Wcnppt Unit-5 Jwfiles

7/28/2019 Wcnppt Unit-5 Jwfiles

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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


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:


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

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