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IEEE 802.11 Wireless Local Area

Networks

The future is wireless

Presented by Tamer Khattab and George Wong

Prepared for EECE571N - Advanced Networking

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WLAN Technology Overview

Physical layer technologies Architecture Transmission

Medium access control technologies

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ArchitectureArchitecture

• Network connectivity life time

•Ad-hoc

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ArchitectureArchitecture (Cont.) (Cont.)

• Network connectivity life time

BSS 2

BSS 1

Distribution System

DS

AP

APSTASTA

STASTA

direct connection

Wired 802.3 LAN

Portal

•Infrastructure

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ArchitectureArchitecture (Cont.) (Cont.)

• Connection type

•Point-to-point

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ArchitectureArchitecture (Cont.) (Cont.)

• Connection type

•Broadcast

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Transmission MediumTransmission Medium

• Radio frequency transmission• Narrow-band transmission

• Spread spectrum transmission• direct sequence

• frequency hopping

• Infrared transmission• Laser diode sources

• Light emitting diode sources

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Wireless LAN Standards

•IEEE 802.11 wireless LAN standards

•ETSI HIPERLAN wireless LAN standards

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Wireless LAN Standards

•IEEE 802.11 wireless LAN standards

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Physical Layer

IEEE 802.11 Wireless Ethernet

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IEEE 802.11 Physical Layer

Medium type 2.4 GHz FHSS (2400 - 2483.2 GHz) 2.4 GHz DSSS (2400 - 2483.2 GHz) Diffused infrared DFIR (850 - 950 nm)

Rates: basic=1 Mbps, enhanced=2 Mbps

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FHSS

Band 2400-2483.5 MHz GFSK (Gaussian Frequency Shift

Keying) Sub-channels of 1 MHz Only 79 channels of the 83 are used Slow hopping 3 main sets each with 26 different

hopping sequences

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FHSS (Cont.)

Sequences within same set collide at max. on 5 channels

Min. hopping distance of 6 channels. No CDMA within same BSS Coexisting BSS in the same coverage

area use different sequences from the same hopping set.

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FHSS (Cont.)

Frequency

Time

Hopping distance >= 6 sub-channels(The distance in frequency between two consecutive hops)Sub-channel

1 MHz

400 ms

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FHSS (Cont.)

MAC data could be at 1 Mb/s or 2 Mb/s

Sync pattern 80 bit

SFD16 bit

PLW12 bit

HEC16 bit

4 bit Payload data(variable length)

PSF

PLCP preamble PLCP header PLCP_PDU

Physical layer header and preamble always at 1 Mb/s

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DSSS

Band 2400-2483.5 MHz DBPSK (Differential Binary Phase Shift

Keying) Band divided into 11 overlapping channels

each with bandwidth 11 MHz Coexisting BSS in the same coverage area

use channels separated by at least 30 MHz. 11 bit Barker sequence is used for

spreading No CDMA used within one BSS

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DSSS (Cont.)

Frequency (MHz)

Channel number

11 MHz

11

10

9

8

7

6

5

4

3

2

1

2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462

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DSSS (Cont.)

Sync pattern 128 bit

SFD16 bit

SG8 bit

HEC16 bit

SR8 bit

Payload data(variable length)

PLCP preamble PLCP header PLCP_PDU

MAC data could be at 1 Mb/s or 2 Mb/s

LN16 bit

Physical layer header and preamble always at 1 Mb/s

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Infra Red

Wave length near visible light 850-950 nm

PPM (Pulse Position Modulation) Diffused transmission technique

used Only used for indoor transmission

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IEEE 802.11a

5 GHz (5.15-5.25, 5.25-5.35, 5.725-5.825GHz)

OFDM (Orthogonal Freq. Div. Multiplexing)

52 Subcarriers BPSK/QPSK/QAM Forward Error Correction

(Convolutional) Rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps

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IEEE 802.11b

2.4 GHZ band DSSS (11-chip) Rates 5.5 and 11 Mbps M-arry modulation. Convolutional Codes Shorter Preamble

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Product Samples

2.4 GHz FHSS ISA2.4 GHz FHSS PCMCIA

5 GHz DSSS PCMCIA

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MAC Layer

IEEE 802.11 Wireless Ethernet

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Overview of the Protocol Layers IEEE 802.11 specifies a MAC layer that is designed to operate over wireless

channel IEEE 802.11 is in the same protocol layer as the IEEE 802.3

IEEE 802.2 Logical Link Control (LLC)

MAC Layer

Data Link Layer

Network Layer

IEEE 802.3 Ethernet

IEEE 802.11 Wireless EthernetPhysical

Layer

IEEE 802.5 Token Ring

IEEE 802.4Token Bus

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IEEE 802.11 – CSMA/CA CS – Carrier Sense

Each transmitter listens to the physical link before transmitting

MA – Multiple Access Many nodes are connected to the same

physical link.

CA – Collision Avoidance Methods used to avoid collision

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CSMA/CA Why not CSMA/CD?

Difficult to detect collision in a radio environment

Radio environment is not as well controlled as a wired broadcast medium, and transmissions from users in other LANs can interfere with the operation of CSMA/CD

Radio LANs are subject to the hidden-station problem

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Hidden-Station Problem A knows the existence of B C knows the existence of B B knows the existence of A and C However, A does not know the existence of C

BA C

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Hidden-Station Problem Since A and C are sufficiently distant from each other that they cannot

hear each other’s transmission (Carrier Sense doesn’t work!) This condition will result in the transmissions from the two stations, A and

C, proceeding and colliding at the intermediate station B (However, A and C cannot hear the collision!)

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Hidden-Station Problem CSMA/CA medium access control was developed

to prevent this type of collision

Data Frame Data Frame

A transmits data frame

B

A B C

C transmits data frame and collides

with A at B

(a)

(b)

Data FrameCA

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Exposed Node Problem Suppose B is sending to A. C is aware of this

communication because it hears B’s transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’ transmission

This is not a problem since C’s transmisstion to D will not interfere with A’s ability to receive from B

B CA D

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Collision Avoidance IEEE 802.11 address these two problems,

hidden-station and exposed node problems, with an algorithms called Multiple Access with Collision Avoidance

Sender and receiver exchange control frames with each other before the sender actually transmit any data

The sender transmits a Request to Send (RTS) frame to the receiver and the receiver then replies with a Clear to Send (CTS) frame

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Collision Avoidance RTS includes a field that indicates how

long the sender wants to hold the medium CTS reserves channel for sender, notifying

(possible hidden) station For Neighbors

See CTS: keep quite See RTS but no CTS: ok to transmit

Receiver sends ACK when it receives the frame Neighbors keep silent until seeing ACK

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Collision Avoidance

B CA D

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UBC Wireless Network UBC is currently deploying

the wireless network http://www.UNP.ubc.ca/

“It’s changed students’ lives” Christopher Macdonald, School of Architecture

“It was like winning the lottery” Alan Steeves, Research Engineer

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Questions?