Tiu lun chuyn mch ATM

Tiu lun chuyn mch ATM

I HC QUC GIA TP. H CH MINHTRNG H CNG NGH THNG TIN-----(((((-----

TIU LUN K thut chuyn mch ATM

Gio vin hng dn: Th.S Chu Tin Dng

Nhm SV thc hin: 1. Dng nh Huy

2. L Trn Anh Tn

3. Hunh Hiu Trung Lp: K2013NT

Nha Trang, 09/20153LI NI U

4I. GII THIU V K THUT CHUYN MCH

1. Qu trnh pht trin ca k thut chuyn mch:....552. Mt s khi nim c bn:

51.1. nh ngha chuyn mch

61.2. H thng chuyn mch

61.3 Phn loi chuyn mch

71.4. Cc thnh phn trong mng vin thng

8II. CC K THUT CHUYN MCH C BN

81. Chuyn mch knh

1.1. Gii thiu v chuyn mch knh91.2. nh ngha trng chuyn mch101.3. Phn loi chuyn mch knh10102. Chuyn mch gi

112.1. Gii thiu v chuyn mch gi

132.2. nh ngha chuyn mch gi

132.3. C s k thut chuyn mch gi

142.4. Nguyn tc c bn ca chuyn mch gi

16III. K THUT CHUYN MCH ATM

161. Gii thiu chung v ATM

161.1 S ra i ca ATM

161.2. Khi nim ATM

161.3. t im ca cng ngh ATM

172. Cu trc phn lp ca mng ATM

172.1. Cu trc t bo ATM:

2.2 Cc loi t bo ATM:222.3 M hnh tham chiu B-ISDN:23262.4 Lp vt l

282.5 Lp ATM

2.6 Lp thch ng ATM (AAL)31KT LUN34

LI NI U

Cng vi s pht trin ca cng ngh tin tin l xu hng hi t mng truyn thng gia mng c nh v mng di ng v internet sang mng th h k tip NGN (Next Generation Network). H tng mng thay i khng ngng nhm p ng cc nhu cu ngy cng cao ca ngi s dng, s tc ng ny lin quan nh hng ti rt nhiu lnh vc trong vin thng trong bao gm c k thut chuyn mch. K thut chuyn mch l mt trong nhng k thut nn tng ca cc mng truyn thng. Chuyn mch ATM l phng thc truyn ti khng ng b, kt hp chuyn mch knh, chuyn mch gi, v phng php ghp TDM thng k Tiu lun ny, chng em nghin cu v cng ngh chuyn mch ATM.Trong qu trnh lm tiu lun, em xin chn thnh cm n s hng dn tn tnh ca thy Chu Tin Dng, trong qu trnh tm hiu khng trnh c s thiu st, rt mong nhn c s gp ca thy v cc bn.

I. GII THIU V K THUT CHUYN MCH

1. Qu trnh pht trin ca k thut chuyn mch:

Chuyn mch l qu trnh thc hin u ni tuyn lin lc gia hai thu bao (in thoi, my tnh, fax.) thng qua mt hay nhiu h thng. H thng c gi l chuyn mch. Khi nim chuyn mch thoi c ngay t khi pht minh ra my in thoi vo nm 1786, vo thi gian qu trnh thit lp tuyn ni c thc hin nh in thoi vin v ban u ni, hnh thc chuyn mch ny cn c gi l chuyn mch nhn cng. Cng vi s pht trin cc ngnh cng nghip, tng i cng tng bc c ci tin v hon thin, t tng i nhn cng ln tng i t ng s dng c in, tng i in t v tng i in t SPC, tng i bng rng vv

Tng i nhn cng u tin c a vo khai thc ti thnh ph NewHeivene bang Conneckticut (USA) vo nm 1878 ch sau 2 nm sng ch ra my in thoi ca A.G Bell. T n nay, mng in thoi pht trin ht sc mnh theo nhu cu thng tin lin lc in thoi. Do vy rt nhanh chng tng i nhn cng t ti gii hn kh nng ca n v tng t ng ha c anh em A.B.Strowger thc y Tng i t ng do A.B.Strowger sang ch c tn gi l tng i c in h tng nc (th h 1) c a vo s dng nm 1892 trn c s b tm chn tng nc c anh em A.B.Strowger sang ch nm 1889. Tip nhm nng cao cht lng v kinh t, tng i R le (t my th h 2), tng i ngang dc iu khin trc tip c sang ch nm 1926 v vo nm 1938 tng i Crossbar-No1 vi phng php iu khin ghi pht l tng i th h 3. Nhng tin b v thnh tu trong cng ngh in t v my tnh thc y tng ng dng vo lnh vc tng i in thoi. Qu trnh chuyn i t chuyn mch in c sang chuyn mch in t (th h 4), c bit l tng i s c c trng bi vic to ra h thng thng nht chuyn mch v truyn dn thng tin. Vo khong thp nin 60 ca th k 20, xut hin sn phm tng i in t s l s kt hp gia cng ngh in t vi k thut my tnh. Tng i in t s cng cng u tin ra i c iu khin theo chng trnh ghi sn SPC(Stored Program Control), c gii thiu ti bang Succasunna,Newjersey, USA vo thng 5 nm 1965. Trong nhng nm 70 hng lot cc tng i thng mi in t s ra i.

Mt trong nhng tng i l tng i E10 ca CIT-Alcatel c s dng ti Lannion (France). V thng 1 nm 1976 Bell gii thiu tng i in t s cng cng 4ESS. Hu ht cho n giai on ny cc tng i in t s u s dng h thng chuyn mch l s v cc mch giao tip thu bao thng l analog, cc ng trung k l s. Mt trng hp ngoi l l tng i DMS100 ca Northern Telecom a vo nm 1980 dng ton b k thut s u tin trn th gii. H thng 5ESS ca hang AT &T c a vo nm 1982 ci tin rt nhiu t h thng chuyn mch 4ESS v c cc chc nng tng thch vi cc dch v ISDN. Sau hu ht cc h thng chuyn mch s u a ra cu hnh h tr cho cc dch v mi nh ISDN, dch v cho mng thng minh, v cc tnh nng mi tng thch vi s pht trin ca mng li. Vo nhng nm 1996 khi mng Internet tr thnh bng n trong th gii cng ngh thng tin, n tc ng mnh m n cng nghip vin thng v xu hng hi t cc mng my tnh, truyn thng, iu khin, vin thng tr thnh mt bi ton cn gii quyt. Cng ngh vin thng ang bin i theo hng tt c cc loi hnh dch v hnh nh m thanh, thoi s c tch hp v chuyn mch qua cc h thng chuyn mch. Mt mng c th truyn bng rng vi cc loi hnh dch v thoi v phi thoi, tc cao v m bo c cht lng phc v(QoS) thnh cp thit trn nn tng ca mt k thut mi: K thut truyn ti khng ng b ATM v trn l cc ng dng cho thoi v phi thoi. Cc h thng chuyn mch in t s cng phi dn thay i theo hng ny cc tng i chuyn mch bng rng ra i. Hin nay rt nhiu cc cu kin v thit b chuyn mch quang c nghin cu, pht trin v c trin khai mt s nc trong tng lai khng xa cc h thng chuyn mch quang bng rng s thay th cho h thng chuyn mch hin ti cung cp cc chuyn mch tc cao v rng bng ln.

S khc bit ny bt u t nhng nm 1980, PSTN chuyn hng tip cn phng thc truyn ti bt ng b ATM h tr a phng tin v QoS, sau chuyn hng sang cng ngh kt hp vi IP chuyn mch nhn a giao thc hin nay. Trong khi Internet a ra mt tip cn hi khc so vi PSTN qua gii php trin khai kin trc phn lp dch v CoS (class of service) v hng ti m bo cht lng dch v QoS thng qua m hnh tch hp dch v IntServ v phn bit dch v DiffServ, cc chin lc ca Internet theo hng tng thch vi IP, mng quang v hng ti mng chuyn mch nhn a giao thc tng qut GMPLS (generalized multiprotocol label switch). Cng ngh chuyn mch nhn a giao thc MPLS ra i vo nm 2001 l s n lc kt hp hai phng thc chuyn mch hng kt ni (ATM,ER) vi cng ngh chuyn mch phi kt ni (IP), cng ngh chuyn mch nhn a giao thc MPLS nh ngha khi nim nhn(Label) nm trn mt lp gia lp 2 v 3 trong m hnh OSI, vi mc tiu tn dng ti a cc u im ca chuyn mch phn cng (ATM, ER) s mm do, linh hot ca cc phng php nh tuyn trong IP. Mt s quc gia c h tng truyn ti cp quang pht trin tt c xu hng s dng cc k thut chuyn mch quang v s dng cc cng ngh trn nn quang nh GMPLS, IP qua cng ngh ghp bc song quang WDM (wavelength division multiplexing), kin trc chuyn mch trong mng th h k tip NGN.2. Mt s khi nim c bn:

1.1. nh ngha chuyn mch

Chuyn mch l mt qu trnh thc hin u ni v chuyn thng tin cho ngi s dng thng qua h tng mng vin thng. Chuyn mch trong mng vin thng bao gm chc nng nh tuyn cho thng tin v chc nng chuyn tip thng tin. Nh vy theo kha cnh thng thng khi nim chuyn mch gn lin vi lp mng v lp lin kt d liu trong m hnh OSI ca t chc tiu chun quc t ISO. 1.2. H thng chuyn mch

Qu trnh chuyn mch c thc hin ti cc nt chuyn mch, trong mng chuyn mch knh thng gi l h thng chuyn mch (tng i) trong mng chuyn mch gi thng c gi l thit b nh tuyn (b nh tuyn).

1.3 Phn loi chuyn mch

Xt v mt cng ngh, chuyn mch chia thnh hai loi c bn: chuyn mch knh v chuyn mch gi. Mt khc, chuyn mch cn c chia thnh bn kiu: chuyn mch knh, chuyn mch gi v chuyn mch t bo

Cc khi nim c s v cng ngh chuyn mch c thc hin trong hnh 1.1 (a, b, c) di y.

Hnh 1.1.a) Chuyn mch knh: hai dng thng tin trn hai mch khc nhau

Hnh 1.1 b) Chuyn mch gi: cc tuyn ng c lp trn mng chia s ti nguyn

Hnh 1.1 c) Chun mch gi knh o: cc gi tin i trn knh oMng chuyn mch knh thit lp cc mch (knh) ch nh ring cho kt ni trc khi qu trnh truyn thng thc hin. Nh vy, qu trnh chuyn mch c chia thnh 3 giai on phn bit: thit lp, truyn v gii phng. thit lp, gii phng v iu khin kt ni (cuc gi) mng chuyn mch knh s dng cc k thut bo hiu thc hin. i ngc vi mng chuyn mch knh l mng chuyn mch gi, chia cc lu lng d liu thnh cc gi v truyn i trn mng chia s. Cc giai on thit lp, truyn v gii phng s c thc hin ng thi trong mt khong thi gian v thng c quyt nh bi tiu gi tin.

1.4. Cc thnh phn trong mng vin thng

L mt tp hp bao gm cc nt mng v cc ng truyn dn kt ni gia hai hay nhiu im xc nh thc hin cc cuc trao i thng tin gia chng. Mng vin thng cung cp a dng cc loi hnh dch v vin thng cho khch hng, t dch v truyn thng nh in thoi, Fax, truyn s liu cho n cc dch v mi nh Internet, VOD, thng mi in t..

Hnh 1.2. Cc thnh phn ca mng vin thng

Thit b u cui l cc trang b ca ngi s dng giao tip vi mng cung cp dch v. Hin nay c nhiu chng loi thit b u cui ca nhiu hng khc nhau ty thuc tng dch v (v d nh my in thoi, my fax.). Thit b u cui thc hin chc nng chuyn i thng tin cn trao i thnh cc tn hiu in v ngc li.

H thng chuyn mch l thnh phn ct li ca mng vin thng c chc nng thit lp ng truyn gia cc thu bao. Ty theo v tr ca h thng chuyn mch trn mng ngi ta chia thnh tng i chuyn tip quc t, tng i chuyn tip lin tnh, tng i ni ht

Thit b truyn dn c s dng ni cc thit b u cui hay gia cc tng i vi nhau v truyn cc tn hiu mt cch nhanh chng v chnh xc. Thit b truyn dn c phn loi thnh thit b truyn dn thu bao v thit b truyn dn chuyn tip, ni gia cc tng i.II. CC K THUT CHUYN MCH C BN

1. Chuyn mch knh

1.1. Gii thiu v chuyn mch knhy l phng php c s dng t lu trong mng in thoi PSTN. Ngy nay phng php ny vn c s dng trong mng ISDN. N s dng phng php ghp knh theo thi gian TDM. Trong thng tin trn 1 knh c truyn theo 1 chu k u n 125 Us 1 khe thi gian c nh, tp hp cc khe thi gian trong khong 125Us to thnh 1 khung thi gian. Knh truyn trong mng chuyn mch knh l knh thc c thit lp trc khi yu cu thit lp cuc gi trong mng. Do phng php nythiu stinhs mm do do thng tin phi truyn theo 1 tn s c nh dn ti gii hn v mt tc v khng thch hp cho vic truyn cc dch v bng rng c c im khc nhau.

Hnh 2.1 Chuyn mch knh

khc phc s thiu mm do ca ch truyn n tc trong chuyn mch knh ngi ta a ra h thng chuyn mch knh a tc MRCS (maltirate circuit swiching). Cc ng ni trong MRCS c chia thnh n knh c bn gm khung thi gian c di khc nhau, mi cuc lin lc c th xy dng t n knh ny. Thng thng cc knh c bn cho 1 cuc ni l:

+ Mt knh c tc l 1024Kbit/s

+ 8 knh H1 c tc l 2048Kbit/s

+ Mt knh H4 c tc l 139.

MRCS rt phc tp do mi knh c s ca 1 ng ni phi gi ng b vi cc knh khc nhau m bo tnh trong sut v mt thi gian. Ngoi vic s dng ti nguyn chung ca MRCS khng t hiu qu: khi mi knh H1 bn th khng th thip lp thm 1 knh no khc trong khi c th H4 vn ri. Do vy y cha phi l gii php cho mng bng rng.

Cc ti nguyn trong h thng chuyn mch knh tc cao FCS(fast circuit swiching) ch c cung cp khi thng tin c gi i. Sau khi gi xong thng tin ti nguyn c gii phng tr li. S cung cp ny c thit lp mi ln gi nh trong trng hp chuyn mch gi nhng di s iu khin ca tn hiu bo hiu lin kt nhanh (fast associated signaling) ch khng nm trong chuyn mch gi.

Khi thit lp cuc gi ngi s dng yu cu rng ca bng tn bng s nguyn ln rng bng ca knh c bn. H thng lc ny cha cung cp ti nguyn ngay m n ghi li cc thng tin v chuyn mch, thng tin v rng bng theo yu cu, thng tin v a ch ca ch c chn. Khi bn pht bt u gi thng tin lc ny h thng bo hiu rng bn pht c thng tin c gi i yu cu chuyn mch phn phi ti nguyn ngay lp tc. Qua y c th thy FCS kh phc tp v khng thch hp cho B-ISDN v kh nng thit lp, hy b cuc ni v iu khin c h thng rt phc tp, khng p ng c yu cu v mt thi gian.

1.2. nh ngha trng chuyn mch

Chuyn mch knh c hiu l k thut chuyn mch m bo vic thit lp cc ng truyn dn dnh ring cho vic truyn tin ca mt qu trnh thng tin gia hai hay nhiu thu bao khc nhau

.Chuyn mch knh c ng dng cho vic lin lc mt cch tc thi m qu trnh chuyn mch c thc hin mt cch khng to cm gic v s chm tr (tnh thi gian thc).

Chuyn mch knh tn hiu s l qu trnh kt ni, trao i thng tin cc khe thi gian gia mt s on ca tuyn truyn dn TDM s.1.3. Phn loi chuyn mch knh

Da vo dng tn hiu i qua trng chuyn:

+ Chuyn mch tong t

+ Chuyn mch s.* Nhc im ca chuyn mch tng t Trng chuyn mch c cu trc phn tng, nhiu tng nn qu trnh khai thc s c tn tht ln, ng thi vic khc phc nh hng ln nhau gia cc phn t chuyn mch rt kh khn.

Tn hiu i qua trng chuyn mch b suy hao nhiu v suy hao ny rt khc nhau i vi cc tip im chuyn mch khc nhau.

Khng xc nh c tr thi gian cho tn hiu trong knh thoi khi qua trng chuyn mch.

Gy tp m ln

Trng chuyn mch cng knh v c gi thnh cao.

Cc nhc im ny s c khc phc khi s dng trng chuyn mch s.

2. Chuyn mch gi

2.1. Gii thiu v chuyn mch gi

K thut chuyn mch gi ngy nay tr thnh mt k thut rt c tim nng v quan trng trong lnh vc vin thng bi v n cho php cc ngun ti nguyn vin thng s dng mt cch hiu qu nht. Chuyn mch gi c th thch ng vi rt nhiu kiu loi dch v v yu cu ca ngi s dng. Trn th gii ngy nay, mng chuyn mch gi cng ang c pht trin rt mnh m v s dng ch yu cho cc dch v truyn thng s liu gia cc my tnh. Tuy vy chuyn mch gi cng ang th hin hiu qu v tnh hp dn ca n cho cc dch v vin thng khc nh in thoi, Video v cc dch v bng rng khc.

Hnh 2.2: Nguyn l phn on v to giNguyn l ca chuyn mch gi l da trn kh nng ca cc my tnh tc cao v cc cch thc tc ng vo bn tin cn truyn sao cho c th phn on cc cuc gi, cc bn tin hoc cc giao dch (Transaction) thnh cc thnh phn nh gi l Gi tin. Tu thuc vo vic thc hin v hnh thc ca thng tin m c th c nhiu mc phn chia. V d mt cch thc hin ph bin c p dng ca chuyn mch gi hin nay l bn tin ca Ngi s dng c chia thnh cc Segment (on) v sau cc Segment li c chia tip thnh cc gi (Packet) c kch thc chun ho. Cc Segment sau khi c chia t Bn tin ca ngi s dng s c x l chun ho tip bng cch dn u (Leader) v ui (Trailer), nh vy chng cha ba trng s liu l: u cha a ch ch cng cc thng tin iu khin m mng yu cu v d nh s th t ca Segment #, m knh Logic tch cc thng tin ngi s dng ghp knh, nh du Segment u tin v Segment cui cng ca bn tin v nhiu thng tin khc lin quan ti chc nng qun l v iu khin t u cui-ti-u cui.

i vi cc gi tin truyn qua mng chuyn mch gi cn phi cha cc mu to khung nh du im u v im cui ca mi gi. Tiu (Header) ca gi tng t nh u ca Segment, ngoi ra n cn c thm cc thng tin m mng yu cu iu khin s truyn ti cu cc gi qua mng, v d nh thng tin cn b sung vo tiu ca gi l a ch ngun, a ch ch, s th t ca gi v cc khi s liu iu khin chng vng lp, qun l QoS, suy hao, lp gi v.v.

Trng kim sot li CRC cho php h thng chuyn mch gi pht hin sai li xy ra trong gi nu c, nh m bo yu cu rt cao v chnh xc truyn tin.

Tng s tin cha trong cc trng s liu u ca Segment v Tiu ca Gi l rt quan trng. Thng thng cc trng s liu ny c khong t 64 n 256 bit trong tng s N khong 1000 bit.

Hnh 2.3: Mng chuyn mch giCc gi tin s c chuyn qua mng chuyn mch gi t nt chuyn mch ny ti nt chuyn mch khc trn c s Lu m v pht chuyn tip, ngha l mi nt chuyn mch sau khi thu mt gi s tm thi lu gi mt bn sao ca gi vo b nh m cho ti khi c hi pht chuyn tip gi ti nt tip theo hay thit b u cui ca ngi s dng c m bo chc chn. Bi v mi qu trnh thng tin c ct nh thnh cc gi ging nhau nn cc bn tin d di hay ngn u c th chuyn qua mng vi s nh hng ln nhau t nht v nh s chuyn ti cc gi qua mng c thc hin trong thi gian thc nn chuyn mch c th p ng c yu cu hot ng mt cch nhanh chng k c khi c s thay i mu lu lng hoc khi c s c mt thnh phn khc ca mng.

Hnh 2.3 minh ho nguyn tc hot ng ca mng chuyn mch gi. Cc bn tin ca ngi s dng t thit b u cui pht A s khng c gi i mt cch tc th v trn vn qua mng ti thit b u cui thu B nh trong mng chuyn mch bn tin, m s c ct v to thnh cc gi chun nt chuyn mch gi ngun PSWS. Mi gi s c pht vo mng mt cch ring r c lp v chng s dch chuyn v nt chuyn mch gi ch PSW theo mt ng dn kh dng tt nht ti bt k thi im no, ng thi mi gi s c kim tra gim st li trn dc ng i.

2.2. nh ngha chuyn mch gi

L mt loi k thut gi d liu t my tnh ngun n my tnh ch qua mng dng mt loi giao thc tha mn 3 iu kin sau:

+ D liu cn vn chuyn chia nh ra thnh cc gi (hay khung) c kch thc (size) v nh dng (format) xc nh.

+ Mi gi s c chuyn ring r v c th n ni nhn bng cc ng truyn (router) khc nhau. Nh vy chng c th dch chuyn trong vng thi gian.

+ Khi ton b cc gi d liu n ni nhn th chng s c hp li thnh d liu ban u.

+ c im chnh ca chuyn mch gi l s dng phng php kt hp tuyn truyn dn theo yu cu. Mi gi c truyn thng tin i ngay sau khi ng thng tin tng ng ri. Nhu vy cc ng truyn dn c th phi hp s dng mt s ln cc ngun tng i t hot ng.

2.3. C s k thut chuyn mch gi

K thut chuyn mch gi da trn nguyn tc chuyn i thng tin qua mng di dng gi. Gi tin l thc th truyn thng hon chnh gm 2 phn: tiu mang cc thng tin iu khin ca mng hoc ngi s dng v ti tin l d liu thc cn chuyn qua mng. Qa trnh chuyn thng tin qua mng chuyn mch gi c coi l mng chia s ti nguyn. Cc gi tin s chuyn giao t cc nt mng ny ti cc nt mng khc trong mng chuyn mch gi theo nguyn tc lu m v chuyn tip, nn mng chuyn mch gi cn c coi l mng giao trong khi mng chuyn mch knh c coi l mng trong sut i vi d liu ngi s dng.

Trn hng tip cn tng i n gin t kha cnh dch v cung cp, cc dch v c cung cp trn mng vin thng chia thnh dch v thoi v dch v phi thoi, trong i din cho dch v phi thoi l dch v s liu. Chng ta hiu rng, s ha v gi ha thoi l hai vn hon ton khc nhau. Trong mng chuyn mch in thoi cng cng PSTN hin nay tn hiu thoi c s ha, v k thut chuyn mch truyn thng c p dng l k thut chuyn mch knh. D liu thoi ch c gi l gi ha nu nhng gi ny c chuyn ti trn mng chuyn mch gi. Trong mc ny chng ta s xt nhng vn k thut chuyn mch c bn c ng dng trong h thng chuyn mch: K thut chuyn mch knh v k thut chuyn mch gi. Mng in thoi cng cng c pht trin trn mng chuyn mch knh cung cp cc dch v thoi truyn thng. Cc mng d liu nh cc mng cc b LAN, mng Internet l mng chuyn mch gi rt thch hp trao i d liu.

Bng 2.1 So snh mt s c im ca dch v thoi v d liu

c imDch v thoiDch v d liu

Bng thng

Bng pht bng thng

Nhy cm vi li

Pht li thng tin

tr

Kiu kt niC nh v thp(di 4kb)

Khng

Thoi li nu c li

Khng th thc hin c

Thp v n nh

Hng kt niThay i

Ln(100/1000:1)

Khng cho php li

Thc hin d dng

Ln v c th thay i

C th l phi kt ni

Cc dch v thoi trong mng PSTN hin nay s dng k thut iu ch PCM v chim bng thng ln hn cho mi cuc gi th cht lng cuc gi thoi cng khng v th m tt hn. Tri li i vi cc dch v d liu bng thng rt quan trng.Mt s ng dng i hi bng thng ti 1Gb/s hoc cao hn.

2.4. Nguyn tc c bn ca chuyn mch gi

Unicast v multicast: cc kt ni trong mng vin thng thng l cc kt ni im ti im. V vy lu lng a vo mt u vo ca ma trn chuyn mch ch yu cu duy nht mt u ra. Tuy nhin cc ng dng nh video/audio hi ngh hay qung b d liu li yu cu nhiu u ra ng thi. h tr chuyn mch a hng h thng chuyn mch c b sung thm c ch sao cho php d liu gia cc u vo v u ra.

thng qua v tc (speedup) : thng qua ca trng chuyn mch c nh ngha nh l t s ca tc u ra trung bnh trn tc u vo trung bnh khi tt c cc u c lu lng chim 100% ti tc ng dy. thng qua ca trng chuyn mch l dng v nh hn bng 1.

Nghn v tranh chp u ra: vn tc nghn v tranh chp u ra thng xy ra ti cc trng chuyn mch khng gian, i vi trng chuyn mch thi gian vn tc nghn c x l ti b ghp knh. Hin tng tc nghn c ng cc xung t khng t gii quyt ti cc v tr khc nhau ca trng chuyn mch. Tranh chp u ra xy ra khi c nhiu hn mt yu cu u vo cng mun kt ni ti u ra cng mt thi im .

c tnh kt ni: mt quy trnh kt ni gm 3 giai on tch bit: thit lp kt ni, truyn thng tin v gii phng kt ni

III. K THUT CHUYN MCH ATM

1. Gii thiu chung v ATM

1.1 S ra i ca ATM

ATM phng thc truyn ti khng ng b, cung cp cc dch v bng rng tng lai.

ATM ln u tin dc nghin cu ti trung tm nghin cu CNET (ca france telecom) v Bell Lads vo nm 1983, sau tip tc pht trin ti trung tm nghin cu Allatebell t nm 1984. Cc trung tm ny tch cc nghin cu nhng nguyn l c bn v gp tch cc trong cng vic thit lp cc tiu chun u tin v ATM.

Hin nay cng ngh ATM pht trin ti kh hon ho v n nh. Cng ngh ny c nghin cu v trin khai ti nhiu nc trn th gii. Nhiu mng ATM c trin khai, bc u cung cp dch v bng rng vi khch hng. Vic ng dng cng ngh ATM vo mng vin thng c bt u vo nm 1990.

ATM l s kt hp ca cng ngh truyn dn v cng ngh chuyn mch qua mng giao tip chun, da vo cng ngh ATM phn chia v ghp ting ni, s liu, hnh nh. Vo trong mt khi c chiu di c nh gi l t bo.

1.2. Khi nim ATM

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

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

1.3. t im ca cng ngh ATM

Cng ngh ATM da trn c s ca phng php chuyn mch gi, thng tin c nhm vo cc gi tin c chiu di c nh, ngn; trong v tr ca gi khng ph thuc vo ng h ng b v da trn nhu cu bt k ca knh cho trc. Cc chuyn mch ATM cho php hot ng vi nhiu tc v dch v khc nhau.

ATM 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, cc t bo nh vi tc truyn ln s lm cho tr truyn v bin ng tr gim nh i vi cc dch v thi gian thc, cng s to iu kin cho vic hp knh tc cao c d dng hn.

- Th hai, ATM c kh nng nhm mt vi knh o thnh mt ng o nhm gip cho vic nh tuyn c d dng.ATM khc vi nh tuyn IP mt s im. N l cng ngh chuyn mch hng kt ni. Kt ni t im u n im cui phi c thit lp trc khi thng tin c gi i. ATM yu cu kt ni phi c thit lp bng nhn cng hoc thit lp mt cch t ng thng qua bo hiu. Mt khc, ATM khng thc hin nh tuyn ti cc nt trung gian. Tuyn kt ni xuyn sut c xc nh trc khi trao i d liu v c gi c nh trong sut thi gian kt ni. Trong qu trnh thit lp kt ni, cc tng i ATM trung gian cung cp cho kt ni mt nhn. Vic ny thc hin hai iu: dnh cho kt ni mt s ti nguyn v xy dng bng chuyn t bo ti mi tng i. Bng chuyn t bo ny c tnh cc b v ch cha thng tin v cc kt ni ang hot ng i qua tng i.

Bng so snh cng ngh IP v ATMCng nghIPATM

Bn cht Cng ngh- L mt giao thc chuyn mch gi c tin cy v kh nng m rng cao

- Do phng thc nh

tuyn theo tng chng nn

iu khin lu lng rt

kh thc hin.- S dng gi tin c

chiu di c nh 53

byte gi l t bo

(cell).

- Nguyn tc nh tuyn chuyn i VPI/VCI

- Nn tng phn cng

tc cao

u im- n gin, hiu qu-Tc chuyn mch cao, mm do h tr QoS theo yu cu

Nhc im-Khng h tr QoS- Gi thnh cao, khng

mm do trong h tr nhng ng dng IP v VoA

2. Cu trc phn lp ca mng ATM

2.1. Cu trc t bo ATM:

Ta bit rng, c im ca ATM l hng lin kt. Do khc vi mng chuyn mch gi, a ch ngun v ch, s th t gi l khng cn thit trong ATM. Hn th, do cht lng ca ng truyn rt tt nn cc c ch chng li trn c s t lin kt ti lin kt cng c b qua. Ngoi ra ATM cng khng 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 trong t bo ATM l nhn dng cuc ni o.Nh xem xt la chn t bo ATM th t bo ATM l t bo c nh, c 53 Bytes: 5 Bytes tiu v 48 Bytes d liu.

ATM cell c cu trc ging nhau cho bt k loi dch v no.

Hnh 3.1 Cu trc cell

- Header: 5 Octet (5 bytes). Thng tin cha trong Header gip cho vic tm ng ca cc ATM cell qua mng. Do mng ATM hot ng theo cch kt ni c hng nn cc cell ch c th lun chuyn qua cc vng m cc kt ni tn ti. Lu : Cc cell Header khng dng khi to bt tay trong cc kt ni.

- Payload: 48 Octet (48 bytes). Cha Data ca ngi s dng, v cc tn hiu iu khin tng ng. Sau khi pht xong cc cell, bn phn thu s t chc li cc lung Data, gi Data cho ging nh ban u.

Phn tiu ca t bo ATM c hai dng: Mt dng l cc t bo c truyn trn giao din gia ngi s dng v mng UNI, dng cn li l cc t bo c truyn gia cc nt chuyn mch NNI.87654321Bit

VPI1 Octet

VPIVCI2

VCI3

VCIPTCLP4

HEC5

Phn d liu (48 octet)

Hnh 3.2 Cu trc t bo ATM ti giao din NNI87654321Bit

GFCVPI1 Octet

VPIVCI2

VCI3

VCIPTCLP4

HEC5

Phn d liu (48 octet)

Hnh 3.3 Cu trc t bo ATM ti giao din UNI c im ca cc trng trong cu trc t bo:

S hiu nhn dng knh o VCI (Virtual Channel Identifier) v ng o VPI (Virtual Path Identifier)

Hnh 3.4 knh o v ng o

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 ng thi trn ng truyn dn. Thng thng trn mt knh c truyn ng thi 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).

Do mng ATM c c im hng lin kt nn mi cuc ni c gn mt s hiu nhn dng VCI ti thi im thit lp. Mi gi tr VCI ch c ngha ti tng lin kt t nt ti nt ca mng. Khi cuc ni kt thc, VCI c gii phng dng cho cuc ni khc. Ngoi ra VCI cn c u im trong vic s dng cho cc cuc ni a dch v nh dch v in thoi truyn hnh, m thanh v hnh nh s c truyn trn hai knh c VCI ring bit, do ta c th b sung hoc hy 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 cuc ni knh o VCC. VPI cho php n gin ha cc th tc chn tuyn cng nh qun l, n c di 8 bit hoc 12 bit ty thuc t bo ATM ang c truyn qua giao din UNI hay NNI.

T hp c VCI v VPI to thnh mt gi tr duy nht cho mi cuc ni. Ty thuc vo v tr i vi hai im cui m nt chuyn mch ATM s nh ng da trn gi tr ca VPI v VCI hay ch da trn gi tr VPI. Tuy vy cn lu rng VCI v VPI ch c ngha trn tng chng lin kt ca cuc ni. Chng uc s dng vic chn ng trn cc chng ny c d dng hn. Do s VPI v VCI qu nh nn chng khng th uc s dng nh mt s hiu nhn dng ton cc v kh nng xy ra hai cuc ni s dng ngu nhin cng mt s VPI v VCI l rt cao. khc phc, ngi ta cho 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 cho mi cuc ni trn on . Khi qua nt chuyn mch, VPI v VCI nhn cc gi tr mi ph hp vi on tip theo. Trng kiu t bo PT (Payload Type): PT l mt trng gm 3 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 gim st, vn hnh, bo dng (OAM).

Nu bit u PT c gi tr = 0 th y l t bo ngi s dng. Trong t bo ngi s dng, bit s 2 bo hiu tc nghn trong mng v bit 3 c chc nng bo hiu cho lp tng thch ATM l AAL.

Nu bit u PT ch gi tr = 1 th y l t bo bo mang cc thng tin qun l mng.

Cu trc trng PT trong t bo mang thng tin OAMDng bitChc nng

100T bo OAM lp F5 lin quan ti lin kt

101T bo OAM lp F5 lin quan ti u cui

110T bo qun l ti nguyn

111Dnh cho vic s dng trong tng lai

Trng ch mc u tin mt t bo CPL (Cell Loss Priority): CLP l trng dng phn loi cc cuc ni khc nhau theo mc u tin khi cc ti nguyn trong mng khng cn l ti u na. Th d trong trng hp qu ti, ch c cc cuc ni c mc u tin thp l b mt thng tin. C hai loi u tin khc nhau l u tin v mt ni dung v u tin v mt thi gian. Trong ch u tin v mt thi gian, vi t bo c th c tr trong mng di hn cc t bo khc. Trong ch u tin v mt ni dung, cc t bo c u tin cao hn s c xc sut mt gi t hn. Cc mc u tin c th c n nh trn c s cuc ni (qua mi VCI hoc VPI) hoc trn c s mi t bo. Trong trng hp th nht, tt c 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 v mt knh o hoc ng o s c cc mc u tin khc nhau.

+ Nu CLP = 0 : u tin cao

+ Nu CLP = 1: u tin thp Trng iu khin li tiu HEC (Header Error Control)

Trng iu khin li tiu HEC cha m d vng CRC (Cyclic Redundancy Check). M ny tnh ton cho 5 bytes tiu . Do phn tiu b thay i sau tng chng nn CRC cn c kim tra v tnh ton li vi mi chng.

a thc sinh c dng y l a thc: g(x) = x8 + x2 + x + 1.

Trng iu khin lung chung GFC (Generic Flow Control)

giao din gia ngi s dng v mng UNI, phn tiu c vi khc bit so vi giao din NNI. S khc nhau cn bn nht l trng VPI b rt ngn li cn 8 bits (so vi 12 bits giao din NNI), thay vo ch 4 bits ca VPI l trng iu khin lung chung GFC. C ch ca GFC cho php iu khin lung cc cuc ni ATM giao din UNI. N c s dng lm gim tnh trng qu ti trong thi gian ngn c th xy ra trong mng ca ngi s dng. C ch GFC dng cho c cc cuc ni t im ti im v t im ti nhiu im.

Khi kt hp mng ATM vi cc mng khc nh DQDB (dictributed Queue Dual Bus), SMDS (Swiched 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 mng u c mt logic iu khin tng ng dng GFC cho cc giao thc truy nhp ca ring cc mng ny. Do trong trng hp ny, GFC thc cht l mt b cc gi tr chun nh ngha 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 c bn ca ATM, n to ra s khc nhau gia cc t bo ti giao din UNI v NNI do cc giao thc trong ATM khng phi l giao thc ng nht. Trong mng s dng cc giao thc ng 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.2.2 Cc loi t bo ATM:- Idle cell (T bo ri hay t bo trng): ng truyn l dng cc cell lin tc ni tip nhau. Nu lp ATM khng gi cell no th lp vt l s chn vo cc cell trng c dng cell lin tc.

- Valid cell (T bo hp l): cha gi tr hu ch, hay cell li nhng c sa cha.

- Invalid cell:cell li nhng khng c sa cha. Lp vt l s loi b cc cell ny.

- Assigned cell: T bo mang thng tin dch v s dng cho cc dch v lp ATM- Unassigned cell: L cc t bo khng mang thng tin dch v, khng c s dng. 2.3 M hnh tham chiu B-ISDN:

a. ISDN l g?

ISDN (Intergated Services Digital Network Mng s tch hp dch v) l tp cc giao thc nhm kt hp mng in thoi s v dch v truyn d liu. Ni mt cch khc, mng ISDN cho php tt c cc thng tin thoi (phone), s liu (data) v hnh nh (video) c th truyn qua mt ng dy thu bao (subscriber line) vi tc cao v cht lng tt. Mc tiu ca ISDN l nhm to ra mt mng din rng c cung cp kh nng kt ni im-im trong mi trng s. iu ny thc hin c bng cch tch hp thnh mt tt c cc dch v truyn ring bit m khng cn thm kt ni hay ng dy thu bao mi.b. Mng s tch hp dch v bng rng B ISDN (Broadband Integrated Services Digital NetWork):Xu hng cc dch v ngy nay v trong tng lai gn l cc yu cu dch v bng rng ang tng ln

Cc dch v phc v cho cc thu bao gia nh: Cc dch v quan trng cho cc thu bao gia nh l nhng dch v truyn hnh cp, truyn hnh s chun SDTV (Standard Digital TV) hay l dch v truyn hnh phn gii cao HDTV (High Definition TV). Mt ng dng quan trng na l dch v in thoi truyn hnh trong cc hnh nh cht lng cao c truyn i tc t 2 ti 5 Mbit/s. Cc dch v phc v trong lnh vc kinh doanh v giao dch: Cc thu bao trong phm vi cng s, vn phng c nhng c im hon ton khc so vi cc thu bao gia nh. im chung duy nht gia hai lnh vc ny l in thoi truyn hnh. Tuy vy, dch v ny cng phi c m rng tin ti in thoi hi ngh truyn hnh, sao cho ngi s dng c th dng dch v in thoi truyn hnh lin lc vi im cng mt lc.

T s cn thit phi t hp cc dch v ph thuc ln nhau chuyn mch knh v chuyn mch gi vo mt mng bng rng duy nht. Cn thit phi tha mn tnh mm do cho cc yu cu v pha ngi s dng cng nh ngi qun l mng (v mt tc truyn, cht lng dch v, ). V vy cng nh ngi qun tr mng. Mng B-ISDN ra i nhm p ng cc iu kin trn (bng rng, bo dng, mm do, kinh t, ) m mng bng hp N.ISDN khng p ng c. So vi cc mng khc, dch v t hp v mng t hp c nhiu u im v kinh t, pht trin, thc hin, vn hnh v bo dng hn.

Vy mng t hp dch v s bng rng (Broadband Integrated Services Digital Network B-ISDN) cung cp cc cuc ni thng qua chuyn mch, cc cuc ni c nh (Permanent) hoc bn c nh (Semi-Permanent), cc cuc ni t im ti im hoc t im ti nhiu im v cung cp cc dch v theo yu cu, cc dch v dnh trc hoc cc dch v yu cu c nh. Cuc ni trong B-ISDN phc v cho c cc dch v chuyn mch knh, chuyn mch gi theo kiu a phng tin (Multimedia), n phng tin (Monomedia), theo kiu hng lin kt (Connection Oriented) hoc khng lin kt (Connectionless) v theo cu hnh n hng hoc a hng. B-ISDN l mng thng minh c kh nng cung cp cc dch v ci tin, cung cp cc cng c bo dng v vn hnh (OAM), iu khin v qun l mng rt hiu qu. c. M hnh tham chiu B-ISDNCu trc phn lp logic c s dng trong ATM da trn m hnh tham chiu lin kt cc h thng m OSI. Tuy vy m hnh ATM s dng khi nim cc lp v cc mt phng ring 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 giao thc B.ISDN (B.ISDN Protocol Reference Model hay B.ISDN - PRM).

Hnh 3.5 M hnh tham chiu B.ISDN

B.ISDN PRM c cu trc phn lp t trn xung, bao gm cc chc nng truyn dn, chuyn mch, cc giao thc bo hiu v iu khin, cc ng dng v dch v. Nh hnh trn B.ISDN PRM bao gm 3 mt phng: mt phng qun l, mt phng ca ngi s dng v mt phng iu khin.

- User Plane: Gi cho dng thng tin xuyn sut t ngi s dng A ti ngi s dng B trn mng thng qua cc lp, sa cha li truyn, gim st dng data.

- Control plane: Mt phng iu khin cng c cu trc phn lp. N c chc nng: thit lp, gim st, gii phng cc ng ni hoc cuc gi.

- Management plane: Tt c cc chc nng lin quan ti ton b h thng (t u cui ti u cui) u nm mt phng qun l. Nhim v l to s phi hp lm vic gia nhng mt phng khc nhau. C 2 chc nng chnh l chc nng qun l lp (Layer Management) v chc nng qun l mt phng (Plane Management).

- Layer management: c chia thnh cc lp khc nhau nhm thc hin cc chc nng qun l c lin quan ti cc ti nguyn v thng s nm cc thc th c giao thc. C nhim v ging nh Meta-Signaling (thit lp tn hiu kt ni) hay x l dng thng tin OAM (Operatrion, Administration, and Maintenance).

- Plane management: Phi hp gia user plane v control plane.- Bn lp ca m hnh tham chiu B-ISDN:

+ Higher layer: Thc cht y chnh l lp ng dng: Frame relay, SMDS/CBDS.

+ ATM Adaption Layer (AAL): C nhim v chia nh dng Data ca Higher Layer thnh cc on 48 Bytes, hay khi phc li dng Data t cc ATM cell. Nhim v ca AAL ph thuc vo t tnh ca yu cu ng dng.

+ ATM Layer: Nhim v chnh l truyn Data m n nhn c t AAL n ch.

+ Physical Layer: Ph thuc vo b phn truyn trung gian. Nhim v ca n l pht bit vt l v tng thch vi lp ATM.ApplicationUpper Layer

Presentation

Session

TransportAAL

NetworkATM

Layer

Data Link

Physical

LayerPhysical

Layer

Hnh 3.6 M hnh cc lp ca ATM so vi m hnh OSI Lp ATM c chc nng tng t chc nng lp Data Link v lp Network trong m hnh OSI.

Lp ATM Adaption Layer c chc nng tng t chc nng lp Transport trong m hnh OSI.

Lp cn li trong m hnh ATM c chc nng tng t vi cc lp cn li trong m hnh OSI (Session Layer, Presentation Layer, Application Layer).2.4 Lp vt l

Hnh 3.7 lp vt lLp vt l c hai phn chnh :

Lp con ng truyn vt l PM (Physical Medium Sublayer): l mt phn ca lp vt l c cc chc nng ph thuc hon ton vo mi trng truyn dn vt l c th. N c chc nng sau:

Pht / nhn bit.

Truyn bit.

M ha ng dng bit theo m ng truyn.

Chuyn i t in sang quang (nu cn)

ng b bit. Lp con hi t truyn TC (Transmission Convergence Sublayer):

C cc chc nng sau:

To v kim tra m HEC: u pht, m HEC c xc nh bi 4 bytes u trong phn tiu ca t bo ATM, kt qu tnh ton c a vo byte th 5. Trng HEC c to ra c kh nng pht hin v sa li n v d tm c li khp trong phn header (4 Bytes) ca t bo trong qu trnh truyn v nhn t bo.

Thm vo hoc ly ra cc t bo trng: Khi khng c t bo cha thng tin hu ch, t bo khng xc nh hoc t bo OAM mc vt l th cc t bo trng s c truyn trn ng truyn dn.

Nhn bit gii hn t bo: Chc nng ny cho php bn thu nhn bit gii hn mt t bo. S nhn bit ny da trn s tng quan ca cc bit tiu v m HEC tng ng. Trong c ch ny, u tin trng thi bt ng b 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 hiu rng phn tiu ng v chuyn sang trng thi tin ng b PRESYNCH. y h thng kim tra lin tc m HEC ca cc t bo lin tip. Nu lin tip mt s ln m HEC ng th h thng chuyn sang trng thi ng b SYNCH, nu khng th tr li trng thi HUNT. Qu trnh thc hin nh hnh v

Bin i dng t bo thnh cc khung truyn dn (Transmission Frame Adaptation):

Ti u pht chc nng ny c nhim v lm cho dng t bo ti t cc lp trn thch ng vi khung s liu c s dng trong h thng truyn dn. Ti u thu, cc dng t bo c khi phc li t cc khung truyn dn.

Pht v khi phc cc khung truyn dn: L chc nng di cng trong lp con TC. N c nhim v to ra cc khung truyn dn v ghp cc t bo ATM vo nhng khung ny. Kch thc khung truyn dn ph thuc t truyn. Ti u thu, cc khung truyn dn c nhn bit v khi phc li dng t bo ATM. Cu trc cc khung truyn c th khc nhau ty thuc tng h thng truyn dn c th.

Nhim v lp vt l l nhn ATM cell t ATM layer v chuyn ATM cell t lp vt l ln ATM layer2.5 Lp ATM

Lp ATM cung cp tt c cc chc nng cho vic vn chuyn cc ATM cell v truyn chng trong cc h thng kt ni ATM. Lp ATM s gi /nhn cc payload t AAL. Bn cnh ,n cng gi/nhn cc cell t lp vt l.

Hnh 3.8 Lp ATMa. Cu trc t bo v m ha:

T bo ATM c 2 dng header l UNI v NNI

UNI (User Network Interface): mt phn ca cu hnh B-ISDN. N m t giao tip gia thit b u cui B-ISDN v mng. NNI (User Network Interface): mt phn ca cu hnh B-ISDN. N m t giao tip gia 2 ATM node.

Hnh 3.9 2 dng header UNI v NNIb. Chuyn mch, hp knh v phn knh t bo ATM:

C 2 loi kt ni ATM: VPC (Virtual Path Connection) v VCC (VirtualChannel Connection).- VPC:l v tr gia im u cui vi im kt ni k tip hay gia im kt ni VC.

- VCC: Kt ni cc thc th trn lp ATM

- VCI c th c thay i trong VCC v VPI cng c th thay i gi tr trong VPC.

- Trong cng mt hng truyn cc ATM cell t nhiu ngun VCC v VPC c multiplex thnh mt lung cell.

- hng thu, lung ATM cell s c demultiplex thnh cc cell ring bit cung cp cho cc VCC,VPC.

Hnh 3.10 VCC,VPC- Vi mi kt ni, cc cell phi c nhn dng r rng.Ngha l trong ATM cross-connect (thc hin bi chuyn mch ng,ATM-cc ch phc v cho vic kt ni, cc kt ni ATM-cc c th c thit lp hay hy b bi ngi qun tr),v ATM switch (cng s dng chuyn mch ng ca kt ni ATM, cc thng tin ca user c th dng cho vic khi to kt ni). Cc gi tr VPI v VCI phi tng thch.

Hnh 3.11 VP switch / VP cross connect- VP switch / VP cross connect: L thit b dng chuyn i cc gi tr ca VPI v chuyn cc knh sang VPI tng ng.

Hnh 3.12 VC switch / VC cross connect

- VC switch / VC cross connect: Lm thay i gi tr ca VPI v VCI

V d:

Hnh 3.13 v dc. To v gii nn header t bo:

- Pht: ATM layer nhn cell payload t AAL (bao gm cc tham s). Sau to cell header ngoi tr trng HEC. Vi cell payload, cell header, ATM layer to ra ATM cell.

- Thu: ATM layer phn r ATM cell thnh ATM header, payload. Lp ATM kim tra header : nu cell b relay mt header mi c to ra vi payload s dng to ra ATM cell mi, nu cell c nhn bi thit b u cui (khng relay) th payload s c a ln lp cao hn - AAL.2.6 Lp thch ng ATM (AAL)

AAL ph thuc vo loi dch v. Chc nng chnh ca n l pht ra cell Payload v cu hnh li khi data hay chuyn tip cc tn hiu hu ch. Hay ni cch khc lp AAL c nhim v to ra s tng thch gia cc dch v c cung cp bi lp ATM vi cc lp cao hn.

AAL nhn cc n v s liu giao thc PDU t Higher Layer chia nh n ra v a chng v trng d liu (Payload) ca t bo ATM.

Sau AAL chuyn cc Payload n ATM layer

AAL gm 2 lp con:

Convergence sublayer (CS).

Segmentation and Reassembly sublayer (SAR).

Chc nng chnh ca AAL l:

SAR: Phn on cc PDU mt cch thch hp (48Bytes Payload) a vo ATM cell v ly thng tin trong Payload ca ATM cell khi phc cc PDU hon chnh.

CS: Ph thuc vo cc loi dch v, n cung cp cc dch v ca lp AAL cho cc lp cao hn thng qua im truy nhp dch v.

Tnh ph thuc cc dch v (Service dependence) ca AAL l nguyn nhn phn loi cc lp v cc loi dch v tng ng.

Hnh 3.14 Phn loi gia cc lp ATM

Nhm A: N phc v cho cc dch v loi ny thng l ting ni v tn hiu video c tc khng i.

Nhm B: N phc v cho cc dch v ca n thng l tn hiu Audio v Video c tc thay i.

Nhm C:N phc v cho cc dch v truyn s liu hng lin kt v bo hiu.

Nhm D: c s dng cho cc dch v truyn s liu khng lin kt.

Da vo cch phn loi trn ITU T

* Cc Loi AAL:

AAL kiu 1: Phc v cho cc dch v thuc nhm A, n thu hoc pht cc SDU (Service Data Unit) ca lp trn theo thi gian thc vi tc truyn khng i. Cc chc nng c bn ca ALL 1 bao gm:

- Phn tch v to li (SAR) thng tin ca ngi s dng

- X l tr truyn v to t bo

- X l li khi mt hoc chn nhm t bo

- Khi phc ng b u thu

- Pht hin li trong trng thng tin iu khin t bo v khi phc li cu trc thng tin ti bn nhn. AAL kiu 2: S dng cho cc dch v c tc thay i c truyn theo thi gian thc (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 nh sau:

- Trao i s liu c tc thay i gia lp cao hn vi lp ATM

- X l tr t bo

- Phn tch v khi phc li thng tin cho ngi s dng

- X l cc loi li t bo cng nh tch tn hiu ng b u thu.

AAL kiu 3/4: AAL 3/4 c pht trin t AAL 3 (phc v cho cc dch v loi C) v AAL 4 (phc v cho cc dch v loi D). Ngy nay, hai kiu AAL trn hp li thnh AAL 3/4, lp AAL ny tha mn cc dch v thuc loi C v D. AAL 3/4 cung cp hai dch v c bn l: dch v kiu thng ip (Message Mode Service) truyn cc s liu c ng thnh khung v dch v kiu dng bit (Streaming Mode Service) truyn s liu tc thp vi yu cu tr nh.

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 yu cu v truyn s liu. Tuy vy, ITU-T a ra AAL 5 nhm mc ch gim di phn thng tin iu khin giao thc PCI (Protocol Control Information). AAL 5 c cc chc nng v giao thc hot ng nh AAL 3/4. im khc nhau chnh ca hai loi ny l AAL 5 khng a ra kh nng phn/hp knh, do n khng c trng MID. AAL 5 ch yu s dng cho bo hiu trong mng ATM.

KT LUN

ATM l mt cng ngh mng tc -cao c thit k dng cho c mng cc b (LAN) v mng din rng (WAN). N l cng ngh chuyn mch hng kt ni, ngha l mt mch dnh ring c thit lp gia hai h thng cui trc khi mt phin lin lc c th bt u. ATMl mt kt qut nhin ca cch thng nh dng truyn dn d liu khc nhau c cp trong phn trc, mc d c mt vi tranh lun im ny. Nhng ngc li cc nh dng c m t th tho mn nhng s cn thit ca th gii d liu, ATM cung cp mt nh dng ti u hay l mt dng giao thc cho nhng truyn thng bng d liu, thoi v hnh nh, ni m cc tbo ca mi phng tin c th c trn ln qua mng. Mt cch in hnh, nhng t bo ATM ny c th c truyn ti trn SONET, SDH, E1/DS1, v nhng nh dng s khc. Cc t bo c th cnc truyn ti mt cch lin tc m khng di nh dng mng s.

Khi mi trng ca x hi thng tin c hon thin, th mng giao tip thng tin bng rng cn thit phi t ra thch nghi vi cc tnh nng nh tc cao, bng rng, a phng tin. V v vy phi tnh n vic thit lp mng thng tin tc siu cao tm quc gia. Mng thng tin tc siu cao da vo s dng cng ngh ATM (phng thc truyn ti khng ng b) to ra mng li quc gia rng khp vi tnh kinh t v hiu qu cho php cc nh cung cp dch v c th cung cp nhiu loi hnh dch v thng tin khc nhau. Cng ngh ATM c hnh thnh t cng ngh ATD (Asynchronous Time Division - phn chia theo thi gian khng ng b) c a ra trn mng vin thng ca Php nm 1983 v FPS (Fast Packet Switchinh - chuyn mch gi tc cao) ca Bell Lab ca nc M.

ATM l s kt hp ca cng ngh truyn dn v cng ngh chuyn mch qua mng giao tip chun, da vo cng ngh ATM phn chia v ghp ting ni, s liu, hnh nh,... vo trong mt khi c chiu di c nh c gi l t bo. c im chnh ca ATM l thng tin c cu to t cc t bo trong mt khi thch hp ca thi gian thc truyn ti thng tin v cch thc truyn ti c th chng minh rng tt c cc dch v bng rng khng nh hng ti tc thng tin. Trong mng ATM tin tc l cc t bo c gi t thit b u cui c xp xp trong tn hiu s sao cho mng vi tc x l khong vi Gbps c th c s dng truyn hoc chuyn mch cc t bo , cng nh vy ton b cc thng tin c truyn bng cc t bo vi chiu di c nh. T y ta c th thit lp mng lin kt a phng tin m n c th x l nhiu loi hnh thng tin khc nhau nh ting ni, s liu, hnh nh, ... mt cch ng nht. iu khin

Chuyn mch

i tng s dng

i tng s dng

Transaction / Message c di L

u Trng tin CRC

Trng tin c di ti M bit

(M>=N)

Tiu Ti tin (Ti Nbit) CRC

To khung bt u

To khung kt thc

Segment#1 Segment#2 .. Segment#n

Bn tin

Segment

Gi

EMBED PBrush

EMBED PBrush

User A

PSW

PSWS

User A

So snh chuyn mch knh v chuyn mch gi

Trnh by cu trc t bo ATM

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