wireless lans - university of...

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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

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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

3

Applications


Wireless LAN Applications

• Retail

• Warehousing

• Healthcare

• Hotel & Restaurant

• Conference Hosting

• SOHO’s

• Certain Enterprise LANs

Technologies

4


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

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• 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

Rx

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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

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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

PC Client

PC Client

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Overlapping Cells

Wired Network

Cell A Cell B Cell C

AccessPoint

AccessPoint

AccessPoint


General LAN Configuration

Wired Network

AccessPoint


Wireless Bridges

Network B

Local Bridge

Remote Bridges

Network A

Network C

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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

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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

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• 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

BSS 1

DistributionSystem

BSS 2

AccessPoint

AccessPoint

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• 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

MAC

PHY

OSI Layer 2(Data Link)

OSI Layer 1(Physical)

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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

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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

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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

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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

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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

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• 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

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• 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

BITS

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

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DSSS PHY


SYNCStart FrameDelimiter

Signal Service FCS MPDU

128 16 8 8 16 Variable

# BITS

PLCPPreamble

PLCPHeader

Length

PPDU

16


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

Slots

PLCPPreamble

PLCPHeader

PSDU

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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

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802.11a PHY (OFDM)

• PLCP frame format

PLCPPreamble

RATE RESERVED PARITY SERVICELEN TAIL TAILPSDU

12 4 1 12 1 6 16 6 V

PAD

V

BITS


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

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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

NAV=0

Success!

IDLE ? Collision?

NO NO YES

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802.11 MAC Layer

contentionmedium busy next framenext frame

DIFS DIFS

PIFS

SIFS


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

BOe B

BOe B

BOe B BOrBOe

BOe BOr

DCF with CSMA/CA

Station 1

Station 2

Station 3

Station 4

Station 5

B

DIFS DIFS

BOe BOr

BOe BOr

DIFS DIFS

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C B

B

ACK

DCF with CSMA/CA

• Unicast ACKs

Sender

Receiver

OtherStations

DIFS

SIFS

DIFS

data

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

DIFS

SIFS SIFS

DIFS

SIFS

defer access

NAV (CTS)

NAV (RTS)

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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

BYTES

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Frame Control Field

PROTOCOLVERSION

TYPE SUBTYPEFROM

DSRETRY

TODS

MOREFRAG

MOREDATA

PWRMGMT

2 2 4 1 1 1 1 1

BITS

WEP ORDER

1 1 1

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

DS

-DASABSSID01To AP

-SABSSIDDA10FromAP

-BSSIDSADA00Ad hoc

Addr

4

Addr

3

Addr

2

Addr

1

From

Ds

To

DS

Tx

Type

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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

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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

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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

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