wireless lans bt

8/9/2019 Wireless Lans Bt

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Advanced NetworkingAdvanced NetworkingWireless LANsWireless LANs

Almerindo GrazianoAlmerindo Graziano


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Wireless LANsWireless LANs

The 802.11 Protocol StackThe 802.11 Protocol Stack

The 802.11 Physical LayerThe 802.11 Physical Layer

The 802.11 MAC Sublayer ProtocolThe 802.11 MAC Sublayer Protocol ServicesServices


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The 802.11 Protocol StackThe 802.11 Protocol Stack

Part of the 802.11 protocol stackPart of the 802.11 protocol stack


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IEEE 802.11 StandardsIEEE 802.11 Standards

802.11802.11 802.11a802.11a 802.11b802.11b 802.11g802.11g

StandardStandard

ApprovedApproved19971997 19991999 19991999 ExpectedExpected

mid 2003mid 2003

FrequencyFrequencyBandBand

2.4GHz2.4GHz 5GHz5GHz 2.4GHz2.4GHz 2.4GHz2.4GHz

Data Rate perData Rate perChannelChannel

Up to 2Up to 2MbpsMbps

Up to 54Up to 54MbpsMbps

up to 11up to 11MbpsMbps

up to 11up to 11MbpsMbps

ModulationModulationTechnologyTechnologyUsedUsed

DSSS &DSSS &FHSSFHSS

OFDMOFDM DSSSDSSS DSSS &DSSS &FHSSFHSS


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InfraredInfrared

FrequencyrangeFrequencyrange In the terahertz regions (10In the terahertz regions (101212Hz)Hz) beyond the control of the FCCbeyond the control of the FCC

TypesTypes

DiffusedDiffused Line of sightLine of sight

CharacteristicsCharacteristics Higher securityHigher security No RF InterferenceNo RF Interference

susceptible to ambient radiationsusceptible to ambient radiation(e.g., sunlight and indoor(e.g., sunlight and indoorlighting)lighting)

Limited rangeLimited range ProprietaryproductsProprietaryproducts

Infrared Data AssociationInfrared Data Association(IRDA)(IRDA) Not part of the 802.11Not part of the 802.11 Consortium of vendors forConsortium of vendors for

lowlow--cost IR communicationscost IR communications Line of sight communicationsLine of sight communications

of up to one meter (3ft)of up to one meter (3ft) 115115--Kbps and 4Kbps and 4--MbpsMbps

connectivityconnectivity

Ad hoc connectivityAd hoc connectivity

802.11802.11 Diffused at 1Diffused at 1--2 Mbps2 Mbps


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The 802.11 Physical LayerThe 802.11 Physical Layer

Industrial, Scientific, and MedicalIndustrial, Scientific, and Medical(ISM) bands(ISM) bands 902902--928 MHz928 MHz

2.42.4--2.4835 GHz2.4835 GHz

5.7255.725--5.825 GHz5.825 GHz

Frequency Hopping SpreadFrequency Hopping SpreadSpectrum (FHSS)Spectrum (FHSS) 79 channels of 1 MHz each79 channels of 1 MHz each

FSKFSK

Pseudorandom generator (PRG)Pseudorandom generator (PRG)determines the hopping sequencedetermines the hopping sequence Dwell timeDwell time time spent at eachtime spent at each

frequency (< 400 msec)frequency (< 400 msec)

All stations must share the same seedAll stations must share the same seedto the PRGto the PRG

Good resistance to radio interferenceGood resistance to radio interference

Low bandwidth (1Low bandwidth (1--2 Mbps)2 Mbps)

Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum(DSSS)(DSSS) PSK usingPSK usingchipschips(specific sequence of bits)(specific sequence of bits)

each bit is sent as a sequence of bitseach bit is sent as a sequence of bits

Spreading codeSpreading codeis the sequence of chips used tois the sequence of chips used tomodulate the carriermodulate the carrier

Higher costs, power consumption,Higher costs, power consumption,bandwidth (> 2 Mbps)bandwidth (> 2 Mbps)

More sensitive to noiseMore sensitive to noise Use directional antennas (building to buildingUse directional antennas (building to building

links)links)

Orthogonal FrequencyDivisionOrthogonal FrequencyDivisionMultiplexing (OFDM)Multiplexing (OFDM)

802.11a (up to 54 Mbps in the 5GHz band)802.11a (up to 54 Mbps in the 5GHz band) 802.11g (up to 54 Mbps in the 2.4 ISM band)802.11g (up to 54 Mbps in the 2.4 ISM band)

High RateDirect Sequence SpreadHigh RateDirect Sequence SpreadSpectrum (HRSpectrum (HR--DSSS)DSSS) 802.11b (up to 11 Mbps)802.11b (up to 11 Mbps)


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The 802.11 MAC Sublayer ProtocolThe 802.11 MAC Sublayer Protocol

(a)(a) The hidden station problemThe hidden station problem (b)(b) The exposed station problemThe exposed station problem


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EarlyWireless MACsEarlyWireless MACs

MACAMACA Send ReadyTo Send (RTS) and Clear To Send (CTS)Send ReadyTo Send (RTS) and Clear To Send (CTS)

RTS and CTS contains the length of the data frame to be sentRTS and CTS contains the length of the data frame to be sent

Multiple RTS can be sent bytwo or more stations (collision)Multiple RTS can be sent bytwo or more stations (collision) Wait using the binaryexponential backoff algorithmWait using the binaryexponential backoff algorithm

MACAW (MACA for Wireless)MACAW (MACA for Wireless) Use an ACK frame to acknowledge a successful data frameUse an ACK frame to acknowledge a successful data frame Carrier Sensing is also addedCarrier Sensing is also added


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The 802.11 MAC Sublayer ProtocolThe 802.11 MAC Sublayer Protocol

We cannot use the Ethernet CSMA/CD MACWe cannot use the Ethernet CSMA/CD MAC Hidden station/Exposed station problemsHidden station/Exposed station problems

Most communication are half duplexMost communication are half duplex

cannot transmit and listen for noises at the same time on a singlecannot transmit and listen for noises at the same time on a singlefrequencyfrequency

802.11 has two modes of operation802.11 has two modes of operation

Distributed Coordination Function (DCF)Distributed Coordination Function (DCF) "Similar" to Ethernet"Similar" to Ethernet

Must be implementedMust be implemented

Point Coordination Function (PCF)Point Coordination Function (PCF) Uses a base station to control all activities in its cellUses a base station to control all activities in its cell


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Distributed Coordination FunctionDistributed Coordination Function

DCF has two modes of operation (both use CSMA/CA) No collision detection (CD)

Exponential backoff algorithm (like Ethernet) if a collision occurs

First mode Transmit when the channel is idle (no CD)

Doesn't use RTS-CTS Second mode (based on MACAW)

Uses RTS/CTS (virtual channel sensing)

Stations hearing RTSs and CTSs produce a Network Allocation Vector (NAV) to assertchannel status NAV is not sent


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Frame FragmentationFrame Fragmentation

Wireless networks are more unreliable than wired ones

large frames can be corrupted withhigher probability

802.11 allows frames to be fragmented into smaller parts, each with its ownchecksum

Fragments are individuallynumbered and ACKed using stop-and-wait (k beforek+1)

Multiple fragments can be sent in a row (fragment burst)


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Point Coordination Function (PCF)Point Coordination Function (PCF)

The base station polls the other stations to checkThe base station polls the other stations to checkwhether theyhave frames to sendwhether theyhave frames to send

Polling order/frequency/service is not defined bythePolling order/frequency/service is not defined bythe

standardstandard

A beacon frame is sent periodically(10 to 100 times perA beacon frame is sent periodically(10 to 100 times persecond)second)

hopping sequence, dwell time, clock synchronization etc.hopping sequence, dwell time, clock synchronization etc.

The base station can also perform power managementThe base station can also perform power managementof other stationsof other stations

Send them to sleep and manage incoming framesSend them to sleep and manage incoming frames


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DCF& PCFDCF& PCF

DCF and PCF can coexist within one cell using appropriate interframe intervalsDCF and PCF can coexist within one cell using appropriate interframe intervals Short InterFrame Spacing (SIFS)Short InterFrame Spacing (SIFS)

time waited between packets in an ongoing dialog (RTS, CTS, data, ACK, next fragment)time waited between packets in an ongoing dialog (RTS, CTS, data, ACK, next fragment)

PCFInterFrame Spacing (PIFS)PCFInterFrame Spacing (PIFS) base station waits PIFS and jumps in (beacon or poll frame)base station waits PIFS and jumps in (beacon or poll frame)

DCFInterFrame Spacing (DIFS)DCFInterFrame Spacing (DIFS) base station does nothing, so anystation can jump in (SIFS is reset)base station does nothing, so anystation can jump in (SIFS is reset)

Extended InterFrame Spacing (EIFS)Extended InterFrame Spacing (EIFS) Bad or unknown frame report (lowest priority)Bad or unknown frame report (lowest priority)


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802.11 Services802.11 Services

Distribution ServicesDistribution Services: cell: cellmembership and interactionmembership and interaction

with stations outside the cellwith stations outside the cell AssociationAssociation connection to theconnection to the

base stationbase station DisassociationDisassociation disconnectiondisconnection

from the base stationfrom the base station

ReassociationReassociation "nest effort""nest effort"handover to another basehandover to another basestationstation

DistributionDistribution routing of framesrouting of frames

IntegrationIntegration address translationaddress translationbetween different WLANsbetween different WLANs

Intracell ServicesIntracell Services::management of activitiesmanagement of activities

within a cellwithin a cell AuthenticationAuthentication secure joinsecure join

(based on a challenge(based on a challenge--response)response)

DeauthenticationDeauthentication leave theleave thenetworknetwork

PrivacyPrivacy achieved throughachieved throughRC4RC4--encryptionencryption

DataDeliveryDataDelivery DCF/PCFDCF/PCF


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IEEE 802.16IEEE 802.16

"Air Interface for Fixed Broadband Wireless Access"Air Interface for Fixed Broadband Wireless AccessSystems" aka Wireless MAN or Wireless Local LoopSystems" aka Wireless MAN or Wireless Local Loop

Whywas a new standard needed?Whywas a new standard needed?

Buildings are "less" mobile and contain manymore usersBuildings are "less" mobile and contain manymore usersthan there usuallyare in a LANthan there usuallyare in a LAN More bandwidth is required (need the 10More bandwidth is required (need the 10--66 GHz range)66 GHz range)

Budget for interconnecting buildings is higherBudget for interconnecting buildings is higher Better hardware, full duplex communicationsBetter hardware, full duplex communications

Buildings can be verydistantBuildings can be verydistant

Wireless MANs need to provide support to voice and dataWireless MANs need to provide support to voice and data(QoS important)(QoS important)

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