Tổng quan về WIFI

7/24/2019 Tng quan v WIFI

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Trn Xun NamKhoa V tuynin tHc vin Kythut Qun s

T

ng quan v Wifi


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Trn Xun Nam

Gii thiu vIEEE 802.11

Chun ca IEEE vmng WLAN nhmpngnhu cu truy nhp ccng dnga phng tin

qua v tuyn So vi mng Ethernet LAN

C khanng ding (thp)

D lpt Chi ph thp

L mt phn trong htiu chun IEEE 802 vmng LAN


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Trn Xun Nam

IEEE 802.11 trong M hnh 7 lp

802.11 nh ngha lp Vt ly(Physical Layer)valpiu khin Truy nhp Mi tr ng (MAC)


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Trn Xun Nam

Phn loi cc tiu chun 802.11

- 802.11: bng tn ISM, lnu tinnh ngha CSMA/CA- 802.11a: xcnh vic sdng bng tn khng cp php (UNII:

Unlicense National Information Infrastructure bng tn 5GHz)

- Khc nhau c bn PHY, lp MAC ging nhau


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Phn loi cc tiu chun 802.11

~5000m~50m54 Mbit/s23 Mbit/s3.7 GHzJune 2008

(est.)802.11y

~250m~70m248 Mbit/s74 Mbit/s2.4 GHz5 GHz

June 2009(est.)

802.11n

~140m~38mDSSS

OFDM54 Mbit/s19 Mbit/s2.4 GHz2003802.11g

~140m~38mDSSS11 Mbit/s4.3 Mbit/s2.4 GHz1999802.11b

~120m~35mOFDM54 Mbit/s23 Mbit/s5 GHz1999802.11a

~100m~20mDSSS

/FHSS2 Mbit/s0.9 Mbit/s2.4 GHz1997Legacy

Range (Radius

Outdoor)

Loss includes

one wall

Range

(Radius

Indoor)

Depends, #

and type of

walls

Modulation

Technique

Data Rate

(Max)

Throughput

(Typ)

Op.

FrequencyRelease DateProtocol


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Trn Xun Nam

IEEE 802.11-1997 (Legacy)

Phin bn gc ca chun IEEE 802.11 Released in 1997 and chnh sa li nm 1999, Xcnh 2 tc(raw) 1 Mbps va2 Mbps bng tn 2.4

GHz ca ISM frequency band. iu ch: IR (not yet implemented), FHSS, DSSS Access: CSMA/CA

Chp nhn bomt phn dung lng knh cho CSMA/CA

tng cht lng truyn dn qua mi trng v tuyn VD: Alvarion (PRO.11 and BreezeAccess-II), BreezeCom,Digital / Cabletron (RoamAbout) , Lucent, NetwaveTechnologies (AirSurfer Plus and AirSurfer Pro),

Nhcim: c nhiu la chn nni khi khotngthch thc tl bn "beta-specification" hn l mtspecification

Phin bn DSSS sauoc pht trin thnh 802.11b.

ISM: Industrial Scientific Medical


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Trn Xun Nam

IEEE 802.11a

IEEE 802.11a-1999 or 802.11a l bsung choIEEE 802.11 cho php tng bit rate ln 54 Mbps

nhsdng bng tn 5 GHz. Sdng cng giao thc li nh bn 802.11

iu ch: 52-subcarrier OFDM cho maximum raw

data rate 54 Mbps va throughput c thtcvo khong 20 Mbps.

C thgim data rate reduces to 48, 36, 24, 18,

12, 9 then 6 Mbit/s nu cn


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Trn Xun Nam

IEEE 802.11a Bit rates


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Trn Xun Nam

IEEE 802.11a Channelization


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Trn Xun Nam

IEEE 802.11a OFDM PHY


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Trn Xun Nam

Nhcim ca 802.11a

Tn s5GHz c mt snhcim

Cly gim do sng bhp thu bi tng, vacc

vt thkhc


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Trn Xun Nam

IEEE802.11b

802.11b c tcmaximum 11 Mbit/s vathrougput thc tl 5.9 Mbit/s using TCP and 7.1Mbit/s using UDP.

iu ch: DSSS (Direct-sequence spreadspectrum)

Tctng svi chun 802.11-1997 (2Mpbs) Typical indoor range is 30m at 11 Mbit/s va90m at

1 Mbit/s

Nhcim: chunh hng ca cc thit bcng gii tn (microwave ovens, Bluetoothdevices, baby monitors and cordless telephones)


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Trn Xun Nam

IEEE 802.11b Channelization

14 channels c ginh cho di 2.4 GHz cch nhau 5MHz (trhai knh cui cch nhau 12 MHz).

No.11b only[4]

NoNoNoNo248414

YesYesYesNoYesNo247213

YesYesYesNoYesNo246712

YesYesYesYesYesYes246211

YesYesYesYesYesYes245710

YesYesNoNoYesYes24529









Australia[3]

Japan[2]

France[2]

Spain[2]

Europe[2]U.S. and

Canada[2]frequency

(MHz)Channel


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Trn Xun Nam

IEEE802.11b PHY

tc tc5Mbps va11Mbps

Sdngiu chDSSS vi 8-chip CCK cho

11Mbps va 4-chip CCK cho 5.5 Mbps Chipping rate: 11 Mcps

Optional (tng tcln 22Mbps)

Thay CCK modulation bng packet binaryconvolutional coding (HR/DSSS/PBCC)

Gim preample ca PLCP (HR/DSSS/short or HR/

DSSS/PBCC/short)

CCK: Complementary Code Keying

PLCP: Physical Layer Convergence Procedure


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Long PLCP format


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Trn Xun Nam

Short PLCP format


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Trn Xun Nam

IEEE802.11g (2003)

Cng bng tn 2.4 GHz vi 802.11b

Tc54 Mbit/s (19 Mbit/s net throughput ) tng

t802.11a Backwards compatible with 802.11b hardware


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Trn Xun Nam

IEEE802.11g

Modulation scheme sdng 802.11g l OFDMvaDSSS

Data rates: 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/svi OFDM vachuyn vCCK (802.11b standard)cho 5.5 and 11 Mbit/s

Even though 802.11g operates in the samefrequency band as 802.11b, it can achieve higher

data rates because of its heritage to 802.11a


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Trn Xun Nam

Cu trc mng 802.11

802.11 nh ngha 3 loi cu trc mng:

Independent Basic Service Set (IBSS),

Basic Service set (BSS) and Extended Service Set (ESS)

Thit bu cui trong 802.11 c gi lStation


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Trn Xun NamKhoa V tuynin tHc vin Kythut Qun s

Cu trc mng 802.11


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Trn Xun Nam

Mng BSS (Basic Service Set)

Cc Station khng lin lc trc tip vi nhau m qua AP (AccessPoint)

AP thc hin chuyn cc khung ttrm khi tonch


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Mng IBSS (Independent Basic Service Set)

IBSS cnc gi l mng Ad hoc

Cc tr m ttchc lin lc trc tip vi nhau khng cn thngqua AP

Phm vi hp, khng truy nhp ti backbone c


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Trn Xun Nam

Mng ESS (Extended Service Set)

Sdng DS(Distribution

System) ktni cc AP vinhau


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Trn Xun Nam

Kythut MAC

Ntn vaphng php Trnh va chm 802.11 sdng s truy nhp mi tr ng da

trn cnh tranh tng tnh Ethernet (802.3)

Tuy nhin CSMA/CD bhn chtrong mi trngv tuyn Cm nhn sng mang vatch va chm bng sbin

i mc tn hiu trong dy khng p dngctrong v tuyn do nh hng ca phainh

Tn hiu truyni tmt trmc giathit ti tt ccc trm khc, do t c li khng p dngc do

mi trng v tuyn gy li nhiu hn Tn hiu phti tmt trm cha chcac cc

trm khc pht hin hin tng ntn


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Hin tng ntn

Trm A vaC l trmn vi nhau: khi C pht A khng nhn bitcvangc lic thxy ra va chm Khc phc bng CSMA/CA


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CSMA/CA

Carrier Sense Multiple Access with CollisionAvoidance

Trm pht phti RTS (Request To Send)

Trm thu xc nhn bng CTS (Clear To Send)

Cc bn tin ny xcnh thi gian ginh cho ktni

Cc trm thuc RTS/CTS tm dng truyn chon khi 2 tr m kt thc truyn

tit kim overhead truyn RTS/CTS: option

boqua RTS/CTS nu kch thc bn tin ngn hnngng cho trct bi admin


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Trn Xun Nam

C cu cm nhn sng mang

C cu cm nhn sng mang ca Ethernet khngp dngc cho mi tr ng v tuyn

Thc hin bi kt hp CA/CCA (Carrier Sense/Clear Channel Assigment)

ca lp vt ly: tch tn hiu hoc nng lng

NAV (Network Allocation Vector) lp MAC: NAV thc cht l giatrm ngc lu li MAC catt ccc trm. Khi NAV=0, thimi trng trnn t

do

Trm pht RTS t giatrNAV:

u khungframes inter-frameRTS CTS t t t t + + +


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Cu trc Khung Dliu


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Cu trc Khung Dliu (tip)

Frame Control (2bytes)

Protocol Version (2bit): hin ti chc 1 version nn bng 0 Type (2bit) & Subtype (4bit): nh ngha cc khung chc

nng ring. C 3 loi khung: control, management, data To/From DS: chhng frame t/ti DS More Fragment: chra frame l thnh phn ca mt MAC

SDU ln bphnon.

Retry: xcnh frame truyn li Power Management: trm lm vic save mode More Data: trm c thm data pht khng WEP: xcnh phng php m mt Wired Encryption

Privacy c sdng trong dliu hay khng Order: chthitrmang xp sp li trnh tcc khung


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Cu trc Khung Dliu (tip)

Duration ID (2 bytes): thi gian mi tr ng ginhcho trm. Mt sframe c thcha c

Association ID ca trm

Address 1~4: a chca trm ngun, ch, trmpht, trm thu

Sequence Control: 4 bit Fragment Number + 12bit Sequence Number

Frame Body: Phn dliu chuynn MAC

ng cp FCS: 32bit CRC


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Cc Frame Chc Nng


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Traoi gia Trm vaAP

1. Probe Request: tm AP sau khi btngun

2. Probe Response: AP trali trm kmtheo thng tin v nh thi, tn hiu li,tc, vacc tham sPHY

3. Nu trm quytnh ni ti AP thicqu trnh Xc thcxc nhn trm

4. Sau khi xc thc xong, trm gi yucu Association Request cha thng tinvpower save mode vatchtr

5. AP ch

p nh

n va g

i l

i AssociationResponse vi ID c gn vatc

6. Nu trm c dliu, n btu pht tRTS


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Cc kythut lp Vt ly

4 loi tngng vi 4 tiu chun


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Trn Xun Nam

Cu trc lp Vt ly

Thc tlp Vt lyc chia nhothnh 2 lp

Physical Layer Convergence Procedure (PLCP):

nh ngha nh xaMAC PDU (MPDU) thnh ccframe phuhp cho thu pht ca PMD lm gimtng tc gia MAC vahotng vt ly

Physical Medium Dependent (PMD): xcnhctrng va phng php thu/pht dliu gia 2 tr m


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VD 802.11a (OFDM):

Cu trc PPDU Frame:Gm 3 phn: Preamble, SIGNAL vaDATA

C th OFDM


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Cc tham sOFDM

S th / ht


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Trn Xun Nam

S my thu/pht

C ii h h WiFi


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Trn Xun Nam

Cc gii php mrng vng phuWiFi

Sdng Antennh hng

Sdng cu Wifi hnh li

Sdng chun 802.11b/g:OFDM va tn s2.4GHz

Cho php tng tcln trn 54Mbps vacly lnn hng chckm

Wifi trthnhng dng cho c

mng LAN vaMAN (Metro-

Access Solution)

Wifi d A t h h


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Trn Xun Nam

Wifi sdng Antennh hng

Wifi s d ng c hnh l i


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Trn Xun Nam

Wifi sdng cu hnh li

u im ca cu hnh li


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uim ca cu hnh li

Giathnh thp

Cn bng lu lng: chuyn lu lng qua cc

node khc nhau trnh nghn hoc nhiu Mnh mvan hi: khng phuthuc vo mt

node n

Gim nghn mch: cc trm c thkt ni tinhiu trm nn khanng nghn mch khoc thxy ra

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