Mang Khong Day Wlan 852

7/31/2019 Mang Khong Day Wlan 852

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TRNG .KHOA.

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BO CO TT NGHIP

TI:

MNG KHNG DY WLAN


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Kho lun tt nghip TrngH Cng Ngh_ HQG H Ni

Nguyn Th Huyn_ K49- HTVT1

LI MU

Trong thi gian gn y chng ta thng nghe ni v WiFi v Internet khngdy. Thc ra, WiFi khng ch c dng kt ni Internet khng dy m cn dng kt ni hu ht cc thit b tin hc v vin thng quen thuc nh my tnh, my in,

PDA, in thai di ng m khng cn dy cp ni, rt thun tin cho ngi s dng.

Mng khng dy l mt trong nhng bc tin ln ca ngnh my tnh. Truy cp

Internet tr thnh nhu cu quen thuc i vi mi ngi.

Tuy nhin, c th kt ni Internetngi s dng phi truy nhp Internet tmt v tr c nh thng qua mt my tnh kt ni vo mng. iu ny i khi gy ra

rt nhiu kh khn cho nhng ngi s dng khi ang di chuyn hoc n mt nikhng c iu kin kt ni vo mng.

Xut pht t yu cu mrng Internet, WLAN c nghin cu v trin khai

ng dng trong thc t. Vi nhng tnh nng h trpngc bng thng, trinkhai lpt d dng v pngc cc yu cu k thut, kinh t. Chng hn vic s

dng cng ngh Internet khng dy Wi- Fi cho php mi ngi truy cp v ly thngtin bt k v tr no nh bn xe, nh ga, sn bay,

Khi nghin cu trin khai ng dng cng ngh WLAN, ngi ta c bit quan

tm ti tnh bo mt an ton thng tin ca n. Do mi trng truyn dn v tuyn nnWLAN rt d b r r thng tin do tc ng ca mi trng bn ngoi, c bit l stn cng ca cc Hacker.

Do , i i vi pht trin WLAN phi pht trin cc kh nng bo mt WLANan ton, cung cp thng tin hiu qu, tin cy cho ngi s dng.

T nhng yu cu, Lun vn ny s trnh by ti v mng cc b khngdy WLAN v mt s vn bo mt cho mng khng dy Wi- Fi vi ni dung gm 3chng:

Chng 1: Tng quan mng cc b khng dy WLAN

Chng 2: Cc tiu chun ca mng WLAN

Chng 3: Mt s vn bo mt cho mng khng dy Wi- Fi

Trong qu trnh thc hin ti, do hn ch v thi gian v lng kin thc cngnh kinh nghim thc t nn lun vn khng trnh khi nhng thiu st. Knh mongsng gp kin ca thy c v cc bn lun vn c hon thin hn.

Em xin chn thnh cm n!


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CHNG 1

TNG QUAN MNG CC B KHNG DY WLAN

1.1 Gii thiuVi s pht trin nhanh chng ca khoa hc, cng ngh thng tin v vin thng,

ngy nay cc thit b di ng cng ngh cao nh my tnh xch tay laptop, my tnh bti palm top, in thoi di ng, my nhn tin khng cn xa l v ngy cng c s

dng rng ri trong nhng nm gn y. Nhu cu truyn thng mt cch d dng v tpht gia cc thit b ny dn n s pht trin ca mt lp mng di ng khng dy

mi, l mng WLAN. WLAN cho php duy tr cc kt ni mng khng dy, ngis dng duy tr cc kt ni mng trong phm vi ph sng ca cc im kt ni trung

tm. Phng thc kt ni mi ny thc s m ra cho ngi s dng mt s la

chn ti u, b xung cho cc phng thc kt ni dng dy.WLAN l m hnh mng c s dng cho mt khu vc c phm vi nh nh

mt ta nh, khun vin ca mt cng ty, trng hc. N l loi mng linh hot c khnng c ng cao thay th cho mng cp ng truyn thng v bt u pht trin vogia thp k 80 ca th k XX bi t chc FCC (Federal CommunicationsCommission). WLAN s dng sng v tuyn hay hng ngoi truyn v nhn d

liu thng qua khng gian, xuyn qua tng trn v cc cu trc khc m khng cncp. WLAN cung cp tt c cc chc nng v cc u im ca mt mng LAN truynthng nh Ethernet hay Token Ring nhng li khng b gii hn bi cp. Ngoi ra

WLAN cn c kh nng kt hp vi cc mng c sn, WLAN kt hp rt tt vi LANto thnh mt mng nng ng v n nh hn. WLAN l mng rt ph hp cho vicpht trin iu khin thit b t xa, cung cp mng dch v ni cng cng, khch sn,vn phng. S pht trin ngy cng tng nhanh ca cc my tnh xch tay nh gnhn, hin i hn v r hn thc y s tng trng rt ln trong cng nghip

WLAN nhng nm gn y.

WLAN s dng bng tn ISM (bng tn phc v cng nghip, khoa hc, y t :2.4GHz v 5GHz ), v th n khng chu s qun l ca chnh ph cng nh khng

cn cp giy php s dng. S dng WLAN s gip cc nc ang pht trin nhanhchng tip cn vi cc cng ngh hin i, nhanh chng xy dng h tng vin thngmt cch thun li v t tn km.

Trn th trng hin nay c rt nhiu sn phm phc v cho WLAN theo ccchun khc nhau nh: IrDA (Hng ngoi), OpenAir, BlueTooth, HiperLAN 2, IEEE


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802.11b, IEEE 802.11a, 802.11g (Wi-Fi), trong mi chun c mt c im khc

nhau. IrDA, OpenAir, BlueTooth l cc mng lin kt trong phm vi tng i nh:IrDA (1m), OpenAir(10m), Bluetooth (10m) v m hnh mng l dng peer-to-peer

tc l kt ni trc tip khng thng qua bt k mt thit b trung gian no. Ngc li,HiperLAN v IEEE 802.11 l hai mng phc v cho kt ni phm vi rng hn khong

100m, v cho php kt ni 2 dng: kt ni trc tip, kt ni dng mng c s (s dngAccess Point) . Vi kh nng tch hp vi cc mng thng dng nh (LAN, WAN),

HiperLAN v Wi-Fi c xem l hai mng c th thay th hoc dng m rngmng LAN.

ng dng ln nht ca WLAN l vic p dng WLAN nh mt gii php ti ucho vic s dng Internet. Mng WLAN c coi nh mt th h mng truyn s liu

mi cho tc cao c hnh thnh t hot ng tng h ca c mng hu tuynhin c v mng v tuyn. Mc tiu ca vic trin khai mng WLAN cho vic s dng

internet l cung cp cc dch v s liu v tuyn tc cao .

1.2 Qu trnh pht trin ca mng WLAN

Mng WLAN, vi c tnh khng dy n rt linhng trong iu kin ngidng di ng hay trong cc cu hnh tm thi. Cc mng LAN khng dy ang ngycng ca chung v pht trin trn th gii. Vi cc u im ni tri nh: d dngci thin nng sut, ci t nhanh, n gin v linh hot, d cu hnh khng i hi c

s h tng cng knh nh cc mng LAN truyn thng, c bit l hiu qu trong cc

vng kh thc hin bng dy v i hi c thm m cao, WLAN pht trin rtnhanh chng v ang dn thay th cho cc mng c dy trong nhiu lnh vc khc

nhau.

Qu trnh pht trin ca cc mng WLAN c s lc qua:

Cng ngh WLAN ln u tin xut hin vo cui nm 1990, khi nhng nh snxut gii thiu nhng sn phm hot ng trong bng tn 900Mhz. Nhng gii php

ny (khng c thng nht gia cc nh sn xut) cung cp tc truyn d liu1Mbps, thp hn nhiu so vi tc 10Mbps ca hu ht cc mng s dng cp hinthi.

Nm 1992, nhng nh sn xut bt u bn nhng sn phm WLAN s dngbng tn 2.4Ghz. Mc du nhng sn phm ny c tc truyn d liu cao hn

nhng chng vn l nhng gii php ring ca mi nh sn xut khng c cng brng ri. S cn thit cho vic hot ng thng nht gia cc thit b nhng dy tn


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s khc nhau dn n mt s t chc bt u pht trin ra nhng chun mng khng

dy chung.

Nm 1997, Institute of Electrical and Electronics Engineers(IEEE) phchun s ra i ca chun 802.11, v cng c bit vi tn gi WIFI (Wireless

Fidelity) cho cc mng WLAN. Chun 802.11 h tr ba phng php truyn tn hiu,trong c bao gm phng php truyn tn hiu v tuyn tn s 2.4Ghz.

Nm 1999, IEEE thng qua hai s b sung cho chun 802.11 l cc chun

802.11a v 802.11b (nh ngha ra nhng phng php truyn tn hiu). V nhngthit b WLAN da trn chun 802.11b nhanh chng tr thnh cng ngh khngdy vt tri. Cc thit b WLAN 802.11b truyn pht tn s 2.4Ghz, cung cp tc

truyn d liu c th ln ti 11Mbps. IEEE 802.11b c to ra nhm cung cpnhng c im v tnh hiu dng, thng lng (throughput) v bo mt so snhvi mng c dy thng thng.

Nm 2003, IEEE cng b thm mt s ci tin l chun 802.11g m c thtruyn nhn thng tin c hai dy tn 2.4Ghz v 5Ghz v c th nng tc truyn d

liu ln n 54Mbps. Thm vo , nhng sn phm p dng 802.11g cng c thtng thich ngc vi cc thit b chun 802.11b.

Hnh 1.1: Qu trnh pht trin ca mng WLAN

1.3 Phn loi mng WLAN

Cc mng WLAN c thc phn loi thnh mng WLAN v tuyn v WLANhng ngoi. Cc mng WLAN v tuyn c th da trn qu trnh truyn dn bng hp

hay truyn dn tri phtrong khi i vi cc WLAN hng ngoi c th l khuychtn hay c nh hng. Di y cp c bn cc mng WLAN v tuyn v hngngoi, c nh gi im mnh cng nh im yu ca mi loi.

1.3.1 Cc WLAN v tuyn

2.4 GHz

1 & 2 Mb s860 Kb s

900 MHz

Pro rietar

11 Mb s

Theo tiu chu n

IEEE802.11

- c h chun

2.4 GHzRadio

Network

Speed 1 & 2 Mb s860 Kb s

900 MHz

c u n

1986 1988 1990 1992 1994 1996 1998 2000 2002


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a s cc h thng mng WLAN s dng cng ngh tri ph. Khi nim v tri

phm bo qu trnh truyn thng tin cy v an ton. Tri ph cp n cc s tn hiu da trn mt s dng m ho (c lp vi thng tin c pht i) v chng s

dng bng thng ln hn nhiu so vi yu cu truyn tn hiu. Bng thng ln hnc ngha l nhiu v cc hiu ng fading a ng chnh hng mt phn n qu

trnh truyn dn tri ph. V vy m nng lng tn hiu thu hu nh khng i theothi gian. iu ny cho php tch sng ddng khi my thu c ng b vi cc

tham s ca tn hiu tri ph. Cc tn hiu tri ph c khnng hn ch nhiu v gykh khn cho qu trnh pht hin v chn tn hiu trn ng truyn. C hai k thuttri ph: Tri ph chui trc tip (DSSS) v tri ph nhy tn (FHSS).

1.3.1.1 Tri phchui trc tip (DSSS)

y l cng ngh tri ph tn s rng s dng phng php to ra mt mu bt

tha cho mi bt s truyni, bt ny c gi l chp hoc m chp. M chp cng dikh nng khi phc tn hiu gc cng cao. Kh khn trong phng php ny l tn

nhiu bng thng.

T l chp s dng trn mt bt gi l t l tri ph. T l ny cng cao s gipcho kh nng chng nhiu khi truyn tin hiu, trong khi t l ny thp s gip tng

bng thng cho cc thit b di dng. Thut ton c bitc s dng khi phc lithng tin m khng yu cu gi li gi tin

C th hiun gin hn l mi bt c m ho thnh mt chui cc bit

V d: 1 c m ho thnh 10011100011

v 0 sc m ho l: 01100011100

th khi vic truyn chui 101 i s thnh gii chui:

100111000110110001110010011100011

Cc m chp thng thng nghcho ln nhau, iu ny lm cho DSSS i ph

tti vi nhiu.

Bi v DSSS tri rng trn ton ph, nn s lng cc knh b chng ln nhau

trong di tn 2.4 Ghz l rt it (thng thng l ba knh), v vy s lng cc mngcng hotngc lp trong mt phm vi m khng b nhiu l rt hn ch.

1.3.1.2 Tri ph nhy tn (FHSS)


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Cng ngh tri ph ny s dng bng tn hp truyn thng tin. Vi FHSS,

mt chui gi ngu nhin c s dng thay i t ngt nhng tn s v cho phpmt trm nhy t tn s ny sang tn s khc. Tuy nhin mi thit b WLAN vn hnh

theo cch ny s thay i tn s s dng cng mt thut ton, thut ton FHSS s phttn hiu trn mt tn s trong mt thi gian ngn, ri t ng nhy sang tn s khc

truyn tn hiu.

Cc thit b truyn v nhn tn hiu FHSS s phi c ng b ho sao chochng c cng tn s ti cng mt thi im, tn hiu c m bo trong sut qutrnh kt ni.

Theo FHSS, n c kh nng hn ch ti a nhiu trn bng tn hp t bn ngoi.

Bi v nu FHSS b nhiu ti mt knh no th n s chuyn sang knh tn khc gi tn hiu.

Theo quy nh ca FCC s lng knh ti thiu c s dng trong FHSS l 75

knh, sau ny gim xung cn 15 v tr ti a l 400ms trn mi knh.

Phng php FHSS cho php xy dng nhiu knh m khng chng ln lnnhau, n cng cho php s dng nhiu im truy cp trong mt vng lm vic nunh cn tng thm lng bng thng hoc cn tng thm s ngi truy nhp ti a.

Cui cng l s khuych i cng sut l rt hiu qu, cc thit b FHSS s tiuth t nng lng hn,v nh vy cc thit b nh cc thit b di dng s c th kt ni

vi thi gian lu hn m khng phi thay sc pin.

1.3.2 Cc mng WLAN hng ngoi

Mng WLAN u tin c pht trin s dng truyn dn hng ngoi cch ykhong chng 20 nm. Cc h thng ny khai thc cc im thun li do s dng vtuyn hng ngoi nh l mt mi trng cho truyn dn v tuyn. Chng hn, tia hngngoi c bng thng khng cp php rt di do, n loi b c nhiu v tuyn, cc

thit b hng ngoi nh v tiu th t cng sut.

Khng ging nh cc sng v tuyn, cc tn s hng ngoi l qu cao thchin iu ch ging nh i vi cc tn s v tuyn. V vy, cc ng truyn hng

ngoi thng da trn c s iu ch xung bt- tt v tch sng tn hiu quang. Qutrnh truyn dn xung bt- tt c thc hin bng cch bin i cng (bin )dng in trong my pht hng ngoi nh l laser diode hay diode pht quang chnghn. Theo cch ny, d liu c mang i bi cng (ch khng phi l pha hay

tn s) ca sng nh sng. Cc h thng hng ngoi s dng hai thnh phn vt l


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khc nhau (cc b pht v cc b tch) pht v thu tn hiu sng quang. iu ny

tri ngc vi cc h thng v tuyn v s dng mt anten chung pht v thutn hiu.

Cc mng WLAN hng ngoi khc vi cc mng WLAN v tuyn nhiu im.

Ni chung, cc h thng v tuyn lun to ra vng ph rng hn. Mt khc, tn hiu vtuyn lun c rng bng thng hp hn cc tn hiu quang mc d cc h thng

thng mi vn cha khai thc c ht bng thng tn hiu quang.

1.4 ng dng ca h thng mng WLAN

Lc u WLAN ch c s dng bi cc t chc, cng ty ln nhng ngy nay,th WLAN c gi c chp nhn c m ta c th s dng. Sau y l mt s ngdng chung v ph hp ca WLAN.

1.4.1 Vai tr truy cp (Access role)

WLAN ngy nay hu nh c trin khai lp access, ngha l chng c s

dng mt im truy cp vo mng c dy thng thng. Wireless l mt phngphp n gin ngi dng c th truy cp vo mng. Cc WLAN l cc mng lp

data- link nh tt c nhng phng php truy cp khc. V tc thp nn WLAN tc trin khai core v distribution.

Cc WLAN cung cp gii php cho mt vn kh kh l: kh nng di ng.

Gii php s dng cellular c tc thp v mc. Trong khi WLAN th c cng slinh hot nhng li r hn. Cc WLAN nhanh, r v c th xc nh mi ni.

Hnh 1.2: Access Role


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1.4.2 M rng mng (Network extension)

Cc mng khng dy c th c xem nh mt phn m rng ca mt mng c

dy. Khi mun m rng mt mng hin ti, nu ci t thm ng cp th s rt tnkm. Hay trong nhng to nh ln, khong cch c th vt qu khong cch ca

CAT5 cho mng Ethernet. C th ci t cp quang nhng nh th s yu cu nhiuthi gian v tin bc hn, cng nh phi nng cp switch hin ti h tr cp quang.

Cc WLAN c th c thc thi mt cch d dng. V t phi ci t cp trong

mng khng dy.

Hnh 1.3: M rng mng

1.4.3 Kt ni cc to nh

Trong mi trng mng campus hay trong mi trng c 2 to nh st nhau, cth c trng hp nhngngi dng t to nh ny mun truy cp vo ti nguyn cato nh khc. Trong qu kh th trng hp ny c gii quyt bng cch i mt

ng cp ngm gia 2 to nh hay thu mt ng leases- line t cng ty in thoi.S dng k thut WLAN, thit b c th c ci t mt cch d d ng v nhanh

chng cho php 2 hay nhiu to nh chung mt mng. Vi cc loi anten khng dyph hp, th bt k to nh no cng c th kt ni vi nhau vo cng mt mng trong

mt khong cch cho php.

C 2 loi kt ni: P2P v P2MP. Cc lin kt P2P l cc kt ni khng dy gia 2to nh. Loi kt ni ny s dng cc loi anten trc tip hay bn trc tip mi ulin kt.


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Hnh 1.4: Kt ni cc to nh

Cc lin kt P2MP l cc kt ni khng dy gia 3 hay nhiu to nh, thng

dng hub- and- spoke hay kiu kt ni star, trong mt to nh ng vai tr trungtm tp trung cc im kt ni. To nh trung tm ny s c core network, kt niinternet, v server farm. Cc lin kt P2MP gia cc to nh thng s dng cc loianten a hng trong to nh trung tm v anten chung hng trn cc spoke.

C hai kiu kt ni ny:

1.4.3.1 Phn pht dliu dm cui (Last Mile Data Delivery)Wireless Internet Service Provider (WISP) cung cp cc dch v phn pht d

liu trn last-mile cho cc khch hng ca h. Last mile cp n h tng giao tip

c dy hay khng dy tn ti gia telco hay cngty cp v ngi dng cui.

Hnh 1.5: Dch v dm cui

Trong trng hp nu c cng ty cp v telco u gp kh khn trong vic mrng mng ca h cung cp cc kt ni bng thng rng cho nhiu ngi dng hn

na. Nu sng trong khu vc nng thn th kh c th truy cp vo kt ni bng thngrng (nh cable modem hay xDSL). S kinh t hn rt nhiu nu cc WISP a ra gii

php truy cp khng dy vo nhng ni xa v cc WISP s khng gp nhng kh

khn nh ca cc cng ty cp hay telco v khng phi ci t nhiu thit b. Cc WISPcng gp phi mt s tr ngi. Nh cc nh cung cp xDSL gp phi vn l khongcch vt qu 5.7 km t CO n nh cung cp cp, cn vn ca WISP chnh l ccvt cn nh mi nh, cy,...


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1.4.3.2 S di ng (Mobility)

Ch l mt gii php lp access, nn WLAN khng th thay th mng c dy

trong tc truyn. Mt mi trng khng dy s dng cc kt ni khng lin tc vc t l li cao. Do , cc ng dng v giao thc truyn d liu c thit k cho

mng c dy c th hot ng km trong mi trng khng dy. Li ch m cc mngkhng dy mang li chnh l tng kh nng di ng b li tc v QoS.

Hnh 1.6: S di ng

Trong tng trng hp, cc mng wireless to nn kh nng truyn d liu m

khng cn yu cu thi gian v sc ngi a d liu, cng nh gim c ccthit b c kt ni vi nhau nh mng c dy. Mt trong nhng k thut mi nht

ca wireless l cho php ngi dng c th roam, ngha l di chuyn t khu vc khng

dy ny sang khu vc khc m khng b mt kt ni, ging nh in thoi di ng,ngi dng c th roam gia cc vng di ng khc nhau. Trong mt t chc ln, khi

phm vi ph sng ca wireless rng th vic roaming kh quan trng v ngi dng cth vn gi kt ni vi mng khi h ra ngoi.

1.4.4 Vn phng nh- Vn phng gia nh (Small Office-Home Office)

Trong mt s doanh nghip ch c mt vi ngi dng v h mun trao i thngtin gia cc ngi dng v ch c mt ng ra Internet. Vi nhng ng dng ny

(Small office-home office-SOHO), th mt ng wireless LAN l rt n gin v

hiu qu. Cc thit b wireless SOHO th rt c ch khi nhngngi dng mun chias mt kt ni Internet.


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Hnh 1.7: SOHO WLAN

1.4.5 Vn phng di dng (Mobile Offices)

Cc vn phng di ng cho php ngi dng c th di chuyn n mt v tr khcmt cch d dng. V tnh trng qu ti ca cc lp hc, nhiu trng hin nay ang s

dng lp hc di ng. c th m rng mng my tnh ra nhng to nh tm thi,

nu s dng cp th rt tn chi ph. Cc kt ni WLAN t to nh chnh ra cc lp hcdi ng cho php cc kt ni mt cch linh hot vi chi ph c th chp nhn c.

Hnh 1.8: Vn phng di ng

1.5 u, nhcim ca mng WLAN

1.5.1 u im

Mng khng dy khng dng cp cho cc kt ni, thay vo , chng s dngsng Radio. u th ca mng khng dy l kh nng di ng v s t do, ngi dng

khng b hn ch v khng gian v v tr kt ni. Nhng u im ca mng khng dybao gm:

- Kh nng di ng v s t do- cho php kt ni bt k u trong khu vc trinkhai mng. Vi s gia tng ngi s dng my tnh xch tay l mtiu rt thun li..


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- Khng b hn ch v khng gian v v tr kt ni: Ngi dng c th duy tr kt

ni mng khi h di chuyn t ni ny n ni khc.

- D lp t v trin khai.png tc thi khi gia tng s lng ngi dng.

- Tit kim thi gian lp t dy cp.

- Khng lm thay i thm m, kin trc ta nh.

- Gim chi ph bo tr, bo dng h thng.

- Vi nhng cng ty m v tr khng tt cho vic thi cng cp nh ta nh c,khng c khong khng gian thi cng cp hoc thu ch t vn phng,

- Hin nay, cng ngh mng khng dy ang dn dn thay th cc h thng cdy v tnh linh ng v nng cp cao.

1.5.2 Nhc im

- Nhiu:

Nhc im ca mng khng dy c th k n nht l kh nng nhiu sngradio do thi tit, do cc thit b khng dy khc, hay cc vt chn (nh cc nh caotng, a hnh i ni)

- Bo mt:

y l vn rt ng quan tm khi s dng mng khng dy. Vic v tnhtruyn d liu ra khi mng ca cng ty m khng thng qua lp vt l iu khin

khin ngi khc c th nhn tn hiu v truy cp mng tri php. Tuy nhin WLANc th dng m truy cp mng ngn cn truy cp, vic s dng m tu thuc vo

mc bo mt m ngi dng yu cu. Ngoi ra ngi ta c th s dng vic m had liu cho vn bo mt.

- Phm vi:

Mt mng chun 802.11g vi cc thit b chun cng ch hot ng tt trong

phm vi vi chc met. N ch ph hp cho khng gian khong cch nh. Nu mun sdng phi s dng thm thit b: Repeater hay AP. Dn n chi ph gia tng.


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CHNG 2

CC TIU CHUN CA MNG WLAN

Cng vi s pht trin mnh m ca mng khng dy, cc chun (v ng thi lcc thit b) cho mng khng dy WLAN ln lt ra i v ngy cng c nng cp,

ci tin. Nhng chun ra i sm nht nh IEEE 802.11 tr nn ph bin. Sau lHiperLAN, HomeRF, OpenAir v gn y l Bluetooth. Mi chun u mang mt s

c tnh, u im ring ca n.

2.1 Cc chun IEEE 802.11

2.1.1 Ngun gc ra i ca chun IEEE 802.11

IEEE (Institute of Electrical and Electronic Engineers) l t chci tin phong

trong lnh vc chun ho mng cc b. n IEEE 802 c trin khai t nhng nm1980 m kt qu l s ra i ca chun thuc h 802.x. y l chun p dng ring

cho mng cc b. Nm 1990, Vin cc k s in v in t IEEE thnh lp mt uban pht trin tiu chun cho cc mng WLAN hot ng tc t 1 n 2 Mbps.

Qu trnh pht trin chun IEEE 802.11 b nh hng mnh bi cc sn phm camng WLAN c mt trn th trng. V vy, mc d cn kh nhiu thi gian hon

thin cc tiu chun (do c kh nhiu xut mang nng tnh cnh tranh t pha cc

nh cung cp thit b), n vn l tiu chun ph bin nht cho n nay.

H tiu chun 802.11 do IEEE pht trin nh ngha giao din v tuyn gia trm

v tuyn v trm gc hay gia hai trm v tuyn vi nhau. Chun u tin m IEEEcho ra i l IEEE 802.11 vo nm 1997. Tc t c l 2Mbps s dng phngphp tri ph trong bng tn ISM khng qun l (bng tn dnh cho cng nghip, khoahc v y hc). H tiu chun 802.11 c nhiu phn m rng trong ba tiu chun

IEEE 802.11b, IEEE 802.11a, IEEE 802.11g l quan trng nht, v mi y nht l sra i ca chun IEEE 802.11i v IEEE 802.11n.

2.1.2 IEEE 802.11b

ca vo nm 1999, tiu chun IEEE 802.11b hay Wi- fi, l phn mrngca tiu chun 802.11. Chun ny cung cp vic truyn d liu trong di tn 2.4 Ghz ,vi cc tc 1- 2 Mbps


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IEEE 802.11b s dng phng php tri ph trc tip DSSS. Tiu chun 802.11b

c xy dng2 lp di cng ca m hnh OSI: PHY v lp con MAC thuc lplin kt d liu.

tng tc truyn ln cho chun 802.11b, vo nm 1998, Lucent v Harris

xut cho IEEE mt chun c gi l Complementary Code Keying(CCK). CCK sdng mt tp 64 t cc m 8bit, do 6 bit c th c i din bi bt k t m no.

V l mt tp hp nhng t m ny c cc c tnh ton hc duy nht cho php chngc bn nhn nhn ra mt cch chnh xc vi cc k thut khc, ngay c khi c shin din ca nhiu.

Vi tc 5.5 Mbps s dng CCK m ho 4 bit mi sng mang, v vi tc

11 Mbps m ho 8 bit mi sng mang. C hai tc u s dng QPSK lm k thutiu ch v tn hiu 1.375 MSps. V FCC iu chnh nng lng u ra thnh 1 wattEffective Isotropic Radiated Power(EIRP). Do vi nhng thit b 802.11, khi di

chuyn ra khi sng radio, radio c th thch nghi v s dng k thut m ho t phctp hn gi d liu v kt qu l tc chm hn.

Mt trong nhng nhc im ca IEEE 802.11b l bng tn d b nghn v hthng d b nhiu bi cc h thng mng khc, l vi ba, cc loi in thoi hot ng

tn s 2.4 GHz v cc mng Bluetooth. ng thi IEEE 802.11b cng c nhng hnch nh: thiu kh nng kt ni gia cc thit b truyn ging ni, khng cung cp

dch v QoS (Quality of Service) cho cc phng tin truyn thng.

Mc d vn cn mt vi hn ch v nhc im nhng chun 802.11b (thnggi l Wifi) l chun thng dng, c s dng ph bin nht hin nay vi s lng

ln cc nh cung cp cho cc i tng khch hng l cc doanh nghip, gia nh haycc vn phng nh.

Hnh 2.1: Cc la chn chun IEEE802.11b

IEEE 802.11b+: TI (Texas Instruments) pht trin mt k thut iu ch gil PBCC (Packet Binary Convolutional Code) m n c th cung cp cc tc tn


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hiu 22Mbps v 33Mbps. TI sn xut cc chipset da trn 802.11b cn h tr PBCC

22Mbps.Cc sn phm kt hp cc chipset ny c bit nh l cc thit b 802.11b+.Chng hon ton tng thch vi 802.11b, v khi giao tip vi nhau c th t c

tc tn hiu 22Mbps. Mt s tng cng m TI c th c s dng gia cc thitb 802.11b+ l ch 4x, n s dng kch thc gi tin ti a ln hn (4000 byte)

gim chng lp v tng thng lng.

2.1.3 IEEE 802.11a

Chun 802.11b s dng k thut m ho da trn DSSS, mt k thut c phttrin bi qun i. Khng ging 802.11b, 802.11a c thit k hot ng bngtn 5 GHz Unlicensed National Information Infrastructure (UNII). Khng ging nh

bng tn ISM (khong 83 MHz trong ph 2.4 GHz), 802.11a s dng gp 4 ln bngtn ISM v UNII s dng ph khng nhiu 300MHz.

Hnh 2.2: Di tn 5 GHz

ch li u tin ca 802.11a so vi 802.11b l chun hot ng ph 5.4 GHz,cho php n c hiu sut tt hn v c tn s cao hn.

ch li th hai da trn k tht m ho s dng bi 802.11a. 802.11a s dngmt phng thc m ho c gi l coded orthogonal FDM(COFDM hay OFDM).Mi knh ph trong s thc thi COFDM c rng khong 300 kHz. COFDM hot

ng bng cch chia nh knh truyn d liu tc cao thnh nhiu knh truyn phc tc thp hn, v sau s c truyn song song . Mi knh truyn tc cao c rng l 20MHz v c chia nh thnh 52 knh ph, mi ci c rng khong300 kHz.

COFDM s dng 48 knh ph cho vic truyn d liu, v 4 knh cn li c s

dng cho sa li. COFDM c tc truyn cao hn v c khnng phc hi li tthn, nh vo k thut m ho v sa li ca n. Mi knh ph c rng khong 300

kHz. m ho 125 kbps th BPSK c s dng cho tc khong 6000 kbps. S


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dng QPSK th c kh nng m ho l6n ti 250 kbps mi knh, cho tc khong

12Mbps. Bng cch s dng QAM 16 mc m ho 4bit/Hertz, v t c tc 24Mbps. Tc 54 Mbps t c bng cch s dng 64 QAM, cho php t 8-10 bit

cho mi vng, v tng cng ln n 1.125 Mbps cho mi knh 300 kHz. Vi 48 knhcho tc 54 Mbps, tuy nhin, tc ti a theo l thuyt ca COFDM l 108 Mbps.

Tt c cc bng tn dng cho Wireless LAN l khng cn ng k, v th n d dngdn n s xung t v nhiu. trnh s xung t ny, c 802.11a v 802.11b u c siu chnh gim cc mc ca tc truyn d liu. Trong khi 802.11b c cc tc truynd liu l 5.5, 2 v 1 Mbps th 802.11a c by mc (48, 36, 24, 18, 12, 9, v 6 )..

2.1.4 IEEE 802.11g

Chun IEEE 802.11g l mt chun mi, c khi tho t nm 2001 nhng min nm 2003 mi hon thnh. Mc d chun 802.11a c tc nhanh (54 Mbps),

hot ng ti bng tn cao (5 GHz ) nhng nhc im ln nht ca n l khng tngthch vi chun 802.11b. V th s khng th thay th h thng ang dng 802.11b mkhng phi tn km qu nhiu. IEEE cho ra i chun 802.11g nhm ci tin

802.11b v tc truyn cng nh bng thng. 802.11g c hai c tnh chnh sau y:

S dng k thut OFDM (Orthogonal Frequency Division Multiplexing), cth cung cp cc dch v c tc ln ti 54Mbps. Trc y, FCC (FederalCommunication Commission- USA) c cm s dng OFDM ti 2,4GHz. Nhng hinnay FCC cho php s dng OFDM ti c hai bng tn 2.4GHz v 5GHz.

Tng thch vi cc h thng 802.11b tn ti trc. Do , 802.11g cng c h

tr CCK v thit b 802.11g cng c th giao tip vi thit b 802.11b c sn

Mt thun li r rng ca 802.11g l tng thch vi 802.11b (c s dng rtrng ri ) v c c tc truyn cao nh 802.11a . Tuy nhin s knh ti a m

802.11g c s dng vn l 3 nh 802.11b. Bn cnh , do hot ng tn s 2,4GHz nh 802.11b, h thng s dng 802.11g cng d b nhiu nh 802.11b.

IEEE 802.11g+: c ci tin t chun 802.11g, hon ton tng thch vi802.11a v 802.11b, c pht trin bi TI. Khi cc thit b 802.11g+ hot ng vi

nhau th thng lng t c c th ln n 100Mbps.

Tm hot ng trung bnh ca cc chun c th t n 90 mt, ty theo tiu

chun, tc v iu kin mi trng lm vic.


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Bng1: Bng tm ttthng s cc chun 802.11 thng dng

Chun WiFiTn

s (GHz)Tc

(Mbps)Khong

cch (m)

IEEE 802.11a 5 54

12m:

54Mb/s90m:

6Mb/s

EEE 802.11b 2.4 11

30m:

11Mb/s

90m:1Mb/s

IEEE 802.11g 2.4 54

15m:

54Mb/s

45m:11Mb/s

2.1.5 IEEE 802.11i

N l chun b sung cho cc chun 802.11a, 802.11b, 802.11g v vn bomt. N m t cch m ha d liu truyn gia cc h thng s dng cc chun ny.

802.11i nh ngha mt phng thc m ho mnh m gm Temporal Key IntegrityProtocol (TKIP) v Advanced Encryption Standard (AES).

2.1.6 IEEE 802.11n

Mt chun Wi-Fi mi ang c Lin minh WWiSE a ra xin ph chun (dkin vo nm 2008), vi mc tiu a kt ni khng dy bng thng rng ln mt tm

cao mi. Cng ngh ny ha hn s y mnh ng k tc ca cc mng cc bkhng dy (WLAN).

Lin minh WWiSE (WorldWide Spectrum Efficiency), bao gm cc cng ty:Airgo Networks, Bermai, Broadcom, Conexant Systems, STMicroelectronics v TexasInstruments, cho bit cng ngh Wi-Fi mi ang c nhm tho lun 802.11n caVin K thut in v in t (IEEE) xem xt. y l b phn gim st mt chun


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Wi-Fi th h k tip c kh nng duy tr tc trao i d liu khng dy vt mc

100Mbps.

Chun Wi- Fi xut da trn cng ngh MIMO- OFDM ( multiple input,multiple output- orthogonal frequency division multiplexing), cung cp tc cao hn

bng cch s dng hai anten mi u ca tn hiu (mt truyn, mt nhn), thayv mt anten mi u nh hin nay.

Cng ngh MIMO s l thnh phn ct yu ca chun 802.11n, cung cp phm vi

ph sng WLAN nnh hn vi t l truyn d liu siu nhanh. N s cho phpngi dng thc hin nhiu cng vic hn vi Wi- Fi, c bit trong cc ng dngaphng tin.

WWiSE cho bit cng ngh mi c tht t l truyn tia ln n 135Mbps

trong cu hnh ti thiu 2 ni 2 (two- by- two), v t l ny c th ln ti 540Mbps qua1 cu trc MIMO 4 ni 4 (four- by- four) v rng knh truyn 40MHz.

2.1.7 Cu trc cbn ca WLAN IEEE 802.11

Mt mng WLAN 802.11 thng thng gm bn thnh phn chnh: H thngphn phi (DS), im truy nhp (AP), Mi trng v tuyn (WM) v Cc trm STA

: Hnh.2.3: Cc thnh phn vt l c bn ca WLAN

2.1.7.1 H thng phn phi (Distribution System)

Thnh phn kin trc dng kt ni cc nhm dch v vi nhau v tch hp vicc mng LAN to thnh mt mng m rng c gi l H thng phn phi DS.

Hay ni cch khc, DS s dng kt ni cc BSS vi nhau , iu phi thng tinn cc trm ch.


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Mt DS cho php h tr cc thit b di ng bng cch cung cp cc dch v

logic cn thit gim st a ch chuyn i ch v tch hp nhiu BSS. D liu dichuyn gia mt BSS v DS qua mt AP. Cc a ch c AP s dng trao i

thng tin trn mi trng v tuyn WM v trn mi trng h thng phn phi DSMkhng nht thit phi ging nhau.

WLAN phn tch mt cch logic mi trng v tuyn vi mi trng h thng

phn phi. Mi mi trng logic khc nhau c s dng cho mi mc ch khcnhau bi mt thnh phn kin trc khc nhau.

Trong thc t, h thng phn phi c xem nh s kt hp gia cu ni(bridge) v mi trng h thng phn phi. N l cc mng xng sng (backbone),

s dng chuyn cc gi tin gia cc im truy nhp.

2.1.7.2 im truy nhp (Access Points)

Thit b gi l im truy nhp ng vai tr nh l cu ni gia mng WLAN vitrng bn ngoi. Chc nng chnh caim truy nhp l m rng mng (m rng

mt vng ph sng v tuyn). Cc im truy nhp b sung c th c trin khai tro ngmt to nh hay khun vin trng i hc nhm to ra cc vng truy nhp v tuynrng ln.

im truy nhp h tr kh nng truy nhp ti h thng phn phi bng cch cungcp cc dch v b sung n hot ng nh mt trm c s. Ngoi ra im truy nhp

cng ng vai tr phn b trong cc cu hnh mng khng ngang hng.

2.1.7.3 Mi trng v tuyn (Wireless Medium)

L mi trng truyn cc sng in t mang thng tin t trm ny n trm khc.y chnh l mi trng khng kh.

2.1.7.4 Cc trm (Station)

Cc mng WLAN c thit k v xy dng nhm mc ch kt ni cc trm vi

nhau. Trm c th l nhng thit b nh my tnh, in thoi cm tay hay bt c thitb no c giao din v tuyn.

Basic service set (BSS)

802.11 nh ngha BSS nh mt khi kt cu cbn ca mng WLAN. Hnh 2.4biu din hai BSS, mi BSS c hai trm.


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Hnh 2.4: Cu trc c bn ca WLAN

BSS ch gm mt nhm cc trm khng dy truyn thng vi nhau trong mt

phm vi gii hn, c xc nh bi cc c tnh ca mi trng truyn. Khi mt trmnm trong vng phc v, n c th lin lc vi tt c cc thnh phn khc trong BSS.

Nu mt trm di chuyn ra ngoi BSS ca n, n s khng lin lc trc tip c vicc thnh vin khc ca BSS.

2.1.8 M hnh ca WLAN IEEE 802.11

Hai m hnh c bn s dng cho WLAN l mng Ad- hoc v mng c s h tng(Infrastructure). Hai m hnh ny c s khc bit nhau r rng v gii hn khng gians dng, cch qun l mng, kin trc mng.

2.1.8.1 Ad- hoc hay cn gi l IBSS (Independent Basic Service Set)

Ad- hoc l m hnh mng m trong ch bao gm cc my trm, khng c

Access Point. Mi thit b kt ni trc tip vi cc thit b khc trong mng, cc nt ding trao i thng tin trc tip vi nhau thng qua cc b bin i v tuyn..V c

bn, hai my tnh c trang b thm Card adapter v tuyn c th hnh thnh mtmngc lp khi chng trong di tn ca nhau. M hnh ny rt thch hp cho vickt ni mt nhm nh cc thit b v khng cn phi giao tip vi cc h thng mngkhc, nh trong cc hi ngh thng mi hoc trong cc nhm lm vic tm thi. Cc

mng hnh thnh theo nhu cu nh vy khng cn thit phi qun l hay thit lp cuhnh t trc. Nt di ng c th truy cp vo cc ti nguyn ca cc my khc mkhng phi qua mt my ch trung tm. Tuy nhin chng c th c nhng nhc im

v vng ph sng b gii hn, mi ngi s dng u nghe c ln nhau.


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Hnh 2.5: M hnh Ad- hoc

2.1.8.2 M hnh mng c s h tng (Infrastructure Basic Service Set)

Infrastructure BSS l mt m hnh m rng ca mt mng WLAN c bng

cch s dng im truy cp Access Point, cc thit b di ng khng giao tip trc tip

vi nhau m giao tip vi cc im truy nhp. im truy cpAP ng vai tr va lcu ni ca mng WLAN vi cc mng khc va l trung tm iu khin s trao i

thng tin trong mng. im truy cp gip truyn v nhn d liu gia cc thit btrong mt vng ln hn. Phm vi v s thit b s dng trong mng c s h tng tu

thuc vo chun s dng v sn phm ca cc nh sn xut. Trong m hnh mng cs h tng c th c nhiu AP to ra mt mng hot ng trn phm vi rng hay ch

c duy nht mt Access Point cho mt phm vi nh nh trong mt cn nh, mt tonh. Mng c s h tng c hai li th chnh so vi mngc lp IBSS:

Infrastructure c thit lp ph thuc vo tm hot ng ca AP. V vy,mun thit lp WLAN tt c cc thit b di ng bt buc phi nm trong vng phsng ca AP v mi cng vic giao tip mng u phi thng qua AP. Ngc li, kt

ni trc tip IBSS trong mng ad- hoc gip hn ch thng tin truyn v nhn ca mngnhng chi ph li gia tng tng vt l bi v tt cc thit b u lun lun phi duy trkt ni vi tt c cc thit b khc trong vng dch v.

Trong mng c s h tng , AP cn cho php cc station chuyn sang ch tit kim nng lng. Cc AP c thng bo khi mt station chuyn sang ch titkim nng lng v to frame m cho chng. Cc thit b ch trng s dng nng

lng (Battery- operated) c th chuyn b thu pht tn hiu ca mnh sang ch ngh v khi hot ng li s nhn c tn hiu c khi phc t cc frame m lutrong AP.


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Hnh 2.6: M hnh mng csh tng Infratructure

Cc cell c th chng ln ln nhau khong 10-15 % cho php cc trm di ng c

th di chuyn m khng b mt kt ni v tuyn v cung cp vng ph sng vi chi phthp nht. Cc my trm s chn AP tt nht kt ni.

Vic thit k WLAN s tng i n gin nu thng tin v mng v qun lcng nm trong mt vng. Mt im truy nhp nm trung tm c th iu khin vphn phi truy nhp cho cc nt tranh chp, cung cp truy nhp ph hp vi mng

ng trc, n nh cc a ch v cc mc u tin, gim st lu lng mng, qun lchuyn i cc gi v duy tr theo di cu hnh mng. Tuy nhin giao thc a truy nhp

tp trung khng cho php cc nt di ng truyn trc tip ti nt khc nm trong cng

vng vi im truy nhp nh trong m hnh mng Ah- hoc. Trong trng hp ny,mi gi s phi c pht i 2 ln (t nt pht gc v sau l im truy nhp) trckhi n ti nt ch, qu trnh ny s lm gim hiu qu truyn dn v tng tr truyn

dn. Tuy nhin cc h thng nh vy thng cung cp cc thng lng d liu caohn, vng ph sng rng hn v c th phc v cc lu lng video, thoi vi thi

gian thc. Ngoi ra mt im truy nhp nm v tr thch hp c th gim ti thiuc cng sut pht v gii quyt c cc vn ca nt n mt cch hiu qu. V

WLAN s dng cc giao thc a truy nhp cm nhn sng mang CSMA/CA nn cth cc nt trong mng c s yu cu ch truyn gi ti im truy nhp. Sau im

truy nhp s chuyn tip cc gi ti ng a ch ch.

2.1.8.3 M hnh mng m rng ESS (Extended Service Set)


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Mng 802.11 m rng phm vi di ng ti mt phm vi bt k thng qua ESS.

Mt ESS l mt tp hp ca cc Infrastructure BSS ni m cc Access Point giao tipvi nhau chuyn lu lng t mt BSS ti mt BSS khc l m cho vic di

chuyn d dng ca cc trm gia cc BSS

Access Point thc hin vic giao tip thng qua h thng phn phi. H thngphn phi l mt lp mng trong mi Access Point m n xc nh ch n cho lu

lng c nhn t mt BSS. H thng phn phi xc nh lu lng nn c tipsng tr li mt ch n trong cng mt BSS, chuyn tip trn h thng phn phi timt Access Point khc, hoc gi ti mng c dy ti ch n khng nm trong ESS.

Cc thng tin nhn bi Access Point t h thng phn phi c truyn ti BSS c nhn bi trm ch.

Hnh 2.7: M hnh ESS

V d vm hnh hon chnh:


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Hnh 2.8: Mng WLAN hon chnh tng qut

2.1.9 M hnh tham chiu WLAN IEEE 802.11

H thng bao gm hai phn chnh: Lp MAC ca lp lin kt d liu

Lp vt l PHY

Nhng lp ny tng ng vi cc lp thp nht trong m hnh tham chiu c

bn OSI.

Hnh 2.9: M hnh tham chiu csIEEE 802.11

2.1.9.1 Phn lp vt l PHY

Lp vt l ca IEEE 802.11 tng ng hon ton vi lp vt l trong m hnhOSI chun. Lp vt l cung cp s kt ni cho php truyn cc khung d liu MAC t


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trm ny n trm khc qua mi trng truyn. Lp vt l PHY c chia thnh 2

phn lp v thc th chc nng qun l lp vt l:

- PMD (Physical Medium Depentdant): Phn lp ph thuc mi trng vt l.

X l cc thuc tnh ca mi trng v tuyn (tc l cc phng php tri ph)

v xc nh cch pht v thu d liu thng qua mi trng (v d nhiu ch v mho).

- PLCP (Physical Layer Covergence Procedure): Phn lp hi t lp vt l.

Xc nh phng php chuyni cc n v d liu giao thc phn lp MACvo mt khun dng gi thch hp cho phn lp PMD. N cng c th thc hin cmbin sng mang (nnh knh) cho phn lp MAC.

+ PLME: Chc nng qun l lp vt l.

Thc hin qun l cc chc nng lp vt l kt hp vi cc thc th qun l

MAC

Tm liPHY cung cp ba chc nng:

u tin, PHY cung cp mt giao din trao i cc frame vi lp MAC trncho vic truyn v nhn d liu.

Th hai, PHY s dng iu ch sng mang tn hiu v ph tri rng truyn

cc frame d liu qua mi trng v tuyn.

Th ba, PHY cung cp mt du hiu cm ng sng mang tr li MAC kimtra hot ng trn mi trng.

802.11 cung cp ba nh ngha PHY khc nhau: c FHSS v DSSS h tr tc

d liu 1 Mbps v 2 Mbps. Mt s m rng ca kin trc 802.11 (802.11a) nh nghacc k thut a thnh phn c th t c tc d liu ti 54 Mbps. Mt s m

rng khc (802.11b) nh ngha tc d liu 11 Mbps v 5.5 Mbps tn dng mt sm rng ti DSSS c gi l High Rate DSSS (HR/DSSS). 802.11b cn nh ngha

mt k thut thay i tc m t mng 11 Mbps xung cn 5.5 Mbps, 2 Mbps, hoc1 Mbps di cc iu kin nhiu hoc hot ng vi cc lp PHY 802.11 tha k.

Khc vi cc mng c dy truyn thng, mng khng dy truyn d liu thng

qua mi trng mng qua hnh thc pht x sng in t trng. Yu cu chung lvng ph sng phi rng, p ng c cc nhu cu ca ngi s dng. Hai loi

mi trng c s dng rng ri nht cho cc ng dng vng cc b l sng hngngoi v sng v tuyn. Hu ht cc my tnh c nhn hin nay u c cng hng

ngoi cho php kt ni nhanh ti my in v cc thit b ngoi vi khc.


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Tuy nhin sng hng ngoi c mt s hn ch, n s d b cn tr bi tng ngn

hoc vt cn. Cn sng v tuyn li c th xm nhp qua hu ht cc vt chng ngitrong phng v cho vng bao ph rng. Do , hu ht cc sn phm 802.11 trn th

trng hin nay u s dng sng v tuyn truyn pht thng tin.

2.1.9.2 Phn lpiu khin truy nhp mi trng MAC

c trng nht ca IEEE 802.11 chnh l lp con MAC. Lp con MAC quy nhcc phng thc truy nhp knh, truyn khung d liu v tng tc vi mi trngmng bn ngoi. Ging nh Ethernet, 802.11 s dng phng thc a truy cp cm

nhn sng mang iu khin vic truy nhp mi trng truyn. Tuy nhin, do sphc tp ca mi trng v tuyn nn giao thc ca n cng phc tp hn.

2.1.9.2.1 Chc nng lp con MACKin trc ca lp con MAC trong tiu chun IEEE 802.11 bao gm hai chc

nng phi hp c bn (Hnh 2.9):

- Chc nng phi hp phn b DCF (Distribution Coordiration Funtion).

- Chc nng phi hp theo im PCF (Point Coordiration Funtion).

Mi chc nng nh ngha mt phng thc hot ng khc nhau cho trm muntruy nhp vo mi trng khng dy. Chc nng phi hp c hiu nh l chc nng

quyt nh vic khi no mt trm trong BSS c php truyn hay nhn mt phn

on n v d liu giao thc MAC (MPDU) mi trng v tuyn.

Ch hot ng DCF l bt buc i vi tt c cc ng dng, cn chc nng

PCF l tu chn, DCF khng s dng bt c loi iu kin trung tm no, bn cht can l mt giao thc MAC a truy cp cm nhn sng mang c trnh xung t

CSMA/CA. Ch cn li PCF s dng trm nn iu khin ton b cc hot ngtrong , n hot ng tng t nh mt h h vng.


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Hnh 2.10:M hnh phn lp MAC

2.1.9.2.2n v d liu giao thc MAC 802.11 tng qut

Hnh 2.10: Khun dng n v d liu giao thc MAC tng qut

Bng2: Thng tin cho cc trng d liu khc nhau trong phn tiu MPDU

Trng Thng tin

iu khin khung Phin bn hin ti ca tiu chun, cc gi cnhn hoc gi i ti h thng phn phi, qun l ngun,phn mnh, gi m ho v nhn thc.

Khong thi gian/Nhn dng Khong thi gian ca vector phn phi mng,nhn dng nt ang hot ng ch bo v ngun.

Cc trng a ch 1-4 Cc a ch ca BSSID, ch, ngun, b pht, v


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b thu.

Chui iu khin Chui s ca gi v phn on gi.

Hnh 2.10 biu din khun dng ca n v d liu giao thc MAC 802.11 tng

qut (MPDU). Cc trng a ch 2, a ch 3, iu khin chui, a ch 4 v d liungi dng ch c trong mt s trng hp nht nh. MPDU c bo v c lp bicc bit kim tra li. C ba kiu gi:

Cc gi d liu;

Cc gi iu khin (v d nh cc gi RTS, CTS, ACK);

RTS: gi yu cu gi, s dng khai bo cho cc trm trnh ng.

CTS: xa gi, gi CTS c truyn t trm khai bo trong gi tin RTS cc

trm khc bitc 1 gi tin spc truyn.ACK: gi tin c s dng xc nhn gi tin c nhn thnh cng.

Cc gi qun l (v d nh n hiu).

Thng tin cho bi cc trng khc nhau trong phn tiu MPDU c lit k

trong Bng 2

2.1.9.2.3 Cc khong thi gian lin khung

Quyn u tin truy nhp ti mi trng v tuyn c iu khin thng qua cckhong khng gian gia cc khung truyn, gi l cc khong thi gian lin khung IFS

(Inter Frame Space). Cc gi tr IFS c xc nh bi lp vt l. Mt trm xc nhxem mi trng c ri khng thng qua chc nng pht hin sng mang trong khong

thi gian ch nh. C ba loi thi gian lin khung thng c s dng cung cpcc mc logic u tin khc nhau cho vic truy nhp n phng tin v tuyn l:

- SIFS (Short Inter Frame Space) khong thi gian lin khung ngn.

SIFS l khong thi gian ngn nht, thng s dng cho khung ACK, CTS,

MPDU th hai hoc tip sau ca mt khi phn on.

SIFS s c s dng khi cc trm chim c mi trng v cn gi trongmt khong thi gian thc hin trao i khung. S dng khe thi gian ngn nhtgia cc ln truyn khung s ngn cho cc trm khc c gng truy nhp mi trng,nhng trm ny c yu cu ch trong mt khong thi gian di hn, iu ny to

iu kin u tin hon thnh mt chu trnh trao i khung.


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- PIFS (PCF Inter Frame Space) khong thi gian lin khung PCF

PIFS s c s dng trong cc hot ng ca cc trm theo PCF tng quyn

u tin truy nhp n phng tin ti thi im u ca khong tranh chp. Mt trms dng PCF s c cho php truyn sau khi c ch cm nhn sng mang ca n xc

nhn rng- DIFS (DCF Inter Frame Space) khong thi gian lin khung DCF

c s dng trong khung hot ng ca cc trm di DCF truyn cckhung d liu MPDU v khung qun l. Mt trm s dng DCF s c cho phptruyn sau khi c ch cm nhn sng mang ca n xc nhn rng mi trng ri sau

mt khong thi gian.

2.1.9.2.4 Chc nng phi hp phn b DCF

Phng thc truy nhp c bn ca MAC WLAN IEEE 802.11 l DCF c bitvi di tn a truy nhp cm nhn sng mang vi c ch trnh xung t. DCF c th

c p dng tt c cc STA, s dng cho c cu hnh IBSS ln cu hnh mng csh tng.

Hnh 2.11: Chc nngphi hp phn bDCF

Khi mt STA mun truyn tn hiu, n s nghe mi trng xc nh xem liuc mt STA khc ang truyn hay khng. Nu mi trng c xc nh l khng

bn, qu trnh chuyn i c th din ra. Cch trruy nhp CSMA/CA bt buc phic mt khe thi gian ti thiu tn ti gia cc khung truyn i lin tc. Mt STA ang

truyn phi m bo rng mi trng ang ri trong khong thi gian ny trc khi


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truyn. Nu mi trng c xc nh l bn, STA s ch cho kt thc qu trnh

truyn hin ti. Sau khi ch, hoc trc khi c gng truyn li ngay lp tc sau mt lntruyn thnh cng, STA s chn mt khong thi gian ngng (backoff) ngu nhin v

s gim b m thi gian ngng.

Giao thc truy nhp mi trng c s l DCF, n cho php chia s phng tint ng gia cc PHY tng thch thng qua s dng c ch CSMA/CA v mt thi

gian ngng ngu nhin sau mt trngthi mi trng bn. Thm vo tt c cc lulng trc tip s dng xc nhn (khung ACK) t ch cc m ti vic truyn dn lic ln k hoch bi bn gi nu khng nhn c ACK no.

Giao thc CSMA/CA c thit k gim xc sut xung t gia nhiu

STA cng truy nhp mt mi trng, ti thi im xung t c kh nng xy ra ln.Ch ngay sau khi phng tin chuyn sang ri l thi im m xc sut xy ra xungt ln nht. iu ny xy ra l do c nhiu STA ang ch mi trng tr li. y l

tnh hung i hi th tc ngng ngu nhin gii quyt cc xung t mi trng.

Pht hin sng mang c th thc hin bng c ch vt l hoc c ch o.

C ch pht hin sng mang o t c bng cch phn tn thng tin yu cu

gi trc, thng tin ny thng bo v s dng sp ti ca mi trng. Trao i cckhung RTS v CTS trc khung d liu thc s l cch phn tn thng tin gi

trc mi trng. Cc khung RTS v CTS cha mt trng thi gian/ID nh nghakhong thi gian m mi trng s c gi trc truyn khung gi liu thc v tr

v khung ACK. Tt c cc trm STA nm trong phm vi nhn ca STA ngun (truynRTS) hoc STA ch (truyn CTS) s bit c yu cu gi mi trng . Do mt

STA c th khng phi l ch nhn d liu ca STA ngun vn c th bit c v ss dng mi trng trc mt.

Mt cch khc phn tn thng tin ginh trc mi trng l trng thi gian/

ID trong khung trc tip. Trng ny a ra thi gian m mi trng s b chim,hoc l ti thi im kt thc ca ACK tip theo, hoc trong trng hp chui phn

on l thi im kt thc ca ACK tip sau phn on k tip.

Vic trao i RTS/CTS thc hin theo kiu xem xt xung t nhanh v kim trang truyn dn. Nu STA pht RTS khng nhn c CTS, STA ngun c th lpli qu trnh nu khung d liu di c truyn i v khng nhn c ACK.

Mt li im khc na ca c ch RTS/CTS l khi nhiu BSS tn dng cng mtknh xp chng. C ch gi trc mi trng lm vic qua cc ranh gii BSA. C ch


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RTS/CTS cng c th tng cng kh nng hot ng trong mt iu kin c th khi

tt c cc STA u c th nhn t AP, nhng khng th nhn t cc STA khc trongBSS.

2.1.9.2.5 Chc nng phi hp im PCF

Ngoi DCF, MAC cng c th kt hp mt phng php truy nhp tu chn gi

l PCF, n ch c th s dng trn cc cu hnh mng csh tng. Phng php truynhp ny s dng mt b phi hp im PC (Point Coordiration) hot ng ti imtruy nhp ca BSS xc nh trm no s c php truyn. V c bn, giao thc

ny hot ng ging nh s thm d (Polling), trong PC ng vai tr ca b phniu khin thm d.

Hnh 2.12: Chc nngphi hpim PCF

PCF s dng c ch pht hin sng mang o c h tr bi mt c ch u tintruy nhp. PCF s phn tn thng tin trong cc khung qun l (Beacom Frame), thu

c quyn qun l mi trng bng cch t ra cc vector cp pht mng NAV trongcc trm. Thm vo , tt c cc truyn dn khung di s iu khin ca PCF u

s dng khong thi gian lin khung IFS nh hn thi gian IFS cho cc khung ctruyn i thng qua DCF. Vic s dng thi gian lin khung IFS nh hn c ngha l

lu lng phi hp im s c quyn u tin truy nhp phng tin truyn thng lnhn cc trm trong ch hot ng trong BSSdi phng php truy nhp DCF.

u tin truy nhp PCF c th c tn dng to ra mt phng php truynhp khng tranh chp (CF Contension Free). PC s iu khin vic truyn dnkhung ca cc trm loi b tranh chp trong mt khong thi gian gii hn no .


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2.1.9.2.6 Phn mnh

Trong tt c cc qu trnh truyn dn gi trong mng WLAN, cc bn tin c

di thay i c s dng trong tiu chun IEEE 802.11. Theo cch ny, tng s ccgi tin pht i l nh nht. iu ny tr nn quan trng t c thng lng cao

do rt nhiu thit b mng b gii hn bi s lng gi tin m chng c th x l trongmt giy. Vic phn mnh d liu c th c ch khi p dng cho cc thit b di ng di

chuyn tc trung bnh. Qu trnh phn mnh gi c th lm gim tc ng caxung t v l mt la chn tt s dng RTS/CTS. Chun 802.11 khuyn nghchiu di gi c phn mnh nn nh hn 3,5 ms (tc l di gi gm 400 octet c

tc d liu 1 Mbps). Tuy nhin, qu trnh phn mnh ho yu cu phn thng tinph nhiu hn do s lng cc gi tin v cc gi ACK c x l tng ln, do phn

thng tin mo u v thng tin tiu trong mi gi tin c phn mnh v do ccSIFS b sung.

Hnh 2.13: Qu trnh phn mnh mt gi d liu unicast

t c nhng thun li ny, mt c ch phn mnh/ti kt hp n gin

c a vo trong lp MAC 802.11 (xem Hnh 1.12). Mi gi bao gm mt chui s s dng cho vic ti kt hp. Mt ngng phn mnh xc nh di ln nht ca

gi trn c phn mnh.

2.2 Tiu chun HiperLAN

S pht trin ca thng tin v tuyn bng rng t ra nhng yu cu mi v

mng LAN v tuyn. l nhu cu cn h tr v QoS, bo mt, quyn s dng,ETSI (European Telecommunications Standards Institute- Nm 1992, Vin cc tiu

chun Vin thng Chu u thnh lp hip hi xy dng tiu chun WLAN dngcho cc mng LAN v tuyn (HiperLAN) hot ng hiu sut cao (High Performance

LAN), tiu chun ny xoay quanh m t cc giao tip mc thp v m ra kh nngpht trin mc cao hn.


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Hnh 2.14: M hnh HiperLAN v OSI

Chun HiperLAN ging nh chun 802.11, chun ny phc v cho c cc mngc lp v cc mng c cu hnh c s. HiperLAN hot ng bng tn 5,15 n 5,3GHz (bng tn c chia thnh 5 knh tn s) vi mc cng sut nh thp khong

1W. Tc d liu v tuyn ti a c th h tr l khong 23,5 Mbps v chun nycng h tr cho cc ngi dng di ng tc thp (khong 1,4 m/s). .

C 4 loi HIPERLAN c a ra: HIPERLAN/1, HIPERLAN/2,

HIPERCESS (3) v HIPERLINK (4).vo nm 1996.

Bng 3: c tnh ca chun HiperLAN

Trong cc chun ca HiperLAN, HiperLAN2 l chun c s dng rng rinht bi nhng c tnh k thut ca n. Nhng c tnh k thut ca HiperLAN2:


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Truyn d liu vi tc cao

Kt ni c nh hng.

H tr QoS.

Cp pht tn s t ng

H tr bo mt

Mng v ng dng c lp

Tit kim nng lng

Tc truyn d liu ca HiperLAN2 c th t ti 54 Mbps. S d c th tc tc v HiperLAN2 s dng phng php gi l OFDM (Orthogonal

Frequence Digital Multiplexing dn knh phn chia tn s). OFDM c hiu qutrong c cc mi trng m sng radio b phn x t nhiu im.

HiperLAN Access Point c kh nng h tr vic cp pht tn s t ng trongvng ph sng ca n. iu ny c thc hin da vo chc nng DFS (DynamicFrequence Selection) Kin trc HiperLAN2 thch hp vi nhiu loi.

mng khc nhau. Tt c cc ng dng chy c trn mt mng thng thng th c thchy c trn h thng mng HiperLAN2.

2.3 Tiu chun OpenAir

Mt nhm cc nh cung cp sn phm tnh ton di ng thnh lp mt t chc c

tn l WLIF Din n tng h cc mng WLAN. Hin nay, WLIF c 38 th nhvin, cc cng ty thnh vin cung cp nhiu sn phm dch v WLAN tng h vi

nhau, v th m thc y s pht trin ca ngnh cng nghip mng WLAN. WLIF cng b giao din OpenAir cho php cc b phn c lp c th pht trin cc sn

phm tng thch v thit lp tin trnh cp bng xc nhn cho cc c tnh tng hca cc sn phm WLAN. Cc c t WLIF da trn mng WLAN FHSS 2,4 GHz

gii thiu vo u nm 1994. H thng ny hot ng tc 1,6 Mbps trn mi munhy tn. Vi 15 mu c lp, tc d liu tng ln n 2,4 Mbps (15 x 1,6 Mbps).

Chun OpenAir hon thnh vo nm 1996. OpenAir MAC da trn CSMA/CA v

RTS/CTS nh 802.11. Tuy nhin OpenAir khng thc hin vic m ha ti lp MAC,nhng li c ID mng da trn mt khu. OpenAir cng khng cung cp chc nng

tit kim cng sut.


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2.4 Tiu chun HomeRF

HomeRF l mt cng nghc s dng trong truyn thng khng dy, c s

dng ch yu trong cc h thng mng gia nh. HomeRF l tn vit tt ca cm tHome Radio Frequency, mt loi mng cc b s dng tn s Radio kt ni v trao

i d liu gia cc phn t ca mng. Cc phn t ca mng ny rta dng: c thl cc my tnh PC, cc thit b Mobile, cc loi thit b cm tay khc (c giao din v

tuyn).

HomeRF c nu ln vo nm 1998. C nhiu tp on cng nghip nh:Compaq, IBM, Intel, Microsoft tp trung nghin cu. Mc ch chung ca h lpht trin mt giao thc chun chung cho mng khng dy trong di tn 2.4GHz v t

l d liu l 1- 2 Mbps, s dng k thut lai TDMA/ CSMA. Giao thc truy nhp vtuyn dng chung SWAP- Shared Wireless Access Protocol (Lp MAC trongHomeRF), c thit k cho c d liu v ting ni. Chun ny cng c th tng h

vi mng in thoi chuyn mch cng cng v mng Internet. Cc sn phm theochun SWAP hot ng di tn 2,4 GHz s dng FHSS. Cng ngh SWAP btngun t cc tiu chun in thoi khng dy tin tin dng k thut s v chunWLAN IEEE 802.11 hin c. SWAP cho php cung cp cc dch v khng dy mi trong nh, SWAP h tr TDMA ( cung cp thoi tng tc v cc dch v thi gian)

v CSMA/CA ( cung cp truyn thng cc gi s liu tc cao khng ng b).

Bng 4: Cc thng s chnh ca h thng HomeRF

Tham s c t

Tc nhy 50mu/s (cng cc mu nhy nh 802.11)Vng tn s Bng 2,4 GHz ISM

Cng sut v

tuyn pht

20 dBm

Tc s liu 1 Mbps (2-FSK), 2 Mbps (4-FSK)

Vng ph Ti 50m

S lng nt Ti 127 thit b cho mt mng

Cc kt ni thoi Ti 6 phin m thoi song cng c kimtra li

2.5 Tiu chun Bluetooth


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Nhm chuyn trch Bluetooth c thnh lp vo nm 1998 bi cc cng ty ln

(Intel, IBM, Toshiba) v cc cng ty in thoi t bo (Nokia, Ericsson) cung cpkt ni v tuyn gia c s my tnh PC di ng, in thoi t bo v cc thit b in

t khc.

Bluetooth l cng ngh radio phm vi hp kt ni gia cc thit b khng dy.Hotng trong di bng tn ISM (2.4 GHz). Chun ny xc nh mt ng truyn

v tuyn phm vi hp song cng tc 1Mbps kt ni c ti 8 thit b v tuyn cmtay. Phm vi ca Bluetooth ph thuc vo nng lng ca lp radio.

Mng Bluetooth c gi l Piconet. Trng hpn gin n l 2 thit bcni trc tip vi nhau. Mt thit b lMaster(ch), cn thit b kia l Slave (t). ng

dng ch yu l ng dngim- im. y chnh l cu trc Ad- hoc trong mngWLAN. Kt ni Bluetooth l kt ni Ad- hoc in hnh. iu c ngha l mngc thit lp ch cho nhim v hin ti v c g b kt ni sau khi d liu

truyn xong.Cng ngh Bluetooth s dng k thut tri ph nhy tn, ngha l cc gi c

truyn trong nhng tn s khc nhau. Trong hu ht cc quc gia, 79 knh c sdng. Vi t l nhy nhanh (1600 ln nhy trn giy) vic chng nhiut kt qu tt.

2.6 Bng tm tt cc chun

Bng5: Bng tm ttcc chun

Chun Tc truyn dliu

Cc cch

Bo mt Ghi ch

IEEE802.11

Ti a 2

Mbps ti bngtn

2.4 Ghz

FHSS

hayDSSS

WEP &WPA

c ci

tin v mrng802.11b

IEEE80

2.11a (Wi-Fi)

Ti a 54

Mbps ti bngtn

5GHz

OFDM WEP &

WPA

Sn phm

s dng chunny c

chng nhnWi- Fi

Ti a

11Mbps ti DSSS vi WEP &

Sn phm

s dng chun


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IEEE802.11b (Wi-Fi)

bng tn

2.4 GHz

CCK WPA ny c

chng nhnWi- Fi

IEEE802.11g (Wi-Fi)

Ti a 54

Mbps ti bngtn

2.4 Ghz

OFDM

cho tc trn

20 Mbps

DSSS vi

CCK cho tc di 20

Mbps

WEP &WPA

Sn phms dng chunny cchng nhn

Wi- Fi

OpenAir

Tc ta 1.6Mbps

ti bng tn 2.4GHz

FHSS

Gnging 802.11,

khng c cch bo mt

HomeRF

Ti a 10

Mbps ti bngtn

2.4 GHz

FHSS

a ch

IP c lp chomi mng.Dng 56 bitcho m ho d

liu

HiperLAN/1

Ti a 20Mbps ti bng

tn

5 GHz

CSMA/CA

nhdang v m

ho cho misecsion

Ch sdng Chu

u

HiperLAN/2

Ti a

54Mbps tibng tn

5 GHz

OFDM Bo mtcao

Ch s

dng Chuu. ng dngcho mngATM


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CHNG 3: MT S VN BO MT

CHO MNG KHNG DY WI- FI

3.1 Gii thiu

Bo mt l vn ht sc quan trng i vi ngi dng trong tt c cc hthng mng (LAN, WLAN). Nhng do bt ngun t tnh c hu ca mi trngkhng dy. kt ni ti mt mng LAN hu tuyn cn phi truy cp theo ng

truyn bng dy cp, phi kt ni mt PC vo mt cng mng. Vi mng khng dyWi- Fi ch cn c thit b trong vng sng l c th truy cp c nn vn bo mt

cho mng khng dyWi- Fi l cc k quan trng v lm au u nhng ngi s dngmng.

iu khin cho mng hu tuyn l n gin: ng truyn bng cp thng

thng c i trong cc ta nh cao tng v cc port khng s dng c th lm chon disable bng cc ng dng qun l. Cc mng khng dy (hay v tuyn) s dng

sng v tuyn xuyn qua vt liu ca cc ta nh v nh vy s bao ph l khng giihn bn trong mt ta nh. Sng v tuyn c th xut hin trn ng ph, t cc

trm pht t cc mng Wi- Fi ny, v nh vy ai cng c th truy cp nh vo ccthit b thch hp. Do mng khng dy ca mt cng ty cng c th b truy cp t

bn ngoi ta nh cng ty ca h.Hnh 3.1 th hin mt ngi l c th truy cp n mt LAN khng dy t bn

ngoi nh th no. Gii php y l phi lm sao c c s bo mt cho mng

chng c vic truy cp theo kiu ny.


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Hnh 3.1: Mt ngi l truy cp vo mng

Khng ging nh cc h thng hu tuyn c bo v vt l, cc mng v tuynkhng c nh trong mt phm vi. Chng c di chuyn ra xa khong1000 bc chn

ngoi ranh gii ca v tr gc vi mt laptop v mt anten thu. Nhng iu ny lmcho mng Wi- Fi rt d b xm phm.

Bo mt l vn rt quan trng v c bit rt c s quan tm ca nhngdoanh nghip. Khng nhng th, bo mt cng l nguyn nhn khin doanh nghip engi khi ci t mng cc b khng dy WLAN. H lo ngi v nhng im yu trongbo mt WEP (Wired Equivalent Privacy), v quan tm ti nhng gii php bo mtmi thay th an ton hn.

IEEE v Wi-Fi Alliance pht trin cc gii php c tnh bo mt hn l: Bo

v truy cp WPA (Wi-Fi Protected Access), v IEEE 802.11i (hay cn c gi lWPA2), bo mt bng xc thc 802.1x v mt gii php tnh th khc mang tn VPN

Fix cng gip tng cng bo mt mng khng dy cho mi trng mng khng dycc b.

Theo nh Webtorial, WPA v 802.11i c s dng tng ng l 29% v 22%.Mt khc, 42% c s dng cho cc "gii php tnh th" khc nh: bo mt h thngmng ring o VPN (Vitual Private Network) qua mng cc b khng dy.

3.2 Mt s hnh thc tn cng xm nhp mng Wi- Fi ph bin

3.2.1 Tn cng khng qua chng thc

Tn cng khng qua chng thc (Deauthentication attack) l s khai thc gnnh hon ho li nhn dng trong mng 802.11. Trong mng 802.11 khi mt nt mi

gia nhp vo mng n s phi i qua qu trnh xc nhn cng nh cc qu trnh c lin


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quan khc ri sau mi c php truy cp vo mng. Bt k cc nt v tr no

cng c th gia nhp vo mng bng vic s dng kho chia s ti v tr nt bitc mt khu ca mng. Sau qu trnh xc nhn, cc nt s i ti cc qu trnh c

lin quan c th trao i d liu v qung b trong ton mng. Trong sut qu trnhchng thc ch c mt vi bn tin d liu, qun l v iu khin l c chp nhn.

Mt trong cc bn tin mang li cho cc nt kh nng i hi khng qua chng thct mi nt khc. Bn tin c s dng khi mt nt mun chuyn gia hai mng

khng dy khc nhau. V d nu trong cng mt vng tn ti nhiu hn mt mngkhng dy th nt s s dng bn tin ny. Khi mt nt nhn c bn tin khngqua chng thc ny n s t ng ri khi mng v quay tr li trng thi gc ban

u ca n.

Trong tn cng khng qua chng thc, tin tc s s dng mt nt gi mo tmra a ch ca AP ang iu khin mng. Khng qu kh tm ra a ch ca AP bi

n khng c bo v bi thut ton m ho, a ch ca chng c th c tm thynu chng ta lng nghe lu lng gia AP v cc nt khc. Khi tin tc c c a ch

ca AP, chng s gi qung b cc bn tin khng chng thc ra ton mng khin chocc nt trong mng ngay lp tc dng trao i tin vi mng. Sau tt c cc nt

s c kt ni li, chng thc li v lin kt li vi AP tuy nhin do vic truyn cc bntin khng qua chng thc c lp li lin tc khin cho mng ri vo tnh trng b

dng hot ng.

3.2.2 Tn cng truyn li

Tn cng truyn li (Replay Attack) l tin tc ng chn ngang vic truyn thngtin hp l v ri s dng li n. Tin tc khng thay i bn tin m ch gi li n trongthi im thch hp theo s la chn ca tin tc.

Trong mng 802.11, tn cng truyn li to ra kiu tn cng t chi dch v vkhi nt nhnc mt bn tin hp l n s chim dng bng thng v tnh ton thigian gii m bn tin . Cc li d b tn cng nht trong 802.11 rt nhy vi hnhthc tn cng ny l cc bn tin khng c th t mt cch r rng. Trong 802.11

khng c cch no d v loi b cc bn tin b truyn li.

3.2.3 Gi mo AP

Gi mo AP l kiu tn cng man in the middle c in. y l kiu tn cngm tin tc ng gia v trm lu lng truyn gia 2 nt. Kiu tn cng ny rt


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mnh v tin tc c th trm tt c lu lng i qua mng. Rt kh khn to mt

cuc tn cng man in the middle trong mng c dy bi v kiu tn cng ny yucu truy cp thc s n ng truyn. Trong mng khng dy th li rt d b tn

cng kiu ny. Tin tc cn phi to ra mt AP thu ht nhiu s la chn hn AP chnhthng. AP gi ny c th c thit lp bng cch sao chp tt c cc cu hnh ca AP

chnh thng l: SSID, a ch MAC ...

Bc tip theo l lm cho nn nhn thc hin kt ni ti AP gi. Cch th nht li cho ngui dng t kt ni. Cch th hai l gy ra mt cuc tn cng t chi dchv DoS trong AP chnh thng do vy ngui dng s phi kt ni li vi AP gi. Trong

mng 802.11 s la chn AP c thc hin bi cng ca tn hiu nhn. iu duynht tin tc phi thc hin l chc chn rng AP ca mnh c cng tn hiu mnh

hn c. c c iu tin tc phi t AP ca mnh gn ngi b la hn l APchnh thng hoc s dng k thut anten nh hng. Sau khi nn nhn kt ni ti AP

gi, nn nhn vn hot ng nh bnh thng do vy nu nn nhn kt ni n mt APchnh thng khc th d liu ca nn nhn u i qua AP gi. Tin tc s s dng cc

tin ch ghi li mt khu ca nn nhn khi trao i vi Web Server. Nh vy tin tcs c c tt c nhng g anh ta mun ng nhp vo mng chnh thng.

Kiu tn cng ny tn ti l do trong 802.11 khng yu cu chng thc 2 hng

gia AP v nt. AP pht qung b ra ton mng. iu ny rt d b tin tc nghe trmv do vy tin tc c th ly c tt c cc thng tin m chng cn. Cc nt trongmng s dng WEP chng thc chng vi AP nhng WEP cng c nhng l hngc th khai thc. Mt tin tc c th nghe trm thng tin v s dng b phn tch m

ho trm mt khu ca ngi dng.

3.2.4 Tn cng da trn s cm nhn sng mang lp vt l

Tn s l mt nhc im bo mt trong mng khng dy. Mc nguy himthay i ph thuc vo giao din ca lp vt l. C mt vi tham s quyt nh s chung ca mng l: nng lng my pht, nhy ca my thu, tn s RF, bng thngv s nh hng ca anten.

Trong 802.11 s dng thut ton a truy cp cm nhn sng mang (CSMA)

trnh xung t. CSMA l mt thnh phn ca lp MAC. CSMA c s dng chcchn rng s khng c xung t d liu trn ng truyn.. Kiu tn cng ny khng

s dng tp m to ra li cho mng nhng n s li dng chnh chun . Thm chl k thut s dng tri ph tun t trc tip (DSSS), m sa sai FEC hay CRC u v

ch vi kiu tn cng ny.


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C nhiu cch khai thc giao thc cm nhn sng mang vt l. Cch n gin

l lm cho cc nt trong mng u tin tng rng c mt nt ang truyn tin ti thiim hin ti. Cch d nht t c iu ny l to ra mt nt gi mo truyn tin

mt cch lin tc. Mt cch khc l s dng b to tn hiu RF. Mt cch tn cng tinhvi hn l lm cho card mng chuyn vo ch kim tra m n truyn i lin tip

mt mu kim tra. Tt c cc nt trong phm vi ca mt nt gi l rt nhy vi sngmang v trong khi c mt nt ang truyn th s khng c nt no c truyn. Theo

nh tin tc th l kiu rt d b tn cng v n khng i hi thit b c bit.

3.2.5 Gi mo a chi MAC

Trong 802.11 a ch MAC l mt cch ngn ngi dng bt hp php gianhp vo mng. Vic gi a ch MAC l mt nhim v kh d dng i vi tin tc.

Trong khi gi tr c m ho trong phn cng l khng th thay i th gi tr c

a ra trong phn sn (chng trnh c s) ca phn cng li c th thay i c. Cnhiu chng trnh s dng cho cc h iu hnh khc nhau c th thay i c ach MAC c a ra trong b iu hp mng. Th tc ny thc s l rt d v c th

c thc hin trong vi pht. Thm ch sau khi gi a ch MAC tr nn ph bin,802.11 vn cn s dng phng php chng thc ny bi v a ch MAC 48 bit l

di ngn chn cc cuc tn cng vo n. Nhiu chng trnh mi c to ra cho php tin tc vt qua c s kh khn ny. Tin tc khng phi i tm a ch

MAC bi v n c pht qung b ra ton mng do chun 802.11 yu cu nh vy.Ch c mt vi gi tin m tin tc cn chn li ly a ch MAC v do vy bng vic

gi mo a ch MAC tin tc c nhn dng nh mt ngi dng hp php camng.

3.2.6 Tn cng tchi dch v

y l hnh thc tn cng lm cho cc mng khng dy khng th phc v cngi dng, t chi dch v vi nhng ngi dng hp php. Trong mng c dy ccc hnh thc tn cng t chi dch v DoS (Denial of Service) ph bin nh Ping of

Death, SYN Flooding. Cc hnh thc ny da trn c ch ca b giao thc TCP/IP, c

th khin cho my ch b treo. Mng khng dy tn ti nhng im yu tn cngDoS khc vi mng c dy v d nh khi sng radio truyn trong mi trng, n rt d

b nh hng bi cc yu t khch quan cng nh ch quan. Mt k tn cng c thto ra cc sng c cng tn s vi tn s truyn tn hiu gy nhiu cho ngtruyn. iu ny i hi mt b pht sng m bo tn hiu n nh cho mng.


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3.3 Mt s phng php bo mt cho mng khng dy Wi- Fi

An ton truy cp v bo mt cho khng dy Wi- Fi s dng cc phng phpthuc 3 nhm sau:

Firewall, phng php lc - S dng phng php lc gi tin, kha port,lc a ch MAC

Xc thc - S dng cc phng php: VPN Fix (Virtual Private NetworkFix), 802.1x.

M ha d liu truyn - S dng cc phng php: WEP (Wired EquivalentPrivacy), WPA (Wifi Protected Access), 802.11i (WPA2).

Hnh 3.2: M hnh bo mt Wi- Fi

3.3.1 Firewall, cc phng php lc

Lc (Filtering) l mt c ch bo mt cn bn m c th dng b sung cho WEPv/hoc AES. Lc theo ngha en l chn nhng g khng mong mun v cho php

nhng g c mong mun. Filter lm vic ging nh l mt danh sch truy nhp trn

router: bng cch xc nh cc tham s m cc trm phi gn vo truy cp mng.Vi Wi- Fi th vic xc nh xem cc my trm l ai v phi cu hnh nh th no.C ba loi cn bn ca Filtering c th thc hin trn Wi- Fi:

- Lc SSID.

- Lc a ch MAC.


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- Lc giao thc.

Mc ny s miu t mi loi ny l g, n c th lm g cho ngi qun tr v

phi cu hnh n nh th no.

3.3.1.1 Lc SSID

Lc SSID (SSID Filtering) l mt phng php lc s ng nn c dng chohu ht cc iu khin truy nhp. SSID (Service Set Identifier) ch l mt thut ng

khc cho tn mng. SSID ca mt trm Wi- Fi phi khp vi SSID trn AP (ch cs, infracstructure mode) hoc ca cc trm khc (ch c bit, Ad -hoc mode) chng thc v lin kt Client thit lp dch v.

V l do SSID c pht qung b trong nhng bn tin dn ng m AP hoc

cc Station gi ra, nn d dng tm c SSID ca mt mng s dng mt b phn

tch mng, Sniffer. Nhiu AP c kh nng ly cc SSID ca cc khung thng tin dnng (beacon frame). Trong trng hp ny client phi so khp SSID lin kt viAP. Khi mt h thng c cu hnh theo kiu ny, n c gi l h thng ng,

closed system. Lc SSID c coi l mt phng php khng tin cy trong vic hnch nhng ngi s dng tri php ca Wi- Fi. Mt vi loi AP c kh nng g b

SSID t nhng thng tin dn ng hoc cc thng tin kim tra. Trong trng hpny, gia nhp dch v mt trm phi c SSID c cu hnh bng tay trong vic

thit t cu hnh driver.

Mt vi li chung do ngi s dng Wi- Fi to ra khi thc hin SSID l:

- S dng SSID mc nh: S thit lp ny l mt cch khc a ra thng tinca mng. N n gin s dng mt b phn tch mng ly a ch MAC khingun t AP, v sau xem MAC trong bng OUI ca IEEE, bng ny lit k cc tint a ch MAC khc nhau m c gn cho cc nh sn xut. Cch tt nht khc

phc li ny l: Lun lun thay i SSID mc nh.

- Lm cho SSID c g lin quan n cng ty: Loi thit lp ny l mt mohim v bo mt v n lm n gin ha qu trnh mt hacker tm thy v tr vt l ca

cng ty. Khi tm kim Wi- Fi trong mt vng a l c bit th vic tm thy v tr vtl ca cng ty hon thnh mt na cng vic. Khi mt ngi qun tr s dng SSIDm t tn lin quan n tn cng ty hoc t chc, vic tm thy Wi- Fi s l rt ddng. Do hy nh rng: lun lun s dng SSID khng lin quan n Cng ty.


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- S dng SSID nh nhng phng tin bo mt mng Wi - Fi: SSID phic ngi dng thay i trong vic thit lp cu hnh vo mng. N nn c sdng nh mt phng tin phn on mng ch khng phi bo mt, v th hy:

lun coi SSID ch nh mt ci tn mng.

- Khng cn thit qung b cc SSID: Nu AP ca mng c kh nng chuynSSID t cc thng tin dn ng v cc thng tin phn hi kim tra th hy cuhnh chng theo cch . Cu hnh ny ngn cn nhng ngi nghe v tnh khi vicgy ri hoc s dng WLAN.

Khi m truyni chng SSID c tc dng lm cho ngi dng trong mng ddng kt ni ti mng hn. Vic truyn di chng nh vy cng lm cho ngi dng

khc trong khu vc cng kt ni d dng. Mcch ca truyni chng l lm chomng rng m. Nhng phn ln thi gian bn mun Wi- Fi ch c sn i vi mtnhm ngi dng gii hn v v l do th bn nn ngng truyni chng.

Ch l cc nh cung cp AP th li khng quan trng trong vic truyn ichng SSID l vnc bit. V vy m cc AP thng c cu hnh mcnh cho

vic kch hot truyni chng.

Tc dng ca vic tt ch truyn i chng l bn cn phi bit SSID camng trc khi kt ni vi n, nh vy s lm tng tnh bo mt.

3.3.1.2 Lc a ch MAC

Wi- Fi c th lc da vo a ch MAC ca cc trm khch. Hu ht tt c cc

AP, thm ch c nhng ci r tin, u c chc nng lc MAC. Ngi qun tr mngc th bin tp, phn phi v bo tr mt danh sch nhng a ch MAC c php v

lp trnh chng vo cc AP. Nu mt Card PC hoc nhng Client khc vi mt a chMAC m khng trong danh sch a ch MAC ca AP, n s khng th n c im

truy nhp .


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Hnh 3.3: Lc a ch MAC

Tt nhin, lp trnh cc a ch MAC ca cc Client trong mng WLAN vo cc

AP trn mt mng rng th khng thc t. B lc MAC c th c thc hin trn viRADIUS Server thay v trn mi im truy nhp. Cch cu hnh ny lm cho lc MAC

l mt gii php an ton, v do c kh nng c la chn nhiu hn. Vic nhpa ch MAC cng vi thng tin xc nh ngi s dng vo RADIUS kh l n gin,

m c th phi c nhp bng bt c cch no, l mt gii php tt. RADIUS Serverthng tr n cc ngun chng thc khc, v vy cc ngun chng thc khc phic h tr b lc MAC. B lc MAC c th lm vic tt trong ch ngc li. Xt

mt v d, mt ngi lm thu b vic v mang theo c Card Lan khng dy ca h.Card Wlan ny nm gi c cha kha WEP v b lc MAC v th khng th h cn

c quyn s dng. Khi ngi qun tr c th loi b a ch MAC ca my khch ra khi danh sch cho php. Mc d Lc MAC trng c v l mt phng php bo

mt tt, chng vn cn d b nh hng bi nhng thm nhp sau:

- S n trm mt Card PC trong c mt b lc MAC ca AP.

- Vic thm d Wi- Fi v sau gi mo vi mt a ch MAC thm nhp vomng.

Vi nhng mng gia nh hoc nhng mng trong vn phng nh, ni m c mt

s lng nh cc trm khch, th vic dng b lc MAC l mt gii php bo mt hiuqu. V khng mt hacker thng minh no li tn hng gi truy nhp vo mt mngc gi tr s dng thp.

3.3.1.3 Circumventing MAC Filters

a ch MAC ca Client Wi- Fi thng c pht qung b bi cc AP vBridge, ngay c khi s dng WEP. V th mt hacker m c th nghe c lu lng

trn mng ca ta c th nhanh chng tm thy hu ht cc a ch MACm c chophp trn mng khng dy ca ta. mt b phn tch mng thy c a ch MACca mt trm, trm phi truyn mt khung qua on mng khng dy, y chnh lc s a n vic xy dng mt phng php bo mt mng, to ng hm trong

VPN, m s c cp phn sau. Mt vi card PC khng dy cho php thay i ach MAC ca h thng qua phn mm hoc thm ch qua cch thay i cu hnh hthng. Mt hacker c danh sch cc a ch MAC cho php, c th d dng thay ia ch MAC ca card PC ph hp vi mt card PC trn mng ca ta, v do truy

nhp ti ton b mng khng dy ca ta. Do hai trm vi cng a ch MAC khng thng thi tn ti trn mt Wi- Fi, hacker phi tm mt a ch MAC ca mt trm m


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hin thi khng trn mng. Chnh trong thi gian trm di ng hoc my tnh xch tay

khng c trn mng l thi gian m hacker c th truy nhp vo mng tt nht. LcMAC nn c s dng khi kh thi, nhng khng phi l c ch bo mt duy nht trn

my ca ta.

3.3.1.4 Lc giao thc

Mng WLan c th lc cc gi i qua mng da trn cc giao thc lp 2-7.Trong nhiu trng hp, cc nh sn xut lm cc b lc giao thc c th nh hnhc lp cho c nhng on mng hu tuyn v v tuyn ca AP. Tng tng mt

hon cnh, trong mt nhm cu ni khng dy c t trn mt Remote buildingtrong mng Wi- Fi ca mt trng i hc m kt ni li ti AP ca ta nh k thut

trung tm. V tt c nhng ngi s dng trong remote building chia s bng thng5Mbs gia nhng ta nh ny, nn mt s lng ng k cc iu khin trn cc s

dng ny phi c thc hin. Nu cc kt ni ny c ci t vi mc ch c bitca s truy nhp Internet ca ngi s dng, th b lc giao thc s loi tr tt c cc

giao thc, ngoi tr SMTP, POP3, HTTP, HTTPS, FTP. . .

Hnh 3.4: Lc giao thc

3.3.2 Xc thc

3.3.2.1 Phng php VPN Fix

Phng php ny ch c xem nh l mt gii php tnh th v khi nhn ra syu km ca WEP, nhng ngi s dng doanh nghip khm ph ra mt cch hiu

qu bo v mng khng dy Wi- Fi ca mnh, c gi l VPN Fix. tng c bnca phng php ny l coi nhng ngi s dng Wi- Fi nh nhng ngi s dng

dch v truy cp t xa.

Trong cch cu hnh ny, tt cc nhng im truy cp Wi- Fi, v cng nh ccmy tnh c kt ni vo cc im truy cp ny, u c nh ngha trong mt mng


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LAN o (Vitual LAN). Trong c s h tng bo mt, cc thit b ny c i x nh

l "khng tin tng". Trc khi bt c cc thit b Wi- Fi c kt ni, chng s phic s cho php t thnh phn bo mt ca mng LAN. D liu cng nh kt ni ca

cc thit b s phi chy qua mt my ch xc thc nh RADIUS chng hn... Tip ,kt ni s c thit lp thnh mt tuyn kt ni bo mt c m ho bi mt giao

thc bo mt v d nh IPSec, ging nh khi s dng cc dch v truy cp t xa quaInternet.

Tuy nhin, gii php ny cng khng phi l hon ho, VPN Fix cn lu lngVPN ln hn cho tng la, v cn phi to cc th tc khi to cho tng ngi s

dng. Hn na, IPSec li khng h tr nhng thit b c nhiu chc nng ring nhthit b cm tay, my qut m vch... Cui cng, v quan im kin trc mng, cu

hnh theo VPN ch l mt gii php tnh th.

3.3.2.2 Phng php 802.1x

Chun WLAN 802.11 khng c s xc nhn thng minh, v vy chun cngnghip thng qua giao thc 802.1x cho s xc nhn ca n. 802.1x a ra cch thciu khin truy cp mng c bn, n s dng EAP (Extensible AuthenticationProtocol) v RADIUS server. 802.1x khng a ra giao thc xc nhn mt cch c th

nhng ch r EAP trong vic h tr s lng cc giao thc xc nhn nh l CHAP-MD5, TLS v Kerberos. EAP c th c m rng v vy cc giao thc xc nhn mic th c h tr nh trong cc phin bn sau ca n.EAP c a ra hot ngtrn giao thc Point- to- Point (PPP); n tng thch vi cc giao thc ca lp lin

kt d liu khc (nh l Token Ring 802.5 hay Wireless LANs 802.11) EAP OverLANs (EAPOL) c pht trin.:

Hnh 3.5: M hnh xc nhn


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802.1x EAP-TLS c s dng trong cc mi trng c bn v an ton cao. S

trao i ca cc message EAP-TLS cung cp s xc nhn ln nhau, s bt tay ca giaothc m ha v s trao i kha bo v gia mt client Wi- Fi v mng. EAP-TLS l

mt k thut cung cp cc kha m ha ng cho ngi dng v session. iu ny cithin mt cch ng k v vt qua nhiu im yu trong cc mng khng dy.

Hnh di y ch ra mt chui cc s kin xut hin khi mt Client c xc

nhn bng 802.1x EAP-TLS. Hai chng ch digital c yu cu y: mt trnRADIUS server (v d EAS) v mt trn Client khng dy. Ch rng s truy cpkhng dy c cung cp cho ti khi s xc nhn thnh cng v cc kha WEP ng

c thit lp.

Hnh 3.6: Xc nhn 802.1x EAP-TLS

802.1x EAP-TLS vi EAS trong Controller Mode c th hin trn hnh sau. Clientkhng dy c chng ch digital (c ci t t trc). Client khng dy truyn thng viEAS thng qua AP. Tt c ba thnh phn (Wireless client, AP v EAS) h tr qu trnh802.1x EAP-TLS. Client khng dy c th s dng Windows XP (c xy dng h trcho 802.1x EAP-TLS) hay Windows 98/Me/2000 bng vic s dng Madge Wireless LANUtility (WLU). Khi xc nhn, d liu ngi dng cng c th c s dng EAS m ccu hnh trong Gateway Mode.


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Hnh 3.7: 802.1x EAP-TLS trong controller mode

3.3.3 M ha d liu truyn3.3.3.1 WEP

Khi thit k cc yu cu k thut cho mng khng dy, chun 802.11 ca IEEE

tnh n vn bo mt d liu ng truyn qua phng thc m ha WEP.Phng thc ny c a s cc nh sn xut thit b khng dy h tr nh mt

phng thc bo mt mc nh. Tuy nhin, nhng pht hin gn y v im yu cachun 802.11 WEP gia tng s nghi ng v mc an ton ca WEP v thc y

s pht trin ca chun 802.11i. Tuy vy, a phn cc thit b khng dy hin ti vang s dng WEP v n s cn tn ti kh lu trc khi chun 802.11i trin khai rng

ri.

Giao thc WEP

WEP (Wired Equivalent Privacy) ngha l bo mt tng ng vi mng c dy(Wired LAN). Khi nim ny l mt phn trong chun IEEE 802.11. Theo nh ngha,WEP c thit k m bo tnh bo mt cho mng khng dy t mc nh

mng ni cp truyn thng. i vi mng LAN (nh ngha theo chun IEEE 802.3),bo mt d liu trn ng truyn i vi cc tn cng bn ngoi c m bo quabin php gii hn vt l, tc l hacker khng th truy xut trc tip n h thng

ng truyn cp. Do chun 802.3 khng t ra vn m ha d liu chng licc truy cp tri php. i vi chun 802.11, vn m ha d liu c u tin hngu do c tnh ca mng khng dy l khng th gii hn v mt vt l truy cp nng truyn, bt c ai trong vng ph sng u c th truy cp d liu nu khng c bo v.


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c gi n my nhn dng khng m ha. My nhn s s dng gi tr IV v kha

gii m gi d liu.

Cch s dng gi tr IV l ngun gc ca a s cc vn vi WEP. Do gi trIV c truyn i dng khng m ha v t trong header ca gi d liu 802.11 nn

bt c ai ly c d liu trn mng u c th thy c. Vi di 24 bit, gi tr caIV dao ng trong khong 16.777.216 trng hp. Nhng chuyn gia bo mt ti i

hc California-Berkeley pht hin ra l khi cng gi tr IV c s dng vi cngkha trn mt gi d liu m ha (va chm IV), hacker c th bt gi d liu v tm rac kha WEP. Thm vo , ba nh phn tch m ha Fluhrer, Mantin v Shamir

(FMS) pht hin thm nhng im yu ca thut ton to IV cho RC4. FMS vch ra mt phng php pht hin v s dng nhng IV li nhm tm ra kha WEP.

Thm vo , mt trong nhng mi nguy him ln nht l nhng cch tn cngdng hai phng php nu trn u mang tnh cht th ng. C ngha l k tn cng

ch cn thu nhn cc gi d liu trn ng truyn m khng cn lin lc vi AccessPoint. iu ny khin kh nng pht hin cc tn cng tm kha WEP y kh khn vgn nh khng th pht hin c.

Hin nay, trn Internet sn c nhng cng c c kh nng tm kha WEP nh

AirCrack , AirSnort, dWepCrack, WepAttack, WepCrack, WepLab. Tuy nhin, sdng nhng cng c ny i hi nhiu kin thc chuyn su v chng cn c hn ch

v slng gi d liu cn bt c.

Gii php WEP ti u

Vi nhng im yu nghim trng ca WEP v s pht tn rng ri ca cc cngc d tm kha WEP trn Internet, giao thc ny khng cn l gii php bo mt c

chn cho cc mng c mc nhy cm thng tin cao. Tuy nhin, trong rt nhiu ccthit b mng khng dy hin nay, gii php bo mt d liu c h tr ph bin vn

l WEP. D sao i na, cc l hng ca WEP vn c th c gim thiu nu ccu hnh ng, ng thi s dng cc bin php an ninh khc mang tnh cht h tr.

gia tng mc bo mt cho WEP v gy kh khn cho hacker, cc bin

php sau c ngh:

S dng kha WEP c di 128 bit: Thng cc thit b WEP cho php cuhnh kha ba di: 40 bit, 64 bit, 128 bit. S dng kha vi di 128 bit gia tngs lng gi d liu hacker cn phi c phn tch IV, gy kh khn v ko di thigian gii m kha WEP. Nu thit b khng dy ch h tr WEP mc 40 bit (thng

gp cc thit b khng dy c), cn lin lc vi nh sn xut ti v phin bn cpnht firmware mi nht.


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Thc thi chnh sch thay i kha WEP nh k: Do WEP khng h tr phng

thc thay i kha t ng nn s thay i kha nh k s gy kh khn cho ngi sdng. Tuy nhin, nu khng i kha WEP thng xuyn th cng nn thc hin t

nht mt ln trong thng hoc khi nghi ng c kh nng b l kha.

S dng cc cng c theo di s liu thng k d liu trn ng truyn khngdy: Do cc cng c d kha WEP cn bt c s lng ln gi d liu v hacker c

th phi s dng cc cng c pht sinh d liu nn s t bin v lu lng d liu cth l du hiu ca mt cuc tn cng WEP, nh ng ngi qun tr mng pht hinv p dng cc bin php phng chng kp thi.

3.3.3.2 WPA (Wifi Protected Access)

Wi-Fi Alliance a ra gii php gi l Wi-Fi Protected Access (WPA). Mttrong 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 migi tin. Cc cng c thu thp cc gi tin ph kho m ho u khng th thc hinc vi WPA. Bi WPA thay i kho lin tc nn hacker khng bao gi thu thp d liu mu 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 thayi 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 truycp 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. Theo Wi-Fi Alliance, c khong 200 thit b ccp chng nhn tng thch WPA.

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 cho doanhnghip 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, v c coi l loi tr mi l hng db tn cng ca WEP, nhng ngi s dng vn khng thc s tin tng vo WPA. C

mt l hng trong WPA v li ny ch xy ra vi WPA Personal. Khi m s dng hmthay i kho TKIP c s dng to ra cc kho m ho b pht hin, nu hacker


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c th on c kho khi to hoc mt phn ca mt khu, h c th xc nh c

ton b mt khu, do c th gii m c d liu. Tuy nhin, l hng ny cng sb loi b bng cch s dng nhng kho khi to khng d on (ng s dng

nhng t nh "PASSWORD" lm mt khu).

iu ny cng c ngha rng k thut TKIP ca WPA ch l gii php tm thi ,cha cung cp mt phng thc bo mt cao nht. WPA ch thch hp vi nhng cng

ty m khng truyn d liu "mt" v nhng thng mi, hay cc thng tin nhy cm...WPA cng thch hp vi nhng hot ng hng ngy v mang tnh th nghim cngngh.

3.3.3.3 802.11i (WPA2)

Mt gii php v lu di l s dng 802.11i tng ng vi WPA2, c chngnhn bi Wi-Fi Alliance. Chun ny s dng thut ton m ho mnh m v c gil Chun m ho nng cao AES (Advanced Encryption Standard). AES s dng thut

ton m ho i xng theo khi Rijndael, s dng khi m ho 128 bit, v 192 bithoc 256 bit.

nh gi chun m ho ny, Vin nghin cu quc gia v Chun v Cng

ngh ca M, NIST (National Institute of Standards and Technology), thng quathut ton m i xng ny. V chun m ho ny c s dng cho cc c quan

chnh ph M bo v cc thng tin nhy cm.

Trong khi AES c xem nh l bo mt tt hn rt nhiu so vi WEP 128 bit

hoc 168 bit DES (Digital Encryption Standard). m bo v mt hiu nng, qutrnh m ho cn c thc hin trong cc thit b phn cng nh tch hp vo chip.Tuy nhin, rt t ngi s dng mng khng dy quan tm ti vn ny. Hn na,hu ht cc thit b cm tay Wi-Fi v my qut m vch u khng tng thch vichun 802.11i.

Mt s sai lm ph bin v bo mt cho mng Wi- Fi:

C 5 ngi dng mng khng dy ti nh th c n 4 ngi khng kch hot bt

k ch bo mt no. Mc nh, cc nh sn xut tt ch bo mt cho vic thit

lp ban u c d dng, khi s dng phi m li. Tuy nhin, cn phi cn thn khikch hot tnh nng bo mt, di y l mt s sai lm thng gp phi.

Khng thay i mt khu ca nh sn xut. Khi ln u tin ci t router khngdy (AP router) hay Access Point, chng ta rt d qun thay i mt khu mc nh

ca nh sn xut. Nu khng thay i, c th ngi khc s dng mt khu mc nh


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truy cp vo router v thay i cc thit lp thoi mi truy cp vo mng. V vy

nn lun thay i mt khu mc nh.

Khng kch hot tnh nng m ha. Nu khng kch hot tnh nng m ha,chng ta s qung b mt khu v e-mail ca mnh n bt c ai trong tm ph sng,

ngi khc c th c tnh dng cc phm mm nghe ln min ph nh AirSnort(airsnort.shmoo.com) ly thng tin ri phn tch d liu. V vy hy bt ch m

ha truyn d liu an ton.

Khng kim tra ch bo mt. Chng ta mua mt router khng dy, kt niInternet bng rng, lp c my in vo, ri c th mua thm nhiu thit b khng dykhc na. C th vo mt ngy no , my in s t ng in ht giy bi v chng ta

khng thit lp cc tnh nng bo mt. V vy khng nn cho rng mng ca chng ta an ton. Hy nh nhng ngi am hiu kim tra h.

Qu tch cc vi cc thit lp bo mt. Mi card mng khng dy u c mt a

ch phn cng (a ch MAC) m router khng dy c th dng kim sot nhngmy tnh no c php ni vo mng. Khi bt ch lc a ch MAC, c kh nng

chng ta s qun thm a ch MAC ca my tnh chng ta ang s dng vo danhsch, nh th s t c lp chnh mnh, tng t nh b cha kha trong xe hi ri cht

ca li. V vy phi kim tra cn thn khi thit lp tnh nng bo mt.

Cho php mi ngi truy cp. C th chng ta l ngi u tin c mng khngdy v mun 'khoe' bng cch t tn mng l 'truy cp thoi mi' chng hn. Hngxm c th dng kt ni ny ti rt nhiu phim nh v mng s tr nn chm chp.V vy mng khng dy gip chia s kt ni Internet d dng, tuy nhin, ng b ng

v s c ngi lm dng.

3.4 Bo mt trong thc t

Mt cch tng qut, c s tho hip gia kh nng bo mt v gi thnh snphm, hiu nng v cch s dng n gin. Mng c bo mt tt hn th chi ph thchin s ln hn, c kh nng l kh khn hn khi s dng v lo ngi v hiu nng

mng thp hn. Trong mi mi trng, tu thuc vo mc yu cu ca ngi s

dng c nhng i hi nht nh v mc bo mt, y xem xt mt vi trnghpnh sau:

3.4.1 Khu vc nh v vn phng nh Yu cu an ninh thp

i vi ngi s dng trong phm vi nh v vn phng nh chi ph trin khaimng l mt vn , ngi s dng c cc server nhn thc trung tm hoc cc thnh


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phn h tng trung tm khc. H thng khng c ngun ti nguyn thng tin no kh

dng. Cc vic lm sau y cho php bo v an ninh cp 1:

Phn mm: Cp nht cc NIC v cc im truy nhp ti phn mm g ny nht. Khi ng khi mua thit b v kim tra thng xuyn.

Kch hot WEP: Thit lp kch thc kho mt m cao nht c th c.Trong khi vn tn ti cc im yu th i vi cc hacker mc trung bnh vic b gyWEP l tng i kh khn nht l trong cc mi trng m lu lng truyn ti

thp.

Thay i tn mc nh SSID thnh mt tn duy nht. Khng s dng cc

tn gi c gi nh tn ng ph hay a ch, cng ty hay nh ring.

Khng kch hot SSID qung b: Windows XP v cc cng c gim stkhc s t ng d tm cc khung n hiu 802.11 thu c SSID. Khi qu trnhqung b SSID b ngt, im truy nhp s khng cha SSID trong mt khung n hiulm cho hu ht cc cng c d tm tr nn v dng.

Lc a ch MAC: Mt vi im truy nhp c kh nng chp nhn cc kt

ni ch i vi cc a ch MAC ng tin cy l cc a ch duy nht trn mng (khngtrng khp nhau). Thc hin iu ny l rt kh khn trong mt mi trng ng vi

hn 20 ngi s dng do vic thit lp im truy nhp rt mt thi gian i vi tt ccc khch hng tin cy. Tuy nhin, n c th c thit lp mt cch n gin trong

mi trng nh v mi trng vn phng nh.

Cc tng la phn cng c lp mt mng WLAN v gi chng trnh

xa ngi s dng khng c php.

S dng WPA khi kh dng. Hu ht cc nh cung cp thit b s dng

WPA thng qua qu trnh nng cp phn mm.

3.4.2 Vn phng nhv ngi dng xa Yu cu an ninh trung bnh

Cc vn phng nh v ngi dng xa c th l mt phn ca mt t chc lnhn. m bo an ninh thng tin l vn quan trng v phi cn bng vi nng sut

tng v s liu ln hn nhng ngi s dng.

Phn mm: Cp nht NIC v cc im truy nhp ti phn mm gn nht.Khi ng khi mua thit b v kim tra thng xuyn.


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Kch hot WEP: Thit lp kch thc kho m ho cao nht c th c.

Trong khi vn tn ti nhng im yu th i vi cc hacker mc trung bnh vic bgy WEP vn cn kh khn nht l trong cc mi trng lu lng d liu thp.

VPN: Trong trng hp c th c s dng cc im truy nhp kch hot

IPSec hoc cc tng la m chng c th thit lp cc ng ng VPN t ngi dngu cui ti im truy nhp. Phn mm VPN cho php qu trnh nhn thc v m h

Mang Khong Day Wlan 852

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