IEEE 802.11 Wireless LAN Standard
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Outline
n Wireless LAN Technology
n Medium Access Control for Wireless
n IEEE 802.11
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Wireless LAN Applications
n LAN Extensionn Cross-building interconnectn Nomadic Access
Ad hoc networkingn Ad hoc networking
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LAN Extension
n Wireless LAN linked into a wired LAN on same premisesn Wired LAN
n Backbonen Backbonen Support servers and stationary workstations
n Wireless LANn Stations in large open areasn Manufacturing plants, stock exchange trading
floors, and warehouses
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Multiple-cell Wireless LAN
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Cross-Building Interconnect
n Connect LANs in nearby buildingsn Wired or wireless LANs
n Point-to-point wireless link is usedDevices connected are typically bridges n Devices connected are typically bridges or routers
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Nomadic Access
n Wireless link between LAN hub and mobile data terminal equipped with antennan Laptop computer or notepad computern Laptop computer or notepad computer
n Uses:n Transfer data from portable computer to
office servern Extended environment such as campus
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Ad Hoc Networking
n Temporary peer-to-peer network set up to meet immediate need
n Example:n Group of employees with laptops convene n Group of employees with laptops convene
for a meeting; employees link computers in a temporary network for duration of meeting
n Military applicationsn Disaster scenarios
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Wireless LAN Parametersn Throughputn Number of nodesn Connection to backbone LANn Service arean Battery power consumption
Wireless Networks Spring 2005
n Battery power consumptionn Transmission robustness and securityn Collocated network operationn License-free operationn Handoff/roamingn Dynamic configuration
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Wireless LAN Categories
n Infrared (IR) LANsn Spread spectrum LANsn Narrowband microwave
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Strengths of Infrared Over Microwave Radion Spectrum for infrared virtually unlimited
n Possibility of high data rates
n Infrared spectrum unregulatedn Equipment inexpensive and simplen Reflected by light-colored objects
Wireless Networks Spring 2005
n Reflected by light-colored objectsn Ceiling reflection for entire room coverage
n Doesn’t penetrate wallsn More easily secured against eavesdroppingn Less interference between different rooms
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Drawbacks of Infrared Medium
n Indoor environments experience infrared background radiationn Sunlight and indoor lightingn Ambient radiation appears as noise in an n Ambient radiation appears as noise in an
infrared receivern Transmitters of higher power required
n Limited by concerns of eye safety and excessive power consumption
n Limits range
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Spread Spectrum LANs
n Multiple cell arrangementn Most popular type of wireless LANn Two configurations:
Hub topology: infrastructure mode
Wireless Networks Spring 2005
n Hub topology: infrastructure moden Peer-to-peer topology: multi-hop ad hoc
network
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Spread Spectrum LAN configurations
n Hub topology: n Mounted on the ceiling and connected to backbonen Need MAC protocoln May act as multiport repeatern Automatic handoff of mobile stationsn Stations in cell either:n Stations in cell either:
n Transmit to / receive from hub onlyn Broadcast using omnidirectional antenna
n Peer-to-peer mode:n No hubn Need a distributed MAC protocol
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IEEE 802 Protocol Layers
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Protocol Architecture
n Functions of physical layer:n Encoding/decoding of signals
n Preamble generation/removal (for synchronization)synchronization)
n Bit transmission/reception
n Includes specification of the transmission medium
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Protocol Architecture
n Functions of medium access control (MAC) layer:n On transmission, assemble data into a frame with
address and error detection fields
n On reception, disassemble frame and perform address recognition and error detectionrecognition and error detection
n Govern access to the LAN transmission medium
n Functions of logical link control (LLC) Layer:n Provide an interface to higher layers and perform flow
and error control
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Separation of LLC and MAC
n The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control
n For the same LLC, several MAC options n For the same LLC, several MAC options may be provided
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MAC Frame Format
n MAC controln Contains Mac protocol information
n Destination MAC addressn Destination physical attachment pointn Destination physical attachment point
n Source MAC addressn Source physical attachment point
n CRCn Cyclic redundancy check
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Logical Link Control
n Characteristics of LLC not shared by other control protocols:n Must support multiaccess, shared-medium
nature of the linknature of the link
n Relieved of some details of link access by MAC layer
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LLC Servicesn Unacknowledged connectionless service
n No flow- and error-control mechanismsn Data delivery not guaranteed
n Connection-mode serviceLogical connection set up between two usersn Logical connection set up between two users
n Flow- and error-control provided
n Acknowledged connectionless servicen Cross between previous twon Datagrams acknowledgedn No prior logical setup
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Differences between LLC and HDLC
n LLC uses asynchronous balanced mode of operation of HDLC (type 2 operation)
n LLC supports unacknowledged connectionless service (type 1 operation) connectionless service (type 1 operation)
n LLC supports acknowledged connectionless service (type 3 operation)
n LLC permits multiplexing by the use of LLC service access points (LSAPs)
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IEEE 802.11 Architecture
n Distribution system (DS)
n Access point (AP)
n Basic service set (BSS) n Stations competing for access to shared wireless n Stations competing for access to shared wireless
medium
n Isolated or connected to backbone DS through AP
n Extended service set (ESS) n Two or more basic service sets interconnected by DS
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IEEE 802.11 Services
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Overview, 802.11 Architecture
STASTASTA
APAP
ESS
BSSBSS
Existing Wired LAN
STASTA
STA STA
BSS
BSSBSS
BSS
Infrastructure Network
Ad Hoc Network
Ad Hoc Network
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Distribution of Messages Within a DS
n Distribution servicen Used to exchange MAC frames from station in
one BSS to station in another BSS
n Integration servicen Integration servicen Transfer of data between station on IEEE
802.11 LAN and station on integrated IEEE 802.x LAN
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Transition Types Based On Mobility
n No transitionn Stationary or moves only within BSS
n BSS transitionn Station moving from one BSS to another BSS n Station moving from one BSS to another BSS
in same ESS
n ESS transitionn Station moving from BSS in one ESS to BSS
within another ESS
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Association-Related Services
n Associationn Establishes initial association between station and AP
n Reassociationn Enables transfer of association from one AP to another,
allowing station to move from one BSS to another
n Disassociationn Association termination notice from station or AP
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Access and Privacy Services
n Authenticationn Establishes identity of stations to each other
n Deathenticationn Invoked when existing authentication is n Invoked when existing authentication is
terminated
n Privacyn Prevents message contents from being read by
unintended recipient
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IEEE 802.11 Medium Access Control
n MAC layer covers three functional areas:n Reliable data delivery
n Access control
n Securityn Security
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Reliable Data Deliveryn More efficient to deal with errors at the MAC
level than higher layer (such as TCP)n Frame exchange protocol
n Source station transmits datan Destination responds with acknowledgment (ACK)n Destination responds with acknowledgment (ACK)n If source doesn’t receive ACK, it retransmits frame
n Four frame exchangen Source issues request to send (RTS)n Destination responds with clear to send (CTS)n Source transmits datan Destination responds with ACK
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Access Control
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Medium Access Control Logic
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Interframe Space (IFS) Valuesn Short IFS (SIFS)
n Shortest IFSn Used for immediate response actions
n Point coordination function IFS (PIFS)Midlength IFSn Midlength IFS
n Used by centralized controller in PCF scheme when using polls
n Distributed coordination function IFS (DIFS)n Longest IFSn Used as minimum delay of asynchronous frames
contending for access
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IFS Usagen SIFS
n Acknowledgment (ACK)n Clear to send (CTS)n Poll response
n PIFSn PIFSn Used by centralized controller in issuing pollsn Takes precedence over normal contention traffic
n DIFSn Used for all ordinary asynchronous traffic
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MAC Frame Format
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MAC Frame Fields
n Frame Control – frame type, control information
n Duration/connection ID – channel allocation time
n Addresses – context dependant, types include source and destinationsource and destination
n Sequence control – numbering and reassembly
n Frame body – MSDU or fragment of MSDU
n Frame check sequence – 32-bit CRC
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Frame Control Fields
n Protocol version – 802.11 version
n Type – control, management, or data
n Subtype – identifies function of frame
n To DS – 1 if destined for DSn To DS – 1 if destined for DS
n From DS – 1 if leaving DS
n More fragments – 1 if fragments follow
n Retry – 1 if retransmission of previous frame
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Frame Control Fields
n Power management – 1 if transmitting station is in sleep mode
n More data – Indicates that station has more data to send
n WEP – 1 if wired equivalent protocol is implemented
n Order – 1 if any data frame is sent using the Strictly Ordered service
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Control Frame Subtypes
n Power save – poll (PS-Poll)
n Request to send (RTS)
n Clear to send (CTS)
Acknowledgmentn Acknowledgment
n Contention-free (CF)-end
n CF-end + CF-ack
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Data Frame Subtypesn Data-carrying frames
n Datan Data + CF-Ackn Data + CF-Polln Data + CF-Ack + CF-Polln Data + CF-Ack + CF-Poll
n Other subtypes (don’t carry user data)n Null Functionn CF-Ackn CF-Polln CF-Ack + CF-Poll
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Management Frame Subtypes
n Association request
n Association response
n Reassociation request
Reassociation responsen Reassociation response
n Probe request
n Probe response
n Beacon
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Management Frame Subtypes
n Announcement traffic indication message
n Dissociation
n Authentication
Deauthenticationn Deauthentication
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Wired Equivalent Privacy
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Authentication
n Open system authenticationn Exchange of identities, no security benefits
n Shared Key authenticationShared Key assures authenticationn Shared Key assures authentication
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Physical Media Defined by Original 802.11 Standardn Direct-sequence spread spectrum
n Operating in 2.4 GHz ISM bandn Data rates of 1 and 2 Mbps
n Frequency-hopping spread spectrumOperating in 2.4 GHz ISM bandn Operating in 2.4 GHz ISM band
n Data rates of 1 and 2 Mbps
n Infraredn 1 and 2 Mbpsn Wavelength between 850 and 950 nm
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IEEE 802.11a and IEEE 802.11bn IEEE 802.11a
n Makes use of 5-GHz bandn Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbpsn Uses orthogonal frequency division multiplexing
(OFDM)(OFDM)n Subcarrier modulated using BPSK, QPSK, 16-QAM or
64-QAM
n IEEE 802.11bn Provides data rates of 5.5 and 11 Mbpsn Complementary code keying (CCK) modulation
scheme
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