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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 Extension
n Cross-building interconnect
n Nomadic Access
nAd hoc networking
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LAN Extension
n Wireless LAN linked into a wired LAN onsame premises
n Wired LAN
n Support servers and stationary workstations
n Wireless LAN
n Stations in large open areas
n
Manufacturing plants, stock exchange tradingfloors, and warehouses
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Multiple-cell Wireless LAN
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Cross-Building Interconnect
n Connect LANs in nearby buildings
n Wired or wireless LANs
n Point-to-point wireless link is used
n ev ces connec e are yp ca y r gesor routers
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Nomadic Access
n Wireless link between LAN hub andmobile data terminal equipped withantenna
n Uses:
n Transfer data from portable computer tooffice server
n Extended environment such as campus
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Ad Hoc Networking
n Temporary peer-to-peer network set upto meet immediate need
n Example:
n for a meeting; employees link computers ina temporary network for duration ofmeeting
n Military applicationsn Disaster scenarios
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Wireless LAN Parameters
n Throughput
n Number of nodes
n Connection to backbone LAN
n Service area
Wireless Networks Spring
n Transmission robustness and security
n Collocated network operation
n License-free operation
n Handoff/roaming
n Dynamic configuration
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Wireless LAN Categories
n Infrared (IR) LANs
n Spread spectrum LANs
n Narrowband microwave
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Strengths of Infrared OverMicrowave Radio
n Spectrum for infrared virtually unlimited
n Possibility of high data rates
n Infrared spectrum unregulated
n Equipment inexpensive and simple
Wireless Networks Spring
n -n Ceiling reflection for entire room coverage
n Doesnt penetrate walls
n More easily secured against eavesdropping
n Less interference between different rooms
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Drawbacks of InfraredMedium
n Indoor environments experienceinfrared background radiation
n Sunlight and indoor lighting
infrared receiver
n Transmitters of higher power required
n Limited by concerns of eye safety and
excessive power consumptionn Limits range
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Spread Spectrum LANs
n Multiple cell arrangement
n Most popular type of wireless LAN
n Two configurations:
Wireless Networks Spring
n Hub topology: infrastructure mode
n Peer-to-peer topology: multi-hop ad hocnetwork
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Spread Spectrum LAN configurations
n Hub topology:n Mounted on the ceiling and connected to backbone
n Need MAC protocol
n May act as multiport repeater
n Automatic handoff of mobile stations
n a ons n ce e er:n Transmit to / receive from hub only
n Broadcast using omnidirectional antenna
n Peer-to-peer mode:n No hub
n 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
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
recogn on an error e ec on
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
,may be provided
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MAC Frame Format
n MAC control
n Contains Mac protocol information
n Destination MAC address
n
Destination h sical attachment ointn Source MAC address
n Source physical attachment point
n CRC
n 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 linkn Relieved of some details of link access by
MAC layer
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LLC Services
n Unacknowledged connectionless service
n No flow- and error-control mechanisms
n Data delivery not guaranteed
n Connection-mode service
n og ca connec on se up e ween wo usersn Flow- and error-control provided
n Acknowledged connectionless service
n Cross between previous two
n
Datagrams acknowledgedn No prior logical setup
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Differences between LLC and
HDLC
n LLC uses asynchronous balanced mode ofoperation of HDLC (type 2 operation)
n LLC supports unacknowledgedconnectionless service (type 1 operation)
n LLC supports acknowledged connectionlessservice (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 com etin for access to shared wirelessmedium
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
ExistingWired LAN
STASTA
STA STA
BSS BSS
Infrastructure Network
Ad HocNetwork
Ad HocNetwork
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Distribution of Messages Within
a DS
n Distribution service
n Used to exchange MAC frames from station in
one BSS to station in another BSS
n n egra on serv cen 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 transition
n Stationary or moves only within BSS
n BSS transition
n
Station movin from one BSS to another BSSin same ESS
n ESS transition
n Station moving from BSS in one ESS to BSSwithin another ESS
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Association-Related Services
n Association
n Establishes initial association between station and AP
n Reassociation
n Enables transfer of association from one AP to another,
allowing station to move from one BSS to another
n Disassociation
n Association termination notice from station or AP
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Access and Privacy Services
n Authentication
n Establishes identity of stations to each other
n Deathentication
n
Invoked when existin authentication isterminated
n Privacy
n Prevents message contents from being read byunintended recipient
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Reliable Data Delivery
n More efficient to deal with errors at the MAClevel than higher layer (such as TCP)
n Frame exchange protocol
n Source station transmits data
n es na on respon s w ac now e gmenn If source doesnt receive ACK, it retransmits frame
n Four frame exchange
n Source issues request to send (RTS)
n Destination responds with clear to send (CTS)
n Source transmits data
n 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) Values
n Short IFS (SIFS)
n Shortest IFS
n Used for immediate response actions
n Point coordination function IFS (PIFS)
n engn Used by centralized controller in PCF scheme when
using polls
n Distributed coordination function IFS (DIFS)
n Longest IFS
n Used as minimum delay of asynchronous framescontending for access
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IFS Usage
n SIFS
n Acknowledgment (ACK)
n Clear to send (CTS)
n Poll response
n
n Used by centralized controller in issuing polls
n Takes precedence over normal contention traffic
n DIFS
n 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 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 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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Data Frame Subtypes
n Data-carrying frames
n Data
n Data + CF-Ack
n Data + CF-Poll
- -n Other subtypes (dont carry user data)
n Null Function
n CF-Ack
n CF-Poll
n CF-Ack + CF-Poll
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Management Frame Subtypes
n Association request
n Association response
n Reassociation request
n 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
n Deauthentication
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Wired Equivalent Privacy
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Authentication
n Open system authentication
n Exchange of identities, no security benefits
n Shared Key authentication
n are ey assures au en ca on
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Physical Media Defined by
Original 802.11 Standard
n Direct-sequence spread spectrum
n Operating in 2.4 GHz ISM band
n Data rates of 1 and 2 Mbps
n Frequency-hopping spread spectrum
n pera ng n . z ann Data rates of 1 and 2 Mbps
n Infrared
n 1 and 2 Mbps
n Wavelength between 850 and 950 nm
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IEEE 802.11a and IEEE 802.11b
n IEEE 802.11a
n Makes use of 5-GHz band
n Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
n Uses orthogonal frequency division multiplexing
n Subcarrier modulated using BPSK, QPSK, 16-QAM or64-QAM
n IEEE 802.11b
n Provides data rates of 5.5 and 11 Mbps
n Complementary code keying (CCK) modulationscheme