Wireless Lans Bt
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Transcript of 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)