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CHAPTER 8:WIFI

ng L Khoa

Email: [email protected]

Facuty of Electronics & Telecommunications, HCMUS

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Outline

802.11 specifications overview

Carrier Sensing:problems, issues, and solutions

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802.11 specifications overview

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802.11 Specifications

MAC

Specification of layers below LLC

Associated management/control interfaces

MIB

Applications

DSSS FH IR OFDM

PHY

WEP

LLC

MACMgmt

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PLCP SublayerPHY layer

ManagementPMD Sublayer

MAC

sublayer

MAC Layer

Management

PHY

Service

Interface

PHY Mgmt

Service

Interface

LLCMAC

ServiceInterface

MAC MgmtServiceInterface

LLC

MIB

DSSS FH IR OFDM

PHY

MACWEP MAC

Mgmt

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PHY Layer PHY Management

MAC sublayer MAC Management

PHY ServiceInterface (clause 12) PHY Mgmt Service

Interface (clause 13)

LLCMAC ServiceInterface

(clause 6)

MAC framing (clause 7)MAC operation (clause 9)

WEP (clause 8)State Machines (Annex C)

Protocols (clause 11)State Machines (Annex C)

MIBs (Annex D)

FH (clause 14)DSSS (clause 15)Infrared (clause 16)

OFDM (clause 17)High rate DSSS (clause 18)

MAC Mgmt ServiceInterface (clause 10)

MIBs (Annex D)

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802.11 System Architecture

Basic Service Set (BSS): a set of stations which communicatewith one another

Independent Basic ServiceSet (IBSS)

only direct communicationpossible

no relay function

Infrastructure Basic ServiceSet (BSS)

AP provides connection to wired network relay funct ion

stations not allowed tocommunicate directly

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Extended Service Set

ESS and all of its stations appear to be a single MAC layer AP communicate among themselves to forward traffic Station mobility within an ESS is invisible to the higher layers

ESS: a set of BSSs interconnected by a distribution system (DS)

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802.11 PHY

MIB

Applications

DSSS FH IR OFDM

PHY

MACWEP

LLC

MACMgmt

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802.11 PHY

MAC Protcol DataUnit (MPDU)


PLCPheader


PLCPheader


Sender Receiver

Physical MediaDependent (PMD) layer

PMD layer

MAC

PHY

High rate (DSSS) PHY11, 5.5 Mbps

802.11b

Direct Sequence Spread

Spectrum (DSSS) PHY1,2 Mbps

Frequency HoppingSpread Spectrum

(FHSS) PHY1, 2 Mbps

Infrared (IR)PHY

1,2 Mbps

Higher rate (DSSS) PHY20+ Mbps802.11g

2.4 GHz

Orthogonal FrequencyDivision Multiplexing

(OFDM) PHY6,9,12,18,24,36,48,54 Mbps

802.11a

5.7 GHz10

DSSS PHY

Baseband signal is spread using Barker word (10 dB processing gain)

Spread signal occupies approximately 22 Mhz bandwidth

Receiver recovers the signal by applying the same Barker word

DSSS provides good immunity against narrowband interferer

CDMA (multiple access) capability is not possible

MPDUPreamble Header

1 Mbps 1, 2 Mbps

DPSKmodulation

Transmitterbaseband signal

MPDUPreamble Header

1 Mbps 1, 2 Mbps

Received signal afterdespreading

DPSKde-modulationSpread the signal using Barker word (11 bits)

+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1

Transmitted signal after spreading

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DSSS PHY

Direct sequence spread spectrum Each channel is 22 Mhz wide

Symbol rate

1 Mb/s with DBPSK modulatio

2 Mbps with DQPSK modulation

11, 5.5 Mb/ps with CCK modulation

Max transmit power

100 Mw

22 Mhz

83.5 Mhz

Ch 1 Ch 6 Ch 11

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Fragments transmission

SourceFragment 0

Destination

SIFS

Fragment transmission supported to improve transmission reliability under noisy

environments

Transmitter holds the channel until the end of fragment transmission burst

If the source does not receive and ACK frame, it will transmit the failed MPDUafter performing the backoff procedure and the contention process

Receiver may receive duplicate fragments and is responsible for detecting and

discarding duplicate fragments

ACK 0

SIFS

Fragment 1

ACK 1

SIFS

Fragment 2

ACK 2

SIFS

DIFS

SIFSBackoffwindow

Fragment burst

SIFS

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802.11 - MAC layer

Priorities defined through different inter frame spaces

no guaranteed, hard priorities SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response

PIFS (PCF IFS) medium priority, for time-bounded service using PCF

DIFS (DCF, Distributed Coordination FunctionIFS)

lowest priority, for asynchronous data service

t

medium busySIFS

PIFS

DIFSDIFS

next framecontention

direct access ifmedium is free DIFS

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Carrier Sensing: problems, issues,and solutions

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Problems with carrier sensing

Z

W

YX

Exposed terminal problem

Z is transmittingto W

Y will not transmit to Xeven though it cannot interfere

Presence of carrier ===> hold off transmission/

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Problems with carrier sensing

Y

Z

W

Hidden terminal problem

W finds that medium is freeand it transmits a packet to Z

no carrier ===> OK to transmit/

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Solving Hidden Node problem with RTS/CTS

Y

ZX

W

RTS CTS

listen RTS ==> transmitter is close to melisten CTS ==> receiver is close to me

- listen RTS- wait long enoughfor the requestedstation to respond

with CTS- if (timeout) then

ready to transmit

- listen CTS- wait long enoughfor the transmitterto send its data

Note: RTS/CTS does not solve exposed terminal problem. In the example above,X can send RTS, but CTS from the responder will collide with Ys data.

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Frame Control Field

2

Frame

control

2

ProtVerType Subtype

To

DS FromDS

More

Frag Order

2 4 1 1 1 1

Retry

Pwr

Mgmt

More

Data

1

WEP

1 1 1

bitsbytes

01Control

00Mgmt

10Data

11Reserved

Association reqAssociation resp

Re-association reqRe-association respProbe req

Probe respBeacon

Announcement TrafficIndication Request (ATIM)

Disassociation

AuthenticationDe-authentication

Power save (PS)-pollRequest to Send (RTS)

Clear to send (CTS)Acknowledgement (ACK)

Contention free (CF)-ENDCF-END + CF-ACK

Data

Data + CF+ACKData + CF-PollData + CF-ACK + CF-Poll

NullCF-ACK

CF-PollCF-ACK + CF-Poll

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