CSC 581 Communication Networks II Chapter 6c: Local Area Network (Wireless LAN – 802.11) Dr. Cheer-Sun Yang

CSC 581Communication

Networks II

Chapter 6c: Local Area Network

(Wireless LAN – 802.11)

Dr. Cheer-Sun Yang

2

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

3

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.

4

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.

5

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.

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

6

B D

CA

Figure 6.65

7

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.


8

Data Frame Data Frame

A transmits data frame

B

AB C

C transmits data frame and collides with A at B

(a)

(b)

C senses medium, station A is hidden from C

Data FrameCA

Figure 6.64

9

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

10

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


11

A2 B2

B1A1

AP1AP2

Distribution SystemServer

Gateway toInternetportal

portal

BSS A BSS B

Figure 6.66

12

MAC Frame Structure and Addressing

• Frame Header• MAC Header• Frame Body• CRC Checksum


13

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4

FrameBody

CRC

ProtocolVersion

Type SubtypeToDS

FromDS

MoreFrag

RetryPwrMgt

MoreData

WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

ToDS

FromDS

Address1

Address2

Address3

Address4

0 0Destination

AddressSource

AddressBSSID N/A

0 1Destination

AddressBSSID

SourceAddress

N/A

1 0 BSSIDSource

AddressDestination

AddressN/A

1 1ReceiverAddress

TransmitterAddress

DestinationAddress

SourceAddress

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

2 2

MAC Header (bytes)

4 1 1 1 1 1 1 1 1

Figure 6.67

14

802.11 MAC Timing


15

Physical

Distribution coordination function(CSMA-CA)

PCF

Contention-freeservice

Contentionservice

MAC

Figure 6.68


16

BusyMedium

DIFS

DIFS

PIFS

SIFS

ContentionWindow

NextFrame

Defer AccessWait for

Reattempt Time

Time

Figure 6.69


17

RTS

CTS CTS

Data frame

A requests to send

B

C

A

A sends

B

B

C

C remains quiet

B announces A ok to send

(a)

(b)

(c)

Figure 6.70


18

DataDIFS

SIFS

Defer accessWait for

reattempt time

ACK

DIFS

NAV

Source

Destination

Other

Figure 6.71


19

Data

SIFS

Defer access

Ack

DIFSNAV (RTS)

Source

Destination

Other

RTSDIFS

SIFSCTS

SIFS

NAV (CTS)

NAV (Data)

Figure 6.72


20

CF End

NAV

PIFS

BD1 + Poll

SIFS

U 1 + ACK

D2+Ack+Poll

SIFS SIFS

U 2 + ACK

SIFS SIFS

Contention-Free Repetition Interval

Contention Period

CF_Max_Duration

Reset NAV

D1, D2 = frame sent by Point CoordinatorU1, U2 = frame sent by polled stationTBTT = target beacon transmission timeB = Beacon Frame

TBTT

Figure 6.73

21

Physical Layer

• Physical Layer Convergence Procedure (PLCP)

• Physical Medium Dependent (PMD)


22

PhysicalLayer

LLC

Physical LayerConvergence

Procedure

Physical MediumDependent

LLC PDU

MAC SDU MACLayer

MACHDR CRC

PLCPPRMBL

PLCPHDR

PLCP PDU

Figure 6.74


23

SyncStart FrameDelimiter

Length Signaling CRC Payload data

80 bits 16 12 4 16 Variable length

PLCP preamble PLCP header

Figure 6.75


24

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

Figure 6.76


25

SyncStart framedelimiter

Signal Length CRC Payload data

128 bits 16 8 8 16 Variable length


Service

16

Figure 6.77


26

SyncStart framedelimiter

Datarate

Length CRC Payload data

57-73 slots 4 3 32 16 Variable length


DC leveladjust

16

Figure 6.78

27

Reading Assignment

• Section 6.6.4

Documents

CSC 581 Communication Networks II Chapter 6c: Local Area Network (Wireless LAN – 802.11) Dr. Cheer-Sun Yang