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Mng my tnh&
H thng thng tin cng nghi
o c ThnhBM K thut o & THCN
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Truyn song song.
z My tnh lu v x l s liu theo tng t ( c di -8,-16,-32,-64 bit).
z D liu s c cp theo dng song song mi ln mt t, mi mt bit c mt ng dn ring.
z Ta c 8 (16,32,64) dy dn song song ni gia 2 im truyn ng thi 8 (16,32,64) mc in p (0/1).
z Nh vy truyn song song l truyn tng byte ( t c di 8,16,32,64 bit).
z Phng php truyn song song c tc truyn cao, n thng c s dng khi truyn bn trong cc thit b hay gia cc linh kin trn cg mt tm mch in,
z Tuy nhin khi truyn khng cch xa phng php ny c nhc im l tn dy dn v c s sai khc v mt thi gian ca cc tn hiu
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Truyn song song.
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Truyn ni tip.
z S dng hai dy dn ni gia hai im. Cc mc in p ON/OFF s c truyn tun t vi mt chun thi gian theo hai dy dn.
z Nh vy truyn ni tip l truyn tng bitz Phng php ny tuy c tc thp hn phng php song
song nhng n khc phc c cc hn ch ca phng php song song khi truyn khong cch xa.
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Truyn ni tip.
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Truyn ni tip.
z My tnh d tn ti dng no u c cc bVXL v bus song song v x l s liu song song. V vy, c th dng phng php truyn ni tip, ta cn c cc b chuyn i song song v ni tip
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Truyn ni tip.
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Truyn khng ng b
z C thi gian ngh gia cc khung bn khng c nh.z Vic truyn c bt u bi 1 Start bit, cc bit c truyn vi
mt thng s nh trc. z Mt c im quan trng l thng s ca cng truyn phi
ging nhau bn pht v bn thu m bo di ca chui bit d liu l nh nhau.
z Trong truyn bt ng b, ng h thu chy mt cch bt ng b vi tn hiu thu. x l thu hiu qu, cn phi c k hoch dng ng h thu ly mu tn hiu n, ngay im gia thi bit ca d liu
z t c iu ny, tn hiu ng h thu nhanh gp N ln ng h pht v mi bit c dch vo SIPO sau N chu k xung ng h
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Truyn khng ng b
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Truyn khng ng b
Nguyn tc ng b k t
Mch iu khin truyn nhn c lp trnh hot ng vi sbit bng nhau trong mt k t k c s stop bit, start bit v bit kim tra gia thu v pht. Sau khi pht hin v nhn start bit, vic ng b k t t c ti u thu rt n gin, ch vic m ng s bit c lp trnh. Sau s chuyn k t nhn c vo thanh ghi m thu ni b v pht tn hiu thng bo vi thit b iu khin (CPU) rng nhn c mt k t mi. V s i cho n khi pht hin mt start bit k tip.
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Truyn khng ng b
Nguyn tc ng b frame
Khi thng ip gm mt khi cc k t thng xem nh mt frame thng tin (information frame) c truyn, bn cnh vic ng b bit v ng b k t, my thu cn phi xc nh c im bt u vim kt thc mt frame. iu ny c gi l sng b frame
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Truyn khng ng b
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Truyn ng b
So vi truyn khng ng b, truyn ng b c mt s khc bit nh l:
z S liu c truyn lin tc, khng c cc bit start, stopz C khung bn tin ln hnz Cn c giao thc iu khin v n nh lung d liu.z Tuy nhin, cng ging nh truyn bt ng b chng ta ch
chp nhn phng php no cho php my thu t c sng b bit, ng b k t v ng b frame.
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Truyn ng b
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Truyn ng b
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Truyn ng b
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Truyn ng b
Truyn ng b hng k t: thc hin vic ng b ny, my pht thm vo cc k t
iu khin truyn, gi l cc k t ng b SYN, ngay trc cc khi k t truyn. Cc k t iu khin ny phi c hai chc nng: trc ht, chng cho php my thu duy tr ng b bit. Th hai, khi iu ny c thc hin, chng cho php my thu bt u bin dch lung bit thu theo cc ranh gii k t chnh xc s ng b k t.
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Truyn ng b-Hng bit
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Truyn ng b
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Truyn ng b
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Truyn 1 chiu ( simplex)
Truyn n cng (Simplex): L h c thit k truyn sliu theo mt chiu khng cung cp chiu ngc li.
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Truyn 2 chiu ( duplex)
H song cng (Duplex): L h c thit k truyn s liu theo c hai chiu
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Truyn Half-duplex)
Bn song cng ( Half Duplex): L h c th truyn s liu theo c hai chiu nhng ti mi thi im ch thc hin mt chiu.
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Truyn Full-duplex)
H song cng ( Full Duplex): L h c th truyn s liu hai chiu mt cch ng thi
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Truyn ti c s
Mt tn hiu mang mt ngun thng tin c th biu din bng tng ca nhiu dao ng c tn s khc nhau nm trong mt phm vi hp, c gi l di tn c s ( Base Band) hay di hp. Tn hiu c truyn i cng chnh l tn hiu c to ra sau khi m ha bit, nn c tn s c nh hoc nm trong mt khong hp no , ty thuc vo phng php m ha bit
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Truyn ti di mang
Trong mt s trng hp, di tn c s khng tng thch trong mi trng lm vic. khc phc tnh trng ny, ngi ta smt tn hiu khc - gi l tn hiu mang, c tn s nm trong mt di tn thch hp - gi l di mang. Di tn ny thng ln hn nhiu so vi tn s nhp. D liu cn truyn ti s dng iu ch tn s, bin hoc pha ca tn hiu mang
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Truyn ti di rng
Mt tn hiu c th cha ng nhiu ngun thng tin khc nhau bng cch s dng kt hp mt cch thng minh nhiu thng s thng tin. Sau khi nhiu ngun thng tin khc nhau c m ho bt, mi tn hiu c to ra s dng iu bin mt tn hiu khc, thng c tn s ln hn nhiu, gi l tn hiu mang. Cc tn hiu mang c iu bin c tn s khc nhau, nn c th pha trn, xp chng thnh mt tn hiu duy nht c ph tn tri rng. y chnh l k thut dn knh phn tn trong truyn ti thng tin, nhm mc ch s dng hiu quhn ng truyn. Pha bn nhn s thc hin vic gii iu bin v phn knh, hi phc cc tn hiu mang cc ngun thng tin khc nhau.
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Truyn ti di rng
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Lin kt
* Lin kt: Lin kt (link) l mi quan h vt l hoc logic gia hai hoc nhiu i tc truyn thng. i vi lin kt vt l, cc i tc chnh l cc trm truyn thng c lin kt vi nhau qua mt mi trng vt l.
C th phn bit cc kiu lin kt sau y:-Lin kt im-im (potnt-to-point): Mt mi nn kt ch c hai i tc tham gia. - Lin kt im-nhiu im (mu/ti-drop): Trong mt mi lin kt c nhiu i tc tham gia, tuy nhin ch mt i tc c nh duy nht (trm ch) c kh nng pht trong khi nhiu i cc cn li (cc trm t) thu nhn thng tin cng mt lc.
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Lin kt
- Lin kt nhiu im (Multipoint): Trong mt mi lin kt c nhiu i tc tham gia v c th trao i thng tin qua li t do theo bt k hng no. Bt c mt i tc no cng c quyn pht v bt c trm no cng nghe c. Cng nh kiu lin kt im-nhiu im c th s dng mt cp dn duy nht ni mng gia cc i tc.
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Topology
* Topology:
Topology l cu trc lin kt ca mt mng, hay ni cch khc chnh l tng hp ca cc lin kt. Topology c th hiu l cch sp xp, t chc v mt vt l ca mng, nhng cng c th lcch sp xp logic ca cc nt mng, cch nh ngha v tchc logic cc mi lin kt gia cc nt mng.
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Cu trc mng dng sao
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Cu trc mng dng sao
- Cu trc hnh sao l mt cu trc mng tt c cc trm c ni vo mt trm trung tm
- Trm trung tm ny s iu khin hot ng truyn thng ca ton mng.
- Tu theo yu cu truyn thng trm trung tm c th l mt b chuyn mch (switch), mt b chn ng (router) hay n gin l mt b phn knh.
-Ta c th nhn thy y kiu lin kt v mt vt l l im-im. Tuy nhin, lin kt v mt logic vn c th l nhiu im. Nu trm trung tm ng vai tr tch cc, n c th m ng nhim v kim sot ton b vic truyn thng ca mng, cn nu khng s ch nh mt b chuyn mch.
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Cu trc mng dng sao
- Nhc im ca cu trc hnh sao l s c trm trung tm s lm t lit ton b cc hot ng truyn thng trong mng, tn dy dn v di ca dy ni vi trm trung tm hn ch.
- u im ca cu trc hng sao l lp t n gin, d kim sot v khc phc s c. Do s dng lin kt im-im do vy c th tn dng ti a tc truyn ca ng truyn vy l.
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Cu trc mng dng vng
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Cu trc mng dng vng
- Cu trc mch vng c thit k sao cho cc thnh vin trong mng c ni t im ny n im kia mt cch tun ttheo mt mch vng khp kn.
- Trong vng, tn hiu c truyn i theo mt chiu qui nh. Mi trm nhn c d liu t trm ng trc v chuyn tip sang trm ln cn ng sau. Qu trnh ny c lp li ti khi d liu quay tr v trm gi.
- u im c bn ca mng cu trc theo kiu ny l mi mt nt ng thi c th l mt b khuch i, do vy khi thit k mng theo kiu cu trc vng c th thc hin vi khong cch v s trm rt ln. Mi trm c kh nng va nhn va pht tn hiu cng mt lc. Bi mi thnh vin ngn cch vng ra lm hai phn.
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Cu trc mng dng vng
- Vi kiu mch vng khng c iu khin trung tm, cc trm u bnh ng nh nhau trong quyn nhn v pht tn hiu. Nh vy vic kim sot ng dn s do cc trm t phn chia. - Vi kiu c iu khin trung tm, mt trm ch s m nhim vai tr kim sot vic truy nhp ng dn.- Cu trc mch vng thc cht da trn c s lin kt im-im, v vy thch hp cho vic s dng cc phng tin truyn tn hiu hin i nh cp quang, tia hng ngoi, v.v. - Mt u im tip theo ca cu trc mch vng l kh nng xc nh v tr xy ra s c, v d t dy hay mt trm ngng lm vic. Tuy nhin, s hot ng bnh thng ca mng cn trong trng hp ny ch c th tip tc vi mt ng dy dphng. Mng dng vng i hi phi c mt giao thc iu khin truy nhp ng truyn kh phc tp.
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Cu trc mng dng Bus
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Cu trc mng dng Bus
- Trong cu trc n gin ny, tt c cc thnh vin ca mng u c ni trc tip vi mt ng dn chung. - Khi mt trm gi tn hiu ra Bus th n s qung b ti tt ccc trm cn li. - c im c bn ca cu trc bus l vic s dng chung mt ng dn duy nht cho tt c cc trm, v th tit kim c cp dn v cng lp t
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Cu trc mng dng Bus
-Bn cnh vic tit kim dy dn th tnh n gin, d thc hin l nhng u im chnh ca cu trc bus, - Mng cn phi c mt giao thc iu khin vic truy nhp ng truyn.Ngoi vic cn phi kim sot truy nhp ng truyn, cu trc bus c nhng nhc im sau:- Mt tn hiu gi i c th ti tt c cc trm v theo mt trnh t khng kim sot c, v vy phi thc hin phng php gn a ch (logic) theo kiu th cng cho tng trm. Trong thc t, cng vic gn a ch ny gy ra khng t kh khn.
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Cu trc mng dng Bus
- Tt c cc trm u c kh nng pht v phi lun lun nghe ng dn pht hin ra mt thng tin c phi gi cho mnh hay khng, nn phi c thit k sao cho ti vi s trm ti a. y chnh l l do phi hn ch s trm trong mt on mng. Khi cn m rng mng, phi dng thm cc b lp.- Chiu di dy dn thng tng i di, v vy i vi cu trc ng thng xy ra hin tng phn x ti mi u dy lm gim cht lng ca tn hiu. khc phc vn ny ngi ta chn hai u dy bng hai tr u cui (Terminator). Vic sdng cc tr u cui cng lm tng ti ca h thng.
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Cu trc mng dng Bus
- Trng hp ng dn b t, hoc do ngn mch trong phn kt ni bus ca mt trm b hng u dn n ngng hot ng ca c h thng. Vic nh v li y cng gp rt nhiu kh khn.- Cu trc ng thng, lin kt a im gy kh khn trong vic p dng cc cng ngh truyn tn hiu mi nh s dng cp quang.
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Cu trc mng dng cy
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iu khin truy nhp ng truyn
- Trong mt mng c cu trc bus,hay dng vng, cc thnh vin phi chia nhau mt ng dn chung. - trnh s xung t v tn hiu gy ra sai lnh v thng tin, mi thi im trn mt ng dn ch duy nht mt in tn c php truyn i. Chnh v vy mng phi c iu khin sao cho ti mt thi im nht nh ch mt thnh vin trong mng c gi thng tin i. Cn s lng thnh vin trong mng mun nhn thng tin th khng hn ch. - Mt trong nhng vn quan trng hng u nh hng ti cht lng ca mi h thng l phng php phn chia thi gian gi thng tin trn ng dn hay phng php truy nhp ng truyn.
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iu khin truy nhp ng truyn
- Phng php truy nhp ng truyn l mt trong nhng vn c bn i vi cc h thng , bi mi phng php c nhng nh hng khc nhau ti cc tnh nng k thut ca h thng. C th, ta phi quan tm ti t nht 3 kha cnh: tin cy, tnh nng thi gian, hiu xut s dng ng truyn.Cc phng php iu khin truy nhp ng truyn c thchia thnh hai nhm chnh:- iu khin truy nhp ngu nhin: vic truy nhp khng c qui nh cht ch trc m xy ra hon ton ngu nhin theo nhu cu ca cc trm.- iu khin truy nhp c iu khin: Trnh t truy nhp c xc nh r rng t trc. Vic truy nhy c kim sot cht ch theo cch tp trung hay phn tn bi cc thnh vin.
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CSMA/CD
Carrier Sense Multiple Access with Collision Detection-Phng php a truy nhp s dng sng mang c php hin xung t
- Phng php truy nhp ngu nhin ny c s dng cho topo dng bus, trong tt c cc trm ca mng c ni trc tip vo bus. - Mi trm u c th truy nhp vo bus chung (a truy nhp) mt cch ngu nhin v do vy rt c th dn n xung t (hai hoc nhiu trm ng thi truyn d liu).-D liu c truyn i theo khun dng chun trong c vng thng tin iu khin cha a ch ca d liu.
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CSMA/CD
- CSMA/CD l phng php ci tin t phng php CSMA, hay cn gi l LBT (Listen Before Talk - Nghe trc khi ni). T tng ca n l: mt trm cn truyn d liu trc ht phi "nghe" xem ng truyn xem ang ri hay bn. Nu ri th truyn d liu i (theo khun dng chun). Ngc li, nu ng truyn ang bn ( c d liu khc) thtrm phi thc hin theo mt trong 3 gii thut sau (thng gi l cc gii thut "kin nhn"-persistent algorithms) :
(1) Trm tm "rt lui" ch i trong mt thi on ngu nhin no ri li bt u "nghe" ng truyn (Non persistent) .
(2) Trm tip tc "nghe" n khi ng truyn ri th truyn d liu i vi xc sut bng 1 (1-persistent)
(3) Trm tip tc "nghe" n khi ng truyn ri th truyn i vi xc sut p xc nh trc (0
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CSMA/CD
- Vic xy ra xung t thng l do tr truyn dn : mt trm truyn d liu (cng sng mang) i ri nhng do tr truyn dn nn mt trm khc lc ang "nghe" ng truyn stng l ri v c th truyn d liu i. Mu cht vn l ch : v cc trm ch "nghe trc khi ni" m khng "nghe trong khi ni" nn thc t c xung t nhng cc trm vn khng hay bit g v vn c tip tc truyn d liu i, gy ra vic chim dng ng truyn mt cch v ch.
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CSMA/CD
c th pht hin xung t, CSMA/CD-hay cn gi lLWT(Listen While Talk-Nghe trong khi ni) b sung thm qui tc :- Khi mt trm ang truyn n vn tip tc "nghe" ng truyn.Nu pht hin thy xung t th n ngng ngay vic truyn nhng vn tip tc gi tn hiu sng mang thm mt thi gian na m bo rng tt c cc trm trn mng u c th"nghe" c s kin xung t .- Sau trm ch i trong mt thi on ngu nhin no ri th truyn li theo cc qui tc ca CSMA
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CSMA/CD
- R rng vi CSMA/CD, thi gian chim dng v ch ng truyn c gim xung bng thi gian dng pht hin mt xung t. - u im ca CSMA/CD l tnh cht n gin, linh hot, vic ghp thm hay b i mt trm trong mng khng nh hng gti hot ng ca h thng. - Nhc im ca CSMA/CD l tnh bt nh ca thi gian phn ng, hiu xut s dng ng truyn v th cng thp.
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CSMA/CA
(Carrier Sense Multiple Access with Collislon Avoidance) - S dng cho Topo mng dng Bus. Tng t nh CSMA/CD, mi trm u phi nghe ng dn trc khi gi cng nh sau khi gi thng tin.- y s dng mt phng php m ha bit thch hp trong trng hp xy ra xung t, mt tn hiu "tri" (dominant) s ln t tn hiu kia "ln" (recessive). - Nu mt trm gi i tn hiu "ln" m gim st v tn hiu "tri" th n s mt quyn u tin v phi dng truyn. Sau trm s ch mt thi gian ngu nhin no v th nghe li ng truyn.
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CSMA/CA
- Mi bc in u c bt u bng mt dy bit c bit c gi l c hiu, sau l ti cc phn khc nh thng tin kim sot, a ch,... - Phng php CSMA/CA, c th s dng mc u tin cho mi trm (hoc theo loi thng tin) v gn m u tin vo phn ng sau c hiu ca mi bc in.- Nh c phng php s dng mc u tin m tnh nng thi gian thc ca h thng c ci thin. C th thy r, tuy b hn ch v tc truyn v chiu di y dn, hiu sut sdng ng truyn phng php ny rt cao. Cc trm chgi thng tin i khi c nhu cu v nu xy ra xung t th mt trong hai bc in vn tip tc c gi i.
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TDMA (Time Divsion Multiple Access):
- S dng cho Topo mng dng Bus.Trong phng php kim sot truy nhp phn chia thi gian TDMA, mi trm c phn mt thi gian truy nhp bus nht nh. Cc trm c th ln lt thay nhau gi thng tin trong khong thi gian cho php gi lkhe thi gian hay lt thi gian (time slot, tim slice ) theo mt tun t qui nh sn. Vic phn chia nyc thc hin trc khi h thng i vo hot ng (tin nh). - H thng c th hot ng khng c trm ch. Trong trng hp c mt trm ch th vai tr ca n ch hn ch mc kim sot vic tun th m bo gi ng lt thi gian ca cc trm khc. Mi trm u c kh nng m nhim vai tr chng trong giao tip trc tip vi cc trm khc.
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TDMA (Time Divsion Multiple Access):
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TDMA (Time Divsion Multiple Access):
- Ngoi cc lt thi gian phn chia c nh cho cc trm dng d trao i d liu nh k (nh s t 1 ti N), thng cn c mt khong d tr dnh cho vic trao i d liu bt thng theo yu cu, v d gi thng tin cnh bo, mnh lnh t cu hnh, d liu tham s, setpoint..
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Master/Slave
- S dng cho cu trc mng dng Bus. Trong phng php ch/t, mt trm ch (master) c trch nhim ch ng phn chia quyn truy nhp bus cho cc trm t (slave). - Cc trm t ng vai tr b ng, ch c quyn truy nhp bus v gi tn hiu i khi c yu cu. Trm ch c th dng phng php hi tun t (polling) theo chu k kim sot ton b hot ng giao tip ca c h thng.
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Master/Slave
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Master/Slave
- Trong mt s h thng, thm ch cc trm t khng c quyn giao tip trc tip vi nhau, m bt c d liu cn trao i no cng phi qua trm ch. Nu hot ng giao tip din ra theo chu k, trm ch s c trch nhim ch ng yu cu d liu t trm t cn gi v sau s chuyn ti trm t cn nhn. Trong trng hp mt trm t cn trao i d liu bt thng vi mt trm khc phi thng bo yu cu ca mnh khi c trm ch hi n v sau ch c phc v. Trnh t tham gia giao tip, hay trnh t hi/p ca cc trm t c th do ngi dng qui nh trc (tin nh) bng cc cng c t cu hnh.
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Master/Slave
- Phng php ch/t c mt u im l vic kt ni mng cc trm t n gin, tn km bi gn nh ton b "tr tu" tp trung ti trm ch. Mt trm ch thng l mt thit b iu khin, v vy vic tch hp thm chc nng x l truyn thng liu khng kh khn.
- Mt nhc im ca phng php kim sot tp trung ch/tl hiu sut trao i thng tin gia cc trm t b gim do phi d liu phi i qua khu trung gian l trm ch, dn n gim hiu sut s dng ng truyn.
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Master/Slave
- Mt hn ch na ca phng php ny l tin cy ca hthng truyn thng ph thuc hon ton vo mt trm ch duy nht. Trong trng hp c xy ra s c trn trm ch th ton b h thng truyn thng ngng lm vic. Mt cch khc phc l s dng mt trm t ng vai tr gim st trm ch v c khnng thay th trm ch khi cn thit.
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Token Bus
- Phng php ny s dng cho topo mng dng Bus. - Nguyn l ca phng php ny l : cp pht quyn truy nhp ng truyn cho cc trm ang c nhu cu truyn dliu, mt th bi c lu chuyn trn mt vng lgic thit lp bi cc trm .- Khi mt trm nhn c th bi th n c quyn s dng ng truyn trong mt thi gian xc nh trc. Trong thi gian n c th truyn mt hay nhiu n v d liu. Khi ht dliu hoc ht thi gian cho php, trm phi chuyn th bi n trm tip theo trong vng lgic.
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Token Bus
- Nh vy, cng vic phi lm u tin l thit lp vng lgic (hay cn gi l vng o) bao gm cc trm ang c nhu cu truyn d liu c xc nh v tr theo mt chui th t m trm cui cng ca chui s tip liu sau bi trm u tin. Mi trm c bit a ch ca cc trm k trc v sau n. - Th t ca cc trm trn vng lgic c th c lp vi th tvt l. Cc trm khng hoc cha c nhu cu truyn d liu thkhng c a vo vng lgic v chng chi c th tip nhn d liu.
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Token Bus
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Token Bus
Vic thit lp vng lgic trong chng trnh l khng kh, nhng vic duy tr n theo trng thi thc t ca mng mi l kh. C th phi thc hin c cc chc nng sau :- B sung mt trm vo vng lgic : cc trm nm ngoi vng lgic cn c xem xt nh k nu c nhu cu truyn d liu th b sung vo vng lgic.- Loi b mt trm khi vng lgic : khi mt trm khng cn nhu cu truyn d liu cn loi n ra khi vng lgic ti u ha vic iu khin truy nhp bng th bi.- Qun l li : mt s li c th xv ra, chng hn trng a ch (hai trm u ngh rng n lt mnh) hoc "t vng" khng trm no ngh ti lt mnh.- Khi to vng lgic : khi ci t mng hoc sau khi "t vng", cn phi khi to li vng.
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Token Ring
- Phng php ny p dng cho Topo dng vng.- Phng php ny cng da trn nguyn l dng th bi cp pht quyn truy nhp ng truyn. Nhng y th bi lu chuyn theo vng vt l ch khng cn thit lp vng lgic nh i vi phng php Token Bus.
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Token Ring
- Th bi l mt n v d liu c bit trong c mt bit biu din trng thi s dng ca n (bn hoc ri). Mt trm mun truyn d liu th phi i n khi nhn c mt th bi "ri" (free) . Khi trm s i bt trng thi ca th bi thnh "bn" (busy) v truyn mt n v d liu cng vi th bi i theo chiu ca vng. Gi y khng cn th bi "ri" trn vng na, do cc trm c d liu cn. truyn cng phi i. D liu n trm ch s c sao li, sau cng vi th bi i tip cho n khi quay v trm ngun. Trm ngun s xa b d liu vi bt trng thi tr v "ri" v cho lu chuyn tip trn vng cc trm khc c th nhn c quyn truyn d liu.
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Token Ring
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Token Ring
- S quay v li trm ngun ca d liu v th bi nhm to mt c ch bo nhn (acknowledgment) t nhin : trm ch c th gi vo n v d liu phn header cc thng tin v kt qutip nhn d liu ca mnh. Chng hn, cc thng tin c thl : (1)trm ch khng tn ti hoc khng hot ng ; (2) trm ch tn ti nhng d liu khng c sao chp; (3) d liu c tip nhn; (4) c li.
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Token Ring
- Trong phng php ny cn gii quyt hai vn c th dn n ph v h thng. Mt l vic mt th bi lm cho trn vng khng cn th bi lu chuyn na. Hai l mt th bi "bn" lu chuyn khng dng trn vng. C th c nhiu gii php khc nhau cho hai vn ny. Sau y l mt gii php c khuyn ngh :- i vi vn mt th bi, c th qui nh trc mt trm iu khin ch ng (active monitor). Trm ny s pht hin tnh trng mt th bi bng cch dng c ch ngng thi gian (time-out) v phc hi bng cch pht i mt th bi "ri" mi.
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Token Ring
- i vi vn th bi "bn" lu chuyn khng dng, trm monitor s dng mt bit trn th bi (gi l monitor bit) "nh du" (t gi tr 1) khi gp mt th bi "bn" i qua n. Nu n gp li mt th bi "bn" vi bit nh du th c ngha l trm ngun khng nhn li c n v d liu ca mnh v th bi "bn" c quay vng mi. Lc , trm monitor s i bit trng thi ca th bi thnh "ri" v chuyn tip trn vng. Cc trm cn li trn vng s c vai tr b ng : chng theo di pht hin tnh trng s c ca trm monitor ch ng v thay th vai tr .
