hệ thống atm và ứng dụng vào mạng cục bộ

7/30/2019 h thng ATM v ng dng vo mng cc b

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Li ni u

Nhu cu v giao tip trao i thng tin i vi loi ngi tr nn khng th thiu

trong cuc sng hng ngy v nhu cu y ngy cng tng cng vi s pht trin rt caoca tr tu loi ngi. Bt u t chic my in thoi, l mt s nhy vt ln trongthng tin ca loi ngi, con ngi dn dn tin ti vic truyn d liu ch, truyn s liui t im ny ti im khc. tip l vic truyn hnh nh lm con ngi gn b vinhau hn v by gi ngi ta mun dng tt c cc loi thng tin khc nhau nh ting ni,hnh nh, s liu trong cng mt lc( nh cu truyn hnh) truyn t mt im nnhiu im hoc t mt im n mt im. C mi ln nh vy, cng vi s tin btrong thng tin, nhng h thng thng tin cng vi s cung cp cho n ngy cng lndn ln, n v ang t ra nhng thch thc mi v mt qun l cho con ngi.

Mng dch v t hp s bng hp N - ISDN ra i vo u nhng nm 80 nh l mtcu cnh cho s pht trin ny. N cho php mt mng c th cung cp tt c cc dchv hin c. Tuy nhin, s pht trin mnh m ca cng ngh thng tin v nhu cu traoi thng tin ngy cng tng nhanhv a dng ho ca x hi i hi phi cp bch ccdch v truyn nh bao gm c nh tnh v nh ng cht lng cao v truyn s liu,truyn file tc siu cao m chng yu cu tc bt ti vi trm Mb/s, thm ch tihng chc Gb/s. Ni chung mng ISDN bng hp khng th p ng, tho mn c ccyu cu b xung nu trn. mt mng vin thng thng nht p ng tt c cc loi hnhdch v vin thng v x l tin vi tc yu cu rt khc nhau t mt vi Kb/s nhng chc Gb/s, thm ch hng Tb/s gi l mng s lin kt a dch v bng rng (B-

ISDN) v ch c B- ISDN vi c kh nng cung cp dch v a phng tin. Ngy naymt s gii php k thut c xut nhm ci thin thc hin ca mng vinthng v tin ti thc hin B-ISDN.Trong s cc Cng ngh thng tin khc nhau phi k n cng ngh quan trng nht l phng thc truyn khng ng b ( Asynchronous transfer Mode - ATM). ATMc th ng dng trong cc mi trng khc nhau nh mng LAN, mng WAN, mngcng cng, mng cp truyn hnh. Do vy ITU-T quyt nh rng kiu truyn khngng b ATM s l phng php truyn cho mng B-ISDN trong tng lai v a ra cckhuyn ngh v ATM, t c s cho mng ATM cng nh phn ln cc tham s ca n.Ti Vit Nam hin nay cng vi s pht trin nhanh chng ca ngnh Bu chnh Vin

thng, cng ngh ATM cng bt u c ch trng nghin cu nhm p ng cnhng nhu cu ngy cng tng ca x hi. Bn n ny l mt phn trong qu trnhnghin cu, tm ti nhm tin ti nhanh chng p dng cng ngh mi nhn ny tiVit Nam, ni dung ch yu l : nghin cu h thng ATM v ng dng ATM trongmng cc b (ATM-LAN).

n gm hai phn :

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Phn A: Trnh by l thuyt chung v cng ngh truyn ti khng ng b ATM nhc im chung, cc dch v, cc tnh ton thit lp tham s ATM, cc giao thc cc tip gip khc nhau, chuyn mch truyn dn.

Phn B: Trnh by cc ng dng ca cng ngh ATM v c bit l ng dng trongmng ATM-LAN. Phn ny ti c gng trnh by trong phm vi c th v kin thc

mng ATM-LAN, ng dng quan trng ca ATM l m phng ATM-LAN.B-ISDN l mt k thut rt mi, ATM- LAN cng cha c s dng rng ri do cn mts vn cn gii quyt, do bn n khng trnh khi mt s thiu st v nhiuvn vn cha c trnh by v gii quyt. V vt ti rt mong c s gp v gip ca cc thy c v cc bn.

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Mc lc

Li ni u.Phn I: C s l thuyt chung v cng ngh chuyn ti khng ng b ATM.Chng I : ATM gii php cho cc dch v vin thng bng rng

1.1 Gii thiu.1.1.1 Cc c im ca mng vin thng ngy nay.1.1.2 S ra i ca h thng vin thng mi B- ISDN

1.2 Gii thiu v ATM v kh nng ca ATM.1.2.1 Khi nim c bn v ATM.1.2.2 Cc lnh vc cng ngh mi quyt nh s ra i v pht

trin ca ATMChong II : Xy dng cc tham s c bn cho B-ISDN

2.1. Cc tham s ca h thng.

2.1.1.Tc bit t nhin, tc bit trung bnh, tc bit cci v tc truyn dch v ca mng2.1.2. Cc tham s c trng cho cht lng mng.

2.2. La chn kiu truyn cho B-ISDN.2.2.1. chuyn mch knh2.2.2.chuyn mch knh a tc 2.2.3 chuyn mch knh tc cao2.2.4. chuyn mch gi.2.2.5. dng truyn khng ng b ATM.

2.3. Tnh ton cc tham s c bn cho ATM.

2.3.1. tr.2.3.2. T l li.2.4. Xc nh di cho t bo ATM.

2.4.1. La chn gia hai gii php di c nh hay thayi2.4.2. La chn kch thc ca t bo ATM.

2.5. Tm tt.Chng III : k thut mng ATM

3.1. M u .3.2. c im k thut ca B-ISDN3.3. K thut lin kt mng trong B-ISDN

3.3.1. M hnh sp xp cc lp mng ca B- ISDN3.3.2. Mt s khi nim c lin quan n knh o v ng o3.3.3. Cc ng dng ca cuc ni knh o v ng o.

3.4. Cu trc t bo ATM.3.4.1.S liu nhn dng knh o VCD v ng o VPI.3.4.2 Kiu t bo.3.4.3 CLP3.4.4HEC

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3.4.5GFC3.5 Nguyn l chuyn mch v bo hiu trong ATM.

3.5.1 Nguyn l chuyn mch3.5.2 Nguyn l bo hiu

3.6 Cu trc phn lp ca mng ATM

3.6.1Tng quan3.6.2 Cc lp thp trong B- ISDN3.6.3 Cc lp cao trong B-ISDN

3.7 Mng ca ngi s dng v cc vn thng tin linmng

3.7.1c im giao din UNI3.7.2M hnh mng CN

3.8 Tm ttChng IV : h thng truyn dn trong ATM

4.1 Tng quan

4.2 Cc thit b truyn dn bng rng4.2.1 Bghp knh SDH4.2.2 B phn knh SDH4.2.3 B hn lung thng tin ng b4.2.4 B tp trung v b dn ATM4.2.5 B ni xuyn ATM4.2.6 Chuyn mch ATM4.2.7 Cc khi dch v4.2.8 phn t kt ni lin mng IWU

4.3 Cc chc nng truyn dn bng rng

4.3.1 To ra cc t bo ATM t dong thng tin ln tc4.3.2 Truyn dn t bo ATM4.3.3 Ghp knh v tp trung dng thng tin4.3.4 Phn lung v trung chuyn dng t bo

4.4 Mng truyn dn SONET/SDH4.5 Cu trc truyn dn bng rng

4.5.1 Cu trc mng a phng4.5.2 Cu trc mng trung k

4.6 Tm tt

Phn B : tng quan m hnh cu trc mng bng rng

Chng V :tng quan v m hnh mng ATM5.1 Cu trc mng cc b5.2 Phng tin truyn5.3 Quan h gia phng tin truyn v cu trc

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5.4 Cc giao thc iu khin truy nhp phng tin truyn5.4.1Truy nhp ngu nhin CSMA/CD5.4.2Token BUS5.4.3Token Ring

5.5 Kin trc ATM - LAN

5.6 M phng ATM - LANChng IV: M phng mng ATM-LAN6.1 Tng quan6.2 Kin trc giao thc6.3 M phng LAN

6.3.1 Gii thiu6.3.2 Client v Server trn LAN m phng6.3.3 Ton cc ca m phng LAN6.3.4 Dng khung LAN m phng

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Chng I : ATM gii php cho cc dch v vin thng bng rng1.1Gii thiu.1.1.1.Cc c im ca mng vin thng ngy nay.Cho n nay cc mng vin thng l cc mng chuyn dng, ng vi mi loi dch vthng tin li c t nht mt loi mng vin thng phc v dch v .

V d ; mng telex : dng gi cc bc in di dng k t c m 5 bit ( m Baudot).Tc truyn rt thp ( t 75 n 300 bit/s ).

Mng in thoi cng cng : cn gi l mng POTS ( plain old telephone service ), y thng tin ting ni c s ho v chuyn mch qua h thng mch in thoicng cng PSTN ( Public switched telephone network )

Mng truyn s liu bao gm mng truyn mch gi trao i gia cc my tnhda trn cc th tc X25 v mng truyn s liu chuyn mch knh X21.

Cc tn hiu truyn hnh c th truyn theo 3 cch : truyn bng sng v tuyn,truyn qua h thng mng truyn hnh CATV ( community antenna TV) bng cp

ng trc hoc qua h thng v tinh, cn gi h thng truyn hnh trc tip DBS( direct broadcast system ).

Trong phm vi c quan vic truyn s liu gia cc my tnh c thc hin bi mngcc b LAN ( local area network ) m ni ting nht l mng Ethernet, token bus vtoken ring

Mi mng trn c thit k cho cc dch v ring bit v khng th s dng cho ccmc ch khc. V d : ta khng th truyn h thng ni qua mng truyn mch giX25 v tr qua mng ny qu ln. Hu qu l hin nay c rt nhiu loi mng khcnhau cng song song tn ti. Mi mng li yu cu phng php thit k, sn xut, vnhnh, bo dng khc nhau. Nh vy mng vin thng hin ti c rt nhiu nhc im mquan trng nht l :- Ch truyn c cc dch v c lp tng ng vi tng loi mng .- Thiu mm do : s ra i ca cc thut ton nn ting ni, nn nh v tin b trong

cng ngh VLSI nh hng mnh m ti tc truyn tn hiu.Ngoi ra cn c nhiu dch v truyn thng trong tng lai m hin nay cha d on trcc, mi loi dch v s c tc truyn khc nhau. Ta d dng nhn thy rng hthng hin nay rt kh thch nghi yu cu ca cc dch v khc trong tng lai.- Km hiu qu trong vic bo dng, vn hnh cng nh vic s dng ti nguyn. V ti

nguyn sn c trong mt mng khng th chia s cho cc mng khc cng s dng.

1.1.2.S ra i ca h thng vin thng mi B- ISDNNh nu trn, yu cu c mt mng vin thng duy nht ngy cng tr nn bc thit,ch yu l do cc nguyn nhn sau :- Cc yu cu dch v bng ang tng ln- Cc k thut x l tn hiu, chuy mch truyn dn tc cao ( khong vi trm

Mbit/s ti vi Gbit/s) tr thnh hin thc.- Nhng tin b v kh nng x l nh v s liu- S pht trin ca mng ng dng phn mm trong lch vc tin hc v vin thng.

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- S cn thit phi t hp cc dch v ph thuc ln nhau chuyn mch knh vchuyn mch gi vo mt mng bng rng duy nht. So vi c c mng khc, dch vt hp v mng t hp c nhiu u im v mt kinh t, pht trin, thc hin vnhnh v bo dng.

- S cn thit phi tho mn tnh mn do cho cc yu cu v pha ngi s dng, cng

nh ngi qun tr mng ( v mt tc truyn, cht lng dch v ).- Khuyn ngh ITU -T121 a ra tng quan v kh nng ca B-ISDN nh sau :

B-ISDN cung cp cc cuc ni thng qua chuyn mch cc cuc ni c nh( parmanent ) hoc bn c nh ( Semi parmanent ), cc cuc ni t im ti im,hoc t im ti nhiu im v cung cp cc dch v theo yu cu c nh. Cuc nitrong B- ISDN phc v cho cc dch v chuyn mch knh, chuyn mch gi theo kiua phng tin( Multimedia ), n phng tin ( monomedia) theo kiu hng lin kt( connectionless) v theo cu hnh n hng hoc a hng B- ISDN l mt mch thngminh c kh nng cung cp cc dch v ci tin, cung cp cc cung c bo dng v vn

hnh (OAM), iu khin v qun l mch rt hiu qu.1.2. Gii thiu v ATM v kh nng ca ATM.1.2.1. Khi nim c bn v ATM.B-ISDN theo ITU-T da trn c s truyn khng ng b ATM. Nh vy ATM s l nntng ca B- ISDN trong tng lai.

Hnh 1.1. Cu trc khung thi gian trong ATM

Knh1

Knh2

Knhn

Knh1

Knh2

Knhn

Khe thi gian

Kkung thi gian 125

Hnh 1.1 : Cu trc khung thi gian trong STM

Knh1

Knh5

Knh1 Knh1

Knh5

Knh1

Knh khng s dng

Phn tiu ca t bo ATM

Phn thng tin ca ngi s dngHnh 1.2 : cu trc lung thng tin trong ATM

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Trong kiu truyn khng ng b, thut ng truyn bao gm c lnh vc truyn dnv chuyn mch, do dng truyn m ch c ch truyn dn v chuyn mchthng tin trong mng.Thut ng khng ng b gii thch cho mt kiu truyn trong cc gi trong

cng mt cuc ni c th lp li mt cch bt thng nh lc chng oc to ra theo yu

cu c th m khng theo chu k. minh ho, hnh 1.1 v 1.2 biu din s khc nhau gia dng truyn ng b v dngtruyn khng ng b. Trong dng truyn ng b STM ( Synchronous transfer mode ),cc phn t s liu tng ng vi knh cho c nhn bit bi v tr ca n trongkhung truyn ( Hnh 1 ) trong khi ATM cc gi thuc v mt cuc ni li tng ngvi cc knh o c th v c th xut hin ti bt k v tr no ( Hnh 2).ATM cn c hai c im quan trng Th nht : ATM s dng cc gi c kch thc nh v c nh gi l cc t bo ATM

( ATM cell), cc t bo nh cng vi tc truyn ln s l cho tr truyn v binng tr ( delay jitter) gim nh i vi dch v thi gian thc, ngoi ra kch thc

nh cng s to iu kin cho vic hp knh tc cao c d dng hn. Th hai : cn c mt c im rt quan trng l kh nng nhm mt vi knh o( Virtual path)

1.2.2.Cc lnh vc cng ngh mi quyt nh s ra i v pht trin ca ATM.C hai yu t nh hng ti ATM l :- S pht trin nhanh chng ca cng ngh bn dn cng nh cng ngh quang in

t.- S pht trin cc tng mi v khi nim h thng .

1.2.2.1. Cc tin b v mt cng ngh. Cng ngh bn dn :

Cng ngh CMOS l cng ngh rt c trin vng bi tch hp ln, tc cao( khong vi trm Mbit/s ti vi Gbit/s, tiu tn nng lng thp.

Cng ngh quang :Cc ng truyn dn quang c cc u im nh : suy gim thp ( dn ti khong cchtruyn ln), rng bng truyn ln, kch thc nh, mn do c hc cao, trnh cnhiu ca trng in t, xc sut truyn li thp v khng c nhiu xuyn m.

1.2.2.2. Cc tng mi v khi nim h thng.

Cc quan im mi v h thng oc pht trin mnh m trong nhng nm gn y, l h thng phi c mn do thch hp, rng bng ca h thng tu thuc co yucu ca tng dch v c th, cc dch v thi gian thc oc truyn theo phng phptruyn mng gi.Cc tng ny phi tho mn hai chc nng chnh ca mng l :

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- Tnh trong sut v mt ni dung ( Semantic transparency ). Tnh trong sut v nidung l chc nng m bo vic truyn ng t u pht ti u thu ( tc l schnh xc v mt ni dung )

-

Hnh 1.3 : iu khin li y trn mi lin kt ca mngChuyn mch gi

Trong cc mng chuyn gi khi mi ra i, cht lng truyn s liu cn thp, do m bo cht lng truyn chp nhn c, ngi ta phi thc hin chc nng iukhin li trn mi lin kt ( link by link ). Vic iu khin li nay c thc hin bi cc

th tc HDL ( Hight level Data Link control) bao gm cc chc nng : gii hn khung (frame delimiting ), m bo truyn bit chnh xc, kim tra li ( kim tra m d vngCRC - Cyclic Redundancy Check ), sa li bng cc th tc truyn li. Hnh 1.3 trnhby th tc iu khin li y ca mng chuyn mch gi thng qua m hnh linkt cc h thng m OSI. Ta thy rng qu trnh iu khin c thc hin trn lp 2. y qu trnh iu khin li c thc hin trn mi lin kt thng qua nt chuyn mch,do nt chuyn mch phi x l rt nhiu thng tin lm nh hng ti x l v tin cy ca h thng.

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Hnh 1.4 : iu khin li c gii hn mng chuyn tip khung.

Hnh 1.5 : chuyn mch t bo trong mng ATM vi cc chc nng ti thiu.

Chc nng Chuyn mch gi Chuyn tip khung ATMTruyn li gi X - -Gii hn khung X X -Kim tra li X X -

Bng 1.1 : Cc chc nng thc hin nt mng ca X25 chuyn tip khung ATM.

i vi b- isdn tng ny cn c m rng hn na, cc chc nng iu khin likhng cn c cung cp cc nt chuyn mch trong mng na m trong trng hpcn thit, s c cung cp bi cc thit b u cui. Nh vy cc chc nng iu khin

trong mng c gim t iu khin li y ( full error control ) mng chuyn giX 25 xung cn cc k ti thiu mng ATM, do cc nt ATM c phc tp tithiu v v th c tc truyn rt cao, c th ln ti 600 Mbit/s ( Hnh 1.5). Bng 1.1trnh by cc chc nng c thc hin ti nt mng chuyn mch gi th h c vmng chuyn mch gi th h mi ( phng php chuyn tip khung) ca mng ATM.R rng nt mng ATM hu nh khng phi x l thng tin iu khin no trong khi nt mng ca h thng chuyn gi th h c phi x l rt nhiu thng tin.- trong sut v mt thi gian ( time transferency).Cc dch v thi gian thc yu cu dng bit c tr rt ngn khi c truyn t u pht

ti u thu. C th phn bit hai loi tr : tr do chuyn mch v tr do truyn t im

u ti im cui.H thng chuyn mch gi v chuyn tip khung rt kh khn khi thc hin cc dch

v thi gian thc v tr cao. Do phc tp ca nt chuyn mch, chng ch c thhot ng tc va v thp. Mng ATM ch cn cc chc nng ti thiu ntchuyn mch , do n cho php truyn s liu vi tc rt cao, tr trn mng v ccbin ng gim xung cn vi trm s , do quan h thi gian c m bo nh trongtrng hp chuyn mch knh.1.3.Tm tt.

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Chng ny trnh by cc c im ca cc mng vin thng hin hu cng nhcc hn ch ca chng v nhu cu dch v bng rngang tng ln. T vn t ral phi c mt mng t hp bng rng duy nht ( B-ISDN) thay th tt c cc mng vinthng ni trn. Chnh trn c s ny m ATM hnh thnh v pht trin. S pht trinca k thut ATM l kt qu trc tip ca cc tng mi v khi nim h thng c h

tr bi cc thnh tu to ln trong cng ngh bn dn v cng ngh quang in t. ATMc kh nng p ng c mt lot cc dch v bng rng khc nhau, k c trong lnhvc gia nh cng nh trong thng mi.

Chng II :cc tham s v c im c bn ca B-ISDN.

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2.1. cc tham s ca h thng.Sau y s trnh by mt s tham s ca cc dch v trong mng bng rng sau

s trnh by ti cc tham s v tnh ton v li cng nh tr trong mng. Cc tham s nyrt quan trng v c th da vo nh gi cht lng mng.

2.1.2 . Tc t nhin, tc trung bnh, tc bit cc i v tc truyn dch v

ca mng.Mng bng rng tng lai cn c truyn mt s lng ln cc dch v, t cc dch v tc thp nh : o lng t xa, bo ng t xa, ting ni, fax, ti cc dch v tc trngbnh nh : m nhc, in thoi truyn hnh, truyn s liu tc cao hoc cc dch v ctc rt cao nh : HDTV, th vin vidio... Cc dch v ny c tc t vi bit/s ti vitrm Mbit/s, thi gian truyn t va giy ti vi gi ( Hnh 2.1).

Hnh 2.1. c im tc thi gian truyn ca cc dch v bng rng.C th biu din tc bit t nhin ca dch v bng hm S(t), hm ny ko di trongthi gian truyn thng tin T. C hai gi tr quan trng l : tc bit cc i ( the peaknatural bit rate ) v tc trung bnh E(S(t)) c tnh trong khoangr thi gian T. Qungthi gian T cng vi hai gi tr E(S(t)) v S l cc tham s quan trng ca dch v.

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Tc ruyn tren mng

Tc truyn t nhin

ca dch v0 t(s)Hnh 2.3 : Cht lng gim do tc truyn nh hn tc bit cc iMt khc, nu tc truyn lun ln hn hoc bng tc bit cc i ca dch v thcc thng tin v ngha s c s dng in y vo khong chnh lch gia tc bit t nhin v tc truyn, do s tiu ph rng bng truyn. iu ny c minhho trn hnh 2.4.

Tc (bit/s) tiu ph rng bng truyn

Tc truyn trn mng

Tc bit t nhin ca dch v

0 t(s)

Hnh 2.4 : Tiu ph rng bng truyn do tc truyn ln hn tc bit cc iQua hai th d trn, c th kt lun rng cn phi la chn tc truyn thch hp tu

theo yu cu dch v.

2.1.2. Cc tham s c trng cho cht lng mng.Nh trnh by 1.2.2.2 truyn thng tin mt cch tin cy, mng phi m bo hai

ch tiu :- Trong sut v mt ni dung.- Trong sut v mt thi gian.

2.1.2.1 : Cc tham s lin quan n tnh trong sut v mt ni dung.Tnh trong sut v mt ni dung m bo cho mng kh nng truyn thng tin mt chchnh xc t ngun ti ch vi s li cho php. Truyn thc t c 3 loi li :

- Li n v s liu d ( residual error data unit rate) : l cc li khng th khi phc -c.

- Li s liu b phn phi nhm (misdelivered data unit rate) : l li khi s liu btruyn ti cc ch sai.

- Li s liu khng c truyn i ( not delivered data unit rate) : l li khi s liukhng c truyn ti a ch cho trc.

Nh vy cc loi li trn c trng cho tnh thng sut v mt ni dung v gy ra mt tl li tr no , chng c th c nh ngha bi cc tham s sau :

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- T l li bit : c c trng bng tham s t l bit li BER ( bit error rate) :

BER = Tng s bit li / Tng s cc bit c gi i

Cc bit li c th xy ra ring bit ( li n) hay xy ra lin tc thnh nhm.- T l li gi : trong cc mng hot ng da trn nguyn l chuyn mng gi, mt

nhm cc li c th xy ra do mt nhm thng tin b mt hoc b nh ng nhm( misrouted).

T l gi c c trng bng tham s t l li gi PER ( packet error rate ) :

PER =S gi b li/Tng s gi c gi

Trong thc t, li gi thng xy ra do hai nguyn nhn : cc gi b mt do nh ngsai hoc do tc nghn ; c c trng bng t l mt gi PLR (packet loss rate ) :

PLR = Tng s gi b mt / Tng s gi c gi

Cc gi ti cc ch khng mong mun nhng cc ch ny li chp nhn chng nh ccgi ng. i lng c trng cho trng hp ny c gi l t l chn gi PIR (packetinsertion rate ) :

PIR = Tng s gi chn vo ch nhm/tng s cc gi c gi iTrong mng cc li c th xut hin phn truyn dn ti cc b tp trung knh hocti nt chuyn mch.

2.1.2.2 : Cc tham s lin quan n tnh trong sut v mt thi gian. trong sut v mt thi gian m bo tr nh cho cc dch v thng tin trnmng, c bit l cc dch v thi gian thc. Trong sut v mt thi gian c c trngbi hai tham s l tr D v bin ng tr J ( delay jitter).Ni chung i tr D gia u thu v u pht gia cc t bo ATM c th khc nhau -

c biu th bng tr cc i DM v tr cc tiu Dm. S khc nhau gia DM vDm dn ti bin ng tr : J =DM-Dm.

Ch : bin ng tr l tr khng ng u ca cc thng tin ti cng mt imcui ti cc thi im khc nhau. N dn ti vic khi phc tin hiu khng chnh xctrong cc dch v yu cu thi gian thc.Theo ITU-T, nu tr gia hai u cui ln hn 25msth phi lp thm cc b kh tingvang. Tuy vy tr trong mng phn ln c gi tr xp x mt gi tr trung bnh Do no, xc sut xy ra tr khc xa gi tr Do ny rt nh.

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Xc sut tr1

trD0

Hnh 2.5 : Hm mt tr ca cc dch v truyn trn mng.Tham s tr D li chia thnh hai loi gm :

- Tr truyn : Dt- Tr x l : Dp.Lc D c tnh : D = Dt + DpTr Dt xy ra ti cc nt chuyn mch v c cu to vt l ca mi nt cng nh phngphp x l thng tin cu chng. Theo ITU-T tr trung bnh trong chuyn mch s

phi nh hn 450 s . Trong ATM, gi tr ny cn b hn do tc x l tng lnng k.Bng 2.2 : Th hin t l li v tr cho php ca cc loi dch v khc nhau

Dch v BER PLR PIR TrThoi 10-7 10-3 10-3 25ms/500ms

Truyn s liu 10-7 10-6 10-6 1000msTruyn hnh qung b 10-6 10-8 10-8 1000ms

m thanh cht lng cao 10-5 10-7 10-7 1000msX l iu khin t xa 10-3 10-3 10-3 1000ms

Bng 2.2 : T l li v tr chp nhn c i vi tng dch v ca mng ATM.

2.2. la chn kiu truyn chuyn B-ISDN.Nh trnh by trong 1.2.1 ITU-T chn kiu truyn khng ng b l dng truyn

cho B-ISDN truyn tng lai ; nguyn nhn la chn ATM nh nn tng caB-ISDN( gii thiu s qua 1.1) s r rng hn sau khi ta xem mt h thng thng tin khcnhau.

2.2.1. Chuyn mch knh.Phng php chuyn mch ny c s dng t lu trong mng in thoi, nhy

nay n vn c s dng trong N- ISDN. Chuyn mch knh s dng phng php ghpknh theo thi gian DTM ( time division multiplexing). Chuyn mch knh rt thiuphn mm do do cc thng tin phi truyn theo mt tn s c nh dn ti gii hn vmt tc v khng thch hp cho vic truyn cc dch v bng rng c c im khcnhau.

2.2.2. chuyn mch knh a tc . khc phc s thiu mn do ca ch truyn n tc trong chuyn mch

knh, ngi ta a ra h thng chuyn mch knh a tc MRCS ( Multirate circuit

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switching ). Cc ng ni trong MRCS c chia thnh n knh c bn gm cc khungthi gian vi cc khe thi gian c di khc nhau, mi cuc lin lc c th c xydng t n knh ny. h thng chuyn mch MRCS rt phc tp do mi knh c s camt ng ni phi gia ng b vi cc knh khc. Do ITU-T cng khng coiMRCS l gii php cho mng bng rng.

2.2.3. chuyn mch knh tc cao.Cc ti nguyn trong h thng chuyn mch knh tc cao FCS ( Fast circuitsuitching) ch c cung cp khi thng tin c gi i v khi gi xong thng tin tinguyn c gii phng tr li. S cung cp ny c thit lp mi ln gi nh trong trnghp chuyn mch gi, nhng di s iu khin ca tn hiu bo hiu lin kt nhanh ( fastassociated signalling) ch khng nm tiu nh trong chuyn mch gi.S kt hp gia FCS v MRCS c gi l h thng chuyn mch nhanh a tc

MRFCS ( Multirate fast circuit switching ). Tuy vy n cng cn mt vi nhc im,c bit l phc tp khi thit k v iu khin mt h thng nh vy, v y yu cukh nng thit lp v hu b cuc ni trong mt khong thi gian rt ngn. FCS v

MRCS cng khng c la chn lm gii php cho mch bng rng.2.2.4. chuyn mch gi. m bo cht lng truyn chp nhn c t u cui ti u cui, cn c

phc tp ca X25 nhm x l li v iu khin lung gia cc ng lin kt ( Link bylink). Mt khc nhng gi c di khc nhau yu cu qun l b m rt phc tp do tc hot ng khng cao.

Trong X25 lp hai s dng truy nhp ng lin kt bng LAPB( balalced link accessprocedure ). LAPB c s dng thc hin cc chc nng nh nhn bit gii hn khung; chn tch cc bit chng li, truyn li cc khung b mt bng th tc ARQ ( automaticrepeat request) iu khin lung.

Cc h chuyn mch gi sau ny c ci tin thnh hai h thng thnh chuyn mchkhung ( Frame switching) v chuyn tip khung (Frame relaying).Trong chuyn tip khung vic truyn li cc khung s liu b li ch c truyn t

u cui ti u cui ( gia cc thit b u cui ngi s dng). Ti nt mng ch cchc nng pht hin li hu b cc khung v khng cn thit phi truyn cc khungny, ngoi ra cng khng c chc nng iu khin lung hoc phn knh hp knh.Trong chuyn mch khung, cc chc nng pht hin li v iu khin lung vn cngia li nt mch do vic truyn li khung v iu khin lung bng ca s trtvn c thc hin trn c s cc lin kt.

Hai h thng chuyn mch khung v chuyn tip khung c rt nhiu u im, tuy vy

chng vn khng c kh nng thc hin cc dch v thi gian thc do tr lp.2.2.5.Dng truyn khng ng b.

Sau khi xem xt rt nhiu h thng khc nhau, cui cng ITU-T quyt nh lyATM l mng phc v cho cc dch v bng rng bi v ATM tho mn c cc yucu t ra, cc u im ca n l :- Mm do v ph hp vi cc dch v trong tng lai.- C hiu qu trong vic s dng ti nguyn.- Ch s dng mt mng duy nht cho tt c cc dch v.

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ATM cn c cc tn gi khc nh ATD( asynchronous time division ), IBCN( intergrated broadband commulication network).2.3. Tnh ton cc tham s c bn cho ATM.

Nh cp trn cc tham s ATM u c tnh ton da trn c s hai yuchnh l trong sut v mt thi gian c trng bi tr v trong sut v mt ni

dung, c trng bi t l li.2.3.1. tr.Tr khi truyn thng qua mng ATM c quyt nh bi cc phn khc nhau ca

mng, mi phn ng gp vo tr tng ca mng. Hnh 2.7 trnh by tr qua cc khukh nhau ca mng.

TD1 TD2 TD3 TD4PD FD1+QD1

(a) Tr trong mng ATM thun tu

TD1 TD2 TD3 TD4

PD FD1+QD1(b) tr trong mng kt hp gia ATM v mng ng bTrong :

Hnh a:-PD : Tr to gi -TD: tr truyn

-FD : Tr chuyn mch -QD: Tr hng i -DD: Tr tho giHnh b:

-FD: tr ca mng ng bHnh 2.7 : cc loi tr khc nhau trong mng ATM.

Trong mng ATM thun tu, thng tin c ng gi thnh cc t bo v khi phc

thnh cc trng thi u ni nhn tin. Cc t bo c s dng khp mi ni trnmng. Ngoc li trong mng kt hp mt phn ca mng hot ng vi t bo phnkhc hot ng, vi cc khung thi gian. Cc loi tr bao gm :- Tr truyn ( TD) ph thuc vo khong cch im u v im cui thng thng cgi tr t 4-5 s /km.- Tr to gi : ( PD) xy ra khi cc thng tin c ng gi vo cc t bo.- Tr chuyn mch c nh : (FD) xy ra khi mt t bo ATM i qua chuyn mch, n

c gi tr c nh.

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MngATM Chuyn

FD +QD DD

ChuynmchAT Chuyn

FD +QD DD

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- Tr hng i (QD) tr ny c gi tr thay i xy ra ti cc hng i trong h thngchuyn mch.

- Tr tho gi : (DD) xy ra ti u thu ca mng ATM v ti gianh gii gia mngATM v mng ng b trong trng hp kt hp.Nh vy ta c th tnh ton tr tng cng nh sau :

- Trong mng thun tu ATM :D1=TDi +FDj + maxqQDj + PD- Trong mng kt hp.

D2=TDi +FDj + maxqQDj + k.PD +SD1

Trong :i: - on lin kt ca ng truyn

j: - S chuyn mch ATM

k: - S ln to gi / tho gi gia mng ATM v mng ng bSD1- tr trong cc mng ng b

Bng 2.3: Trnh by tr qua tng khu trong mng

Tc 150 Mbit/s 600 Mb/sKch thc t bo

(byte)16 32 64 16 32 64

TD() 4000 4000 4000 4000 4000 4000

FD()64 128 256 16 32 64

QD() 200 400 800 50 100 200PD() 2000 4000 8000 2000 4000 8000SD() 900 900 900 900 900 900D1() 6264 8528 12256 6166 8123 12364D2() 9365 13828 21956 9016 13132 21364

2.3.2. T l li.Cng nh cc h thng khc cc li xy ra trong ATM l do s khng hon ho ca

h thng chuyn dn hoc ca h thng chuyn mch.2.3.2.1. S mt t bo do li phn tiu .

Li truyn s dn ti s thay i khng mong mun cc thng tin truyn. Nu lixy ra phn s liu ca t bo th c t bo vn c truyn ti im cui do ATMkhng c bt c mt c ch chng li no khi truyn t kin kt ti lin kt. Nu lixy ra phn tiu th chuyn mch ATM s khng dch nhm phn tiu ny v

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sau t bo c th b nh ng sai. iu ny xy ra khi phn tiu mang gi tr camt ng ni khc. Nu phn tiu mang mt gi tr khng tn ti th t bo d bhu b. Trong c hai trng hp u c th xy ra li nhn (multiplication error) do chcn mt bit trong phn tiu cng dn ti c t bo. T li c gi l B.

2.3.2.2. S mt t bo do trn hng i.

Do kch thc thch hp ca c c hng i trong mng s mt t bo do trn hng igim xung t gi tr chp nhn c ,gi tr ny khong 10-8. Vic tnh ton kch thchng i c gim nh rt nhiu bi tnh cht hng lin kt (connection - oriented))caATM v n to kh nng mng cho php hoc t chi mt cuc ni nh hn hoc lnhn ti con li trong hng i .2.4:Xc inh di cho t bo ATM

Sau khi tnh ton cc tham s tr v t l ni ,ta i n tnh ton kch thc ca tbo ATM ,c th l la chn cc gii php : di tt bo l c nh hay thay i ra sao cho tr ,t l ni cng nh phc tp khi thc hin l ti thiu .

2.4.1La chn gi 2gii php di c nh hoc thay i.

2.4.1.1: V mt hiu xut bng truynC hai quan im khc nhau v di gi , di c nh hoc thay i .Nhiunhn t khc nhau nh hng ti u im v nhc im ca c hai gii php nhng ccyu t quan trng nht cn phi xem xt khi la chn l hiu qu ca bng truyn dnmc phc tp ca chuyn mch v tr. Ta c cng thc chung tnh hiu sut bngtruyn :

=S byte thng tin /(S byte thng tin + S byte tiu )

di gi c nh :Trong trng hp di ATM c nh, hiu sut bng truyn ctnh theo cng thc O:

= X/[(int (X/L))(L+H)]

trong :L : Kch thc truyn s liu ca gi tnh bng byte.H : Kch thc phn tiu .X : Tng s byte thng tin hu ch c truyn.Int(Z) : Phn nguyn ca Z.Nh vy hiu sut s ti u khi ton b thng tin c truyn i chia ht cho kch thc tr-

ng thng tin.

Lc , gi tr hiu sut bng truyn s l :Fopt =L/(L+H)

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Ngi ta nhn thy rng hiu sut ph thuc rt nhiu vo cc byte thng tin hu chc truyn i, s byte thng tin hu ch di th hiu sut ti u cng d t c.

Do lung thng tin trong ATM gm : ting ni, tn hiu video, s liu tc cao lnhiu sut gn t c gi tr ti u, k c khi s dng t bo c di c nh. Gi c di thay i :

- i vi gi c di thay i cc thng tin b xung v phn tiu bao gm cc c nhn bit gii hn gi mt vi bit c trn thm nhn bit cc c chnh xc. Ngoi ra cn phi cng thm vo phn u khung phn bo hiu digi, lc hiu sut l :

= X/ (X+H+hv)

Trong : hv l phn thng tin bt buc phi b xung thm bo hiu di githay i ca gi ATM. Hiu sut truyn ca gi c di thay i rt cao, vi cc gi

c di ln hiu sut ny t gn ti 100%.Kt lun :Xt v mt hiu sut truyn ni chung gi c di thay i tt hn l gi c di

c nh. Tuy nhin khi xem xt trong tng trng hp c th, u th ny li rt hn ch dolung thng tin ca mch bng rng s bao gm s kt hp ca ting ni, tn hiuvideo v s liu u l nhng tn hiu c dng bit lin tc./2.4.1.2. V mt tc chuyn mch v phc tp.

phc tp ca vic thc hin chuyn mch i vi gi c di c nh hay thayi ph thuc vo cc chc nng ca chng cng nh cc yu cu k thut tng ng vicc chc nng ny. Hai nhn t quan trng nht l tc hot ng v yu cu vkch

thc b nh hng i. tc hot ng :Ph thuc vo cc chc nng cn phi thc hin v thi gian thc cc chc nng .- X l phn tiu : i vi cc gi c di c nh khong thi gian x l phn

tiu l c nh.Trong trng hp gi c di thay i, thi gian x l l khng c nh v ph thucvo di gi.- Qun l b nh ca hng i : trong trng hp kch thc gi c nh, h thng qun

l b nh c th a ra cc khi nh vi kch thc c nh tng ng vi kch thc cat bo ATM. hot ng ny ht sc n gin nh trong trng hp qun l b nh tdo. Trong trng hp gi c di thay i, h thng qun l b nh phi c khnng a ra cc khi b nh c kch thc khc nhau sao cho cc hot ng nh tm ccon thng tin , tm on u tin,...c tin hnh tc cao. Vic qun l b nht do cng tr ln phc tp hn.

yu cu v kch thc hng i :trong trng hp di gi c nh, yu cu v kch thc hng i cng phi cng ln.Trong trng hp gi c di thay i, tnh ton kch thc hng i phc tp hn nhiu

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v s ph thuc vo di gi. n gin nht l nhng kch thc hng i tng ng vigi c di ln nht, lc kch thc hng i s ln hn rt nhiu so vi trng hpgi c kch thc c nh. Vic tnh ton kch thc hng i mt cch ti u trong trnghp ny s ht sc kh khn.

Kt lun : sau khi i chiu vi yu cu v tc hot ng v kch thc b nh

hng i, gii php gi c kch thc c nh l hp l nht i vi cc dch v bngrng ca ATM.2.4.1.3. tr.

Nh ta bit trong mc 1.2.1 kch thc ca gi ATM cn phi gii hn khngpht sinh ra tr trong mng. Tr ting ni s rt ln nu kch thc gi ln.2.4.1.4. Kt lun.

Trong mng bng rng ni cc ng dng chnh c s dng l ting ni, tn hiuvideo v gi s liu li ch thu v hiu sut truyn i vi cc gi c kch thc thay inh hn rt nhiu so vi li ch thu c khi s dng cc gi c kch thc c nh, nu sosnh v mt tc hot ng v phc tp. Mt khc nu s dng t bo c kch thc

thay i th di ca cc t bo ny khng nhng khng th tu m cn rt b hnch m bo tr nh. Do vo nm 1988, ITU-T chn gii php s dng t boATM c kch thc c nh.

2.4.2. la chn kch thc ca t bo ATM.Sau khi quyt nh s dng gi c di c nh vn t ra l chn t bo c

kch thc bao nhiu. kch thc c t bo s nh hng ti cc ch tiu sau :- Hiu sut bng truyn.- Tr ( tr to gi,tr hng i, tr tho gi, tr chuyn mch, tr truyn dn...)- phc tp khi thc hin.2.4.2.1.Hiu sut bng truyn.

Hiu sut bng truyn c quyt nh bi t l gia kch thc phn tiu v kchthc truyn d liu cng di th hiu qu cng cao ( xem 2.4.1.1.).2.4.2.2. Tr.

Tr ca h thng bao gm mt s loi tr nh tr to gi,tr hng i, tr tho gi, trchuyn mch, tr truyn dn...nh trnh by mc 2.3.1.Tr to gi ph thuc vo kch thc ca trng s liu trong t bo. Hnh 2.9 th hinhiu sut truyn i vi cc t bo c di khc nhau ( so snh gia hai t bo H = 4v H= 5 ) cng nh tr to gi ca chng ( so snh gia hai tc truyn ting ni l 64Kbit/s v 32Kbit/s).

Tr(s) (%) Hiu sut bng truyn4 (H=4)

2 1004 90

4 (H=5) 808 70

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16 60

8 16 32 64 128Hnh 2.9 : HIu sut truyn v tr to gi i vi cc trng d liu c di khc nhau.

Tr hng i b nh hng bi t l gia ln ca trng d liu L v ln phn tiu. Hnh 2.9 th hin s ph thuc ca tr hng i v t l L/H. Ta nhn thy rng trb nht khi L/H c gi tr t 8 - 16 tng ng vi kch thc ca t bo t 32 + 4 byte ti64 + 4 byte.

Tr tho gi c quyt nh bi bin ng tr l nguyn nhn ca tr tng ca mtvi hng i. Tr tho gi cng b nh hng bi di ca t bo.

Tr hng i (s)

280 32+4 64+4 128+4

240200160

12080

40L/H

0 8 10 16 20 30 32 60

Trong : Hiu sut ti0.850.800.750.700.65

Hnh 2.10 : Tr hng i ph thuc vo t l L/H vi cc hiu sut ti khc nhau.Tr ton mng theo khuyn ngh Q161 ca ITU-T cn phi c gii hn sao cho gi

tr ca n nh hn 25ms. Nu tng tr ln hn gi tr ny th cn phi lp thm b khting vng. Theo kt qu nghin cu ca ITU-T di t bo c nh hng trc tip titr :- i vi cc t bo c di tng i ngn ( 32 byte hoc nh hn) th tr tng rt

nh.- i vi cc gi c di trung bnh khong gia 32- 64 byte, phn ln cc trng

hp ta khng cn b kh ting vng.2.4.2.3. phc tp khi thc hin.

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- phc tp ca h thng ph thuc vo hai thng s c bn l tc x l vdung lng b nh cn thit.

P M

2 P 64004 32008

16 1600

32 M 8000

Kch thc t bo16 32 64 128 256 (byte)

Trong : P : thi gian x l mt t bo (s).M: kch thc b nh (bit).

Hnh 2.11 : S ph thuc ca kch thc t bo ti thi gian x l v kchthc b nh. gii hn t l mt t bo, ta cn cung cp mt hng i c kch thc ln. V vy

kch thc t bo cng ln th kch thc hng i cng phi cng ln. Mt khc khi cmt gi ti nt chuyn mch th phn tiu ca n cng cn phi c x l ngay trongkhong thi gian mt t bo, do kch thc t bo cng ln th vic thi gian dnh

cho vic thc hin cng nhiu v tc yu cu cng thp.Th d trn hnh 2.11, i vi t bo 16 byte, ta ch cn 8000 bit hng i nhng thigian x l phi nh hn 1 s . i vi t bo 256 byte ta cn hn 64000 bit cho mthng i nhng thi gian x l cho php hn 15 .

Tuy vy, tc khng phi l vn quan trng nht bi v cng ngh hin nay chophp x l rt nhiu thng tin trong khong thi gian 1s, nh vy vn chnh l gii hn b nh.2.4.2.4. Kt lun.

Cc gi tr di kch thc gia 32 - 64 byte c a chung hn c. S la chn nyph thuc vo ba thng s chnh cp trn. Cui cng ITU-T t c mt thohip s dng t bo c trng s liu 48 byte v 5 byte trng tiu .2.5. tm tt.

Trong chng II, ta xem xt cc tham s quan trng ca mng ATM l cc thams lin quan n li v tr. T cc tham s ny cng nh cc yu cu v dch v c thngi ta xem xt v la chn cc tham s truyn trong mng ATM nh di hngi, tc x l, kch thc t bo ATM. Cui cng ITU-T chn gii php t bo ATMkch thc c nh c di 53 byte, trong trng s liu di 48 byte, phn tiu di5 byte.

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Chng III: k thut mng ATM.3.1. M u.Sau khi xt cc tham s quan trng ca mng ATM chng II, chng III s tip tc cp ti cc k thut ca ATM v cc chc nng ca tng lp. y cn cp ncu trc ca t bo ATM, nguyn tc chuyn mch bo hiu, chc nng vn hnh gimst bo dng OAM trong mng ATM.3.2. c im k thut ca B-ISDN.

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TE LFC

Cc dch

Cc dch v cho N-ISDN c tc 64

Chc nng bo

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Bo hiu ngi s dng ti ngi s dng ( hoc t ngi s dng ti mng )

LFC ( local function capability) cc chc nng c cung cp bi nt chuyn mch cab.TE ( terminal equipment ) thit b u cui.Hnh 3.1. Cc kh nng ca B-ISDN.Cc c im chnh ca h thng B-ISDN c ITU-T a ra trong khuyn ngh I.327,theo cc kh nng v bo hiu, truyn dn ca B-ISDN bao gm :

- Kh nng cung cp cc dch v bng rng.- Kh nng cung cp cc dch v cho N-ISDN vi tc c s l 64 Kbit/s.- Cung cp cc th tc bo hiu t ngi s dng ti mng.- Cung cp cc th tc bo hiu t ngi s dng ti ngi s dng .

Nh phn tch phn t s liu c dng trong ATM l cc t bo ATM. chng c di c nh l 53 byte. Tnh ton vn ca chui t bo c m bo khi truyn quamng ATM. ni cch khc cc t bo thuc v cng mt knh o lun c truyn theomt th t nht nh. ATM s dng k thut hng lin kt. Mt cuc ni lp ATMbao gm mt hoc nhiu lin kt, mi lin kt c gn mt s liu nhn dng khngi trong sut cuc ni. Tuy vy, ATM cng cung cp th tc cho cc dch v truyns liu khng lin kt.Cc thng tin bo hiu ca mt cuc ni s dng mt knh truyn khc vi knh truynthng tin ca cuc ni , tc l n s dng mt s liu nhn dng khc. v vy bohiu trong ATM l bo hiu ngoi bng.3.3. k thut lin kt mng trong B-ISDN.

3.3.1. m hnh sp xp cc lp mng ca B-ISDN.

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B- ISDN

Cc chcnng lp cao

MngtruynATM

Cc chc nngTruyn dn lpATM

Cc chc nngtruyn dn vt l

Mc knh o VC

Mc ng o VP

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Hnh 3.2 : Cu trc phn lp mng ca ATM.

3.3.1.1. Lp vt l.Trong k thut lin kt mng, lp vt l bao gm 3 mc :- Mc ng truyn dn : mc ng truyn dn lin kt cc phn t c chc nng lp

ghp hoc bo thng tin hu ch trong h thng truyn dn, thng tin hu ch cngvi cc thng tin iu khin to ra mt khung truyn dn hon chnh.

- Mc nhm, tch s : cc phn t mng c nhim v nhm hoc tch dng bit lintc c mc ni vi nhau mc ny ( nhm hoc tch cc khung truyn dn thnhmt dng truyn).

- Mc pht : l mt phn t ca mc nhm/ tch s, n c nhim v truyn tn hiugia im k nhau.

3.3.1.2. Lp ATM.Lp ATM bao gm 2 mc :- Mc knh o : l mc c chc nng truyn n hng cc t bo ATM tng ng vi

mt gi tr nhn dng chung duy nht VCI.- Mc ng o : l mc c chc nng truyn n hng cc t bo ATM thuc v

nhiu knh o khc nhau nhng li c chung mt gi tr nhn dng ng o VPI.Trong mt ng truyn dn c th bao gm vi ng o VP, trong mi VP c mt vi

knh o VC. Mi VP v VC trong ng truyn dn u c mt gi tr VPI v VCI ring,s cc VP v VC ph thuc vo di ca VPI v VCI trong tiu ca t bo ATM.Hnh 3.3 th hin mi quan h gia VP,VC v ng truyn dn.

VPVC VCVC VCVC VC

VP

Hnh 3.3. Mi quan h gia ng o, knh o v ng truyn dn.

3.3.2. Mt s khi nim lin quan ti ng o, knh o.3.3.2.1. ng o VP v knh o VC.Nh trnh by 3.3.1.2

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3.3.2.2. Lin kt knh o v lin kt ng o.Theo ITU-T lin kt knh o l s truyn n hng cc t bo ATM gia im m ti

cc gi tr VCI c gn vo t bo v im m cc gi tr b thay i hoc b xo. Lin kt ng o l lin kt gia hai im m ti gi tr VPI c gn thay i

chuyn xo .3.3.2.3. Cuc ni knh o VCC v cuc ni knh o VPC.Cuc ni knh o .Cuc ni knh o VCC l tp hp ca mt s lin kt knh o. Theo nh ngha caITU-T : VCC l s mc ni ca cc lin kt knh o gia hai im truy nhp vo lptng thch ATM .Thc cht VCC l mt ng ni logic gia hai im dng truyn cc t bo ATM.

Thng qua VCC, th t truyn cc t bo ATM s c bo ton. C 4 phng php cs dng thit lp mt quc ni knh o ti giao din gia ngi s dng v mng :+ Vic thit lp v gii phng i vi cc cuc ni c thc hin thng qua cc knh

dnh sn m khng cn cc th tc bo hiu. Phng php ny c p dng cho cc cucni c nh v bn c nh.+ Qua cc th tc bo hiu trao i : phng php ny s dng knh bo hiu trao i o thit lp hoc gii phng cc knh bo hiu a thng thng.

Cuc ni kt knh o

Lin kt knh oKhi knh o

(OAMF5)

Cuc ni ng o

Lin kt ngoKhi ngo

(OAMF4)

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ng truyn dnMc ngtruyn dn

(OAMF3) Nhm /tch sMc nhmtch s(OAMF2)

Mc pht(OAMF1)

u cui ca tng lp

im lin kt ca cac lp

Hnh 3.4 : trnh by mi lin h gia cc lp mng ATM qua cuc ni ng o, cucni knh o...+ Qua th tc bo hiu gia ngi s dng v mng, cc VCC bo hiu c s dng thit lp hoc gii phng cc cuc ni knh o t u cui ti u cui.+ Qua th tc bo hiu t ngi s dng ti ngi s dng : Nu mt VPC tn ti giacc giao din VNI ca hai ngi s dng th mt VCC no trong VPC ny c th cthit lp hoc gii phng thng qua VCC bo hiu gia hai ngi s dng ny.

- Cuc ni ng o :Cuc ni ng o VPC l s mc ni lin kt ca mt s ng o. VPC l s lin kthp logic ca cc VCC. Trong mt VPC, mi lin kt knh o u c mt s liu VCIring, tuy vy nhng VC thuc v cc VP khc nhau c th c cc VCI ging nhau.Mi VC c nhn dng (duy nht) thng qua t hp hai gi tr VPI v VCI. C hai ph-ng php c s dng thit lp cuc ni ng o.+ Thit lp VPC khng cn n th tc bo hiu. Vic thit lp hoc gii phng mtVPC c thc hin qua knh dnh sn. ( trn c s thu bao).+ Thit lp VPC c iu khin bi ngi s dng hoc mng. Cc gi tr VPI c cungcp bi thit b ca ngi s dng hoc cc im cung cp dch v trong mng.

3.3.3. cc ng dng ca cc cuc ni knh o, ng o.Cc VPC v VCC c s dng gia :

- Ngi s dng v ngi s dng.- Ngi s dng v mng.- Mng v mng.

Cc t bo tng ng mt VPC v VCC s c truyn qua mng trn cng mt tuyn.Cc VCC gia ngi s dng v ngi s dng c dng mang thng tin ngi s

dng v thng tin bo hiu gia hai u cui. VCC gia ngi s dng v mng c th

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c dng truy cp n cc chc nng nm nt chuyn mch a phng c linquan ti cuc ni ( nh bo hiu ti giao din UNI). VCC gia cc nt mng dng mang cc thng tin v qun l lu lng mng nh tuyn v bo hiu ti giao din UNI.

VPC lin kt nhng ngi s dng cung cp cho h nhng ng truyn dn, t chccc VC s ph thuc vo ng ny. nhng VPC gia ngi s dng v mng c s dng

kt hp nhng lung thng tin t ngi s dng ti cc phn t ca mng nh chuynmch a phng. VPC gia mng v mng c s dng t chc lung thng tin cangi s dng theo cc s nh tuyn c sn cho vic chuyn mch cc tuyn haythng tin qun l mng.3.4. Cu trc t bo ATM.

Nh bit c im ca ATM l hng lin kt. Do khc vi mng chuyn mnggi, a ch ngun v ch, s th t gi ( sp xp li th t cc gi) l khng cnthit trong ATM. Hn na do cht lng ca ng truyn rt tt nn cc c ch chng litrn c s t lin kt ti lin kt cng c b qua. ( xem 1.2.2.2.). Ngoi ra ATM cngkhng cung cp cc c ch iu khin lung gia cc nt mng do c cu iu khin

cuc gi ca n. V vy chc nng c bn cn li ca phn tiu truyn t bo ATMl nhn dng cuc ni o.Phn t bo ca tiu c hai dng : mt dng l cc t bo c truyn trn giao dingia ngi s dng v mng UNI, dng cn li l dng cc t bo c truyn gia ccnt chuyn mch ( gia NNI). Hnh 3.5 v 3.6 th hin cu trc ATM giao din NNIv UNI.

3.4.1. S liu nhn dng knh o VCI v ng o VPI.Do knh truyn ATM c th truyn vi tc t vi Kbit/s ti vi trm Mbit/s ti

mt thi im no , do VCI c dng nhn dng cc knh c truyn ngthi trn ng truyn dn. Thng thng trn mt ng truyn c hng ngn knh nh vy

v th VCI c di 16 bit ( tng ng vi 65535 knh)

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

1 12 2

Byte3 34 45 5

Do mng ATM c c im hng lin kt nn mi cuc ni c gn mt s liunhn dng VCI ti thi im thit lp. Mi gi tr VCI ch c gi tr ti tng lin kt t

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VCI VCI

VCI

VCI PT CLP

HEC

Phn d liu(48 b te)

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC

Phn d liu (48b te)

Hnh 3.5: Cu trc t boT i iao di n NNI

Hnh 3.6: Cu trc t boT i iao di n UNI


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nt ti nt ca mng. Khi cuc ni kt thc VCI cn c u im trong vic s dngtrong cc cuc ni a dch v. Th d trong dch v in thoi, truyn hnh, m thanh vhnh nh s c truyn trn hai knh c VCI ring bit, do ta c th b xung hochu b mt dch v trong khi ang thc hin mt dch v khc.VPI c s dng thit lp cuc ni ng o cho mt s cc knh o VCC. VPI cho

php n gin ho cc th tc chn tuyn cng nh qun l n c di 8 bit hoc 12bit tu thuc t bo ATM ang c truyn qua giao din UNI hay NNI.T hp ca VCI v VPI to thnh mt gi tr duy nht cho mi cuc ni. Tu thuc

vo v tr i vi hai im cui ca cuc ni m nt chuyn mch ATM s nh ngda trn ga tr ca VPI v VCI hay ch da trn g tr VPI. Tuy vy cn lu rng VCIv VPI ch c ngha trn tng chng lin kt ca cuc ni. Chng c s dng chn ng trn cc chng ny c d dng hn. Do s VPI v VCI qu nh nn chngkhng th c s dng nh mt s liu nhn dng ton cc v kh nng xy ra hai cucni c s dng ngu nhin cng mt s VPI v VCI l s rt cao. khc phc ngi tacho VCI v VPI l duy nht trn mi on lin kt. Trn tng on lin kt ny, hai

nt chuyn mch s dng VPI v VCI nh s hiu nhn dng cuc ni trn mi on .Khi qua nt chuyn mch, VPI v VCI nhn cc gi tr mi ph hp vi on tiptheo.

3.4.2. Kiu t bo.PT l mt trng gm ba bit c nhim v phn bit cc kiu t bo khc nhau nh : t

bo mang thng tin ca ngi s dng, t bo mang cc thng tin v ngi gim st, vnhnh, bo dng. Nu bit u PT c gi tr th y l t bo ca ngi s dng. Trong loit bo ny bt s 2 bo hiu tc nghn trong mng v bit 3 c chc nng bo hiu cholp tng thch ATM AAL ( s c trnh by 3.6.3). Nu bit u ca PT c gi tr bng1 th y l t bo mang thng tin qun l mng. Hnh 3.7 v bng 3.1 trnh by cu

trc trng PT.

T bo ca ngi bit bo hin lp AALS dng

Bit bo hiu tc nghn1 nu tc nghn

Hnh 3.7 : cc cu trc trng PT trong t bo mang thng tin ca ngi s dng.

Bng 3.1 : Cu trc trng PT trong t bo mang thng tin OAM.Dng bit Chc nng100 T bo OAM lp F5 lin quan ti lin kt101 T bo OAM lp F5 lin quan ti u cui110 T bo qun l ti nguyn111 Dnh sn cho cc chc nng sau ny

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Ngoi ra cn c hai kiu t bo c bit l t bo khng xc nh v t bo trng. Tbo khng xc nh v t bo trng u c c im chung l chng khng mang thngtin ngi s dng. Tuy vy t bo trng ch tn ti mc vt l cn t bo khng xcnh tn ti c c mc ATM ln mc vt l ( xem 3.6.1). T bo khng xc nh cgi i khng c thng tin hu ch dnh sn trn u pht.

3.4.3. CLP.CLP l trng dng phan loi cc trng ni khc nhau theo mc u tin khi ti

nguyn trong mm khng cn l ti u na. Th d trong trng hp qu ti, ch c cccuc ni vi mc u tin thp o b mt thng tin. C hai loi u tin khc nhau l : u tinv mt ni dung v u tin v mt thi gian. Trong ch u tin v mt ni dung, cc tbo c u tin cao hn s c xc sut mt gi t hn, cn trong ch u tin v mtthi gian, vi t bo c th c tr trong mng di hn cc t bo khc.

Cc mc u tin c th c n nh trn c s cuc ni ( qua mi VCI hoc VPI)hoctrn cc s mi t bo. trong trng hp th nht cc t bo thuc v mt knh o hoc

ng o s c mt mc u tin xc nh. Trong trng hp th hai, mi t bo thuc vmt knh o hoc ng o s c cc mc u tin khc nhau.CLP c di mt bit. Gi tr bit ny c th c xc lp bi khch hng hoc nh cungcp dch v dng cho mc ch iu khin tc nghn. Cc t bo trong CLP =0 cmc u tin cao v CLP = 1 c mc u tin thp hn. cc t bo cCLP =1 s b loi b khi xy ra tc nghn trong mng.

3.4.4. Trng hp iu khin li tiu HEC.Tm bit cui cng trong phn tiu ca t bo l dng cho trng iu khin l tiu

HEC cha m d vng CRC. M ny tnh ton cho 5 bit tiu . a thc sinh yc dng l a thc : x8 + x2 + x + 1 . a thc ny c th sa ton b cc li n

v pht hin ra phn ln cc li nhm. Sa li HEC l cn thit bi v t bo trong khitruyn trn mng nu gi tr VPI/VCI ca n sai dn n c truyn i sai a ch.Ngoi ra nh trong SONET, HEC cn c dng ph t hin ra bin gii gia tng tbo. Thc ra cng c hai cch x l vi HEC. Cch th nht : nu pht hin sai, t bos b loi b. Cch th hai : l sa li bit b li. Thc ra th vic dng cch th nht hoccch th hai ph thuc vo ng truyn. Nu ng truyn l cp quang th cch thnht c th p dng th s khng qu nhiu hoc ch mt bit li. Nhng i vi ngtruyn cp xon thng thng th dng cch th nht c th khng mang li kt qu ttv nhiu c th tc ng n mt nhm cc bit. Do phn tiu b thay i theo tngchng ( thay i VPI/VCI) nn CRC cn kim tra v tnh ton li vi m chng. a

thc sinh c th sa ton b cc li n v pht hin ra phn ln cc li nhm.3.4.5. GFC.

giao din gia ngi s dng v mng UNI, phn tiu c vi giao din khc bitso vi giao din NNI. S kh c nhau cn bn nht l trng VPI b rt ngn li cn 12 bit( so vi 16 bit giao din NNI) thay vo ch 4 bit ca VPI l trng iu khin lungchung GFC. ( hnh 3.6).

Chc nng ca GFC c nu ra truyn khuyn ngh I-150 ca ITU-T. C ch caGFC cho php iu khin lung cc cuc ni ATM giao din LNI. N c s dng

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lm gim tnh trng qu ti trong thi gian ngn c th xy ra trong mng ca ngi sdng. C ch GFC cn dng cho c cc cuc ni t im ti im v t im ti nhiuim.

Khi kt hp mng ATM vi cc mng khc nh DQDB (Distributed queue Dual Bus),SMDS( Switched Multi- megabit Data Service); GFC a ra 4 bit nhm bo hiu cho cc

mng ny, lm th no hp knh cc t bo ca cc cuc ni khc nhau. Mi mngu c mt th tc truy nhp ring, do hu nh mi mng u phi c mt logic iukhin tng ng hoc GFC thnh dng cc th tc truy nhp ca ring n. Do trong trng hp ny, GFC thc cht l mt b cc gi tr chun nh ngha cc mc u tin ca ATM i vi cc quy lut truy nhp vo cc mng khc nhau.

Vic buc phi s dng trng iu khin lung chung GFC l mt nhc im cbn ca ATM, l toa s khc nhau gia cc t bo ti giao din UNI v NNI, v vycc th tc ca ATM khng phi l th tc ng nht. Trong mng s dng cc th tcng nht, cc thit b vin thng c th c lp t vo bt k mt ni no trong mng.Trong khi trong ATM, ta phi ch xem thit b c lp t c thch hp vi giao

din cho hay khng.3.5. Nguyn l chuyn mch v bo hiu trong ATM.3.5.1. Nguyn l chuyn mch .Hnh 3.8 : Cuc ni knh o thng qua nt chuyn mch v b ni xuyn.

Trong mi trng ATM, vic chuyn mch cc t bo c thc hin trn c s cc gitr VPI v VCI. Nh ni, cc gi tr VPI, VCI ni chung ch c hiu lc trn mt -ng lin kt. Khi t bo ti nt chuyn mch, ga tr ca VPI hoc c VPI v VCI cthay i cho ph hp vi lin kt tip theo. Vic chuyn mch ch thc hin trn gi trVPI c gi chuyn mch VP, nt ni xuyn hoc b tp trung. Nu thit b chuynmch thay i c hai gi tr VPI v VCI ( cc gi tr VPI v VCI thay i u ra) th

n c gi l chuyn mch VC hoc chuyn mch ATM.

VPY =y1VCI=a1 VCI=a2

VPX=x2VPI=x1 VPI=y2

Cuc ni knh o VCC

Trong :

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- D1,D2 : B ni xuyn- T : Chuyn mch ATM- A,B : Thit b u cui

Hnh 3.8 Minh ho cuc ni knh o VCC thng thng. T l nt chuyn mch ni mgi tr VPI v VCI b thay i. A v B l hai thit b u cui; D1 v D2 l cc b ni

xuyn, ni ch thay i gi tr VPI ; ai, xn ln lt l cc gi tr VCI, VPI tng ng.Hnh 3.9 : S gii thch nguyn l chuyn mch VP.

Hnh 3.10 gii thch nguyn tc chuyn mch VC.

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3.5.2. Nguyn l bo hiu3.5.2.1. Tng quan :

B-ISDN s dng nguyn tc bo hiu ngoi bng ging nh ca N-ISND.

Trong B-ISDN, knh o VC l phng tin logic tch cc knh bo hiu rakhi knh ngi s dng . Ngi s dng c th c cc thc th bo hiu khcnhau c ni vi h thng iu khin qun l cuc gi ca mng thng quacc VC ring r.

3.5.2.2. Cc yu cu bo hiu trong B-ISDNBo hiu ca B-ISDN phi cung cp v h tr cho cc dch v c tc 64kbit/s

ca N-ISDN cng nh cc dch v bng rng, nh vy c nggha l bo hiu ca B-ISDNphi bao gm cc chc nng bo hiu c ca N-ISDN theo khuyn ngh Q-931 caITU-T, ngoi ra cn phi c cc chc nng bo hiu mi.

Cc chc nng bo hiu mi ca ATM c nhim v:

Thit lp (hoc thit lp li ) cc tham s v lu lng ca cuc nii vi cc cuc gi a truyn thng (multi-media) mt vi ng ni cn phi thit lpgia hai im cng mt lc truyn ting ni , hnh nh v s liu . Lc ny bo hiuATM cn phi c kh nng gii phng mt hoc vi ng ni trong cuc gi ng thib sung cc ng ni mi. Cc chc nng c thc hin chuyn mch i phng cacc u cui tham gia vo cuc gi ch khng phi cc nt chuyn mch trunggian . Trong dch v thng tin hi ngh, mt vi cuc gi nhiu ng (multi-connection)cn phi thit lp gia vi im . Lc ny bo hiu c nhim v thit lp v gii phngmt cuc gi hi ngh cng nh b sung hoc loi b mt thnh vin no .

Ngoi ra trong ATM, cc cuc ni khng i xng rt quan trng ( th d nh

truyn chng trnh TV) do bo hiu trong ATM cng cn h tr cho cc cuc niny.

Ch : Cuc ni khng i xng l cuc ni ch truyn s liu theo mt chiuhoctruyn tc thp mt chiu v tc cao trong chiu ngc li.3.5.2.3.Bo hiu trn knh o SVC (signaling virtual channel )

Cc thng ip bo hiu trong ATM c truyn trn knh o bo hiu SVC, knhSVC l cc knh o c nhim v truyn cc thng tin v bo hiu. Cc loi knh SVCkhc nhau c th hin trn bng 3.2

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Hnh 3.2: Cc knh bo hiu trn giao din gia ngi s dng v mng UNI

Kiu SVC Hng S SVC/UNIKnh bo hiu trao i 2 hng 1 knhSVC truyn thng chung n h-

ng1 knh

SVC truyn thng c la chn n h-ng c th mt hoc mt vi knh

SVC t im ti im 2 hng mt knh trn mi im cui bohiu

Knh bo hiu trao i (Meta - signalling SVC-MSVC) trn mi giao din UNI cmt knh MSVC. MSVC c nh v c truyn theo hai hng. N l mt kiu knhqun l giao din c s dng thit lp, kim tra v gii phng cc knh SVC tim ti nhiu im v cc SVC truyn thng c la chn.

Knh bo hiu t im ti im (point to point SVC). Knh bo hiu im ti

im c dng thit lp iu khin v gii phng cc cuc ni knh o VCC ca ng-i s dng. N l knh bo hiu hai hng.Knh bo hiu truyn thng (broadcast SVC-BSVC) BSVC l knh bo hiu n

hng (t mng ti ngi s dng), n c dng gi cc thng ip bo hiu cho tt ccc im cui nh trong trng hp SVC truyn thng chung (General Broadcast SVC)hoc cho mt s im cui c nh trc trong trng hp truyn thng c la chn(Selective Broadcast SVC). truyn thng c la chn, tn hiu bo hiu c gi titt c cc u cui c cng mt loi dch v.

Trong cu hnh bo hiu cho cc cuc ni t im ti im, mng s dng mt knhbo hiu o c thit lp t trc (bi MSVC). Trong cu hnh truy cp bo hiu t im

ti nhiu im, mt knh bo hiu trao i c x l qun l cc SVC. Knh bohiu trao i khng c s dng trong trng hp bo hiu gia cc nt mng. Trong tr-ng hp mt v d lin kt gia hai nt mng c cha mt s knh o bo hiu SVC thcc gi tr VCI b sung cho bo hiu trn VP ny s c thit lp t trc (pre established)3.6.Cu trc phn lp ca mng ATM

3.6.1 M hnh tham chiu th tc B-ISDN:Cu trc phn lp logic c s dng trong ATM a trn m hnh tham chiu lin

kt cc h thng m OSI. M hnh ATM s dng khi nim cc lp v cc mt phngring r cho tng chc nng ring bit nh chc nng dnh cho ngi s dng, chc nng

iu khin qun l mng. Khi nim ny c gi l m hnh tham chiu th tc B-ISDN(B-ISDN protocol refrence model hay B-ISDN- PRM).

B-ISDN- PRM c cu trc phn lp t trn xung, bao gm cc chc nng truyndn, chuyn mch, cc th tc bo hiu v iu khin, cc ng dng v dch v. Mhnh tham chiu th tc B-ISDN bao gm 3 mt phng c trnh by nh trn hnh 3.11,cc mt phng l: mt phng qun l, mt phng ca ngi s dng v mt phngiu khin (hay bo hiu)

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Mt phng qun l: Bao gm 2 chc nng chnh l chc nng qun l lp (layermanagement) v chc nng qun l mt phng (plane management) tt c cc chcnng lin quan ti ton b h thng (t u cui ti u cui) u nm mt phngqun l. Nhim v ca n l to s phi hp lm vic gia nhng mt phng khc nhau.Trong khi chc nng qun l mt phng khng c cu trc phn lp th chc nng qun

l lp li c chia thnh cc lp khc nhau nhm thc hin cc chc nng qun l clin quan ti cc ti nguyn v thng s nm cc thc th c th tc (nh bo hiuchng hn). i vi mi lp, qun l lp x l dng thng tin OAM tng ng, s ctrnh by trong 3.8.2

Mt phng ngi s dng: Nhim v ca mt phng ny l truyn thng tin cangi s dng t im A ti im B trn mng. Tt c cc c ch c lin quan nh iukhin lung, iu khin tc nghn, chng li u thc hin mt phng ny, n cngc cu trc phn lp, mi lp thc hin mt chc nng ring bit lin quan n viccung cp dch v cho ngi s dng.

Mt phng iu khin (hoc bo hiu): Mt phng iu khin cng c cu trc lp.

Mt phng ny c nhim v thc hin cc chc nng iu khin ng ni (connectioncontrol) v cuc gi (call control). Chng thc hin cc chc nng bo hiu c linquan ti vic thit lp, gim st v gii phng ng ni hoc cuc gi.

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3.6.2: Cu trc chc nng ca ATMCu trc tham chiu chc nng ca ATM ch ra cc chc nng c thc hin bi

mng ca mi lp c th trn B-ISDN- PRM. Bng 3.3 trnh by cc chc nng :

Qun

l

m

ng

Cc lp cao hn

AAL

CS

Nhn /gi cc PDU ti cc lp cao hn v to dng CS PDUKim tra s khi phc chnh xc cc CS-PDUPht hin s mt cc t bo ca CS-PDUCung cp mt vi chc nng AAL trong phn tiu CS PDUChn mt vi t bo b sung vo CS PDUiu khin lung, gi cc thng ip tr li hoc yucu truyn li cc t bo li

SAR

To cc t bo t CS-PDU, khi phc cc CS-PDU t

t boTo ra trng kiu on nh BOM, COM, EOM, SSMKim tra m d vng CRC ca trng d liu ca t boTo ra 2 byte tiu v 2 byte cui ca SAR-PDU

ATMiu khin lung chnhTo ra hoc tch phn tiu ca t boc v thay i phn tiu t boThc hin phn knh/hp knh cc t bo

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Lp

Lpvtl

Lpconhi ttruyn

Thm vo hoc ly ra cc t bo trngTo v kim tra m HECNhn bit gii hn t boBin i dng t bo thnh cc khung ph hp vi hthng truyn dn

Pht/khi phc cc khung truyn dnLpcon -ngtruynvt l

ng b bit

Thu pht s liu

Bng 3.3: Chc nng cc lp ca B-ISDN y:

AAL l lp tng thch ATM

PDU: n v s liu th tcCS: Lp con hi t truyn dnSAR: Lp con to v tho t bo

3.6.2. Cc lp thp trong B-ISDN3.6.2.1. Chc nng lp vt l:

Lp vt l c chia thnh hai lp con ng truyn vt l v lp con hi t truyn3.6.2.1.1. Lp con ng truyn vt l l lp thp nht, cc chc nng ca n hon tonph thuc vo mi trng vt l c th. Lp ny cung cp cc kh nng truyn dn bit,no cng lm nhim v m ho ng truyn v nu cn thit thc hin bin i quangin. Lp con PM cn c nhim v ng b bit. Trong ch hot ng bnh thng,

cc bit ng b trn ng truyn thng da vo cc bit ng b thu c qua giao din,tuy vy h thng cng c th s dng h thng ng b ring ca mnh. Mng B-ISDNtrong tng lai s ch yu s dng cc ng truyn dn l cp quang, k c mng trungk v mng truy nhp ATM.

Cc t bo ca ngi s dng, cc t bo bo hiu v OAM (tr t bo OAM lpvt l) c tc l 149,760 Mb/s trong trng hp giao din 155,520 Mb/s. i vi giaodin 622,080 Mbit/s, tc thc t ca dng thng tin hu ch l 599,040 Mb/s (bng 4ln 149,760 Mb/s )3.6.2.1.2 Lp con hi t truyn (transmission convergence TC)

Lp con hi t truyn l lp con th hai thuc v lp vt l. N c 5 chc nngsau:- Khi thm vo hoc ly ra cc t bo trng (call rate decoupling). Khi khng c cc

t bo cha thng tin hu ch, t bo khng xc nh hoc t bo OAM mc vt lth cc t bo trng s c truyn trn cc ng truyn cho tc dng cc t boph hp vi tc truyn dn cho trcca ng truyn. Mi octec ca t bo trngtrong phn trng thng tin s l 01101010, cu trc ca phn tiu ca t botrng c th hin trong bng 3.4

octet1 octet 2 octet3 octet 4 octet 5

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0000000 00000000 00000000 00000001 M HECBng 3.4: Cu trc phn tiu ca t bo trng

- To v kim tra m HEC: u pht, m HEC c xc nh bi 4 octet u trongphn tiu ca t bo ATM, kt qu tnh ton c a vo octet th 5 . Gi tr HECl phn d cho php chia mun 2 ca tch 4 octet u tin nhn X 8 vi a thc sinh

X8

+ X2

+X +1. a thc sinh trn c kh nng sa cc li bit n v pht hin lichm tiu ca t bo. 2 kh nng ny c s dng u thu theo gin trngthi c m t hnh 3.12. Bnh thng u thu c t ch sa sai. Khi phthin ra mt li n trong tiu ca t bo ATM th li ny s c sa. Nu xuthin li nhm th c t bo s b hu. Trong c hai trng hp, sau khi tm ra c lin hoc li chm, h thng t ng chuyn qua ch pht hin li. trng thiny, khi c li n hoc li chm xut hin th t bo s b hu. H thng duy tr ch pht hin li cho ti khi khng tip tc pht hin ra t bo li na, lc ns t ng quay tr li ch sa sai

pht hin ra li (gi b hu) Pht hin ra li nhm (gi li b ph hu)

Khng pht hin thyli pht hin ra li n(sa sai) khng c li

- Nhn bit gii hn t bo (cell delimition): chc nng ny cho php bn thu nhn

bit gii hn ca mt t bo. S nhn bit ny da trn c s s tng quan ca ccbit tiu v m HEC tng ng. Trong c ch nhn bit gii hn t bo, u tintrng thi HUNT thc hin vic kim tra tng bit ca tiu t bo va nhn c,nu lut m ho HEC c tun th, c ngha l t bo khng b li th h thng hiurng phn tiu c nhn dng ng v chuyn sang trng thi PRESYNCH. Presynch h thng thc hin kim tra lin tc m HEC ca cc t bo lin tip.Nu ln lin tip m HEC c nhn bit l ng th h thng chuyn sang trng

thi SYNCH, nu khng n tr li trng thi HUNT. T trng thi SYNCH, h thngli quay v HUNT nu tm ra ln lin tip m HEC sai (xem hnh 3 vn t ral lm sao nhn bit c im bt u ca t bo)

Hnh 3.13: C ch nhn bit gii hn khung

HEC ngKim tra tng bit Kim tra tng t bo

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Ch phthi n li

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HEC sai

ln m HEC ln m HEC

sai ng

- Bin i dng t bo thnh cc khung truyn dn (transmission frame adaption). Tiu pht , chc nng ny c nhim v lm cho dng t bo ti t cc lp trn thchng vi cc cu trc khung s liu c s dng trong h thng truyn dn. Ti uthu, dng t bo c khi phc li t cc khung truyn dn. Cc h thng truyn dnthng c dng l h thng phn cp s ng b SDH v h thng truyn dn datrn c s cc t bo.

- Chc nng di cng trong lp con TC l pht v khi phc cc khung truyn dn.N c nhim v to ra cc khung truyn dn v ghp cc t bo ATM vo nhngkhung ny. Kch thc khung truyn dn ph thuc vo tc truyn. Ti u thucc khung truyn dn c nhn bit v khi phc li. T cc khung ny ta c thkhi phc li dng t bo ATM. Cu trc cc khung truyn c th khc nhau tuthuc vo tng h thng truyn dn c th. Trong ch truyn SDH cc t boATM c ng vo cc khung truyn gi l container, phn u ca cccontainer ny cha cc thng tin iu khin. Hnh 3.14a minh ho khung SDH giao din 155,520 Mbit/s.

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SOH

AU4-PTR

SOH

B3

C2

G

F2

H4

Z3

Z5

J1

Z4

3 b te

1 b te

5 byte

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Trong phng php truyn khung trn c s t bo, cc t bo ATM c truyn lintc m khng tun theo cc khung thi gian nh kiu truyn ng b. Trn ng truyn,c nhiu nht sau 26 t bo hu ch li c mt t bo lp vt l c chn vo, mcch l lm dng t bo truyn ph hp vi tc bit cho ca giao din. T l26 : 27 tng ng vi t l 149,760 Mbit/s :155,520 Mbit/s v 599,040 Mbit/s : 622,080

Mbit/s do n p ng c yu cu v tnh tng thch vi ng truyn (xem hnh3.14b)

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Lp AAL c nhim v to ra s tng thch gia cc dch v c cung cp bi lpATM vi cc lp cao hn. Thng qua lp AAL, cc n v s liu th tc PDU cclp cao hn c chia nhv a vo trng d liu ca t bo ATM. AAL c chia nhthnh hai lp con l lp con thit lp v tho t bo SAR (Segmentation andReassembly) v lp con hi t (Convergence sublayer). Chc nng chnh ca SAR l

chia cc PDU ca lp cao hn thnh cc phn tng ng vi 48 byte ca trng d liutrong t bo ATM ti u pht. Ti u thu SAR ly thng tin trong trng d liu ca tbo ATM khi phc li cc PDU hon chnh.

Lp con SC ph thuc loi dch v. N cung cp cc dch v ca lp AAL cho cclp cao hn thng qua im truy nhp dch v SAP (Service Access Point). gimthiu cc th tc ca lp AAL, ITU-T chia AAL thnh 4 nhm khc nhau tu thuc voc im ca chng. S phn nhm cc lp AAL ny ch yu da trn 3 tham s l:mi quan h v mt thi gian, tc bit v dng truyn (hay kiu lin kt ). Bng 3.4trnh by s phn loi ny

Bng 3.4: Bng phn loi cc nhm AAL

Nhm A Nhm B Nhm C Nhm DMi quan h thi giangia ngun v ch

C yu cu Khng yu cu

Tc truyn Khng i Thay iKiu lin kt Hng lin kt Khng lin

kt

- Nhm A (m phng chuyn mch knh): phc v cc dch v yu cu thi gianthc, tc truyn khng i kiu truyn hng lin kt. Cc dch v thuc v loiny thng l ting ni v tn hiu video c tc khng i.

- Nhm B: L cc dch v thi gian thc, tc truyn thay i, kiu truyn hng linkt. Cc dch v ca n thng l tn hiu audio v video c tc d thay i.

- Nhm C: l cc dch v khng yu cu thi gian thc tc truyn thay i, phngphp truyn hng lin kt v bo hiu.

- Nhm D: Bao gm cc dch v khng yu cu thi gian thc, tc thay i, kiutruyn khng lin kt. c s dng cho cc dch v truyn s liu khng lin kt.Da vo phn loi trn ITU - T a ra mt vi kiu AAL. Sau y ta s ln lt xem

xt tng loi.3.6.3.2. Cc loi AAL3.6.3.2.1: AAL kiu 1

AAL kiu 1 phc v cho cc dch v thuc nhm A bi v n thu hoc pht cc nv s liu dch v SDU (Service data unit) ca lp trn thi gian thc vi tc truynkhng i. Cc chc nng c bn ca AAL 1 bao gm phn tch v to li

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(Segmentation and reassembly ) thng tin ca ngi s dng, x l tr truyn v to tbo, x l li khi mt hoc chn nhm t bo, khi phc ng b u thu, pht hinli trng thng tin iu khin t bo v khi phc li cu trc thng tin ti bn nhn.- Lp con SAR:

4 bit 4 bit 47 byteSN SNP Trng d liu ca SAR PDU

C SC CRC P

Tiu SAR PDU

C: bit CSIHnh 3.15: Dng SAR PDU ca AAL kiu 1

n v s liu th tc SAR - PDU gm c 48 byte. Octet u tin l trng thngtin iu khin th tc PCI. PCI bao gm 4 bit ch th t SN (sequence number) v 4 bitm chng li SNP (sequence number protection ) cho SN. Trng SN li c chia nhthnh bit bo hiu lp con hi t CSI (Convergence Sublayer Indication) v 3 bit mth t SC (Sequence counted). Hnh 3.15 th hin dng SAR- PDU ca AAL 1.

Gi tr SC to ra kh nng pht hin cc t bo b mt hoc truyn nhm. Bit CSIc th c s dng truyn thng tin ng b hoc cc thng tin v cu trc d liu.Trng SNP cha m CRC pht hin v sa li cho SN, bit cui cng l bit P (parity)kim tra chn l c 7 bit u ca PCI.- Lp con CS:

Cc chc nng lp con CS hon ton ph thuc vo dch v, bao gm mtt s chcnng c bn nh x l gi tr tr t bo, x l cc t bo b mt hoc chn nhm sa litheo c ch FEC, khi phc ng b theo phng php nh du thi gian d ng bSRTS (Synchronous residual time stamp ). Trong phng php STRS, mi thi gian d(residual time stamp), c s dng o c v mang thng tin v khc nhau giang h ng b chung ly trong mng v ng h ca thit b cung cp dch v.3.6.3.2.2. AAL kiu 2

AAL kiu 2 s dng cho cc dch v c tc thay i c truyn theo thi gianthc (thuc v nhm B). Cc chc nng ca AAL 2 vn cha c nh ngha r rng.Tuy vy c th cho rng AAL 2 c pht trin t AAL 1, n c cc chc nng sau: Trao

i s liu c tc d thay i gia cc lp cao hn vi lp ATM, x l tr t bo, phntch v khi phc li thng tin cho ngi s dng, x l cc loi li t bo cng nh tchtn hiu ng b u thu.3.6.3.2.3. AAL kiu 3/4

AAL kiu 3/4 c pht trin t AAL 3 (phuc v cho cc dch v loi C) v AAL 4(phc v cho cc dch v loi D). Ngy nay 2 kiu AAL trn hp li thnh AAL 3/4tho mn cc dch v thuc loi C v D. Hnh 3.16 l cu trc chnh ca AAL 3/4 trong

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lp con CS c chia thnh 2 phn l phn chung CPCS (common part CS ) v phnph thuc dch v SSCS (service specific CS).

SSCS CSCPCSSAR SAR

-SAP: im truy cp dch v-CS: lp con hi t-AAL 3/4 cung cp 2 kiu dch v c bn l dch v kiu thng ip (message modeservice) truyn cc s liu c ng thnh khung (nh cc khung HDLC) v dch vkiu dng bit truyn s liu tc thp vi yu cu tr nh

AAL - SDU

CS - PDU

48 octet SAR-PDU

s liu Phn ui ca CS - PDU Khi m

Phn tiu ca SAR PDU Phn tiu ca CSPDU

Phn ui ca SAR PDU Khng s dng

Hnh 3.17: Dch v kiu thng ip

48 octet

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Hnh 3.18: Dch v kiu dng bitTrong dch v kiu thng ip, mt n v s liu dch v AAL - SDU c truyn

trong mt hoc vi SAR - PDU li c to ra t cc CS - PDU ny. Hnh 3.17 minh hophng php ny.

Trong dch v kiu dng bit, mt vi AAL - SDU c kch thc c nh c truyn

trong mt CS - PDU (hnh 3.15)Hai th tc hot ng ng cp (peer to peer )c s dng cho c hai kiu dch vtrn, l hot ng c bo m (assured operation) v hot ng khng bo m(none - assured operation). Trong hot ng c bo m, cc AAL - SDU b mt hocc li s c truyn li. hot ng khng bo m khng c vic truyn li gi.- Lp con SAR:

Ni chung cc CS - PDU c di thay i, v vy SAR PDU bao gm 44 octetl s liu ca CS - PDU. 4 octet cn li c dnh cho cc trng thng tin iu khin. Tr-ng kiu on ST (Segment type) c di 2 bit, n ch ra loi ca CS - PDU c chatrong SAR - PDU nh : phn u ca CS - PDU BOM (beginning of message), phn cui

EOM (End of message) v cc CS - PDU n SSM (single - segment message). Ch th di trng thng tin LI (length indicator) ch ra s octet ca CS - PDU c cha trongtrng d liuca SAR - PDU. LI c di 6 bit. Ngoi ra trong SAR- PDU cn c trngs th t gi SN (sequence number) di 4 bit. Mi khi nhn c SAR - PDU thuc vmt cuc ni, gi tr ca SN li tng ln mt n v.

Pht hin li l chc nng th hai ca lp con SAR. Trng chng li CRC di 10bit thc hin vic kim tra li bit trong SAR - PDU. Gi tr CRC c tnh cho trng tiu, trng d liu, LI. Lp con SAR cn c kh nng pht hin cc gi SAR - PDU bmt hoc chn nhm.

Chc nng th 3 ca SAR l ng thi phn knh hoc hp knh cc CS -PDU ca

cc cuc ni n mc ATM. Chc nng ny s dng trng s liu nhn dng hpknh MID (Multiplexing Indetifier )di 10 bit. Cc SAR - PDU vi s liu nhn dngMID khc nhau s thuc v cc CS - PDU ring bit. Vic phn/hp knh phi c thchin trn c s t u cui ti u cui. Nhng ng ni lp ATM bao gm cc cucni lp AAL khc nhau s c x l nh mt thc th n. Hnh 3.19 trnh by cu trckhung SAR - PDU ca AAL 3/4.

ST SN MID Trng thng tin ca SAR-PDU LI CRC2bit 4bit 10bit 44 octet 6bit 10bitPhn tiu SAR - PDU Phn ui SAR-PDU

SAR - PDU

Hnh 3.19. Dng SAR - PDU ca AAL 3/4

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- Lp con CS:CPI Btag Basize Trng thng tin ca CPCS-PDU PAD AL Etag Length8bit 8bit 16bitPhn tiu SAR - PDU Phn ui SAR-PDU

SAR PDU

Hnh 3.20. Cu trc ca CPCS PDU ca AAL 3/4Nh bit, lp con CS c chia thnh 2 phn l phn chung CPCS v phn ph

thuc dch v SSCS. Chc nng nng v cu trc ca SSCS hin ti vn cha r rng, ni hi cn phi nghin cu thm. Phn CPCS truyn cc khung s liu ca ngi sdng vi di bt k trong khong t 1 octet n 65535 octet. Cc chc nng caCPCS nm trong 4 octet phn tiu (header) v 4 octet phn ui (tailer). Trng ch

th phn chung CPI (Common part indicator) c s dng qun l phn cn li catiu v phn ui. Trng nhn hiu u Btag (beginning tag) v nhn hiu kt thcEtag (Ending tag) cho php to nn s lin kt mt cch chnh xc gia phn tiu vphn ui ca khung. Trng kch thc b m cung cp BA size (Bufer Allocation Size)bo hiu cho du thu b m kch thc ti a cn thit nhn CPCS - SDU. Trngm PAD (padding) m bo sao cho trng d liu ca CPCS - SDU l mt s nguynln ca 4 octet do n c di t 0 octet n 3 octet. Trng sp xp AL(Aligment)c s dng sp xp phn ui 32 bit ca CPCS - PDU. Trng di (length) ch ra di trng d liu. Hnh 3.20 minh ho cu trc CPCS - PDU ca AAL kiu 3/43.6.3.2.4. AAL kiu 5:

AAL kiu 5 phc v cho cc dch v c tc thay i, khng theo thi gian thc.Cng ging nh AAL 3/4, AAL 5 c s dng ch yu cho cc nhu cu truyn s liu.Tuy vy, ITU - T a ra kiu 5 nhm mc ch gim di trng thng tin iu khinth tc (PCI - bao gm phn tiu v phn ui). AAL 5 c cc chc nng v th tcnh AAL3/4. im khc nhau chnh ca hai loi ny l AAL 5 khng a ra kh nngphn hp knh, do n khng c trng MID. AAL 5 ch yu s dng cho bo hiutrong mng ATM.- Lp con SAR:

Lp con SAR chp nhn cc SDU c di l mt s nguyn ln ca 48 octet cgi xung t CPCS. N s khng b sung thm cc trng thng tin iu khin (nh phn

tiu v phn ui) vo cc SDU va nhn c. SAR ch thc hin cc chc nngphn tch (sementation) u pht v to li gi u thu. nhn bit c im btu v kt thc ca SAR - PDU. AAL 5 s dng trng AUU. AUU nm trong trng PT(payload type - xem 3.4.2). tiu ca t bo ATM, gi tr AUU = 1 ch ra im ktthc, trong khi AUU = 0 ch ra im bt u hoc tip tc ca SAR - SDU- Lp con CS:

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Trng thng tin ca CPCS - PDU PAD UU CPI Length CRC0-47octet 8bit 8bit 16bit 32bit

Phn ui ca CPCS-PDUSAR PDU

Hnh 3.21. Cu trc CPCS - PDU ca AAL5Phn CPCS cung cp chc nng truyn khung s liu ca ngi s dng vi di

bt k t 1 octet ti 65535 octet. Thm vo , trong CPCS - PDU cn c trng UU(CPCS user to user indication) di 1 octet mang thng tin CPCS ca ngi s dng. Tr-ng di length ch ra di ca phn d liu trong CPCS - PDU. Mi CRC di 32 bitc s dng chng li. Hnh 3.2.1 th hin cu trc ca CPCS - PDU ca AAL 5.3.7. Mng ca ngi s dng v vn thng tin lin mng:

3.7.1. c im ca giao din B-ISDN UNIHnh 3.22 m t cu hnh ca giao din B-ISDN UNI.N bao gm:- Cc khi chc nng: Bao gm thit b kt cui mng B - NT1 (Broadband network

termination 1), thit b kt cui mng B - NT2 (Broadband network termination 2)v thit b u cui bng rng B - TE 1 (Broadband terminal equipment 1)

- im truy nhp: Tb v Sb

MngCN

Tb Sb UNI

Hnh 3.22 : Cu hnh giao din UNIB-NT1 ch thc hin cc chc nng truyn dn lin quan ti ng truyn. B-NT2 c

th l tng i c quan hoc mng ATM - LAN, chng thc hin cc chc ng nh phnhp knh v chuyn mch cc t bo ATM. Mng CN chnh l ni s dng dng truy cp vo mng cng cng thng qua thit b u cui ca mnh. N trng vi khichc nng B-NT2 (i khi c BNT1)

3.7.2.M hnh mng CNKhuyn ngh I.413 ca ITU - T a ra cc cu hnh vt l khc nhau cho CN (hnh

3.23). Hnh 3.23a minh ho cu hnh hnh sao trong thit b u cui c ni trctip vo B-NT2 B-NT2 c s dng nh h thng tp trunghoc phn phi. Ngoi ra,ITU - T cn a ra mt s cu hnh mng CN mi trong cc thit b u cui c ni

trc tip vo mt knhdng chung (trong hnh 3.23b,c,d). u im chnh ca cc mhnh knh dng chung cn phi c thm cc chc nng truy cp ng truyn. TrongATM, nhng chc nng ny c thc hin qua cc th tc trong trng iu khin lungchnh GFC. C ch GFC cung cp kh nng truy cp mt cch cng bng v c trt tvo knh chung cho cc thit b u cui

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B-TE

B-TE

B-TE

B-NT2 B-NT1

BNT2

B-TE B-TE B-TE

B-TE B-TE B-TE

BNT1

B-TE B-TE B-TE BNT2 BNT1

B-TE B-TE

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3.8. Tm tt :Chng 3 trnh by tt c cc khi nim c bn nht xy dng nn mng

ATM. Ta xem xt m hnh lin kt mng B-ISDN v cc khi nim lin quan

ti cuc ni (VCC,VPC,VC...). Nguuyn tc bo hiu v chuyn mch cu trct bo ATM. M hnh tham chiu ATM - PRM v chc nng ca tng lp trong

m hnh quy chiu ny c c bit ch trong chng 3 v n th hin c th

cch thc hot ng ca ATM.

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Chng 4: h thng truyn dn

4.1.Tng quanChng ny xem xt phng php truyn thng tin trn mng truyn dn bng rng

.Nh bit trong mi trung chuyn mch ATM,thng tin ucng gi thnh cc tbo 53 byte, Tuy nhin mng truyn dn bng rngATM ,khng phi tt c cc thngtin dng t bo bi v mng truyn dn bng rng phi c thit k sao cho dngthng tin ca cc h thng ng b ang tn ti cng c th c truyn trn cng mtmng vi lung thng tin ATM. Nhim v chnh ca mng truyn dn ny l truyn ccthng tin t ngun ti ch vi tr v xc sut li ti thiu. hon thnh nhim v trn,chng c cc chc nng sau: To ra cc t bo ATM t cc dng thng tin lin tc , Truyn cc t bo ATM v thng tin ca cc h thng truyn dn ng b

STM(Synchronous Tranfer Mode) ang tn ti .

Ghp knh v phn knh dng t bo cng nh dng thng tin s ng b hin ti. Phn lung v trung chuyn( cross- connecting) dng t bo ATM v cc dnh

thng tin ng b . Chuyn mch t bo v cc thng tin ca h thng ng b STM hin hu. p ng c cc chc nng trn , ITU-T chn mng cc quang ng b SONETv h thng phn cp s ng b SDH lm c s cho mnh truyn dn B_ISDN,tuyvy chng khng phi l h thng truyn dn duy nht trong mng ATM. Cc hthng truyn dn dng vai tr thc s quan trng trong B_ISDN,rt nhiu tin b tc lnh vc chuyn mch ,cc mng thng minh l nh s ci tin h thng truyn dn . Mt trong cc ci tin ny l p dng ca cc quang v cc thit b truyn dnquang.4.2 Cc h thng truyn dn bng rng

H thng truyn dn m thanh ATM bao gm rt nhiu thit b khc nhau ,mithit b thc hin mt chc nng truyn dn nhtnh.Sau y s trnh by mt s loithit b truyn dn bng rng.

4.2.1 B ghp knh SDH

155,520Mb/s1 p*155,520Mb/s

STM-1

MUXSTM-n

155,520Mb/s

B ghp knh SDH c p u vo v 1 u ra duy nht nh trn hnh . Cc nhnh u vo vo c th c tc khc nhau .u ra c tc bng tng tc ca ccnhnh du vo .Cc b ghp knh cng c th c kt hp vi nhau thnh nhiu tng

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ghp nhiu loi knh c tc khc nhau mt u thnh mt knh tc cao u ra. 4.2.2.B phn knh SDH

B phn knh SDH thc hin chc nng ngc li vi b ghp knh . N phn knhmt ng b dng thng tin n u vo thnh p u ra ng vi cc u vo ca b

ghp knh SDH.

155,520Mb/sP*155,520 Mb/s 1

STM-1 STM-1DEMUX 155,520Mb/s

pSTM-1

4.2.3 B phn lung thng tin ng b ( Synchronous transport Signal Cross-connect)

B phn lung thng tin ng b c p u vo v p u ra nh hnh v . B phnlung c kh nng ni mt u vo vi mt hoc nhiu u ra tc l n c th gicc tn hiu t mt u vo ti nhng u ra khc nhau . Chc nng chnh ca n lnhm nhng lung thng tin t cc nhnh u vo c cng a ch ch thnh mtlung thng tin duy nht a chng ti cc u ra thch hp .

1 1

2 2 B phn lungp thng tin ng b p

4.2.4 B phn tp trung ( ATM concentror) v b dn ( ATM expender)B phn tp trung ATM c p u vo v mt u ra , n nhn cc t bo ATM

ti cc u vo vi tc khc nhau . Tt c cc t bo trng u b loi b . Cc tbo ATM hu ch nm b m u vo cho ti khi chng c trn vi nhau nhng t bo nhnh khc thnh mt dng duy nht u ra . Tc u ra ph

thuc vo c im ca lung thng tin cc nhnh u vo v mc tp trung ccnhnh ca b tp trung . Nh vy tc u ra lun nh hn tng tc ccnhnh u vo .

B dn ATM thc hin chc nng nguc vi b tp trung . Dng ATM uvo c tch thnh cc lung thng tin khc nhau v c chn cc t bo trng ti to li cc u ra tng ng .4.2.4 B ni xuyn ATM ( ATM Cross-connect)

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B ni xuyn c p u vo v q u ra . Nhim v chnh ca n l phn hocghp lung cc ng o VP (xem 3.3.2.1) u vo v a chng ti cc u rathch hp. Vic phn / ghp lung ny da trn gi tr VPI ca c c t bo,do bni xuyn cn c gi l chuyn mch VP.B ni xuyn ATM cn thc hin chcnng loi b t bo trng ra khi dng thngtin u vo ( y cng l chc nng

ca b tp trung ) . 4.2.5 Chuyn mch ATMCu trc chuyn mch c trnh by chng 3 , tuy vy trong thc t chuyn

mch ATM cn bao gm b tp trung ,b dn ,b ghpknh v phn knh lm cho dng t bo v u ra thch hp vi tc bn trong c xc nh trc .

Ti u vo ,phn tiu ca cc t bo ATM c kim tra nh ng u rathch hp. Thng tin phn tiu c s dng nh mt con tr trong bng truynt , bng ny cha cc thng tin nh ng cho t bo .

Nt chuyn mch ATM1 12 2

p p

Hnh 4.1. Nt chuyn mch ATM 4.2.6 Cc khi dch v

Cc khi dch v l mt phn ca mng , chng thc hin cc chc nng ca cc

lp cao hn lp ATM nh lp AAL,mt phng iu khin( xem 3.6.1) . Di y l mt s khi dch v in hnh :- Khi dch v thu nhn cc thng ip bo hiu , thc hin cc chc nng x l

cuc gi v iu khin bng ma trn chuyn mch .- Khi dch v phc v cho vic truyn s liu khng lin kt ( Connectionless

data service) c tc cao ,chng thc hin cc chc nng to v tho gi ,kim tra phn tiu gi lp 3 v nh ng cho cc gi ti ch cn thithoc ti mt kh dch v s liu khng lin kt khc.

Cu phn phi (Multicast bridge) thu nhn t bo t mt u vo ,nhn bn cc t bony v gi chng ti nhiu ch khc nhau .

4.2.7 Phn t kt ni lin mng IWU (Interworking Unit)Phn t kt ni lin mng IWU c nhim v kt ni cc mng khc nhau vo mngB-ISDN. Thng thng n thc hin cc chc nng bin i cc th tc ca mng hinhu cho ph hp vi cc th tc ca mng bng rng.

4.3 Cc chc nng truyn dn bng rngSau y ta s xem xt chi tit cc chc nng c trnh by 4.1

4.3.1 To ra cc t bo ATM dng thng tin lin tc

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Thng thng cc thit b u cui bng rng gi vo knh truyn thng tin cng thnh cc t bo ATM di 53 byte . Trong trng hp ny khng cn b xung ccchc nng ng gi . Tuy vy vic to gi cn thit khi kt ni cc thit b thng tinkhng phi l ATM vo mng bng rng .Lc ny b ch to t bo ATM s ct knhtruyn nh b STM ( Synchronous Transfer Mode) thnh nhiu mu nh thch hp vi

trng thng tin ca t bo ,n cng lm cc thng tin dng chuyn mch gi ph hpvi kch thc t bo ATM . S chuyn i ny thng c thc hin thit b kt cuimng B-ISDN ( xem 3.7.1.1) hoc bt k u trong mng bng rng ,ni c lin ktvi mt mng khng ATM. Ngc li ,ta cng cn c nhng thit b tho gi chuyncc t bo ATM thnh dng thng tin thng . Hnh 4.2 th hin vic tho v to t boti B-NT2.

Sn

TnB-NT1

S hoc R B-NT2

B to / tho t bo ATM

Hnh 4.3 S dng b to / tho t bo ti B-NT2

4.3.2 Truyn dn t bo ATM4.3.2.1 Cc tiu chun ca h thng truyn dn

V nguyn tc ,t bo ATM c th c truyn trn h thng truyn dn khcnhau.ITU-T nh ngha 2 ch truyn dn ti giao din ngi s dng- mng , lch truyn dn SDH v ch truyn dn da trn c s t bo ATM(xem3.6.2.1.2),in hnh l h thng phn cp s cn ng b PDH, c nh ngha trong khuyn nghG.703 ca ITU-T. H thng PDH hot ng tc 2,3,34,140 Mb/s theo tiu chunCHU U hoc 1,5 v 45 Mb/s theo tiu chun BC MY.

Cc giao din ca h thng truyn dn phi lun tun theo chun vch ra biITU-T. Nhng giao din khc nhau s c s dng vo loi dch v c truyn , tc v lu lng truyn ti cc thu bao hoc nt truy nhp chuyn mch a phng . Timng trung k v ng di, cc giao din nh nhau cng c s dng truyn ting ni

,s liu hnh nh, mi loi giao din li c phng thc truyn khc nhau ( v mt khungtruyn ,cch truyn l cp quang hoc in v m ng truyn) . Bng 4.4Minh ho cc giao din ca h thng truyn dn bng rng.

Tn giao din Tc truyn Kiu tn hiuT1 1,544Mbit/s in

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DS3 45,76Mbit/s inOC1 51,84 Mbit/s inOC3 155,52Mbit/s QuangOC12 622,08Mbit/s QuangOC24 1,244Mbit/s Quang

OC48 2,488Mbit/s QuangOC192 9,6Mbit/s Quang

4.3.2.2 ng b mngNh trnh by trong 3.6.2.1.2 c hai h thng s c ng dng rng ri trong k

thut truyn dn ATM l h thng truyn dn trn c s t bo v h thng truyn dnng b m in hnh l mng SONET/SDH.

Mng truyn dn da trn c s t bo ATM cn phi c kh nng ng b tng bitcng nh ng b c t bo . Ti mi u vo ca b ghp knh hoc nt chuyn mch

ATM cn phi cung cp cc tn hiu ng b c lp ng b t bo u vo vih thng bn trong .Khi gii hn khong cch cc i gia mt chu k con l 26 t bo( xem 3.6.1.2) , lch ng b gia ng h bn ngoi v ng h bn trong s ciu chnh ( t bo OAM mc vt l mang cc thng tin ng b ngoi ) . trnnguyn tc ,mt ng truyn dn trong h thng truyn ny khng nht thit phi ngb vi cc ng truyn dn khc.

Ngc li trong cc mng truyn dn ng b ca B-ISDN tn hiu ng b mng

phi c to ra t nhng ngun ng h tin cy v n nh .Tt c cc phn t trong

mng u c ng b trc tip t mt ng h n .Tuy vy do khng th c nglin kt trc tip t ngun ng h n ti mi phn t trong mng nn thng tin

ng b ca mt nt mng thng c ly t nt k vi n .Nt mng ny ti lt n li

chuyn tn hiu ng b ti nt tip theo ,nh vy ton b thit b trong mng u c

ng b.Bi v ng h l phn t rt quan trng trong cc mng truyn dn ng b

nn cn phi c mt ngun khc d tr. Trong B-ISDN ,mi nt mng cn c kh nng

nhanh chng chuyn sang s dng ngun ng b d tr trong trng hp ngun chnh

b hng hay thit b b mt ng b trong mt khong thi gian ngn. Thng thng

ngun ng b d tr nm ngay bn trong nt mng .Hnh 4.5 minh ho mt v d ng

b trong mng B-ISDN

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Ngun ng h d tr Ngun ng h d Mgun dng h d Ngun ng h d tr

Thitb Nttruynhp Nt mng Nt mng Nt Thitu mngATM ATM Truy bcui nhp u

Ngung ng ATM cuiH chnh

ng cung cp ngun ng b chnhng cung cp ngun ng b d tr

Hnh 4.5:Cung cp ng b cho mng truyn dn ATM 4.3.3 Ghp knh tp trung dng thng tin

Trong b ghp knh SDH cc t bo trng khng c loi khi lung thng tin uvo ,nh vy c ngha l b ghp khng c chc nng x l cc

container SDH mang t bo ATM( xem 3.6.2.1.2).Hnh 4.6 trnh by mt b ghpknh SDH. 155,520Mb/s

1STM-1 B tp trung 155,520Mb/s

ATM STM-1155,520Mb/s

P STM-1

Hnh 4.6. B tp trung ATMNgc li b tp trung ATM ,tt c cc t bo trng u c tch ra khi dng t

bo mang thng tin c ch .V vy dung lng thngtin ti u ra ca b tp trung ATMnh hn tng dung lng thng tin u vo , iu ny c minh ho trn hng 4.6. B tptrung ATM tho cc t bo ATM ra khi khung truyn dn ( trn hng v l khungSTM-1).Loi b nhng t bo trng hoc b li sau ghp cc t bo mang thng tinhu ch vo khung STM-1 . Trong th d trn hnh 4.6 t l tp trung l 1/p,tuy vy tl ny ch ng vi s p nh (cho ti 8) v tc truyn ca dng thng tin hu ch timi nhnh khng qu 20 Mb/s .

4.3.4 Phn lung v trung chuyn dng t bo

Dng dy thu baoLung A

Lung Bng dy thu bao

Chuyn mch a phngNt ni xuyn ATM

Hnh 4.7. Phn lung thng tin nt ni xuynChc nng phn lung thng tin v trung chuyn dng t bo c thc hin nt nixuyn .cng nh chuyn mch ATM ,nt ni xuyn c th c d dng lin kt cc knho VC , ng o VP u vo vi cc VC,VP u ra v do cho php thit lp cc

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cuc ni knh o VCC v cuc ni ng o VPC qua mng (xem 3.3.2.3) . N cn thchin cc chc nng vn hnh ,qun l ,bo dng OAM cn thit lp vt l v lpATM . B ni xuyn c th c s dng trong mng truy nhp phn tch lungthng tin ca khch hng thnh mt lung i ti nt chuyn mch a phng v cclung c nh i ti mt im no trong mng . Hnh 4.7 minh ho vic phn lung

thng tin lp ni xuyn . Trong khi chuyn ATM thit lp v gii phng cuc nithng qua cc th tc bo hiu th nt ni xuyn li c iu khin bi mt phng qunl .

4.4 Mng truyn dn SONET /SDHSONET/ SDH l mt h thng ng vai tr quan trng trong s pht trin B-ISDN .

SONET (Synchronous Optical network ) l mt giao din truyn dn quang c hngBellcore a ra v c chun ho bi ANSI (American Nation Standard institude ) . T-ng ng vi SONET , ITU-T cng a ra chun ring ca mnh l h thng phn cps ng b SDH (Synchronous Digital Heachy ),Nhng tiu chun ca h thng ny nm trong khuyn ngh G.707, G.708, G.709.

Trong qu trnh pht trin hai chun truyn dn trn , ngi ta nhn thy iu ht sccn thit l phi ng nht v mt tiu chun cng nh yu cu ca c ITU-T cng nhANSI . Cui cng hai t chc trn i n mt tho hip cho php to tnh tngthch gia hai h thng SONET v SDH . V vy v mt cu trc cng nh tc truyn , hai h thng ny c rt t im khc bit v mt thc t , h thng SONET/SDH c xy dng v pht trin nhm t mt s mc ch sau : a ra cc chun phn cp phn cp truyn dn mi c th mang cc knh

truyn dn c s ca c CHU U v BC MY (th d : Cp DS3 45,76 Mbpsca BC MY hoc knh PCM 2.048 ca CHU U ).

To nn cc chun truyn dn quang ng nht kt ni thit b ca cc nh sn

xut khcnhau . a ra kh nng gim st , vn hnh v bo dng mi . To ra giao din thun tin , n gin vi nt chuyn mch , nt ni xuyn v b

ghp / tch lung thng tin . a ra mt cu trc mn do nhm tho mn cc dch v a tc bng rng ca t-

ng lai . Cung cp kh nng truy nhp d dng vo mt vi knh thng tin no nm trong

khi thng tin a knh tc cao

4.5 Cu trc mng truyn dn bng rng 4.4.1 Cu trc mng a phngCu hnh mng a phng n gin nht l cu hnh hnh sao . trong cu hnh ny , mithu bao u c mt ng dy truy nhp ring. Hinh 4.9 trnh by cc cu hnh mnga phng khc nhau . Cu hnh hnh sao kp,v hnh vng cng ng vai tr quantrng trong mng a phng . ti u ho lung thng tin ti ngi s dng ,cc ngtruy nhp t thu bao thng c ni vi b ni xuyn trc khi i ti nt chuyn mhcng nh cc ni khc nhau trong mng

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Hnh 4.9 : Mt s cu hnh mng a phng

4.4.2 Cu trc mng trung k

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u cui

u cui

u cui

Chuyn mch

a phng

u cui

u cui

Chuyn mcha phng

u cui

Nt ni xuyn

u cui

u cui

M n

u cui

u cui

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Mng trung k l mng dng lin kt cc nt chuyn mch ( hoc mng a ph-ng ) li vi nhau . T thu bao , mt cuc gi ng di s c truyn qua nt chuynmch a phng v mng trung k ti c thu bao b gi . Mng trung k thng cto thnh t b ni xuyn v nt chuyn mch ATM, trong cc nt ni xuyn hocb ghp knh dng vai tr trung gian gia chuynATM v cac thit b truyn dn

SONET. Thit b ny c nhim v bin i dng t bo ATM thnh dng thng tinquang thch hp vi ng truyn . Hnh 4.10 l mt v d vMng trung k.

Hnh 4.10 Mng trung k

4.5 Tm ttchng ny chng ta cp n ti cc vn c lin quan ti h thng truyn dn

bng rng v chc nng ca chng . Cc giao din ca mng truyn dn v vn ngb mng cng c nghin cu. Phn cui ca chng trnh by cc nguyn tc c bn thit lp cu hnh cho mng a phng v mng trung k.

Chng 5: tng quan v mng my tnh

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Lung thng tinT mng a

Phng

ChuynMchATM

Nt niXuyn

Nt niXuyn

Chuyn

Mch

ATM

NtNixuyn

Chuyn

MchATM

Lung thng tinTi mng a

Phng

Lung thng tinTi mng a

Phng

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Cc b (lan )hin hu5.1 kin trc ca mng cc b

cc mng cc b thng c 3 loi kin trc chnh (hnh5.1)

hnh sao (star)

Hnh vng (ring)

Hnh cy (tree)

Bus l trng hp c bit ca hnh cy c mt thn , khng c cnh ,nn ta s dngt ghp bus/cy.

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Trong dng hnh sao ,mt phn t chuyn mch trung tm c dng ni vi ttc cc nt trong . Mt trm mun truyn d liu cn phi gi mt yu cu lin kt vimt trm ch no ti trm chuyn mch trung tm v trm trung tm ny s dngchuyn mch knh thit lp , d liu c trao i gia hai trm nh th chng c nivi nhau bi mt lin kt im- im.

Dng hnh vng bao gm mt chu trnh trong mi nt c ni vi mt phn tlp .d liu lu thng quanh vng theo mt chui lin tip cc lin kt d liu im -im gia cc phn t chuyn tip . Mt trm mun truyn phi i n lt v gi dliu vo vng di dng mt packet trong c cha cc a ch ca ngun v ch cngnh d liu cn chuyn . Khi packet ti , trm ch s sao d liu vo b nh m ca nv packet tip tc lu thng cho n khi n tr v li nt ngun , to ra mt kiu xcnhn t nhin.

Dng bus/cy c c trng bi vic s dng mt phng tin truyn tin dng qungb ,a truy nhp .V tt c cc thit b phn chia mt phng tin truyn thng chungnn mi thi im ch c mt thit b c th truyn v cng ging trng hp dng

vng , d liu c gi trong mt packet c cha cc a ch ngun v ch. Mi trmkim sot phng tin truyn v sao cho cc gi dnh cho n .5.2 Phng tin truyn

Bng 2.1 cho ta danh sch cc phng tin truyn thch hp vi mng cc b : cp dyxon ,cp

hệ thống atm và ứng dụng vào mạng cục bộ

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