cng n tp mn h thng vin thngCu 1:trnh by s khi h thng thng tin s v cc chc nng cc khi?

1.2.2.1. Ngun tin

Ni sn ra tin:

- Nu tp tin l hu hn th ngun sinh ra n c gi l ngun ri rc.

- Nu tp tin l v hn th ngun sinh ra n c gi l ngun lin tc.

Ngun tin c hai tnh cht: Tnh thng k v tnh hm .

Vi ngun ri rc, tnh thng k biu hin ch xc sut xut hin cc tin l khc nhau.

Tnh hm biu hin ch xc sut xut hin ca mt tin no sau mt dy tin khc nhau

no l khc nhau.

V d: P(y/ta) P(y/ba)

1.2.2.2. My pht L thit b bin i tp tin thnh tp tn hiu tng ng. Php bin i ny phi l n tr hai chiu (th bn thu mi c th sao li c ng tin gi i). Trong trng hp tng qut, my pht gm hai khi chnh.

- Thit b m ho: Lm ng mi tin vi mt t hp cc k hiu chn nhm tng mt , tng kh nng chng nhiu, tng tc truyn tin.

- Khi iu ch: L thit b bin tp tin ( hoc khng m ho) thnh cc tn hiu bc x vo khng gian di dng sng in t cao tn. V nguyn tc, bt k mt my pht no cng c khi ny.

1.2.2.3. ng truyn tin L mi trng vt l, trong tn hiu truyn i t my pht sang my thu. Trn ng truyn c nhng tc ng lm mt nng lng, lm mt thng tin ca tn hiu.

1.2.2.4. My thu L thit b lp li (sao li) thng tin t tn hiu nhn c. My thu thc hin php bin i ngc li vi php bin i my pht: Bin tp tn hiu thu c thnh tp tin tng ng.

My thu gm hai khi:

- Gii iu ch: Bin i tn hiu nhn c thnh tin m ho.

- Gii m: Bin i cc tin m ho thnh cc tin tng ng ban u (cc tin ca ngun gi i).

1.2.2.5. Nhn tin C ba chc nng:- Ghi gi tin (v d b nh ca my tnh, bng ghi m, ghi hnh,)

- Biu th tin: Lm cho cc gic quan ca con ngi hoc cc b cm bin ca my th cm c x l tin (v d bng m thanh, ch s, hnh nh,)

- X l tin: Bin i tin a n v dng d s dng. Chc nng ny c th thc hin bng con ngi hoc bng my.

1.2.2.6. Knh truyn tin L tp hp cc thit b k thut phc v cho vic truyn tin t ngun n ni nhn tin.

1.2.2.7. Nhiu L mi yu t ngu nhin c nh hng xu n vic thu tin. Nhng yu t ny tc ng xu n tin truyn i t bn pht n bn thu. cho gn, ta gp cc yu t tc ng vo mt trn hnh 1.2.

Hnh 1.2 l s khi tng qut nht ca mt h truyn tin s. N c th l: h thng v tuyn in thoi, v tuyn in bo, raa, v tuyn truyn hnh, h thng thng tin truyn s liu, v tuyn iu khin t xa.

2.iu ch la gi ?-iu ch tn hiul qu trnh bin i mt hay nhiuthng sca mttn hiu tun hontheo s thay i mt tn hiu mangthng tincn truyn i xa. Tn hiu tun hon gi lsng mang. Tn hiu mang thng tin gi l tn hiu c iu ch. u thu b gii iu ch s da vo s thay ithng s ca sng mang ti to li tn hiu mang thng tin ban u. Cc thng s ca sng mang c dng trong qu trnh iu ch c th lbin ,pha,tn s.

Nguyn li iu ch :

iu ch nhm bin i mt tn hiu u1(t) nguyn thy thnh mt tn hiu u2(t). iu ch.Qu trnh ngc li l s gii iu ch. iu ch v gii iu ch t ra quy tc bin i, c tnh thun nghch.

Tuy nhin, trong qu trnh truyn tn hiu thc t, do c can nhiu nn s b mo dng. Tn hiu trn knh truyn, tn hiu iu ch u ra knh l u1(t) khc u2(t). Do tn hiu sau gii iu ch l u1(t) khc u1(t). Kh nng min nhin nh hng ca nhiu m mo dng l ty thuc vo dng iu ch. Vic chn li iu ch sao cho thch hp phi ty theo tng iu kin c th.

Cc dng iu ch c chia lm 2 nhm chnh: iu ch tng t (analog modulation)v iu ch s (digital modulation).

*Mc ch : iu ch l mt php ton bin i t 1 tn hiu mang tin tc ny sang 1 tn hiu khc m khng lm thay i v ni dung mang tin tc mang theo.

Cho php tn hiu thch nghi vi iu kin truyn ca mi trng ngay c khi trong mi trng b can nhiu m vn m bo cht lng tn hiu mun truyn i.

Cho php trn nhiu knh thng tin trn cng mt mi trng truyn.

Hai mc ch ny khng th c tha mn cng lc do ta ch dng 1 loi iu ch. Trng hp ny ngi ta phi dng bin php iu ch kp.

V d: tn hiuting nictn s thp, khng th truyn i xa c. Ngi ta dng mttn hiu hnh sinctn s cao( c th truyn i xa c) lm sng mang. Bin i bin ca tn s sin theo tn hiu ting ni. u thungi ta da vo s thay i bin ca tn hiu thu c ti to li tn hiu ting ni ban u.

3.so sanh iu ch tng t va iu ch s:

*iu ch tng t : (AM,FM,PM)

-Trong iu ch tng t, vic iu ch c thc hin lin tc theo tn hiu thng tin tng t.Cc phng php iu ch tng t thng dng l: + iu bin (Amplitude modulation).

+ iu ch hai bng (DSB-Double-sideband modulation).

+ iu ch hai bng khng trit sng mang (DSB-WC) dng trong radio bng AM.

+ iu ch hai bng trit sng mang (DSB-SC).

+ iu ch hai bng nn sng mang (DSB-RC).

+ iu ch n bng (SSB hoc SSB-AM).

+ iu ch n bng trit sng mang (SSB-SC).

+ iu ch Vestigial sideband (VSB hoc VSB-AM).

*iu ch s :

-Trong iu ch s, mt sng mang tng t s c bin i theo mt chui bit c chiu di c nh hoc thay i. y cng c th c coi l mt dngbin i tng t-s. Hnh dng ca sng mang c ly t mt tp hp hu hn ccsymbol. Sau y l nhng phng php c bn ca iu ch s :

-TrongCW, ngi ta dng on-off keying ca tn hiu c chiu di thay i.

-TrongPSK, ngi ta dng mt s hu hn pha.

-TrongFSK, ngi ta dng mt s hu hn tn s.

-TrongASK, ngi ta dng mt s hu hn bin .

*Truyn qua bng tn gc (truyn gii nn):

-Truyn tn hiu qua bng tn gc ( baseband transmission) ngha l tn hiu c truyn trc tip sau khi n xut pht t ngun, ti bng tn gc ca n. Phng php truyn ny i hi cc mi trng truyn khc nhau cho mi loi tn hiu, chng hn :

-Knh thoi : Bng tn 100 Hz 5 kHz, truyn trn dy song hnh i xng, trong phm vi a phng hoc khong cch gn.

-Knh truyn hnh : Bng tn 50Hz + 5MHz, truyn trn khong cch ngn vi cp ng trc.

-Knh truyn s liu: Cho cc tn hiu m ha ni lin cc terminals.

Ta quy c bng tn gc ca tn hiu c biu din bng 1 tam gic t en, dc v hng tn s tng ln nh hnh 1.1 , trong tn gii gc ca tn hiu la B = fmax fmin .

f

fmin B fmax

Hnh 1.2

*Truyn tn hiu tng t:

Khng quan tm ni dung d liu cha ng bn trong.

Suy gim tn hiu theo khong cch.

Dng amplifier khuch i tn hiu (k c nhiu).

*Truyn tn hiu s:

Cn ch ni dung d liu cha ng bn trong.

Khong cch truyn ngn.

Dng repeater tng khong cch truyn.

Nhiu khng b khuch i .

*u im

Cng ngh s:

+ Cng ngh LSI/VLSI lm gim gi thnh Ton vn d liu:

+ Nhiu v suy gim tn hiu khng b tch ly bi cc repeater

+ Truyn khong cch xa hn trn cc ng truyn km cht lng

Hiu qu knh truyn:

+ TDM > FDM

Bo mt:

+ Cc k thut m ha bo mt d liu d p dng.

Tch hp:

+ D liu s v analog c x l tng t nhau.

4. phat biu inh li nyquist va y nghia cua inh li

-nh l ly mu NyquistShannonl mt nh l c s dng trong lnh vc l thuyt thng tin, c bit l trong vin thng v x l tn hiu doHarry NyquistvClaude Shannonpht minh.Ly mul qu trnh chuyn i mttn hiu(v d, l mthmlin tc theo khng thi gian) thnh mt chui s (mt hm ri rc theo khng thi gian). nh l ly mu c pht biu nh sau:

Mt hm s tn hiux(t)khng cha bt k thnh phn tn s no ln hn hoc bng mt gi trfmc th biu din chnh xc bng tp cc gi tr ca n vi chu k ly muT = 1/(2fm). Nh vy, tn s ly mu phi tho mn iu kinfs 2fm. Tn s gii hnfs/2ny c gi ltn s Nyquistv khong(-fs/2; fs/2)gi l khong Nyquist. Thc t, tn hiu trc khi ly mu s b gii hn bng mt b lc tn s tn hiu nm trong khong Nyquist.

nh l ny thng c gi lnh l ly mu Nyquist, nhng k t khi n c pht hin mt cch c lp bi E.T. Whittaker, bi Vladimir Kotelnikov, v bi nhng ngi khc, th n cn c gi lNyquist-Shannon-Kotelnikov,Whittaker-Shannon-Kotelnikov,Whittaker-Nyquist-Kotelnikov Shannon,WKS, haynh l c bn ca l thuyt ni suy. Nhng n thng c gi n gin lnh l ly mu**y nghia: V bn cht, nh l cho thy mttn hiu tng t(analog) c tn s gii hn c ly mu c th c ti to hon ton t mt chui v s cc mu nu t l ly mu ln hn2fmmu trong 1giy,fml cctn sln nht ca tn hiu ban u. Shannon cho rng nu tn hiu c cha mt thnh phn c tn s chnh xc bngfmHz, th cc mu s cch nhau ng1/(2fm)giy s khng th khi phc hon ton chnh xc tn hiu.iu kin ny c th khng chnh xc nh trong phn tho lun ti ly mu tn hiu non-baseband di y.

Cc pht biu ca nh l gn y i khi cn thn loi tr gi tr bng, c ngha l iu kin nux(t)khng c tn s no cao hn hocbng fm; iu ny l tng t pht biu ca Shannon tr khi hm s bao gm mt dng sng sin n nh c cha thnh phn ti tn s chnh xc fm.

Trong x l tn hiu,ly mul chuyn i mttn hiu lin tcthnh mttn hiu ri rc. Mt v d ph bin l vic chuyn i ca mtsng m thanh(mt tn hiu lin tc) thnh mt chui cc mu (mt tn hiu thi gian ri rc).

Mt mu cha mt gi tr hoc tp hp cc gi tr ti mt im trn trc thi gian (v/hoc khng gian).

Ly mu c th c thc hin cho cctn hiukhc nhau trong h ta khng thi gian, hoc vi cc h ta bt k, v kt qu tng t thu c trong h ta 2 hoc nhiu chiu.

i vi cc tn hiu khc nhau theo thi gian, v dl mt tn hiu lin tc c ly mu, v vic ly mu c thc hin bng cch o cc gi tr ca tn hiu lin tc thi im mi giy,Tc gi l khong thi gian ly mu. Nh vy, tn hiu sau khi c ly muc a ra bi:

, vin = 0, 1, 2, 3,... Trong thc t, cc tn hiu lin tc c ly mu bng cch s dng mt mch chuyn i tn hiu tng t sang tn hiu s (ADC), y l 1 thit b khng l tng. iu ny dn n xut hin mt lch trong qu trnh ti dng li tn hiu so vi l thuyt, gi chung l b mo.Cu 6 v sao tn s UL lon hon DL

Suy hao l mt hm ph thuc tn s, tn s cng cao th suy hao cng ln. Trong cc thit b vin thng gim suy hao th ngi ta thng dng phng php l tng cng sut pht. Trong thng tin v tinh th tn s uplink cao hn downlink l v:khi tin hiu truyn t v tinh n trm mt t v ngc li do khong cch rt xa, mi trng truyn lun c cc yu t gy suy hao nh ma..nu tn s downlink bng tn cao th i hi phi tng cng sut pht ca v tinh ln nhiu ln, iu ny dn n tng kch thc cng nh khi lng ca v tinh, y l vic lm khng kh thi v khi bn mt v tinh ln qu o th gi thnh t l vi khi lng ca n. khi uplink tn s cao th chng ta d dng iu khin cng sut pht tram mt t.

7. Cc loi anten sau c th c s dng t trn v tinh:

Anten ng hng: khng c hng tnh trong khng gian t do, do , li anten l G=0dB

Hao ph nng lng pht.

Anten chn t: c hng tnh cc i theo mt phng thng gc vi trc chnh anten. Mt phng ny ca anten trn v tinh phi c hng v Tri t, n nh bng chuyn ng t xoay spin ca V Tinh quanh trc song song vi trc chnh ca chn t.

Anten loa vi gc m trng vi gc nhn t V Tinh v Tri t (170). li anten chng 20dB, nhng anten phi lun hng v Tri t.

Anten parabol vi hng tnh rt cao (spot beam). Vng bao ph song ca anten ch gii hn trong mt a lc, mt vng hoc mt quc gia, li anten s t l nghch vi gc m ca tia sng.

8. Fading l hin tng ti ni thu nhn c ng thi 2 hay nhiu sng cng n 1 lc, cc sng ny xut pht cng 1 ngun nhng i theo nhiu ng khc nhau

Tu thuc vo hiu cc ng i m hiu pha ca chng cng khc nhau

Nu hiu pha = 2n. th cng chng tng cng nhau

Nu hiu pha = (2n+1). th cng chng trit tiu nhau

Hin tng Fading gy ra s thu chp chn, gy gin on thng tin trong mt thi gian ngn, trong k thut truyn hnh, to ra hin tng bng ma

khc phc Fading, ngi ta s dng phn tp tn s hay phn tp khng gian

9.u im va nhc im cua si n mode va si a modeb) Si a mode

c im ca si a mode l truyn ng thi nhiu bc sng

S mode sng truyn c trong mt si quang ph thuc vo cc thng s ca si,trong c tn s c chun ha V (Normalized Frequency). Tn s c chun ha c xc nh nh sau:

Vi

a: bn knh li si quang

: bc sng lm vic

NA: khu s ca si quang

Mt cch tng qut,s mode sng truyn trong si quang c xc nh gn ng:

(g l s m trong hm chit sut)

T suy ra:

S mode truyn trong si SI:

S mode truyn trong si GI:

Si a mode c ng knh li v khu ln:

-ng knh li: d = 50 -ng knh lp bc: D = 125 -Gi l si a mode 50/125 -Chit sut li: n1 = 1,47 ( = 1300 nm)

-Khu s: NA = 0.2 0.29

nh sng i trong si a mode

nh sng i trong si a mode

c) Si n mode.

Si n mode l si trong ch c mt mode song c bn lan truyn

Theo l thuyt,iu kin si lm vic ch n mode l tha s sng V ca si ti bc sng lm vic V < Vc1 = 2,405

Si n mode c ng knh li v khu nh:

-ng knh li: d = 9 -ng knh lp bc: D = 125 -Gi l si a mode 50/125 -Chit sut li: n1 = 1,465 ( = 1300 nm)

-Khu s: NA = 0.13 0.18

nh sng i trong si n mode

nh sng i trong si n mode

10.s khi cua h thng thng tin quang

1.2.2 Cu trc c bn ca h thng thng tin quang

Gm:-B pht quang

-B thu quang

-Mi trng truyn dn l cp si quang

Truyn dn quang lin lc theo mt hng

Truyn dn quang lin lc theo hai hng

Thc hin truyn dn gia hai im cn c hai si quang

Nu c ly thng tin qu di th cn mt n hai trm lp.Cu trc n gin ca mt trm lp (cho truyn dn mt hng)

Cu trc n gin ca mt trm lp quang

-Khi E/O: B pht quang c nhim v nhn tn hiu in a n,bin tn hiu in thnh tn hiu quang v a tn hiu quang ny ln ng truyn (si quang). l chc nng chnh ca khi E/O b pht quang.Thng ngi ta gi khi E/O l ngun quang. Hin nay linh kin c dng lm ngun quang la LED v LASER.

-Khi O/E: Khi tn hiu quang truyn n u thu, tn hiu quang ny s c thu nhn v bin tr li thnh tn hiu in nh u pht. l chc nng ca khi O/E b thu quang.Cc linh kin hin nay c s dng lm chc nng ny l PIN v APD v chng thng c gi l linh kin tch sng quang (photo detector).

-Trm lp: Khi truyn trn si quang, cng sut tn hiu quang b suy yu dn (do si quang c suy hao). Nu c ly thng tin qu di th tn hiu quang ny c th khng n c u thu hoc n u thu vi cng sut cn rt thp u thu khng nhn bit c, lc ny ta phi s dng trm lp(trm tip vn). Chc nng chnh l thu nhn tn hiu quang suy yu, ti to chng tr li thnh tn hiu in. Sau sa dng tn hiu in ny,khuch i tn hiu sa dng,chuyn i tn hiu khuch i thnh tn hiu quang . V cui cng a tn hiu quang ny ln ng truyn truyn tip n u thu. Nh vy, tn hiu ng vo v ng ra ca trm lp u dng quang v trm lp c c khi O/E v E/O.

11 .cu tao si cap quang Cp quang c cu to gm dy dn trung tm l si thy tinh hoc plastic c tinh ch nhm cho php truyn i ti a cc tn hiu nh sng. Si quang c trng mt lp lt nhm phn chiu tt cc tn hiu nh sng v hn ch s gy gp ca si cp quang.

Si cp quang c cu to t ba thnh phn chnh:-li (core)-Lp phn x nh sng (cladding)-Lp v bo v chnh (primary coating hay cn gi coating, primary buffer).

Core c lm bng si thy tinh hoc plastic dng truyn dn nh sng. Bao bc core l cladding lp thy tinh hay plastic nhm bo v v phn x nh sng tr li core. Primary coating l lp v nha PVC gip bo v core v cladding khng b bi, m, try xc.

Hai loi cp quang ph bin l GOF (Glass Optical Fiber) cp quang lm bng thu tinh v POF (Plastic Optical Fiber) cp quang lm bng plastic. POF c ng knh core kh ln khong 1mm, s dng cho truyn dn tn hiu khong cch ngn, mng tc thp. Trn cc ti liu k thut, bn thng thy cp quang GOF ghi cc thng s 9/125m, 50/125m hay 62,5/125m, y l ng knh ca core/cladding; cn primary coating c ng knh mc nh l 250m.

Bo v si cp quang l lp v ngoi gm nhiu lp khc nhau ty theo cu to, tnh cht ca mi loi cp. Nhng c ba lp bo v chnh l lp chu lc ko (strength member), lp v bo v ngoi (buffer) v lp o gip (jacket). Strength member l lp chu nhit, chu ko cng, thng lm t cc si Kevlar. Buffer thng lm bng nha PVC, bo v trnh va p, m t. Lp bo v ngoi cng l Jacket. Mi loi cp, ty theo yu cu s dng s c thm cc lp jacket khc nhau. Jacket c kh nng chu va p, nhit v chu mi mn, bo v phn bn trong trnh m t v cc nh hng t mitrng.

12.chuyn mach la gi?

NH NGHA CHUYN MCHL mt qu trnh thc hin u ni v chuyn thng tin cho ngi s dng qua h tng mng vin thng. Ni cch khc chuyn mch trong mng vin thng bao gm chc nng nh tuyn cho thng tin v chc nng chuyn tip thng tin.Nh vy theo kha cnh thng thng n gn lin vi lp mng (lp 3) v lp lin kt d liu (lp 2) trong m hnh OSI (Open System Interconnection).

Qu trnh chuyn mch c thc hin cc nt mng, trong mng chuyn mch knh cc nt mng thng gi l cc tng i, trong mng chuyn mch gi thng gi l Thit b nh tuyn (B nh tuyn).Trong mt s mng c bit phn t thc hin nhim v chuyn mch c th va ng vai tr TBC (thit b u cui) va ng vai tr chuyn mch v chuyn tip thng tin13.chuyn mach knh la gi

Chuyn mch knh

Chuyn mch mch in(circuit switching) hay chuyn mch knh

Chuyn mch mch in c s dng ch yu trong mng in thoi.

Loi chuyn mch ny thng l hai chiu, s dng mt mi trng truyn(chuyn mch 2 dy) hoc hai mi trng ring bit(chuyn mch 4 dy).

Vic thit lp mt chuyn mch mch in tri qua 3 giai on sau:

Thit lp kt ni: trong giai on ny, b chuyn mch phi pht hin yu cu kt ni ni thu bao pht, nhn v gii m t thu bao pht pht hin thu bao cn kt ni n, tm v chim gi ng c th kt ni n thu bao thu, bo cho thu bao thu nhu cu kt ni, nhn dng p ng ca thu bao thu v sau l kt ni thng gia hai thu bao.Thc cht qu trnh ny l lin kt cc tuyn gia cc trm trn mng thnh mt tuyn (knh) duy nht dnh ring cho cuc gi. Knh ny i vi PSTN l 64kb/s (do b m ha PCM c tc ly mu ting ni 8kb/s v c m ha 8 bit).

Duy tr kt ni: gi vng ng kt ni trong sut thi gian hai thu bao thng tin vi nhau,gim st kt ni v tnh cc lin lc.

Gii ta kt ni: chuyn mch s gii ta kt ni khi c yu cu ca mt trong hai thu bao, gii ta tt c cc knh v cc thit b phc v cngcng v quay v trng thi ch.

Qua , ta nhn thy mng chuyn mch knh c nhng u im ni bt nh cht lng ng truyn tt, n nh, c tr nh. Cc thit b mng ca chuyn mch knh n gin, c tnh n nh cao, chng nhiu tt. Nhng ta cng khng th khng nhc ti nhng hn ch ca phng thc truyn d liu ny nh:

S dng bng thng khng hiu qu: Tnh khng hiu qu ny th hin qua hai yu t. Th nht, rng bng thng c nh 64k/s. Th hai l knh l dnh ring cho mt cuc gi nht nh. Nh vy, ngay c khi tn hiu thoi l lng (khng c d liu) th knh vn khng c chia s cho cuc gi khc.

Tnh an ton: Do tn hiu thoi c gi nguyn bn trn ng truyn nn rt d b nghe trm. Ngoi ra, ng dy thu bao hon ton c th b li dng n trm cc vin thng.

Kh nng m rng ca mng knh km: Th nht l do c s h tng kh nng cp v tng thch vi cc thit b c. Th hai, l hn ch ca h thng bo hiu vn c s dng t trc khng c kh nng ty bin cao.

14.so snh gia chuyn mch knh v chuyn mch gi

Ging nhau: l s thit lp kt ni theo yu cu t mt ng vo yu cu n mt ng ra, yu cu trong mt tp ng vo v ng ra

Mc ch: thit lp mt ng truyn dn thng tin t ngun n ch theo mt cu trc c nh hoc bin ng qua cc mng v trung tm.

Chuyn mch knh

nguyn tc l loi chuyn mch phc v vic trao di thng tin bng cch cp knh truyn dn trc tip cho hai i tng s dng.

x l cuc gi tin hnh qua 3 giai on: thit lp knh dn, duy tr knh dn, gii phng knh dn.

c dim:

Thc hin trao i thng tin gia hai i tng theo thi gian thc.

i tng s dng lm ch knh dn trong sut qu trnh trao i thng tin.

Hiu sut thp.

Lng ph thi gian do c giai on thit lp v gii phng knh truyn.

Ni dung thng tin khng mang thng tin a ch.

Ph hp vi dch v thoi.

Khi lu lng tng n mt mc ngng no th mt s cuc gi c th b kha, mng t chi mi kt ni cho n khi c th.

Chuyn mch gi

nguyn tc:

Gi tin c chia lm nhiu gi nh c chiu di thay i, mi gi c gn thm a ch cng vi nhng thng tin a ch cn thit. Cc gi i vo trong mt node c lu vo trong b nh m cho n khi c x l sau c xp hng trong hng i cho n khi c truyn trong chuyn tip theo,

Ti trung tm tin nhn cc gi c hp li thnh mt bn tin v c sp xp li a ti thit b nhn s liu.

c im

Cc ng truyn dn c th phi hp s dng mt s ln cc ngun tng i hot ng. Do hiu sut s dng knh tng.

tr trung bnh ca cc tuyn truyn dn ph thuc vo cc ti trong mng.

Hn ch c tnh trng tr v thng lng ca mng suy gim khi lng thng tin n qu ln cc node. tin cy cao.

chng li, chuyn mch gi s dng phng thc t ng hi li.

Ti trung tm nhn tin, x l cc tn hiu kim tra li xc nh xem gi c li.

18