Microsoft Word - Tieu chuan IEEE 802.11 va Cong nghe Wifi

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ti:Tiu chun IEEE 802.11 v cng ngh Wifi

1.L Vn Thng085205992.L Trng An085200043.Nguyn Vn Thiu08520596GVHDThS. Ng Hn Chiu SVTHNhm 10

TP HCM, 11/2011

MC LC

ti: Tiu chun IEEE 802.11 v cng ngh WifiGVHD: ThS. Ng Hn Chiu

-1-MC LC1LI NI U4DANH MC BNG BIU V HNH V5THUT NG VIT TT8NI DUNG 12Chng I: Tng quan v t chc IEEE v h chun IEEE 80212Gii thiu v t chc IEEE12Cc tiu chun IEEE13Gii thiu 13Cc b tiu chun thuc h IEEE 802 14Quan h gia cc chun IEEE v m hnh OSI 15Chng II: Mt s chun thng dng trong h chun IEEE 80217Chun ha mng LAN/MAN hu tuyn17IEEE 802.1 - cc giao thc LAN tng cao 17IEEE 802.2 iu khin lin kt logic (LLC) 18IEEE 802.3 tiu chun cho cng ngh Ethernet 182.1.4.IEEE 802.4, .5, .6, .9, .12, .14 222.1.5.IEEE 802.17 24Chun ha mng LAN/MAN khng dy25IEEE 802.11 cng ngh WiFi mng WLAN 26IEEE 802.15 Cng ngh Bluetooth, ZigBee mng WPAN 26IEEE 802.16 cng ngh WiMAX mng WMAN 29IEEE 802.20 34IEEE 802.21 35IEEE 802.22 mng WRAN 35Chun ha cc thnh phn khc 35IEEE 802.10 36IEEE 802.19 36Chng III: IEEE 802.11 v chun ha mng LAN khng dy (WLAN) 37Gii thiu b tiu chun IEEE 802.11 37Kin trc ca chun IEEE 802.11 37Cc thnh phn kin trc 37Kin trc cc lp trong m hnh OSI ca chun IEEE 802.11 39Phng php truy cp c bn: CSMA/CA 41Cc chng thc mc MAC 43Phn on v Ti hp 43Cc khng gian khung Inter (Inter Frame Space) 44Gii thut Exponential Backoff 45So snh kiu C s h tng v kiu Ad Hoc 46L thuyt v mng khng dy 48Cch mt trm ni vi mt cell hin hu (BSS) 48Roaming 48Gi ng b 49Tit kim nng lng 49Cc kiu khung 50Khun dng khung 50Cc khung nh dng ph bin nht 54Hm Phi hp im (PCF) 56Cc mng Ad-hoc 56Cc tiu chun trong b tiu chun IEEE 802.11 56IEEE 802.11 56IEEE 802.11b 57IEEE 802.11a 57IEEE 802.11d 58IEEE 802.1x (Tbd) 58IEEE 802.11i 58IEEE 802.11g 59IEEE 802.11h 59IEEE 802.11n 60Bo mt trong mng WLAN 69C s bo mt mng WLAN 69WEP 74WPA (Wi-Fi Protected Access) 75WPA2 76Trng thi bo mt mng WLAN 84Cc v d kin trc bo mt mng WLAN 85Bo mt 88Kin trc khuyn ngh 88KT LUN V PHNG HNG TIP CN TIP THEO 91TI LIU THAM KHO 92

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LI NI U

X hi ngy cng pht trin, nhu cu i sng ngy cng cao, con ngi cn c nhiu phng tin tha mn nhng nhu cu v cng vic v sinh hot. Vic kt ni v truyn ti gia cc thit b cng ngh ngy cng c yu cu cao v cng ngh cng nh tc truyn ti. p ng nhu cu ngy cng cao , Vin IEEE pht trin tiu chun 802 dnh cho cc mng LAN v mng MAN. Tri qua thi gian pht trin, cc k s ca vin IEEE pht trin rt nhiu cc chun khc nhau thuc h chun IEEE 802. Cc chun c dng rng ri nht l dnh cho h Ethernet, Token Ring, mng LAN khng dy, cc mng LAN dng bridge v bridge o.

Ti liu ny tp trung tm hiu tng quan v h chun IEEE 802. Bn cnh , ti liu cng i su tm hiu v tiu chun IEEE 802.11, mt trong nhng b tiu chun quan trng trong h chun IEEE 802. Ti liu c chia lm 3 chng gm:

1. Chng I: Tng quan v t chc IEEE v h chun IEEE 802.

2. Chng II: Mt s chun thng dng trong h chun IEEE 802.

3. Chng III: IEEE 802.11 v chun ha mng LAN khng dy (WLAN)

Cui cng chng em xin chn thnh cm n s hng dn v to iu kin ca thy ThS. Ng Hn Chiu trong qu trnh chng em hon thnh ti ny.

ti: Tiu chun IEEE 802.11 v cng ngh WifiGVHD: ThS. Ng Hn Chiu

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DANH MC BNG BIU V HNH V

BNG BIUBng 1.1: Cc b tiu chun thuc h chun IEEE 80215Bng 2.1 : So snh ZigBee Wifi - Bluetooth28Bng 3.1. So snh cc lp vt l ca chun IEEE 802.1139Bng 3.2: Tng kt cc cch dng a ch54Bng 3.3: Cc c t k thut ch yu ca 802.11n64Bng 3.4: Cc c im k thut ca IEEE 802.11n66Bng 3.5. Cc c tnh ca cc kin trc bo mt mng WLAN88

HNH VHnh 1.1: M hnh phn tng ca mng LAN16Hnh 1.2: Quan h gia mt s chun IEEE v m hnh OSI16Hnh 1.3: Cc th v v cch tip cn i vi m hnh OSI17Hnh 2.1: Mt s loi mng Ethernet vi ng truyn vt l21Hnh 2.2: V d v mt mng Ethernet21Hnh 2.3: Vng RPR25Hnh 2.4: Cc lnh vc ng dng ZigBee29Hnh 2.5: Wimax Relay31Hnh 3.1. Mng WLAN IEEE 802.11 tiu biu38Hnh 3.2. Lp MAC40Hnh 3.3. Giao dch gia hai trm A v B, v s thit lp NAV43

Hnh 3.4. Khung MSDU44Hnh 3.5. S c ch truy cp46Hnh 3.6. So snh kiu Ad Hoc v kiu c s h tng47Hnh 3.7. Khun dng khung chun IEEE 802.1150Hnh 3.8. Khun dng khung MAC51Hnh 3.9. Trng iu khin khung (Frame Control)52Hnh 3.10: Khun dng khung RTS54Hnh 3.11: nh dng khung CTS55Hnh 3.12: nh dng khung ACK55Hnh 3.13:Logo chng nhn sn phm t chun 802.11n61Hnh 3.14: Cng ngh MIMO63Hnh 3.15 :Cc chui d liu ca 802.11n63Hnh 3.16: S tp hp ci thin hiu qu trong ch Mixed68Hnh 3.17: Cc mc bo v mng70Hnh 3.18. Cc mu lan truyn RF ca cc anten ph bin72Hnh 3.19. Chng thc h thng m73Hnh 3.20. Chng thc kha chia s74Hnh 3.21: Key Generation trong WEP, WPA v WPA277Hnh 3.22: Per-Packet Key Generation79Hnh 3.23: Authentication Architecture80Hnh 3.24: EAPOL81Hnh 3.25: Authentication Overview82Hnh 3.26: Hot ng ca ASE Counter Mode83Hnh 3.27: CBC MAC84

-6-Hnh 3.28. Chng thc LEAP/RADIUS Cisco87Hnh 3.29. Kin trc mng WLAN c xng89

100VG-

AnyLAN

THUT NG VIT TT

0-9

100VG (Voice Grade) AnyLanMt loi cng ngh mng

3GThird-GenerationCng ngh truyn thng th h th 3

4GFourth-GenerationCng ngh truyn thng th h th 4

A

ADSLAsymmetric Digital Subscriber Lineng dy thu bao bt i xng AMCAdaptive Modulation and CodingANSIAmerican National Standards InstituteVin Tiu chun Quc gia Hoa K APAccess Pointim truy nhpARCNETAttached Rersource Computer NetworkMt loi mng cc b Token-passing ATMAsynchronous Transfer ModeGiao thc truyn thng ATMB

BANBody Area Network TechnologiesMt loi cng ngh truyn thng

BSBase StationTrm pht

BSSBasic Service SetTp dch v c s

C

CCKComplementary Code KeyingK thut iu ch kha m b

CSMA/CDCarrier Sense Multiple Access/ Collision AvoidanceD

DOCSISData Over Cable Service Interface

Specification

a truy nhp cm nhn sng mng nhm trnh xung i

L mt c t

DQDBDistributed-queue dual-busCp quang hnh trng dng bus kp

DSdistributed systemH thng phn tn DSSSDirected Sequence Spread SpectrumTri ph chui trc tipE

EV-DOEvolution-Data OptimizedCng ngh pht trin ti u ha d liFFDDIFiber Distributed Data InterfaceGiao din D liu Phn b theo Cpsi quangGPRS GSMHARQIEEEIP ISL ISOLAN LLCMAC MAN MBWA MIH MIMO MPLSGGeneral Packet Radio Service Global System for Mobile CommunicationsHHybrid Automatic Repeat RequestIInstitute of Electrical and Electronics EngineersInternet Protocol Inter-Switch LinkInternational Organization for StandardizationLLocal Area Network Logical Link ControlMMedia Access Control Metropolitan Area NetworkMobile Broadband Wireless Access Media Independent Handover Multi Input Multi OutputMulti Protocol Label SwitchingODch v v tuyn gi tng hpH thng thng tin di ng ton cuK thut sa li bng d lpHc Vin k ngh in v in TGiao thc lin mngCc giao thc Cisco Inter-Switch Link T chc tiu chun ha quc tMng my tnh cc b iu khin logiciu khin truy cp mi trng truyn Mng th MANMng di ng bng rngK thut phn chia a ng Mt loi cng ngh truyn thngu

OSIOpen Systems Interconnection ReferenceM hnh tham chiu kt ni cc h

thng m

Q

QoS

QPDSQuality of Service

Queued-Packet, Distributed-SwitchCht lng dch v

Gii thut sp xp hng phn tn

R

RIFSReduced Inter-Frame SpacingGim khng gian gim cc frame PRPResilient Packet RingTiu chun thit k vn chuyn tiu d liu.

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SDH/SON ET

S

Synchronous Digital Hierarchy/ Synchronous Optical Networking

Mt chun mng truyn thng

SOFDMAMt k thut tri ph ci tin t

OFDMA

SSSucriber StationNgi s dng

TAGTechnical AdviSory GroupMt t chc

T

TVTelevisionTruyn hnh

U

UMTSUniversal Mobile Telecommunications System

L mng di ng th h th 3

V

VLANVirtual Local Area NetworkMng Lan o

W

Wi-FiWireless FidelityH thng mng khng dy

WiMAXWorldwide Interoperability for Microwave Access

L mt cng ngh vin thng cho php kt ni Internet bng rng khong cch ln

WLAN

WMANWireless LAN

Wireless Metropolitan Area NetworkMng LAN khng dy

Mng khng dy nh v th hay

gin l mng MAN khng dy

WMMWi-Fi MultimediaWi-Fi a phng tin

WPANWireless Personnal Area NetworkMng khng dy c nhn

WRANWireless Regional Area NetworkMng khng dy vng a l

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NI DUNG

1. Chng I: Tng quan v t chc IEEE v h chun IEEE 802

Trong chng ny s trnh by tng quan v t chc IEEE v cc h chun IEEE 802 cng nh

cc thng tin lin quan n h chun ny.

Mc ch thy c:

S lc v t chc IEEE v h tiu chun IEEE 802.

Cc b tiu chun v ang c chun ha.

Phm vi: Khng i su vo chi tit v t chc IEEE v cc tiu chun c th, m ch nu ln khi nim s lc, cch tip cn cng nh hin trng ca cc tiu chun thuc h chun IEEE 802.

1.1. Gii thiu v t chc IEEE

IEEE (Institute of Electrical and Electronics Engineers ngha l "Hc Vin k ngh in v in T") (pht m trong ting Anh nh i triple e), l t chc chuyn mn k thut ln nht trn th gii vi mc tiu thc y s sng to v chuyn ngnh cng ngh v li ch con ngi, c thnh lp vo nm 1884 bi mt s cc chuyn gia in nh Thomas Edison, Alexander Graham Bell New York, M. T chc ny chnh thc hot ng u nm 1963. IEEE l t chc hng u trong cc lnh vc t cc h thng khng gian v tr, my tnh v vin thng n k thut ha sinh, nng lng in, in t tiu dng vi 39 hi chuyn ngnh. IEEE ang ngy cng ln mnh, hin nay c 380.000 hi vin l cc nh khoa hc, cc nh gio dc, cc chuyn gia u ngnh, cc k s trong nhiu ngnh ngh t hn 150 nc, hot ng trong 325 chi hi 10 vng lnh th trn ton th gii. Vi 1300 tiu chun ban hnh v hn 400 tiu chun ang c son tho. IEEE cn l c quan pht trin cc tiu chun quc t hng u trong cc lnh vc vin thng, cng ngh thng tin, thit b sn xut nng lng v dch v,

T chc IEEE thnh lp mt y ban chun ha ring cho mng LAN/MAN c gi l IEEE 802. y ban ny tin hnh chun ha cc thnh phn cng nh tng chi tit trong cu trc mng LAN/MAN v cc qu trnh kt ni gia chng. Cc tiu chun do y ban ny chun ha c chia thnh cc nhm nh, tng ng vi tng lnh vc ca cc chun . y ban chun hamng LAN/MAN IEEE 802 c mt nguyn tc c bn l duy tr v khuyn khch s dng cc

chun ha IEEE/ANSI v cc chun tng ng IEC/ISO JTC trong lp 1 v lp 2 ca m hnh tham chiu OSI. y ban ny gp nhau t nht 3 ln mt nm t khi c thnh lp nm 1980. Theo yu cu ca mt s nc thnh vin, tp cc chun IEEE 802 c quc t ha trong chun JTC1. Tp cc chun ny c bit n vi k hiu l 802.xxx v cc chun tng ng ca JTC1 c k hiu l 8802-nm. IEEE 802 quan nim khi nim local (trong LAN) ngha l cc khu trng hc, c quan, cn khi nim metropolitan (trong MAN) ngha l trong mt thnh ph, th.

Trong h chun IEEE 802, IEEE a ra cc chun v cng ngh Ethernet u tin, cc cng ngh v mng LAN khng dy (Wireless LAN, WPAN, WiMAX),

1.2. Cc tiu chun IEEE

1.2.1. Gii thiu

IEEE 802 l cc h chun IEEE dnh cho cc mng LAN v mng MAN. C th hn, cc chun IEEE 802 c gii hn cho cc mng mang gi tin c kch thc a dng. (Khc vi cc mng ny, d liu trong cc mng cell-based c truyn theo cc n v nh c cng kch thc c gi l cell. Cc mng Isochronous, ni d liu c truyn theo mt dng lin tc cc octet, hoc cc nhm octet, ti cc khong thi gian u n, cng nm ngoi phm vi ca chun ny).Cc dch v v giao thc c t trong IEEE 802 nh x ti hai tng thp (tng lin kt d liu v tng vt l ca m hnh 7 tng OSI). Thc t, IEEE 802 chia tng lin kt d liu OSI thnh hai tng con LLC (iu khin logic lin kt) v MAC (iu khin truy cp mi trng truyn). C th, ta c th lit k nh sau: Tng lin kt d liu o Tng con LLC o Tng con MAC Tng vt l

H chun IEEE 802 c bo tr bi LMSC (Ban Tiu Chun LAN/MAN IEEE 802) c thnh lp nm 1980. LMSC pht trin rt nhiu tiu chun cho mng LAN/MAN trong ph bin nht l cc tiu chun dnh cho h Ethernet, Token Ring, mng LAN khng dy. Mi lnh vc c mt Working Group c lp tp trung nghin cu.

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H chun IEEE 802 hin c 3 tiu chun c chun ha:

Tiu chun 802-2001 IEEE cho cc mng LAN v MAN: tng quan v kin trc chung, tiu chun ny l mt phn ca h tiu chun 802 LAN/MAN v nu tng quan v h giao thc ny. ng thi nh ngha s tun th vi h tiu chun IEEE 802, m t mi quan h ca cc tiu chun IEEE 802 vi m hnh tham chiu OSI v mi quan h ca nhng tiu chun ny vi cc giao thc lp cao hn. Tiu chun ny cng a ra mt kin trc chun v a ch LAN MAC v s nhn dng cc giao thc chung, ring v chun. Tiu chun IEEE 802a-2003 cho mng LAN v MAN ni v cc loi Ethernet cho cc loi giao thc khc nhau v trin khai c th ca tng nh cung cp thit b. Tiu chun IEEE 802b-2004 cho mng LAN v MAN ni v qu trnh ng k v nhn dng cc mc tiu. P802/D29 (C/LM) ni v tng quan v kin trc ca mng LAN v MAN. Trong d n ny nhm im li cc chun c lin quan xut bn trc cng nh tho lun v cc chun ny.

1.2.2. Cc b tiu chun thuc h IEEE 802

IEEE l t chc i tin phong trong lnh vc chun ha mng cc b vi d n IEEE 802 bt u c trin khai v kt qu l hng lot chun h IEEE 802 ra i, to nn tng quan trng cho vic thit k v ci t mng cc b trong thi gian qua. V tr ca h chun ny cng cao hn khi ISO xem xt v tip nhn chng thnh chun quc t mang tn ISO 8802.x. n nay h IEEE 802 bao gm cc b tiu chun sau:

Tiu chunLnh vc nghin cuTrng thi hot ng

IEEE 802.1

IEEE 802.2Cc giao thc LAN tng cao

iu khin lin kt logic

ngng pht trin

IEEE 802.3

IEEE 802.4

IEEE 802.5Ethernet Token Bus Token Ring

gii tn

IEEE 802.6Metropolitan Area Network gii tn

IEEE 802.7Broadband LAN using Coaxial Cable gii tn

IEEE 802.8Fiber Optic TAG gii tn

IEEE 802.9Integrated Services LAN gii tn

IEEE 802.10Interoperable LAN Security gii tn

IEEE 802.11Wireless LAN (Wi-Fi certification)

IEEE 802.12Cng ngh 100 Mbits/s plus

IEEE 802.13Khng s dng

IEEE 802.14Modem cp gii tn

IEEE 802.15

IEEE 802.15.1

IEEE 802.15.4

IEEE 802.16

IEEE 802.16e

Wireless PAN Bluetooth certificationZigBee certification

Broadband Wireless Access (WiMAX certification)(Mobile) Broadband Wireless Access

IEEE 802.17Resilient packet ring IEEE 802.18Radio Regulatory TAG IEEE 802.19Coexistence TAGIEEE 802.20Mobile Broadband Wireless Access IEEE 802.21Media Independent HandoffIEEE 802.22Wireless Regional Area Network

Bng 1.1: Cc b tiu chun thuc h chun IEEE 802

1.2.3. Quan h gia cc chun IEEE v m hnh OSI

Ngoi m hnh OSI dng cho vic chun ha cc mng ni chung, vic chun ha mng LAN/MAN cng c thc hin trong mt khong thi gian di. Do c trng ring, vic chun ha mng LAN/MAN ch c thc hin trn hai tng thp nhp, tng ng vi tng vt l v tng lin kt d liu trong m hnh OSI.

Hnh 1.1: M hnh phn tng ca mng LAN

Trong LAN, tng lin kt d liu c chia lm hai tng con: LLC v MAC. LLC m bo tnh c lp ca vic qun l cc lin kt d liu vi ng truyn vt l v phng php truy cp ng truyn MAC. Tng con LLC trng vi nhiu mi trng truyn vt l khc nhau (chng hn Ethernet, token ring, WLAN). Trong khi , MAC qun l truy cp ng truyn, hot ng vi vai tr mt giao din gia tng con LLC v tng vt l ca mng.

Hnh 1.2 sau s m t s so snh v tr tng i ca mt s chun IEEE 802.x trn khi so snh vi m hnh OSI:

Hnh 1.2: Quan h gia mt s chun IEEE v m hnh OSI

-16-Hnh 1.3 s m t cc th d khc nhau v cch tip cn ca t chc IEEE i vi m hnh OSI

ti: Tiu chun IEEE 802.11 v cng ngh WifiGVHD: ThS. Ng Hn Chiu

Hnh 1.3: Cc th v v cch tip cn i vi m hnh OSI

2. Chng II: Mt s chun thng dng trong h chun IEEE 802

Trong chng ny s trnh by chung v cc b tiu chun trong h chun IEEE 802 v nhn mnh mt s chun quan trng, c p dng rng ri trong cuc sng.Mc ch thy c:

S lc v cc b tiu chun thuc h IEEE 802.

c im, tnh cht ca cc b tiu chun quan trng.

Phm vi: Khng i su vo chi tit cc chun thng thng, khng quan trng. Trong khi , o su vo cc chun quan trng, c ng dng rng ri.

2.1. Chun ha mng LAN/MAN hu tuyn

Bao gm cc b chun IEEE 802.1, .2, .3, .4, .5, .6, .9, .12, .14, .17. Nhng b chun ny ch yu chun ha v mng LAN/MAN hu tuyn, v cng ngh Ethernet cng nh cc phng thc truy nhp v bo hiu vt l cho cc cho cc cng ngh mng LAN/MAN hu tuyn gm: Token Bus, Token Ring, DQDB, cc dch v tch hp, u tin theo yu cu.

C th nh sau:

2.1.1. IEEE 802.1 - cc giao thc LAN tng cao

IEEE 802.1 l chun c t kin trc mng, kt ni gia cc mng v vic qun tr mng i vi cc mng cc b. Tiu chun IEEE 802.1 c pht trin cho cc lnh vc sau: kin trc mng LAN/WAN, kt ni gia cc mng LAN, mng MAN v cc mng li rng khu vc khc, bo mt, tng th mng li qun l v giao thc cc lp pha trn MAC v LLC.Trong h tiu chun ny hin c nhiu chun c th cho tng vn nh:

Chun IEEE P802.1AB/D10 l phin bn nhp hin cha c thng qua ni v cc trm v qu trnh khm ph iu khin truy nhp mi trng. Chun IEEE 802.1F-1993 (R2004) ni v cc nh ngha v cc th tc chung cho thng tin qun l IEEE 802 Chun IEEE 802.1D-2004 v cu ni iu khin truy nhp mi trng (MAC).

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Chun IEEE 802.1G, 1998 ni v k thut thng tin, vin thng v qu trnh trao i thng tin gia cc h thng. Chun IEEE 802.1X-2001 ni v iu khin truy nhp mng da vo cng.

Chun IEEE P802.1X/D11 l phin bn nhp hin vn cha c thng qua cng ni

v iu khin truy nhp mng da trn cng.

P802.1t/D10 (C/LM) l chun v k thut thng tin, vin thng v trao i thng tin gia cc h thng mng LAN v mng MAN-cc c tnh chung - phn 3: cc cu ni iu khin truy nhp mi trng (MAC)- cc hiu chnh v k thut v ni dung. P802.1w/D10 (C/LM) chun ny cng nh chun P802.1t/D10 trn, tuy nhin phn ny ni v qu trnh cu hnh li nhanh. P802.1X/D11(C/LM) ni v iu khin truy nhp mng da trn cng.

P802.1y (C/LM) ni v cu ni iu khin truy nhp mi trng trong mng LAN/MAN - bn sa i th 3: Cc hiu chnh v ni dung v k thut v c thng qua vo thng 12 nm 2005. P802.1aa (C/LM) ni v iu khin truy nhp mng da trn cng trong mng LAN v mng MAN

2.1.2. IEEE 802.2 iu khin lin kt logic (LLC)

IEEE 802.2 l chun c t tng LLC (dch v, giao thc) ca mng LAN. C 3 kiu giao thc LLC chnh c nh ngha: LLC type 1: L giao thc kiu khng lin kt, khng bo nhn.

LLC type 2: L giao thc kiu c lin kt.

LLC type 3: L giao thc dng khng lin kt, c bo nhn.

Cc giao thc ny c xy dng da theo phng thc cn bng ca giao thc HDLC v c cc khun dng d liu v cc chc nng tng t, c bit l trong trng hp LLC type 2.Hin ti, b tiu chun ny khng cn hot ng na.

2.1.3. IEEE 802.3 tiu chun cho cng ngh Ethernet

Vi i hi ni mng cc my tnh vi nhau, mng LAN ra i. Cng vi l cc b giao thc cho php kt ni LAN (FDDI, TokenRing,) ra i theo tuy nhin pht trin nht vn l Ethernet.

Ethernet l mt h ln v a dng gm cc cng ngh mng da khung d liu (frame-based) dnh cho mng LAN. Ethernet nh ngha mt lot cc chun ni dy v pht tn hiu cho tng vt l, hai phng tin truy nhp mng ti phn MAC (iu khin truy nhp mi trng truyn dn) ca tng lin kt d liu (data-link), v mt nh dng chung cho vic nh a ch.

Ethernet v m hnh kin trc c bn c hnh thnh vo nhng nm 1970 v tr thnh cng ngh ch o xy dng mng LAN vo nhng nm 1980. Tri qua hn hai thp k pht trin, vi mc tiu xuyn sut l xy dng mt giao din mm do, c linh hot v tin cy ln, gim gi thnh lp t mng, thun tin cho vic vn hnh v bo dng, p ng c nhng i hi ngy cng cao ca mng chuyn mch gi, Ethernet ngy nay tr nn ph bin trong cc im tp trung lu lng ca mng Internet, v ti cc kt ni ca cc my tnh trong mng vn phng. Cng vi s pht trin ca cng ngh thng tin, tc Ethernet c ci thin t Mbps ln Gbps. Song song vi n, cu hnh mng my tnh s dng cng ngh Ethernet cng pht trin t cu trc bus dng chung ln cu trc mng chuyn mch hnh sao. y l nhng nhn t quan trng xy dng cc mng my tnh c dung lng cao, cht lng cao, hiu sut cao, p ng c nhng i hi ngy cng kht khe ca yu cu v cht lng (QoS).

Do , y ban IEEE 802 chun ha Ethernet thnh IEEE 802.3. IEEE 802.3 quy nh cc phng thc truy nhp v bo hiu vt l cho cc k thut mng MAN v LAN hu tuyn theo CSMA/CD. Theo , cu trc mng hnh sao, hnh thc ni dy cp xon (twisted pair) ca Ethernet tr thnh cng ngh LAN c s dng rng ri nht t thp k 1990 n nay. N thay th cc chun LAN cnh tranh khc nh Ethernet cp ng trc (coaxial cable), Token Ring, FDDI v ARCNET.

Vi s pht trin mnh m ca cng ngh, tc kt ni trong Ethernet khng ngng c nng cao. Vo nm 1995, Fast Ethernet ra i, IEEE dng 802.3u quy chun cho cc tiu ch c

lin quan n Fast Ethernet. Tip n l 802.3z (10Gbps qua cp quang), 802.3ab (10 Gbps qua cp UTP), 802.3ae(10Gbps),..

Hin c 4 tiu chun ni v mng LAN/MAN thuc h ny.

Chun IEEE 802.3-2002 ni v k thut thng tin, vin thng v s trao i thng tin gia cc h thng mng LAN v mng MAN. Chun IEEE 802.3af-2003, chun ny cng nh chun IEEE 802.3-2002 nhng ni

v cng sut thit b u cui s liu (DTE) qua giao din c lp vi mi trng (MDI).

Chun IEEE 802.3aj-2003, chun ny cng nh hai chun trn nhng phn ny ni v qu trnh bo dng trong mng. Chun 802.3ak-2004, chun ny ni v cc tham s qun l v lp vt l cho hot

ng ti tc 10 Gb/s, loi 10GBase-CX4.

P802.3ah (C/LM) v k thut thng tin - vin thng v qu trnh trao i thng tin gia cc h thng mng LAN v mng MAN. P1802.3/D3.2 (C/LM) ni v phng php kim tra ph hp cho cc chun ca IEEE v mng LAN v MAN.Tuy nhin, trong nhng nm gn y, Wi-Fi dng LAN khng dy c chun ha bi b tiu chun IEEE 802.11 c s dng bn cnh hoc thay th cho Ethernet trong nhiu cu hnh mng.

Mt s thng tin thm v cc quy nh v mng khung d liu IEEE 802.3 quy nh mt s loi mng c ng truyn vt l nh sau:

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Hnh 2.1: Mt s loi mng Ethernet vi ng truyn vt l

V d v mt mng Ethernet:

Hnh 2.2: V d v mt mng Ethernet

2.1.4.IEEE 802.4, .5, .6, .9, .12, .14

Cc phng thc truy nhp v bo hiu vt l cho cc cho cc cng ngh mng MAN/LAN hu tuyn gm: Token Bus, Token Ring, DQDB, cc dch v tch hp, u tin theo yu cu. Nhng chun nh 802.4, 802.5, 802.6, 802.9 v 802.12 hin ang b lng qun v khng hot ng cn 802.14 va b gii tn.Mt s thng tin v cc chun ny:

IEEE 802.4

IEEE 802.4 l chun c t mng cc b vi tnh trng bus s dng token iu khin truy cp ng truyn. IEEE 802.4 cng bao gm c tng vt l v tng con MAC vi cc c t sau: c t dch v MAC

Giao thc MAC

c t dch v tng vt l

c t ng truyn

Nguyn l phng php truy nhp c iu khin: cp pht quyn truy nhp ng truyn cho cc trm ang c nhu cu truyn d liu, mt token c lu chuyn trn mt vng logic thit lp bi cc trm . Token l n v d liu c bit dng cp pht quyn truyn d liu. Cc i tng c nhu cu truyn d liu s bt tay vi nhau to thnh mt vng logic v token s c lu truyn trong vng logic ny. Sau khi truyn xong data hoc ht thi gian cm token th token c chuyn sang trm k tip trong vng logic. y thc cht l phng php thm nhp mng theo kiu pht tn tn hiu thm d token qua cc trm v ng truyn bus.

IEEE 802.5

IEEE 802.5 l chun c t mng LAN vi hnh trng vng s dng th bi iu khin truy cp ng truyn. IEEE 802.5 cng bao gm c tng vt l v tng con MAC vi cc c t sau: c t dch v MAC.

Giao thc MAC.

c t thc th tng vt l.

c t ni trm.

Nguyn l: IEEE 802.5 dng cho mng dng xoay vng v trn c s dng tn hiu thm d token. Mi trm khi nhn c tn hiu thm d token th tip nhn token v bt u qu trnh truyn thng tin di dng cc frame. Phng php xm nhp mng ny quy nh nhiu mc u tin khc nhau cho ton mng v cho mi trm, vic quy nh ny va do ngi thit k va do ngi s dng t quy nh.

IEEE 802.6

IEEE 802.6 l chun c t mt mng tc cao ni kt nhiu LAN thuc cc khu vc khc nhau ca mt th. Mng ny s dng cp quang vi hnh trng dng bus kp (dual-bus), v th cn c gi l DQDB. Lu thng trn mi bus l mt chiu v khi c cp bus cng hot ng s to thnh mt cu hnh chu li. Phng php iu khin truy cp da theo mt gii thut xp hng phn tn c tn l QPDS (Queued-Packet, Distributed-Switch).

y l mt ci tin ca mt tiu chun c hn (c to bi ANSI) m n s dng cu trc mng FDDI. Cc tiu chun da trn FDDI b tht bi do thc hin tn km v thiu tnh tng thch vi cc chun LAN hin hnh. Cc tiu chun IEEE 802.6 s dng DQDB nn n h tr ln ti 150 Mbit/s tc truyn ti. Tiu chun ny khng thnh cng, ch yu l do tiu chun FDDI m n da theo cng khng thnh cng.

IEEE 802.9

IEEE 802.9 l chun c t mt mng tch hp d liu v ting ni bao gm 1 knh d b 10 Mbps cng vi 95 knh 64 Kbps. Gii thng tng cng 16 Mpbs. Chun ny c thit k cho cc mi trng c lu lng lu thng ln v cp bch. Tiu chun ny thng c gi l isoEthernet. C mt s nh cung cp h tr cho isoEthernet, nhng n b mt th trng do vic pht trin nhanh chng ca Fast Ethernet v cc nhm lm vic pht trin b tiu chun ny gii tn.

IEEE 802.12

IEEE 802.12 l chun c t mng cc b da trn cng ngh c xut bi AT&T, IBM v HP, gi l 100 VG AnyLAN. Mng ny s dng hnh trng mng hnh sao v mt phng php

truy cp ng truyn c iu khin tranh chp. Khi c nhu cu truyn d liu, trm s gi yu cu

n hub v trm ch c th truyn d liu khi c hub cho php.

Chun ny nhm cung cp mt mng tc cao (100 Mbps v c th ln hn) c th hot ng trong cc mi trng hn hp Ethernet v Token Ring, bi th n chp nhn ca hai dng khung. 100VG AnyLAN l i th cnh tranh ng gm ca 100BASE-T (Fast Ethernet) nh mt s tnh nng ni tri hn, chng hn v khong cch i cp ti a cho php.

IEEE 802.14

Vo nhng nm 1990, y ban IEEE 802 thnh lp mt tiu ban (802.14) pht trin mt tiu chun cho cc h thng modem cp. Trong khi tin b ng k, nhm ny b gii tn khi cc nh iu hnh h thng Bc M ng h cc c im k thut mi m, non tr hn l DOCSIS.

2.1.5.IEEE 802.17

IEEE 802.17 hay l RPR (Resilient Packet Ring), giao thc lp MAC ang c IEEE chun ha, l gii php cho vn bng n nhu cu kt ni tc cao v chi ph thp trong khu vc thnh ph. Bng cch ghp thng k gi IP truyn trn h tng vng si quang, c th khai thc hiu qu dng vng quang v tn dng u im truyn gi nh Ethernet. Khi c li node hay lin kt xy ra trn vng si quang, RPR thc hin chuyn mch bo v thng minh i hng lu lng i xa khi ni b li vi tin cy t ti thi gian nh hn 50 ms.

RPR s dng vng song hng gm hai si quang truyn ngc chiu nhau, c hai vng ng thi c s dng truyn gi d liu v iu khin. RPR cho php nh cung cp dch v gim chi ph thit b phn cng cng nh thi gian v chi ph ca vic gim st mng. Trong RPR khng c khi nim khe thi gian, ton b bng thng c n nh cho lu lng. Bng cch tnh ton kh nng mng v d bo yu cu lu lng, RPR ghp thng k v phn phi cng bng bng thng (fairness) cho cc node trn vng trnh tc nghn c th mang li li ch hn nhiu so vi vng SDH/SONET da trn ghp knh phn chia theo thi gian.

RPR l giao thc lp MAC vn hnh lp 2 ca m hnh OSI, n khng nhn bit lp 1 nn c lp vi truyn dn nn c th lm vic vi WDM, SDH hay truyn dn da trn Ethernet (s dng GBIC - Gigabit Interface Converter). Ngoi ra, RPR i t thit b a lp n dch v mng

thng minh lp 3 nh MPLS. MPLS kt hp thit b ra mng IP lp 3 vi thit b lp 2 nh ATM, Frame Relay. S kt hp tin cy v kh nng phc hi ca RPR vi u im qun l lu lng v kh nng m rng ca MPLS VPN v MPLS TE c xem l gii php xy dng MAN trn th gii hin nay.

Mt nhc im ca cc phin bn u tin RPR l n khng cung cp ti s dng khng gian truyn frame n t a ch MAC khng c mt trn vng. Vn ny c gii quyt bi IEEE 802.17b, trong xc nh mt khng gian tng con ty chn (SAS). iu ny cho php ti s dng khng gian truyn frame n t a ch MAC khng c mt trong vng.

Hin nay RPR l vn kh phc tp v cha c chun ho y , nhiu nh sn xut c sn phm RPR 802.17 nhng kh nng tng thch gia sn phm ca cc hng khc nhau l khng chc chn.

Hnh 2.3: Vng RPR

2.2. Chun ha mng LAN/MAN khng dy

Bao gm cc b tiu chun IEEE 802.11, .15, .16, .20, .21, .22. Nhng b chun ny ch yu tp trung vo cc mng LAN khng dy nh cc mng WLAN, WPAN, WRAN, cng nh quy nh v cc chun cng ngh nh cng ngh WiFi (802.11), Bluetooth, ZigBee (802.15), WiMax (802.16), C th nh sau:

2.2.1. IEEE 802.11 cng ngh WiFi mng WLAN

IEEE 802.11 l mt tp cc chun bao gm cc c im k thut lin quan n h thng mng khng dy. Chun IEEE 802.11 m t mt giao tip truyn qua khng kh (ting Anh: over-the- air), s dng sng v tuyn truyn nhn tn hiu gia mt thit b khng dy v tng i hoc im truy cp (access point - AP), hoc gia hai hay nhiu thit b khng dy vi nhau(m hnh ad-hoc).

B chun ny bao gm cc tiu chun con nh IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, B chun ny l mt b chun quan trng, s c cp chi tit trong chng 3.

2.2.2. IEEE 802.15 Cng ngh Bluetooth, ZigBee mng WPAN

IEEE 802.15 l b tiu chun th 15 thuc h IEEE 802. B tiu chun ny chuyn v Wireless PAN (Personal Area Network). Nhm lm vic IEEE 802.15 bao gm 7 nhm lm vic con nh sau:

3Nhm 1: WPAN/ Bluetooth nhm lm vic chuyn v tiu chun Bluetooth.

3Nhm 2:

3Nhm 3: High Rate WPAN

3Nhm 4: Low Rate WPAN

3Nhm 5: Mesh Networking

3Nhm 6: BAN

3Nhm 7: VLC

Mng WPAN (Wireless Personal Area Network) hay cn gi l mng c nhn khng dy c s dng phc v truyn thng tin trong nhng khong cch tng i ngn. Khng ging nh mng WLAN, mng WPAN c th lin lc hiu qu m khng i hi nhiu v c s h tng. Tnh nng ny cho php c thm cc hng gii quyt r tin, nh gn m vn em li hiu sut cao trong lin lc nht l trong mt bng tn eo hp.

Trong thi gian khong gia nhng nm thp k 80 th k XX, chun IEEE 802.15 ra i phc v cho nhm chun WPAN. Nhm chun ny tp trung gii quyt cc vn v iu khin d liu trong nhng khong khng gian nh (bn knh 30m). Tnh nng ca chun mng WPAN l suy hao nng lng nh, tiu tn t nng lng, vn hnh trong vng khng gian nh, kch thc b. Chnh v th m n tn dng c tt nht u im ca k thut s dng li knh tn s, l gii quyt c cc vn hn ch v bng tn nh hin nay.

IEEE 802.15 c th phn ra lm 3 loi mng WPAN, chng c phn bit thng qua tc

truyn, mc tiu hao nng lng v cht lng dch v (QoS).

3 WPAN tc cao (chun IEEE 802.15.3) ph hp vi cc ng dng a phng tin yu cu cht lng dch v cao.3 WPAN tc trung bnh (chun IEEE 802.15.1 / Bluetooth) c ng dng trong cc mng in thoi n my tnh c nhn b ti PDA v c cht lng dch v QoS ph hp cho thng tin thoi.3 WPAN tc thp (chun IEEE 802.15.4 / LR-WPAN) dng trong cc sn phm cng nghip dng c thi hn, cc ng dng y hc ch i hi mc tiu hao nng lng thp, khng yu cu cao v tc truyn tin v QoS. Chnh tc truyn d liu thp cho php LR-WPAN tiu hao t nng lng. Trong chun ny th cng ngh ZigBee/IEEE802.15.4 chnh l mt v d in hnh.

Chng ta s ni r thm v cng ngh ZigBee, mt xu hng ca mng khng dy trong iu khin t ng.

c im ca cng ngh ZigBee l tc truyn tin thp, tiu hao t nng lng, chi ph thp v l giao thc mng khng dy hng ti cc ng dng iu khin t xa v t ng ha. T chc IEEE 802.15.4 bt u lm vic vi chun tc thp c mt thi gian ngn th tiu ban v ZigBee v t chc IEEE quyt nh st nhp v ly tn ZigBee t cho cng ngh mi ny. Mc tiu ca cng ngh ZigBee l nhm ti vic truyn tin vi mc tiu hao nng lng nh v cng sut thp cho nhng thit b c thi gian sng t vi thng n vi nm m khng yu cu cao v tc truyn tin nh Bluetooth. Mt iu ni bt l ZigBee c th dng c trong cc mng mt

li (mesh network) rng hn l s dng cng ngh Bluetooth. Cc thit b khng dy s dng cng ngh ZigBee c th d dng truyn tin trong khong cch 10-75m ty thuc vo mi trng truyn v mc cng sut pht c yu cu vi mi ng dng. Tc d liu l 250kbps di tn 2.4Ghz (ton cu), 40kbps di tn 915Mhz (M + Nht) v 20kbps di tn 868Mhz ( chu u).Cc nhm nghin cu ZigBee v t chc IEEE lm vic vi nhau ch r ton b cc khi giao thc ca cng ngh ny. IEEE 802.15.4 tp trung nghin cu v 2 tng thp ca giao thc (tng vt l v lin kt d liu). ZigBee cn thit lp c s cho nhng tng cao hn trong giao thc (t tng mng n tng ng dng) v bo mt, d liu, chun pht trin m bo chc chn rng cc khch hng d mua sn phm t cc hng sn xut khc nhau nhng vn theo mt chun ring lm vic vi nhau c m khng tng tc ln nhau.Hin nay th IEEE 802.15.4 tp trung vo cc chi tit k thut ca tng vt l v tng con MAC ng vi mi loi mng khc nhau (mng hnh sao, mng hnh cy, mng mt li). Cc phng php nh tuyn c thit k sao cho nng lng c bo ton v tr trong truyn tin l mc thp nht c th bng cch dng cac khe thi gian m bo (GTSs_guaranteed time slots). Tnh nng ni bt ch c cng ngh ZigBee l gim thiu c s hng hc dn n gin on kt ni ti mt nt mng trong mng mesh. Nhim v c trng ca tng vt l gm c pht hin cht lng ca ng truyn (LQI) v nng lng truyn (ED), nh gi knh truyn (CCA), gip nng cao kh nng chung sng vi cc loi mng khng dy khc.Vi cc tnh nng ni bt nh trn, ha hn trong tng lai s xut hin cng ngh ZigBee mi vi chun IEEE 802.15.4 c ph bin rng ri.

Bng 2.1 : So snh ZigBee Wifi - Bluetooth

Hnh 2.4: Cc lnh vc ng dng ZigBee

2.2.3. IEEE 802.16 cng ngh WiMAX mng WMAN

IEEE 802.16 l h thng tiu chun truy cp khng dy bng rng (Broadband Wireless Access Standards) cung cp c t chnh thc cho cc mng MAN khng dy bng rng trin khai trn ton cu. H thng tiu chun ny do nhm lm vic IEEE 802.16 c thnh lp nm 1999, nghin cu v xut. Nhm ny l mt n v ca hi ng tiu chun LAN/MAN IEEE 802. H tiu chun IEEE 802.16 chnh thc c gi l WirelessMAN (WMAN).

Cng ngh WiFi IEEE 802.11 trong vi nm gn y gt hi c nhng thnh cng rc r vi minh chng l n c trin khai rng ri khp ni. Hu nh tt c cc my tnh c nhn, in thoi thng minh, PDA u c tch hp WiFi. Tc d liu ca WiFi c th t c 54Mpbs. Tuy nhin vng ph sng ca WiFi ch hn ch tm vi chc n vi trm mt. p ng nhu cu ph sng xa hn, cng ngh WiMAX (IEEE 802.16) xut hin.

Chun WiMAX u tin ra i vo thng 10 nm 2001. Khc vi WiFi ch s dng mt bng tn, WiMAX c th hot ng trong bng tn t 2-66 Ghz. Cc ng dng khc nhau s dng nhng bng tn khc nhau trnh s giao thoa. C th, cc ng dng di ng (802.16e) dng bng tn t 2-11 GHz. nhiu nc chu u, bng tn 3.5 GHz c dnh ring cho WiMAX di ng. Cc ng dngicinhi(802.16d)1th1dng1bng1tn1t110-661GHz.

Cc chun khc nhau ca WiMAX

Chun c bn 802.16 basic:

Chun 802.16 ban u c to ra vi mc ch l to ra nhng giao din (interface) khng dy da trn mt nghi thc MAC chung. Kin trc mng c bn ca 802.16 bao gm mt trm pht (BS - Base Station) v ngi s dng (SS - Sucriber Station). Trong mt vng ph sng, trm BS s iu khin ton b s truyn d liu (traffic). iu c ngha l s khng c s trao i truyn thng gia hai SS vi nhau. Ni kt gia BS v SS s gm mt knh uplink v downlink. Knh uplink s chia s cho nhiu SS trong khi knh downlink c c im broadcast. Trong trng hp khng c vt cn gia SS v BS (line of sight), thng tin s c trao i trn bng tn cao. Ngc li, thng tin s c truyn trn bng tn thp chng nhiu.

Cc chun b sung (amendments) ca WiMAX

802.16a : Chun ny s dng bng tn c bn quyn t 2 11 Ghz. y l bng tn thu ht c nhiu quan tm nht v tn hiu truyn c th vt c cc chng ngi trn ng truyn. 802.16a cn thch ng cho vic trin khai mng Mesh m trong mt thit b cui (terminal) c th lin lc vi BS thng qua mt thit b cui khc. Vi c tnh ny, vng ph sng ca 802.16a BS s c ni rng. 802.16b: Chun ny hot ng trn bng tn t 5 6 Ghz vi mc ch cung ng dnh v vi cht lng cao (QoS). C th chun u tin truyn thng tin ca nhng ng dng video, thoi, real-time thng qua nhng lp dch v khc nhau (class of service). Chun ny sau c kt hp vo chun 802.16a. 802.16c : Chun ny nh ngha thm cc profile mi cho di bng tn t 10-66GHz vi mc ch ci tin interoperability. 802.16d : C mt s ci tin nh so vi chun 802.16a. Chun ny c chun ha 2004. Cc thit b pre-WiMAX c trn th trng l da trn chun ny. 802.16e : c chun ha. c im ni bt ca chun ny l kh nng cung cp cc dch v di ng (vn tc di chuyn ln nht m vn c th dng tt dch v ny l 100km/h).

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802.16j: By gi IEEE ang bt tay vo chun ha 802.16j phc v cho vic Relay

(Wimax Mesh network). minh ha wimax relay, cc bn c th xem hnh 2.5 y

Hnh 2.5: Wimax Relay

Li ch ca vic dng nhng relay BS c lit k trong hnh 2.5. C th k n cc li ch sau: Thay v lin lc trc tip vi BS, user c th lin lc thng qua nhiu Relay BS vi

ng truyn tt hn v tc cao hn, hiu qu truyn cao hn, v.v

Relay BS c th dng tng vng ph sng ca mng WiMAX (relay BS r hn

lp t BS wimax).

User s khng cn tiu tn mt nng lng ln lin lc vi BS (tit kim nng lng tiu th thit b di ng). 802.16m: ang c nghin cu v chun ha. Chun ny hng ti tng tc truyn ca WiMAX ln 1Gbps bng cch dng MIMO v cc dy angten. Ngoi ra cn c nhiu chun b sung khc ang c trin khai hoc ang trong giai on chun ha nh 802.16g, 802.16f, 802.16h...

c im ni bt ca WiMAX di ng

WiMAX di ng cng c nhng c im ging EV-DO hoc HSxPA nhm tng tc truyn thng (data rate). Nhng c im bao gm: M ha v iu ch thch nghi (Adaptive Modulation and Coding - AMC), k thut sa li bng d lp (Hybrid Automatic Repeat Request- HARQ), Phn b nhanh (Fast Scheduling) v chuyn giao mng (handover) nhanh v hiu qu.

Khng ging nh cng ngh 3G da trn CDMA c xy dng nhm vo dch v thoi, WiMAX c thit k p ng dch v truyn d liu dung lng ln (trong c c dch v thoi VoIP). WiMAX s dng k thut tri ph SOFDMA v h tng mng xy dng trn nn IP.WiMax cung cp kh nng kt ni Internet khng dy nhanh hn so vi WiFi, tc uplink v downlink cao hn, s dng c nhiu ng dng hn, v quan trng l vng ph sng rng hn, v khng b nh hng bi a hnh. WiMAX c th thay i mt cch t ng phng thc iu ch c th tng vng ph bng cch gim tc truyn v ngc li. tng vng ph, chun WiMAX hoc s dng mng Mesh hoc s dng antenna thng minh hoc MIMO. D liu truyn trong mng WiMAX c phn chia thnh 5 lp dch v vi nhng u tin khc nhau nhm cung ng QoS. Ngoi ra bo mt cng l mt c im vt tri ca WiMAX so vi WIFI.

ng dng ca WiMAX

Ni ti WiMAX , ngi ta c th ngh ti rt nhiu gii php thay th m cng ngh ny c th mang li. chnh l kh nng thay th ng xDSL gip tip cn nhanh hn cc i tng ngi dng bng rng m khng cn phi u t ln. c bit WiMAX rt hu ch cung cp dch v bng thng rng nhng vng xa xi m gii php ADSL hoc cp quang l rt tn km. nhng nc ang pht trin nh Vit Nam, ni m Internet bng thng rng cha ph bin, WiMAX l mt gii php kinh t. Ngoi ra WiMAX cn gip vic trin khai WiFi thm nhanh chng do cc hotspot WiFi s khng cn ng leased-line m s ni trc tip vi WiMAX BS. Kh nng roaming gia cc dch v Wi-Fi v WiMAX s mang li nhiu li ch cho ngi s dng.

Hnh 2.6: Minh ha 1 s ng dng last-mile ca WiMAX

c th dng dch v Internet bng thng rng ca WiMAX (fixed WiMAX), nh cung cp dch v ch cn lp t mt ang-ten BS gia khu dn c. Mi ngi dng s c cung cp mt ang-ten thu (CPE), lp trn mi nh/ca s. CPE c th c ni trc tip vi my vi tnh hoc thng qua mt Access Point WiFi. Vic trin khai kh n gin, m gi thnh li thp hn nhiu so vi cng ngh hin hnh.Bn cnh dch v c nh, WiMAX cn cung ng cc dch v di ng. Trong tng lai, cc thit b mobile m hin nay c tch hp WiFi s c tch hp WiMAX. Khi , ngi dng c th kt ni mng mi lc mi ni thng qua WiMAX, v c bit l vn c th dng cc dch v ging nh nhng dch v ca mng cellular 3G. Hn na, tc truyn ca WiMAX cao hn hn 3G m gi ha hn s r. i vi cc nh cung cp mng, gi thnh ca mt WiMAX BS r hn rt nhiu so vi gi ca mt BS UMTS. Do , c th nh cung ng mng 3G s dng WiMAX thay th 3G11nhng1khu1vc1tha1dn1c.

Hnh 2.7: Minh ha ng dng ca WiMAX Mobile

2.2.4. IEEE 802.20

IEEE 802.20 hay l MBWA (Mobile Broadband Wireless Access). Chun ny bt ngun t mng Wi-Fi, chuyn qua cc tiu chun c nh IEEE 802.16e, IEEE 802.16m. Tiu chun ny c th h tr ngay c khi ang di chuyn vi tc ln n 250km/h. Trong khi chuyn vng (roaming) ca WiMAX nhn chung b gii hn trong mt phm vi nht nh, th chun IEEE802.20 ging nh 3G c kh nng h tr chuyn vng ton cu. Ngoi ra, cng ging nh WiMAX, IEEE 802.20 cng h tr cc k thut QoS nhm cung cp nhng dch v c yu cu cao v tr, Trong mng IEEE 802.20, vic ng b ng ln v ng xung u c thc hin hiu qu. D kin, chun IEEE 802.20 tng lai s kt hp vi mt s tnh nng ca IEEE 802.16e v cc mng d liu 3G, nhm cung cp v to ra mt mng truyn thng a dng (rich communication). IEEE 802.20 cng vi IEEE 802.16m, IEEE 802.22 c xem l nhng cng ngh tin 4G.

2.2.5. IEEE 802.21

IEEE 802.21 l mt tiu chun IEEE mi cn ang trong qu trnh pht trin. Tiu chun ny h tr cc thut ton cho php chuyn giao lin mng gia cc mng cng loi cng nh bn giao gia cc loi mng khc nhau, cng hay c gi l MIH (Media Independent Handover). Tiu chun ny cung cp thng tin cho php bn giao n v i t cc mng GSM, GPRS, WiFi, Bluetooth, IEEE 802.11, IEEE 802.16 thng qua cc c ch chuyn giao khc nhau.

Cc nhm lm vic IEEE 802.21 bt u lm vic vo thng 3 nm 2004. Nhiu hn 30 cng ty gia nhp nhm lm vic. Cc nhm sn xut mt d tho u tin cho tiu chun gm cc nh ngha giao thc. Qu trnh b phiu cng c thc hin v bn sa i tip theo ca d tho cng ang c tin hnh pht trin. Trong tng lai s c nhng tiu chun thuc b IEEE802.21 i.

2.2.6. IEEE 802.22 mng WRAN

IEEE 802.22 l mt tiu chun cho WRAN (Wireless Regional Area Network) s dng khong trng trong ph tn truyn hnh ph tn m TV analog khng s dng c. S pht trin ca tiu chun IEEE 802.22 WRAN l nhm mc ch s dng k thut v tuyn cho php s dng nhng ph tn khng s dng cho dch v pht sng truyn hnh, trn c s khng can thip vo cc ph tn TV analog. Vng ph ca cng ngh ny c th ln tm 40 100km, do n mang li kh nng truy cp bng rng n nhng vng a l kh khn, kh tip cn, nhng khu vc c mt dn s thp, xa xi. iu lm cho chun IEEE 802.20 c tim nng cho mt ng dng rng ton cu.

IEEE 802.22 WRAN c thit k hot ng trong bng tn pht sng truyn hnh trong khi m bo rng khng c nhiu c hi cho cc hot ng truyn thng hin ti. Tiu chun ny d kin s c p dng rng ri trn ton cu.

2.3. Chun ha cc thnh phn khc

Bao gm cc b chun cn li. Nhim v ch yu tp trung vo cc vn v bo mt mng cng nh iu phi cc mng vi nhau, trnh can thip ln nhau.

C th nh sau:

2.3.1. IEEE 802.10

IEEE 802.10 l mt tiu chun cho cc chc nng bo mt c th c s dng trong c hai mng LAN v MAN. IEEE 802.10 quy nh c th vic qun l an ninh v qun l ch cht, cng nh kim sot truy cp, bo mt d liu v tnh ton vn d liu.

Cc tiu chun IEEE 802.10 tm dng hot ng vo thng 1 nm 2004 v nhm pht trin tiu chun ny ca IEEE 802 b gii tn. Cc giao thc Cisco Inter-Switch Link (ISL) h tr VLAN trn mng LAN Ethernet v cc cng ngh tng t da trn chun IEEE 802.10 phn ln c thay th bi IEEE 802.1q. Bo mt cho mng khng dy th ang c pht trin sang cho IEEE 802.1i.

2.3.2. IEEE 802.19

IEEE 802.19 hay cn gi l Coexistence TAG ( Technical AdviSory Group) l mt nhm lm vic trong y ban IEEE 802, nhm gii quyt cc vn chung sng gia cc mng khng dy khng c giy php (tc l cc mng khng dy t pht ca ngi s dng). Nhiu chun khng dy IEEE 802 s dng nhng ph tn khng c giy php v do cn gii quyt vn cng tn ti. Nhng thit b khng dy khng c giy php c th hot ng trong cng mt bng tn khng c giy php cng mt v tr. iu ny c th dn n s can thip ln nhau gia hai mng khng dy. (Hai mng khng dy khng c giy php c cho l cng tn ti nu chng c th hot ng trong cng mt v tr a l m khng gy nhiu vi nhau ng k).

V d nh cc chun khng dy c th cng tn ti:

IEEE 802.11 WLAN

IEEE 802.15 WPAN

IEEE 802.16 WMAN

IEEE 802.22 WRAN

3. Chng III: IEEE 802.11 v chun ha mng LAN khng dy (WLAN)

Phn ny cp chi tit, tm hiu su v cc kha cnh ca b tiu chun IEEE 802.11Mc

ch thy c tnh cht v ng dng ca cc b tiu chun ny.

3.1. Gii thiu b tiu chun IEEE 802.11

IEEE 802.11 l mt tp cc chun ca t chc IEEE bao gm cc c t k thut lin quan n h thng mng khng dy. Mc ch ca chun IEEE 802.11 nh IEEE nh ngha l " cung cp kt ni khng dy ti cc thit b, hoc cc trm t ng m yu cu trin khai nhanh, v xch tay hoc cm tay, hoc c gn ln cc phng tin chuyn ng bn trong mt vng".Chun IEEE 802.11 m t mt giao tip "truyn qua khng kh" (over-the-air) s dng sng v tuyn truyn nhn tn hiu gia mt thit b khng dy v tng i hoc im truy cp (AP), hoc gia 2 hay nhiu thit b khng dy vi nhau (m hnh ad-hoc).

3.2. Kin trc ca chun IEEE 802.11

3.2.1. Cc thnh phn kin trc

Chun IEEE 802.11 nh ngha tng vt l v tng MAC cho mt mng WLAN. Chun ny nh ngha ba tng vt l khc nhau cho mng WLAN 802.11, mi tng hot ng mt di tn khc nhau v s dng cc tc 1 Mbps v 2 Mbps. Thnh t c bn ca kin trc 802.11 l t bo (cell) vi tn gi trong 802.11 l BSS (c gi l Tp hp dch v c bn, Basic Service Set). Mi BSS thng gm mt vi my trm khng dy v mt trm c s trung tm c gi l AP (access point). Cc my trm (c th di ng hoc c nh) v trm trung tm lin lc vi nhau bng giao thc MAC IEEE 802.11 khng dy. C th kt ni nhiu trm AP vi nhau bng mng hu tuyn Ethernet hoc mt knh khng dy khc to mt h thng phn tn (DS distributed system). i vi cc giao thc tng cao hn, h thng phn tn ny nh l mt mng 802 n.

Cc my trm dng chun IEEE 802.11 c th nhm li vi nhau to thnh mt mng ad hoc - mng khng c iu khin trung tm v khng c kt ni vi "th gii bn ngoi". Trong trng hp ny, mng c hnh thnh tc thi khi mt s thit b di ng tnh c thy mnh ang gn nhau trong khi ang c nhu cu lin lc m khng tm thy mt c s h tng mng sn c ti ch (chng hn mt BBS 802.11 vi mt trm AP). Mt v d v mng ad hoc c hnh thnh

l khi mt vi ngi mang my tnh xch tay gp nhau ti mt bn tu v mun trao i d liu m khng c mt trm AP gn . Tng t trong nh mng Ethernet hu tuyn 802.3, cc my trm trong mng WLAN 802.11 phi phi hp vi nhau khi dng chung mi trng truyn dn (tn s radio). Giao thc MAC c nhim v iu khin s phi hp ny. MAC IEEE 802.11 l giao thc CSMA/CA.

Ton b lin kt li mng LAN khng dy bao gm cc cell khc nhau, cc im truy cp v h phn phi tng ng, c xem xt thng qua m hnh OSI, nh mt mng n chun IEEE 802, v c gi l Tp hp dch v c m rng (ESS).

Hnh 3.1 m t mt chun mng LAN IEEE 802.11 tiu biu:

Hnh 3.1. Mng WLAN IEEE 802.11 tiu biu

Chun cng nh ngha khi nim Portal, l mt thit b lin kt gia mng LAN chun IEEE 802.11 v mng LAN chun IEEE 802 khc. Khi nim ny m t v l thuyt phn chc nng ca cu chuyn dch.Mc d chun khng yu cu s ci t tiu biu tt yu phi c AP v Portal trn mt thc th vt l n.

3.2.2. Kin trc cc lp trong m hnh OSI ca chun IEEE 802.11

Chun IEEE 802.11 nh ngha lp vt l (PHY) v lp iu khin truy cp mi trng (MAC) cho cc mng WLAN. N nh ngha lp vt l hot ng tc d liu 1Mbps v 2 Mbps trong bng tn RF 2.4 GHz v trong hng ngoi (IR). N c m rng hai ln vo nm 1999 thnh chun IEEE 802.11a nh ngha lp vt l cho bng 5GHz tc 54 Mbps, v chun IEEE 802.11b nh ngha lp vt l cho bng tn 2.4 GHz tc 5.5 v 11 Mbps.

3.2.2.1. Lp vt l

Chun IEEE 802.11 quy nh cc lp vt l nh bng 3.1.

Chun

Tn s v tuyn (RF)

Hng ngoi (IR)

C chTc d liu cc i (Mbps)

IEEE 802.112.4 GHzDSSS2

IEEE 802.112.4 GHzFHSS2

IEEE 802.11850 - 950 nmIR2

IEEE 802.11a5 GHzOFDM54

IEEE 802.11b2.4 GHzDSSS11

Bng 3.1. So snh cc lp vt l ca chun IEEE 802.11.

H thng tri ph nhy tn FHSS 2.4 GHz v h thng IR ca chun IEEE 802.11 t khi c s dng. Lp vt l OFDM 5 GHz c phm vi hn ch (xp x 15m) nn n t c s dng. a s cc sn phm hin ti thc hin cng ngh tri ph chui trc tip (DSSS) theo chun IEEE 802.11b tc d liu ln trn ti 11 Mbps do li th kh nng thc hin v gi thnh ca n.Mc ch ca cng ngh tri ph l tng thm thng lng v tin cy ca truyn dn bng cch s dng nhiu di tn. DSSS hot ng bng cch chuyn i mi bit truyn thnh mt chui"chip" m thc cht l mt chui s 1 v 0. Sau chip ny c gi song song qua mt di tn

rng. V s dng nhiu di tn, nn n tng cng tin cy truyn dn khi c giao thoa. V mi bit c biu din bi mt chui chip, nn nu phn no ca chui chip b mt v giao thoa, th gn nh phn chip nhn c s vn phn bit bit gc.

3.2.2.2. iu khin truy cp mi trng (Lp MAC)

Hnh 3.2. Lp MAC

Trong khi lp vt l chun IEEE 802.11 khc vi chun IEEE 802.3 Ethernet, th ch tiu k thut ca MAC tng t nh ch tiu k thut ca MAC Ethernet chun IEEE 802.3 cng vi iu khin lin kt Logic (LLC) chun IEEE 802.2, n lm cho khng gian a ch MAC chun IEEE802.11 thch hp vi khng gian a ch MAC ca cc giao thc IEEE 802. Trong khi MAC Ethernet chun IEEE 802.3 thc cht l CSMA/CD - a truy cp nhy sng mang pht hin xung t, th MAC chun IEEE 802.11 l CSMA/CA - a truy cp nhy sng mang trnh xung t. S khc nhau ny l do khng c phng cch thit thc truyn v nhn cng lc trn mi trng khng dy (mi trng WM). CSMA/CA c gng trnh cc va chm trn mi trng WM bng cch t mt khong thi gian thng tin trong mi khung MAC, cc trm thu xc nh thi gian cn li ca khung trn mi trng WM. Nu khong thi gian ca khung MAC trc ht v mt kim tra nhanh trn mi trng WM ch ra rng n khng bn, th trm truyn c php truyn. Bng cch ny, n cho php ni gi truyn bt k lc no m mi trng khng bn.Ngoi cc tnh nng chun c thc hin bi cc lp MAC, lp MAC chun IEEE 802.11 cn thc hin chc nng khc lin quan n cc giao thc lp trn, nh Phn on, Pht li gi d liu, v Cc ghi nhn.

Lp MAC nh ngha hai phng php truy cp khc nhau, Hm phi hp phn tn v Hm phi hp im.

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3.2.3. Phng php truy cp c bn: CSMA/CAy l mt c ch truy cp c bn, c gi Hm phi hp phn tn, v c bn l a truy cp cm bin sng mang vi c ch trnh xung t (CSMA/CA). Cc giao thc CSMA c bit trong cng nghip, m ph bin nht l Ethernet, l giao thc CSMA/CD (CD ngha l pht hin xung t).Giao thc CSMA lm vic nh sau: Mt trm truyn i cc cm bin mi trng, nu mi trng bn (v d, c mt trm khc ang pht), th trm s tr hon truyn mt lc sau, nu mi trng t do th trm c cho php truyn.Loi giao thc ny rt c hiu qu khi mi trng khng ti nhiu, do n cho php cc trm truyn vi t tr hon, nhng thng xy ra trng hp cc trm pht cng lc (c xung t), gy ra do cc trm nhn thy mi trng t do v quyt nh truyn ngay lp tc.Cc tnh trng xung t ny phi c xc nh, v vy lp MAC phi t truyn li gi m khng cn n cc lp trn, iu ny s gy ra tr ng k. Trong trng hp mng Ethernet, s xung t ny c on nhn bi cc trm pht i ti quyt nh pht li da vo gii thut exponential random backoff.Cc c ch d tm xung t ny ph hp vi mng LAN ni dy, nhng chng khng c s dng trong mi trng mng LAN khng dy, v hai l do chnh:

1. Vic thc hin c ch d tm xung t yu cu s thi hnh ton song cng, kh nng

pht v nhn ng thi, n s lm tng thm chi ph mt cch ng k.

2. Trn mi trng khng dy chng ta khng th gi thit tt c cc trm nghe thy c nhau (y l s gi thit c s ca s d tm xung t), v vic mt trm nhn thy mi trng t do v sn sng truyn khng tht s c ngha rng mi trng l t do quanh vng my thu.

vt qua cc kh khn ny, chun IEEE 802.11 s dng mt c ch trnh xung t vi mt

s Ghi nhn tnh tch cc (Positive Acknowledge) nh sau:

Mt trm mun truyn cm bin mi trng, nu mi trng bn th n tr hon. Nu mi trng rnh vi thi gian c ch r (gi l DIFS, Distributed Inter Frame Space, Khng gian khung Inter phn tn), th trm c php truyn, trm thu s kim tra m CRC ca gi nhn c

v gi mt gi chng thc (ACK). Chng thc nhn c s ch cho my pht bit khng c s xung t no xut hin. Nu my pht khng nhn chng thc th n s truyn li on cho n khi n c tha nhn hoc khng c php truyn sau mt s ln pht li cho trc.

Cm bin sng mang o (Virtual Carrier Sense)

gim bt xc sut kh nng hai trm xung t nhau v chng khng th nghe thy nhau, chun nh ngha mt c ch Cm bin sng mang o:Mt trm mun truyn mt gi, trc ht n s truyn mt gi iu khin ngn gi l RTS (Request To Send) gm ngun, ch n, v khong thi gian giao dch sau (v.d. gi v ACK tng ng), trm ch s p ng (nu mi trng t do) bng mt gi iu khin p li gi l CTS (Clear To Send) gm cng thng tin khong thi gian.Tt c cc trm nhn RTS v/hoc CTS, s thit lp ch bo Virtual Carrier Sense ca n (gi l NAV, Network Allocation Vector, Vect nh v mng) cho khong thi gian cho trc, v s s dng thng tin ny cng vi Cm bin sng mang vt l (Physical Carrier Sense) khi cm bin mi trng.C ch ny gim bt xc sut xung t v vng my thu do mt trm n t my pht, lm ngn khong thi gian truyn RTS, v trm s nghe thy CTS v d tr mi trng khi bn cho n khi kt thc giao dch. Thng tin khong thi gian v RTS cng bo v vng my pht khi cc xung t trong thi gian ACK (bi cc trm nm ngoi phm vi trm nhn bit).Cn ch thng tin khong thi gian ACK v cc khung RTS v CTS l cc khung ngn. N cng lm gim bt no ca cc xung t, v chng c nhn dng nhanh hn khi n c nhn dng nu ton b gi c truyn, (iu ny ng nu gi ln hn RTS mt cch ng k, nh vy l chun cho php k c cc gi ngn s c truyn m khng c giao dch RTS/CTS), v iu ny c iu khin bi mt tham s gi l ngng RTS.Cc s sau cho thy mt giao dch gia hai trm A v B, v s thit lp NAV ca cc trm gn chng:

Hnh 3.3. Giao dch gia hai trm A v B, v s thit lp NAV

Trng thi NAV c kt hp vi cm bin sng mang vt l cho bit trng thi bn ca

mi trng.

3.2.4. Cc chng thc mc MACLp MAC thc hin d tm xung t bng cch ch i s tip nhn ca mt ghi nhn ti bt k on c truyn no (Ngoi l cc gi m c hn mt ni n, nh Qung b, cha c tha nhn).

3.2.5. Phn on v Ti hp

Cc giao thc mng LAN tiu biu s dng cc gi vi vi hng trm byte (v d, gi Ethernet di nht di trn 1518 byte) trn mt mi trng mng LAN khng dy. L do cc gi di c a chung s dng cc gi nh l:

V t l li bit BER ca thng tin v tuyn cao hn, xc sut mt gi b h tng thm

theo kch thc gi.

Trong trng hp b hng (v xung t hoc nhiu), gi nh nht vi t mo u hn gy

ra s pht li gi.

Trn mt h thng FHSS, mi trng c ngt nh k mi khi nhy tn (trong trng hp ny l mi 20 mili - giy), nh vy nh hn gi, nh hn c hi truyn b hon li sau thi gian ngng truyn.

Mc khc, n khng c gii thiu nh l mt giao thc mng LAN mi v n khng th gii quyt cc gi 1518 byte c s dng trn mng Ethernet, nh vy IEEE quyt nh gii quyt vn bng cch thm mt c ch phn on/ti hp n gin ti lp MAC.C ch l mt gii thut Send - and - Wait n, trong trm pht khng cho php truyn mt

on mi cho n khi xy ra mt trong cc tnh hung sau y:

1. Nhn mt ACK cho on.

2. Quyt nh rng on cng c truyn li nhiu ln v th vo ton b khung.

Cn phi nh rng chun cho php trm c truyn ch mt a ch khc gia cc pht li ca mt on cho, iu ny c bit hu ch khi mt AP c vi gi ni bt vi cc ch n khc nhau v mt trong s chng khng tr li.

S sau biu din mt khung (MSDU) c chia thnh vi on (MPDUs):

Hnh 3.4. Khung MSDU

3.2.6. Cc khng gian khung Inter (Inter Frame Space)

Chun nh ngha 4 kiu khng gian khung Inter, c s dng cung cp cc quyn u tin

khc nhau:

SIFS - Short Inter Frame Space, c s dng phn chia cc truyn dn thuc mt hi thoi n (v.d. Ack - on), v l Khng gian khung Inter ti thiu v lun c nhiu nht mt trm n truyn ti thi gian cho trc, do n c quyn u tin i vi tt c cc trm khc. l mt gi tr c nh trn lp vt l v c tnh ton theo cch

m trm pht truyn ngc li nhn kiu v kh nng gii m gi vo, trong lp vt l chun IEEE 802.11 FH gi tr ny c thit lp 28 micr - giy. PIFS - Point Cooordination IFS, c s dng bi im truy cp (hoc Point Coordinator, c gi trong trng hp ny), c truy cp ti mi trng trc mi trm khc. Gi tr ny l SIFS cng vi mt khe thi gian (s c nh ngha sau), v d 78 micr - giy. DIFS - Distributed IFS, l khng gian khung Inter c s dng bi mt trm sn sng bt u mt truyn dn mi, m l c tnh ton l PIFS cng thm mt khe thi gian, v d 128 micr - giy. EIFS - Extended IFS, L mt IFS di hn c s dng bi mt trm nhn mt gi khng hiu, n cn ngn trm (trm m khng hiu thng tin khong thi gian Cm bin sng mang o) khi xung t vi mt gi tng lai thuc hi thoi hin thi.

3.2.7. Gii thut Exponential Backoff

Backoff l mt phng php ni ting gii quyt cc tranh ginh gia cc trm khc nhau mun truy cp mi trng, phng php yu cu mi trm chn mt s ngu nhin (n) gia 0 v mt s cho trc, v i s khe thi gian ny trc khi truy cp mi trng, n lun kim tra liu c mt trm khc truy cp mi trng trc khng.Khe thi gian c nh ngha theo cch m mt trm s lun c kh nng xc nh liu trm khc truy cp mi trng ti thi gian bt u ca khe trc khng. iu ny lm gim bt xc sut xung t i mt na.Exponential Backoff c ngha rng mi ln trm chn mt khe thi gian v xy ra xung t, n s tng gi tr theo ly tha mt cch ngu nhin.Chun IEEE 802.11 chun nh ngha gii thut Exponential Backoff c thc hin trong cc

trng hp sau y:

Nu khi trm cm bin mi trng trc truyn gi u tin, v mi trng ang bn

Sau mi ln truyn li

Sau mt ln truyn thnh cng

Trng hp duy nht khi c ch ny khng c s dng l khi trm quyt nh truyn mt gi mi v mi trng rnh cho nhiu hn DIFS.Exponential backoff khin cc nt chu kh ch lu hn khi mc xung t cao.

bit time: thi gian truyn 1 bit.

n l s ln xung t khi truyn mt frame no .

sau n ln xung t, nt s i 512 x K bit time ri truyn li; K c chn ngu nhin trong tp {0,1,2,,2m 1} vi m:=min (n,10).Hnh 3.5 biu din s c ch truy cp:

Hnh 3.5. S c ch truy cp

3.2.8. So snh kiu C s h tng v kiu Ad HocC hai phng php lm vic khc nhau cho thit b chun IEEE 802.11: Ad Hoc (tp hp cc dch v c bn c lp, IBSS) v C s h tng (tp hp cc dch v c m rng, ESS). Mt mng Ad Hoc thng thng l mt mng tn ti trong mt thi gian hu hn gia hai hoc nhiu hn hai thit b v tuyn m khng c ni thng qua mt im truy cp (AP) ti mt mng ni dy. V d, hai ngi dng laptop mun chia s cc file s thit lp mt mng Ad Hoc s dng cc card NIC thch hp chun IEEE 802.11 v chia s cc file qua mi trng WM m khng cn phng tin truyn thng ngoi no (nh a mm, cc card flash).Kiu C s h tng gi thit c mt mt hoc nhiu hn cc AP bc cu phng tin truyn thng khng dy vi phng tin ni dy truyn thng (hnh 3.6). AP iu khin vic chng thc

v lin kt trm ti mng khng dy. Nhiu AP c ni bi mt h phn phi (DS) m rng phm vi ca mng khng dy ra nhiu vng ln hn. Trong cc ci t tiu biu, DS n gin l c s h tng mng IP hin hu. Vi mc ch bo mt, ngi ta thng s dng cc mng LAN o (VLAN) tch ring lu thng mng khng dy vi lu thng mng khc trn DS. Mc d chun IEEE 802.11 cho php cc trm v tuyn lin kt chuyn mch ng t im truy cp ny n im truy cp khc, nhng n khng iu khin cch trm thc hin. Kt qu l, cc thi hnh ca nh cung cp khc nhau ni chung khng tng tc vi nhau trong ng cnh ny. Ti thi im hin nay, kh nng thc hin kiu hot ng ny yu cu mt gii php nh cung cp n.

Hnh 3.6. So snh kiu Ad Hoc v kiu c s h tng.

3.3. L thuyt v mng khng dy

3.3.1. Cch mt trm ni vi mt cell hin hu (BSS)

Khi mt trm mun truy cp mt BSS hin hu (hoc sau ch bt ngun, ch ngh, hoc ch l i vo vng BSS), trm cn c thng tin ng b t im truy cp (hoc t cc trm khc khi trong kiu Ad - hoc).

Trm nhn thng tin ny theo mt trong s hai cch sau:

1. Qut b ng: Trong trng hp ny trm i nhn mt khung n hiu (Beacon) t AP, (khung n hiu l mt khung tun hon cha thng tin ng b c gi bi AP).

2. Qut tch cc: Trong trng hp ny trm c gng tm mt im truy cp bng cch truyn cc khung yu cu d (Probe) v ch p li thng tin d t AP.

Hai phng php u hp l, v mi mt phng php c chn phi hi ho gia kh nng

tiu th in v kh nng thc hin.

3.3.1.1. Qu trnh chng thc

Mi khi trm tm thy mt im truy cp, n s quyt nh ni cc BSS, n thc hin thng qua qu trnh chng thc, l s trao i thng tin ln nhau gia AP v trm, m mi bn chng minh s nhn bit mt khu cho.

3.3.1.2. Qu trnh lin kt

Khi trm c xc nhn, sau n s khi ng qu trnh lin kt, y l s trao i thng tin v cc trm v cc BSS, v n cho php thc hin DSS (tp hp cc AP bit v tr hin thi ca trm). Ch sau khi qu trnh lin kt c hon thnh, th mt trm mi c kh nng pht v nhn cc khung d liu.

3.3.2. Roaming

Roaming l qu trnh chuyn ng t cell ny (hoc BSS) n cell khc vi mt kt ni cht. Chc nng ny tng t nh cc in thoi t bo, nhng c hai khc bit chnh:

Trong mt h thng mng LAN da trn cc gi, s chuyn tip gia cc cell c thc hin gia cc truyn dn gi, ngc vi k thut in thoi trong s chuyn tip xut hin trong thi gian mt cuc ni chuyn in thoi, iu ny lm roaming mng LAN d hn mt t. Trong mt h thng ting ni, mt gin on tm thi khng nh hng cuc ni chuyn, trong khi trong mt gi da vo mi trng, n s gim ng k kh nng thc hin v s chuyn tip c thc hin bi cc giao thc lp trn.Chun IEEE 802.11 khng nh ngha cch roaming c thc hin, nhng nh ngha cc cng c c bn cho n, iu ny bao gm s qut tch cc/b ng, v mt qu trnh ti lin kt, trong mt trm roaming t im truy cp ny sang im truy cp khc s c lin kt vi mt im truy cp mi.

3.3.3. Gi ng b

Cc trm cn gi ng b, gi cho nhy tn c ng b v cc chc nng khc nh tit kim nng lng. Trong mt c s h tng BSS iu ny c thc hin bi tt c cc trm cp nht cc ng h ca chng theo ng h ca AP, s dng c ch sau:AP truyn cc khung tun hon gi l cc khung bo hiu, cc khung ny cha gi tr ca ng h AP ti thi im truyn (Ch rng y l thi im khi truyn dn tht s xut hin v khng phi l thi im truyn khi n c t vo hng i truyn, v khung bo hiu c truyn s dng cc quy tc CSMA, nn truyn dn tr mt cch ng k).Cc trm thu kim tra gi tr ng h ca chng thi im nhn, sa cha n gi ng b vi ng h ca AP, iu ny ngn nga s tri ng h gy ra do mt ng b sau vi gi hot ng.

3.3.4. Tit kim nng lngMng LAN khng dy tiu biu lin quan n cc ng dng di ng, v trong cc kiu ng dng ny ngun pin l mt ngun nhanh ht, l l do ti sao chun IEEE 802.11 trc tip hng vo vn tit kim nng lng v nh ngha c c ch cho php cc trm i vo trong ch ngh ngi cho cc thi hn di m khng mt thng tin.

tng chnh ng sau c ch tit kim nng lng l AP duy tr mt bn ghi c cp nht ti cc trm hin thi ang lm vic trong ch tit kim nng lng v nh m cc gi c gi ti cc trm ny cho n khi c trm yu cu nhn cc gi bng cch gi mt yu cu kim tra tun t, hoc cho n khi chng thay i thao tc ca n.AP cng truyn nh k (mt phn ca cc khung bo hiu) thng tin v trm tit kim nng lng no c cc khung c nh m AP, nh vy cc trm ny cn phi c nh thc nhn mt trong s cc khung bo hiu v nu mt ch bo cho bit c mt khung c lu tr ti AP i phn pht, th trm cn phi trong trng thi hot ng v gi mt thng bo kim tra tun t cho AP c cc khung ny.Qung b v pht thanh c lu tr bi AP, v c truyn mt thi im c bit trc (mi DTIM), ti tt c trm tit kim nng lng mun nhn kim khung ny cn phi hot ng.

3.3.5. Cc kiu khung

C ba kiu khung chnh:

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chng. Khung d liu: cc khung c s dng truyn d liu

Khung iu khin: cc khung c s dng iu khin truy cp ti mi trng (v d RTS, CTS, v ACK). Khung qun l: cc khung c truyn ging nh cc khung d liu trao i thng tin qun l, nhng khng hng ti cho cc lp trn.Mi kiu c chia nh ra thnh cc kiu nh hn khc nhau, ty theo chc nng ca

3.3.6. Khun dng khung

Tt c cc khung chun IEEE 802.11 u c cc thnh phn sau y:

Hnh 3.7. Khun dng khung chun IEEE 802.11

3.3.6.1. Tin t (Preamble)

N ph thuc lp vt l, bao gm:

Synch: Mt chui 80 bit 0 v 1 xen k, c s dng bi bo mt lp vt l la chn anten thch hp (nu tnh s phn tp c s dng) v nh hng ti vic sa li dch tn s trng thi vng ng b vi vic nh thi gian gi nhn c.

SFD: Mt b nh ranh gii khung bt u, n gm 16 bit nh phn 0000 1100 1011

1101, c dng nh ngha nh thi khung.

3.3.6.2. u mc (Header) PLCP

u mc PLCP lun lun c truyn tc 1 Mbit/s v n cha thng tin Logic m s c s dng bi lp vt l gii m khung, gm c:

Chiu di t PLCP_PDU: biu din s byte cha trong gi, n c ch cho lp vt l

pht hin ra chnh xc kt thc gi.

Tng bo hiu PLCP: hin thi, n ch cha ng thng tin tc , c m ha tc

0.5 MBps, tng dn t 1Mbit/s ti 4.5 Mbit/s.

Trng kim tra li u mc: l trng pht hin sai st CRC 16 bit.

3.3.6.3. D liu MAC

Hnh 3.8 cho thy khun dng khung MAC chung, cc phn ca trng trn cc phn ca cc

khung nh m t sau .

Hnh 3.8. Khun dng khung MAC

Trng iu khin khung (Frame Control)

Trng iu khin khung cha ng thng tin sau:

Hnh 3.9. Trng iu khin khung (Frame Control)

Protocol Verson

Trng ny gm 2 bit c kch thc khng i v xp t theo cc phin bn sau ca chun IEEE 802.11, v s c s dng nhn bit cc phin bn tng lai c th. Trong phin bn hin thi ca chun gi tr c nh l 0.

ToDS

Bit ny l tp hp cc bit 1 khi khung c nh a ch ti AP hng n ti h phn phi (gm trng hp m trm ch t li khung ging vi BSS, v AP). Bit l tp hp cc bit 0 trong tt c cc khung khc.

FromDS

Bit ny l tp hp cc bit 1 khi khung ang n t h phn phi.

More Fragments

Bit ny l tp hp cc bit 1 khi c nhiu on hn thuc cng khung theo sau on hin thi ny.

Retry

Bit ny cho bit on ny l mt chuyn tip mt on trc c truyn, n s c s dng bi trm my thu on nhn bn sao c truyn ca cc khung m xut hin khi mt gi Chng thc b mt.

Power mangenment

Bit ny cho bit kiu qun l nng lng trong trm sau khi truyn khung ny. N c s dng bi cc trm ang thay i trng thi t ch tit kim nng lng n ch hot ng hoc ngc li.

More Data

Bit ny cng c s dng qun l nng lng v n c s dng bi AP cho bit rng c nhiu khung c nh m hn ti trm ny. Tm quyt nh s dng thng tin ny tip tc kim tra tun t hoc kiu ang thay i thm ch thay i sang ch hot ng.

WEP

Bit ny cho bit rng thn khung c m ha theo gii thut WEP

Order

Bit ny cho bit rng khung ny ang c gi s dng lp dch v Strictly - Order.

Khong thi gian/ID

Trng ny c hai ngha ph thuc vo kiu khung:

Trong cc bn tin Kim tra tun t tit kim nng lng, th n l ID trm.

Trong tt c cc khung khc, n l gi tr khong thi gian c dng cho Tnh ton NAV.

Cc trng a ch

Mt khung cha ln trn ti 4 a ch ph thuc vo cc bit ToDS v FromDS c nh ngha trong trng iu khin, nh sau: a ch - 1 : Lun l a ch nhn (v d, trm trn BSS m nhn gi tc thi), nu bit ToDS c lp th y l a ch AP, nu bit ToDS c xa th n l a ch trm kt thc. a ch - 2 : Lun lun l a ch my pht (v d, trm ang truyn gi vt l), nu

bit FromDS c lp th y l a ch AP, nu c xa th n l a ch trm.

a ch - 3 : Trong hu ht cc trng hp cn li, mt a ch, trn mt khung vi bit FromDS c lp, sau a ch - 3 l a ch ngun gc, nu khung c bit ToDS lp, sau a ch - 3 l a ch ch. a ch - 4 :c s dng trong trng hp c bit trong mt h phn phi khng dy c s dng, khung ang c truyn t im truy cp ny sang im truy cp khc, trong trng hp ny c cc bit ToDS ln cc bit FromDS c lp, v vy c a ch ch gc v a ch ngun gc u b mt.Bng 3.2 tng kt cc cch dng a ch khc nhau theo cch thit lp bit ToDS v bit FromDS:

Bng 3.2: Tng kt cc cch dng a ch

3.3.6.4. iu khin ni tip

Trng iu khin ni tip c dng biu din th t cc on khc nhau thuc khung, nhn bit cc gi sao, n gm c hai trng con: trng S on v trng S ni tip, m nh ngha khung v s on trong khung.

3.3.7. Cc khung nh dng ph bin nht

3.3.7.1. Khun dng khung RTS

Khung RTS nh sau:

Hnh 3.10: Khun dng khung RTS

RA ca khung RTS l a ch STA, trong mi trng khng dy, n c dnh nhn d liu tip theo hoc khung qun l mt cch tc thi.

TA l a ch ca STA pht khung RTS.

Gi tr Khong thi gian l thi gian, tnh theo micr - giy, c yu cu truyn d liu lin tip hoc khung qun l, cng vi mt khung CTS, cng mt khung ACK, cng ba khong SIFS.

3.3.7.2. Khun dng khung CTS

Khung CTS nh sau:

Hnh 3.11: nh dng khung CTS

a ch my thu (RA) ca khung CTS c copy t trng a ch my pht (TA) ca khung RTS ngay trc n mt p ng CTS no . Gi tr Khong thi gian l gi tr thu c t trng Khong thi gian ca khung RTS ngay trc , tr thi gian (tnh theo micr - giy) c yu cu pht khung CTS v khong SIFS.

3.3.7.3. Khun dng khung ACK

Khung ACK nh sau:

Hnh 3.12: nh dng khung ACK

a ch My thu ca khung ACK c sao chp t trng a ch-2 ca khung ngay trc . Nu nhiu bit on hn c xa (0) trong trng iu khin khung ca khung trc , th gi tr Khong thi gian l 0, nu khng th gi tr Khong thi gian thu c t trng Khong thi gian ca khung trc , tr i thi gian (tnh theo micr - giy) c pht khung ACK v khong SIFS ca n.

3.3.8. Hm Phi hp im (PCF)

Bn cnh Hm Phi hp Phn tn c bn, c mt Hm Phi hp im chn, m s dng thc hin cc dch v bin - thi gian, nh ting ni hoc truyn video. Hm Phi hp im lm cho im truy cp s dng quyn u tin cao hn bng cch s dng mt Khng gian khung Inter (PIFS) nh hn.Bng cch s dng cao hn ny quyn u tin truy cp, cc vn im truy cp kim tra tun t yu cu ca cc trm truyn d liu, do iu khin vic truy cp mi trng. cho php cho cc trm bnh thng kh nng vn cn truy cp mi trng, c mt chun b m im truy cp phi li thi gian cho Truy cp Phn tn trong gia PCF.

3.3.9. Cc mng Ad-hocTrong mt s trng hp cc ngi dng mun lp mt mng LAN khng dy m khng c mt c s h tng (c bit hn khng c mt im truy cp), iu ny bao gm truyn file gia hai ngi dng my notebook, cuc hp gia cc cng tc vin bn ngoi vn phng, vn vn.Chun IEEE 802.11 gii quyt cc nhu cu ny bng cch nh ngha mt m hnh hot ng Ad-hoc, trong trng hp ny khng c im truy cp no hoc phn no tnh nng ca n c thc hin bi cc trm ngi dng cui (nh to bo hiu, ng b, ), v cc chc nng khc khng c h tr (nh t li gia hai trm khng nm trong phm vi, hoc tit kim nng lng).

3.4. Cc tiu chun trong b tiu chun IEEE 802.11

3.4.1. IEEE 802.11Nm 1997, IEEE a ra chun WLAN u tin c gi l 802.11 theo tn ca nhm gim st s pht trin ca chun ny. Lc ny, 802.11 s dng tn s 2,4GHz v dng k thut tri ph

trc tip (Direct-Sequence Spread Spectrum-DSSS) nhng ch h tr bng thng ti a l 2Mbps tc kh chm cho hu ht cc ng dng. V l do , cc sn phm chun khng dy ny khng cn c sn xut na.

3.4.2. IEEE 802.11b

T thng 6 nm 1999, IEEE bt u m rng chun 802.11 ban u v to ra cc c t k thut cho 802.11b. Chun 802.11b h tr bng thng ln n 11Mbps, ngang vi tc Ethernet thi by gi. y l chun WLAN u tin c chp nhn trn th trng, s dng tn s 2,4 GHz. Chun 802.11b s dng k thut iu ch kha m b (Complementary Code Keying - CCK) v dng k thut tri ph trc tip ging nh chun 802.11 nguyn bn. Vi li th v tn s (bng tn nghip d ISM 2,4GHz), cc hng sn xut s dng tn s ny gim chi ph sn xut. Nhng khi y, tnh trng "ln xn" li xy ra, 802.11b c th b nhiu do l vi sng, in thoi m bng con v cc dng c khc cng s dng tn s 2,4GHz. Tuy nhin, bng cch lp t 802.11b khong cch hp l s d dng trnh c nhiu.u im ca 802.11b l gi thp, tm ph sng tt v khng d b che khut.

Nhc im ca 802.11b l tc thp; c th b nhiu bi cc thit b gia dng.

3.4.3. IEEE 802.11a

Song hnh vi 802.11b, IEEE tip tc a ra chun m rng th hai cng da vo 802.11 u tin - 802.11a. Chun 802.11a s dng tn s 5GHz, tc 54Mbps trnh c can nhiu t cc thit b dn dng. ng thi, chun 802.11a cng s dng k thut tri ph khc vi chun 802.11b - k thut tri ph theo phng php a phn chia tn s trc giao (Orthogonal Frequency Division Multiplexing-OFDM). y c coi l k thut tri hn so vi tri ph trc tip (DSSS). Do chi ph cao hn, 802.11a thng ch c s dng trong cc mng doanh nghip, ngc li, 802.11b thch hp hn cho nhu cu gia nh. Tuy nhin, do tn s cao hn tn s ca chun 802.11b nn tn hiu ca 802.11a gp nhiu kh khn hn khi xuyn tng v cc vt cn khc.Do 802.11a v 802.11b s dng tn s khc nhau, hai cng ngh ny khng tng thch vi nhau. Mt vi hng sn xut bt u cho ra i sn phm "lai" 802.11a/b, nhng cc sn phm ny ch n thun l cung cp 2 chun sng Wi-Fi cng lc (my trm dng chun no th kt ni theo chunf).

u im ca 802.11a l tc nhanh; trnh xuyn nhiu bi cc thit b khc.

Nhc im ca 802.11a l gi thnh cao; tm ph sng ngn hn v d b che khut.

3.4.4. IEEE 802.11d

Chun IEEE 802.11d l mt chun IEEE b sung lp s iu khin truy cp (MAC) vo chun IEEE 802.11 y mnh kh nng s dng rng mng WLAN chun IEEE 802.11. N s cho php cc im truy cp truyn thng thng tin trn cc knh v tuyn dng c vi cc mc cng sut chp nhn c cho cc thit b khch hng. Cc thit b s t ng iu chnh da vo cc yu cu a l.Mc ch 802.11d l s thm cc c tnh v cc hn ch cho php mng WLAN hot ng theo cc quy tc ca cc nc ny. Cc nh sn xut thit b khng mun to ra mt s a dng rng ln ca cc sn phm v cc ngi dng chuyn bit theo quc gia m ngi i du lch khng mun mt ti y cc card PC mng WLAN chuyn bit theo quc gia. Hu qu s l cc gii php b chuyn bit theo quc gia.

3.4.5. IEEE 802.1x (Tbd)

Chun IEEE 802.1x (Yu cu mt nh cung cp dch v RADIUS) cung cp cc doanh nghip

& cc nh ring mt gii php chng thc bo mt, bin i c s dng k thut ti kha (re - keying) ng, s chng thc tn v mt khu ngi dng v chng thc ln nhau. K thut ti kha ng, m trong sut vi ngi dng, loi tr phn phi kha khng bo mt v s chi phI thi gian v ngn nga cc tn cng lin quan n cc kha WEP tnh. S chng thc trn nn ngi dng loi tr cc l bo mt xut hin t thit b b trm hoc mt khi s chng thc trn nn thit b c s dng, v s chng thc ln nhau gim nh tn cng da vo cc im truy cp lu c. ng thi, v s chng thc chun IEEE 802.1x thng qua mt c s d liu RADIUS, n cng chia thang d dng iu khin cc s lng ngi dng mng WLAN ang gia tng.

3.4.6. IEEE 802.11i

y l tn ca nhm lm vic IEEE dnh cho chun ha bo mt mng WLAN. Bo mt chun IEEE 802.11i c mt khung lm vic c da vo RSN (C ch Bo mt tng cng). RSN gm c hai phn:1. C ch ring ca d liu

2. Qun l lin kt bo mt.

C ch ring ca d liu h tr hai s c xng: TKIP v AES. TKIP (S ton vn kha thi gian) l mt gii php ngn hn m nh ngha phn mm v cho WEP cung cp mt mc ring t d liu thch hp ti thiu. AES hoc AES - OCB (Advanced Encryption Standard and Offset Codebook) l mt s ring t d liu mnh m v l mt gii php thi hn lu hn.

Qun l lin kt bo mt c nh a ch bi:

a) Cc th tc m phn RSN

b) S Chng thc chun IEEE 802.1x

c) Qun l kha chun IEEE 802.1x.

Cc chun ang c nh ngha cng tn ti mt cch t nhin cc mng pre - RSN m hin thi c trin khai. Chun ny khng k vng s c thng qua cho n khi kt thc nm 2003.

3.4.7. IEEE 802.11g

Nm 2002 v 2003, cc sn phm WLAN h tr chun mi hn c gi l 802.11g ni ln trn th trng; chun ny c gng kt hp tt nht 802.11a v 802.11b. 802.11g h tr bng thng 54Mbps v s dng tn s 2,4GHz cho phm vi ph sng ln hn. 802.11g tng thch ngc vi 802.11b, ngha l cc AP 802.11g s lm vic vi card mng Wi-Fi chunp802.11b...Thng 7/2003, IEEE ph chun 802.11g. Chun ny cng s dng phng thc iu ch OFDM tng t 802.11a nhng li dng tn s 2,4GHz ging vi chun 802.11b. iu th v l chun ny vn t tc 54Mbps v c kh nng tng thch ngc vi chun 802.11b ang ph bin.u im ca 802.11g l tc nhanh, tm ph sng tt v khng d b che khut.

Nhc im ca 802.11g l gi cao hn 802.11b; c th b nhiu bi cc thit b gia dng.

3.4.8. IEEE 802.11h

Chun ny c dng chu u, di tn 5 Ghz. N cung cp tnh nng s la chn knh ng v iu khin cng sut truyn dn TPC, nhm trnh can nhiu. chu u ngi ta ch yu s

dng thng tin v tinh, nn phn ln cc quc gia y s dng ch s dng Wireless LAN

trong nh (Indoor). Chun ny ang giai on chun ha.

3.4.9. IEEE 802.11n

Do tnh tin dng v d trin khai, mng WLAN ngy cng thm nhp khp ni phc v nhu cu trao i thng tin v gii tr. Vi nhu cu ngy cng cao cp, tc 11 Mbps ca chun 802.11b, 54Mbps ca chun 802.11a/g d rt hp dn nhng cng cha tha yu cu ca ngi dng. p ng nhu cu ngy cng cao hn , IEEE hnh thnh nhm lm vic pht trin chun 802.11n. Phm vi lm vic ca nhm ny l nh ngha iu chnh lp vt l v lp MAC chuyn giao mt gi tr nh nht 100Mbps thng qua mt im truy nhp dch v MAC (SAP).802.11n s dng mt quan im pht trin ang s dng ti cc cng ngh hin hu trong khi gii thiu cng ngh mi chng cung cp hiu qu lm vic s pht trin thy c yu cu ca vic pht trin nhng ng dng. S dng li nhng cng ngh qu bu nh OFDM, m ha sa li t pha trc, an xen v nh x iu bin bin cu phng c sa v gia li, gi c gim xung v hon ton tng thch vi cc cng ngh trc .So vi cc chun trc, c t k thut ca 802.11n "thong" hn nhiu: c nhiu ch ty chn, nhiu cu hnh c th cho ra sn phm c cc mc tc ti a khc nhau. (Trc y, tt c cc sn phm 802.11b phi c tc 11Mbps; 802.11a v 802.11g phi c tc 54Mbps). iu ny vch ra ranh gii v hiu nng trn mi thit b 802.11n: cc nh sn xut c th tng hoc iu chnh kh nng h tr ng dng, mc gi... ng vi mi ty chn, 802.11n c th h tr tc ln n 600Mbps, nhng phn cng WLAN khng nht thit phi p dng tt c cc ty chn. V d, nm 2006, hu ht thit b phn cng WLAN 802.11n 1.0 h tr tc 300Mbps.Mt trong nhng iu mong i nht ca ngi dng thit b u cui Wi-Fi khng g khc ngoi tc v tm ph sng. Theo c t k thut, chun 802.11n c tc l thuyt ln n 600Mbps (cao hn 10 ln chun 802.11g) v vng ph sng rng khong 250m (cao hn chun 802.11g gn 2 ln, 140m). Hai c im then cht ny gip vic s dng cc ng dng trong mi trng mng Wi-Fi c ci tin ng k, phc v tt cho nhu cu gii tr a phng tin, nhiu ngi dng c th xem phim cht lng cao (HD, Full HD, Full HD 3D...), gi in thoi qua mng Internet (VoIP), ti tp tin dung lng ln ng thi... m cht lng dch v v tin cy vn lun t mc cao.

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Hnh 3.13:Logo chng nhn sn phm t chun 802.11n

Bn cnh , chun 802.11n vn m bo kh nng tng thch ngc vi cc sn phm trc , chng hn, nu sn phm Wi-Fi chun n s dng ng thi hai tn s 2,4GHz v 5GHz th s tng thch ngc vi cc sn phm chun 802.11a/b/g.Chun 802.11n c IEEE ph duyt a vo s dng chnh thc v cng c Hip hi Wi-Fi (Wi-Fi Alliance) kim nh v cp chng nhn cho cc sn phm t chun. Chng nhn chun Wi-Fi 802.11n l bc cp nht thm mt s tnh nng ty chn cho 802.11n d tho 2.0 (draft 2.0, xem thm bi vit ID: A0905_100) c Wi-Fi Alliance bt u t hi thng 6/2007; cc yu cu c bn (bng tng, tc , MIMO, cc nh dng khung, kh nng tng thch ngc) khng thay i. y l tin vui cho nhng ai ang s hu thit b t chng nhn 802.11n draft 2.0. Chng nhn Wi-Fi n vn m bo cho hn 700 sn phm c cp chng nhn draft 2.0 trc y (gm my tnh, thit b in t tiu dng nh tivi, my ch a phng tin (media server) v cc thit b mng). Tt c thit b c cp chng nhn dngdraft n c iu kin s dng logo "Wi-Fi CERTIFIED n" m khng cn phi kim tra li

(xem hnh 3.13).

Cc ci tin cng ngh ca chun 802.11n:

a) OFDM tt hnYu cu u tin l phi s dng phng php ghp knh phn chia tng s trc giao OFDM c pht trin trn da trn 802.11a/g, s dng tc m ln nht cao hn v nhp di thng rng hn. iu c ngha l 802.11n h tr mt OFDM tt hn. Nhng thay i gip tng tc ln 65 Mbps so vi 52Mbps ca chun 802.11a v 802.11g. Ci thin hiu nng ca MIMO:

Mt trong nhng thnh phn c bit rng ri nht trong c t k thut ca bn d tho l MIMO. MIMO tn dng hin tng t nhin ca sng trung tn c gi l a ng: thng tin c pht xuyn qua tng, ca s v cc vt chn khc, anten thu tn hiu nhiu ln qua cc b nh tuyn khc nhau cc thi im khc nhau. Do , tn hiu a ng nguyn gc c th b "bp mo" dn n kh gii m v ko theo hiu nng Wi-Fi km. MIMO khai thc hin tng a ng vi k thut a phn chia theo khng gian (space-division multiplexing). Thit b pht WLAN chia gi d liu ra thnh nhiu phn, mi phn c gi l chui d liu (Spatial Stream) v pht tng chui d liu qua cc anten ring r n cc anten thu.

Hnh 3.14: Cng ngh MIMO

Hin ti, 802.11n cung cp n 4 chui d liu, cho d phn cng khng yu cu h tr nhiu nh th. (xem hnh 3.15)

Mi mu tng ng vi mt chui d liu

Hnh 3.15 :Cc chui d liu ca 802.11n

Gp i s lng chui d liu ng ngha vi vic tng gp i tc , tuy nhin s ko theo cng sut tiu th tng, kh nng m rng km hn v gi thnh sn phm cao hn. Trong khi c t k thut 802.11n yu cu phi c ch tit kim nng lng (MIMO power-save). iu ny c ngha l ch nn s dng k thut a ng khi t c li ch v hiu nng.

Cc c im ni bt ca MIMO

C 2 tnh nng trong c t k thut 802.11n nhm tp trung ci thin hiu nng MIMO: cc to bp sng (Beam-forming) v s phn tp (Diversity). Beam-forming l k thut iu chnh tn hiu trc tip trn anten, gip tng vng ph sng v hiu sut bng cch hn ch nhiu. Diversity khai thc trn nhiu anten bng cch tng hp cc tn hiu u ra hoc chn tn hiu tt nht trong s cc anten. y l c t k thut quan trng do 802.11n c 4 anten, v th s gp phi trng hp thit b c s lng anten khc vi n. V v, my tnh xch tay dng 2 anten c th kt ni n access point (AP) c 3 anten. Trng hp ny, ch 2 chui d liu c dng d AP h tr n 3 chui d liu. Vi Diversity, thm cng nhiu anten cng tt. Thit b nhiu anten s c phm vi ph sng xa hn. V d, tn hiu pht ra ca 2 anten c th kt hp li thu mt chui d liu khong cch xa. tng ny c th c m rng kt hp cc tn hiu u ra ca 3 anten thu v 2 chui d liu c tc cao, vng ph sng rng... Diversity khng b gii hn trong 802.11n v c WLAN. Thc t, n c ci t cho cc sn phm chun 802.11a/b/g c 2 anten.

Cc c t k thut ch yu ca 802.11n (xem bng 3.3).

Bng 3.3: Cc c t k thut ch yu ca 802.11n

Tnh nng nghaTrng thi

OFDM tt hnH tr bng thng rng hn v tc m ha cao hn

tng tc t ti a 65MbpsBt buc

a phn chia

theo khng gianCi tin hiu sut bng cch phn chia d liu thnh nhiu chui pht n nhiu antenTy chn n ti a 4 chui d liu

DiversityKhai thc s c mt ca nhiu anten ci tin tm ph sng v tin cy. Hnh thc ny c thc thi khi sTy chn n ti a 4 anten

lng anten u thu cao hn s lng anten u pht.

MIMO tit kim

nng lngGii hn cng sut tiu th bt li ca MIMO bng cch ch s dng nhiu anten khi cn thitQuy nh

Cc knh 40MHzTng tc gp i bng cch tng rng bng thng t

20MHz ln 40MHzTy chn

S kt hpCi tin hiu sut bng cch cho php nhiu gi d liu

pht tng tc gia s truyn thng trn caoQuy nh

Gim Inter-frame Spacing (RIFS)Mt trong nhng c im chung trong thit k 802.11n-n l ci tin hiu sut. Thi gian tr trong vic pht OFDM ngn hn so vi 802.11a, 802.11g

Quy nh

Ch

GreenfieldCi tin hiu sut bng cch loi cc thit b 802.11a/b/g ra khi mng 802.11n-nTy chn hin thi

b) Ci tin lu lng v tc truyn d liu

Mt ty chn khc trong 802.11n d tho l nhn i tc bng cch tng bng thng knh truyn WLAN t 20MHz ln 40MHz. iu ny lm gim s lng knh gy bt li cho cc thit b khc. Tn s 2,4GHz c khng gian cho 3 knh 20MHz khng chng ln nhau (non- overlapping), cn knh 40MHz khng c nhiu khng gian cho cc thit b khc tham gia vo mng hay truyn d liu trn cng khu vc vi chng. Do , vic chn knh 40MHz s ci thin hiu nng cho ton WLAN.

Cc c im k thut ca IEEE 802.11 (xem bng 3.4).

Bng 3.4: Cc c im k thut ca IEEE 802.11n

Cc chun802.11a802.11b802.11g802.11n

Nm ph

chunThng 7/1999Thng 7/1999Thng

6/2003Thng 6/2007

Tc ti

a54Mbps11Mbps54Mbps300Mbps

Khong cch ti a100m100m100m150m

K thut

iu ch

OFDM

DSSS hay CCKDSSS hay CCK hay OFDMDSSS hay CCK hay OFDM

Di tn s

trung tn (RF)

5GHz

2,4GHZ

2,4GHZ

2,4GHz hay 5GHz

Chui d liu1111, 2, 3 hay 4

rng

bng thng

20MHz

20MHz

20MHz

20MHz hay 40MHz

S knh

khng chng ln nhau

3

3

23

3 (2,4GHz)

23 (5GHz)

Ngun can nhiuBluetooth, l vi sng, thit b quan st b t xa...Bluetooth, l vi sng, thit b quan st b t xa...in thoi m bng conTng t 802.11b/g (2,4GHz)Tng t 802.11a (5GHz)

c) Hot ng hiu qu cng WLAN hin hnh

c tnh k thut ca 802.11n d tho c kh nng tng thch vi chun trc . Access Point 802.11n s tng thch ngc vi cc thit b chun 802.11a tn s 5GHz cng nh chun 802.11b v 802.11g tn s 2,4GHz. 802.11n s h tr tt hn vi ch "mixed" ca 802.11g. Thc t, hiu sut mng ph thuc vo t l bng thng c s dng pht d liu ch khng phi gi tin u tin (Overhead) hay cc giao thc c s dng qun l vic truyn thng mng. Mt trong cc tnh nng quan trng nht trong c t k thut 802.11n ci tin hiu nng ch hp nht (mixed mode) l s tp hp. Thay v gi tng frame d liu, my trm pht mt gi gm nhiu frame chung vi nhau. Do , vic tp hp d liu s hiu qu hn (xem hnh 3.16).

Hnh 3.16: S tp hp ci thin hiu qu trong ch Mixed

iu ny d dng cho cc thit b 802.11n cng tn ti vi 802.11g v 802.11a v tt c u dng phng thc iu ch OFDM. V th, c nhiu tnh nng trong c t k thut ca thit b nh tng hiu nng cho cc mng ch s dng OFDM. Chng hn nh tnh nng gim khong khng gia frame (Reduced Inter-Frame Spacing, hay RIFS), chi tit ny c thi gian tr hon gia 2 ln pht ngn.

t hiu nng tt, c t k thut ca 802.11n cung cp mt ch gi l "cnh ng xanh"

(Greenfield) - ch cc thit b 802.11n hot ng trong mng.

d) Hin thc phn cng ca chun IEEE 802.11n

Cc hng sn xut chip Wi-Fi ln nh Atheros, Broadcom xut xng cc chip h tr chun 802.11n v t c chng nhn ca Wi-Fi Alliance, chng hn chip BCM943224HMS, BCM94313HMGB ca Broadcom, chip AR5B195 ca Atheros. Cc chip ny h tr y 4 chun 802.11a/b/g/n vi rt nhiu tnh nng ty chn (Short Guard Interval, Greenfield Preamble, A-MPDU, STBC, 40MHz trn tn s 2,4GHz hay 5GHz...), ch bo mt WPA2 cao cp, tnh nng WMM (Wi-Fi Multimedia) h tr gii tr a phng tin v cc tnh nng tin ch khc nh ci t m ha Wi-Fi nhanh theo dng PIN (Personal Identification Number) hay PBC (Push button configuration)...

u, nhc im ca 802.11n:

u im ca 802.11n l tc nhanh nht, vng ph sng tt nht; tr khng ln hn chng

nhiu t cc tc ng ca mi trng.

Nhc im ca 802.11n gi cao hn 802.11g; s dng nhiu lung tn hiu c th gy nhiu

vi cc thit b 802.11b/g k cn.

Tuy nhin, vi u im nhiu hn khuyt im nh vy, tng lai chun IEEE 802.11n s ph

bin trn ton cu v thay th cc chun IEEE 802.11 khc.

3.5. Bo mt trong mng WLAN

3.5.1. C s bo mt mng WLAN

Chun IEEE 802.11 c vi c tnh bo mt, nh h thng m v cc kiu chng thc kha dng chung, nh danh t dch v (SSID) v gii thut WEP. Mi c tnh cung cp cc mc bo mt khc nhau v chng c gii thiu trong phn ny. Phn ny cng cung cp thng tin v cch dng anten RF hn ch lan lan truyn trong mi trng WM.

3.5.1.1. Lin kt v Chng thc

Chun IEEE 802.11 nh ngha mt trm cui l nh x AP cc trm khc trn mng ni dy v mng khng dy c phng tin giao tip vi trm cui. nh x ny c gi "lin kt". Trong khi cc trm cui c php lin kt ng n cc AP khc th ti bt k im cho trc mt trm cui ch c lin kt n mt AP. Mt trm cui "c lin kt" vi mt AP kh ging vi mt trm cui Ethernet c t vo trong cu ni (bridge) ca mt switch. Khng c c ch ny, AP khng c cch xc nh thc y cc khung nhn c trn cng Ethernet ti cng khng dy hay khng.Lin kt l mt qu trnh ba trng thi:

1. Khng c lin kt v khng c xc thc

2. Khng c lin kt nhng c xc thc

3. c lin kt v c xc thc.

Cc bn tin i qua trong thi gian thc hin cc bc ny c gi l cc khung qun l. iu quan trng trong qu trnh ny l lin kt s khng xy ra cho n khi chng thc xy ra. S chng thc theo chun IEEE 802.11 c ni k trong phn 3.5.1.5.

3.5.1.2. Cc mc bo v an ton mng

V khng c mt gii php an ton tuyt i nn ngi ta thng phi s dng nhiu mc bo v khc nhau to thnh nhiu lp "ro chn" i vi hot ng xm phm. Vic bo v thng tin trn mng ch yu l bo v thng tin ct gi trong cc my tnh, c bit l trong cc server ca mng. Hnh 3.17 m t cc lp ro chn thng dng hin nay bo v thng tin ti cc trm ca mng.

Access rightslogin/passworddata encrytionPhysical protectionfirewallsInformation

Hnh 3.17: Cc mc bo v mng

Nh minh ha trong hnh 3.17, cc lp bo v thng tin trn mng gm

Lp bo v trong cng l quyn truy nhp nhm kim sot cc ti nguyn ( y l thng tin) ca mng v quyn hn (c th thc hin nhng thao tc g) trn ti nguyn . Hin nay vic kim sot mc ny c p dng su nht i vi tp.

Lp bo v tip theo l hn ch theo ti khon truy nhp gm ng k tn/mt khu tng ng. y l phng php bo v ph bin nht v n n gin, t tn km v cng rt c hiu qu. Mi ngi s dng mun truy nhp c vo mng s dng cc ti nguyn u phi ng k tn v mt khu. Ngi qun tr h thng c trch nhim qun l, kim sot mi hot ng ca mng v xc nh quyn truy nhp ca nhng ngi s dng khc ty theo thi gian v khng gian.

-70-

Lp th ba l s dng cc phng php m ha (encrytion). D liu c bin i t dng c c sang dng khng c c theo mt thut ton no . Chng ta s xem xt cc phng thc v cc thut ton m ha c s dng ph bin phn di y.

Lp th t l bo v vt l (physical protection) nhm ngn cn cc truy nhp bt hp php vo h thng. Thng dng cc bin php truyn thng nh ngn cm ngi khng c nhim v vo phng t my, dng h thng kha trn my tnh, ci t cc h thng bo ng khi c truy nhp vo h thng..

Lp th nm: Ci t cc h thng tng la (firewall), nhm ngn chn c thm nhp tri php v cho php lc cc gi tin m ta khng mun gi i hoc nhn vo v mt l do no .

3.5.1.3. Gii hn lan truyn RFTrc khi thc hin cc bin php bo mt, ta cn xt cc vn lin quan vi lan truyn RF do cc AP trong mt mng khng dy. Khi chn tt, vic kt hp my pht v anten thch hp l mt cng c bo mt c hiu qu gii hn truy cp ti mng khng dy trong vng ph sng nh trc. Khi chn km, s m rng mng ra ngoi vng ph sng nh trc thnh nhiu vng ph sng hoc hn na.Cc anten c hai c tnh ch yu: tnh nh hng v khuch i. Cc anten a hng c vng ph sng 360 , trong khi cc anten nh hng ch ph sng trong vng hn ch (hnh 3.18). khuch i anten c o bng dBi v c nh ngha l s tng cng sut m mt anten thm vo tnh hiu RF.

Hnh 3.18. Cc mu lan truyn RF ca cc anten ph bin.

3.5.1.4. nh danh thit lp Dch v (SSID)

Chun IEEE 802.11b nh ngha mt c ch khc gii hn truy cp: SSID. SSID l tn mng m xc nh vng c ph sng bi mt hoc nhiu AP. Trong kiu s dng ph bin, AP lan truyn nh k SSID ca n qua mt n hiu (beacon). Mt trm v tuyn mun lin kt n AP phi nghe cc lan truyn v chn mt AP lin kt vi SSID ca n.Trong kiu hot ng khc, SSID c s dng nh mt bin php bo mt bng cch nh cu hnh AP khng lan truyn SSID ca n. Trong kiu ny, trm v tuyn mun lin kt n AP phi sn c SSID nh cu hnh ging vi SSID ca AP. Nu cc SSID khc nhau, cc khung qun l t trm v tuyn gi n AP s b loi b v chng cha SSID sai v lin kt s khng xy ra.V cc khung qun l trn cc mng WLAN chun IEEE 802.11 lun lun c gi n r rng, nn kiu hot ng ny khng cung cp mc bo mt thch hp. Mt k tn cng d dng nghe cc khung qun l trn mi trng WM v khm ph SSID ca AP.

3.5.1.5. Cc kiu Chng thc

Trc khi mt trm cui lin kt vi mt AP v truy cp ti mng WLAN, n phi thc hin chng thc. Hai kiu chng thc khch hng c nh ngha trong chun IEEE 802.11: h thng m v kha chia s.

a. Chng thc h thng m

Chng thc h thng m l mt hnh thc rt c bn ca chng thc, n gm mt yu cu chng thc n gin cha ID trm v mt p li chng thc gm thnh cng hoc tht bi. Khi thnh cng, c hai trm c xem nh c xc nhn vi nhau.

Hnh 3.19. Chng thc h thng m.

b. Chng thc kha chia s

Chng thc kha chia s c xc nhn trn c s c hai trm tham gia trong qu trnh chng thc c cng kha chia s. Ta gi thit rng kha ny c truyn ti c hai trm sut knh bo mt no trong mi trng WM. Trong cc thi hnh tiu biu, chng thc ny c thit lp th cng trn trm khch hng v AP. Cc khung th nht v th t ca chng thc kha chia s tng t nh cc khung c trong chng thc h thng m. Cn cc khung th hai v khung th ba khc nhau, trm xc nhn nhn mt gi vn bn yu cu (c to ra khi s dng b to s gi ngu nhin gii thut WEP (PRNG)) t AP, mt m ha n s dng kha chia s, v gi n tr li cho AP. Sau khi gii m, nu vn bn yu cu ph hp, th chng thc mt chiu thnh cng. chng thc hai pha, qu trnh trn c lp li pha i din. C s ny lm cho hu ht cc tn cng vo mng WLAN chun IEEE 802.11b ch cn da vo vic bt dng mt m ha ca mt p ng bit trc, nn dng chng thc ny l mt la chn km hiu qu. N cho php cc hacker ly thng tin nh mt m ha WEP v cng l l do ti sao chng thc kha chias khng bao gi khuyn ngh.

S dng chng thc m l mt phng php bo v d liu tt hn, v n cho php chng thc m khng c kha WEP ng. Bo mt gii hn vn c duy tr v trm s khng th pht hoc nhn d liu chnh xc vi mt kha WEP sai.

Hnh 3.20. Chng thc kha chia s

3.5.2. WEP

WEP c thit k bo v ngi dng mng WLAN khi b nghe trm tnh c v n c cc thuc tnh sau:

Mt m ha mnh, ng tin cy. Vic khi phc kha b mt rt kh khn. Khi di kha cng di th cng kh khi phc.

T ng b ha. Khng cn gii quyt mt cc gi. Mi gi cha ng thng tin cn gii m n.

Hiu qu. N c thc hin ng tin cy trong phn mm.

Gii thut WEP thc cht l gii thut gii m ha RC4 ca Hip hi Bo mt D liu RSA. N c xem nh l mt gii thut i xng v s dng cng kha cho mt m ha v gii mt m UDP (Protocol Data Unit) vn bn gc. Mi khi truyn, vn bn gc XOR theo bit vi mt lung kha (keystream) gi ngu nhin to ra mt vn bn c mt m. Qu trnh gii mt m ngc li.

Gii thut hot ng nh sau:

Ta gi thit rng kha b mt c phn phi ti c trm pht ln trm thu theo ngha bo mt no .

Ti trm pht, kha b mt 40 bit c mc ni vi mt Vect Khi to (IV) 24

bit to ra mt seed (ht ging) cho u vo b PRNG WEP.

Seed c qua b PRNG to ra mt lung kha (keystream) l cc octet gi ngu nhin.

Sau PDU vn bn gc c XOR vi keystream gi ngu nhin to ra PDU

vn bn mt m ha.

PDU vn bn mt m ha ny sau c mc ni vi IV v c truyn trn

mi trng WM.

Trm thu c IV v mc ni n vi kha b mt, to ra seed m n chuyn cho b PRNG.

B PRNG ca my thu cn phi to ra keystream ng nht c s dng bi trm pht, nh vy khi no c XOR vi vn bn mt m ha, PDU vn bn gc c to ra.

PDU vn bn gc c bo v bng mt m CRC ngn nga can thip ngu nhin vo vn bn mt m ang vn chuyn. Khng may l khng c bt k cc quy tc no i vi cch s dng ca IV, ngoi tr ni rng IV c thay i "thng xuyn nh mi MPDU". Tuy nhin, ch tiu k thut khuyn khch cc thc thi xem xt cc nguy him do qun l IV khng hiu qu.

3.5.3. WPA (Wi-Fi Protected Access)

Nhn thy c nhng kh khn khi nng cp ln 802.11i, Wi-Fi Alliance a ra gii php khc gi l Wi-Fi Protected Access (WPA). Mt trong nhng ci tin quan trng nht ca WPA l s dng hm thay i kho TKIP (Temporal Key Integrity Protocol). WPA cng s dng thut ton RC4 nh WEP nhng m ho y 128 bit. V mt c im khc l WPA thay i kho cho mi gi tin. Cc cng c thu thp cc gi tin ph kho m ho u khng th thc hin c vi WPA. Bi WPA thay i kho lin tc nn hacker khng bao gi thu thp d liumu tm ra mt khu. Khng nhng th, WPA cn bao gm kim tra tnh ton vn ca thng tin

(Message Integrity Check). V vy, d liu khng th b thay i trong khi ang trn ng truyn. Mt trong nhng im hp dn nht ca WPA l khng yu cu nng cp phn cng. Cc nng cp min ph v phn mm cho hu ht cc card mng v im truy cp s dng WPA rt d dng v c sn. Tuy nhin, WPA cng khng h tr cc thit b cm tay v my qut m vch.WPA c sn 2 la chn: WPA Personal v WPA Enterprise. C 2 la chn ny u s dng giao thc TKIP v s khc bit ch l kho khi to m ho lc u. WPA Personal thch hp cho gia nh v mng vn phng nh, kho khi to s c s dng ti cc im truy cp v thit b my trm. Trong khi , WPA Enterprise cn mt my ch xc thc v 802.1x cung cp cc kho khi to cho mi phin lm vic. Trong khi Wi-Fi Alliance a ra WPA c coi l loi tr mi l hng d b tn cng ca WEP nhng ng