IEEE 802.11 Tarik Cicic University of Oslo December 2001 Overview - - PDF document

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IEEE 802.11

Tarik Cicic University of Oslo December 2001

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Overview

• Wireless LAN architecture:

– components – operation

• Frequency Hopping vs. Direct Sequence
• Recent developments

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Components

• Basic service sets, controlled by their base

station (Access Points, AS)

• base stations connected by a backbone ---

distribution system, usually Ethernet

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Sample 802.11 LAN

AP1 AP2

Distribution system (Backbone)

BSS1 BSS2

Extended service set

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IEEE 802.11 layers

• Architecture similar to other 802.x

technologies

• three physical layers supported:

– Frequency Hopping Spread Spectrum – Direct Sequence Spread Spectrum – Infra Red

• additional functionality in MAC:

– fragmentation – packet retransmission – acknowledgments

802.11 MAC 802.2

Link Layer

FH DS IR

PHY Layer

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Spread spectrum

• IEEE 802.11 uses Spread Spectrum radio

transmission:

– Frequency hopping (FHSS) – Direct Sequence (DSSS)

• diffused infrared transmission is the third

physical layer technology

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Frequency hopping

• Pseudorandom sequence of frequencies is

chosen within a BSS

• all communicating parts follow the

sequence

• 2.4 GHz, 79.1 MHz bandwidth

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Frequency hopping (2)

Time

2.40 2.41 2.42 2.43 2.44 2.45 2.46 2.47 2.48

GHz

(fictive frequencies) 9

Direct Sequence

1 1 1

Data 4-bit random Result Data 4-bit random Result 1010 => 1111000011110000 1011101110111011 0100101101001011

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Direct Sequence (2)

• Different senders use different frequencies
• dominant technology today:

– IEEE 802.11 b – up to 11 Mb/s (~50 m distance)

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Diffuse infrared ( in IEEE 802.11)

• Millimetre (and shorter) waves
• Close to the visible light --- cannot pass through

walls

• but is reflected by walls, no “line of sight” needed
• IR can be used on distances of ~10m, while radio

transmission can be used ~100m in buildings and 1km outside

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CSMA/CA

• Carrier Sense Multiple Access with

Collision Avoidance

• sense the medium; transmit if free
• collision detection as in wired LAN is not

possible:

– full duplex needed – no other stations heard means not free medium

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CSMA: hidden nodes

• If A and C send in the same time to B, they cannot

notice the collision

A and C cannot see each other (hidden nodes)

A B C

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CSMA: exposed nodes

• B transmits to A. C hears it, but does not transmit to D
• this is a mistake, since A cannot hear C and D cannot

hear B --- poor resource utilization

A B D C

C is exposed to B’s transmission

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Positive Acknowledgment

• If the medium is free for DIFS (Distributed Inter

Frame Space) period, transmit

• the receiver checks the CRC and, if OK,

sends an ACK

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Collision Avoidance

• Virtual Carrier Sense decreases the collision

possibility

• sender station first sends a Request To Send

(RTS), a short packet with source and destination ID and transaction duration

• the destination responds with “Clear To Send”
• all stations hearing this set their Network

Allocation Vector to the given duration

• stations delay the sending (+ sense the medium)

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Collision Avoidance

Src Dest Other RTS CTS

SIFS

DATA ACK NAV(RTS) NAV(CTS)

DIFS

Next MPDU

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MAC Functionality

• 1. Acknowledgments (no ACKs in wired

LANs)

• 2. fragmentation & reassembly:
• small packets at radio LANs, 20 ms FH and

2Mb/s give 5242 Bytes

• 1518 Bytes in Ethernet
• 3. exponential backoff procedure used

(modified compared to Ethernet)

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Joining a cell (BSS)

• Active / passive scanning (beacon message)
• the association process (capability

exchange)

• Roaming through re-association!

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Security

• Authentication through knowledge of a

secret key

• eavesdropping hindered by use of pseudo

random number generator; packets are scrambled with a random sequence

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Other features

• Power save
• Ad-hoc networks

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Current Developments

• First 802.11 implementations were based on

FH, 3 Mb/s communication

• IEEE 802.11b standard opens for 11 Mb/s

communication using DS

• 802.11b is de-facto standard today (despite

shorter distances and worsened security)

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Discussion: Security in 802.11

• Why is WLAN technology often regarded

unsecure?