Transcript
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Trn Xun NamKhoa V tuynin tHc vin Kythut Qun s
T
ng quan v Wifi
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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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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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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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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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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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IEEE 802.11a Bit rates
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IEEE 802.11a Channelization
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IEEE 802.11a OFDM PHY
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Nhcim ca 802.11a
Tn s5GHz c mt snhcim
Cly gim do sng bhp thu bi tng, vacc
vt thkhc
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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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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
YesYesNoNoYesYes24478
YesYesNoNoYesYes24427
YesYesNoNoYesYes24376
YesYesNoNoYesYes24325
YesYesNoNoYesYes24274
YesYesNoNoYesYes24223
YesYesNoNoYesYes24172
YesYesNoNoYesYes24121
Australia[3]
Japan[2]
France[2]
Spain[2]
Europe[2]U.S. and
Canada[2]frequency
(MHz)Channel
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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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Short PLCP format
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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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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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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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Cu trc mng 802.11
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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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Mng ESS (Extended Service Set)
Sdng DS(Distribution
System) ktni cc AP vinhau
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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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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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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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S my thu/pht
C ii h h WiFi
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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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Wifi sdng Antennh hng
Wifi s d ng c hnh l i
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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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