hung-cong nghe atm

8/8/2019 Hung-Cong Nghe ATM

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mng s dng cc cng ngh truyn ti v iu khin khc nhau) thnh mt mng adch v thng nht, da trn cng ngh chuyn mch gi - mng th h sau.

S hi t ny c th hin trong hnh I.1, trong cc mng ring bit hin

ti vi h thng truy nhp, truyn ti v chuyn mch ring, cung cp n dch v nhmng in thoi PSTN/ISDN, mng di ng t bo PLMN, mng d liu/IP v mngtruyn hnh cp bng rng CATV s tin ti hi t thnh mt mng a dch v thngnht vi cc ng dng client/server v da trn mt mng xng sng - mng chuynmch gi.

Hnh I.1. S pht trin ca kin trc mng

I.1 GII THIU CHUNG

I.1.1. NGN l g?Khi nim mng th h mi (NGN) l khi nim mi c cc nh thit k mng

s dng cho vic minh ho quan im ca h i vi mng vin thng tng lai. Tthi im u tin trong chu k nghin cu trong nm 2000, khi nim NGN vn cnrt m. Cc quan im khc nhau v NGN c biu din bi cc nhm nghincu, cc nh khai thc, nh sn xut, v nh cung cp dch v ti cc cuc hi tho,mong mun tin n mt hiu bit chung v NGN v thit lp tiu chun cho NGN. l nguyn nhn v sao ITU quyt nh bt u tin trnh tiu chun ho v NGN

theo m hnh d n do nhm nghin cu 13 chun b. Ti cuc hp ca SG 13 vothng 1/2002, vn NGN li mt ln na c cp n, tp trung vo mi qua


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h gia c s h tng thng tin ton cu v NGN. V cng thi im, vin tiu chunVin thng Chu u cng thnh lp nhm nghin cu NGN vi nhim v phi xut chin lc chun ho ca h trong lnh vc NGN.

C th nh ngha mt cch khi qut mng NGN nh sau: Mng vin thng th h mi l mt mng c h tng thng tin duy nht da trn cng ngh gi c th trin khai nhanh chng cc loi hnh dch v khc nhau da trn s hi t giathoi v s liu gia c nh v di ng .

c im quan trng ca mng NGN l cu trc phn lp theo chc nng v phntn cc tim nng (intelligence) trn mng. Chnh iu ny lm cho mng mm ho(progamable network) v s dng rng ri cc giao din m API kin to cc dchv m khng ph thuc nhiu vo cc nh cung cp thit b v khai thc mng.

c im v kh nng ca mng NGN Vi s hi t mng chuyn t tch hp cc mng n dch v theo chiu dc sang

mng a dch v cu trc theo cc lp ngang, mng NGN c nhng c im v khnng chnh nh sau:

Mt trong cc c tnh chnh ca NGN l tch ring cc dch v v mng, cho php a chng ra mt cch ring bit v pht trin c lp. Do trong cccu trc NGN a ra c s phn chia r rng gia cc chc nng ca dch vv cc chc nng truyn ti. NGN cho php cung cp c cc dch v ang tnti v cc dch v mi khng ph thuc vo mng v kiu truy nhp c sdng.

NGN s phi cung cp cc nng lc (c s h tng, cc giao thc...) c thto ra, pht trin v qun l tt c cc loi dch v hoc s c. Cc dch vtrn c th l Multimedia (audio, visual, audiovisual), Unicast, Boadcast,nhn tin,... dch v truyn d liu n gin, yu cu/ khng yu cu thi gianthc, nhy cm vi tr hay chp nhn tr, hoc yu cu rng bng thng

khc nhau t vi kbit/s ti hng trm Mbit/s. Trong mng NGN cc dch v tu bin theo khch hng ca cc nh cung cp dch v ngy cng quan trng. NGN s dng giao din lp trnh ng dng API (Application ProgrammeInterface) h tr vic to, cung cp v qun l cc dch v.

Trong NGN, cc thc th chc nng iu khin hot ng, cc phin, cc tinguyn, phn pht dch v, bo mt, ...c th c phn tn khp c s h tngmng bao gm c cc mng ang tn ti v mng mi. Mng NGN lin kthot ng vi cc mng ang tn ti nh PSTN, ISDN v GSM qua ccGateway.


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NGN h tr c cc thit b u cui nhn bit NGN v cc dch v ang tn ti.V th, cc thit b kt ni ti NGN bao gm cc thit b thoi tng t, myfax, cc thit b ISDN, in thoi di ng t bo, u cui SIP,....

i vi vic chuyn cc dch v thoi ti c s h tng NGN, cht lng dchv lin quan ti cc dch v thi gian thc (m bo bng thng, tr, mtgi ...) cng nh vn bo mt, NGN cn cung cp c ch i vi cc thngtin nhy cm khi qua c s h tng ca n, bo v chng li vic s dngian ln cc dch v c cung cp bi cc nh cung cp dch v v bo v bnthn c s h tng ca n trc s tn cng t bn ngoi.

Mng NGN s h tr tnh di ng chung (generalized mobility). Ngy nay, ccmng c nh v di ng cung cp nhiu dch v tng t nhau cho ngi s

dng. Tuy nhin, h vn c xem l cc khch hng khc nhau vi cu hnhdch v khc nhau v khng c cu ni gia cc dch v khc nhau . Mt cim ni bt khc ca mng NGN l tnh di ng chung, n cho php cungcp nht qun cc dch v cho ngi s dng. iu ny c ngha l ngi sdng s c xem l duy nht khi h s dng cc cng ngh truy nhp khcnhau, vi bt c loi thit b no.

Tuy nhin, mng NGN cng gp phi cc vn kh khn nh vic chuyncc dch v thoi sang h tng NGN, vn QoS lin quan n cc dch vthoi thi gian thc (m bo v bng thng, tr, mt gi) cng nh vicm bo an ninh, bo mt. Nhng c im v kh nng ny ca mng NGN c nh hng trc tip v

t ra nhiu yu cu mi i vi h thng qun l mng NGN. S hi t ca nhimng khc nhau mt mt lm tng s phc tp v thch thc trong qun l mng vdch v nh phi qun l nhiu phn t mng phn tn vi cng ngh v nh cung cpkhc nhau, phi m bo QoS t u cui n u cui cho cc loi dch v khc

nhau, vn tng quan li, tnh cc, an ninh u phc tp hn. Mt khc, stch bit gia mng v dch v, gia chc nng kt ni truyn ti v chc nng iukhin dch v cho php n gin hn vic qun l mng nh d liu lin quan ncuc gi v cc d liu logic phc tp khc c tp trung, trin khai nhanh cc loihnh dch v khc nhau.

Hin nay, c nhiu hng cung cp thit b a ra cc m hnh khc nhau nhmtha mn cc yu cu ca mng NGN: Alcatel vi E10MM, Ericsson vi ENGINE,Siemens vi SURPASS


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I.1.2. XU HNG PHT TRIN CNG NGH VIN THNG

Trong qu trnh pht trin, cc ng lc thc y s tin b ca k thut vinthng l:

- Cng ngh in t vi xu hng pht trin hng ti s tch hp ngy cngcao ca cc vi mch.

- S pht trin ca k thut s.

- S kt hp gia truyn thng v tin hc, cc phn mm hot ng ngy cnghiu qu.

- Cng ngh quang lm tng kh nng tc v cht lng truyn tin, chi ph

thp Nhng xu hng pht trin cng ngh an xen ln nhau v cho php mng li

tho mn tt hn cc nhu cu ca khch hng trong tng lai.

Vi s gia tng c v s lng v cht lng ca cc nhu cu dch v ngycng phc tp t pha khch hng kch thch s pht trin nhanh chng ca thtrng cng ngh in t-tin hc-vin thng.

Nhng xu hng pht trin cng ngh v ang tip cn nhau, an xen ln

nhau nhm cho php mng li tho mn tt hn cc nhu cu ca khch hng trongtng lai. Th trng vin thng trn th gii ang ng trong xu th cnh tranh v pht trin hng ti mng vin thng ton cu to ra kh nng kt ni a dch v trn phm vi ton th gii.

Xu hng pht trin cng ngh in t-vin thng-tin hc ngy nay trn thgii c ITU th hin mt cch tng qut trong hnh v sau y (Hnh I.2). Cc dchv thng tin c chia thnh hai xu th:

- Hot ng kt ni nh hng (Connection Oriented Operation).- Hot ng khng kt ni (Connectionless Operation).

Cc cuc gi trong mng vin thng, PSTN, ISDN l cc hot ng kt ninh hng, cc cuc gi c thc hin vi trnh t: quay s-xc lp kt ni-gi vnhn thng tin-kt thc. Vi cht lng mng tt, cc hot ng kt ni nh hnglun lun m bo cht lng dch v thng tin. Cng ngh ATM cho php pht trincc dch v bng rng v nng cao cht lng dch v.


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Hnh I.2. Cc xu hng pht trin trong cng ngh mng (ITU TSB)

Khc vi cc cuc gi quay s trc tip theo phng thc kt ni nh hng,cc hot ng thng tin da trn giao thc IP nh vic truy nhp Internet khng yu

cu vic xc lp trc cc kt ni, v vy cht lng dch v c th khng c bo. Tuy nhin, do tnh n gin, tin li vi cho ph thp, cc dch v thng tin theo phng thc hot ng khng kt ni pht trin rt mnh theo xu hng nng caocht lng dch v v tin ti cnh tranh vi cc dch v thng tin theo phng thckt ni nh hng.

Hai xu hng pht trin ny dn tim cn v hi t vi nhau tin ti ra icng ngh ATM/IP, l ngun gc ng lc cho ra i v pht trin cc cng ngh

mng mi nh MPLS.S pht trin mnh m ca nhu cu dch v v cc cng ngh mi tc ng

trc tip n s pht trin ca cu trc mng (Hnh I.3).

IP

Cnh tranh v i CO

ATM

PSTN/ISDNMi trng vin thng

QoS khng c m bo QoS c m bo QoS cao

CL

CO

CO = Hot ng kt ni nh hngCL = Hot ng khng kt ni= Song hng


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Hnh I.3. Xu hng pht trin mng v dch v

C th khng nh giai on hin nay l giai on chuyn dch gia cng nghth h c (chuyn mch knh) sang dn cng ngh th h mi (chuyn mch gi)iu khng ch din ra trong h tng c s thng tin m cn din ra trong cc cngty khai thc dch v, trong cch tip cn ca cc nh khai thc th h mi khi cungcp dch v cho khch hng.

Cc dch vhin nay camng hin ti

Cc dch v pht trin tiptheo ca mnghinhin ti

Cc dch v phttrin tip theo

ca mng th hsau

th h sau

Cc dch vhin nay camng th h

S pht trin mng

S

ph

ttrindch

dchv

Xen k CLEC, ISP,

ITSP

IEC v LECtruyn thng


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CHNG 2

CNG NGH ATM

2.1. GI I THIU CHUNG V ATM 2.1.1. Khi nim v ATM.

ATM l phng thc truyn khng ng b k thut chuyn mch gi chtlng cao. C phng thc truyn ti nh hng, chuyn gi nhanh da trn ghpkhng ng b phn chia theo thi gian.

Trong kiu truyn khng ng b tn ti hai thut ng:

* Thut ng truyn bao gm c lnh vc truyn dn v chuyn mch trong dng truyn m ch c ch truyn dn v chuyn mch thng tin trong mng.* Thut ng khng ng b gii thch cho mt kiu truyn thng, trong ccgi tin trong cng mt cuc ni c th lp i lp li mt cch bt thng nh chngc to ra theo yu cu c th m khng theo chu k.

ATM kt hp tt c nhng li th ca k thut chuyn mch trc y vomt k thut truyn thng duy nht. S dng cc gi c nh gi l cc t bo, n cth truyn ti mt hn hp cc dch v bao gm thoi, hnh nh, s liu, c th cungcp cc bng thng theo yu cu. ATM c th loi tr c cc nt c chai thngxy ra cc mng LAN v WAN hin nay.

Cng ngh ATM xut hin vi mng din rng, a dch v bng rng, tc cao. Nh cc cng ngh ATM, ta c th kt hp cc dch v B-ISDN khc nhau, lnhng dch v bng rng, bng hp khc nhau cng tn ti trong mng vin thng ccng mt kch c t bo ATM.

ATM cng chp nhn loi dch v kt ni trong knh o c to ra truyn cc thng tin dch v. ID kt ni c ch nh khi thit lp knh v ID cgii phng khi kt thc kt ni. Trnh t ATM ca cc t bo ATM ca knh o cto nn bi chc nng ca lp ATM v thng tin bo hiu cho vic thit lp kt ni,c truyn i theo cc t bo ATM khc nhau.


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2.1.2. Cc c im ca ATM.ATM truyn ti theo phng thc khng ng b, tc l cc thng tin c

truyn t u pht ti u thu mt cch khng ng b v c th hin nh sau:thng tin xut hin ti u vo ca h thng c np vo cc b nh m, sau chng c chia nh thnh cc t bo v truyn ti qua mng. ATM c hai c imquan trng l:

Th nht: ATM s dng cc gi c kch thc nh v c nh gi l t boATM (ATM cell), cc t bo nh cng vi tc truyn ln s lm cho trtruyn v bin ng tr gim nh i vi cc dch v thi gian thc.ngoi rakch thc nh cng s to iu kin cho vic hp knh tc cao c ddng hn.

Th hai: ATM cn c mt c im rt quan trng l nhm mt vi knh othnh mt ng o mhm gip cho vic nh tuyn c d dng.Phng thc truyn ti trong ATM gn ging vi phng thc chuyn mch

gi. V n c mt s c im khc vi chuyn mch gi nh sau:

ph hp vi vic truyn tn hiu thi gian thc th ATM phi t tr nh, tc l cc t bo phi c di ngn hn cc gi thng tin trong chuynmch gi.

Cc t bo c on mo u nh nht nhm tng hiu qu s dng v ccng truyn c tc rt cao.

m bo tr nh th cc t bo c truyn nhng khong thi giaxc nh, khng c khong trng gia cc t bo.

Trong ATM th t cc t bo bn pht v bn thu phi ging nhau (m bonht qun v th t). Nhng c im ny gip cho mng ATM c s mn do v linh hot v n c

th to ra s tng thch v mt tc truyn ca cc t bo (tc ca thng tin) vtc ca thng tin c to ra (tc thay i ngun tn hiu).

ATM c th iu khin tt c cc kiu lu lng: Voice, Audio, Video, Text,

Data..., c ghp knh v chuyn mch trong mt mng chung. Trong mng ATM rng bng c th gn li trong thi gian thc cho bt k kiu lu lng khc nhau


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no theo yu cu, c th thy rng y l mt cng ngh cho mi mi trng LAN,GAN, PSTN... y l nguyn nhn ni bt lm cho ATM c la chon lm cngngh chuyn mch v truyn dn chung cho cc dch v trong mng B-ISDN.

Cc tnh nng u vit ca ATM v mi trng ATM l: Ghp knh khng ng b (ATDM) v thng k cho mi kiu lu lng. Gn rng knh rt linh hot v mm do. Gim cc mng ring. Chp nhn mng hin c nh kt ni chng vi mng ATM mi.

Tc truy cp cao (155 Mbt/s 16 Gbt/s) Tit kim gi thnh OA&M (Operation Administrantion and Maintenance) nh

cng ngh cao v ng nht.Bn cht ca ATM l lin kt truyn cc t bo vi cc thng tin c to ra v

ATM cung cp kh nng ghp knh thng k vi ng truyn. Do trong ATM tn dng c dung lng truyn dn trong cc thi im c hot ng thp cngun thng tin vi thay v truyn i cc t bo khng c ch, l cc t bo truyn i

trong khong thi gian ny, s c cc ngun thng tin khc nhau c thay th. Trongtrng hp c nhiu ngun thng tin c thay i (VBR) truyn i trn cng mtng truyn th kh nng ghp knh thng k l rt cao.

T bo ATM c kch thc c nh v kt hp vi ghp knh, gip cho vic thp nhiu ngun tn hiu khc nhau trn mt ng truyn c d dng, t ccnh khai thc c th cung cp nhiu dch v cho khch hng trn cng mt ngtruyn.

Tuy nhin ATM khng phi khng c nhc im:_ Thi gian t hp t bo v tr bin ng t bo._ Tr bin ng t bo sinh ra bi cc gi tr tr khc nhau ti nhng im chuyn

mch v cc thit b tch/ghp knh, dn n khong cch cc t bo b thay i.Trong tn hiu thoi s b nh hng rt nhiu nu xy ra tr ny.


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Trca mng Gi trtrtng Gi trtrgim

Hnh 2.1 M t s bin i tr ca t bo

2.1.3. Cu trc t bo ATM

Cu trc mt t bo ATM

6

52

53

Phn mang thng tin dch vInformation Section

Phn tiu Header Section

Byte123

4

5

8 7 6 5 4 3 2 1

Hnh 2.2 Cu trc mt t bo ATM

c im ca ATM l hng lin kt nn khc vi chuyn mch gi l a chngun, ch v s th t cc gi tin l khng cn thit. ATM cng khng cung cp c ch iu khin lung gia cc nt mng nhng c kh nng nhm mt vi knh othnh mt ng onhm gip cho vic nh tuyn c d dng hn. V vy chcnng c bn ca phn tiu trong t bo ATM l nhn dng cc cuc ni o.


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Da vo cu trc phn cp ATM theo s :

Hnh 2.3 Cu trc phn cp ATM

Tng ng vi hai cp giao din trn, ngi ta a hai dng cu trc phn tiu tng ng:+ Cu trc phn tiu giao din gia ngi s dng v mng UNI.+ Cu trc phn tiu giao din gia cc nt mng NNI.

Hnh 2.4 Cu trc tiu t bo ATM

Mng cngcng

Mng cngcng

NNI(Giao din gia cc nt m ng)

Mngring

Mngring

Cc tr m ktcui s dng

Cc tr m ktcui s dng

Cc tr m ktcui s dng

Cc tr m ktcui s dng

UNI(Giao din mng - khch

hng)

* Cu trc t bo ATM ti UNI

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC

Phn mang thng tin c angi s dng 48 byte

Bit 8 7 6 5 4 3 21

1

2

3

4

5......53Byte

VPI

VPI VCI

VCI

VCI PT CLP

HEC

Phn mang thng tin c angi s dng 48 byte

Bit 8 7 6 5 4 3 21 Octet

1

2

3

4

5......53Byte* Cu trc t bo ATM ti NNI


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ngha cc trng trong phn tiu . _ GFC ( General Flow control) l trng iu khin lung chung. Trng

ny ch dng cho giao din UNI trong cu hnh im - im, c di gm 4 bit,trong 2 bit dng cho iu khin v 2 bit dng lm tham s. C cu ny ctiu chun ha.

_ VPI (Virtual Path Identyfier) vVCI ( Virtual Channel Identyfier) l haitrng nh tuyn cho cc t bo trong qu trnh chuyn mch:

Vi UNI th c 8 bit VPI v 16 bit VCI.Vi NNI th c 12 bit VPI v 16 bit VCI.

_ Hai trng ny ghi nhn dng lung o v knh o. c tnh c bn ca ATM lchuyn mch xy ra trn c s gi tr trng nh tuyn:

Nu chuyn mch xy ra trn VPI th gi l kt ni ng o. Nu chuyn mch xy ra trn VPI v VCI th gi l kt ni knh o.

_ PT ( Payload Type) l trng ti thng tin xc nh xem t bo ny mangthng tin khch hng hay thng tin iu khin. N cng xc nh qu ti ca t bothng tin khch hng. Trng ny c c hai giao din v c di 3 bit. _ CLP ( Cell Loss Prioryti) l trng u tin b t bo dng ch ra kh nng cho php hoc khng cho php b cc t bo khi c hin tng qu ti xy ra.

Nu cc t bo c CLP = 0 th c mc u tin cao Nu cc t bo c CLP = 1 th c mc u tin thpTrng ny ch nhn hai gi tr 0 hoc1 nn c di 1 bit v tn ti c hai

giao din.

_ HEC (Heacler Error Check) l trng kim tra li phn tiu . Trng ny c di 8 bit. N dng pht hin li ghp bit v sa li cho ng cc li ghp bit n. Cng vic ny c thc hin lp vt l.


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2.1.4. K thut ghp knh trong ATM Nhoc im c bn ca STM l lng ph kh nng truyn ti ca h thng v

kh x l ng thi tt c cc dch v yu cu (thm ch khng th x l c) c tc dng bit rt khc nhau.

Kho st s b k thut dng trong ch truyn ti ng b STM l k thutghp knh theo thi gian ng b STDM (Synchronous Time Divission Multiplexing).STDM thc hin ghp knh ng b vi ng b h thng v cc khung tn hiu phi b tr sp xp theo mt th t c nh v lp li theo mt chu k hon ton xc nh bi ng h h thng .

Mi khe thi gian Tsi ca mt khung c gn cho mt knh lin lc c nhtrong sut thi gian ca qu trnh thng tin, do vy thng xy ra lng ph ngun tinguyn v knh gn dnh ring cho mt qu trnh thng tin th cho d n khngc s dng (khi khng c thng tin truyn) cng khng th dng cho cc qutrnh thng tin khc.

C

B

A

A

B

C

A1 A3

B1 B2

C2 C3

A1 A3

B1 B2

C2 C3

A1 B1 B2 C2 A3 C3

A1 B1 B2 C2 A3 C3

STDM

ATDM

Hnh 2.5 So snh STDM v ATDM

Khc vi ch ghp knh ng b, trong k thut ghp knh khng ng bATDM (Asynchronous Time Divission Multiplexing) khng cn nhim v gn khe

thi gian cho cc qu trnh thng tin c th na m c c bt k khe thi gian no ri


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th ATDM ghp gi tin cn truyn vo. Ni cch khc, ATDM thc hin k thutghp knh thng k, ngha l cc gi tin chun ca ngun tin c th ghp vo ngthi nhiu khe thi gian c ch s khe khc nhau v do vy ATDM t c mm

do, linh hot, hiu qu cao vi nhiu kiu dch v, mi tc bit v kiu lu lnkhc nhau.

Gii php kt hp cc u im, khc phc cc nhc im ca k thut chuynmch knh v chuyn mch gi, s dng ATDM s c kh nng p ng tt cc yucu trong B-ISDN l vn ch yu ca cng ngh truyn ti khng ng b ATM.

Nh trnh by, thut ng truyn bao gm c lnh vc truyn dn vchuyn mch trong mng, mng ATM c kh nng ch gi s liu lin quan ti mtcuc ni khi n c thc s c s liu cn truyn v khng c khe thi gian gn ringcho cuc ni . iu ny hon ton khc so vi c ch ng b khi ng vi cucni cn phi c mt khe thi gian dnh ring xc nh, v my thu STM khng thkhi c thng tin cha trong cc khe thi gian khc.

2.1.5. Nguyn l c bn ca ATM

Nguyn l c bn ca ATM l kt hp cc u im ca chuyn mch knh vichuyn mch gi v ATDM. Trong cng ngh k thut chuyn mch gi, v d tronggiao thc X.25 cc gi tin c phn tiu kh phc tp, kch thc kh ln v khngchun ho di gi tin. Nh vy c ngha l x l chuyn mch gi tng i kh,kch thc ln nn tr ln, x l v truyn dn chm ng thi kh qun l qutrnh.

Khc phc nhc im ny ca chuyn mch gi ATM ngi ta to ra cc

gi tin gi l t bo ATM, n c chun ho khch thc v nh dng cho phhp nht, d qun l nht, hiu qu nht v tiu n gin nht.

Tht vy i khi cch tt nht qun l lng thng tin ln l chia thnh ccgi tin nh nh vy d qun l hn. ATM khng quan tm thng tin l ci g v n tu n. n gin l ATM ct bn tin cn pht thnh cc t bo ATM c kch thcnh v bng nhau, gn tiu cho cc t bo sao cho c th nh hng chng tic ch mong mun, m bo cc yu cu trong sut qu trnh truyn tin. Mi t bo ny theo ITT-T a ra kch thc l 53 byte, trong c 5 byte tiu v 48 byte


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trng thng tin. Trng thng tin mang thng tin ca khch hng v phn tiu gil mo u mang thng tin mng nh thng tin nh tuyn.

V i trn cng mt ng truyn nn c th c nhiu t bo t cc ngun tn

hiu khc nhau ghp li vi nhau to nn mt lung t bo c chung mt ngun tnhiu. Vic ny c thc hin bng thng tin mo u ca t bo.

Tr-ng thng tin Mou

Hnh 2.6 Cu trc nguyn l dng t bo.

Vi trng thng tin th c truyn ti thng sut qua mng ATM v khng b x l trong qu trnh vn chuyn (khng c iu khin li nh trong chuyn mchgi).

Tm li: ATM l ch truyn ti cc gi tin khng ng, n khc ch chuyn mch gi nhng ni chung ATM c c trng ca chuyn mch gi ng thicng c cc c tnh tr v tc cao nh cnng ngh chuyn mch knh (v kchthc nh v tiu n gin hn chuyn mch gi nhiu).

2.1.6. Cu trc phn lp ca mng ATMTheo m hnh tham chiu lin kt h thng m OSI (Open System

Interconnection ) ca t chc tiu chun ho quc t ISO ( International StandrdOrganization ). Mi h thng m u c cc h thng con c sp xp theo th t.


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-ngtruynvt l

Lpcaonht

LpN+1

LpN

LpN-1

Lpthpnht

HthngA HthngB

..

....

Hnh 2.7 M hnh phn lp OSI

Thc thN+1

Thc thN Thc thN

Thc thN+1

Giao thc ng cpN

Giao thc ng cpN+1

Lp s dngdch v

Lp cungcp dch v

PDU

N-PDU

Hnh 2.8 Mi quan h gia cc thc th v cc lp trong OSI

Mt PDU lp N bao gm thng tin iu khin giao thc PCI ( Protocol ControlInformation ) lp N v s liu ti t lp N+1 thng tin iu khin trao i gia ccthc th lp N.


18/34

PDU lp N+1

SDU lp N+1

PDU lp N+1

PCI lp N

Lp N+1Lp N

Hnh 2.9 Cc kiu n v s liu v quan h gia chng

2.1.7. La chn di cho t bo.2.1.7.1. La chn di c nh hoc thay i.

Vic la chn di t bo c nh hay thay i c quyt nh sau khi cnnhc cc u nhc im ca nhng phng n ny thng qua mt lot cc yu t nhhiu sut bng truyn, tr, tc v phc tp ti nt chuyn mch.

V mt hiu sut bng truyn.

Ta c cng thc chung tnh hiu sut bng truyn:

S byte thng tinn =

S byte thng tin + S byte tiu

* Trng hp di gi c nh:Trong trng hp di t bo ATM c nh, hiu sut bng truyn c tnh

theo cng thc:X

nF = X

(int )( L + H)

L


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Trong :nF : Hiu sut bng truyn ca t bo c di c nh.

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

kch thc trng thng tin:

X X = int L L

Lc gi tr hiu sut bng truyn s l:

LnFopt =

L + H

Ngi ta nhn thy rng hiu sut ph thuc rt nhiu vo cc byte thng tin hu chc truyn i. S byte thng tin cng nhiu 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

nn hiu sut gn t c gi tr ti u, k c khi s dng t bo c di c nh.

* Trng hp di gi thay i:i vi gi c di thay i, cc thng tin b sung vo phn tiu bao gm

cc c nhn bit gii hn gi, mt vi bit c chn thm vo nhn bit ccc chnh xc. Ngoi ra cn phi cng thm vo phn u khung phn bo hiu digi, lc hiu sut l:


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Xnv =

X + H + hv

Trong hv l phn thng tin bt buc phi b sung thm bo hiu dithay i ca gi ATM. Hiu sut truyn ca gi c di thay i l rt cao, vi ccgi c di ln hiu sut ny t gn ti 100%.

V mt tc chuyn mch v phc tp. phc tp ca vic chuyn mch cc gi c di c nh hay thay i ph

thuc vo nhng chc nng m chng cn thc hin cng nh yu cu k thut tngng vi cc chc nng ny. Hai yu t quan trng nht l: tc hot ng v yucu v kch thc b nh ca hng i.

* Tc hng i: Ph thuc vo s lng cc chc nng cn thc hin vthi gian thc hin cc chc nng .

* X l phn tiu : i vi cc gi c di c nh, khong thi gian cnthit x l phn tiu l c nh. Trong trng hp gi c di thay i, thigian x l ny khng c nh m ph thuc vo di ca gi, thng thng tc x l cn ln hn rt nhiu mi t c tc truyn tin nh trong trng hp gc di c nh. Kch thc cng nh th yu cu tc x l cng ln. Bng thhin tc x l cn thit trong hai trng hp tc 150 Mb/s. Ta thy l tc

trong trng hp di gi thay i yu cu ln hn rt nhiu so vi trng hp di gi c nh.

* Qun l b nh ca hng i: Trong trng hp kch thc gi c nh, hthng qun l b nh c th a ra cc khi nh vi kch thc c nh tng ng vikch thc ca t bo ATM. Hot ng ny ht sc n gin nh trong trng hpqun l b nh t do. Trong trng hp gi c di thay i, h thng qun l bnh phi c kh nng a ra cc khi b nh c kch thc khc nhau sao cho cc


21/34

hot ng nh tm cc on thng tin, tm on u tin, c tin hnh tc cao. Vic qun l b nh cng tr nn phc tp hn.

* Yu cu v kch thc hng i: Trong trng hp di gi c nh, yu

cu v kch thc hng i ph thuc vo ti v t l mt gi ti, t l mtgi ti cng ln th th yu cu hng i cng phi cng ln. Trong trng hp gi c di thay i, tnh ton kch thc hng i phc tp hn nhiu v s ph thuc vo di gi. n gin nht l nh kch thc hng i tng ng vi gi c di lnnht, lc kch thc hng i s tr nn ln hn rt nhiu so vi trng hp gi c di c nh. Vic tnh ton kch thc hng i mt cch ti u trong trng hpny s rt kh khn.

Kt lun:

Sau khi i chiu vi yu cu v tc hot ng v kch thc hng i, gii php gi c kch thc c nh l hp l nht i vi cc dch v bng rng.

Tr.Kch thc ca gi trong ATM cn phi gii hn khng pht sinh ra tr

trong mng. Tr ting ni s rt ln nu kch thc gi ln. Hnh minh ho mtMultiplexer hp knh cc ng truyn s liu khc nhau vo b chuyn mch, ccgi s liu trong trng hp ny c kch thc thay i. u vo pha trn l mt muting ni c truyn cho thi gian thc, u vo pha di l mt gi s liu rt diDo mu ting ni ti sau gi s liu mt cht nn phi i s liu truyn xong mi t

lt n c truyn ti u ra. Nu gi s liu qu di th mu ting ni s b tr nk. Ngc li, nu s dng cc t bo nh v c nh th tr s gim xung ti mt gtr chp nhn c.


22/34

I n p u t

O u t p

I n p u t

I n p u t

I n p u t

O u t p

a

b

M U X

M U X

Hnh 2.10. a) Gi c kch thc nh b tr sau gi ln.

b) Gi c kch thc b v c nh trong ATM m bo tr nh.

Kt lun.

Trong mng bng rng, cc ng dng chnh c s dng l ting ni, tn hiuvideo, s liu gi. Li ch thu c v mt hiu sut truyn i vi cc gi c kchthc thay i nh hn rt nhiu so vi li ch thu c khi s dng cc gi c kchthc c nh nu so snh v mt tc hot ng v phc tp. Mt khc nu sdng t bo c kch thc thay i th di ca cc t bo ny khng th tu mcn rt b hn ch m bo tr nh. Do vo nm 1988 ITU-T chn gii php sdng t bo ATM c kch thc c nh.

2.1.7.2. 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 ca t bo s nh hng ti cc ch tiu sau:+ Hiu sut bng truyn.+ Tr (tr to gi, tr hng i, tr tho gi, bin ng tr).+ phc tp khi thc hin.


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Hiu sut bng truyn.Hiu sut bng truyn c quyt nh bi t l gia kch thc phn tiu

v kch thc trng d liu. Kch thc trng d liu cng ln th hiu sut cng

cao. ( trnh by 7.1.1).

Tr.+ Tr to gi: Ph thuc vo kch thc trng d liu trong t bo th hin hnhhiu sut truyn i vi cc t bo c di khc nhau (so snh 2 t bo c H=5 vH=4) v tr to gi ca chng (so snh gia 2 tc truyn ting ni 64Kb/s v32Kb/s).

2 9 0

1 0 0

T r ( m s )H i u s u t b n g

d i t r - n g s

D ( 3 2 K / b i t )

( % )

( H = 4 )4

( H + 5 )5


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Hnh 2.11. Hiu sut truyn v tr to gi i vi trng s liu c di khc.

+ Tr hng i: B nh hng bi t l gia ln ca trng s liu L v

ln trng tiu H.Hnh 2.12 th hin s ph thuc ca tr hng i v t l L/H. Ta nhn thy tr

b nht khi L/H c gi tr t 8 - 16, tng ng vi kch thc t bo t 32+4 byte ti64+4 byte.

T r h n g i ( s )

0

4 0

8 0

1 2 0

1 6 0

2 0 0

2 4 0

2 8 0

8

1 0 2 0 3 0

1 6 3 2 6 0 L / H

3 2 + 46 4 + 41 2 8 + 4

H i u s u0 . 8

0 . 8

0 . 7

0 . 7

0 . 6

Hnh 2.12 Tr hng i ph thuc vo t l L/H vi cc hiu sut ti khc nhau.

+ Tr tho gi: c quyt nh bi bin ng tr, l nguyn nhn ca tr tngca mt vi hng i. Tr tho gi cng b nh hng bi di t bo. Tr tonmng theo khuyn ngh Q.161 ca ITU-T cn phi c gii hn sao cho gi tr can nh hn 25 ms. Nu tng tr ln hn gi tr ny th cn phi lp thm b kh tingvang. Theo kt qu nghin cu ca ITU-T, di ca t bo c nh hng trc tipti tr:


25/34

i vi cc t bo c di tng i ngn (32 byte hoc nh hn) th tr tngrt nh, do trong hu ht cc trng hp u khng cn b kh ting vang.

i vi cc t bo c di ln (hn 64 byte) th tr tng ln ng k, do

lc ny s c hai gii php: lp b kh vang cho hu ht cc cuc thoi. ivi cc cuc thoi, ta ch in mt phn ca trng s liu gim tr, tuy v phng php ny lm gim hiu sut truyn.

i vi cc gi c di trung bnh trong khong 32 64 byte, phn ln cctrng hp ta u khng cn s dng b kh ting vang nu s nt chuynmch, s ln chuyn gia mng ATM v mng ng b, khong cch truynkhng qu ln.

phc tp khi thc hin. phc tp ca h thng ph thuc vo hai thng s c bn, l tc x l

v dung lng b nh cn thit. gii hn t l mt t bo, ta cn phi cung cp mhng i c kch thc ln. V vy kch thc t bo cng ln th kch thc hngi cng phi cng ln. Mt khc, khi c mt gi ti nt chuyn mch th phn tiu ca n cn phi c s l ngay trong khong thi gian mt t bo, do kch thc

t bo cng ln th thi gian dnh cho vic thc hin cng nhiu v tc yu cucng thp.

Tuy vy tc khng phi l vn quan trng nht, bi v cng ngh hinnay cho php x l rt nhiu thng tin trong khong 1s, nh vy vn chnh l giihn b nh.

Kt lun.Cc gi tr di kch thc gia 32 byte v 64 byte c a chung hn c.

S la chn ny ph thuc vo ba thng s chnh cp trn. Cui cng ITU-Tla chn gii php t bo ATM vi kch thc c nh c di 53 byte, trong phn trng d liu l 48 byte, phn tiu l 5 byte.


26/34

2.2. M HNH GIAO THC CHUN CA B-ISDN.

2.2.1. M hnh tham chiu chun ca B-ISDN.Da trn m hnh tham chiu lin kt cc h thng m OSI. M hnh ATM s

dng khi nim cc lp v cc mt phng ring r cho tng chc nng ring bit nhchc nng dnh cho ngi s dng, chc nng iu khin, chc nng qun l mng.Khi nim ny c gi l m hnh tham chiu giao thc B-ISDN (B-ISDN ProtocolReference Model hay BISDN-PRM).

Hnh 2.13 M hnh tham chiu giao thc B-ISDN.

M hnh tham chiu giao thc B- ISDN c cu trc phn lp t trn xung baogm cc chc nng truyn dn, chuyn mch, cc giao thc bo hiu v iu khincc ng dng v dch v. M hnh tham chiu giao thc B- ISDN bao gm 3 mt

Qu nlm

tph ng

i u

khi n

vbo

hi uS h

i u

CLNS

S hi u

CONS

Video

Tho

i

Lp cu to v tho t bo(SAR)

.... lp tng thch ATM(AAL)

lp con h i t

Lp ATMLp con t o v tho t bo(SAR)

..................................... l pvt l

Lp con ng truy n vt l(IM)

Mtphng

iu khin

Mt ph ng cangi s dng

Mt ph ngqun l

Qu nll p

- CLNS:s liukhng lin kt- CONS: sliu hng linkt


27/34

phng c trinh by nh trong hnh v l: Mt phng qun l mt phng cangi s dng v mt phng iu khin.

M hnh ny gm 3 mt phng: Mt phng qun l: C 2 chc nng:- Qun l lp (Layer Managerment) v Qun l mt phng (Plane

Managerment). Nhim v l to ra s phi hp gia cc mt phng khc vinhau. Qun l mt phng khng c cu trc phn lp, qun l lp c cc lpkhc nhau nhm thc hin cc chc nng qun l c lin quan ti cc tinguyn v thng s nm cc thc th c giao thc.

Mt phng ngi s dng:- C nhim v truyn cc thng tin ca ngi s dng t im A n im B

trn mng. Cc c ch nh iu khin lung, iu khin tc nghn, chng liu thc hin ti mt phng ny, mt phng ny cng c cu trc phn lp.

Mt phng iu khin:- C cu trc phn lp, vi chc nng thc hin cc iu khin ng ni

(Connection Control) v cuc gi (Call Control). Chng thc hin cc chcnng bo hiu c lin quan n vic thit lp, gim st v gii phng ng nihay cuc gi.

- Mt phng qun l v mt phng iu khin c th phn ra lm cc lp sau:

Lp bc cao: Tng ng vi 3 lp trn cng ca m hnh OSI.

Lp vt l: ng vi lp 1 (lp vt l) trong m hnh OSI.

Lp ATM: c th coi nh nm ti l di ca lp 2 (lp truyn d liu)trong m hnh OSI.

Lp AAL: c nhim v thc hin vic kt ni vi cc giao thc ca lp

cao hn.


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Theo ITU-T cc lp cn c th chia nh thnh cc phn lp vi cc chc nngnh sau:

- Lp vt l: tng t lp 1 ca m hnh OSI v ch yu thc hin cc chc

nng trn mc bit.- Lp kt ni s liu ATM c th c b tr ch yu ti bin di ca lp k

ni s liu thuc m hnh OSI.- Lp thch ng ATM thc hin vic thch ng cc giao thc lp trn cho t bo

ATM c nh.

2.2.2. Cc lp thp trong B-ISDN.2.2.2.1. Lp vt l (PL-Physical Layer).

Chu trch nhim truyn ti thc t cc gi ATM t mt im ti mt imkhc. N bao gm vic bin i cc tn hiu sang khun dng in hoc quang thchhp v np cc t bo ny thnh cc khung truyn dn thch hp. Trong PL c haigiao din c bn thc hin cc kt ni: Giao din ngi dng Mng (UNI) kni mi ngi s dng ti mt mng, v giao din Mng Mng (NNI) kt ni

nt mng trong mt mng. Giao din UNI c phn chia lm 2 loi: cng cng vring.

Nm 1993 Din n ATM qui nh bn giao din ca lp vt l, hai giaodin c cc tc s liu l 44,736 Mbit/s, hai giao din c cc tc s liu l155,52 Mbit/s. Trong s cc giao din th SONET (Synchrounous Optical Network) hoc SDH (Synchrounous Digital Hierachy) c th em li nhiu trin vngcho ATM.

Cc chc nng ca lp vt l:Lp vt l c phn chia thnh hai lp con:- Lp con 1: Mi trng vt l (PM-Physical Medium).- Lp con 2: Hi t truyn dn (TC-Transmission Convergence).

Chc nng chnh ca hai lp con ny nh sau:

Lp con vt l Cc chc nng1. Cung cp kh nng truyn dn bit, k c ng


29/34

1. Mi trng vt l (PM)chnh bit2. Thc hin s m ho ng dy v c s bin iin/quang

3. Cc chc nng nh thi bit (to ra v thu nhncc dng sng thch hp cho mi trng) cng xenvo v tch ra cc thng tin nh thi bit

2. Hi t truyn dn (TC)

1. To ra v phc hi khung truyn dn2. Thch ng khung truyn dn theo cu trc trngti truyn dn3. Chc nng m t t bo (cho php thit b thu

phc hi bin gii cc t bo nh s dng trn vkhi phc trn (descrambling)4. To trnh t HEC5. Kh ghp tc t bo (xen cc t bo trng theohng truyn dn nhm lm thch ng tc caATM

2.2.2.2. Lp ATM.Mt phn ca lp ATM ny tng ng vi lp 2 ca OSI (Lp lin kt s liu

DLL). Lp ATM chuyn ti tt c cc thng tin trong cc t bo ATM c di 53 byte.

Chc nng ca lp ATM. To ra/loi b phn tiu t bo:Chc nng ny l b xung tiu t bo

ATM (tr gi tr HEC) cho trng thng tin t bo thu c t AAL tronghng pht. Cc gi tr VPI/VCI c c l nh thng dch t phn t nhndng im truy nhp dch v SAP (Service Access Point). N thc hin ccchc nng ngc li, c ngha l hu b phn tiu trong hng thu. Ch ctrng thng tin t bo l c chuyn tip sang AAL.

Ghp knh v tch knh t bo:Chc nng ny ghp knh cho cc t bot cc VP v VC ring r thnh mt lung t bo trn vn theo hng pht. N


30/34

phn chia lung t bo i ti thnh cc dng t bo ring r ng vi VP hocVC theo hng thu.

Thng dch VPI v VCI:Chc nng ny c thc hin ti nt chuyn mch

ATM v/hoc cc nt kt ni cho. Ti chuyn mch VP, gi tr ca trngVPI ca mi t bo i ti c thng dch thnh mt gi tr VPI mi ca t boi ra. Cc gi tr ca VPI v VCI c dch thnh cc gi tr mi ti mtchuyn mch VC.

iu khin lung chung (GFC Generic Flow Control):Chc nng ny tr gip vic iu khin lung lu lng ATM trong mt mng khch hng. Nxc nh ti giao din gia khch hng B-ISDN vi mng (UNI).

2.2.3. Cc lp cao trong B-ISDN.

2.2.3.1. Lp thch ng ATM (AAL). ATM tr gip c nhiu th loi dch v, vi nhng c trng lu lng khc

nhau v nhng yu cu h thng khc nhau, vic lm thch ng cc lp ng dng khc

nhau vi lp ATM l rt cn thit. Chc nng c bn ca lp thch ng ATM (AAL)l ng gi cc phn s liu (ti 64Kbyte) ca cc giao thc lp cao hn thnh cc t bo (48 byte) ca lp ATM. Cc phn s liu ny c th l cc gi d liu, cc mum thanh hoc cc khung video. Nh vy, AAL phi ph thuc dch v trong vic phn chia ghp on cc d liu thnh cc t bo v t cc t bo. Cc chc nng ca lp thch ng ATM:Lp thch ng ATM c phn chiathnh hai lp con theo chc nng ca n. l lp con hi t v lp con phn onv ti ng gi.* Lp con hi t (CS-Convergence Sublayer)cung cp dch v AAL ti im truynhp dch v AAL SAP cho cc lp cao hn v n ph thuc vo dch v.* Lp con phn on v ti ng gi (SAR Segmentation and Reassembly)cnhim v phn on v ti sp xp cc thng tin ca lp cao hn thnh kch thc ph hp cho trng thng tin ca t bo ATM.


31/34

Cc chc nng ca lp con thch ng ATM c th hin qua bng sau:

Lp con thch ngATM

Cc chc nng

1. Lp con hi t1. ng cc gi khi d liu dch v ngi dngtrong mt tiu v phn ui (cc dch v c

yu cu).2. X l li v u tin d liu3. Nhn dng tin bo (message) v phc hi thigian xung nhp

2. Lp con phn onv ti ng gi

1. Thu nhn khi s liu ca giao thc hi t vt n trong t bo ATM

2. Thm tiu cho mi on, cha ng thng tinc s dng ti ng gi cc khc d liu tich.

2.2.3.2. Cc loi AAL.Cc loi thng tin c lp thch ng ATM (AAL) chuyn ti c phn chia

thnh 4 cp tu thuc vo cc c tnh v cc dch v nh sau:

1. Nu thng tin cn chuyn ti l c lp hoc ph thuc thi gian th nhtthit phi ti to s ph thuc thi gian ca tn hiu ti ch, c ngha l thoiPCM 64Kbit/s.2. Tc bit bin i hoc c nh.3. Chuyn ti thng tin kiu phi kt ni hoc nh tuyn kt ni.


32/34

Bng th hin bn cp dch v tiu chun m AAL s dng:

Bng phn loi dch v ca AAL:Cc lpdch v A B C DQuan h

nh thi Yu cu Khng yu cu

Cp

dchv

Mi quan h

nh thi giangun v ch

Tc bit Kiu kt ni Th d

CpA

Yu cu Khng i nh hng ktni

Video tc bitkhng i

CpB

Yu cu Bin i nh hng ktni

Video tc bit bin i

Cp

C

Khng yu cu Bin i nh hng kt

ni

Tc s liu nh

hng kt niCpD

Khng yu cu Bin i Phi kt ni Chuyn ti s liu phi kt ni


33/34

Tc bit Khng i Bin iPhng

thc kt ni nh hng kt ni Phi kt ni

Loi AAL 1 23 4

5

2.2.4. S cu hnh chun ca B-ISDN.Mng ATM/B- ISDN bao gm cc thit b khch hng, cc nt chuyn mch

v cc chng truyn dn ni thit b khch hng v nt chuyn mch. Khuyn nghca CCITT I. 431 nh ngha cc nhm chc nng B - TE1, B - TE2, B - NT1, B - NT2 v cc im tham chiu TB, SB, UB v R ca mng B- ISDN c miu t tronghnh v sau :

Hnh 2.18. Cu hnh giao thc chun ca B-ISDN

B - TE2hoc TE2 B - TA B - NT2 B - NT1

PC hocthit b Router

Lp AALLp ATM Lp vt l

S TR

(A)

B - TE1

Router

S

IWU

Router

S

B - NT2

Lp AALLp ATM

B - NT1

Lp vt l

T ULAN hocMAN ring

(B)


34/34

Cu hnh giao thc chun B- ISDN (c cu trc tng t nh cu hnh caISDN) xc nh cc giao din v cc chc nng khc nhau gia cc thc th ca mngB-NT1 thc hin cc chc nng ca cc lp bn di nh kt cui ng truyn, x

l giao din truyn dn lin quan n cc kt cui quang v in ti a im thit bkhch hng. B-NT1 c iu khin bi nh cung cp mng v l ranh gi gia ccmng, B-NT2 thc hin cc chc nng lp cao hn bao gm ghp / phn tch lulng, x l bng tn, chuyn mch cc kt ni ni b, x l giao thc bo hiu, x ldung lng b m v xc nh ti nguyn. Cc thit b u cui l thit b khchhng s dng B-ISDN.

B - TE1 kt cui vi giao din chun B - ISDN v thc hin kt cui i vi ttc cc loi giao thc ca cc bc thp cng nh bc cao. Cc giao din khc vn chac ITUchun ho y . B -TE2 c dng cho cc giao din kt ni vi thit bchun B - ISDN hin c. Cc thit b ny cn c thit b chuyn i (TA) kt nivi giao din. TA thc hin tt c cc chc nng cn thit thc hin giao tip B ISDN k c vic chuyn i tc . im chun R cung cp giao din khng phi BISDN gia thit b khch hng, khng phi B - ISDN vi thit b chuyn i. imchun TB phn tch thit b nh cung cp mng vi thit b khch hng, im chun SBtng ng vi gaio din ca cc thit b B-ISDN ring r v tch bit thit b khchhng vi cc chc nng truyn thng lin quan ti mng.

hung-cong nghe atm

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