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WLAN and WLAN and 802.11 802.11

ArchitectureArchitectureAshish BabbarAshish Babbar

EE668EE668

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OverviewOverview What are WLANsWhat are WLANs Why use WLANsWhy use WLANs Technology OverviewTechnology Overview 802.11 standard802.11 standard Reference modelReference model Various standardsVarious standards 802.11 Architecture802.11 Architecture

Basic Service SetBasic Service Set Ad hoc modeAd hoc mode Infrastructure modeInfrastructure mode

Distribution SystemDistribution System Extended Service SetExtended Service Set Integration with wired LANsIntegration with wired LANs

Channel ReuseChannel Reuse Channel AllocationChannel Allocation Channel Access in 802.11Channel Access in 802.11 Comparison between 802.11a and 802.11bComparison between 802.11a and 802.11b

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What are WLAN’sWhat are WLAN’s A supplement to wired LANS!! LANs that use radio waves as a transport medium

Transmission can occur using radio, microwaves or Infrared.

Configured with transmitter/receiver (access point or AP) that is connected to a wired network

Accessed by end users with wireless LAN adapters (usually cards in notebook computers)

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Why WLAN’sWhy WLAN’s LANs were originally created as short LANs were originally created as short

distance data communication networks.distance data communication networks. Ease of Installation. Mobility Timesaving / Convenient / Logistically easy Allows users to work where it’s most

convenient Cost advantages

Hard-to-wire auditoriums and classrooms Impossible-to-wire outdoor spaces Expandable at any time

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Technology overview

How does is work? Access Points — stationary transceivers APs act as broadcast and receive base

stations for wireless network traffic APs can offer encryption, security and

other services, including access Once you have an Access Point in

operation, wireless networking is enabled in its vicinity

Radio frequency signal coverage typically ranges from 100 to 300 feet

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802.11 Standard802.11 Standard 802.11 refers to specifications developed by the IEEE 802.11 refers to specifications developed by the IEEE

for wireless LAN technology.for wireless LAN technology.

802.11 specifies an over-the-air interface between a 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless client and a base station or between two wireless clients.wireless clients.

802.11 is based on the Open System interconnection 802.11 is based on the Open System interconnection (OSI) model.(OSI) model.

IEEE 802.11 framework defines the lowest two layers IEEE 802.11 framework defines the lowest two layers of the OSI model- the physical and the data link of the OSI model- the physical and the data link layers.layers.

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Reference ModelReference Model The 802.11 deals with only the data link layer and The 802.11 deals with only the data link layer and

the physical layer.the physical layer. The Data link layer is divided into the MAC layer The Data link layer is divided into the MAC layer

and LLC layer.and LLC layer. The objective of the standard is to define a The objective of the standard is to define a

mechanism for delivery of MAC service data units mechanism for delivery of MAC service data units (MSDUs) between the Logic Link controls(MSDUs) between the Logic Link controls

The subdivision makes it possible to support The subdivision makes it possible to support different wireless media, such as radio frequency different wireless media, such as radio frequency signaling and infrared transmission.signaling and infrared transmission.

For the radio frequency signaling the frequency For the radio frequency signaling the frequency hopping spread spectrum (FHSS) or direct hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS)sequence spread spectrum (DSSS) is usedis used

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Reference ModelReference Model

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Spread Spectrum Spread Spectrum methodsmethods

Frequency Hoping Spread SpectrumFrequency Hoping Spread Spectrum A narrowband signal moves or hops from A narrowband signal moves or hops from

one frequency to another using one frequency to another using pseudorandom sequence to control pseudorandom sequence to control hopping.hopping.

Signal lingers in the pre-defined Signal lingers in the pre-defined frequency for a short period of timefrequency for a short period of time

The possibility of interference from The possibility of interference from another signal source generating power another signal source generating power at the specific hop frequency is reduced.at the specific hop frequency is reduced.

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Spread Spectrum Spread Spectrum methodsmethods

Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum The data entering a transmitter is combined The data entering a transmitter is combined

with a higher data rate bit sequence.with a higher data rate bit sequence. This bit sequence is called chipping code This bit sequence is called chipping code

and is known to both the transmitter and and is known to both the transmitter and the receiver.the receiver.

Chipping code forms the basis of spreading Chipping code forms the basis of spreading data over a wide range of frequencies.data over a wide range of frequencies.

At receiver the same chipping code is At receiver the same chipping code is employed to recover the original data.employed to recover the original data.

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Various 802.11 Various 802.11 StandardsStandards

802.11 Specification of WLAN MAC and PHY layers (IR, FH and DSSS at 2.4 GHz), ratified in 1997/1999

802.11a PHY layer at 5 GHz (54 Mbps OFDM), ratified in 1999

802.11b 11 Mbps DSSS at 2.4 GHz, ratified in 1999 802.11c Improvements of the MAC layer (Internal

Sub-Layer Service) 802.11d Update (frequency spectrum regulations) 802.11e Improvements of the MAC layer (Quality of

Service) 802.11f Inter-Access Point Protocol (IAPP) 802.11g Higher Data rate (>20 Mbps) at 2.4 GHz 802.11h Dynamic Channel Selection and Transmit

Power Control mechanisms 802.11i Authentication and Security

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802.11 Frequency Bands802.11 Frequency Bands

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802.11 Architecture

The basic building block of an 802.11 network is the Basic Service Set (BSS), which is simply a group of stations communicate with each other.

Communication take place within a somewhat fuzzy area, called the Basic Service Area.

When a station is in the basic service area, it can

communicate with the other member of the BSS

When a station moves out of its BSS, it can no longer directly communicate with each other.

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STA1

STA2

STA3

STA4BSS1

BSS2

Basic Service SetBasic Service Set

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Basic Modes of BSSBasic Modes of BSS An ad-hoc or peer-to-peer mode - One “Basic Service Set”, BSS

- A set of wireless stations communicate directly with one another.

- No need of a central access point or wired network connection

- Limited coverage area

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Ad-Hoc modeAd-Hoc mode

STA 1 STA 2

STA 4STA 3

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Basic Modes of BSSBasic Modes of BSS

An infrastructure mode - At least one central access point connected

to a wired network.

- Access points are used for all communications in infrastructure networks

- Communications between mobile nodes in the same service area is also done through Access points.

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Infrastructure modeInfrastructure modeWLAN Distributed System

STA 1

STA 2

STA 3

Access Point

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Distribution System Distribution System ConceptConcept

PHY limitations determine the direct PHY limitations determine the direct station-to- station distance supported.station-to- station distance supported.

Instead of existing independently, the BSS Instead of existing independently, the BSS may interconnected.may interconnected.

The Distribution system (DS) enables the The Distribution system (DS) enables the interconnection of the BSS.interconnection of the BSS.

Data moves between the BSS and the DS Data moves between the BSS and the DS via an access point.via an access point.

The DS can represent an existing LAN The DS can represent an existing LAN infrastructure or a twisted pair wire that infrastructure or a twisted pair wire that simply interconnects two access points.simply interconnects two access points.

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Distribution SystemsDistribution Systems

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Extended Service Set Extended Service Set (ESS)(ESS)

The DS and BSSs allow IEEE 802.11 to create a wireless network of arbitrary size and complexity, called the ESS.

An ESS is created by chaining BSSs together with a backbone

The stations within the ESS may communicate and mobile stations may move from one BSS to another (within the same ESS).

Within an ESS the BSSs may overlap or may be physically disjoint.

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Extended Service SetExtended Service Set

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Integration With Wired Integration With Wired LANsLANs

To integrate the IEEE 802.11 architecture To integrate the IEEE 802.11 architecture with a traditional wired LAN, a final with a traditional wired LAN, a final architectural component called the Portal architectural component called the Portal is used.is used.

A portal is the logical point at which the A portal is the logical point at which the MSDUs from a wired LAN enter the IEEE MSDUs from a wired LAN enter the IEEE 802.11 distribution system802.11 distribution system

All data from non IEEE 802.11 LANs enter All data from non IEEE 802.11 LANs enter the IEEE 802.11 architecture via a portal.the IEEE 802.11 architecture via a portal.

It is possible for one device to provide the It is possible for one device to provide the functions of both the AP and the portal.functions of both the AP and the portal.

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STA 1STA 2

AP STA 4

AP

Portal

Wired LAN

BSS 1

BSS 2

Distribution System

Complete ArchitectureComplete Architecture

STA 3

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Channel ReuseChannel Reuse The 802.11b operates in the 2.4GHz The 802.11b operates in the 2.4GHz

band.band. The transmitted signal uses The transmitted signal uses

approximately 30MHz which is approximately 30MHz which is approximately 1/3approximately 1/3rdrd of the band. of the band.

Thus 802.11b provides only 3 non Thus 802.11b provides only 3 non overlapping channels, i.e. channel overlapping channels, i.e. channel number 1, 6 and 11.number 1, 6 and 11.

These 3 channels can be reused and These 3 channels can be reused and allocated to access points in different allocated to access points in different regions for effective channel re-use. regions for effective channel re-use.

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Channel AllocationChannel Allocation

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Non Overlapping Non Overlapping channelschannels

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Channel Reuse 802.11b

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Channel Access in 802.11Channel Access in 802.11 A Request To Send (RTS) frame is sent by a A Request To Send (RTS) frame is sent by a

potential transmitter to a receiver.potential transmitter to a receiver. In response the receiver sends a Clear To Send In response the receiver sends a Clear To Send

(CTS) frame back to the transmitter.(CTS) frame back to the transmitter. When an RTS or CTS frame is sent out by a When an RTS or CTS frame is sent out by a

transmitter or receiver the requesting station transmitter or receiver the requesting station notifies all stations within the radio range not to notifies all stations within the radio range not to initiate any transmissions for a given timeinitiate any transmissions for a given time

This is called the Net Allocation Vector (NAV)This is called the Net Allocation Vector (NAV) The NAV indicates the amount of time that must The NAV indicates the amount of time that must

elapse before the current transmission session is elapse before the current transmission session is complete and the channel can be sampled again complete and the channel can be sampled again for idle statusfor idle status

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1 2

3 4

RTS2RTS2 CTS1CTS1

MEDIUM BUSYRTS2

DIFSSIFS

CTS1

SIFS

DATA

SIFS

ACK

NAV(RTS)

NAV(CTS)

Node 1 has to communicate with Node 2Node 1 has to communicate with Node 2

3 and 4 have to remain silent till 1 finishes 3 and 4 have to remain silent till 1 finishes transmission to 2transmission to 2

Channel Access in 802.11Channel Access in 802.11

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Comparison between 802.11a Comparison between 802.11a & 802.11b& 802.11b

802.11b operates in the 802.11b operates in the 2.4 GHz band.2.4 GHz band.

Supports bit rates up to Supports bit rates up to 11Mbps.11Mbps.

Uses the Direct Uses the Direct Sequence Spread Sequence Spread Spectrum (DSSS). Spectrum (DSSS).

Only 3 non overlapping Only 3 non overlapping channels can be used channels can be used without interference. without interference.

Backward compatibility Backward compatibility with 802.11 productswith 802.11 products

Better range as Better range as compared to 802.11a.compared to 802.11a.

802.11a operates in the 802.11a operates in the 5 GHz band.5 GHz band.

Supports bit rates up to Supports bit rates up to 54 Mbps.54 Mbps.

Uses Orthogonal Uses Orthogonal frequency division frequency division multiplexing (OFDM)multiplexing (OFDM)

Up to 8 non overlapping Up to 8 non overlapping channels can be used channels can be used without interference. without interference.

No backward No backward compatibility with the compatibility with the 802.11 and 802.11b 802.11 and 802.11b products.products.

Reduced range owing to Reduced range owing to higher operating higher operating frequencyfrequency

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ReferencesReferences

“ “Gil Held”- Deploying Wireless LANs –Gil Held”- Deploying Wireless LANs –McGraw HillMcGraw Hill

““Benny Bing”-Wireless Local Area Networks Benny Bing”-Wireless Local Area Networks – Wiley Interscience– Wiley Interscience

““Asuncion Santamaria” –Wireless LAN Asuncion Santamaria” –Wireless LAN standards and applications – Artech Housestandards and applications – Artech House

““James Larocca, Ruth larocca” -802.11 James Larocca, Ruth larocca” -802.11 Demystified – McGraw HillDemystified – McGraw Hill

““Neil Reid and Ron Seide” - 802.11 Neil Reid and Ron Seide” - 802.11 Networking HandbookNetworking Handbook

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