Bao Cao Wireless

8/2/2019 Bao Cao Wireless

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LI NI U

Wireless Lan l mt trong nhng cng ngh truyn thng khng dy

c p dng cho mng cc b. S ra i ca n khc phc nhng hn

ch m mng ni dy khng th gii quyt c, v l gii php cho xu

th pht trin ca cng ngh truyn thng hin i. Ni nh vy thy

c nhng li ch to ln m Wireless Lan mang li, tuy nhin n khng

phi l gii php thay th ton b cho cc mng Lan ni dy truyn thng.

Da trn chun IEEE 802.11 mng WLan i n s thng nht

v tr thnh mng cng nghip, t c p dng trong rt nhiu lnh

vc, t lnh vc chm sc sc khe, bn l, sn xut, lu kho, n cc

trng i hc. Ngnh cng nghip ny kim li t vic s dng cc

thit b u cui v cc my tnh notebook truyn thng tin thi gian

thc n cc trung tm tp trung x l. Ngy nay, mng WLAN ang

c n nhn rng ri nh mt kt ni a nng t cc doanh nghip. Litc ca th trng mng WLAN ngy cng tng.

V vy, nhm chng em chn ti tm hiu cng ngh

Wirelesss Lan.

Nhm sinh vin thc hin


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CHNG I

GII THIU V MNG WLAN

Mng WLAN l mt h thng thng tin lin lc d liu linh hot c thc hinnh phn m rng, hoc thay th cho mng LAN hu tuyn trong nh hoc trong cc cquan. S dng sng in t, mng WLAN truyn v nhn d liu qua khong khng, tigin nhu cu cho cc kt ni hu tuyn. Nh vy, mng WLAN kt ni d liu vingi dng lu ng, v thng qua cu hnh c n gin ha, cho php mng LAN ding.

Cc nm qua, mng WLAN c ph bin mnh m trong nhiu lnh vc, t lnhvc chm sc sc khe, bn l, sn xut, lu kho, n cc trng i hc. Ngnh cngnghip ny kim li t vic s dng cc thit b u cui v cc my tnh notebook truyn thng tin thi gian thc n cc trung tm tp trung x l. Ngy nay, mngWLAN ang c n nhn rng ri nh mt kt ni a nng t cc doanh nghip. Litc ca th trng mng WLAN ngy cng tng.

1.1 Cc ng dng ca Mng WLAN

Mng WLAN l k thut thay th cho mng LAN hu tuyn, n cung cp mngcui cng vi khong cch kt ni ti thiu gia mt mng xng sng v mng trongnh hoc ngi dng di ng trong cc c quan. Sau y l cc ng dng ph bin caWLAN thng qua sc mnh v tnh linh hot ca mng WLAN:

Trong cc bnh vin, cc bc s v cc h l trao i thng tin v bnh nhn mtcch tc thi, hiu qu hn nh cc my tnh notebook s dng cng ngh mngWLAN.

Cc i kim ton t vn hoc k ton hoc cc nhm lm vic nh tng nngsut vi kh nng ci t mng nhanh.

Nh qun l mng trong cc mi trng nng ng ti thiu ha tng ph i li,

b sung, v thay i vi mng WLAN, do gim bt gi thnh s hu mng LAN. Cc c s o to ca cc cng ty v cc sinh vin cc trng i hc s dngkt ni khng dy d dng truy cp thng tin, trao i thng tin, v nghin cu.

Cc nh qun l mng nhn thy rng mng WLAN l gii php c s h tngmng li nht lp t cc my tnh ni mng trong cc ta nh c.

Nh qun l ca cc ca hng bn l s dng mng khng dy n gin havic ti nh cu hnh mng thng xuyn.

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Cc nhn vin vn phng chi nhnh v trin lm thng mi ti gin cc yu cuci t bng cch thit t mng WLAN c nh cu hnh trc khng cn cc nhqun l mng a phng h tr.

Cc cng nhn ti kho hng s dng mng WLAN trao i thng tin n c

s d liu trung tm v tng thm nng sut ca h.

Cc nh qun l mng thc hin mng WLAN cung cp d phng cho ccng dng trng yu ang hot ng trn cc mng ni dy.

Cc i l dch v cho thu xe v cc nhn vin nh hng cung cp dch v nhanhhn ti khch hng trong thi gian thc.

Cc cn b cp cao trong cc phng hi ngh cho cc quyt nh nhanh hn v hs dng thng tin thi gian thc ngay ti bn hi ngh.

1.2 Cc li ch ca mng WLAN tin tng cao trong ni mng ca cc doanh nghip v s tng trng mnh

m ca mng Internet v cc dch v trc tuyn l bng chng mnh m i vi li chca d liu v ti nguyn dng chung. Vi mng WLAN, ngi dng truy cp thng tindng chung m khng tm kim ch cm vo, v cc nh qun l mng thit lp hoc

b sung mng m khng lp t hoc di chuyn dy ni. Mng WLAN cung cp cchiu sut sau: kh nng phc v, tin nghi, v cc li th v chi ph hn hn cc mngni dy truyn thng.

Kh nng lu ng ci thin hiu sut v dch v - Cc h thng mng WLANcung cp s truy cp thng tin thi gian thc ti bt c u cho ngi dng mngtrong t chc ca h. Kh nng lu ng ny h tr cc c hi v hiu sut vdch v m mng ni dy khng th thc hin c.

n gin v tc nhanh trong ci t- Ci t h thng mng WLAN nhanhv d dng v loi tr nhu cu ko dy qua cc tng v cc trn nh.

Linh hot trong ci t- Cng ngh khng dy cho php mng i n cc nim mng ni dy khng th.

Gim bt gi thnh s hu - Trong khi u t ban u ca phn cng cn chomng WLAN c gi thnh cao hn cc chi ph phn cng mng LAN hu tuyn,nhng chi ph ci t ton b v gi thnh tnh theo tui th thp hn ng k.Cc li ch v gi thnh tnh theo tui th l ng k trong mi trng nng ngyu cu thng xuyn di chuyn, b sung, v thay i.

Tnh linh hot- Cc h thng mng WLAN c nh hnh theo cc kiu topokhc nhau p ng cc nhu cu ca cc ng dng v cc ci t c th. Cuhnh mng d thay i t cc mng c lp ph hp vi s nh ngi dng ncc mng c s h tng vi hng nghn ngi s dng trong mt vng rng ln.

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Kh nng v hng:cc mng my tnh khng dy c th c cu hnh theocc topo khc nhau p ng cc nhu cu ng dng v lp t c th. Cc cuhnh d dng thay i t cc mng ngang hng thch hp cho mt s lng nhngi s dng n cc mng c c s h tng y dnh chohng nghn ngi

s dng m c kh nng di chuyn trn mt vng rng.1.3 Bng so snh u v nhc im gia mng khng dy v c dy:

1. Phm vi ng dng

Mng c dy Mng khng dy- C th ng dng trong tt c cc m

hnh mng nh, trung bnh, ln, rt ln

- Gp kh khn nhng ni xa xi, a

hnh phc tp, nhng ni khng n nh,

kh ko dy, ng truyn

- Ch yu l trong m hnh mng nh v

trung bnh, vi nhng m hnh ln phi

kt hp vi mng c dy

- C th trin khai nhng ni khng

thun tin v a hnh, khng n nh,

khng trin khai mng c dy c

2. phc tp k thut

Mng c dy Mng khng dy- phc tp k thut ty thuc tng

loi mng c th

- phc tp k thut ty thuc tng

loi mng c th

- Xu hng to kh nng thit lp cc

thng s truyn sng v tuyn ca thit b

ngy cng n gin hn

3. tin cy

Mng c dy Mng khng dy- Kh nng chu nh hng khch quan

bn ngoi nh thi tit, kh hu tt

- Chu nhiu cuc tn cng a dng, phc

tp, nguy him ca nhng k ph hoi v

tnh v c tnh

- t nguy c nh hng sc khe

- B nh hng bi cc yu t bn ngoi

nh mi trng truyn sng, can nhiudo thi tit

- Chu nhiu cuc tn cng a dng, phc

tp, nguy him ca nhng k ph hoi v

tnh v c tnh, nguy c cao hn mng c

dy

- Cn ang tip tc phn tch v kh nng

nh hng n sc khe

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4. Lp t, trin khai

Mng c dy Mng khng dy- Lp t, trin khai tn nhiu thi gian

v chi ph

- Lp t, trin khai d dng, n gin,

nhanh chng

5. Tnh linh hot, kh nng thay i, pht trin

Mng c dy Mng khng dy- V l h thng kt ni c nh nn tnh

linh hot km, kh thay i, nng cp,

pht trin

- V l h thng kt ni di ng nn rt

linh hot, d dng thay i, nng cp,

pht trin

6. Gi c

Mng c dy Mng khng dy- Gi c ty thuc vo tng m hnh

mng c th

- Thng th gi thnh thit b cao hn so

vi ca mng c dy. Nhng xu hng

hin nay l cng ngy cng gim s

chnh lch v gi

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CHNG II

NGUYN TC HOT NG CA WLAN

2.1 Cch lm vic ca mng WLANMng WLAN s dng sng in t (v tuyn v tia hng ngoi) truyn thng

tin t im ny sang im khc m khng da vo bt k kt ni vt l no. Cc sngv tuyn thng l cc sng mang v tuyn bi v chng thc hin chc nng phn phtnng lng n gin ti my thu xa. D liu truyn c chng ln trn sng mangv tuyn n c nhn li ng my thu. l s iu bin sng mang theo thngtin c truyn. Mt khi d liu c chng (c iu ch) ln trn sng mang vtuyn, th tn hiu v tuyn chim nhiu hn mt tn s n, v tn s hoc tc

truyn theo bit ca thng tin bin iu c thm vo sng mang.Nhiu sng mang v tuyn tn ti trong cng khng gian ti cng mt thi im

m khng nhiu vi nhau nu chng c truyn trn cc tn s v tuyn khc nhau. nhn d liu, my thu v tuyn bt sng (hoc chn) mt tn s v tuyn xc nh trongkhi loi b tt c cc tn hiu v tuyn khc trn cc tn s khc.

Trong mt cu hnh mng WLAN tiu biu, mt thit b thu pht, c gi mtim truy cp (AP - access point), ni ti mng ni dy t mt v tr c nh s dngcp Ethernet chun. im truy cp (access point) nhn, lu vo b nh m, v truynd liu gia mng WLAN v c s h tng mng ni dy. Mt im truy cp n h tr

mt nhm nh ngi s dng v vn hnh bn trong mt phm vi vi mt ti vi chcmt. im truy cp (hoc anten c gn ti n) thng thng c gn trn cao nhngthc t c gn bt c ni u min l khong v tuyn cn thu c.

Cc ngi dng u cui truy cp mng WLAN thng qua cc card giao tipmng WLAN, m c thc hin nh cc card PC trong cc my tnh notebook, hocs dng card giao tip ISA hoc PCI trong cc my tnh bn, hoc cc thit b tchhp hon ton bn trong cc my tnh cm tay. Cc card giao tip mng WLAN cungcp mt giao din gia h iu hnh mng (NOS) v sng tri (qua mt anten). Bncht ca kt ni khng dy l trong sut vi NOS.

2.2 Cc cu hnh mng WLANMng WLAN n gin hoc phc tp. C bn nht, hai PC c trang b cc

card giao tip khng dy thit lp mt mng c lp bt c khi no m chng nm trongphm vi ca nhau. N c gi l mng ngang hng. Cc mng ny khng yu cu squn tr hoc s nh cu hnh trc. Trong trng hp ny mi khch hng ch truy cpti ti nguyn ca khch hng khc v khng thng qua mt nh phc v trung tm.

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Hnh 2.1. Mt mng ngang hng khng dy

Vic thit lp mt im truy cp m rng phm vi ca mt mng, phm vi ccthit b lin lc c m rng gp i. Khi im truy cp c ni ti mng ni dy,mi khch hng s truy cp ti cc ti nguyn phc v cng nh ti cc khch hngkhc. Mi im truy cp iu tit nhiu khch hng, s khch hng c th ph thuc vos lng v c tnh truyn. Nhiu ng dng thc t vi mt im truy cp phc v t15 n 50 thit b khch hng.

Hnh 2.2. Khch hng v im truy nhp

Cc im truy cp c mt phm vi hu hn, 152,4m trong nh v 304,8m ngoitri. Trong phm vi rt ln hn nh kho hng, hoc khu vc c quan cn thit phi lpt nhiu im truy cp hn. Vic xc nh v tr im truy da trn phng php khost v tr. Mc ch s ph ln vng ph sng bng cc cell ph sng chng lp nhau cc khch hng di chuyn khp vng m khng mt lin lc mng. Kh nng cc khchhng di chuyn khng ghp ni gia mt cm ca cc im truy cp c gi roaming.Cc im truy cp chuyn khch hng t site ny n site khc mt cch t ng mkhch hng khng hay bit, bo m cho kt ni lin tc.

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Hnh 2.3. Nhiu im truy cp v Roaming

gii quyt cc vn c bit v topology, nh thit k mng chn cch sdng cc im m rng (Extension Point - EP) lm tng cc im truy cp ca mng.Cch nhn v chc nng ca cc im m rng ging nh cc im truy cp, nhng

chng khng c ni dy ti mng ni dy nh l cc AP. Chc nng ca EP nhmm rng phm vi ca mng bng cch lm tr tn hiu t mt khch hng n mt APhoc EP khc. Cc EP c ni tip nhau truyn tin t mt AP n cc khch hngrng khp, nh mt on ngi chuyn nc t ngi ny n ngi khc n mt mchy.

Hnh 2.4. Cch s dng ca mt im m rng (EP)

Thit b mng WLAN cui cng cn xem xt l anten nh hng. Gi s c mtmng WLAN trong ta nh A ca bn, v bn mun m rng n ti mt ta nh chothu B, cch 1,609 km. Mt gii php l s lp t mt anten nh hng trn mita nh, cc anten hng v nhau. Anten ti ta nh A c ni ti mng ni dy quamt im truy cp. Tng t, anten ti ta nh B c ni ti mt im truy cp trongta nh , m cho php kt ni mng WLAN thun tin nht.

Hnh 2.5. Cch s dng anten nh hng

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2.2.1 Mng WLAN c lp (mng ngang hng)

Cu hnh mng WLAN n gin nht l mng WLAN c lp (hoc ngang hng)ni cc PC vi cc card giao tip khng dy. Bt k lc no, khi hai hoc hn card giaotip khng dy nm trong phm vi ca nhau, chng thit lp mt mng c lp (hnh

1.6). y, cc mng ny khng yu cu s qun tr hoc s nh cu hnh trc.

Hnh 2.6. Mng WLAN c lp Hnh 2.7. Mng WLAN c lp phm

vi c m rng s dng im truy

cp nh mt b chuyn tip

Cc im truy cp m rng phm vi ca mng WLAN c lp bng cch ngvai tr nh l mt b chuyn tip (hnh 1.7), c hiu qu gp i khong cch gia cc

PC khng dy.2.2.2. Mng WLAN c s h tng (infrastructure)

Trong mng WLAN c s h tng, nhiu im truy cp lin kt mng WLANvi mng ni dy v cho php cc ngi dng chia s cc ti nguyn mng mt cchhiu qu. Cc im truy cp khng cc cung cp cc truyn thng vi mng ni dy mcn chuyn tip lu thng mng khng dy trong khu ln cn mt cch tc thi. Nhiuim truy cp cung cp phm vi khng dy cho ton b ta nh hoc khu vc c quan.

Hnh 2.8. Mng WLAN C s h tng

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2.2.3 Microcells v roaming

Thng tin v tuyn b gii hn bi tn hiu sng mang i bao xa khi cng sut ra cho trc. Mng WLAN s dng cc cell, gi l cc microcell, tng t h thngin thoi t bo m rng phm vi ca kt ni khng dy. Ti bt k im truy cp

no trong cng lc, mt PC di ng c trang b vi mt card giao tip mng WLANc lin kt vi mt im truy cp n v microcell ca n, hoc vng ph sng. Ccmicrocell ring l chng lp cho php truyn thng lin tc bn trong mng ni dy.Chng x l cc tn hiu cng sut thp v khng cho ngi dng truy c p khi h i quamt vng a l cho trc.

Hnh 2.9. Handing off gia cc im truy cp

2.3 Cc ty chn cng nghCc nh sn xut mng WLAN chn nhiu cng ngh mng khc nhau khi thit

k gii php mng WLAN. Mi cng ngh c cc thun li v hn ch ring.

2.3.1 Tri ph

a s cc h thng mng WLAN s dng cng ngh tri ph, mt k thut tns v tuyn bng rng m trc y c pht trin bi qun i trong cc h thngtruyn thng tin cy, an ton, trng yu. S tri ph c thit k hiu qu vi s nhi di thng ly tin cy, kh nng tch hp, v bo mt. Ni cch khc, s dngnhiu bng thng hn trng hp truyn bng hp, nhng i li to ra tn hiu mnhhn nn d c pht hin hn, min l my thu bit cc tham s ca tn hiu tri phca my pht. Nu mt my thu khng chnh ng tn s, th tn hiu tri ph ging nhnhiu nn. C hai kiu tri ph truyn i bng v tuyn: nhy tn v chui trc tip.

2.3.2 Cng ngh tri ph nhy tn (Frequency Hopping pread Spectrum)

Tri ph nhy tn (FHSS) s dng mt sng mang bng hp thay i tn strong mt mu c my pht ln my thu. c ng b chnh xc, hiu ng mng sduy tr mt knh logic n. i vi my thu khng mong mun, FHSS lm xut hincc nhiu xung chu k ngn.

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Hnh 2.10. Tri ph nhy tn

FHSS nhy tn t bng hp sang bng hp bn trong mt bng rng. c bithn, cc sng v tuyn FHSS gi mt hoc nhiu gi d liu ti mt tn s sng mang,

nhy n tn s khc, gi nhiu gi d liu, v tip tc chui nhy - truyn d liuny. Mu nhy hay chui ny xut hin ngu nhin, nhng tht ra l mt chui c tnhchu k c c my thu v my pht theo di. Cc h thng FHSS d b nh hng canhiu trong khi nhy tn, nhng hon thnh vic truyn dn trong cc qu trnh nhy tnkhc trong bng tn.

Hnh 2.11. Tri ph chui trc tip

2.3.3 Cng ngh tri ph chui trc tip (Direct Sequence Spread Spectrum)

Tri ph chui trc tip (DSSS) to ra mt mu bit d cho mi bit c truyn.Mu bit ny c gi mt chip (hoc chipping code). Cc chip cng di, th xc sut md liu gc b loi b cng ln (v tt nhin, yu cu nhiu di thng). Thm ch khi mthoc nhiu bit trong mt chip b h hi trong thi gian truyn, th cc k thut cnhng trong v tuyn khi phc d liu gc m khng yu cu truyn li. i vi mythu khng mong mun, DSSS lm xut hin nhiu bng rng cng sut thp v cloi b bi hu ht cc my thu bng hp.

B pht DSSS bin i lung d liu vo (lung bit) thnh lung symbol, trong mi symbol biu din mt nhm cc bit. Bng cch s dng k thut iu bin phathay i nh k thut QPSK (kha dch pha cu phng), b pht DSSS iu bin hay

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nhn mi symbol vi mt m ging nhiu gi l chui gi ngu nhin (PN). N cgi l chui chip. Php nhn trong b pht DSSS lm tng gi to di bng c dng

ph thuc vo di ca chui chip.

2.3.4 Cng ngh bng hp (narrowband)

Mt h thng v tuyn bng hp truyn v nhn thng tin ngi dng trn mttn s v tuyn xc nh. V tuyn bng hp gi cho di tn tn hiu v tuyn cng hpcng tt ch cho thng tin i qua. S xuyn m khng mong mun gia cc knh truynthng c trnh bng cch kt hp hp l cc ngi dng khc nhau trn cc knh ctn s khc nhau.

Mt ng dy in thoi ring rt ging vi mt tn s v tuyn. Khi mi nhln cn nhau u c ng dy in thoi ring, ngi trong nh ny khng th nghecc cuc gi trong nh khc. Trong mt h thng v tuyn, s dng cc tn s v tuynring bit hp nht s ring t v s khng can thip ln nhau. Cc b lc ca my

thu v tuyn lc b tt c cc tn hiu v tuyn tr cc tn hiu c tn s c thit k.2.3.5 Cng ngh hng ngoi ( Infrared )

H thng tia hng ngoi (IR) s dng cc tn s rt cao, ch di tn s ca nhsng kh kin trong ph in t, mang d liu. Ging nh nh sng, tia hng ngoiIR khng th thm nhp cc i tng chn sng; n s dng cng ngh trc tip (tmnhn thng) hoc cng ngh khuch tn. Cc h thng trc tip r tin cung cp phm virt hn ch (0,914m) v tiu biu c s dng cho mng PAN nhng thnh thongc s dng trong cc ng dng WLAN c bit. Cng ngh hng ngoi hng khnng thc hin cao khng thc t cho cc ngi dng di ng, v do n c s

dng thc hin cc mng con c nh. Cc h thng IR WLAN khuch tn khng yucu tm nhn thng, nhng cc cell b hn ch trong cc phng ring l.

2.4 Cc ch tiu k thut ca mng WLANSo vi mng LAN hu tuyn, mng WLAN linh hot hn trong ci t, nh cu

hnh v t do vn c trong mng lu ng. Cc khch hng mng WLAN cng nh ccnhn vin k thut cn xem xt cc ch tiu k thut sau.

2.4.1 Phm vi/Vng ph sng

Khong cch m qua cc sng RF truyn thng l mt nhim v ca vic thit

k sn phm (bao gm thit k my thu v cng sut pht) v ng truyn dn mngLAN, c bit trong mi trng trong nh. Cc tng tc vi cc i tng xy dngtiu biu, bao gm tng nh, kim loi, v thm ch c con ngi, nh hng n cchtruyn nng lng, v nh vy tnh c phm vi v vng ph sng ca h thng. a scc h thng mng WLAN s dng sng RF v cc sng v tuyn thm nhp qua tngv cc b mt trong nh. Phm vi (hoc bn knh ph sng) tiu biu ca h thng mngWLAN thay i t di 30,48m ti hn 152,4m. Vng ph sng c m rng, v st do ch thc ca kh nng lu ng thng qua roaming, c cung cp qua ccmicrocell.

2.4.2 Lu lng

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Nh cc h thng mng LAN hu tuyn, lu lng thc t trong mng WLAN lsn phm v c cu ph thuc. Cc nhn t nh hng ti lu lng bao gm s tcnghn sng (s lng ngi dng), cc h s truyn, kiu h thng mng WLAN sdng, cng nh gc tr v cc c chai trn cc phn ni dy ca mng WLAN. Tc

d liu tiu biu t 1 n 11 Mbps.Mng WLAN cung cp lu lng cho cc ng dng vn phng ph bin trnnn mng LAN, bao gm s trao i email, truy cp chia s thit b ngoi vi, v cctruy cp ti c s d liu v cc ng dng nhiu ngi dng.

2.4.3 S ton vn v tin cy

Cc cng ngh d liu khng dy c chng minh qua hn nm mi nms dng cc ng dng khng dy trong cc h thng c thng mi ln qun i. Nhiuv tuyn gy ra s gim st lu lng, nhng chng him c ti ni lm vic. Cc thitk ni bt ca cng ngh mng WLAN v gii hn khong cch tn hiu truyn dn ti

cc kt ni ca mng ny mnh hn cc kt ni in thoi t bo, v mng cung cp khnng thc hin ton vn d liu bng hoc hn mng ni dy.

2.4.4 Kh nng kt ni vi c s h tng mng ni dy

a s cc h thng mng WLAN cung cp kt ni chun cng nghip vi cc hthng ni dy, bao gm Ethernet (IEEE 802.3) v Token Ring (IEEE 802.5). Kh nngkt ni trn nn chun lm cc phn khng dy ca mng trong sut hon ton vi phncn li ca mng. Cc nt mng WLAN lc h tr bi cc h iu hnh mng theocch ging nh cc nt mng LAN khc qua trnh iu khin. Mt khi c ci t, cch iu hnh mng xem cc nt mng nh mi thnh phn khc ca mng.

2.4.5 Kh nng kt ni vi c s h tng mng khng dy

C th c vi kiu kt ni gia cc mng WLAN. iu ny ph thuc c cchla chn cng ngh ln cch thc hin ca nh cung cp thit b c th. Cc sn phm tcc nh cung cp khc nhau s dng cng cng ngh v cng cch thc hin cho phptrao i gia cc card giao tip v cc im truy cp. Mc ch ca cc chun cngnghip, nh cc c t k thut IEEE 802.11, s cho php cc sn phm tng hp vnhnh vi nhau m khng c s hp tc r rng gia cc nh cung cp.

2.4.6 Nhiu

i vi cc WLAN hot ng bng tn v tuyn 2,4 GHz cc l vi sng l mtngun nhiu quan trng. Cc l vi sng cng sut ln ti 750W vi 150 xung trn giyv c bn knh bc x hot ng khong 10 m. Nh vy i vi tc d liu 2 Mbit/s di gi ln nht phi nh hn 20.000 bit hoc 2.500 octet. Bc x pht ra qut t 2,4GHz n 2,45 GHz v gi n nh theo chu k ngn tn s 2,45 GHz. Cho d cc khi

b chn th phn ln nng lng vn gy nhiu ti truyn dn WLAN. Cc ngun nhiukhc trong bng tn 2,4 GHz gm my photocopy, cc thit b chng trm, cc m tthang my v cc thit b y t.

2.4.7 Tnh n gin v d dng trong s dng

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Ngi dng cn rt t thng tin mi nhn c thun li ca mng WLAN. Vbn cht khng dy ca mng WLAN l trong sut i vi h iu hnh mng ngidng, nn cc ng dng hot ng ging nh chng hot ng trn mng LAN hutuyn. Cc sn phm mng WLAN hp nht s a dng ca cc cng c chn on

hng vo cc vn lin quan n cc thnh phn khng dy ca h thng; tuy nhin,cc sn phm c thit k hu ht cc ngi dng him khi cn n cc cng cny.

Mng WLAN n gin ha nhiu vn ci t v nh cu hnh m rt phintoi i vi cc nh qun l mng. Ch khi cc im truy cp ca mng WLAN yu cuni cp, cc nh qun l mng c gii phng khi vic ko cp cho cc ngi ucui mng WLAN. Khng c ni cp cng lm di chuyn, b sung, v thay i cc hotng bnh thng trn mng WLAN. Cui cng, bn cht di ng ca mng WLANcho php cc nh qun l mng nh cu hnh trc v sa li ton b mng trc khi

lp t chng ti cc v tr t xa. Mt kho c nh cu hnh, mng WLAN c dichuyn t ch ny n ch khc m t hoc khng c s ci bin no.

2.4.8 Bo mt

V cng ngh khng dy bt ngun t cc ng dng trong qun i, nn t lu bo mt l mt tiu chun thit k cho cc thit b v tuyn. Cc iu khon bo mtin hnh c xy dng bn trong mng WLAN, lm cho chng tr nn bo mt hnso vi hu ht cc mng LAN hu tuyn. Cc my thu khng mong mun (cc nginghe trm) kh c kh nng bt c tin ang lu thng trong mng WLAN. K thutm ha phc tp lm cho cc gi mo tt nht truy cp khng php n lu thng

mng l khng th. Ni chung, cc nt ring l phi cho php bo mt trc khi chngc php tham gia vo lu thng mng.

2.4.9 Chi ph

Mt mng WLAN thc hin y bao gm c chi ph c s h tng, cho ccim truy cp khng dy, ln chi ph ngi dng, cho cc card giao tip mng WLAN.Cc chi ph c s h tng ph thuc ch yu vo s lng im truy cp c trinkhai; khong chi ph ca cc im truy cp t 800$ ti 2000$. S lng im truy cp

ph thuc tiu biu vo vng ph sng c yu cu v/hoc s v kiu ngi dngc dch v. Vng ph sng t l bnh phng vi phm vi sn phm. Cc card giao

tip mng WLAN c yu cu trn nn my tnh chun, v khong chi ph t 200$ ti700$. Chi ph lp rp v bo tr mt mng WLAN ni chung thp hn gi lp rp v botr ca mt mng LAN hu tuyn truyn thng, v hai l do. u tin, mt mng WLANloi tr cc chi ph trc tip ca vic ni cp v chi ph lao ng lin quan n lp rpv sa cha n. Th hai, v mng WLAN n gin ha vic di chuyn, b sung, v thayi, nn chng gim bt cc chi ph gin tip v thi gian ngh ca ngi dng v tng

ph hnh chnh.

2.4.10 Tnh linh hot

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Cc mng khng dy c thit k n gin v cng hoc kh phc tp. Ccmng khng dy h tr s lng nt mng v/hoc cc vng vt l ln ln bng cchthm cc im truy cp vo vng ph sng c m rng hoc tng.

2.4.11 Tui th ngun pin cho cc sn phm di ng

Cc sn phm khng dy ca ngi dng u cui c kh nng c gii phnghon ton dy nh, v hot ng qu ngun pin trong my tnh notebook hoc my tnhcm tay ch. Cc nh cung cp mng WLAN dng cc k thut thit k c bit lmtng tui th pin v cch dng ngun nng lng ca my tnh ch.

2.4.12 An ton

Cng sut ra ca cc h thng mng WLAN rt thp, t hn nhiu in thoi tbo cm tay. Khi cc sng v tuyn yu dn nhanh chng qua khong khng th c rt thng nng lng RF cung cp n cc vng ca h thng LAN khng dy. MngWLAN phi thch hp vi s qun l nghim v cc quy tc cng nghip m bo an

ton. Mng WLAN khng c hi cho sc khe cng ng.

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CHNG III

CHUN IEEE 802.11

3.1 Li gii thiuMc ch chng ny s cung cp tng quan v chun IEEE 802.11 mi vi cckhi nim c bn, cc nguyn l hot ng, v vi l do ng sau cc c tnh v ccthnh phn ca chun. Chng ny hng vo cc kha cnh MAC v cc chc nngchnh ca n.

3.2 Kin trc IEEE chun IEEE 802.113.2.1 Cc thnh phn kin trc

Chun mng LAN IEEE 802.11 da vo kin trc t bo, l kin trc trong hthng c chia nh ra thnh cc cell, mi cell (c gi l Tp hp dch v c bn,

hoc BSS) c kim sot bi mt trm c s (gi l im truy cp, hoc AP).Mc d, mt mng LAN khng dy c th c hnh thnh t mt cell n, vi

mt im truy cp n, nhng hu ht cc thit lp c hnh thnh bi vi cell, ti cc im truy cp c ni ti mng xng sng (c gi h phn phi, hoc DS),tiu biu l Ethernet, v trong c mng khng dy.

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

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

Hnh 4.1. Mng WLAN IEEE 802.11 tiu biu

Chun cng nh ngha khi nim Portal, l mt thit b lin kt gia mngLAN 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.

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Mc d chun khng yu cu s ci t tiu biu tt yu phi c AP v Portaltrn mt thc th vt l n.

3.2.2 M t cc lp chun IEEE 802.11

Nh bt k giao thc chun IEEE 802.x khc, giao thc chun IEEE 802.11 bao

gm MAC v lp vt l, chun hin thi nh ngha mt MAC n tng tc vi ba lpvt l (tt c hot ng tc 1 v 2Mbit/s):

FHSS hot ng trong bng tn 2.4GHz

DSSS hot ng trong bng tn 2.4GHz, v

Hng ngoi

Hnh 4.2. Lp MAC

Ngoi cc tnh nng chun c thc hin bi cc lp MAC, lp MAC chunIEEE 802.11 cn thc hin chc nng khc lin quan n cc giao thc lp trn, nhPhn on, Pht li gi d liu, v Cc ghi nhn.

Lp MAC: Lp MAC nh ngha hai phng php truy cp khc nhau,Hm phi hp

phn tn vHm phi hp im.

3.2.3. Phng php truy cp c bn: CSMA/CA

y l mt c ch truy cp c bn, c giHm phi hp phn tn, v c bnl a truy cp cm bin sng mang vi c ch trnh xung t (CSMA/CA). Cc giaothc CSMA c bit trong cng nghip, m ph bin nht l Ethernet, l giao thcCSMA/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 chophp cc trm truyn vi t tr hon, nhng thng xy ra trng hp cc trm phtcng lc (c xung t), gy ra do cc trm nhn thy mi trng t do v quyt nhtruyn ngay lp tc.

Cc tnh trng xung t ny phi c xc nh, v vy lp MAC phi t truynli gi m khng cn n cc lp trn, iu ny s gy ra tr ng k. Trong trng hpmng 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.

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Cc c ch d tm xung t ny ph hp vi mng LAN ni dy, nhng chngkhng 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 songcng, 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 trmnghe thy c nhau (y l s gi thit c s ca s d tm xung t), vvic mt trm nhn thy mi trng t do v sn sng truyn khng tht s cngha 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 trnhxung 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 FrameSpace,Khng gian khung Inter phn tn), th trm c php truyn, trm thu s kimtra m CRC ca gi nhn c v gi mt gi chng thc (ACK). Chng thc nhnc s ch cho my pht bit khng c s xung t no xut hin. Nu my pht khngnhn chng thc th n s truyn li on cho n khi n c tha nhn hoc khngc 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 mtgi 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 Senseca n (gi l NAV,Network Allocation Vector,Vect nh v mng) cho khong thigian 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 tmy pht, lm ngn khong thi gian truyn RTS, v trm s nghe thy CTS v dtr mi trng khi bn cho n khi kt thc giao dch. Thng tin khong thi gian vRTS cng bo v vng my pht khi cc xung t trong thi gian ACK (bi cc trmnm ngoi phm vi trm nhn bit).

Cn ch thng tin khong thi ACK v cc khung RTS v CTS l cc khungngn, N cng lm gim bt mo u ca cc xung t, v chng c nhn dng nhanhhn khi n c nhn dng nu ton b gi c truyn, (iu ny ng nu gi lnhn RTS mt cch ng k, nh vy l chun cho php k c cc gi ngn s c

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truyn m khng c giao dch RTS/CTS, v iu ny c iu khin bi mt tham sgi l ngng RTS).

Cc s sau cho thy mt giao dch gia hai trm A v B, v s thit lp NAVca cc trm gn chng:

Hnh 4.3. 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 trngthi bn ca mi trng.

3.2.4 Cc chng thc mc MAC

Lp MAC thc hin d tm xung t bng cch ch i s tip nhn ca mt ghinhn 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 (vd, 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 tngthm 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 (trongtrng 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 nkhng th gii quyt cc gi 1518 byte c s dng trn mng Ethernet, nh vy IEEEquyt nh gii quyt vn bng cch thm mt c ch phn on/ti hp n gin tilp MAC.

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C ch l mt gii thut Send - and - Wait n, trong trm pht khng chophp truyn mt on mi cho n khi xy ra mt trong cc tnh hung sau y:

1. Nhn mt ACK cho on, hoc

2. Quyt nh rng on cng c truyn li nhiu ln v th vo ton b khungCn 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 nibt 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 4.4. 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 ccquyn u tin khc nhau:

SIFS - Short Inter Frame Space, c s dng phn chia cc truyn dn thucmt hi thoi n (v.d. Ack - on), v l Khng gian khung Inter ti thiu, vlun c nhiu nht mt trm n truyn ti thi gian cho trc, do n cquyn u tin i vi tt c cc trm khc. l mt gi tr c nh trn lp vtl v c tnh ton theo cch m trm pht truyn ngc li nhn kiu vkh nng gii m gi vo, trong lp vt l chun IEEE 802.11 FH gi tr nyc thit lp 28 micr - giy.

PIFS - Point Cooordination IFS, c s dng bi im truy cp (hoc PointCoordinator, c gi trong trng hp ny), c truy cp ti mi trngtrc mi trm khc. Gi tr ny l SIFS cng vi mt khe thi gian (s cnh 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 thmmt khe thi gian, v d 128 micr - giy.

EIFS - Extended IFS, L mt IFS di hn c s dng bi mt trm nhnmt gi khng hiu, n cn ngn trm (trm m khng hiu thng tin khong

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3.2.7 Gii thut Exponential Backoff

Backoff l mt phng php ni ting gii quyt cc tranh dnh gia cc trmkhc nhau mun truy cp mi trng, phng php yu cu mi trm chn mt s ngunhin (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 xyra xung t, n s tng gi tr theo ly tha mt cch ngu nhin.

Chun IEEE 802.11 chun nh ngha gii thut Exponential Backoffc thchin trong cc trng hp sau y:

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

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 nhtruyn mt gi mi v mi trng rnh cho nhiu hn DIFS.

Exponential backoff khin cc nt chu kh chlu 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 Kbit time ri truyn li; Kcchn ngu nhin trong tp {0,1,2,,2m 1} vi m:=min (n,10).

Hnh sau biu din s c ch truy cp:

Hnh 4.5. S c ch truy cp

3.3 Cch mt trm ni vi mt cell hin hu (BSS)

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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 kiuAd - 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 nhiu (Beacon) t AP, (khung n hiu l mt khung tun hon cha thng tinng b c gi bi AP), hoc

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

Hai phng php u hp l, v mi mt phng php c chn phi hi hogia kh nng tiu th in v kh nng thc hin.

3.3.1 Qu trnh chng thc

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

3.3.2 Qu trnh lin kt

Khi trm c xc nhn, sau n s khi ng qu trnh lin kt, y l s traoi 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.3.4 RoamingRoaming 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 bitchnh:

Trong mt h thng mng LAN da trn cc gi, s chuyn tip gia cc cellc 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, iuny lm roaming mng LAN d hn mt t, nhng

Trong mt h thng ting ni, mt gin on tm thi khng nh hng cuc nichuyn, trong khi trong mt gi da vo mi trng, n s gim ng k khnng 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 nhngha cc cng c c bn cho n, iu ny bao gm s qut tch cc/b ng, v mtqu trnh ti lin kt, trong mt trm roaming t im truy cp ny sang im truycp khc s c lin kt vi mt im truy cp mi.

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Cc trm cn gi ng b, gi cho nhy tn c ng b, v cc chc nngkhc 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 cch sau:

AP truyn cc khung tun hon gi l cc khung bo hiu, cc khung ny chagi tr ca ng h AP ti thi im truyn (Ch rng y l thi im khi truyn dntht 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 trmt cch ng k).

Cc trm thu kim tra gi tr ng h ca chng thi im nhn, v sa chan gi ng b vi ng h ca AP, iu ny ngn nga s tri ng h gy ra domt ng b sau vi gi hot ng.

3.6 Tit kim nng lng

Mng LAN khng dy tiu biu lin quan n cc ng dng di ng, v trongcc kiu ng dng ny ngun pin l mt ngun nhanh ht, l l do ti sao chunIEEE 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 mtthng tin.

tng chnh ng sau c ch tit kim nng lng l AP duy tr mt bn ghic 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 phic 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 trngthi 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 imc bit trc (mi DTIM), ti tt c trm tit kim nng lng mun nhn kimkhung ny cn phi hot ng.

3.7 Cc kiu khungC ba kiu khung chnh:

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), v

Khung qun l: cc khung c truyn ging nh cc khung d liu trao ithng 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 nngca chng.

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3.8 Khun dng khung

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

Hnh 4.6. Khun dng khung chun IEEE 802.11

3.8.1. Tin t (Preamble)

N ph thuc lp vt l, v 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 vicsa 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.

3.8.2 u mc (Header) PLCP

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

Chiu di t PLCP_PDU: biu din s byte cha trong gi, n c ch cho lp vtl 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, v Trng kim tra li u mc: l trng pht hin sai st CRC 16 bit

3.8.3 D liu MAC

Hnh sau cho thy khun dng khung MAC chung, cc phn ca trng trn ccphn ca cc khung nh m t sau .

Hnh 4.7. Khun dng khung MAC

3.8.3.1 Trng iu khin khung (Frame Control)

Trng iu khin khung cha ng thng tin sau:

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a. Phin bn giao thc (Protocol Verson)

Trng ny gm 2 bit c kch thc khng i v xp t theo cc phin bn sauca 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.

b. ToDS

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

c. FromDS

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

d. More Fragments

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

e. Retry

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

f. Power mangenment (Qun l nng lng)

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.g. More Data (Nhiu D liu hn)

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 dngthng tin ny tip tc kim tra tun t hoc kiu ang thay i thm ch thay isang ch hot ng.

h. WEP

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

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i. Order (Th t)

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

3.8.3.2 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, v

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

3.8.3.3 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), nubit ToDS c lp th y l a ch AP, nu bit ToDS c xa th n l a ch trmkt 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 khungvi bit FromDS c lp, sau a ch - 3 l a ch ngun gc, nu khung c bitToDS lp, sau a ch - 3 l a ch ch.

a ch - 4 c s dng trong trng hp c bit trong mt h phn phikhng dy c s dng, v khung ang c truyn t im truy cp ny sang imtruy cp khc, trong trng hp ny c cc bit ToDS ln cc bit FromDS c lp, vvy c a ch ch gc v a ch ngun gc u b mt.

Bng sau tng kt cc cch dng a ch khc nhau theo cch thit lp bit ToDSv bit FromDS:

3.8.3.4 iu khin ni tip

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

4.8.3.5 CRC

CRC l mt trng 32 bit cha mt m kim tra d s chu k 32 bit (CRC)

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3.9 Cc khung nh dng ph bin nht3.9.1 Khun dng khung RTS

Khung RTS nh sau:

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 khungACK, cng ba khong SIFS.

3.9.2 Khun dng khung CTS

Khung CTS nh sau:

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 thigian l gi tr thu c t trng Khong thi gian ca khung RTS ngay trc , trthi gian (tnh theo micr - giy) c yu cu pht khung CTS v khong SIFS.

3.9.3 Khun dng khung ACK

Khung ACK nh sau:

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

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khung trc , th gi tr Khong thi gian l 0, nu khng th gi tr Khong thi gianthu c t trng Khong thi gian ca khung trc , tr i thi gian (tnh theomicr - giy) c pht khung ACK v khong SIFS ca n.

3.11 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 cchs 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 cpkim tra tun t yu cu ca cc trm truyn d liu, do iu khin vic truy cpmi trng. cho php cho cc trm bnh thng kh nng vn cn truy cp mitrng, c mt chun b m im truy cp phi li thi gian cho Truy cp Phntn trong gia PCF

3.12 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 nybao gm truyn file gia hai ngi dng my notebook, cuc hp gia cc cng tc vinbn ngoi vn phng, vn vn.

Chun IEEE 802.11 gii quyt cc nhu cu ny bng cch nh ngha mt mhnh hot ng Ad hoc, trong trng hp ny khng c im truy cp no hoc phnno tnh nng ca n c thc hin bi cc trm ngi dng cui (nh to bo hiu,ng b, vn vn), 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).3.13 H chun IEEE 802.113.13.1 Chun IEEE 802.11a

L mt ch tiu k thut IEEE cho mng khng dy hot ng trong di tn s 5GHz (5.725 GHz ti 5.85 GHz) vi tc truyn d liu cc i 54 Mbps. Di tn s 5GHz khng nhiu nh tn s 2.4 GHz, v ch tiu k thut chun IEEE 802.11 nghnhiu knh v tuyn hn so vi chun IEEE 802.11b. S b sung cc knh ny giptrnh giao thoa v tuyn v vi ba.

3.13.2 Chun IEEE 802.11b (Wifi)

L chun quc t cho mng khng dy hot ng trong di tn s 2.4 GHz (2.4GHz ti 2.4835 GHz) v cung cp mt lu lng ln trn 11 Mbps. y l mt tn srt thng s dng. Cc l vi ba, cc in thoi khng dy, thit b khoa hc v y hc,cng nh cc thit b Bluetooth, tt c lm vic bn trong di tn s 2.4 GHz.

3.13.3 Chun 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 WLANchun 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 khchhng. Cc thit b s t ng iu chnh da vo cc yu cu a l.

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Mc ch 11d l s thm cc c tnh v cc hn ch cho php mng WLANhot 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 theoquc 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 phn sn chuyn bit theo quc gia.3.13.4 Chun IEEE 802.11g

Tng t ti chun IEEE 802.11b, chun lp vt l ny cung cp mt lu lngln ti 54 Mbps. N cng hot ng trong di tn s 2.4 GHz nhng s dng mt cngngh v tuyn khc tng di thng ton b. Chun ny c ph chun cui nm2003.

3.13.5 Chun 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 v

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 choWEP 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 liumnh 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 v

c) Qun l kha chun IEEE 802.1x.

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

3.13.6 Chun IEEE 802.1x (Tbd)

Chun IEEE 802.1x (Yu cu mt nh cung cp dch v RADIUS) cung cp ccdoanh nghip & cc nh ring mt gii php chng thc bo mt, bin i c s dngk thut ti kha (re - keying) ng, s chng thc tn v mt khu ngi dng vchng 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 linquan n cc kha WEP tnh. S chng thc trn nn ngi dng loi tr cc l bomt xut hin t thit b b trm hoc mt khi s chng thc trn nn thit b c sdng, 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 anggia tng.

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CHNG IV

BO MT TRONG MNG WLAN

Chng ny phc tho cc giao thc, cc c ch bo mt lin quan, v cc kintrc ca chun IEEE 802.11 - mng WLAN v thc hin cc khuyn ngh ti mt thihnh c thc hin dn ca cc mng WLAN.

4.1 Mt s hnh thc tn cng mng

C th tn cng mng theo mt trong cc hnh thc sau y:

4.1.1 Da vo nhng l hng bo mt trn mng: nhng l hng ny c th ccim yu ca dch v m h thng cung cp, v d nhng k tn cng li dng cc

im yu trong cc dch v mail, ftp, web xm nhp v ph hoi.

Cc l hng ny trn mng l cc yu im quan trng m ngi dng, hacker da tn cng vo mng. Cc hin tng sinh ra trn mng do cc l hng ny mang lithng l : s ngng tr ca dch v, cp thm quyn i vi cc user hoc cho phptruy nhp khng hp php vo h thng.

Hin nay trn th gii c nhiu cch phn lai khc nhau v l hng ca h thngmng. Di y l cch phn loi sau y c s dng ph bin theo mc tc hi

h thng, do B quc phng M cng b nm 1994.a. Cc l hng loi C

Cc l hng loi ny cho php thc hin cc phng thc tn cng theo DoS(Denial of Services - T chi dch v). Mc nguy him thp, ch nh hng ti chtlng dch v, c th lm ngng tr, gin on h thng; khng lm ph hng d liuhoc t c quyn truy nhp bt hp php

DoS l hnh thc tn cng s dng cc giao thc tng Internet trong b giao thcTCP/IP lm h thng ngng tr dn n tnh trng t chi ngi s dng hp php

truy nhp hay s dng h thng. Mt s lng ln cc gi tin c gi ti server trongkhong thi gian lin tc lm cho h thng tr nn qu ti, kt qu l server p ngchm hoc khng th p ng cc yu cu t client gi ti.

Mt v d in hnh ca phng thc tn cng DoS l vo mt s Web Site ln lmngng tr hot ng ca web site ny: nh www.google.com, www.ebay.com,www.yahoo.com v.v

Tuy nhin, mc nguy him ca cc l hng loi ny c xp loi C; t nguy

him v chng ch lm gin on cung cp dch v ca h thng trong mt thi gian m

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khng lm nguy hi n d liu v nhng k tn cng cng khng t c quyn truynhp bt hp php vo h thng.

b. Cc l hng loi B

Cc l hng cho php ngi s dng c thm cc quyn trn h thng m khng cnthc hin kim tra tnh hp l. i vi dng l hng ny, mc nguy him mc trung bnh. Nhng l hng ny thng c trong cc ng dng trn h thng; c th dnn mt hoc l thng tin yu cu bo mt.

Cc l hng loi B c mc nguy him hn l hng loi C, cho php ngi sdng ni b c th chim c quyn cao hn hoc truy nhp khng hp php.

Nhng l hng loi ny thng xut hin trong cc dch v trn h thng. Ngi sdng cc b c hiu l ngi c quyn truy nhp vo h thng vi mt s quyn

hn nht nh.

Mt s l hng loi B thng xut hin trong cc ng dng nh l hng ca trnhSendMail trong h iu hnh Unix, Linux... hay li trn b m trong cc chng trnhvit bng C.

Nhng chng trnh vit bng C thng s dng mt vng m, l mt vng trongb nh s dng lu d liu trc khi x l. Nhng ngi lp trnh thng s dngvng m trong b nh trc khi gn mt khong khng gian b nh cho tng khi dliu. V d, ngi s dng vit chng trnh nhp trng tn ngi s dng; qui nhtrng ny di 20 k t.

Do h s khai bo:

char first_name [20];

Vi khai bo ny, cho php ngi s dng nhp vo ti a 20 k t. Khi nhp dliu, trc tin d liu c lu vng m; nu ngi s dng nhp vo 35 k t; sxy ra hin tng trn vng m v kt qu 15 k t d tha s nm mt v tr khngkim sot c trong b nh. i vi nhng k tn cng, c th li dng l hng ny

nhp vo nhng k t c bit, thc thi mt s lnh c bit trn h thng. Thngthng, l hng ny thng c li dng bi nhng ngi s dng trn h thng t c quyn root khng hp l.

Vic kim sot cht ch cu hnh h thng v cc chng trnh s hn ch c ccl hng loi B.

c. Cc l hng loi A

Cc l hng ny cho php ngi s dng ngoi c th truy nhp vo h thng bt

hp php. L hng ny rt nguy him, c th lm ph hy ton b h thng.

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Cc l hng loi A c mc rt nguy him; e da tnh ton vn v bo mt cah thng. Cc l hng loi ny thng xut hin nhng h thng qun tr yu kmhoc khng kim sot c cu hnh mng.

Nhng l hng loi ny ht sc nguy him v n tn ti sn c trn phn mm sdng; ngi qun tr nu khng hiu su v dch v v phn mm s dng s c th bqua nhng im yu ny.

i vi nhng h thng c, thng xuyn phi kim tra cc thng bo ca cc nhmtin v bo mt trn mng pht hin nhng l hng loi ny. Mt lot cc chngtrnh phin bn c thng s dng c nhng l hng loi A nh: FTP, Gopher, Telnet,Sendmail, ARP, finger...

nh hng ca cc l hng bo mt trn mng WLAN

Phn trn chng ta phn tch mt s trng hp c nhng l hng bo mt,nhng k tn cng c th li dng nhng l hng ny to ra nhng l hng khc tothnh mt chui mt xch nhng l hng. V d, mt k ph hoi mun xm nhp voh thng m anh ta khng c ti khon truy nhp hp l trn h thng . Trong trnghp ny, trc tin k ph hoi s tm ra cc im yu trn h thng, hoc t cc chnhsch bo mt, hoc s dng cc cng c d xt thng tin (nh SATAN, ISS) trn hthng t c quyn truy nhp vo h thng. Sau khi mc tiu th nht tc; k ph hoi c th tip tc tm hiu cc dch v trn h thng, nm bt c cc

im yu v thc hin cc hnh ng ph hoi tinh vi hn.Tuy nhin, khng phi bt k l hng bo mt no cng nguy him n h thng.

C rt nhiu thng bo lin quan n l hng bo mt trn mng WLAN, hu ht trongs l cc l hng loi C, v khng c bit nguy him i vi h thng. V d, khinhng l hng v sendmail c thng bo trn mng, khng phi ngay lp tc nhhng trn ton b h thng. Khi nhng thng bo v l hng c khng nh chcchn, cc nhm tin s a ra mt s phng php khc phc h thng.

Da vo k h ca cc l hng ny, k xu s xy dng cc hnh thc tn cng

khc nhau nhm khng ch v nm quyn kim sot trn mng. Cho n nay, cc hacker ngh ra khng bit bao nhiu kiu tn cng t xa qua mng khc nhau. Mi cuc tncng thng m u bng vic trc tip hoc gin tip chui vo mt hoc nhiu mytnh ang ni mng ca ngi khc. Sau khi vo c h thng mng, hacker c thi n cc bc khc nh xem trm, ly cp, thay i v thm ch ph hu d liu hoclm treo cc hot ng ca mt h thng thng tin in t. Cc hacker cng c th gi

by nhng ngi s dng thiu cnh gic hoc nh la nhng h thng thng tin kmphng b. Chng hn, chng su tm cc a ch email v gi th km virus n hoc

lm nghn tc mng bng cch gi tht nhiu cc bc th in t n cng mt a ch.

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i khi cc hacker xm nhp vo mt mng my tnh no m n pht hin ra li v li thng bo cho ngi qun tr mng, t hn na l chng ci virus hoc phn mmno theo di v ly i nhng thng tin ni b. Di y l mt s k thut tncng mng ch yu c s dng nhiu trn thc t.

4.1.2 S dng cc cng c ph hoi: v d s dng cc chng trnh ph khamt khu truy cp vo h thng bt hp php;lan truyn virus trn h thng; ci tcc on m bt hp php vo mt s chng trnh.

Nhng k tn cng mng cng c th kt hp c 2 hnh thc trn vi nhau tc mc ch.

- Mc 1: Tn cng vo mt s dch v mng : nh Web, Email dn n ccnguy c l cc thng tin v cu hnh mng. Cc hnh thc tn cng mc ny c th

dng D hoc spam mail.- Mc 2: K ph hoi dng ti khn ca ngi dng hp php chim ot ti

nguyn h thng ( da vo cc phng thc tn cng nh b kha, nh cp mtkhu); k ph hoi c th thay i quyn truy cp h thng qua cc l hng bo mthoc c cc thng tin trong tp tin lin quan n truy nhp h thng nh /etc/paswd

- T mc 3 n mc 5: K ph hoi khng s dng quyn ca ngi dng thngthng m c thm mt s quyn cao hn i vi h thng, nh quyn kch hot mt sdch v, xem xt cc thng tin khc trn h thng.

- Mc 6: K tn cng chim c quyn root trn h thng.

4.2 C s chun IEEE 802.11Chun 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 dliu 1Mbps v 2 Mbps trong bng tn RF 2.4 GHz v trong hng ngoi (IR). ChunIEEE 802.11 l mt thnh vin ca h chun IEEE 802 c pht hnh bi IEEE mgm chun IEEE 802.3 (Ethernet) v chun IEEE 802.5 (Token Ring). N c mrng 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 2.4GHz tc 5.5 v 11 Mbps.

Mc ch ca chun IEEE 802.11 nh IEEE nh ngha l " cung cp kt nikhng dy ti cc thit b, hoc cc trm t ng m yu cu trin khai nhanh, v xchtay hoc cm tay, hoc c gn ln cc phng tin chuyn ng bn trong mt vng".

4.2.1 Lp vt l

Chun IEEE 802.11 quy nh cc lp vt l nh bng 2.1.

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

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ChunTn s v

tuyn (RF)Hng ngoi

(IR)C ch

Tc d liucc i(Mbps)

IEEE 802.11 2.4 GHz DSSS 2

IEEE 802.11 2.4 GHz FHSS 2

IEEE 802.11 850 - 950 nm IR 2

IEEE 802.11a 5 GHz OFDM 54

IEEE 802.11b 2.4 GHz DSSS 11

H thng tri ph nhy tn FHSS 2.4 GHz v h thng IR ca chun IEEE802.11 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 chuitrc tip (DSSS) theo chun IEEE 802.11b tc d liu ln trn ti 11 Mbps do lith kh nng thc hin v gi thnh ca n.

Mc ch ca cng ngh tri ph l tng thm thng lng v tin cy catruyn 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 nyc 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 nhnc s vn phn bit bit gc.

4.2.2 iu khin truy cp mi trng (MAC)

Trong khi lp vt l chun IEEE 802.11 khc vi chun IEEE 802.3 Ethernet, thch tiu k thut ca MAC tng t nh ch tiu k thut ca MAC Ethernet chunIEEE 802.3 cng vi iu khin lin kt Logic (LLC) chun IEEE 802.2, n lm chokhng gian a ch MAC chun IEEE 802.11 thch hp vi khng gian a ch MAC cacc 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 IEEE802.11 l CSMA/CA - a truy cp nhy sng mang trnh xung t. S khc nhau ny ldo khng c phng cch thit thc truyn v nhn cng lc trn mi trng khngdy (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 thuxc nh thi gian cn li ca khung trn mi trng WM. Nu khong thi gian cakhung MAC trc ht v mt kim tra nhanh trn mi trng WM ch ra rng nkhng bn, th trm truyn c php truyn. Bng cch ny, n cho php ni gitruyn bt k lc no m mi trng khng bn.

4.2.3 So snh kiu C s h tng v kiu Ad Hoc

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C 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 cm rng, ESS). Mt mng Ad Hoc thng thng l mt mng tn ti trong mt thigian 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 munchia s cc file s thit lp mt mng Ad Hoc s dng cc card NIC thch hp chunIEEE 802.11 v chia s cc file qua mi trng WM m khng cn phng tin truynthng ngoi no (nh a mm, cc card flash).

Kiu C s h tnggi thit c mt mt hoc nhiu hn cc AP bc cu phngtin truyn thng khng dy vi phng tin ni dy truyn thng (hnh 2.1). AP iukhin vic chng thc v lin kt trm ti mng khng dy. Nhiu AP c ni bi mth 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 tngmng IP hin hu. Vi mc

ch bo mt, ngi ta thng s dng cc mng LAN o (VLAN) tch ring luthng mng khng dy vi lu thng mng khc trn DS. Mc d chun IEEE 802.11cho php cc trm v tuyn lin kt chuyn mch ng t im truy cp ny n imtruy cp khc, nhng n khng iu khin cch trm thc hin. Kt qu l, cc thi hnhca 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 phpnh cung cp n.

Hnh 4.1. So snh kiu Ad Hoc v kiu c s h tng.

4.2.4 Lin kt v Chng thc

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Chun IEEE 802.11 nh ngha mt trm cui l nh x AP cc trm khc trnmng ni dy v mng khng dy c phng tin giao tip vi trm cui. nh x nyc 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 votrong 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 cxc thc; (2) khng c lin kt nhng c xc thc; (3) c lin kt v c xcthc.

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

4.2.3.4.3 Cc mc bo v an ton mng

V khng c mt gii php an ton tuyt i nn ngi ta thng phi s dng nhiumc 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 sau m t cc lp ro chn thng dng hinnay bo v thng tin ti cc trm ca mng.

I n f o r m a t i o n

Accessrights

login/password

dataencrytion

Physicalprotection

firewalls

Hnh 2 - Cc mc bo v mng

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

- Lp bo v trong cng l quyn truy nhp nhm kim sot cc ti nguyn ( yl thng tin) ca mng v quyn hn ( c th thc hin nhng thao tc g) trn tinguyn . 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/ v mtkhu tng ng. y l phng php bo v ph bin nht v n n gin, t tn km vcng rt c hiu qu. Mi ngi s dng mun truy nhp c vo mng s dng cc

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ti nguyn u phi ng k tn v mt khu. Ngi qun tr h thng c trch nhimqun l, kim sot mi hot ng ca mng v xc nh quyn truy nhp ca nhngngi s dng khc ty theo thi gian v khng gian.

- Lp th ba l s dng cc phng php m ha (encrytion). D liu c bini 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 sdng ph bin phn di y.

- Lp th t: l bo v vt l ( physical protection) nhm ngn cn cc truy nhpbt hp php vo h thngd. Thng dng cc bin php truyn thng nhngn cm ngi khng c nhim v vo phng t my, dng h thng khatrn 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 cthm nhp tri php v cho php lc cc gi tin m ta khng mun gi i hocnhn vo v mt l do no .

4.4 C s bo mt mng WLANChun 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 tnhcung 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 mitrng WM.

4.4.1 Gii hn lan truyn RF

Trc khi thc hin cc bin php bo mt, ta cn xt cc vn lin quan vilan 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 timng khng dy trong vng ph sng nh trc. Khi chn km, s m rng mng rangoi 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 trongvng hn ch (hnh 3.2). khuch i anten c o bng dBi v c nh ngha ls tng cng sut m mt anten thm vo tnh hiu RF.

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Hnh 4.2. Cc mu lan truyn RF ca cc anten ph bin.

4.4.2 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 kius dng ph bin, AP lan truyn nh k SSID ca n qua mt n hiu (beacon). Mttrm v tuyn mun lin kt n AP phi nghe cc lan truyn v chn mt AP linkt vi SSID ca n.

Trong kiu hot ng khc, SSID c s dng nh mt bin php bo mt bngcch nh cu hnh AP khng lan truyn SSID ca n. Trong kiu ny, trm v tuynmun lin kt n AP phi sn c SSID nh cu hnh ging vi SSID ca AP. Nucc SSID khc nhau, cc khung qun l t trm v tuyn gi n AP s b loi b vchng 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 cgi n r rng, nn kiu hot ng ny khng cung cp mc bo mt thch hp. Mt ktn cng d dng nghe cc khung qun l trn mi trng WM v khm ph SSID caAP.

4.4.3 Cc kiu Chng thc

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

4.4.3.1 Chng thc h thng m

Chng thc h thng m (hnh 2.3) 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 gmthnh cng hoc tht bi. Khi thnh cng, c hai trm c xem nh c xc nhn vi

nhau.

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Hnh 4.3. Chng thc h thng m.

4.4.3.2 Chng thc kha chia s

Chng thc kha chia s (hnh 4.4) c xc nhn trn c s c hai trm thamgia 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 ccthi 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 ctrong chng thc h thng m. Cn cc khung th hai v khung th ba khc nhau, trmxc nhn nhn mt gi vn bn yu cu (c to ra khi s dng b to s gi ngunhin gii thut WEP (PRNG)) t AP, mt m ha n s dng kha chia s, v gi ntr li cho AP. Sau khi gii m, nu vn bn yu cu ph hp, th chng thc mt chiuthnh cng. chng thc hai pha, qu trnh trn c lp li pha i din. C sny 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 lmt la chn km hiu qu. N cho php cc hacker ly thng tin nh mt mha WEP v cng l l do ti sao chng thc kha chia s khng bao gi khuynngh.

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

Hnh 4.4. Chng thc kha chia s.

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4.4.4 WEP

WEP c thit k bo v ngi dng mng WLAN khi b nghe trm tnhc 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 thngtin 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 mtD liu RSA. N c xem nh l mt gii thut i xng v s dng cng kha chomt 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 vnbn 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 thutheo ngha bo mt no .

Ti trm pht, kha b mt 40 bit c mc ni vi mt Vect Khi to (IV) 24bit to ra mtseed(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 raPDU vn bn mt m ha.

PDU vn bn mt m ha ny sau c mc ni vi IV v c truyn trnmi trng WM.

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

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

PDU vn bn gc c bo v bng mt m CRC ngn nga can thip ngunhin vo vn bn mt m ang vn chuyn. Khng may l khng c bt k cc quy tcno i vi cch s dng ca IV, ngoi tr ni rng IV c thay i "thng xuynnh mi MPDU". Tuy nhin, ch tiu k thut khuyn khch cc thc thi xem xtcc nguy him do qun l IV khng hiu qu.

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Nhn thy c nhng kh khn khi nng cp ln 802.11i, Wi-Fi Alliance a ragii php khc gi l Wi-Fi Protected Access (WPA). Mt trong nhng ci tin quan trngnht 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 phkho m ho u khng th thc hin c vi WPA. Bi WPA thay i kho lin tcnn 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). Vvy, d liu khng th b thay i trong khi ang trn ng truyn. Mt trong nhngim 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 dngv 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 us dng giao thc TKIP v s khc bit ch l kho khi to m ho lc u. WPAPersonal thch hp cho gia nh v mng vn phng nh, kho khi to s c s dngti cc im truy cp v thit b my trm. Trong khi , WPA cho doanh nghip cn mtmy ch xc thc v 802.1x cung cp cc kho khi to cho mi phin lm vic. Trongkhi Wi-Fi Alliance a ra WPA, v c coi l loi tr mi l hng d b tn cng caWEP nhng ngi s dng vn khng thc s tin tng vo WPA. C mt l hng trongWPA v li ny ch xy ra vi WPA Personal. Khi m s dng hm thay i kho TKIPc s dng to ra cc kho m ho b pht hin, nu hacker c th on c khokhi to hoc mt phn ca mt khu, h c th xc nh c ton b mt khu, do cth gii m c d liu. Tuy nhin, l hng ny cng s b loi b bng cch s dng

nhng kho khi to khng d on. iu ny cng c ngha rng k thut TKIP caWPA ch l gii php tm thi, cha cung cp mt phng thc bo mt cao nht. WPAch thch hp vi nhng cng ty m khng khng truyn d liu "mt" v nhng thngmi, hay cc thng tin nhy cm... WPA cng thch hp vi nhng hot ng hng ngyv mang tnh th nghim cng ngh.

4.5 Trng thi bo mt mng WLANChun IEEE 802.11b hnh thnh di s khuyn khch t nhiu hng. C

nhiu ti liu ca cc nh nghin cu khc nhau ch ra cc l hng bo mt quantrng trong chun. H ch ra rng gii thut WEP khng hon ton cung cp tnhring t trn mt mng khng dy. H khuyn ngh:

Cc lp lin kt xut khng c bo mt.

S dng cc c ch bo mt cao hn nh IPsec v SSH, thay cho WEP.

Xem tt c cc h thng c ni qua chun IEEE 802.11 nh l phn ngoi. ttt c cc im truy cp bn ngoi bc tng la.

Gi thit rng bt c ai trong phm vi vt l u c th lin lc trn mng nhmt ngi dng hp l. Nh rng mt i th cnh tranh c th dng mt anten tinh

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vi vi nhiu vng nhn sng rng hn c th c tm thy trn mt card PC chunIEEE 802.11 tiu biu.

4.6 Cc v d kin trc bo mt mng WLANCc kin trc mng WLAN sau y c ngha khi ta nghin cu ton b cc cch

tip cn c th. N khng hng vo cc vn mt m ha lp cao ca d liu trnmi gi trong mi trng WM, nh mt mng ring o (VPN). Trong tt c cc trnghp, ta gi thit rng mt gii php VPN c u tin hn so vi cc kin trc khc tng mc bo mt. Bin php bo mt c tho lun di y nhm bo v s luthng mng c truyn gia cc AP v radio khch hng. Do , ta gi thit rng mngni dy hin ti tht s c bo v bi mt bin php no chp nhn c.

SSID cung cp rt t mc bo mt v bn cht vn bn sch ca n v do takhng quan tm n SSID khi tho lun v cc kin trc bo mt.

Sau y l mt danh sch kin trc mng WLAN v cc tn thnh cng nh ccphn i i vi chng. Bng 2.2 so snh cc c tnh ca cc kin trc bo mt mngWLAN.

Chng thc m khng c gii thut WEP (hnh 4.3)

Cc tn thnh: khng c mo u qun l; bt k khch hng no cng c thlin kt n AP m khng c bt k cu hnh b sung no.

Cc chng i: khng c bo mt no khc ngoi a ch MAC da vo k thutlc.

Chng thc m c gii thut WEP (hnh 4.3)Cc tn thnh : tnh bo mt tt ngn cn bt k k xm phm tnh c no;

c mo u qun l kh.

Cc chng i: cc kha gii thut WEP b tha hip.

Chng thc kha chia s vi gii thut WEP (hnh 4.4)

Cc tn thnh: tnh bo mt tt ngn cn bt k cc k xm nhp no; cmo u qun l kh.

Cc chng i: s dng mt c ch yu cu/p ng khng bo mt; cc khagii thut WEP b tha hip.

Chng thc m LAWN/MOWER

LAWN/MOWER l mt kin trc s dng cc giao thc chung v phn mmngun m tch ngi dng trn mng WLAN ra khi mng cho n khi h c xcnhn bi mt h thng tnh ton. Mt khi c xc nhn, cc quy tc c thm vorouter n cho php khch hng giao tip trong mng ni dy. Nh mt bin php bomt b sung, a ch MAC v IP ca khch hng c m ha cht cng trong cache

nh MOWER ARP.

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Cc tn thnh: c lp (ch B trnh duyt c kh nng SSL c yu cu); davo phn mm ngun m sn c t do; chng thc kh mnh m (SSL v Kerberos 128

bit).

Cc chng i: khng c truy cp ngoi mng WLAN m khng c chng thc.

Cng Gateway Firewall khng dy Ames ca NASA (WFG)

WFG tng t vi LAWN/MOWER ch c iu c s d liu trn nn RADIUSthay v trn nn Kerberos. WFG c thit k quanh mt nn n c kh nng nhtuyn, lc gi, chng thc, v DHCP. N hot ng bng cch gn cc a ch IP sutDHCP, xc nhn cc ngi dng qua mt trang Web c mt m ha SSL, cho phptruyn thng cho IP chng thc thng qua cng gateway, v ng nhp (logging). KhiDHCP c gii phng, c s dng li, b ht hiu lc hoc c thit lp li, WFGg b cc firewall theo a ch . iu ny nh a ch tng phn lin quan thng quahijacking (bt cc) mt IP chng thc sau khi ngi dng hp php ri mng.

Cc tn thnh: c lp nn; da vo phn mm ngun m; qun trusername/password trung tm.

Cc chng i: khng truy cp bn ngoi mng WLAN m khng c chng thc.

Cisco LEAP/RADIUS (gii thut WEP theo phin + Chng thc Mt khu) (hnh 4.5)

Cc tn thnh: chng thc username/password; qun tr username/passwordtrung tm; gii thut WEP theo phin c c t bt ngun t username/password.

Cc chng i: mc du Cisco s hu nhng n da phn ln vo cc chun

AAA (ngoi tr LEAP); phc tp; khi s dng VPN vi chi ph qun l ng k; phnmm khch hng (cc trnh iu khin, cc phn sn, cc tin ch) c cn li.

Hnh 4.5. Chng thc LEAP/RADIUS Cisco.

Bng 4.2. Cc c tnh ca cc kin trc bo mt mng WLAN.

c tnh Chng thc m LAWN/MOWER WFG LEAP/RADIUS

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gii thutw/WEP

Mt m ha gi X X

Kha WEP theo ngidng/theo phin

X

Username/password X X X

Logging (ng nhp) X X X X

c lp nn X X X

Mo u qun l thp X X

Ngun m X

4.7 Bo mtBo mt l mt trong cc quan tm hng u ca ai mun trin khai mt mng

LAN khng dy, y ban chun IEEE 802.11 hng vo vn ny bng cch cungcp WEP (Wired Equivalent Privacy)

Quan tm chnh ca ngi dng l mt k quy ry khng c kh nng :

Truy cp cc ti nguyn mng bng cch s dng thit b mng LAN khng dytng t, v

C th chim c lu thng mng LAN khng dy (nghe trm)

4.7.1 Ngn nga truy cp ti ti nguyn mng

N c thc hin bng cch s dng mt c ch chng thc trong mt trmcn chng minh s nhn bit kha hin thi, n tng t nh mng LAN ring ni dy,n pht hin k xm nhp (bng cch s dng mt kho vt l) ni trm lm vic cahn ti mng LAN ni dy.

4.7.2 Nghe trmVic nghe trm c ngn nga bng cch s dng gii thut WEP, n l mtB to s gi ngu nhin (PRNG) c khi to bi mt kho b mt dng chung.PRNG ny to ra mt chui kha cc bit gi ngu nhin c chiu di bng vi chiu dica gi ln nht m c kt hp vi gi n/i ang to ra gi c truyn trongkhng gian.

Gii thut WEP l mt gii thut n gin c da vo gii thut RC4 caRSA, n c cc thuc tnh sau:

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tin cy mnh m: cc tn cng mnh m ti gii thut ny kh thc hin biv mi khung c gi vi mt vector khi to (IV) bt u li PRNG chomi khung.

T ng b: Gii thut ng b da vo mi bn tin, n c cn lm vic

trong mt mi trng khng kt ni, ti cc gi b mt (nh bt k mngLAN no).

4.8 Kin trc khuyn ngh

Phn ny xng mt kin trc mng WLAN da vo cc nguyn l sau y:

Mng khng dy c xem xt nh mt mng khng bo mt c hu. Nh vy,n cn phi c firewall bn ngoi.

S mt m ha theo gii thut WEP d b b gy vi cc gii thut thng thng,khng tin cy bo mt d liu.

WEP cung cp t nht mt s bo v khi xm nhp v n nn c s dng nuc chi ph qun l thp.

Khi yu cu mt m ha d liu mnh, cn s dng gii php VPN/IPsec

V truy cp ti mng khng dy kh iu khin hn so vi cc truy cp ti mngni dy, nn cn thc hin bo dng khi cung cp truy cp t mng WLAN n ccmng khc (thm ch l mng Internet) m khng c chng thc trc.

Kin trc tng quan

Hnh 4.6. Kin trc mng WLAN c xng.

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Kin trc c xng (hnh 4.6) c th thay th mng khng dy bn ngoifirewall. Ngoi ra, n s dng cc kha WEP tnh trong mng WLAN c chi phqun l thp v cung cp mt phng tin D tm Xm nhp Mng (NID) theo dicc cuc tn cng bt ngun t mng WLAN n mng Internet v cc mng khc.

Ngi ta khuyn ngh rng phm vi a ch IP v tn min ca mng khng dyu lin kt vi mng ni b hin hu bt k. iu ny s cho php tch cc lu thngkhng dy tt hn v gip nhn din v lc lu thng ti/ra khi mng ny.

Kin trc c xng hp nht hu ht cc nguyn l thit k ban u trongkhi cho php mt vi mc truy cp ti mng Internet t mng khng - VPN, t ngidng khng c xc thc. Gi s lan truyn RF gii hn trong vng kho st v thitlp cng sut anten v my pht thch hp, mng WLAN khng biu hin bt k duhiu quan trng no e da n mng ni b nh mng Internet.

V roaming gia cc AP vn nm trong min s hu, ngi ta khuyn co cao

rng tt c AP phi c mua t cng nh cung cp. iu ny s bo m mt trmcui c trang b vi bt k card NIC tng thch chun IEEE 802.11 s roam gia ccAP. Ngoi ra, bt k ci tin bo mt chuyn bit mi no c gii thiu yu cu ccAP ng nht.

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CHNG V

GII PHP MNG WIRELESS CHO

HC VIN CHNH TR KHU VC III

5.1 Kho st hin trng

Hc vin chnh tr khu vc III gm c 2 khu:

+ Khu 1: l vn phng v phng k thut ( 14 phng ban) lp t 1 Router khng dy, 3my ch dng chung 1 a ch Subnet, IP c cp ng cho cc my Client.

+ Khu 2: l khu ging dy v k tc x (gm 13 khoa v 4 ta nh) lp t 2 Switch,mi ta nh 5 tng,cao 200m, cch nhau 10 m, mi ta t 2 AP chun g.

-Mng c dy: gm 200 nt mng, 7 Switch 2960.

5.2 Thit k mng v gii php bo mt cho mng Wireless

M hnh logic ca mng Wireless

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Trong khi cu hnh cc thit b khng dy cn ch :

a/ Cu hnh router khng dy

Hy s dng mt cp mng i km vi router khng dy, bn cn tm kt ni my tnh

ca mnh ti mt trong nhng cng cn trng ca router khng dy (bt c cng no mkhng c nhn Internet, WAN, hoc WLAN). Bn hy bt my ca mnh, PC ca bn st ng kt ni vo router.

M IE v g a ch IP cu hnh router, c th s phi g mt khu. Tn mt khu va ch s rt khc nhau, ph thuc vo router m bn mua, bn cn xem hng dntrong ti liu i km. Bng di y l mt cu hnh cc a ch, tn mt khu thngc s dng mc nh ca hng sn xut.

Router Address Username Password

3Com http://192.168.1.1 admin admin

Linksys http://192.168.1.1 admin admin

Netgear http://192.168.0.1 admin password

IE s hin th trang cu hnh router ca bn. Hu ht cc cu hnh mc nh u tt, tuynhin bn cn ch :

- Tn mng khng dy, thng gi l SSID: Ci tn ny xc nh mng ca bn.

Do , cn phi t tn khc v khng ging nh ci tn m hng xm ca bnang s dng.

- M ha khng dy WEP, v bo v truy cp khng dy (WPA): s gip mngkhng dy ca bn bo mt hn. Hu ht cc router, bn cn cung cp v k t router ca bn t sinh cc kha. Bn hy g cc k t ny duy nht ng lp li( bn cng khng cn phi nh cc k t ny). Sau bn hy ghi li cc kha mrouter t sinh.

- Mt khu qun tr, cha kha cu hnh mng khng dy: Cng ging nh ccmt khu khc, mt khu cho router khng th l mt t no trong t in, ncn phi l s kt hp cc k t, s, biu tng. nhng cng rt quan trng l bn

phi nh chng, bi bn s phi g mt khu khi ng nhp cu hnh li router.- Cc bc cu hnh c th rt khc gia cc router, nhng bao gi trong mi ln

thit lp cu hnh cng l c cc mc nh: Save Settings, Apply, v OK lu licc thay i ca bn.By gi, bn c th tt kt ni mng t my bn ang dng cu hnh.

b/ Cu hnh AccessPoint & ISA server cp quyn truy cp internet v ti nguyn nib cho nhn vin.

*Cu hnh DHCP ca AP: Dng my 1 truy cp web cu hnh AP (http://192.168.1.1)Trang 48


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- Lan: IP address: 192.168.1.3, s.mask: 255.255.255.0, d.gateway:192.168.1.1

- DHCP: start IP: 192.168.1.11 end IP: 192.168.1.100

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- Cu hnh default route trn AP

- Kt qu

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* Thc hin ti my 2: Log on Domain Administrator. iu chnh access rule HTTPoutbound, tab From: thm network VPN Clients

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* Thc hin ti my 3: Log on Administrator.

Gi lp laptop nhn vin:b1. Ngt kt ni AP.

b2. To conection VPN kt ni server 192.168.1.2 (ISA)

b3. Kt ni AP.

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b4. Kt ni VPN. Username: NV1, password: 123

b5. Truy cp d liu trn M1 (DC ).

b6. Truy cp internet.

Gii php wireless cho hc vin chnh tr khu vc III p ng c:

- Khng lm nh hng n h tng kin trc cn phng

- Cung cp kt ni tm thi vi mng cp c sn: ngi s dng mng wireless c

th truy nhp thng tin thi gian thc ti bt k ni no trong phng m khng cn

quan tm n ch cm card mng.

- Ci t rt nhanh chng v d dng hn nhiu so vi mng cp (wired).

- Gi h hn so vi mng cp m vn m bo tc v cht lng cuc truyn

- H tr kh nng m rng mng mt cch nhanh chng v linh ng

S ngi trong phng cng truy nhp ng thi vo mng cp s dng cc dch v

c cung cp qua thit b Access Point ln ti 2048 ngi vi mi ngi l i din

cho mt thit b u cui. iu ny c bit hu ch trong trng hp mt cuc hi

tho, cuc hp hay bui o to c t chc ti y. Mi ngi u c th ng thi

truy nhp vo mng qua thit b Access Point vi cc my tnh c ci Client Adapter

ph hp.

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KT LUN V HNG PHT TRIN CA TI

Mng khng dy hin nay pht trin rt nhanh l nh vo s thun tin ca n.

Hin nay cng ngh khng dy, nht l Wi-Fi hin ang c ng dng ngy cng

mnh m trong i sng. Nhng a s mi ngi u ch s dng Wi-Fi cc lnh vc

lin quan n my tnh m khng bit rng bng sng Wi-Fi, ngi dng dng my tnh

iu khin h thng n, qut, my lnh, l si, my ti, h thng nc Nhng

vn quan trng nht ca mng khng dy hin nay l s bo mt ca n cha c mt

gii php no n nh.

Trong ti ny chng em c gng tng hp tt c nhng c ch bo mt v tt

c nhng kin thc c bn v Cng ngh mng khng dy. Vi kh nng nghin cu,

thi gian cn hn ch cng nh vn v thit b phn cng, phn mm cho mng

khng dy nn vn cn c nhng thiu st trong ti ny. Tuy nhin vi nhng g

nghin cu v tm hiu th: Mng khng dy theo chng em ngh l mt gii php hay

v thi i, n gip cho chng ta tit kim c thi gian cng nh cng sc trong vic

lp t cng nh s dng.

Trong iu kin cho php, cng vic ch mi dng li ch gii thiu v tm hiu,

nhng nhng cng vic nghin cu s c tip tc khi :

- H tr tnh nng Multi SSID cho php ngi dng phn chia mng thnh nhiu

mng con m bo rng ngi ngoi ch c th truy cp vo internet m khng tip

cn c ti nguyn cng ty khi kt ni vo mng khng dy.

- Tm hiu su hn k thut bo mt hin nay ang c s dng ph bin.- Nghin cu cc l hng v cc cch tn cng mng WLAN tm ra phng

php bo mt hiu qu cho mi ngnh gip cho vic qun tr v trao i ti nguyn gia

cc trm lm vic trong mng WLAN.

Chng em xin chn thnh cm n c Nguyn Th Minh Thi tn tnh gip

chng em trong thi gian thc hin ti v trong ny cng khng trnh khi nhng

thiu st, mong thy c gp chng em c th hon thin tt hn.

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TI LIU THAM KHO

[1] Nguyn Hng Sn, K thut truyn s liu, NXB Lao ng X hi

[2] Nguyn Minh Nht ,Bi ging An ton mng, Khoa Cng Ngh Thng Tini Hc Duy Tn

[3] Nguyn Thc Hi,Mng my tnh v cc h thng m.

[4] Building A Cisco Wireless LAN (Syngress Publishing 2002)

[5] Cc Website :

- http://www.wlana.com

- http://www.quantrimang.com

-http://www.cuocsongso.com.vn

-http://www.haiphongit.com.vn

-http://www.nhatnghe.com.vn

-http://www.adminviet.com.net

-http://www.3c.com.vn

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MC LC

LI NI U ................................................................................................................ 1

CHNG I .................................................................................................................. 2

GII THIU V MNG WLAN ......................................................................................21.1 Cc ng dng ca Mng WLAN .....................................................................................21.2 Cc li ch ca mng WLAN............................................................................................3

CHNG II ..................................................................................................................6

NGUYN TC HOT NG CA WLAN .................................................................... 6

2.1 Cch lm vic ca mng WLAN..........................................................................................62.2 Cc cu hnh mng WLAN...................................................................................................62.2.1 Mng WLAN c lp (mng ngang hng)....................................................................92.2.2. Mng WLAN c s h tng (infrastructure)................................................................92.2.3 Microcells v roaming..................................................................................................10

2.3 Cc ty chn cng ngh......................................................................................................102.3.1 Tri ph........................................................................................................................102.3.2 Cng ngh tri ph nhy tn (Frequency Hopping pread Spectrum)..........................102.3.3 Cng ngh tri ph chui trc tip (Direct Sequence Spread Spectrum)...................112.3.4 Cng ngh bng hp (narrowband)..............................................................................122.3.5 Cng ngh hng ngoi ( Infrared )...............................................................................12

2.4 Cc ch tiu k thut ca mng WLAN..............................................................................122.4.1 Phm vi/Vng ph sng...............................................................................................122.4.2 Lu lng.....................................................................................................................122.4.3 S ton vn v tin cy.............................................................................................132.4.4 Kh nng kt ni vi c s h tng mng ni dy......................................................132.4.6 Nhiu ...........................................................................................................................132.4.7 Tnh n gin v d dng trong s dng.....................................................................132.4.8 Bo mt.........................................................................................................................142.4.9 Chi ph..........................................................................................................................142.4.10 Tnh linh hot.............................................................................................................142.4.11 Tui th ngun pin cho cc sn phm di ng..........................................................152.4.12 An ton.......................................................................................................................15

CHNG III ...............................................................................................................16

CHUN IEEE 802.11 .................................................................................................. 163.1 Li gii thiu.......................................................................................................................163.2 Kin trc IEEE chun IEEE 802.11 ...................................................................................16

3.2.1 Cc thnh phn kin trc..............................................................................................163.2.2 M t cc lp chun IEEE 802.11...............................................................................173.2.3. Phng php truy cp c bn: CSMA/CA.................................................................17

3.2.4 Cc chng thc mc MAC..........................................................................................19

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3.2.5 Phn on v Ti hp ..................................................................................................193.2.6 Cc khng gian khung Inter (Inter Frame Space)........................................................203.2.7 Gii thut Exponential Backoff ...................................................................................21

3.3 Cch mt trm ni vi mt cell hin hu (BSS)................................................................213.3.1 Qu trnh chng thc....................................................................................................22

3.3.2 Qu trnh lin kt..........................................................................................................223.4 Roaming...............................................................................................................................223.5 Gi ng b.........................................................................................................................223.6 Tit kim nng lng..........................................................................................................233.7 Cc kiu khung....................................................................................................................233.8 Khun dng khung..............................................................................................................24

3.8.1. Tin t (Preamble).......................................................................................................243.8.2 u mc (Header) PLCP ............................................................................................243.8.3 D liu MAC................................................................................................................24

3.9 Cc khung nh dng ph bin nht....................................................................................273.9.1 Khun dng khung RTS...............................................................................................27

3.9.3 Khun dng khung ACK..............................................................................................273.11 Hm Phi hp im (PCF)...............................................................................................283.12 Cc mng Ad hoc..............................................................................................................283.13 H chun IEEE

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