wireless lans

Wireless LANs

Dr. Michael S. Boykin Spring 02-2 Local Area Networks 2

Outline

• Benefits

• Applications

• Technologies

• Issues

• Configurations

• Overview of 802.11 Standard

Outline II

• MAC layer protocols

• PHY layer protocols

• Implementation concerns

Introduction

• Standard vs. Proprietary protocols– Interoperable hardware

– Interoperable software

– Reduced prices

– New applications and uses

Benefits

Wireless LAN Benefits

• Mobility• Simple/fast installations

– Temporary connectivity– Difficult to wire situations

• Reliability– Immunity from cable plant problems

• Reduced TCO for MAC’s– Moves, adds and changes

Applications

Wireless LAN Applications

• Retail

• Warehousing

• Healthcare

• Hotel & Restaurant

• Conference Hosting

• SOHO’s

• Certain Enterprise LANs

Technologies

Wireless LAN Technologies

• IEEE 802.11– 1 Mbps and 2 Mbps– 2.4GHz spectrum– 2-PHYs FHSS, DSSS

• IEEE 802.11b– Most common– Extended data rates thru 11 Mbps– 2.4 Ghz

• IEEE 802.11a– Up to 54 Mbps– 5 GHz spectrum– OFDM PHY modulation

• HiperLAN/1– Up to 24 Mbps

– QoS support

– 5 GHz specturm

• HiperLAN/2– in development

– Up to 54 Mbps– QoS support

– Ethernet, ATM and IP transport standards

• HomeRF SWAP– Shared Wireless Access Protocol

– Support communications between PCsand consumer devices

– 1 to 2 Mbps support

– FHSS modulated PHY

– 10 Mbps in developent

• Bluetooth–Radio-based wireless PAN

– PAN: personal area network

– FHSS modulated PHY

– 2.4Ghz spectrum

Issues

Wireless LAN Issues

• Multipath propagation

Office WallTx

Office Furniture

Wireless LAN Issues

• Hidden terminal problem

TerminalA

TerminalB

Barrier

Access Point

Wireless LAN Issues

• Path loss• Signal interference

– Inward interference–Outward interference

• Battery life

InwardW

OutwardW

Wireless LAN Issues

• Interoperability• Network Security

Property A Property B

Bldg Bldg

Public Road

Wireless LAN Issues

• Application Connectivity Problems– Addressing

– Connection reliability

• Installation IssuesOmnidirectional antenna radiation

Configurations

Ad-Hoc Networks (Peer to Peer)

Single CellRadio LAN

PC Client

Overlapping Cells

Wired Network

Cell A Cell B Cell C

AccessPoint

General LAN Configuration

Wired Network

AccessPoint

Wireless Bridges

Network B

Local Bridge

Remote Bridges

Network A

Network C

Infrared LAN Configuration

CeilingInfraredLight

802.11 Overview

Goals of 802.11

• Asynchrounous, time-bounded delivery

• Continuity of servic via distribution network

• Transmission speeds of 1 Mbps and 2 Mbps

• Wide application support

• Multicast services

• Network management services

• Registration and authentication

802.11 Overview

• Target Environments– Inside buildings– Select outdoor areas

• Target concerns– Power management– Bandwidth efficiency– Security– Addressing

Benefits of 802.11

• Appliance interoperability

• Fast product development

• Stable future migration

• Price competition

• Silo avoidance

IEEE 802.11 Topology

• 2 standard topologies composed ofstations, access points, distribution systemsportals.

• IBSS- Independent Basic Service Set– Standalone BSS with no backboneinfrastructure

• ESS- Extended Service Set– Multiple BSS’s interconnected by accesspoints and a distribution system

• Station– any device that contains functionality of the802.11 protocol– The functions named in the standard physically reside in theNIC

• Access point– an addressable station providing aninterface to the distribution system for stations located invarious BSS’s

• Distribution system– network element used to connectBSSs within the ESS via access points

• Portals– logical point of entry and exit for 802.11 frames– The access point for 802-type distribution systems

IBSS Topology

Station A

Station B

Basic Service Set(BSS)

Single CellPropagationBoundary

ESS Topology

DistributionSystem

AccessPoint

• BSS configurations within an ESS– Partially overlap

• Contiguous coverage, no disruption

– Physically disjointed• Non-contiguous coverage, possible disruption

– Physically co-located• Used for redundancy and high performance

• Mobility Support– No-transition

• Fixed or “local” stations only

– BSS-transition• Stations move between BSSs of the same ESS

– ESS transition• Stations move between BSSs of different ESSs

IEEE 802 Family

• Local and Metropolitan Area NetworkStandards Committee

IEEE 802.2Logical Link Control (LLC)

IEEE802.3

CarrierSense

IEEE802.4TokenBus

IEEE802.5TokenRing

IEEE802.11

Wireless

OSI Layer 2(Data Link)

OSI Layer 1(Physical)

802.11 Services

• Functions required by the LLC to sendMAC SDUs

• 2 types– Station services

– Distribution services

802.11 Station Services

• Authentication– Open systems– Shared key

• De-authentication– Irrefutable

• Privacy – optional– All data frames– Some management frames

Optional Privacy Service

Encryption

Decryption

Plain Text

Cipher Text

Plain Text

WirelessMedium

Network Join

• Passive scanning—– Listen for access point beacons with correctSSID

• Active scanning-– Probe request packet with SSID of desirednetwork

– Probe response– Probe broadcast packet to have allreachable network respond

802.11 Distribution Services

• Association– Station associates with single access point– Access point can associate with multiple stations

– First step to support station mobility

– Indicates whether a station is pollable

• Disassociation– When leaving network

– When going offline

802.11 System Services

• Distribution– Transmission of MAC frame across distributionnetwork

• Integration– Media and address translation for non-802.11LANs

• Reassociation– Initiated by mobile station to support BSS-transitions

Station State Transition

Class 1, 2 & 3Frames Permitted Class 1 & 2

Frames Permitted

State 1(Unauthenticated,

Unassociated)

State 2(Authenticated,Unassociated)

State 3(Authenticated,

Associated)

Class 1Frames Permitted

Unassociated)

Associated)

Frames Permitted

SuccessfulAuthentication

DeauthenticationNotification

Unassociated)

Associated)

Frames Permitted

Successful Association,Reassociation

DisassociationNotification

Unassociated)

Associated)

Successful Association,Reassociation

DisassociationNotification

Frames Permitted

Frame Types

• Class 1 Frames– Control Frames –

• RTS/CTS, ACK, Contention-free

– Management Frames –• Probe request/response, beacon,

• Authentication, deauthentication

• Announcement traffic indication message

– Data Frame

Frame Types

• Class 2 Frames– Management Frames

• Association request/response• Reassociation request/response• Disassociation

• Class 3 Frames– Data & Management Frames– Deauthentication– Control Frames– Power Save Poll

Physical Layer

Physical Layer Management

• Convergence Procedure (PLCP)– Communicates with MAC layer to handleTx and Rx of wireless frames

• Media Dependent Sublayer (PMD)– Provides actual Tx and Rx functions

– De/Modulates on wireless medium

• Operations– Carrier Sense

– Transmit

– Receive

• Carrier Sense function at multiple levels– MAC level performs “clear channelassessment”

– CCA causes PLCP to direct PMD to checkwhether the medium is busy or idle

– If station is not transmitting a frame, PLCPsearches for preamble

• Transmit Function– MAC layer sends data and rate directive toPHY

– PHY sends preamble to antenna within20 msec

• Preamble and header sent @ 1 Mbps

• Payload sent at specified rate

• Receive Function– PLCP senses busy medium

– PLCP receives error-free header

– PLCP interprets data rate and length field

– PLCP transfers header to MAC

– PLCP receives payload

– PLCP delivers payload to MAC

• 802.11 PHYs 2.4 GHz– Initially 1 or 2 Mbps– FHSS, DSSS or IR

• 802.11b– DSSS extended to HR-DSSS– Employs CCK for 5.5 and 11 Mbps operation

• 802.11a– Uses OFDM for data rates up to 54 Mbps @ 5 GHz– Employs multiple frequencies each supportingmultiple phase and amplitude modulation

FHSS PHY

• PLCP Frame format

SYNCStart FrameDelimiter PLW PSF

Header ErrorCheck

WhitenedPSDU

80 16 12 4 16 Variable

PLCPPreamble

PLCPHeader

FHSS Modulation

• Hop from channel to channel to channel ina pseudo-random sequence.

• Tx/Rx at each frequency for a specificamount of time– dwell time

• Binary 0 is represented by a fixed negativedeviation from the center frequency

• Binary 1 is represented by a positivedeviation of the same magnitude

DSSS PHY

SYNCStart FrameDelimiter

Signal Service FCS MPDU

128 16 8 8 16 Variable

# BITS

PLCPPreamble

PLCPHeader

Length

DSSS Modulation

• Up to 14 channels each 22 MHz wide

• Differential Binary PSK for 1 Mbps

• Differential Quadrature PSK for 2 Mbps

Infrared (IR) PHY

SYNCStart FrameDelimiter

DataRate

DC LevelAdjustment

Length FCS

57-73 4 3 32 16 16

PLCPPreamble

PLCPHeader

IR Modulation

• 16 PPM for 1 Mbps transmission– Creates a 16 position encoding for 4 bits– 16 entry table in gray-code order

• Column 1 has the 4 data bit values• Column 2 has the 16 position encoding

– Encoding places a 1 in the position equal to thetable row number. E.g.

• Row 1: 0000 0000000000000001• Row 2: 0001 0000000000000010• Row 3: 0011 0000000000000100• Etc.

IR Modulation

• 4 PPM for 2 Mbps transmission– Creates a 4 position encoding for 2 bits– 4 entry table in gray-code order

• Column 1 has the 2 data bit values• Column 2 has the 4 position encoding

– Encoding places a 1 in the position equal to thetable row number.

• Row 1: 00 0001• Row 2: 01 0010• Row 3: 11 0100• Row 4: 10 1000

802.11b PHY (HR-DSSS)

• Same as 802.11 DSSS for 1 and 2 Mbps• Similar frame format

– Shorter sync field 56 bits– Expanded signal field for 5.5 and 11 Mbps– Expanded service field values

• Uses CCK to spread the frequencies• Uses bit-position dependent phases to

signal multiple bits

802.11a PHY (OFDM)

• PLCP frame format

PLCPPreamble

RATE RESERVED PARITY SERVICELEN TAIL TAILPSDU

12 4 1 12 1 6 16 6 V

802.11a PHY (OFDM)

• Divide high-speed serial data stream acrossmultiple lower speed sub-signals

• Transmit the multiple sub-signalssimultaneously at different frequencies

MAC Layer

802.11 MAC Layer

• Transimission Services– Asynchronous data service (mandatory)

– Time-bounded service

• Access Methods– CSMA/CA

– Station reservation (hidden terminal)

– Contention-free polling

802.11 MAC Layer

• Access method Implementation– Distributed Coordination Function (DCF)– Point Coordination Function (PCF)

• Frame Spacing– DIFS– DCF Interframe Spacing

• Lowest priority for asynchronous data

– PIFS– PCF Interframe Spacing• Medium priority for time-bounded data

– SIFS– Short Interframe Spacing• Highest priority for control, polling and acks

DCF with CSMA/CA

sense thechannel

Txframe

RandomBackoff

Success!

IDLE ? Collision?

NO NO YES

802.11 MAC Layer

contentionmedium busy next framenext frame

DIFS DIFS

802.11 MAC Layer

• Contention Interval Issues– Backoff timer management

• Randomized backoff timer

• Residual backoff timer

– Congestion window management• Load dependent

• Exponential increase 7 .. 255– Light load … short delay

– Heavy load… reduce probability of additional collisions

BOe B BOrBOe

BOe BOr

DCF with CSMA/CA

Station 1

Station 2

Station 3

Station 4

Station 5

DIFS DIFS

BOe BOr

DIFS DIFS

DCF with CSMA/CA

• Unicast ACKs

Sender

Receiver

OtherStations

contention

wait time

DCF RTS/CTS

• Hidden terminal solution– Two stations unable to sense each other’stransmission that collides at the receiver

• Goal have senders schedule theirtransmission via the NAV

• Collisions possible only at the initial phase

• Threshold based due to overhead

DCF RTS/CTS

ACKCTS

dataRTSSender

Receiver

OtherStations

SIFS SIFS

defer access

NAV (CTS)

NAV (RTS)

DCF RTS/CTS

• Fragment Solution– May be useful for error-prone environs

– Transparent to LLC and USER

– Send new NAV with each fragment

• Fragment and ACK transmission causeNAV update

PCF Polling

• SuperFrame concept– Contention-free interval reserved via NAV– Contention interval

• Point Coordinator Frame Types– Data Frame– CF Poll Frame– Data+CF Poll Frame– CF End Frame

Frame Format

FRAMECONTROL

DurationID

ADDRESS1

ADDRESS3

ADDRESS4

ADDRESS2

SEQCONTROL

CRCDATA

2 2 6 6 6 2 6 [0-2313] 4

Frame Control Field

PROTOCOLVERSION

TYPE SUBTYPEFROM

DSRETRY

MOREFRAG

MOREDATA

PWRMGMT

2 2 4 1 1 1 1 1

WEP ORDER

Bit 0 Bit 15

Transmission Types

• Ad hoc network

• Infrastructure network, from AP

• Infrastructure network, to AP

• Infrastructure network, within DS

Address Interpretation

SADATARA11W/I

-DASABSSID01To AP

-SABSSIDDA10FromAP

-BSSIDSADA00Ad hoc

Station Synchronization

• Distributed clock management

• Infrastructure-based networks– Access point transmits (quasi) periodicbeacon w/ timestamp

• Ad hoc networks– Each station schedules beacon transmission

– Winning station causes others to reschedule

Power Management

• Goal– Allow stations to enter “sleep” mode toconserve power

– Have senders buffer data for receivers in“sleep” mode

– All stations wake up for beacon

– Delivery outstanding frames

– Allow stations to go back to “sleep”

Power Management

• Infrastructure networks– TIM– traffic indication map

• Sent with beacon message to identify receiverswith outstanding unicast frames

– DTIM– delivery traffic indication map• Special TIM interval for sending broadcast &

multicast traffic

• Multiple of the the TIM interval

Power Management

• Ad hoc networks– ATIMs- ad hoc TIMs

• Sent by all stations during beacon interval

• Lists all stations with buffered frames

Power Management

• Issues– TIM, DTIM sizing

• Too small – little power savings

• Too large – many buffered frames

– ATIM scalability• Large number of sleeping stations imply large

ATIM window. See above.

• Large number of stations with large ATIMs implyhigher collision rate

Mobility Support

• Typical networks require multiple accesspoints to cover all areas

• Access point coverage 10-20m radius

• Moving between access points– Roaming

Mobility Support

• Roaming steps– Scan for access points– Select “best” access point for association

– Transmit association request

– The access point transmits an association response

– Access points updates• Its BSS station database

• the DS to allow it to update its station database

• May inform old access point of update information

wireless lans - university of...

Documents

chapter 6 high-speed lans 1 chapter 6 high-speed lans

switched and wireless lans. 2 shared media lans u limits to...

connecting lans, backbone networks, andvirtual lans ·...

c07 wireless lans

cs3502 , lans. objectives

lans consult profile

4/20/09 wireless lans - university of notre...

ccn4_wired lans ethernet

connecting lans

wireless lans & pans

more on lans

lans and wlans lans and wlans1. network building blocks ...

defining lans

7 wireless lans

introducing wireless lans

virtual lans

understanding virtual lans

capítulo 15 conexões de lans, redes backbone e lans...

pervasive wireless lans