que recibir frames estn despiertos hasta que el frame es entregado

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0 0 7

El Extended Service Set (ESS)


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BSSBSS

AP AP

WLANWLAN LANLAN

No definido por elestndar como realizarloExiste una propuesta deun grupo de empresas:Inter-acces point protocol(IAPP)

Distribution System (DS)

0 6 - 2

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0 0 7

Task Group f


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Scope of Project : to develop recommended practices for an Inter-Access Point Protocol (IAPP)which provides the necessary capabilities to achieve multi-vendor Access Point interoperabilityacross a Distribution System supporting IEEE P802.11 Wireless LAN Links.Purpose of Project : ... including the concepts of Access Points and Distribution Systems.Implementation of these concepts where purposely not defined by P802.11 ... As 802.11 basedsystems have grown in popularity, this limitation has become an impediment to WLAN market

growth.This project proposes to specify the necessary information that needs to be exchanged between Access Points to support the P802.11 DS functions. The information exchanges required will bespecified for, one or more Distribution Systems; in a manner sufficient to enable theimplementation of Distribution Systems containing Access Points from different vendors whichadhere to the recommended practices

Status : Work has been completed and is now part of the Standard as a recommended practice.

0 0 6 - 2

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0 0 6 - 2

0 0 7

Procedimiento de Handoff


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Fase de reassociationCuatro mensajes IAPP

o IAPP Latency > 4 * RTTLos mensajes de Move Request y deMove Response utilizan TCP

STA New AP Old AP

R e a s s o c i a t i o n R e q u e s t

M o v e N o t i f y

S e n d S e c u r i t y B l o c k

A c k S e c u r i

t y B l o c k

M o v e R e s p

o n s e

IAPP Messag

R e a s s o c i a t

i o n R e s p o n s e

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0 0 6 - 2

0 0 7

Wireless Distribution System


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62

IEEE 802.11, WDS meanso Multiple wireless ports inside the access-point, to wirelessly

interconnect cells (access-points connecting to other access-points)

pre-IEEE 802.11, did not support WDS:

o Three ports exist in one access-point (one Ethernet, and two wirelesscells)o One wireless backbone extension can be made (using two radio

modules in the access-point)

WDS allows:o Extending the existing infrastructure with wireless backbone linkso Totally wireless system without any wired backbones, needed in

locations where large areas are to be covered and wiring is not possible

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Channel 1

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WDS, consiste en puentear dos redes cableadas para obtenerconexin entre ellas, por ejemplo entre dos edificios de una mismaempresa

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WDS Wireless Distribution System


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Repeater: Consiste en un AP en modo WDS, que simplemente va arepetir las seales que capte de sus compaeros de bridge

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Operational processes Traffic flow - WDS operation


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STA-1STA-1 STA-2STA-2

BSS-A

BSS-B

Packet for STA-2

ACK

Packet for STA-2 ACK

AP-1000 or AP-500 AP-1000 or AP-500

Avaya Wireless PC-Card Avaya Wireless PC-Card

Association table

Bridge learntable

AP-1000 or AP-500 AP-1000 or AP-500

Avaya Wireless PC-Card Avaya Wireless PC-Card

Association table

Bridge learntable

STA-1

STA-2 2

STA-1

STA-

2STA-1

2STA-

2

2

2

W ire le s s

B a c k b o n e

WDSRelay

WDS

RelayPacket for STA-2

ACK

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Linksys Wireless-G Access Point


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o la capa MACo calidad de servicio: 802.11eo seguridad: WAP y 802.11io MIMO: 802.11no herramientas de gestin

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Limitations of the MAC standard for QoS IEEE802.11e


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DCF (Distributed Coordination Function)o Only support best-effort serviceso No guarantee in bandwidth, packet delay and jitter o Throughput degradation in the heavy load

PCF (Point Coordination Function)o Inefficient and complex central polling schemeo Unpredictable beacon frame delay due to incompatible cooperation

between CP and CFP modes

o Transmission time of the polled stations is unknown

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Overview of 802.11e IEEE802.11e


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U b i c u Task group e formed in Sep. 1999 and Approved in March 2000o Current version: IEEE P802.11e/D6.0, November 2003

o Backwardly compatible with the DCF and PCF

New QoS mechanism: HCF (Hybrid Coordination Function)

o Contention-based channel accessEDCA (Enhanced Distributed Channel Access)

was Enhanced Distributed Coordination Function ( EDCF )

o Controlled channel access (includes polling)HCCA (HCF controlled channel access)

The station that operates as the central coordinator for all otherstations within the same QoS supporting BSS (QBSS) is called thehybrid coordinator (HC).

o The HC reside inside an AP

A BSS that includes an 802.11e-compliant HC is referred to as aQBSS.


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EDCF (Enhanced DCF) IEEE802.11e


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U b i c u Review of DCFo CSMA/CA

o Transmit the frame directly if the medium is found idle for DIFS (DCFInterFrame Space)

o Otherwise, defer the transmission and start the backoff processo Backoff_time = rand[0, CW], CWmin < CW < CWmaxo The backoff timer decreases only when the medium become idle.o Transmit the frame once backoff timer expires

How to provide prioritieso Change the contention window sizenewCW[TC i] = ((oldCW[TC i]) * PF i ) 1PF i is the persistence factor

o Replace DIFS with AIFS (Arbitration InterFrame Space) AIFS[i] = DIFS + TC i

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EDCF IEEE802.11e


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U b i c u CSMA/CA and Exponential Backoff

Eight Traffic Categories (TCs) within one station

TC7

Backoff (AIFS)

TC6

Backoff (AIFS)

TC5

Backoff (AIFS)

TC4

Backoff (AIFS)

TC3

Backoff (AIFS)

TC2

Backoff (AIFS)

TC1

Backoff (AIFS)

TC0

Backoff (AIFS)

High priority Low priority

Scheduler

(resolve virtual collisions by granting permission to highest priority)Transmissionattempt

AIFS:Arbitration Inter-Frame Space

Backoff (AIFS)

Transmissionattempt

DCF

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EDCA parameters for AC IEEE802.11e


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4 access categories ( AC ), AIFS[AC] = SIFS + AIFSN[AC] * aSlotTime , AIFSN[AC] 2.

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EDCA and AC Mapping IEEE802.11e


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HCF superframes IEEE802.11e


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Performance IEEE802.11e


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Redes WLAN: IEEE 802.11o la capa fsicao la capa MACo calidad de servicio: 802.11eo seguridad: WAP y 802.11io MIMO: 802.11no herramientas de gestin

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Wireless LAN Security Issues

Issue 802.11 WEP Solution

WEP y IEEE802.11i


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Wireless LAN(WLAN)

Wireless sniffer can view all WLANdata packets

Anyone in AP coverage area can geton WLAN

Encrypt all data transmitted betweenclient and APWithout encryption key, user cannottransmit or receive data

Wired LAN

Goal: Make WLAN security equivalent to that of wired LANs (Wired EquivalentPrivacy)

client access point (AP)

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Wi d E i l t P i

WEP y IEEE802.11i


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U b i c Wired Equivalent Privacyo No worse than what you get with wire-based systems.

Criteria:o Reasonably strong

o Self-synchronizing stations often go in and out of coverageo Computationally efficient in HW or SW since low MIPS CPUs might be

usedo Exportable US export codes (relaxed in Jan 2000 / Wassenaar

Arrangement)o Optional not required to used it

Objectives:o confidentialityo integrityo authentication

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WEP How It Works

Secret key (40 bits or 104 bits)

WEP y IEEE802.11i


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U b i c Secret key (40 bits or 104 bits)o can use up to 4 different keys

Initialization vector (24 bits, by IEEE std.)o total of 64 or 128 bits of protection.

RC4-based pseudo random number generator (PRNG)Integrity Check Value (ICV): CRC 32

IV(4 bytes)

Data (PDU)( 1 byte)

Init Vector(3 bytes)

1 byte

Pad6 bits

Key ID2 bits

Frame header ICV(4 bytes) FCS

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WEP Encryption Process

1)Compute ICV using CRC 32 over plaintext msg

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U b i c Compute ICV using CRC-32 over plaintext msg.2) Concatenate ICV to plaintext message.

3) Choose random IV and concat it to secret key and input it to RC4 toproduce pseudo random key sequence.

4) Encrypt plaintext + ICV by doing bitwise XOR with key sequence toproduce ciphertext.5) Put IV in front of cipertext.

Initialization Vector (IV)

Secret Key

Plaintext

Integrity Algorithm

Seed WEP PRNG

KeySequence

Integrity Check Value (ICV)

IV

CiphertextMessage

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WEP Decryption Process

1) IV of message used to generate key sequence, k.

WEP y IEEE802.11i


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2) Ciphertext XOR k original plaintext + ICV.3) Verify by computing integrity check on plaintext (ICV) and

comparing to recovered ICV.

4) If ICV ICV then message is in error; send error to MACmanagement and back to sending station.

IVCiphertext

Secret Key

Message

WEP PRNGSeed

KeySequence

Integrity Algorithm

Plaintext

ICV

ICVICV - ICV

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WEP Station Authentication

Wireless Station (WS) sendsAuthentication Request to Access Point WS AP

WEP y IEEE802.11i


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U b Authentication Request to Access Point

(AP). AP sends (random) challenge text T.WS sends challenge response(encrypted T).

AP sends ACK/NACK.

WS AP Auth. Req.

Challenge Text

Challenge Response

Ack

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WEP Weaknesses

Forgery Attacko Packet headers are unprotected can fake src and dest addresses

WEP y IEEE802.11i


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U b o Packet headers are unprotected, can fake src and dest addresses.

o AP will then decrypt data to send to other destinations.o Can fake CRC-32 by flipping bits.

Replayo Can eavesdrop and record a session and play it back later.

Collision (24 bit IV; how/when does it change?)o Sequential: roll-over in < day on a busy neto Random: After 5000 packets, > 50% of reuse.

Weak Keyo If ciphertext and plaintext are known, attacker can determine key.o Certain RC4 weak keys reveal too many bits. Can then determine RC4

base key.

Well known attack described in Fluhrer/Mantin/Shamir paper o Weaknesses in the Key Scheduling Algorithm of RC4 , Scott Fluhrer,Itsik Mantin, and Adi Shamir

o using AirSnort: http://airsnort.shmoo.com/


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War Driving in New Orleans(back in December 2001)

WEP y IEEE802.11i


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802.11i Task Group Recommendations

Mutual AuthenticationDynamic Session Key

WEP y IEEE802.11i


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U Dynamic Session KeyMessage Integrity Check (MIC)Temporal Key Integrity Protocol (TKIP)

o Per-packet Key Hashingo Initialization Vector Sequencingo Rapid Re-Keying

Future

o Stronger encryption schemes such as AES


Redes WLAN: IEEE 802 11


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Ways to increase data rate

Conventional single tx and rx radio systemso Increase transmit power

IEEE802.11n


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101

o Increase transmit power Subject to power amplifier and regulatory limitsIncreases interference to other devicesReduces battery life

o Use high gain directional antennasFixed direction(s) limit coverage to given sector(s)

o Use more frequency spectrumSubject to FCC / regulatory domain constraints

Advanced MIMO: Use multiple tx and / or rx radios!

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Conventional (SISO): Wireless Systems IEEE802.11n


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102

Conventional Single Input Single Output (SISO) systems were favored for simplicityand low-cost but have some shortcomings:

o Outage occurs if antennas fall into nullSwitching between different antennas can help

o Energy is wasted by sending in all directionsCan cause additional interference to others

o Sensitive to interference from all directionso Output power limited by single power amplifier

channelRadioDSPBits

TX

Radio DSP Bits

RX

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MIMO Wireless Systems IEEE802.11n


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