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CSC 581 Communication Networks II. Chapter 6c: Local Area Network (Wireless LAN – 802.11) Dr. Cheer-Sun Yang. Wireless LAN - Physical . Infrared 1Mbps and 2Mbps Wavelength 850-950nm Direct sequence spread spectrum 2.4GHz ISM band Up to 7 channels Each 1Mbps or 2Mbps

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Csc 581 communication networks ii l.jpg

CSC 581CommunicationNetworks II

Chapter 6c: Local Area Network

(Wireless LAN – 802.11)

Dr. Cheer-Sun Yang


Wireless lan physical l.jpg
Wireless LAN - Physical

  • Infrared

    • 1Mbps and 2Mbps

    • Wavelength 850-950nm

  • Direct sequence spread spectrum

    • 2.4GHz ISM band

    • Up to 7 channels

    • Each 1Mbps or 2Mbps

  • Frequency hopping spread spectrum

    • 2.4GHz ISM band

    • 1Mbps or 2Mbps

  • Others under development


Challenges l.jpg
Challenges

  • Radio and infread transmission is susceptible to noise and interference.

  • The strength of a radio transmission varies in time and in space and so coverage is inconsistent and unpredictable.

  • Radio signals can be evedroped.

  • Radio spectrum is limited.

  • Radio spectrum has traditionally been regulated by government. It can be difficult to design products for a global market.


Motivations l.jpg
Motivations

  • Mobility is desirable in many cases since portable computers are ubiquitous.

  • For example, a doctor or nurse in a hospital accessing up-to-date information on a patient may not be able to log off and on frequently. It is beneficial to provide wireless points so that portable devices can communicate with each other via a backbone network.

  • A conference participants may need to create a temporary ad hoc LAN.


Ad hoc network l.jpg
Ad Hoc Network

  • A single BSS can be used to form an ad hoc network.

  • An ad hoc network consists of a group of stations within range of each other.

  • Ad hoc networks are typically temporary in nature.


Slide6 l.jpg

B

D

C

A

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.65


Hidden station problem l.jpg
Hidden Station Problem

  • Why not using wireless Ethernet using CSMA/CD?

    • It is difficult to detect collision in a radio environment.

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

    • Hidden station: Between A and C, there could be another station B. The transmissions of A and C can collide at the intermediate station B.

    • CSMA/CA is a solution to the hidden station problem.


Slide8 l.jpg

(a)

C

A

Data Frame

B

A transmits data frame

C senses medium,

station A is hidden from C

A

C

(b)

Data Frame

B

Data Frame

C transmits data frame and collides with A at B

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.64


Wireless lans l.jpg
Wireless LANs

  • IEEE 802.11 incorporates CSMA/CA as the MAC layer protocol.

  • Basic service set (cell)

    • Set of stations using same MAC protocol

    • Competing to access shared medium

    • May be isolated

    • May connect to backbone via access point (bridge)

  • Extended service set

    • Two or more BSS connected by distributed system

    • Appears as single logic LAN to LLC level


Types of station l.jpg
Types of station

  • No transition

    • Stationary or moves within direct communication range of single BSS

  • BSS transition

    • Moves between BSS within single ESS

  • ESS transition

    • From a BSS in one ESS to a BSS in another ESS

    • Disruption of service likely


Slide11 l.jpg

B1

A1

A2

B2

Gateway to

Internet

portal

Server

Distribution System

portal

AP1

AP2

BSS A

BSS B

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.66


Mac frame structure and addressing l.jpg
MAC Frame Structure and Addressing

  • Frame Header

  • MAC Header

  • Frame Body

  • CRC Checksum


Slide13 l.jpg

Frame

Control

Duration/

ID

Address

1

Address

2

Address

3

Sequence

Control

Address

4

Frame

Body

CRC

Protocol

Version

Type

Subtype

To

DS

From

DS

More

Frag

Retry

Pwr

Mgt

More

Data

WEP

Rsvd

To

DS

0

1

0

1

0

From

DS

1

1

0

BSSID

Destination

Address

Receiver

Address

Destination

Address

Address

1

Transmitter

Address

Source

Address

Source

Address

Address

2

BSSID

BSSID

Destination

Address

Destination

Address

Source

Address

Address

3

Address

4

N/A

N/A

Source

Address

N/A

Meaning

Data frame from station to station within a BSS

Data frame exiting the DS

Data frame destined for the DS

WDS frame being distributed from AP to AP

MAC Header (bytes)

2

2

6

6

6

2

6

0-2312

4

2

2

4

1

1

1

1

1

1

1

1

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.67



Slide15 l.jpg

Contention-free

service

Contention

service

PCF

MAC

Distribution coordination function

(CSMA-CA)

Physical

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.68


Slide16 l.jpg

Contention

Window

DIFS

PIFS

DIFS

Busy

Medium

Next

Frame

SIFS

Time

Wait for Reattempt Time

Defer Access

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.69


Slide17 l.jpg

(a)

B

RTS

C

A requests to send

(b)

CTS

B

CTS

A

C

B announces A ok to send

(c)

B

Data frame

A sends

C remains quiet

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.70


Slide18 l.jpg

DIFS

Data

Source

SIFS

ACK

Destination

DIFS

NAV

Other

Wait for reattempt time

Defer access

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.71


Slide19 l.jpg

DIFS

RTS

Data

Source

SIFS

SIFS

SIFS

CTS

Ack

Destination

DIFS

NAV (RTS)

NAV (CTS)

Other

NAV (Data)

Defer access

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.72


Slide20 l.jpg

U 2 + ACK

D2+Ack+Poll

D1 + Poll

U 1 + ACK

TBTT

Contention-Free Repetition Interval

SIFS

SIFS

SIFS

SIFS

SIFS

Contention Period

CF End

B

PIFS

Reset NAV

NAV

CF_Max_Duration

D1, D2 = frame sent by Point Coordinator

U1, U2 = frame sent by polled station

TBTT = target beacon transmission time

B = Beacon Frame

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.73


Physical layer l.jpg
Physical Layer

  • Physical Layer Convergence Procedure (PLCP)

  • Physical Medium Dependent (PMD)


Slide22 l.jpg

LLC PDU

LLC

MAC

HDR

MAC SDU

CRC

MAC

Layer

Physical Layer

Convergence

Procedure

PLCP

PRMBL

PLCP

HDR

PLCP PDU

Physical

Layer

Physical Medium

Dependent

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.74


Slide23 l.jpg

80 bits

16

12

4

16

Variable length

Sync

Start Frame

Delimiter

Length

Signaling

CRC

Payload data

PLCP header

PLCP preamble

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.75


Slide24 l.jpg

11 chip Barker sequence:

+1

+1 +1+1

+1 +1

-1

-1

-1 -1-1

11 symbol times

To transmit +1, send:

+1

+1 +1+1

+1 +1

-1

-1

-1 -1 -1

11 symbol times

To transmit -1, send:

+1 +1+1

+1

+1

-1

-1 -1 -1

-1 -1

11 symbol times

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.76


Slide25 l.jpg

128 bits

16

8

16

Variable length

8

16

Service

Sync

Start frame

delimiter

Signal

Length

CRC

Payload data

PLCP header

PLCP preamble

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.77


Slide26 l.jpg

57-73 slots

4

3

16

Variable length

32

16

DC level

adjust

Sync

Start frame

delimiter

Data

rate

Length

CRC

Payload data

PLCP header

PLCP preamble

Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

Figure 6.78


Reading assignment l.jpg
Reading Assignment

  • Section 6.6.4


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