chuyên Đề voip over wlan - ptit

8/3/2019 Chuyn Voip Over WLAN - Ptit

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Chuyn K Thut Chuyn Mch Ti: Voip Over WLAN

LI NI U--------------o0o-------------

Trong nhng nm gn y, Internet bng n v pht trin mnh m cng vi n

l mt lot cc cng ngh khc cng pht trin ngy cng ln mnh v chim u th

hn, in hnh l VoIP. ng thi cng ngh khng dy cng tr ln kh ph bin,

ngy cng c nhiu ngi truy nhp Internet qua mng khng dy nh Wi-Fi, Wimax.

VoIP c u im l chi ph tng i r, cht lng cuc gi cng kh tt, tuy nhin

tt c ngi s dng VoIP u gp mt vn l tin hnh c cuc gi th h

phi c mt chic my tnh kt ni Internet. chnh l mt nhc im ln ca

VoIP. Do mt xu hng mi chnh l kt hp hai cng ngh VoIP v cng nghkhng dy, dn ti s ra i ca VOIP over WLAN. Trong bi cnh ny VoIP qua

WLAN xut hin nh mt gii php r rng nht cho cc thng tin lin lc bng ging

ni ca cc loi in thoi di ng, mt trong nhng ng dng khng th thiu hng

ngy ca con ngi. Tuy nhin, kt hp hai cng ngh trn hin nay l rt kh khn v

phc tp, i hi k thut chuyn mn cao.

VOIP over WLAN k tha cc u im ca VoIP v mng khng dy, n to ra

s linh ng trong VoIP, p ng nhu cu ngy cng cao ca con ngi.

T thc tin trn, ti ny s tin hnh nghin cu cng ngh VOIP over

WLAN vi nhng ni dung chnh sau:

Chng 1: Tng quan v VOIP over WLAN

Chng 2: Cc cng ngh c bn ca VOIP over WLAN

Chng 3: Nhng vn c bn trong VOIP over WLAN

Do cn nhiu hn ch v trnh v thi gian nn lun vn khng th trnh khinhng thiu st, rt mong nhn c kin ng gp ca cc thy c v bn c.

Chng Em xin chn thnh cm n!

H Ni, thng 6 nm 2011

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CHNG I: TNG QUAN V VOIP OVER WLAN--------------o0o-------------

1.1 Gii thiu chung

Ngi s dng mng khng dy c lin quan n nhiu lnh vc nh doanh nghip(qun l, CNTT v in thoi di ng khc ca trng cng nhn), gio dc (Hiutrng, gio s, nhn vin bo tr), y t (bc s, y t, k thut vin), sn xut (gim stvin, ngi kim sot cht lng, cc chuyn gia), bn l (qun l, nhn vin hng tnkho, vn chuyn / ngi nhn). Mt s l do lm cho mng WLAN cn thit cho hotng ca h. Nhng ngi dng in thoi di ng cao, hoc v h khng c mt bnlm vic hoc v h khng ngi trn my ca h mt s lng ng k thi gian. Hcn phi c ngay lp tc c th truy cp vo mng (hin cc chin lc truyn thngchnh l ging ni, cng vi tin nhn). H cng yu cu truy cp nhanh n cc d liuquan trng.

Trong bi cnh ny VoIP qua WLAN (VoipWLAN) xut hin nh mt gii phpr rng nht cho cc thng tin lin lc bng ging ni ca cc loi in thoi di ngang l iu khng th thiu hng ngy ca con ngi. in thoi IP yu cu bngthng thp di 64 kb/s, do ngi ta c th gi nh rng VoIP l d dng sdng trn cc mng LAN khng dy. Tuy nhin kt hp hai cng ngh hin nay l khkhn v rt phc tp. Cc th nghim cho thy rng thm ch mt lng nh lu lngtruy cp d liu trn cng mt mng c th dn n cht lng m thanh xung cpnghim trng v b cuc gi, ngay c vi cc tnh nng QoS cho php.

Hnh 1.1: M hnh Voip over WLAN

L do chnh l, khi x l ting ni v lu lng truy cp d liu trn cng mtmng, tranh chp phi c qun l v s chm tr v jitter ch khng phi l t lchuyn tip. Hu ht cc nh cung cp ch bt u iu chnh sn phm ca mnh chohi t d liu ging ni, do hiu sut ca VoIP v thi gian thc cc ng dng nichung trn phng tin truyn thng khng dy c th l mt vn . Lu rng cc

kh khn trong vic tm kim gii php thch hp QoS bt ngun t mt s thuc tnhc hu ca mng WLAN. Trong cc mng khng dy gi t l li c th c trong

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phm vi 10-20%. Hn na tc bit thay i ty theo rng bng thng ca knhtruyn, do thi gian thit lp kt ni khng th gi trong sut thi gian truyn tin.

VOIP over WLAN l cng ngh da trn s kt hp gia thoi qua giao thcInternet v cng ngh khng dy. VOIP over WLAN mang li li ch quan trng trong

vic hi t thoi vi d liu. Vi VOIP over WLAN, cc bnh vin, cng ty, ca hng,khu nh chung c c th gim chi ph in thoi v cc ng dng v di ng.

1.2 Cu trc mng VOIP over WLAN

Kin trc tng qut ca VOIP over WLAN c trnh by trong hnh v:

Hnh 1.2: Kin trc tng qut ca VOIP over WLAN

Wireless IP Phone: Thit b ny tng t nh mt chic in thoi di ng, Tuynhin n phi bao gm b tng thch, giao tip vi mng LAN khng dy,

kt ni ti h thng in thoi. Ngi s dng c th thit lp mt cuc gi ni httrc tip ti mt ngi khc trong mng, tng t vi h thng in thoi cng shay mt cuc gi ra bn ngoi thng qua mng PSTN.

Call manager: Qun l cuc gi trong mt mng.

Voice gateway: Thit b giao tip in thoi IP vi cc loi mng v h thng khc.Th d, mt gateway c th giao tip mt mng VoIP ti mng PSTN truyn thng.iu ny c th to ra mt tuyn truyn thng gia in thoi IP v ngi s dngin thoi thng thng.

Wireless LAN infrastructure: Mt mng LAN khng dy qun l s truyn ti cccuc gi VoIP khng dy thng qua mi trng. Lm cho ngi s dng c th tdo hn khi m thoi.

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B tng thch trong in thoi IP khng dy phi kt ni ti mt Access Pointtrn mng trc khi ngi s dng c th tin hnh cuc gi. Khi ngi s dng btu mt cuc gi, yu cu s c gi qua Access Point v qua mng hu tuyn ti b

phn qun l cuc gi, ti y yu cu c x l. to ra tnh linh ng, b tngthch LAN khng dy trong in thoi IP khng dy ca ngi s dng ang dichuyn phi lin kt vi Access Point khc khi cn thit. cuc gi gia hai ngis dng khng ngt qung th s chuyn tip gia cc im truy cp phi t hn 100msnu khng cuc gi s b ngt kt ni.

Nu ngi s dng quay s ti mt s bn ngoi, b qun l cuc gi s chuyncuc gi ti PBX hoc Internet hon tt cuc gi. Nu PBX khng c tch hp x l lu lng IP, th mt cng VoIP trung gian s chuyn lu lng VoIP thnh tnhiu tng t.

1.3 C s h tng mng

Mt h thng in thoi khng dy yu cu mt c s h tng mng ph hp.Chng ta c th s dng ngay cc Access Point hin ti hoc s dng cc thit b mihn. Tuy nhin, phi m bo v dung lng v tr chuyn vng sao cho cuc gi ccht lng tt nht. Sau y chng ta s xt cc c s h tng mng khng dy c bn.

1.3.1 Mng Access Point truyn thng

Cch thng thng trin khai mng LANs khng dy bao gm vic lp t ccAccess Point thng minh thick, cc Access Point ny s kt ni vi nhau thng quamt mng chuyn mch Ethernet nh trong hnh 1.3

ThickAccess Point

ThickAccess Point

ThickAccess Point

ThickAccess Point

ThickAccess Point

Chuyn mchEthernet

Hnh 1.3: Mng li Access Point

Khi ngi s dng thc hin chuyn vng, thit b client khng dy ca h slin kt vi mt Access Point m c tn hiu bo mnh nht. V Access Point ch l ccthit b LAN khng dy trn mng nn chng phi c mt b x l thng minh nhnra cc thnh phn ch yu ca mt mng LAN khng dy m chun 802.11 khng

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cung cp. Access Point cng thc hin chc nng qun l vo bo mt i vi cc ktni khng dy c bn.

Access Point truyn thng thng kt ni vi nhau s dng chuyn mch chunEthernet, mt loi chuyn mch chim u th trong h tr kt ni hu tuyn. Mt kha

chuyn mch kt ni mt ngi s dng, chng hn mt Access Point, ti mt ngis dng khc m khng gy nh hng ti vic truy cp ca nhng ngi s dng cnli. Switch gip ci thin thng lng so vi mt hub chia s bi v n c t xung t(min) hn. Ngi s dng khng cn phi i cho n khi ngi s dng khc ktthc vic gi d liu. ti u ha hiu sut ca h thng th s dng switch l rtcn thit.

Bt tay (Handoffs) t mt access point thick ti mt Access Point khc c thrt chm trong vic h tr cc ng dng VoIP khi ngi s dng chuyn vng. Vn y l s kt hp ca Access Point v chuyn mch Ethernet truyn thng trong gii

php ny l cha nhanh. Chng hn, mt vi ngi s dng in thoi LAN khng

dy s b rt cuc gi khi chuyn vng t mt Access Point ny ti mt Access Pointkhc. Ty thuc vo tng mng LAN khng dy h tr ng dng thoi, bn c thkim tra tr chuyn vng v tr ny nn nh hn 100ms gim thiu rt cucgi.

Phng php s dng Access Point thick yu cu vic qun l phn quyn h tr chc nng remote. D nu cc hng sn xut access point thick cung cp mth thng qun l tp trung nhng mng LAN khng dy vn phi qun l cc lu lng

b xung cho vic lin lc u n vi access point. Lu lng ny c th lm gimdung lng ca mng LAN khng dy.

1.3.2 Mng chuyn mch khng dy

Nh l mt gii php c hiu qu thay th m hnh Access Point thick truynthng, xt mng chuyn mch khng dy nh trong hnh 1.4. tng chnh y ls dng cc Access Point thin tp trung vo hiu sut v tin cy cao. AccessPoint ch n thun tun theo giao thc 802.11, trong khi switch cung cp s thngminh cn thit mang li s bo mt, qun l v hiu sut c hiu qu. c im nydn n mt gii php c hiu qu gip tha mn nhng ng dng v thoi v gimmt cch ng k tng chi ph cho hu ht cc ng dng.

Gii php chuyn mch khng dy c chi ph thp hn trong vic tng vng phsng v hiu sut v mt cch tng i cc Access Point ny khng t.

Mt chuyn mch khng dy thng minh cung cp chuyn vng nhanh hn giiphp ca mng LAN khng dy thng thng. iu ny cho php h tr hiu qu ccng dng VoIP. Ngi s dng in thoi khng dy c th chuyn vng mt cch ddng trong khi ti lin kt vi Access Point khc.

Chuyn mch khng dy thc hin c hiu qu trong vic pht hin cc AccessPoint gi mo (Rogue Access Point). Nu mt ngi no hay mt hacker truy cpvo mt Access Point gi mo mt hu tuyn ca switch th switch khng cho phptruy cp thng qua Access Point gi mo ti mng. Switch khng dy cha tt c cc

cu hnh bo mt c th c gi trong mt server c kha, n s ngn chnhacker vi cu hnh ca n.

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Lightweight

Access Point

Lightweight

Access Point

LightweightAccess Point



Wireless LANController

Hnh 1.4: Mng chuyn mch khng dy

Mt li ch khc ca gii php chuyn mch khng dy l n kh n gin vicc nhn vin IT. Cng ngh khng dy rt mi vi hu ht cc cng ty v cc nhnvin IT hin ti n khng ging vi cc c im v tuyn ca mng LAN khng dy.Phng php chuyn mch khng dy t tp chung vo sng v tuyn m ch yu tpchung vo chuyn mch l vn quen thuc ca cc nh qun tr mng.

Mng Mesh

Mt mng mesh l mt lot nhng b truyn sng radio. Mi mt my pht cth kt ni ti t nht 2 my pht khc. Chng to ra nhng m my tn hiu radioxuyn sut thnh ph. Tn hiu c chuyn i t router ny n router khc thngqua nhng m my ny.

Trong mt vi mng, tn hiu chuyn t mt ngi nhn ti ngi khc cho nkhi h nhn c mt nt mng c kt ni ti Internet. Nhng mng khc s dngnhng nt mng gi l backhaul nodes. Nhng nt mng ny thu thp tt c nhng dliu t rt nhiu b truyn pht khc v chuyn nhng d liu ny qua Internet bngvic gi chng ti 1 router s dng kt ni c dy. Backhaul nodes thng l nhng

nt mng im-ti-im(point-to-point) hoc nhng nt im ti nhiu im (point-to-multipoint). Chng c th kt ni t mt im kt ni ny n im kt ni khc hocchng c th kt ni mt im ti vi im.

Nu bn s dng my tnh xch tay kt ni ti Internet trong mt mng mesh,bn c th hnh dung kt ni din ra nh sau:

1. My tnh ca bn tm v pht hin ra mt mng ngay , v bn truy cp vomng ny

2. Giao thc iu khin mng mesh s tm ra ng kt ni tt nht cho d liuca bn truyn i. Mng ny s tm ra ng i t my bn n mt kt nic dy hoc nt mng backhaul nodes sao cho tn t nt mng nht

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3. D liu ca bn s c chuyn theo ng ng nh giao thc thit lptrc . Khi d liu ca bn tm n mt nt mng c kt ni c dy, n struyn qua Internet v chuyn n a ch bn cn dng.

Multilayer

switch

Mesh Access Point

Multilayerswitch

WLANController WLAN

Controller

Hnh 1.5: Cu trc mng Mesh khng dy

Mt mng mesh a ra nhiu tuyn ng song song t ngun ti ch. Thutton nh tuyn thng minh cho php mi node quyt nh tuyn ng no forward gi tin qua mng nng cao hiu sut. Nu lin kt gia mt cp node camt tuyn b nghn, th d, thut ton thit lp mt tuyn khc trnh xa lin kt ang bnghn. Nu mt node b hng th route thay th s c chn da trn thut ton nhtuyn.

c im ny lm cho mng mesh thch hp cho nhng khu vc m vic lp tmng LAN khng dy c cc Access Point truyn thng l khng kh thi. Mng meshcng thch hp cho vic thit lp mt mng khng dy tm thi v cc node backhauld lp t v tho g. Th d, c th thit lp nhanh mt mng mesh trong tnh trngkhn cp. Cc doanh nghip cng c li t mng mesh khi h cn mt mng kt nitm thi.

Li ch m mng mesh mang li rt nhiu. Nu bn cn mt gii php tt v Wi-Fi

nhng ni m vic lp t cp ti Access Point l khng th th mng mesh chnh lcu tr li. Chi ph lp t thp hn v s dng t cp hn v n tng tnh hu dngtrong nhng vng kh lp dy (difficult-to-wire) lm cho mng mesh tr ln sng gi.

1.4 Tn hiu trong VOIP over WLAN

H thng VOIP over WLAN s dng tn hiu thoi. Phn ny s cung cp mtci nhn tng quan v cc yu t chnh ca truyn thng thoi, th d nh cc c imca thoi, s ha tn hiu thoi, k thut nn, chun tn hiu cuc gi. Qua chochng ta thy c c s ca truyn thng thoi VoIP, l nn tng hiu r hn ccqu trnh tng tc xy ra trong mng hu tuyn dy v v tuyn dy.

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1.4.1 Tn hiu Thoi

Mt cuc gi IP khng dy truyn v nhn tn hiu cha tn hiu thoi (mthanh). iu ny l s kt hp gia VoIP v giao thc v tuyn. Ting ni ca chngta c dng sng v c tn s ch yu nm trong khong 0,3Hz-3,4KHz. T nhiu nm

nay h thng in thoi u c thit k h tr bng tn ny. Hu ht mi ngiu c th nghe tt bng tn ny. Tn hiu VoIP h tr cuc m thoi c tn s nmtrong bng tn trn rt tt.

Hu ht h thng VoIP khng c thit k truyn dn m thanh trung thc.Mt tn hiu m thanh c bng tn rng ln ti 10.000Hz c cht lng m thanh tthn, nhng n thc s khng cn thit cho hu ht cc ng dng. iu ny c bitng cho cc h thng thoi. Hn na s cn rt nhiu dung lng ca h thng htr m thanh cht lng, nht l i vi mng LAN khng dy th vn dung lnglun c quan tm hng u.

in thoi IP khng dy c nhiu bc tin xa hn so vi chun in thoi tngt. N s dng mt b m ha m thanh chuyn tn hiu thoi tng t thnh tnhiu s s dng mt chui cc s logic 0 v 1. Qu trnh ny c gi l chuyn itng t/s. Sau tn hiu d liu sau c truyn qua mng IP. Ti im cuinhn, mt b gii m s chuyn lung d liu s thnh tn hiu m thanh v c ara loa. Mi mt thit b cui, th d nh in thoi IP khng dy, gm c b m ha vgii m m thanh, thng c gi l audio codec.

Hu ht vic chuyn i tng t/s s dng m ha PCM, ly mu tn hiu 8000ln mi giy. Mt b m ha PCM dng 8 bits m ha cho mi mt mu. Kt quvic m ha l tn hiu s 64Kbps (8000 mu/giy x 8bits/mu) c gi qua mng

qua VoIP, k thut ny c hiu qu i vi tn hiu m thanh trong khong tn s300Hz-3.400KHz, th d nh ging ni ca con ngi. Tuy nhin, i vi cc tn hiuc tn s bin i trong mt khong ln hn, nh l m nhc th vic ly mu phthuc s phn b tn ss ca tn hiu .

1.4.2 Tn hiu Video

Video nh chng ta thy hng ngy qua vic xem ti-vi, video ch n thun lmt chui cc hnh nh tnh (cc khung) xut hin lin tc ti mt tc nht nh din t li chuyn ng ca mt vt no trong mt khung cnh. N cng c th baogm thm mt knh m thanh. So snh vi tn hiu thoi, tn hiu video c nhiu thay

i hn v bng tn, s thay i ny ph thuc vo nhiu yu t, nh tc khungvideo, kch c v phn gii hnh nh, hiu qu mu sc l t l bit ca tn hiu video. gim nh hng ti dung lng ca mng LAN khng dy, hu ht cc my quayWi-Fi ch phn gii i a l 640x480, 15 khung/1s v gii hn su ca musc cho mi pixel. Cht lng ca hnh nh t my quay Wi-Fi khng qu ti cho vicquan st nhng cnh t thay i hoc thay i chm, nhng i vi nhng mc tiu dichuyn nhanh hn nh l mt chic xe ang li trn ng cao tc, th s b bin dnghoc chuyn ng chm v tc khung l tng i chm. N khng nhanh chp ton b chuyn ng. 30khung/1s s cho ta video tng t nh mt chic ti-vi vchng ta c th xem c cc mc tiu ang di chuyn nhanh. Trong thc t, conngi khng th pht hin s khc nhau tc khung cao hn 30 khung/1s

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Mt lung video th, thm ch mt lung khi c x l, hot ng trongphm vi Mb/s. V hu ht cc h thng, nht l cc h thng v tuyn u c dunglng gii hn cho nn vic s dng thit b nn video c ngha quan trng. N gipgim bt bng thng yu cu cn thit h tr lung video. Thng s dng mt bcodec trong my quay khng dy nn hnh nh video s trc khi gi i.

Cc thut ton nn thng thng cho video thng nm trong chun MPEG(Moving Picture Experts Group). MPEG-4 l thut ton nn c tn tht, iu cngha trong qu trnh nn s lm mt mt vi thng tin trong video. Tuy nhin, tronghu ht cc trng hp, tn tht nh ny khng quan trng. Trong thc t, hu ht mingi khng th nhn ra s mt mt ny. Do MPEG-4 l mt chun m ha tt chovideo.

1.4.3 Lu lng VoIP

Lu lng VoIP bt u khi mt ngi s dng in thoi IP khng dy (im

cui A) quyt nh thc hin mt cuc ti mt ngi khc (im cui B). Callmanagernhn yu cu cuc gi ny v chuyn yu cu cho im cui B. Qu trnh ny thitlp mt kt ni trc tip hai im cui VoIP vi nhau. Khi im cui B tr li cucgi, c hai im cui xc nh kiu codec v trao i cng thng tin giao thc iukhin truyn dn (TCP) d dnh kt ni. Sau khi thit lp mt kt ni, s trao ixy ra gia hai im cui c lp vi Callmanager.

Khi mt cuc gi xy ra, im cui gi d liu thoi c m ha thng quagiao thc RTP (Real-time Transport Protocol) vi chun IETF RFC 3550. RTP cungcp chc nng u cui-u cui cho cc ng dng thi gian thc l thoi v video.RTP m mt cng TCP cho lung m thanh v mt cng khc cho iu khin (iukhin phng tin v thng tin phn hi cht lng dch v). Nu video c km theom thanh, th lung m thanh v video c gi thng qua cc phin RTP khc nhau.Gi RTP c tiu v ti cha d liu thoi.

Vi h thng VOIP over WLAN, TCP thit lp kt ni gia cc im cui, nhngUDP (giao thc gi tin ngi dng) c nh ngha trong IETF RFC 768, mang giRTP gia cc h thng cui. Phn ln cc gi ca cuc thoi c truyn qua lin ktv tuyn l nhng gi UDP c gi trong khung d liu 802.11. Trong hu ht cctrng hp, nhng khung ny c gi theo kiu unicast (c nh hng) gia haiim cui.

C rt nhiu chun v giao thc ring cho vic iu khin cc cuc gi qua hthng VoIP. H323, SCCP (Skinny Client Control Protocol) v SIP (Session InitiationProtocol) l nhng giao thc ch yu.

CHNG II: CC CNG NGH C BN VOIP OVER WLAN--------------o0o-------------

2.1 Cng ngh VoIP

2.1.1 Tng quan v VoIP

Voice over IP: c hiu l cng ngh truyn thoi qua mi trng IP. V cim ca mng gi l tn dng ti a vic s dng bng thng m t quan tm ti thi

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gian tr lan truyn v x l trn mng, trong khi tn hiu thoi li l mt dng thi gianthc, cho nn ngi ta b sung vo mng cc phn t mi v thit k cc giao thc

ph hp c th m bo cht lng dch v cho ngi dng. N khng ch truynthoi m cn truyn cho cc dch v khc nh truyn hnh v d liu.

2.1.2 u nhc im ca VoIPu im:

Gim chi ph cc cuc gi.

Tit kim bng thng do phi nn dung lng xung thp a ln mng.

Tn dng c c s h tng sn c.

Hiu sut ng truyn cao do c c ch pht hin khong nng.

Dch v a dng. Nhc im:

Cht lng dch v tng i thp do k thut nn v mt mt gi tin.

Ting vng do tr gy nh hng ln n cht lng cuc gi.

2.1.3 Kin trc mng VoIP

Hnh 2.1: in thoi PSTN v VoIP

Hnh 2.1 biu din s phn loi tng cao ca kin trc VoIP. N bao gm bn imcui truyn thng, hai trong s chng l thit b PSTN (A v B) v 2 ci cn li l thit

b Internet (C v D). Ch rng mt thit b PSTN thng l mt chic in thoitrong khi mt thit b Internet c th l phn mm chy trn PC hoc l mt thit btch ri nh mt chic VoIP cm tay. By gi hy xem xt cc tnh hung sau:

Cuc gi C-D

Trong tnh hung ny, c hai im cui l thit b Internet v do cuc thoi khngbao gi di khi tn min IP. Cc giao thc bo hiu thng s dng l im-imcho kt ni trc tip, SIP hoc H323.

Cuc gi A-D

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Trong trng hp ny, chng ta c hai im cui, mt l thit b Internet v mt lthit b PSTN. cho php truyn thng gia thit b VoIP v thit b PSTN th phis dng mt cng VoIP

y, gateway (cng) l mt thc th logic n kt ni gia hai mng khc nhau, th

d nh PSTN v IP trong trng hp ca VoIP. Cng c th c phn loi da trncc tiu chun khc nhau. Da trn chc nng mt cng c th l cng bo hiu hoccng phng tin. Cng bo hiu bo m kt ni gia bo hiu VoIP v bo hiuPSTN. N thc hin chc nng ny bng cch phin dch lnh bo hiu PSTN thnh

bn tin bo hiu tng ng trong VoIP. Mt khc cng phng tin chu trch nhimbo m rng tuyn thoi t vng PSTN (A-1) ti vng IP (1-D) c xuyn sut.

Cuc gi A-B

Trong trng hp ny chng ta c hai im cui l PSTN. Thng th mt cuc gi shon ton c truyn i trong mng PSTN. Tuy nhin, cc nh cung cp dch v inthoi trong hnh 2.1 quyt nh trin khai Gateway-2 trung k chuyn cuc gi thoiti Internet. Cuc gi c truyn i nh l VoIP gia cc cng 1 v 2 v chuynmch in thoi gia A v 1, B v 2.

Ch rng trong hnh 2.1, chng ta gi thuyt rng Gateway-1 v 2 ng vai tr nhl cng bo hiu v cng phng tin.

2.1.4 Phng thc hot ng

VoIP chuyn i tn hiu ging ni thng qua mi trng mng (IP basednetwork). Do vy, trc ht ging ni (voice) s phi c chuyn i thnh cc bits( digital bits) v c ng gi thnh cc packet sau c truyn ti qua mng

IP network v cui cng s c chuyn li thnh tn hiu m thanh n ngi nghe.Tin trnh hot ng ca VoIP thng qua 2 bc:

Call Setup: trong qu trnh ny , ngi gi s phi xc nh v tr ( thng qua a chca ngi nhn) v yu cu 1 kt ni lin lc vi ngi nhn.Khi a ch nginhn c xc nh l tn ti trn cc proxy server th cc proxy server gia 2 ngis thit lp 1 cuc kt ni cho qu trnh trao i d liu voice

Voice data processing: Tn hiu ging ni (analog) s c chuyn i sang tn hiu s( digital) ri c nn li nhm tit kim ng truyn (bandwidth) sau s c m

ha (tng bo mt). Cc voice samples sau s c chn vo cc gi d liu c vn chuyn trn mng. Giao thc dng cho cc gi voice ny l RTP (Real-TimeTransport Protocol).1 gi tin RTP c cc field u cha d liu cn thit cho vic bindch li cc gi tin sang tn hiu voice thit b ngi nghe. Cc gi tin voice ctruyn i bi giao thc UDP . thit b cui, tin trnh c thc hin ngc li.

2.1.5 Cc giao thc bo hiu

VoIP cn 2 loi giao thc : Signaling protocol v Media Protocol.

Signaling Protocol iu khin vic ci t cuc gi. Cc loi signaling protocols bao

gm: H.323, SIP, MGCP, Megaco/H.248 v cc loi giao thc c bn quyn ringnh UNISTIM, SCCP, Skype, CorNet-IP,

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Media Protocols: iu khin vic truyn ti voice data qua mi trng mng IP. Ccloi Media Protocols nh: RTP ( Real-Time Protocol) ,RTCP (RTP controlProtocol) , SRTP (Secure Real-Time Transport Protocol), v SRTCP (SecureRTCP).

Hnh 2.2: Mng VoIP v cc giao thc s dng trong mng

Cc nh cung cp c th s dng cc giao thc ring hay cc giao thc m da trnnn ca mt trong hai giao thc tiu chun quc t l H.323 v SIP. V d Nortel sdng giao thc UNISTIM (Unified Network Stimulus) Cisco s dng giao thc SCCP

( Signaling Connection Control Part) Nhng giao thc ring ny gy kh khn trongvic kt ni gia cc sn phm ca cc hng khc nhau.

2.1.5.1 Giao thc MGCP

MGCP (Media Gateway Control Protocol) c pht trin iu khin cng ra vo(gateway) cho h thng PSTN. MGCP nh ngha giao thc, hay qui tc iu khin (control) cho cng ra vo ca VoIP m kt ni vi thit b iukhin cuc gi (call-control) bn ngoi, thng c gi l tc nhn gi(callagent), MGCP cung cp tnh nng bo hiu cho nhng thit b r tin v trra nh l cng ra vo, y l nhng thit b m khng c tnh nng to y

tn hiu cho thoi nh l H323. Thc cht, bt k lc no khi xy ra s kinnh qu trnh nhc ng nghe (off hook) ti cng thoi (voice port) ca cng ravo, cng thoi s thng bo s kin n tc nhn gi. Sau tc nhn gi totn hiu n thit b m cung cp dch v, nh l tn hiu mi quay s.

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Hnh 2.3: Cu trc giao thc MGCP

2.1.5.2 Giao thc H323

Hnh 2.4: Cu trc giao thc H323

y l giao thc chun ca ITU, c pht trin cho h thng a phngtin(multimedia) phng tin trong mi trng khng hng kt ni

(connectionless) nh LAN. Giao thc ny chuyn i cc cuc hi thoivoice, video, hay cc tp tin v cc ng dng a phng tin cn tng tc viPSTN. H323 l chun nh ngha tt c tin trnh lin quan n vn ng

b cho thoi, video, v truyn d liu. H323 nh ngha tn hiu cuc gi end-to-end.

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2.1.5.3 Giao thc SIP

Hnh 2.5: Cu trc giao thc SIP

SIP: (Session Initiation Protocol) c pht trin bi IETF (Internet EngineeringTask Force) MMUSIC (Multiparty Multimedia Session Control) Working Group (theoRFC 3261). y l 1 giao thc kiu din k t (text-based protocol-khi client gi yucu n Server th Server s gi thng tin ngc v cho Client), n gin hn giaothc H.323. N ging vi HTTP, hay SMTP. Gi tin(messages) bao gm cc headerv phn thn (message body). SIP l 1 giao thc ng dng (application protocol) v

chy trn cc giao thc UDP, TCP v STCP.

2.2 Cng ngh WLAN

2.2.1 Tng quan v WLAN

Wireless LAN l m hnh mng c s dng cho mt khu vc c phm vinh nh mt ta nh, khun vin ca mt cng ty, trng hc. N l loi mng linhhot c kh nng c ng cao thay th cho mng cp ng. WLAN ra i v btu pht trin vo gia thp k 80 ca th k XX bi t chc FCC (Federal

Communications Commission). Wireless LAN s dng sng v tuyn hay hngngoi truyn v nhn d liu thng qua khng gian, xuyn qua tng trn v cccu trc khc m khng cn cp. Wireless LAN cung cp tt c cc chc nng vcc u im ca mt mng LAN truyn thng nh Ethernet hay Token Ring nhngli khng b gii hn bi cp. Ngoi ra WLAN cn c kh nng kt hp vi ccmng c sn, Wireless LAN kt hp rt tt vi LAN to thnh mt mng nng ngv n nh hn. Wireless LAN l mng rt ph hp cho vic pht trin iu khinthit b t xa, cung cp mng dch v ni cng cng, khch sn, vn phng. Trongnhng nm gn y, nhng ng dng vit cho mng khng dy ngy cngc pht trin mnh nh cc phm mm qun l bn hng, qun l khch sn ...cng

cho ta thy c nhng li ch ca Wireless LAN.

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Hnh 2.6: V d v m hnh WLAN

Wireless LAN s dng bng tn ISM (bng tn phc v cng nghip, khoahc, y t: 2.4GHz v 5GHz) v th n khng chu s qun l ca chnh ph cngnh khng cn cp giy php s dng. S dng Wireless LAN s gip cc ncang pht trin nhanh chng tip cn vi cc cng ngh hin i, nhanh chng xy

dng h tng vin thng mt cch thun li v t tn km.

2.2.2 u nhc im ca WLAN

u im:

Kh nng di ng v s t do cho php kt ni bt k u.

Khng b hn ch v khng gian v v tr kt ni.

D lp t v trin khai.

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,vv

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

Nhc im :

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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

tnh truyn d liu ra khi mng ca cng ty m khng thng qua lp vt l iukhin khin ngi khc c th nhn tn hiu v truy cp mng tri php. Tuynhin Wireless LAN c th dng m truy cp mng ngn cn truy cp, vic s dngm tu thuc vo mc bo mt m ngi dng yu cu. Ngoi ra ngi ta c ths dng vic m ha d liu cho vn bo mt.

2.2.3 M hnh WLAN

Hai m hnh c bn s dng cho Wireless LAN l Ad-hoc v mng c s h

tng (infrastructure). Hai m hnh ny c s khc bit nhau r rng v giihn khng gian s dng, cc qun l mng, kin trc mng.

2.2.3.1 M hnh Ad-hoc (IBSS-Independent Basic Service Set)

Hnh 2.7: M hnh Ad hoc

Ad-hoc l m hnh mng m trong ch bao gm cc my trm, khngc Access Point. Mi thit b kt ni trc tip vi cc thit b khc trong mng.M hnh ny rt thch hp cho vic kt ni mt nhm nh cc thit b v khng cn

phi giao tip vi cc h thng mng khc.

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

Hnh 2.8: M hnh Infrastructure

Infrastructure BSS l mt m hnh m rng ca mt mng Wireless LAN c bng cch s dng Access Point. Access Point ng vai tr va l cu ni camng WLAN vi cc mng khc va l trung tm iu khin s trao i thng tintrong mng. Access Point gip truyn v nhn d liu gia cc thit b trong mtvng ln hn. Phm vi v s thit b s dng trong mng c s h tng tu thucvo chun s dng v sn phm ca cc nh sn xut. Trong m hnh mng c s h

tng c th c nhiu Access Point to ra mt mng hot ng trn phm vi rnghay ch c duy nht mt Access Point cho mt phm nh trong mt cn nh, mt tonh. Mng c s h tng c hai li th chnh so vi mng c lp IBSS:

Infrastructure c thit lp ph thuc vo tm hot ng ca AP. V vy,mun thit lp Wireless LAN tt c cc thit b di ng bt buc phi nm trongvng ph sng 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 truynv nhn ca mng nhng chi ph li gia tng tng vt l bi v tt cc thit b ulun lun phi duy tr kt 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 tit kim nng lng v to frame m cho chng. Cc thit b ch trng s dng nnglng (Battery-operated) c th chuyn b thu pht tn hiu ca mnh sang ch nghv khi hot ng li s nhn c tn hiu c khi phc t cc frame m lu trongAP.

2.2.4 Cc thit b c bn ca mng WLAN

2.2.4.1 Card mng khng dy (Wireless NIC)

Card mng khng dy giao tip my tnh vi mng khng dy bng cchiu ch tn hiu d liu vi chui tri ph v thc hin mt giao thc truy nhpcm ng sng mang. Card mng c dy c th s dng khe cm ISA (hin nay hu

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nh khng cn s dng), khe cm PCI (s dng ph bin), hoc cng USB trn mytnh bn hoc s dng khe cm PCMCIA trn cc laptop. Card mng khng dythng c mt ng-ten ngoi v c th gn vo tng hoc mt v tr no trong

phng.

Hnh 2.9: Card mng khng dy s dng khe cm PCI

2.2.4.2 Cc im truy cp (Access Point)Cc im truy cp khng dy AP (Access Point) to ra cc vng ph sng, ni

cc nt di ng ti cc c s h tng LAN c dy .

Hnh 2.10: Access Point

2.2.4.3 Bridge khng dy (WBridge)

Cc WBridge (Bridge khng dy) tng t nh cc im truy cp khng dy trtrng hp chng c s dng cho cc knh bn ngoi. Ph thuc vo khong cchv vng m cn dng ti ng-ten ngoi. WBridge c thit k ni cc mng vinhau, c bit trong cc to nh c khong cch xa ti 32 km.

2.2.4.4 Cc router im truy cp (Access Point Router)

Mt AP router l mt thit b m n kt hp cc chc nng ca mt AccessPoint v mt router. Khi l Access Point, n truyn d liu gia cc trm khng dyv mt mng hu tuyn cng nh l gia cc trm khng dy. Khi l router, n hotng nh l im lin kt gia hai hay nhiu mng c lp, hoc gia mt mng

bn trong v mt mng bn ngoi.

2.2.5 Cc chun WLAN

Cng vi s pht trin mnh m ca mng khng dy, cc chun (v ng thil cc thit b) mng khng dy 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 sc tnh, u im ring ca n.

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2.2.5.1 IEEE 802.11

Session

Application

Presentation

Network

Data

Link

Physical

Transport

Network

Operating

System

(NOS)

802.11

TCP

IP

Logical Link Control (LLC) 802.2

Media Access Control (MAC),

Security, power, etc

FH, DS, IR

Hnh 2.11: IEEE 802.11 v OSI

Hin nay tiu chun chnh cho Wireless l mt h giao thc truyn tin qua mngkhng dy IEEE 802.11. Do vic nghin cu v a ra ng dng rt gn nhau nn cmt s giao thc thnh chun ca th gii, mt s khc vn cn ang tranh ci vmt s cn ang d tho. Mt s chun thng dng nh: 11802.11b (ci tin t802.11), 802.11a, 802.11h, 802.11g.

1. IEEE 802.11b(Wifi) :L chun quc t cho mng khng dy hot ng trong ditn s 2.4 GHz (2.4 GHz ti 2.4835 GHz) v cung cp mt lu lng ln trn 11

Mbps. y l mt tn s rt 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 trongdi tn s 2.4 GHz.

2. IEEE 802.11a: L mt ch tiu k thut IEEE cho mng khng dy hot ng trongdi tn s 5 GHz (5.725 GHz ti 5.85 GHz) vi tc truyn d liu cc i 54 Mbps.Di tn s 5 GHz khng nhiu nh tn s 2.4 GHz, v ch tiu k thut chun IEEE802.11 ngh nhiu knh v tuyn hn so vi chun IEEE 802.11b. S b sung ccknh ny gip trnh giao thoa v tuyn v vi ba.

3. IEEE 802.11g: Tng t ti chun IEEE 802.11b, chun lp vt l ny cung cp

mt lu lng ln ti 54 Mbps. N cng hot ng trong di tn s 2.4 GHz nhng sdng mt cng ngh v tuyn khc tng di thng ton b. Chun ny c phchun cui nm 2003.

4. IEEE 802.11h: Chun ny c dng chu u ,di tn 5 Ghz. N cung cp tnhnng s la chn knh ng v iu khin cng sut truyn dn TPC, nhm trnh cannhiu. chu u ngi ta ch yu s dng thng tin v tinh, nn phn ln cc qucgia y s dng ch s dng Wireless LAN trong nh (Indoor). Chun ny ang giai on chun ha

Sau y l bng tm tt thng s cc chun IEEE 802.11 thng dng:

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Chun WiFi Tn s (GHz)Tc (Mbps)

Khong cch(m)

802.11a 5 5412m vi 54Mb/s

90m vi 6Mb/s

802.11b 24 1130m vi 11Mb/s

90m vi 1Mb/s

802.11g 2,4 5415m vi 54Mb/s

45m vi 11Mb/s

2.2.5.2 HiperLANS 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 sdng, ETSI (European Telecommunications Standards Institute-Vin tiu chunvin thng chu u) nghin cu xy dng b tiu chun cho cc loi LAN hiusut cao (High Performance LAN), tiu chun ny xoay quanh m t cc giao tip mc thp v m ra kh nng pht trin mc cao hn.

Cc tiu chun ca ETSI HiperLAN:

HiperLAN

1

HiperLAN

2

HiperLAN

3

HiperLAN

4

ng dng Wireless LAN Wireless ATMIndoor Acces

Wireless ATMRemote Acces

Wireless ATMInterconnect

Bng tn 2,4 GHz 5 GHz 5 GHz 17 GHz

Tc t 23,5 Mbps 54 Mbps 54 Mbps 155 Mbps

2.2.5.3 Cc chun khc

HomeRF: l chun hot ng ti phm vi bng tn 2.4 GHz, cung cp bngthng 1.6 MHz vi thng lng s dng l 659 Kb/s. Khong cch phc v ti aca HomeRF l 45m.

OpenAir: l sn phm c quyn ca Proxim. OpenAir l mt giao thc trc802.11, s dng k thut nhy tn (2FSK v 4FSK), c tc 1,6Mbps. OpenAirMAC 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 mtkhu. OpenAir cng khng cung cp chc nng tit kim cng sut.

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Bluetooth: l cng ngh radio phm vi hp kt ni gia cc thit b khng dy.Hot ng trong di bng tn ISM (2.4 GHz). Chun ny xc nh mt ngtruyn v tuyn phm vi hp song cng tc 1Mbps kt ni c ti 8 thit b vtuyn cm tay. Phm vi ca Bluetooth ph thuc vo nng lng ca lp radio.

CHNG III: CC VN C BN TRONG VOIP OVER WLAN--------------o0o-------------

3.1 Dung lng h thng v QoS

3.1.1 Phn loi lu lng

802.11e chia lu lng thnh nhiu loi ty thuc theo yu cu ca lu lng.Nh chng ta bit, lu lng thoi c nhiu yu cu hn so vi lu lng d liu. Lulng thoi yu cu tr nh nht v n ch c th chu c mc tn tht nht nh tythuc vo ti nguyn kh dng ca mng. Mt khc, lu lng d liu chu rt t nhhng bi tr v n cng chu c mt mc tn tht nht nh, nhng mc tn tht

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ny lun ln hn so vi thoi. Do , vic phn loi lu lng l mt bc khi utt cho vic thc hin QoS.

Phn loi lu lng bng cch s dng nhiu hng i trong cc thit b WLANnh trong hnh 3.1. K tha chun 802.11, tt c cc lu lng (khng quan tm ti n

l thoi hay d liu, video) t mt trm c truyn t mt hng i n. iu ny cngha l cc gi thoi (c kch thc nh) c th b tr trong khi truyn dn d liubng n. Nhn thy rng v cc gi d liu ln hn cc gi thoi v lu lng d liuthng bng n mt cch t nhin, iu ny s gy tr cho cc gi thoi v do victruyn cc gi thoi ny s tr ln kh khn. Mt gii php c t ra l s dngnhiu hng i, cc hng i ring cha thoi v d liu.

Hnh 3.1: Phn loi lu lng

Vi vic s dng hng i ny, s to ra mt s u tin cho cc gi thoi mbo rng chng c u tin cao nht trong hng i v c truyn dn trc cc gic u tin thp hn. Tuy nhin, s u tin kht khe nh th ny c th lm mt ccgi d liu. Do , 802.11e a ra mt hng gii quyt mi l gii quyt s tranh

chp trong cc hng i khc nhau bn trong mt trm ging nh gii quyt s tranhchp gia cc trm vi nhau bi vic s dng chc nng phn b phi hp mi ra trong WME, l s dng EDCF.

Ni tm li, tng c bn ca vic phn loi lu lng l mi trm duy trnhiu hng i (ln ti 8 hng i) cho cc loi lu lng khc nhau. Mi hng isau s ng vai tr nh l mt trm o v nhng trm o ny s tranh chp vi nhau chn ra gi no s c truyn dn t trm tht . Quy tc cnh tranh gia cchng i tng t nh quy tc MAC cho cc trm tranh chp knh truyn.

Ch rng 802.11e cho php mt trm phn loi lu lng ca n thnh 8 loikhc nhau, mi loi lu lng c th c nh x ti mt hng i ring. Tuy nhin,WME hn ch s hng i trong mt trm l 4.

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3.1.2 Giao Thc GEDCF

Giao thc EDCF (Enhanced Distributed Coordination Function) l mt phin bnnng cao ca giao thc DCF ca 802.11. Giao thc EDCF c thng qua biWME (Wireless Multimedia Enhancement) bi Wi-Fi Alliance nh l mt chun trc

ca 802.11e.Mc tiu ca giao thc EDCF l m bo u tin truy nhp n knh (v

tuyn), do lu lng thoi c th d dng truy nhp n knh truyn vi mt utin cao hn cc lu lng khc. Ch rng giao thc ny ch xt cho cc hng itrong cng mt trm. Vi mc tiu l bo m rng cc gi c u tin cao hnc u tin truy nhp ti knh truyn.

EDCF thc hin mc tiu ny bi vic duy tr cc tham s tranh chp trn miloi lu lng. Cc tham s m EDCF s dng l: CWmax, CWmin, AIFS, PF.

hiu lm th no nhng tham s ny trn mi lu lng thc hin truy nhp

u tin cho cc loi lu lng khc nhau, chng ta cn xem li vi c im quantrng ca DCF. mt BSS ti va phi, khi mt trm mun truyn dn trn knh vtuyn, n phi i cho n khi knh trng thi t do. Sau khi bit knh trng thi tdo, trm i mt khong thi gian DIFS. Sau khi ht thi gian DIFS, mi trm ithm mt khong thi gian ngu nhin xc nh bi b m back-off (BC). Gi tr BCl ngu nhin chn trng khong t [0, CW], CW l ca s tranh chp.

Hnh 3.2: Hot ng ca EDCF

u tin, thi gian ti thiu i sau khi kim tra knh ph thuc vo tng loiknh. Thay v tt c cc lu lng phi i DIFS trc khi truy nhp mi trng, lulng c mc u tin cao hn (thoi) phi i thi gian nh hn AIFS lu lng c u tin thp hn (d liu) nh trong hnh 3.2. Tip theo EDCF cho php mi hng duytr mt tp hp gii hn cc ca s tranh chp khc nhau. Lu lng c u tin caohn (thoi) c phm vi gi tr CW gii hn hn lu lng c u tin thp hn (dliu). iu ny c ngha rng nu xy ra s tranh chp gia gi d liu v gi thoi thgi thoi c nhiu kh nng hn c truyn i.

Cui cng, s truyn dn khng thnh cng ca mt MSDU dn ti tng gp iCW, do vy lm tng s ln back-off v kh nng truyn dn li gi. S ln back-offny nh hng bt li ti lu lng thoi hn l d liu. gii quyt vn ny,

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802.11e a ra nhn t bn (PF-Persistence factor). PF c xc nh da trn milu lng, n xc nh khong thi gian bao lu mi hng i tr li trc khi thchin truyn dn li. Vi DCF, PF lun l 2, v CW gp i sau khi truyn dn khngthnh cng. Trong 802.11e, PF c s dng tnh ton gi tr CW mi trong cctrng hp truyn dn li nh sau: newCW = ((oldCW+1) * PF) 1. Bng vic sdng mt PF thp hn cho cc hng i thoi c u tin cao, cc gi thoi c thc chp nhn truyn dn li nhanh hn, v th m bo tr thp hn cho cc githoi.

Trong mt trm, nh ni phn trn cc hng i c coi nh l mt trm ovi cc tham s khc nhau. Nu b m back-off ca hai hay nhiu trm o ny t tigi tr 0 ti cng mt thi im th c th coi l mt xung t o. C hi truyn dn(TXOP) c dnh cho lu lng c u tin cao nht trong cc lu lng angxung t.

Gii hn CW, AIFS v PF hot ng cng nhau to u tin truy nhp cho

thoi trong 802.11e/WME. Bn cnh vic ci thin tr cho thoi, EDCF cng ci thinhiu sut h thng trong mi trng phc tp c thoi, video, d liu cng tn ti.

Ch rng gi tr QoS cho mi loi lu lng c a ra bi AP, c chatrong Association Response, Probe Response v bo hiu. iu ny bo m rng ttc cc trm trong BSS, ngoi tr AP c i x cng bng. Mt trong nhng vn lm gii hn dung lng ca h thng v gim QoS trong chun 802.11 .802.11e/WME cho php AP s dng cc tham s tranh chp khc nhau hn cc thams c s dng bi cc trm. iu ny c ngha l AP c th truy cp ti phng tinvi u tin cao hn cc trm trong BSS, iu ny gii quyt vn tc nghn cchai ca AP trong h thng VOIP over WLAN.

3.1.3 Giao Thc HCF

Tng t nh EDCF c thng qua trong chun WME bi Wi-Fi Alliance, HCFc thng qua trong WMM-SA (Wi-Fi MultiMedia-Scheduled Access) xc nhn biWi-Fi Alliance nh l s m rng ca PCF.

Cc lu lng thi gian thc (thoi v video) c u tin cao hn khi n tranhchp truyn dn. Mt khc, cc tham s truy nhp xc nh tr gii hn cho lu lngthi gian thc. HCF thc hin iu ny bng vic s dng phng php thm d(polling) ging nh PCF. Tuy nhin, phng php thm d lm pht sinh cc chi ph

ph nh l khung thm d. Do , k thut EDCF (khng s dng bng thng trongkhung thm d) s thc hin tt di cc iu kin tt cho cc ti nh trong khi kthut HCF thc hin tt di cc iu kin cc ti nng. Cc nh qun tr mng ca hthng VoWLAN phi nm r iu ny khi quyt nh chn la EDCF hay HCF.

Nh chng ta ni, HCF l mt phin bn nng cao ca PCF. Mt trong nhngvn ch yu ca PCF l khi mt trm c truy nhp ti knh truyn, truy nhp can khng ph thuc vo thi gian (n c th chim knh truyn). TXOP gii quyt vn ny bi vic gii hn c hi truyn dn theo thi gian, do vy khi mt trm truynhp ti knh truyn s dng quy tc MAC, trm c thi gian gii hn n c thtruy nhp vo knh truyn. Vic s dng TXOP trnh tr khng d bo trc l khthi trong PCF. Do , c hi truyn dn l thi gian ti a lin k nhau mt trm c ths dng knh truyn khi n c truy nhp vo. Ch rng trong mt TXOP, mt

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trm c cho php truyn nhiu MSDU vi mt khong SIFS gia mt ACK vkhung tip theo khi trm t gii hn TXOP.

HCF s dng giao thc EDCF nh l mt khi tiu chun v m rng khi nimCFP, CP v thm d t PCF. Tng t nh PCF, thi gian c chia thnh cc siu

khung. Mi siu khung bao gm mt CP v mt CFP. Ch HC (c t ti AP) c thtruy nhp ti knh truyn trong thi gian CFP v n thm d cc trm ng chophp truy nhp ti chng trong thi gian ny. Trong thi gian CP, EDCF c s dng quyt nh truy nhp ti knh truyn.

Khng ging vi PCF, HCF cho php HC thm d cc trm thm ch trong thigian CP. Do , trong thi gian mt CP, mt TXOP bt u khi knh c xc nhkh dng theo quy lut EDCF hoc khi trm nhn c khung thm d CF c bit.

Hnh 3.3: Hot ng ca HCF

Ch rng s dng TXOP gii quyt vn tr khng bo trc. Trong HCF,TXOP c th c xc nh trong CF thm d, n c gi ti trm chp nhntrm truy nhp ti knh. Trm s c truy nhp ti knh truyn trong TXOP ny.

Mt vn vi HCF l n yu cu qun l tt tri ph tn. Vi HCF, lch gitrong AP s l thnh phn ch yu. Tuy nhin, thc tt vic lp lch s khng tnh nnhiu t cc knh khc hoc t cc mng khc s dng cng mt ph tn, nu ph tnkhng hon ton c qun l. iu ny s lm phc tp qu trnh lp lch. Do ,HCF c mong i s tr thnh cc gii php cho cc doanh nghip hay t ra c sdng trong mng gia nh.

3.1.4 Thc hin QoS cho VOIP over WLAN

802.11e/WME cung cp mt k thut cho cc thit b 802.11gip u tin cc githoi hn cc gi d liu trong WLAN. Tuy nhin, nhng k thut ny khng khnng thc hin QoS cho cc cuc gi VOIP over WLAN v mt cuc gi VOIP overWLAN c th vt ra ngoi mng WLAN. hiu c iu ny, xt mt chic inthoi IP khng dy (WIPP) trong mt cuc gi thoi vi chic in thoi hu tuyn tcch xa mng IP. Hnh 3.4 chia QoS u cui ti u cui thnh 3 phn: WLAN, LANhu tuyn v mng IP. By gi chng ta s xem xt ci g cn thc hin QoS ca

mi phn ny.

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Hnh 3.4: in thoi Wi-Fi

a. WLAN

Trong ng ln, 802.11e/WME thc hin QoS v cc trm tranh chp vi nhau truyn cc gi ti AP v cc gi thoi c mc u tin cao hn d liu.

u tin, thc hin QoS lung xung khng quan trng v AP cng s dng802.11e/WME u tin lu lng ca n qua mi trng truyn dn. Tuy nhin, cmt vn quan trng l AP cn phn bit r gia cc gi thoi (c bo hiu v a

phng tin) v cc gi d liu u tin mt ci hn ci cn li. Trong ng ln,cc ng dng chy trn trm chu trch nhim phn loi cc gi theo ng loi. Trongth d ca chng ta, cc ng dng thoi trn WIPP c th thng bo cho b phn iukhin WLAN bit nhng gi no thuc loi no. Tuy nhin trong ng xung, APiu khin cc gi t mt im cui pha xa. Do vy, n cn phn loi cc gi d liung xung bi vic kim tra gi. AP nh l mt cy cu bo m s lm vicgia 802.3 v 802.11. Do , theo m hnh phn lp OSI, n s ch truy nhp ti tiu lp 2. Hay ni cch khc, ci chng ta cn l mt trng tiu trong 802.3 lp 2cho cc gi n lung xung hu tuyn ti AP c th kim tra phn loi ng ccgi ny.

b. LAN hu tuyn

Chc chn rng hu ht cc k thut c trin khai thc hin QoS trong mngLAN hu tuyn l s dng 802.11D/Q. 802.11D cho php cc lp MAC khc nhautrong h 802.11 c th lm vic vi nhau. 802.11Q VLAN m rng nh dng khung802.3, v n xc nh mc u tin ngi s dng khung. 802.3 MAC khng h tr bt

k cc knh khc nhau truy nhp ti cc lu lng u tin khc nhau, nhng qua 802.3VLAN, cc khung 802.3 MAC c th cha cc gi tr u tin tng ng, cc gi trny c quay vng v c th c s dng bi MAC 802.1D.

Nu AP h tr c hai 802.11e v 802.11D/Q, n c th c s dng thchin QoS trong mi trng LAN. Tip tc th d ca chng ta, khi mt AP nhn mtgi Ethernet vi mt tiu 802.11D/Q, c c th kim tra bits u tin trong tiu quyt nh truyn dn gi t hng i thoi ca AP c hay khng hoc hng i BEtrong mng v tuyn.

Tuy nhin, vn cn mt vn l vic chn bits u tin nh th no. lung ln,AP c th nhn ra gi thoi v gi d liu bi vic kim tra trng u tin trongtrng iu khin QoS ca tiu 802.11e/WME MAC v c th s dng nhngthng tin ny thit lp cc bits u tin trong tiu 802.1D. Tng t trong lung

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xung, cc gi nhn c ti AP hu tuyn c cc bits u tin 802.11Q trong802.1D l phn tiu ca 802.3 v AP c th nh x chng ti tiu 802.11e.

c. Mng IP

Khi cc gi truyn dn t IPP c nh trc t WIPP nm trong phm viAP, khng c mt con ng hon ho cho AP bit rng chng l cc gi thoi. IPPthit lp cc bits u tin 802.1Q/D trong tiu MAC khng l mt ty chn, v tiu lp 2 (MAC) b thay i trn mi hop v IPP v WIPP c th c ngn cch binhiu hop. V AP khng th phn bit gia cc gi thoi va d liu trong lung xunghu tuyn bi vic xem xt tiu chun 802.3, n khng th quyt nh gi no cnc truyn t hng i thoi ca AP v gi no cn c truyn t hng i thoi dliu (BE). Vic thc hin QoS trong lung xung v tuyn cn s dng k thut khc.y l ni m QoS lp 3 tham gia v cc tiu lp 3 c duy tr u cui ti ucui.

DiffServ thng trin khai QoS lp 3. Trong phng php ny, mi gi d liuIP mang mt m dch v khc nhau (DSCP) trong cc trng dch v khc nhau (DS),n nm trong octet TOS (Type Of Service) ca IPv4 v octet phn loi lu lng IPv6.V IP l giao thc lp 3 (u cui ti u cui), tiu IP lun sn c v c th cs dng bi cc router trong mng IP cung cp cc dch v khc nhau nh QoS.

By gi, xt th d ca chng ta v in thoi IP khng dy (WIPP) trong mtcuc thoi vi mt chic in thoi IP hu tuyn (IPP). Khng cn quan tm IPP trong mt mng IP hay trong cng mt mng LAN vi WIPP, nu chng quyt nhs dng DiffServ, gi tr DSXP c th c s dng phn bit cc gi thoi vi ccgi d liu tng mng, n c thc hin bi Router h tr DiffServ trong mng IP.

Khi mt gi DiffServ ti AP, AP c th xem trong tiu gi IP ly DSCPsau chuyn gi vo hng i thch hp. Thc t, AP s dng DSCP phn bitcc gi thoi v d liu. Ni ng ra phng php ny vi phm kin trc phn lp vDSCP c cha trong tiu IP lp 3, trong khi AP l thit b lp 2. Theo quy tc,trong kin trc phn lp, mt thit b lp 2 (AP) khng th bit c tiu lp 3(DSCP). Tuy nhin, 802.11e hot ng vi DiffServ th n cn nh vy.

Vic s dng phng php ny v gi thuyt rng QoS trong mng LAN hutuyn l khng cn thit, trong th d ca chng ta, c hai WIPP v IPP s thit lptrng DSCP trong tiu IP. Vi cc gi thoi i t WIPP ti IPP (qua AP), khng

c yu cu g c bit v 802.11e cung cp QoS t WIPP ti AP v DSCP c chnvo bi WIPP c th c s dng cung cp QoS trong mng WAN. Vi cc githoi i t IPP ti WIPP (thng qua AP), DSCP c s dng cung cp QoS trongWAN v AP s dng gi tr ny xt u tin, do vy n to ra QoS trong lungxung v tuyn.

3.1.5 Dung lng h thng

Chng ta s xem xt lm th no 802.11e/WME nng cao u tin ca lulng thoi bi vic s dng phn loi lu lng v thut ton MAC nng cao. Nhngc tnh ny gip gim tr cho lu lng thoi v do nng cao cht lng truynthng. Mt u im na l n tng dung lng h thng cho thoi qua WLAN. Nh ta

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bit dung lng ca WLAN dnh cho cc cuc gi thoi b gii hn v nhng nhnt sau:

Kch thc gi thoi nh

Chi ph lp vt l cao

Thoi v d liu cnh tranh cng bng vi nhau trong truy nhp knh truyn

Cc trm v AP cnh tranh vi nhau trong truy nhp knh truyn.

Yu cu ACK cho mi gi.

Cc tham s trong lp PHY 802.11

802.11b 802.11g+b 80.11g-only 802.11a

DIFS 50 50 28 34

SIFS 10 10 10 16

Slot Time 20 20 9 9

Cwmin 32 16 16 16

Supported DataRate (Mbps)

1, 2, 5.5, 111, 2, 5.5, 11, 6,9, 12, 18, 24,36, 48, 54

6, 9, 12, 18, 24,36, 48, 54

6, 9, 12, 18,24, 36, 48,54

Basic Rate(Mbps)

2 2 NA NA

PHY for otherframes

192 20 20 20

ACK frames 131 24 24 24

u tin hy xem 802.11e/WME khng gii quyt c nhng vn g. Nkhng gii quyt vn kch thc gi thoi, l bn cht gii hn ca VoIP. Cc gi

thoi c gi nh gim tr.802.11e/WME khng gii quyt vn chi ph lp vt l, cng l vn lp

PHY. Gii php l chn ng lp PHY cho VOIP over WLAN. Bng 3.1, vic sdng 802.11g hoc 802.11a nh l lp PHY l hp l hn c cho VOIP over WLAN vn gim ng k chi ph lp PHY. iu ny c xc nhn bi phn tch nh trong

bng 3.2.

Bng 3.2: Dung lng VoIP cho 802.11b, 802.11a v 802.11g

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DCFtp trung to

ra s lngc hi truyndn cng

bng trongcc trm.

Ni cchkhc, n m bo rng khi mt trm c c truy nhp ti knh truyn v kt thctruyn dn ca n. N phi cnh tranh li vi cc trm khc truyn dn gi tiptheo. Tuy nhin, khng cng bng vi cc gi thoi c kch thc nh hn v n khngtnh n bao lu trm s trn knh truyn khi n c c truy nhp. 802.11e gii

quyt vn ny bi vic cho php cc gi thoi vi mc u tin cao hn cc gi dliu. C ngha rng cc trm thoi s dng ti trng kch thc nh tn t thi gian chtruyn hn cc gi d liu, iu ny l cng bng v chng chim knh t thi gian khichng c truy nhp vo knh truyn.

Mt kha cnh khc ca s cng bng trong chun 802.11 xut pht t thc trng c AP v trm u s dng cng tham s MAC. iu ny c ngha l nu mt APv trm cnh tranh truy nhp ti knh truyn, c hai u c kh nng ngang nhau.Tuy nhin, trong mt h thng VOIP over WLAN, do tnh hai chiu v khng ixng t nhin ca thoi, AP iu khin nhiu lu lng hn bt k trm no v chngta c mt h thng m node iu khin nhiu lu lng (AP) khng c u tin hncc node khc. iu ny dn ti s tc nghn ti mt im trong mt BSS VOIP overWLAN. EDCF gii quyt vn ny bi vic cho php AP s dng s dng cc thams cnh tranh khc nhau (gii hn CW, AIFS v PF) t trm. N lm cho AP c mcu tin cao hn truy nhp vo knh truyn, iu ny thch hp cho truyn thngthoi. Mt khc, HCF gii quyt vn ny bi vic iu khin hon ton mi trngtruy nhp ti AP trong sut thi gian khng tranh chp (trong thi gian ny truynthng thoi c lp lch). Do , AP c th s dng knh truyn cho lung d liunhiu hn trm.

Cui cng vi e, 802.11e cho ACK ty chn, do n cho php s lng gi qua

mi trng khng kh gim xung mt na. Tuy nhin, lm mi trng v tuyn phctp, khng s dng ACK l mt gii php khng c tnh thc t. Tuy nhin, chng tac th s dng gi ACK m rng, ACK ca N gi c th c gi cng nhau. Vicgi mt ACK cho N gi thay v ACK choi mi gi c mong i l s lm gim slng gi gi trong mi trng do lm tng dung lng h thng. Tuy nhin, mtvi cuc th nghim ch ra rng, WLAN thng hot ng trong mi trng khcnhit th vic s dng ACK cho mi gi l tt nht.

Mc d 802.11e/WME gip nng cao dung lng h thng v s lng cuc ging thi trong mt BSS, mt BSS c th mt s lng cuc gi nht nh. Khi slng cuc gi vt qu ngng ny, BSS c gi l trng thi tc nghn. Mt

BSS b tc nghn dn ti gim cht lng ca tt c cc cuc gi ang hot ng. Rrng y khng phi l tnh hung mong i. Do , cn phi c s iu khin truy

29

MAC VoIP

802.11b 11.2

802.11a 56.4

802.11g-only 60.5

802.11g vi bo v CTS-to-seft 18.9

802.11g vi bo v RTS-CTS 12.7


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nhp bo m s cuc gi ng thi c php. 802.11e cung cp TSPEC thchin iu khin truy nhp.

3.2 Bo mt trong VOIP over WLAN

3.2.1 S cn thit ca bo mt nh gi ng s cn thit ca bo mt cho h thng VOIP over WLAN,

chng ta cn hiu r mi nguy him nu khng thc hin bo mt trn cu hnh hthng. Nhng vn ch yu l gim st th ng, truy cp tri php v t chi dchv nh trong hnh 3.5. Khi trin khai mt h thng VOIP over WLAN, vic nh ginhng nguy c tim n c th thc hin c da trn nhng vn ny, v xc nhgii php bo mt no s hiu qu nht.

Mngkhng dy

Tn cngt chi dch v

Truy nhp triphp

Qun l lulng

Hnh 3.5: Cc nguy c bo mt trong mng khng dy

3.2.1.1 iu khin th ng

Mt AP WLAN c thit lp ban u khng c k thut bo mt. Dn n, tt cd liu gi gia AP v thit b Client, nh l in thoi IP hoc mt my tnh tch hpthit b khng dy, c gi i m hon ton khng c bt c s m ha no. Ttnhin, iu ny rt nguy him, mt hacker c th s dng cng c bt gi tin (packet-sniffing), nh l Ethereal, v c th gim st c lung lu lng gia AP v Clientv c th ly mt khu, email, ti liu, cc cuc gi thoi. Hn na, mt AP vi cuhnh mc nh s c username v mt khu mc nh, m tt c u c th bit. Do ,chng ta phi thng xuyn cp nht, s li c c s bo mt tt nht.

trnh b ri vo cm by ny, ngi s dng phi thng s dng mt khukhc nhau cho mi ti khon. Hn na h thng v tuyn c th thc hin mt dngm ha kh mnh m bo rng hacker d ra bt k th g t d liu m hackergim st.

3.2.1.2 Truy cp tri php

Nu chc nng bo mt ca mt AP khng c kch hot th bt k ngi novi mt chic laptop tch hp thit v tuyn u c th truy nhp vo mng LANkhng dy t ti mt ta nh, mt nh my hay mt bnh vin. AP khng c bo

mt lin tc gi gi tin bo hiu qung b m mt laptop khng dy c th thu c vh thng Microsoft Window s hin th SSID (Service Set Identifier) tm thy trongbn tin bo hiu khi mt mng khng dy l kh dng. Ngi s dng laptop, c th l

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hacker, sau thc s kt ni ti AP, iu ny lm hacker tr thnh mt b phn trongmng ca cng ty. c im ny cho php hacker s dng k hoch v cng c h tr n trm d liu c t ti my ch ca mng. Th d, hacker c th chy phnmm qut cng giao thc TCP v pht hin ra cc giao din ng dng ca ngi quntr khng c bo mt v cc cng c h tr trn my ch. iu ny c th cho phphacker to mt ti khon trn h thng cho hn v bt u s dng n trm nhngfile, ng dng khng cho php.

3.2.1.3 T chi dch v

Mt tn cng t chi dch v l mt cuc tn cng c th lm tn thng hocph hng mt mng WLAN. N chn ngi s dng in thoi IP ca h.

Mt dng ca tn cng DoS l phng php brute-force, n c th n t haidng: hoc l mt lng gi rt ln trn ti chim ht ti nguyn ca mng lm cho n

b dng li, hoc mt tn hiu v tuyn rt mnh vt tri hn cc sng khc truy cp

ti AP lm cho ngi s dng khc khng th truy cp vo. Mt hacker c th thchin kiu tn cng brute-force bng cch s dng cc my tnh khc trn mng gicc gi tin ti my ch. Phng php ny to ra hu qu nghim trng, n chim ht

bng thng ca ngi s dng hp php trn mng.

Vic s dng tn hiu v tuyn rt mnh lm ngt mng l kiu tn cng rtnguy him vi hacker. V mt my pht cng sut ln ti v tr tng i gn phic s dng thc hin kiu tn cng ny, ngi ch ca WLAN c th tm thyhacker bng vic s dng cng c ti nh nh l AirMagnet.

Thnh thong mt v DoS xy ra trn mng c th khng phi do c . Di tn

m 802.11b hot ng rt ng c. Cc thit b khc c di tn 2,4GHz nh l l visng, in thoi i ng, bluetooth c th lm gim ng k hiu sut ca 802.11b.

chng li DoS bn phi cn thn trong thit k, gii hn s sng trong mng.

3.2.2 Cc phng php bo mt cho VOIP over WLAN

An ton truy cp v bo mt cho mng cc b khng dy s dng cc phngphp thuc sau:

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

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

3.2.2.1 Xc thc

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Hnh 3.6: Qu trnh xc thc

Xc thc l mt qu trnh nhn bit mt ngi hoc thnh phn mng da trnd liu xc thc, nh trong hnh 3.6 . Xc thc bo m rng mt ngi s dng hocmt thnh phn ca mng c cho php kt ni vi ngi s dng khc hoc thit b

khc. Qu trnh ny tng t vi vic ngi s dng trnh giy php trc khi i vomt khu vc c bo m. Bo v kim tra tn ngi s dng trong danh sch v xcnhn nh trn giy php ng vi ngi . Nu mi kim tra c thng qua, bo vm ca v ngi s dng c th i vo khu vc .

Cc h thng mng thc hin xc nhn ln nhau. Mt Access Point xc nhn rngmt thit b v tuyn ang mun truy nhp c cho php v thit b ny bo mAcces Point c xc thc. Nu xc nhn ny khng c thc hin th mt hacker cth bt mt Acces point gi mo gn ngi s dng v pht tn hiu lm ngt kt nithit b ca ngi s dng vi Access point ng. Thit b ngi s dng sau s ktni vi Access point gi mo ca hacker. Hacker c th cu hnh Access Point ny thu d liu ca ngi s dng chuyn ti my tnh ca mnh, v thu thp cc thng tinquan trng nh l mt khu. Vic xc nhn ln nhau c th ngn chn tnh hung ny.

Sau y chng ta s xt cc k thut xc nhn c s dng trong mng WLAN:

a, 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 hiuqu bo v mng khng dy WLAN ca mnh, c gi l VPN Fix. tng c bn ca phng php ny l coi nhng ngi s dng WLAN nh nhng

ngi s dng dch v truy cp t xa.Trong cch cu hnh ny, tt cc nhng im truy cp WLAN, v cng nh cc

my tnh c kt ni vo cc im truy cp ny, u c nh ngha trong mtmng LAN o (Virtual LAN). Trong c s h tng bo mt, cc thit b nyc i x nh l "khng tin tng". Trc khi bt c cc thit b WLAN ckt ni, chng s phi c s cho php t thnh phn bo mt ca mng LAN. Dliu cng nh kt ni ca cc thit b s phi chy qua mt my ch xc thc nhRADIUS chng hn... Tip , kt ni s c thit lp thnh mt tuyn kt ni bomt c m ho bi mt giao thc bo mt v d nh IPSec, ging nh khi sdng cc dch v truy cp t xa qua Internet.

Tuy nhin, gii php ny cng khng phi l hon ho, VPN Fix cn lulng VPN ln hn cho tng la, v cn phi to cc th tc khi to cho tngngi s dng. Hn na, IPSec li khng h tr nhng thit b c nhiu chc nngring nh thit b cm tay, my qut m vch... Cui cng, v quan im kin trcmng, cu hnh theo VPN ch l mt gii php tnh th ch khng phi l s kt hpvi WLAN.

b, Phng php 802.1x

Chun LAN khng dy 802.11 khng c s xc nhn thng minh, v vy

chun cng nghip thng qua giao thc 802.1x cho s xc nhn ca n. 802.1xa ra cch thc iu khin truy cp mng c bn, n s dng EAP(Extensible Authentication Protocol) v RADIUS server. 802.1x khng a ra

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giao thc xc nhn mt cch c th nhng ch r EAP trong vic h tr s lngcc giao thc xc nhn nh l CHAP-MD5, TLS v Kerberos. EAP c th c mrng v vy cc giao thc xc nhn mi c th c h tr nh trong cc phin bnsau ca n. EAP c a ra hot ng trn giao thc Point-to-Point (PPP); n tng thch vi cc giao thc ca lp lin kt d liu khc (nh l TokenRing 802.5 hay Wireless LANs 802.11) EAP Over LANs (EAPOL) c

pht trin. M hnh xc nhn cui cng c th hin hnh di:

TLS

PPP

OtherKerberosCHAP-MD5

801.11

Wireless

LAN

802.5

Token Ring802.3

Ethernet

Extensible Authentication Protocol (EAP)

EAP Over Lans (EAPOL)

Authentication

Layer

EAP Layer

Data Link

Layer

Hnh 3.7: M hnh xc nhn

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 bttay ca giao thc m ha v s trao i kha bo v gia mt client khng

dy v mng. EAP-TLS l mt k thut cung cp cc kha m ha ng cho ngidng v session. iu ny ci thin mt cch ng k v vt qua nhiu im yutrong cc mng khng dy.

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

802.1x EAP-TLS vi EAS trong Controller Mode c th hin trn hnh

sau. Client v tuyn c chng ch digital (c ci t t trc). Client khng dytruyn thng vi EAS thng qua AP. Tt c ba thnh phn (Wireless client, AP vEAS) h tr qu trnh 802.1x EAP-TLS. Client khng dy c th s dng WindowsXP (c xy dng h tr cho 802.1x EAP-TLS) hay Windows98/Me/2000 bng vic s dng Madge Wireless LAN Utility (WLU). Khi xcnhn, d liu ngi dng cng c th c s dng EAS m c cu hnhtrong Gateway Mode.

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Wireless Client Access Point Radius Server Network

Client CertificateServer Certificate

NetworkAccessdenied

802.11 Assocaition Network access denied

Wireless client authenticate RADIUS Serve r

(RADIUS Server send its certificate

Wireless client authenticate RADIUS Serve r

(RADIUS Server send its certificate

Access PointDerlves WEP key

Access Point send WEP key

To Wireless Client

WEP encrypttraffic Decrypt traffic passed to network

EAP TSL session

TSLNegotiated

encryption

WEP

encryption

Hnh 3.8: Xc nhn 802.1x EAP-TLS

3.2.2.2 M ha d liu truyn

M ha l mt qu trnh thay i bit d liu ca gi trc khi truyn dn. Phathu s phi gii m cc d liu thu c, qu trnh ny gip che giu d liu thc vihacker.

Hnh 3.9: Qu trnh m ha

Sau y chng ta s xem xt cc kiu m ha c s dng cho mng khng dy.

a, 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 haWEP. Phng thc ny c a s cc nh sn xut thit b khng dy h tr nhmt phng thc bo mt mc nh. Tuy nhin, nhng pht hin gn y v im yu

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ca chun 802.11 WEP gia tng s nghi ng v mc an ton ca WEP v thcy s pht trin ca chun 802.11i. Tuy vy, a phn cc thit b khng dy hin ti v ang s dng WEP v n s cn tn ti kh lu trc khi chun 802.11i trinkhai rng ri.

Giao thc WEP

WEP (Wired Equivalent Privacy) ngha l bo mt tng ng vi mng cdy (Wired LAN). Khi nim ny l mt phn trong chun IEEE 802.11. Theonh ngha, WEP c thit k m bo tnh bo mt cho mng khng dy tmc nh mng ni cp truyn thng. i vi mng LAN (nh ngha theo chunIEEE 802.3), bo mt d liu trn ng truyn i vi cc tn cng bn ngoi cm bo qua bin php gii hn vt l, tc l hacker khng th truy xut trc tipn h thng ng truyn cp. Do chun 802.3 khng t ra vn m had liu chng li cc truy cp tri php. i vi chun 802.11, vn mha d liu c u tin hng u do c tnh ca mng khng dy l khng

th gii hn v mt vt l truy cp n ng truyn, bt c ai trong vng ph sngu c th truy cp d liu nu khng c bo v.

Kha WEP IV

Cipher TextD liu m ha

802.11HeaderPayload

RC4 XOR

Stream Cipher

Hnh 3.10: Quy trnh m ha WEP s dng RC4

Nh vy, WEP cung cp bo mt cho d liu trn mng khng dy qua phng thc m ha s dng thut ton i xng RC4 (Hnh 3.14), c RonRivest, thuc hng RSA Security Inc ni ting pht trin. Thut ton RC4 cho phpchiu di ca kha thay i v c th ln n 256 bit. Chun 802.11 i hi bt buccc thit b WEP phi h tr chiu di kha ti thiu l 40 bit, ng thi m bo ty

chn h tr cho cc kha di hn. Hin nay, a s cc thit b khng dy h tr WEPvi ba chiu di kha: 40 bit, 64 bit v 128 bit. Vi phng thc m ha RC4, WEPcung cp tnh bo mt v ton vn ca thng tin trn mng khng dy, ng thic xem nh mt phng thc kim sot truy cp. Mt my ni mng khng dykhng c kha WEP chnh xc s khng th truy cp n Access Point (AP) v cngkhng th gii m cng nh thay i d liu trn ng truyn. Tuy nhin, cnhng pht hin ca gii phn tch an ninh cho thy nu bt c mt s lng lnnht, nh d liu m ha s dng WEP v s dng cng c thch hp, c thd tm c chnh xc kha WEP trong thi gian ngn. im yu ny l do lhng trong cch thc WEP s dng phng php m ha RC4.

Hn ch ca WEP

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Do WEP s dng RC4, mt thut ton s dng phng thc m ha dng(stream cipher), nn cn mt c ch m bo hai d liu ging nhau s khng chokt qu ging nhau sau khi c m ha hai ln khc nhau. y l mt yu t quantrng trong vn m ha d liu nhm hn ch kh nng suy on kha ca hacker. t mc ch trn, mt gi tr c tn Initialization Vector (IV) c sdng cng thm vi kha nhm to ra kha khc nhau mi ln m ha. IV lmt gi tr c chiu di 24 bit v c chun IEEE 802.11 ngh (khng bt

buc) phi thay i theo tng gi d liu. V my gi to ra IV khng theo nh luthay tiu chun, IV bt buc phi 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 802.11nn bt c ai ly c d liu trn mng u c th thy c. Vi di 24 bit, gitr ca IV 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 cs dng vi cng kha trn mt gi d liu m ha (va chm IV), hacker c th btgi d liu v tm ra c kha WEP. Thm vo , ba nh phn tch m haFluhrer, Mantin v Shamir (FMS) pht hin thm nhng im yu ca thutton to IV cho RC4. FMS vch ra mt phng php pht hin v s dng nhngIV 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 tncng ch cn thu nhn cc gi d liu trn ng truyn m khng cn lin lc viAccess Point. iu ny khin kh nng pht hin cc tn cng tm kha WEP y

kh khn v gn 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 chv s lng gi d liu cn bt c.

b, 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 RC4nh WEP, nhng m ho y 128 bit. V mt c im khc l WPA thay ikho cho mi gi tin. Cc cng c thu thp cc gi tin ph kho m ho ukhng th thc hin c vi WPA. Bi WPA thay i kho lin tc nn hackerkhng 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 IntegrityCheck). V vy, d liu khng th b thay i trong khi ang trn ngtruyn.

Mt trong nhng im hp dn nht ca WPA l khng yu cu nng cp phncng. Cc nng cp min ph v phn mm cho hu ht cc card mng v im truy

cp s dng WPA rt d dng v c sn. Tuy nhin, WPA cng khng h tr cc thitb cm tay v my qut m vch. Theo Wi-Fi Alliance, c khong 200 thit b ccp chng nhn tng thch WPA.

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WPA c sn 2 la chn: WPA Personal v WPA Enterprise. C 2 la chn nyu 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 Cch sdng gi tr IV l ngun gc ca a s cc vn vi WEP. Do gi tr IV c truyni dng khng m ha v t trong header ca gi d liu 802.11 nn bt c ai lyc d liu trn mng u c th thy c. Vi di 24 bit, gi tr ca IV daong trong khong 16.777.216 trng hp. Nhng chuyn gia bo mt ti i hcCalifornia-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 tmra c kha WEP. Thm vo , ba nh phn tch m ha Fluhrer, Mantin vShamir (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 rakha WEP.

Thm vo , mt trong nhng mi nguy him ln nht l nhng cch tn cng

dng hai phng php nu trn u mang tnh cht th ng. C ngha l k tncng ch cn thu nhn cc gi d liu trn ng truyn m khng cn lin lc viAccess Point. iu ny khin kh nng pht hin cc tn cng tm kha WEP ykh khn v gn nh khng th pht hin c.

Hin nay, trn Internet sn c nhng cng c c kh nng tm kha WEP nhAirCrack, AirSnort, dWepCrack, WepAttack, WepCrack, WepLab. Tuy nhin, sdng nhng cng c ny i hi nhiu kin thc chuyn su v chng cn c hn chv s lng gi d liu cn bt c.

c, 802.11i (WPA2)

Mt gii php v lu di l s dng 802.11i tng ng vi WPA2, cchng nhn bi Wi-Fi Alliance. Chun ny s dng thut ton m ho mnh m vc gi l Chun m ho nng cao AES (Advanced Encryption Standard). AES sdng thut ton m ho i xng theo khi Rijndael, s dng khi m ho 128 bit,v 192 bit hoc 256 bit.

nh gi chun m ho ny, Vin nghin cu quc gia v Chun v Cngngh 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 quanchnh 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 bithoc 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.

KT LUN--------------o0o-------------

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Trong ti ny, chng em tm hiu v VOIP over WLAN cng vi hai cng

ngh c bn l VoIP v WLAN. ti trnh by c cc vn sau:

Tng quan v VOIP over WLAN

Tm hiu hai cng ngh c bn ca VOIP over WLAN l VoIP v WLAN

Trnh by nhng vn quan trng trong VOIP over WLAN (dung lng h

thng, QoS, bo mt)

Gii php VOIP over WLAN l mt cng ngh cn kh mi m v mi c

trin khai mt s nc, tuy nhin vi s pht trin nhanh chng ca Internet cng

nh cc cng ngh khng dy th VOIP over WLAN s ha hn l mt gii php y

tim nng pht trin trong tng lai, p ng nhu cu ngy cng a dng ca conngi. Gip gim chi ph dch v v thi gian, to iu kin cho cng ngh mi pht

trin trong tng lai.

Dung lng h thng v cht lng dch v (QoS) lun l vn c quan tm

hng u, chuyn phn tch cc gii php nhm nng cao dung lng cho h

thng VOIP over WLAN v thc hin QoS cho mng. N cng cho ta thy cc nguy

c trong bo mt mng VOIP over WLAN v cc phng php bo mt hin ny angs dng cho mng khng dy nh WEP, WAP. Vi chuyn ny gip chng ta c

mt ci nhn r hn v VOIP over WLAN.

THUT NG VIT TT

--------------o0o-------------

ACK Acknowleged Xc nhn

AES Avanced Encryption Standart Tiu chun m ho nng cao

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AP Access Point im truy cp

CA Collision Avoidance Trnh xung t

CCK Complementary Code Keying Kho m b sung

CRC Cyclic Reduntdancy Check Kim tra d chu trnh

CSMA Carrier Sense Multiple Access a truy cp cm nhn sngmang

CTS Clear To Send Xo pht

DCF Distributed Coordination Function Chc nng phi hp phn b

DES Data Encryption Standard Chun m ho d liu

DHCP Dynamic Host ConfigurationProtocol

Giao thc cu hnh host ng

DIFS DCF- Inter Frame Space Khong thi gian lin khung

DMZ Data Management Zone Khu vc qun l d liu

DoS Denial of Service T chi dch v

DS Distribution System H thng phn phi

DSM Distribution System Medium Mi trng h thng phn phi

DSSS Direct Sequence SpreadSpectrum

Tri ph chui trc tip

EAP Extensible AuthenticationProtocol

Giao thc nhn thc m rng

EDCF Enhanced DistributedCoordination Function

Chc nng phn b phi hpnng cao

EIRP Effective Isotropically RadiatedPower

Bc x ng hng ng u

ESS Extended Service Set B dch v m rng

FDM Frequency Division Multiplexing Hp knh phn chia tn s

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FEC Forward Error Correction Sa li trc

FHSS Frequency Hopping SpreadSpectrum Tri ph nhy tn

HCF Hybrid Coordination Function Chc nng phi hp lai

IBSS Independent Basic Server Set M hnh mng c lp

BSS Basic Service Set M hnh mng c s

IEEE Institute of Electrical andElectronic Engineers

Vin cc k s in v in t

IrDA Infrared Data Association Kt hp d liu hng ngoi

ISM Industrial, Scientific, and Medical Bng tn cng nghip, khoahc v y t

IPsec IP Security An ninh IP

LAN Local Area Network Mng ni ht

MAC Media Access Control iu khin truy nhp mitrng

MGCP Media Gateway Control Protocol Giao thc iu khin cngphng tin

MIMO Multiple input, multiple output Nhiu u vo, nhiu u ra

MPDU MAC Protocol Data Unit n v d liu giao thc MAC

MSDU MAC Service Data Unit n v d liu dch v lp MAC

NAT Network Address Translation Phin dch a ch mng

NAV Network Allocation Vector Vector cp pht mng

NIST National Institute of Standards

and Technology

Vin cc tiu chun v cng

ngh quc gia

OSI Open System Interconnection H thng kt ni m

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OFDM Orthorgonal Frequency DivisionMultiplexing

Ghp knh phn chia theo tns trc giao

PCF Point Coordination Function Chc nng phi hp im

PCM Pulse Code Modulation iu ch xung m

PF Persistance Factor Nhn t bn

PIFS PCF Inter Frame Space Khong trng lin khung PCF

PHY Physical layer Lp vt l

PSTN Public Switched TelephoneNetwork

Mng in thoi chuyn mchcng cng

QAM Quadrature amplitude modulation iu ch bin 1 phn t

QoS Quality of Service Cht lng dch v

QPSK Quadrature phase shift keying iu ch bin dch pha 4 v

tr

RF Radio Frequency Tn s v tuyn

RSSI Received Signal StrengthIndication

mnh tn hiu thu

RTP Real-Time Transport Protocol Giao thc vn chuyn thi gianthc

RTS Request to Send Yu cu truyn

SIFS Short Inter Frame Sort Khong cch lin khung ngn

SNR Signal to Noise Ratio T l tn hiu trn tp m

SS System State Trng thi h thng

SSID Service Set Identifier B nhn dng tp dch v

SWAP Shared Wireless Access Protocol Giao thc truy nhp v tuyn

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dng chung

TDMA Time Division Multiple Access a truy nhp phn chia theothi gian

TKIP Temporary Key Integrity Protocol Giao thc ton vn kho tm thi

TLS Transport Layer Security An ninh lp truyn ti

TSPEC Traffic Specifications Tham s lu lng

UDP User Datagram Protocol Giao thc d liu ngi dng

UNII Unlicensed National InformationInfrastructure

H tng thng tin quc giakhng cp php

VLAN Virtual LAN Mng LAN o

VoIP Voice over Internet Protocol Thoi qua giao thc Internet

VPN Virtual Private Network Mng ring o

WAN Wide Area Network Mng khu vc rng

WEP Wired Equipvalent Privacy Bo mt tng ng hu tuyn

WIPP Wireless IP Phone in thoi IP khng dy

WLAN Wireless Local Area Network Mng ni ht v tuyn

WMA Wireless MultimediaEnhancement

a phng tin nng cao vtuyn

WPA Wi Fi Protected Access Truy nhp c bo v Wi Fi

TI LIU THAM KHO--------------o0o-------------

Sch Tham Kho:

1. Voice Over WLANS The Complete Guide, Michael F. Finneran, Nov.2007.

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2. Voice over 802.11, Frank Ohrtman.

3. Deploying Voice over Wireless LANs, Cisco PressNetworking.

4. Practical VoIP Security, Thomas Porter

5. VoIP over WLAN: Voice capacity, admission control, QoS, and MAC,International Journal of Communication Systems, Int. J. Commun. Syst. 2006

6. VoIP over Wireless LAN Survey, Rzvan Beuran. Internet Research CenterJapan Advanced Institute of Science and Technology (JAIST), April 20, 2006.

Website Tham kho :

1. http://www.google.com.vn/2. http://vnvoip.info/

3. http://www.baonguyen.vn

4. http://www.thegioivoip.net

5. http://www.voip.com

6. http://www.voip-info.org/

7. http://www.vietphattien.com/

MC LC--------------o0o-------------

LI NI U ........................................................................................................................... 1CHNG I: TNG QUAN V VOIP OVER WLAN ......................................................... 2

1.1 Gii thiu chung ............................................................................................................2

1.2 Cu trc mng VOIP over WLAN ...............................................................................3

1.3 C s h tng mng .................................................................................................4

1.3.1 Mng Access Point truyn thng .............................................................................4

43
http://www.google.com.vn/http://www.google.com.vn/http://vnvoip.info/http://vnvoip.info/http://www.baonguyen.vn/http://www.thegioivoip.net/http://www.thegioivoip.net/http://www.voip.com/http://www.voip-info.org/http://www.voip-info.org/http://www.vietphattien.com/http://www.vietphattien.com/http://www.google.com.vn/http://vnvoip.info/http://www.baonguyen.vn/http://www.thegioivoip.net/http://www.voip.com/http://www.voip-info.org/http://www.vietphattien.com/


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1.3.2 Mng chuyn mch khng dy ................................................................................5

1.4 Tn hiu trong VOIP over WLAN ....................................................................................7

1.4.1 Tn hiu Thoi.........................................................................................................8

1.4.2 Tn hiu Video .........................................................................................................8

1.4.3 Lu lng VoIP ......................................................................................................9

CHNG II: CC CNG NGH C BN VOIP OVER WLAN .................................... 92.1 Cng ngh VoIP ............................................................................................................9

2.1.1 Tng quan v VoIP .................................................................................................9

2.1.2 u nhc im ca VoIP .....................................................................................10

2.1.3 Kin trc mng VoIP .............................................................................................10

2.1.4 Phng thc hot ng ........................................................................................11

2.1.5 Cc giao thc bo hiu .........................................................................................11

2.1.5.1 Giao thc MGCP ................................................................................................12

2.1.5.2 Giao thc H323 ..................................................................................................13

2.1.5.3 Giao thc SIP .....................................................................................................14

2.2 Cng ngh WLAN ........................................................................................................14

2.2.1 Tng quan v WLAN .............................................................................................14

2.2.2 u nhc im ca WLAN ...................................................................................15

2.2.3 M hnh WLAN ......................................................................................................16

2.2.3.1 M hnh Ad-hoc (IBSS-Independent Basic Service Set) .....................................16

2.2.3.2 M hnh mng c s h tng (Infrastructure Basic Service Set) .....................17

2.2.4 Cc thit b c bn ca mng WLAN ....................................................................17

2.2.4.1 Card mng khng dy (Wireless NIC) ................................................................17

2.2.4.2 Cc im truy cp (Access Point) ......................................................................18

2.2.4.3 Bridge khng dy (WBridge) ..............................................................................18

2.2.4.4 Cc router im truy cp (Access Point Router) .................................................18

2.2.5 Cc chun WLAN ..................................................................................................18

2.2.5.1 IEEE 802.11 .......................................................................................................19

2.2.5.2 HiperLAN ............................................................................................................20

2.2.5.3 Cc chun khc .................................................................................................20

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CHNG III: CC VN C BN TRONG VOIP OVER WLAN ......................... .213.1 Dung lng h thng v QoS ......................................................................................21

3.1.1 Phn loi lu lng ...............................................................................................21

3.1.2 Giao Thc GEDCF ................................................................................................23

3.1.3 Giao Thc HCF .....................................................................................................24

3.1.4 Thc hin QoS cho VOIP over WLAN ...................................................................25

3.1.5 Dung lng h thng ............................................................................................27

3.2 Bo mt trong VOIP over WLAN .................................................................................30

3.2.1 S cn thit ca bo mt......................................................................................30

3.2.1.1 iu khin th ng ...........................................................................................30

3.2.1.2 Truy cp tri php ..............................................................................................30

3.2.1.3 T chi dch v ...................................................................................................31

3.2.2 Cc phng php bo mt cho VOIP over WLAN ................................................31

3.2.2.1 Xc thc .............................................................................................................31

3.2.2.2 M ha d liu truyn ........................................................................................34

KT LUN ..............................................................................................................................37

TI LIU THAM KHO ......................................................................................................42

chuyên Đề voip over wlan - ptit

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