Do thu lap dat ADSL - Ts Đặng Quốc Anh

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HC VIN CNG NGH BU CHNH VIN THNG

TRUNG TM O TO BU CHNH VIN THNG II

Ti liu lp o to t xa

O TH NH GI V LP T ADSL

Tng v v hai con ti

Thng 3 nm 2005

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Ti liu tham kho

DSL Advances Thomas Starr, Massimo Sorbara, John M. Cioffi, Peter J. Silverman Prentice Hall PTR 2002 ISBN: 0-13-093810-6

The DSL Source Book The Comprehensive Resource On Digital Subscriber Line Technology Paradyne Corporation 2000 ISBN: 0-9706170-0-3

ADSL, VDSL, and Multicarrier Modulation John A. C. Bingham John Wiley & Sons, Inc. 2000 ISBN: 0-471-20072-7

ADSL, thc t, gii php v trin khai ng Quc Anh Trung tm o to Bu chnh Vin thng II 2002

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Mc lc

Ti liu tham kho .........................................................................................2

Mc lc ...........................................................................................................3

Chng 1: K THUT ADSL........................................................................9

xDSL ............................................................................................................9

Lch s hnh thnh xDSL v cc phin bn xDSL .....................................9

K thut iu ch dng trong xDSL.........................................................13

Cc thnh phn ca mt h thng xDSL.................................................15

Cng ngh ADSL ......................................................................................18

K thut c bn ADSL .............................................................................18

Kh nng ca ADSL...........................................................................20

Cng ngh ADSL................................................................................21

Tiu chun ho ADSL..............................................................................22

ADSL Forum.......................................................................................23

ANSI ...................................................................................................24

ETSI....................................................................................................24

ITU-T ..................................................................................................24

Cc website v ADSL.........................................................................25

Cu hnh mng ADSL..............................................................................26

o th, nh gi v lp t ADSL

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Cu hnh mng ADSL.........................................................................26

Kt ni ADSL G.dmt ti thu bao .......................................................29

Kt ni ADSL G.lite ti thu bao.........................................................29

Cc k thut c trng dng trong ADSL................................................30

K thut chng nhiu xung.................................................................30

Bit swapping .......................................................................................31

Sp xp tn s....................................................................................34

Nghi thc kt ni ADSL ...........................................................................35

nh gi cc nghi thc mng dng cho ADSL ..................................35

Mi trng ly ATM lm c s ..........................................................37

Ethernet trc tip qua ADSL ..............................................................45

Cu hi t kim tra...................................................................................48

Chng 2: O TH NH GI MNG CP NG S DNG CHO ADSL.......................................................................................................................52

c tnh mng cp ng..........................................................................52

Cp ng trong thi i mi ....................................................................52

Hin trng cp ng ...........................................................................52

Nhng thay i ...................................................................................53

Hng v tng lai ............................................................................53

c tnh ng dy cp ng .................................................................54

Mch tng ng ng dy thu bao...........................................54

Tr khng c tnh .............................................................................54

Mc lc

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Phi hp tr khng.............................................................................56

Suy hao phn hi (Return Loss).........................................................59

Suy hao chn (Insertion Loss)............................................................60

Cn bng dc (Longitudinal Balance) ................................................60

Xuyn knh (Crosstalk) ......................................................................62

Tc ng ca mi trng ........................................................................65

Nhiu trng (White Noise) ..................................................................68

Mt nhiu (Noise Density) .............................................................69

Nhiu thc (Real Noise) .....................................................................70

Nhiu xung (Impulse Noise) ...............................................................71

Ngt (Interruptions).............................................................................73

Li cp ng ............................................................................................74

Cc loi li trn ng dy ................................................................74

nh v li ng dy .........................................................................77

o th cp ng.......................................................................................80

Tiu chun o th ng dy thu bao ..................................................80

Tiu chun quc t .............................................................................80

Tiu chun ngnh...............................................................................80

Cc tiu chun thit b ADSL..............................................................80

o th tng t.......................................................................................81

Suy hao phn hi................................................................................81

Nhiu ..................................................................................................81


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Xuyn knh u gn...........................................................................81

Cn bng dc .....................................................................................82

o TDR...............................................................................................82

o th nhiu my o ..........................................................................83

Suy hao chn......................................................................................83

Xuyn knh u xa.............................................................................84

nh gi tc bit..............................................................................84

Ngt qung .........................................................................................84

Cc php o dng cu kt ni............................................................84

Kh nng khng nhiu........................................................................85

Mt ph cng sut .........................................................................85

Cc phng php o th khc................................................................85

o th s............................................................................................85

Ghi li h s cho mng ......................................................................86

o tham s .........................................................................................87

nh gi cp ng ...................................................................................87

T chc vic nh gi mng cp ng....................................................87

Nhng yu t cn lu ......................................................................87

S dng my o .................................................................................88

Chin lc nh gi ................................................................................91

Giai on nh gi trc hp ng ...................................................91

nh gi hng lot trc hp ng ...................................................96

Mc lc

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nh gi c chn la .........................................................................98

nh gi ngay khi lp t ...................................................................99

nh gi ngay khi bo dng..........................................................101

Thit b o th ADSL..............................................................................102

Thit b o th ADSL cm tay................................................................102

Dng c o th tng t .......................................................................102

So snh my o TDR thng thng v my o Dynatel 965DSP ca 3M.....................................................................................................102

My o in dung .............................................................................103

My o tm cun ti, nhnh r ..........................................................103

Dng c o th dch v s ADSL ..........................................................104

Cc loi my o khc ............................................................................105

My o cht lng ng dy.........................................................105

Dng c kim tra tn hiu thoi.........................................................105

Cu hi t kim tra.................................................................................106

Chng 3: LP T ADSL ......................................................................109

Lp t ng dy.................................................................................109

u chuyn ti dn MDF .......................................................................109

i vi thu bao c yu cu s dng in thoi ..............................109

i vi thu bao khng c yu cu s dng thoi trn ng dy ADSL ................................................................................................110

o th ng dy .................................................................................111

Lp t trong nh ...................................................................................113


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Thu bao khng s dng in thoi ......................................................113

Thu bao s dng in thoi .................................................................114

Gii thiu Modem router ADSL.............................................................118

c tnh phn cng SpeedTouchTM Pro 530.........................................118

c tnh k thut modem ADSL SpeedTouchTM Pro 530......................118

H tr dch v SpeedTouchTM Pro 530..................................................118

Qun l SpeedTouchTM Pro 530 ............................................................119

Dng bn ngoi SpeedTouchTM Pro 530...............................................119

Ci t modem ADSL .............................................................................121

Nhng lu khi ci t modem ADSL ..................................................121

Ci t IP cho my tnh trong mng ......................................................121

Vi h iu hnh Windows XP......................................................121

Vi h iu hnh Windows ME .....................................................125

Ci t modem router qua web server trn modem router....................128

X l s c lp t .................................................................................131

Thu thp thng tin..................................................................................131

Cc sai hng nghim trng ti my mc ca khch hng.....................132

Cc sai hng nghim trng ng dy cp ng..............................133

Cu hi t kim tra.................................................................................134

Ch mc ......................................................................................................137

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Chng 1: K THUT ADSL xDSL

Cng ngh ADSL

xDSL

Lch s hnh thnh xDSL v cc phin bn xDSL

Hnh 1.1 xDSL theo dng thi gian

Lch s hnh thnh ca DSL v cc phin bn ca n c minh ho vn tt trn hnh v 1.1. Nm 1985 Bell Labs pht minh ra phng php truyn dch v s trn di tn s cao ca dy in thoi xon i truyn thng.


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Nm 1990 cc cng ty in thoi bt u s dng HDSL trn 2 i dy thay th cc ng truyn T1 trn dy xon i vi u im khng cn s dng cc trm tip vn (repeater). HDSL khng cn phi c cc trm tip vn hay cc iu kin c bit cho ng dy. Ngay c mt s nhnh r cng c th tn ti trong cc lin kt HDSL vi s lng khng qu 2 v di cng phi b gii hn. iu hp dn ca HDSL l gi thnh h v HDSL lm cp ng c v nh cp quang trong chng mc tin cy v t s sai bit kh hn cp ng trong T1/E1 nhiu.

Nm 1995 cc cng ty in thoi trin khai ADSL thm nhp th trng video. ANSI tiu chun ho thnh cng ADSL vi tiu chun T1.413. Sau ADSL khng cnh tranh ni vi truyn hnh cp ng trc, truyn hnh v tinh nn b xp x v cui cng c s dng li rt hiu qu vi truy xut Internet tc cao nh bn cht bt i xng ca n.

Bng 1.1 Cc loi ng dy thu bao s

Tn ngha Tc d liu Ch ng dng

V.22 V.32 V.34

Voice-grade modem

1200 bit/s n 33600 bit/s

Song cng Thng tin s liu da trn PSTN

ISDL Integrated Digital Subscriber Line

160 kbps Song cng Dch v ISDN cho thng tin thoi v s liu

HDSL High data rate Digital Subscriber Line

1,544 kbps 2,048 kbps

Song cng

Song cng Mng cung cp T1/E1, truy xut WAN, LAN, truy xut server

SDSL Single-line Digital Subscriber Line

1,544 kbps 2,048 kbps

Song cng Song cng

Nh HDSL nhng thm phn truy xut i xng

ADSL Asymmetric Digital Subscriber Line

1,5 n 9 Mbps 16 n 640 kbps

Downstream Upstream

Truy xut Internet, video demand, interactive multimedia, truy xut LAN t xa

VDSL Very high rate Digital Subscriber Line

13 n 52 Mbps 1,5 n 23 Mbps

Downstream

Upstream

Nh ADSL nhng thm HDT

n nm 1998 th tiu chun ADSL T1.413 c nng cp thnh release 2 vi kh nng thch ng tc v lng overhead gim xung. Cng nm ny vic tiu chun ho SDSL thnh cng. Thng t nm 1998 ETSI ban hnh tiu chun u tin v cc chc nng ca VDSL: TS 101 207-1.

Chng 1: K thut ADSL

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Vo nm 1998, nhm cng tc ADSL ton cu (UAWG: Universal ADSL Working Group) c thnh lp bao gm cc t chc hng u trong cng nghip vin thng mng v my tnh c nhn pht trin mt dng ADSL tc thp, gi thnh h c th nhanh chng ginh ly th trng.

Kt qu cng vic ca nhm cng tc ny l mt chng loi ADSL da trn tiu chun mi l G.lite ra i. G.lite c ITU-T chp nhn khuyn ngh G.922.2 vo thng 6 nm 1999 v c th cung cp tc ln n 1,5 Mbps theo chiu downstream v 512 kbps theo chiu upstream. G.lite c thit k cung cp dch v ny qua ng dy in thoi m khng cn b tch dch v thoi n thun POTS m cc gii php ADSL tc y vn cn n.? Mt phn ca tiu chun G.lite l k thut gi l "fast retrain" gii hn nng lc dng d liu upstream ca tn hiu G.lite khi t hp in thoi ang s dng ti thiu ho xuyn knh v sau phc hi li nng lc ca dng tn hiu upstream khi t hp c gc tr li.

Thng 6 nm 1999 ITU-T ban hnh cng lc hai khuyn ngh u tin v ADSL l G.992.1 cn gi l G.dmt v G.992.2 cn gi l G.lite. Cng nm ANSI ban hnh tiu chun T1.418 cho HDSL2 l loi HDSL s dng mt i dy.

Thng 2 nm 2001 ETSI ra tip tiu chun TS 101 207-2 v c tnh my thu pht VDSL. Thng 11 nm 2001 ITU-T ban hnh khuyn ngh u tin v VDSL: G.993.1.

Thng 7 nm 2002 ITU-T ban hnh tiu chun u tin v ADSL2 (prepublication) trong khuyn ngh G.992.3 v G.992.4. ADSL2 ra i nhm ci tin tt c cc ng dng trn ADSL v c thoi v s liu. Ngoi hai ch truyn dn trn ADSL l ch truyn dn ng b STM v ch truyn dn bt ng b ATM nh ADSL th h u tin ADSL2 cn h tr thm ch truyn dn th ba l ch truyn dn paket v h tr thoi khng gi ho gi l thoi knh ho (CV: Channelized Voice). Thoi knh ho c a vo dng d liu ca ADSL2 m khng cn qua b IAD.

Ngoi ra ADSL2 cn thay i v di tn truyn dn theo hai chiu upstream v downstream. Nh cc ci thin ny m tc truyn dn ca cc ng dy di 3 dm tng t 560Kbps/276Kbps ln n 1560Kbps/596Kbps theo th t tng ng cho chiu downstream/upstream. di ng dy cho mt tc c nh tng c 300m. Vi nhng ci tin nh vy, ADSL2 em n dch v cho n 99% cc ng dy vng thnh th. Hn na ADSL2 li tng hp tt vi ADSL c tiu chun ho trong ITU-T G.992.1.

Mc tiu ca qun l ngun ADSL2 l thc hin tit kim nng lng cho tnh nng "Always on" ca ADSL. Vi hng triu modem ADSL qun l


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ngun cc ch standby, sleep nh PC s tit kim lng in ng k. ADSL2 c 3 ch hot ng ngun l L0, L2 v L3.

L0: cp ngun cho modem t tc d liu cao nht.

L2: hn ch ngun ti a ti ATU-C khi lu lng Internet qua modem gim.

L3: Hn ch ngun ti a ti ATU-C v ATU-R khi khch hng khng s dng ADSL2: ch sleep/standby, khi ng li phi mt 3 giy

L2 l tin b ln nht ca ADSL2

S chuyn i trng thi qua li gia cc ch ngun L0 v L2 l hon ton t ng, ngi s dng khng bit c v khng gy ra sai bit cng nh khng ngt qung hot ng ca cc modem ADSL.

Thng 5 nm 2003 ITU-T a ra khuyn ngh G.992.5 dnh cho ADSL2+.

ReachDSL l cng ngh DSL i xng p ng nhu cu ca thu bao v ng dy DSL tc cao cc khong cch xa. b sung cho cng ngh ADSL tiu chun (DMT hay G.lite), cc sn phm ReachDSL cung cp tc d liu t 128 kbps n 1 Mbps v c thit k lm vic vi iu kin ng dy v i dy trong nh d di hn. Mt trong cc li ch ca ReachDSL l khng cn phi lp t cc b tch dch v thoi n thun POTS. iu ny cho php khch hng hon ton c th t lp t cc b microfilter.

Khc vi cc h thng ADSL c di vng thu bao gii hn trong khong 6 Km k t tng i, cc h thng ReachDSL m rng dch v n hn 6 500 m v hin nay c cc ng dy vt qu 10 Km.

ReachDSL c kh nng tng hp ph tn s. Gii php ReachDSL c s tng hp tn s mc cao. Mt trong cc thnh vin ca gia nh ReachDSL l MVL (Multiple Virtual Lines) l h thng DSL u tin c FCC cng nhn vi ph chun Part 68 ngha l n thn hu vi cc dch v khc trn mng in thoi v bn thn khng phi l mt tc nhn gy nhiu. Cc gii php ReachDSL khc cng ph hp vi tiu chun qun l ph tn s nhm 1 (Spectral Management Class One). ReachDSL c gi thnh sn phm thp v c tc ng, cho php dch v c nh hng khch hng cho cc ng dng khc nhau. Nhng thun li k trn rt quan trng iu chnh gi thnh v nh hng th trng dch v nhm n cc khu vc doanh nghip v ngi tiu dng. Mt phin bn mi nht ca DSL l VDSL (Very High Speed DSL). Cc h thng VDSL vn ang khng

Chng 1: K thut ADSL

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ngng c pht trin nn khng th ni c chnh xc kh nng ti a ca chng. Tuy nhin, cc tiu chun d nh cho dng d liu upstream l 52 Mbps v i xng l 26 Mbps. B li tc cao ny l di vng thu bao rt hn ch, khong 330 m vi tc cao nht v c s thch ng tc gim xung khi di vng thu bao tng ln. Nhng hn ch ny lm cho vic a vo s dng VDSL dn ti s dng mt m hnh hi khc vi cc h thng DSL truyn thng. Trong m hnh VDSL cc b DSLAM c di ra khi tng i ni ht v a v gn thu bao hn. Cc b DSLAM c nui bng cc ng cp quang. Tc cao ca VDSL m ra mt c hi cho cc nh cung cp dch v a ra th h dch v mi ca DSL, vi video tr thnh dch v c s. tc 52 Mbps mt ng dy VDSL c th cung cp nhiu knh video MPEG-2 cht lng cao v ngay c mt hay mt vi knh truyn hnh nt cao (HDTV: High Definition Television). Nhiu nh cung cp dch v bt u th nghim cc h thng VDSL cung cp nhng dch v ny vi u bn pha thu bao ca VDSL dng nh mt my truyn hnh cp cng ng thi vi cc dch v d liu t my tnh c nhn.

Gi thit c bn ca DSL l tr thnh mt cng ngh vng thu bao m cc thit b tng thch c b tr 2 u vng thu bao cp ng bo m cho nhiu cng ngh DSL mi s c a ra theo thi gian. Mc tiu chin lc ca cc nh cung cp dch v l bo m s chn la mt m hnh hay mt cng ngh DSL no cho dch v ngy nay s khng gii hn kh nng theo kp cc cng ngh mi trong tng lai. Nh vy l c nhiu dng DSL c th chn la. S la chn mt cng ngh v b qua cc cng ngh khc l ph thuc vo mt lot cc yu t t loi dch v, dng ch yu ca mng hin ti, d nh ca khch hng v cc dch v trong tng lai.

K thut iu ch dng trong xDSL

Vo cui nm 1992, 3 loi m ng dy c kh nng nht trong vic cung cp cho cc dch v video tc cao xut hin l:

QAM: Quadrature Amplitude and Phase Modulation, mt loi m ng dy dng trong k thut MODEM t 20 nm qua.

CAP lc u c a ra cho HDSL v thc ra l mt bin th ca QAM.

DMT: Discrete MultiTone l k thut ng dy c AT&T Bell Labs ng k bn quyn (nhng khng s dng) t hn 20 nm nay.

Khng ging nh 2B1Q s dng k thut tn hiu di nn (baseband) pht tn hiu gm c tn s 0 Hz (tn hiu mt chiu) cc loi m ng dy s dng di thng di (passband) v c th c thit k hot ng trn bt


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k tm tn s no. ADSL ngay t u c thit k cho dch v dn dng nn cn tn ti c lp vi dch v in thoi thun ty. V vy, thuc tnh di tn ca ADSL phi c tn dng tch (hay ghp phn tn FDM) vi POTS (Plain Old Telephone Service), knh thng tin upstream t ngi s dng dch v ti mng v knh thng tin downstream t mng ti ngi s dng dch v. Ngoi k thut FDM mt vi cng ngh DSL bao gm c DMT c thit k cung cp k thut trit ting di cho cc knh upstream v downstream ti thiu ho vic s dng tn s cao v ti u ho di vng thu bao. Tuy nhin vic s dng cc h thng trit ting di ny s gim khi s dch v cng loi trong mt b cp ngy cng tng.

Hnh 1.2 Tn hiu DMT ca ADSL

Vo nm 1992 v u nm 1993 nhm lm vic ANSI T1E1.4 (ANSI T1E1.4 Working Group) chn mt loi m ng dy n cho tiu chun ADSL Video Dial Tone. Nhm lm vic tp trung vo cc dch v video khc nhau t video ghi sn nn sn (pre-recorded, pre-compressed) MPEG-1 (Motion Picture Experts Group) cho n h thng c th cung cp n 4 knh video MPEG-1 cng lc hay mt knh MPEG-2 m ho thi gian thc tc 6 Mbps. S tp trung cui cng hng n vic ti a ho di vng thu bao vi nhng tc xc nh ti u cho video. Bn cht video ng b bit yu cu mt tc d liu c nh trnh s suy gim cht lng hnh nh video. Mc d v l thuyt cc loi m ng dy u c hiu qu nh nhau nhng ch c DMT l m ng dy u tin c chn dng cung cp dch v 6 Mbps v c chn lm tiu chun chnh thc cho ADSL dng cho dch v Video Dial Tone. Bn cnh cc h thng da trn CAP cng c s dng trn ton th gii cho cc dch v ADSL v Video Dial Tone. Trn nhiu th trng Video Dial Tone kh c th cnh tranh gi c vi truyn hnh cp v truyn hnh v tinh nn n ang bt u nhanh chng bin khi th trng Bc M (Hoa K, Gia N i v M Ty C). Tiu chun gn y nht dnh cho ADSL c ITU (International Telecommunication Union) a ra trong khuyn ngh G.dmt hay G.992 v ANSI (T1.413 Issue 2) cng da trn k thut DMT v l c s cho hu ht cc dch v ADSL ngy nay. Tuy nhin, mt vi nh cung cp vn cn s dng h thng da vo k thut CAP trn mng ca mnh.

Chng 1: K thut ADSL

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Khi ng dng l video ng b bit ng dy DSL phi hot ng tc c nh. Tuy nhin d liu li hot ng mt tm tc rng hn. Tc ng duy nht l tc cng chm s cng mt nhiu thi gian vn chuyn cc file ln. V vy, vi ng dng s liu ta c th tu chn gim tc ng dy cho php dch v c cung cp cho cc ng dy thu bao di hn. C CAP v DMT u c sa i ti u ho dch v da trn c s tng vng thu bao v ng dng ny gi l ng dy thu bao s thch ng tc (RADSL: Rate Adaptive Digital Subscriber Line). Cng ngh RADSL cung cp tu chn cho php my thu, pht bt u bng cch tng dn tc ng dy n tc ti a c th t c m vn tin cy trn mt ng dy c th. Trong khi c tnh ny ban u c thit k n gin ho vic lp t dch v th n cng gip cc nh cung cp dch v mt tu chn gim bt mc dch v khi cht lng vng thu bao gim. Ngy nay c nhiu cng ngh DSL khc cng cung cp tc bin i v cc nh cung cp dch v s dng chc nng thay i tc kim tra mc chp nhn i vi cc dch v khc nhau.

Phng php iu ch DMT tho mn c cc yu cu ca VDSL v thc s hiu qu hn nhiu so vi cc phng php iu ch QAM (Quadrature Amplitude Modulation) hay CAP (Carrierless Amplitude-Phase modulation). V phng php iu ch DMT thc hin gn nh ti u trn tt c cc knh truyn nn n vt qua cc vn cn bn ca ng dy ngay c khi gp mt vng tn s b nhiu trm trng. DMT d dng khc phc c xuyn nhiu RF, thch ng vi cc knh truyn thay i v c nhiu, h tr c tt c cc tc truyn, h tr c vic chuyn thng tin i xng v bt i xng, trnh tc ng n di tn v tuyn nghip d v tng hp ph tt vi cc phin bn DSL khc.

Cc thnh phn ca mt h thng xDSL

Trong mt mng DSL in hnh cn phi thit lp mt vi loi thit b mng s liu thc hin cc dch v s liu tc cao. Phn chnh ca mng DSL l mt DSLAM a dch v c b tr tng i ni ht v mt im kt thc DSL c lp t vn phng hoc nh ring ca thu bao. Thng cc im kt thc DSL l cc b MODEM, cc b nh tuyn (router) hay cc thit b truy xut tch hp (IAD: Integrated Access Device) c kh nng chp nhn c thoi v s liu. Tc truyn dn c th ln n 8 Mbps hoc hn na ty thuc vo mt s yu t bao gm c thit b, di vng thu bao v tnh trng cht lng vng thu bao.

Giao tip truyn dn chnh cho h thng DSLAM thng thng l T1/E1, T3/E3, OC-1, OC-3, OC-12, STS-1 v STS-3.

Mng thu bao ni ht tn dng mng thu bao in thoi lin i lm c s. cung cp kh nng kt ni gia cc nh cung cp dch v v ngi s dng dch v cn phi lp t thm nhiu thit b nh cc h thng chuyn


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mch frame relay, ATM v cc b nh tuyn. Cc thit b chuyn mch ATM ang ngy cng c s dng nhiu v cc th h DSLAM sp ti s bao gm c chuyn mch ATM.

Mt khi nim cn cp n l nt truy nhp (AN: Access Node) l ni t cc thit b chuyn mch v nh tuyn. Ty thuc vo quy m ca mng thu bao v gi thnh lin quan n vic chuyn vn m c th b tr mt hay nhiu AN trn mng thu bao ni ht. Vi cc th h DSLAM mi c tch hp c AN kt hp vi cc h thng chuyn mch ATM.

DSLAM c b tr cnh tng i ni ht v l nn tng ca gii php DSL. V mt chc nng DSLAM tp hp lu lng d liu t cc vng thu bao DSL vo xng sng ca mng kt ni vi phn cn li ca mng. DSLAM cung cp dch v vn chuyn gi s liu, t bo s liu hay cc ng dng ch mch qua vic tp trung cc ng dy DSL vo cc ng ra 10Base-T, 100Base-T, T1/E1 T3/E3 hay ATM. tng cng hn na chc nng tp hp lu lng d liu t cc thu bao ngi ta pht trin cu hnh DSLAM hai cp. Khi tp trung lu lng d liu t cc DSLAM cn gi l DSLAM subs ngi ta b tr cc DSLAM DSLAM Hub.

Ngy nay mt vi DSLAM c nhit i ho c th lp t nhng vng khng c iu ho nhit . iu ny cho php lp t cc DSLAM ti cc remote terminal hay cc t t bn l ng thay v phi lp t ti tng i ni ht. Kh nng di chuyn DSLAM ti nhng v tr xa tng i (cng vi cng ngh vng thu bao m rng) lm tng nhanh nh hng ca cc nh cung cp dch v, cho php cung cp dch v n nhng khch hng m bnh thng DSL khng th vn ti c. Ngoi chc nng tp trung th ty thuc vo tng dch v c cung cp mt DSLAM c th c thm cc chc nng khc. Trong mt vi trng hp mt DSLAM c th c kh nng c yu cu m cc gi s liu thc hin mt tc v no . V d, h tr vic gn a ch IP ng bng DHCP (Dynamic Host Control Protocol) mi gi phi c xem xt chuyn n ch ng ca n.

Thit b kt thc (endpoint) DSL MODEM/router l thit b u cui pha ngi s dng dng kt ni ngi s dng dch v vi vng thu bao DSL. Kt ni kt thc DSL thng l 10Base-T, V.35, ATM hay T1/E1. Cc sn phm i mi ca ngi s dng cng h tr cc phng php giao tip internal khc nh USB, IEEE 1394 hay PCI. Thm vo thit b kt thc DSL ti CPE (Customer Premises Equipment: thit b ti sn khch hng) ang c pht trin vi cc cng c thit k h tr cho tng ng dng ring bit nh cng RJ11 dnh cho dch v thoi, cng video dnh cho dch v video da trn DSL v cc giao tip mng mi nh Home PNA (Home Phoneline Networking Alliance) hay cc giao tip mng nh Ethernet v tuyn 802.11. Cc thit b kt thc DSL ca ti sn khch hng c mt s cu hnh khc nhau ty thuc vo dch v c th c cung cp. Ngoi vic cung cp chc nng c bn ca mt MODEM DSL nhiu thit b kt thc

Chng 1: K thut ADSL

17

cn c thm mt s chc nng nh nh tuyn, ghp knh TDM hay ghp knh ATM.

Cc thit b DSL endpoint phi c cc c tnh sau:

c kh nng cung cp thng k qun l lp 1, lp 2 nh t s SNR chng hn,

c kh nng cung cp thng k MIB lp 3 nh m gi chng hn,

phi hon ton qun l c t pha cc nh cung cp dch v m khng cn c nhn vin n tn ni,

h tr thc hin gim st trc tip nhanh chng d sai hng, cch ly v thc hin sa cha,

c kh nng ti t xa cc phn mm nng cp,

giao tip tt vi cc thit b ca hng khai thc trung gian nh thit b truy xut tch hp IAD chng hn.

Cc thit b kt thc nh tuyn em li s mm do cho IP t v tr khch hng. Vi mt thit b kt thc IP-aware c th to ra v duy tr c cc mng ni b, cho php phn on hiu qu cc LAN t xa cng nh vic nhn dng dng d liu downstream l multicast hay unicast. Vng a dch v cng c th c cc ngi s dng LAN t xa tn dng cng lc. Vng a dch v tr nn quan trng khi ta c mt nhm ln cc ngi s dng cn truy nhp cc nh cung cp dch v khc nhau. Cc b tch dch v (splitter) POTS c lp thm c tng i ni ht v pha ngi s dng dch v cho php s dng vng thu bao cp ng truyn dn ng thi d liu tc cao v mt knh in thoi khi dch v da trn DSL ang s dng c h tr cc dch v ny. Cc b tch dch v POTS thng c 2 cu hnh: cu hnh n c thit k lp nh ring thu bao v cu hnh nhiu dy c thit k lp t tng i ni ht. Lu rng trong khi nhiu s m ng dy DSL h tr mt knh dch v in thoi n thun POTS th mt s s m ng dy DSL khc li khng h tr loi dch v ny.

B tch dch v in thoi n thun POTS c th l dng tch cc hoc dng th ng:

B tch dch v in thoi n thun POTS th ng khng cn phi c cp thm ngun in bn ngoi. B tch dch v in thoi n thun POTS th ng h tr cc dch v thng trc nh s in thoi khn cp 911 (


18

Hp chng quc Hoa K) ngay c khi ngun in nh n b mt DSLAM hay MODEM DSL.

B tch dch v in thoi n thun POTS tch cc phi c cp ngun t bn ngoi cho in thoi v tn hiu DSL hot ng trn i dy cp ng. Khi mt in th mun duy tr cc dch v thng trc nh s in thoi khn cp 911 ( Hp chng quc Hoa K) ngay c khi ngun in nh n b mt DSLAM hay MODEM DSL phi c thm ngun in d phng.

Ngy nay, cc loi DSL nh G.dmt ADSL, G.lite, ReachDSL v RADSL c th c lp t m khng cn b tch dch v in thoi POTS ring thit b ti sn khch hng (CPE: Customer Premises Equipment). Thay vo l cc thit b th ng gi l cc b microfilter c ngi s dng gn gia mi thit b in thoi n thun (nh my in thoi, my in thoi khng dy, my tr li in thoi, my fax nhm 3 hay MODEM tng t) vi ng dy. Mch microfilter thc cht ch l mt b lc thng thp cho php tn hiu di tn m thoi i qua v loi b tn hiu trong di tn DSL loi tr xuyn knh. u im ca phng php ny l trong khi cc b tch dch v POTS truyn thng c ngi lp t ca cc cng ty cung cp dch v lp thit b giao tip mng (NID: Network Interface Device) th cc b microfilter c ngi s dng t gn d dng khi phi gi cho cng ty cung cp n lp t. Vi cc loi dch v da trn DSL thc hin qua cc kt ni in thoi n thun POTS th y l chn la quan trng nht khi lp t.

Cng ngh ADSL

K thut c bn ADSL

Cng ngh ADSL l cng ngh bt i xng. ADSL cho php truyn theo chiu downstream t pha nh cung cp dch v ti khch hng vi tc d liu cao hn chiu upstream t pha khch hng ti nh cung cp dch v. c tnh bt i xng ny cng vi tnh nng lun kt ni (always-on) lm cho ADSL tr nn l tng cho truy xut Internet, dch v video-on-demand v truy xut mng LAN t xa. Ngi s dng ca nhng ng dng ny thng ti thng tin v nhiu hn l gi d liu i. ADSL thc hin truyn n thu bao vi tc trn 6Mbps v truyn 640kbps theo c hai chiu.

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Hnh 1.3 Kt ni ADSL

Tc ny nhanh hn tc modem quay s n c trm ln m khng cn thay cp mi. Tht vy, ADSL lm thay i mng thng tin cng cng hin nay t ch ch ti c thoi, vn bn v cc hnh nh phn gii thp tr thnh h thng em li a phng tin k c video cho mi nh ngay t th k 20.

Hnh 1.4 Mng ADSL

ADSL s ng mt vai tr quan trng trong thp k ny hay s tr thnh ph bin khi cc cng ty in thoi cung cp cc thng tin nh video v a phng tin. Mun lp t cp mi dnh ring cho dch v s liu tc cao phc v tt c cc thu bao trong tng lai phi mt hng chc nm. Thnh cng ca cc dch v ny ph thuc vo s thu bao c th phc v ti a trong ch mt vi nm. Bng cch phc v cc dch v video theo danh mc, cung ng CD-ROM qua mng, mng LAN cng tc, Internet n tn nh ring v doanh nghip nh ADSL s gip cc cng ty in thoi v cc cng ty cung cp dch v thm nhp c vo th trng ny.

Internet Access Server

Work-at-home Server

Video-on-demand Server

Info & Advertiser Server

TCP/IP Router

ATM Switch

Access Node

MUX

ADSL n

ADSL 1

ADSL 1

ADSL n

New Services

New Services

Customer 1

Customer n

Central Office

POTS n

POTS 1


20

Kh nng ca ADSL

ng dy ADSL kt ni modem ADSL mi u cui ng dy in thoi xon i, to ra ba knh thng tin ring bit bao gm mt knh downstream tc cao, mt knh thng tin song cng tc trung bnh v knh in thoi cn bn. Knh dch v in thoi c tch ra khi modem s bng mt b tch splitter bo m khng lm ngt dch v in thoi ngay c khi dch v ADSL khng hot ng hay c s c. Knh n cng tc cao c tc t 1,5 n 6,1 Mbps v knh song cng c tc t 16 ti 640 kbps. Mi knh s liu ni trn li c phn chia na thnh nhng knh tc thp hn.

Modem ADSL cung cp tc d liu theo c tiu chun Bc M ANSI T1 1.544 v tiu chun chu u E1 2.048 Mbps v c th c tr cc ph theo nhiu cp tc khc nhau. Cu hnh ti thiu h tr mt knh 1,5 Mbps hay 2,0 Mbps theo chiu downstream v mt knh song cng 16 kbps. Ngy nay c cc sn phm modem vi chiu downstream ln ti 8 Mbps v tc song cng ln ti 640 kbps cung cp chuyn vn ATM vi cc tc khc nhau v tr i cc phn overhead ca ATM cng nh cc nghi thc IP. Tc chiu down ph thuc vo mt s cc yu t khc nhau bao gm c chiu di ca ng dy thu bao, c dy, s hin din ca nhnh r v xuyn nhiu vo ng dy. Suy hao trn ng dy tng theo di ng dy v tn s tn hiu truyn dn v gim khi tng ng knh dy. B qua tc ng ca cc nhnh r, tc ADSL c lit k bng 1.2.

Bng 1.2 Tc ADSL ph thuc vo ng knh v c dy

Tc d liu (Mbps)

C dy (AWG)

Khong cch (feet)

C dy (mm)

Khong cch (km)

1,5 hay 2 24 18.000 0,5 5,5 1,5 hay 2 26 15.000 0,4 4,6 6,1 24 12.000 0,5 3,7 6,1 26 9.000 0,4 2,7

Mc d s nh gi ca tng cng ty in thoi c th khc nhau nhng kh nng trin khai ADSL c th ln ti 95% s ng dy trn mng ngoi vi tu theo tc ADSL mong mun. Cc khch hng xa hn c th c c ADSL qua cc h thng DLC (Digital Loop Carrier). Khi cc h thng DLC c thng mi ho cc cng ty in thoi c th trong mt thi gian ngn em li dch v bt c ni u.

Nhiu ng dng c th hnh dung c cho ADSL bao gm video nn. L tn hiu thi gian thc video s khng th dng cc th tc sa sai mc lin kt s liu hay mc mng nh trong cc h thng thng tin s liu c. V vy, modem ADSL kt hp vi c ch t sa sai gim ng k sai gy

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ra bi nhiu xung (impulse noise). Vic sa sai da trn c s tng k hiu cng gim c cc sai cm do nhiu tc ng trn ng dy.

Cng ngh ADSL

ADSL ph thuc vo tin b ca k thut x l tn hiu s v phng php sng to dn nhiu thng tin qua ng dy in thoi. Thm vo , c nhiu tin b trong cc k thut cc b bin p, b lc tng t v cc b bin i tn hiu tng t sang s (ADC: analog/digital converters). ng dy in thoi di c th lm suy hao tn hiu tn s 1 MHz (gii hn trn di tn truyn dn tn hiu ADSL) khong 90 dB lm cho tn hiu tng t rt kh nhn c my thu. Nhn bn ngoi, ADSL c v n gin l mt dng s liu ng b nhng tc khc nhau truyn qua ng dy in thoi. Tuy nhin bn trong cc mch transistor mi cho thy ngh thut thit k thit b thu pht ADSL. Hnh 1.6 sau minh ho s khi thit b thu pht ADSL mng.

Hnh 1.5 Cc phng php song cng trong truyn dn tn hiu ADSL

to ra cc knh truyn dn modem ADSL chia di tn ca ng dy in thoi bng hai phng php: song cng phn tn v trit tn hiu di (echo cancellation) nh minh ho hnh sau. Song cng phn tn thc hin hai chiu upstream v downstream trn hai di tn khc nhau. Song cng trit tn hiu di truyn chiu upstream chng di tn ln chiu downstream v tch chng ra bng k thut trit tn hiu di c bit trong cc khuyn ngh v modem V.32 v V.34 ca ITU-T. Trong c hai phng php song cng ny ADSL u tch di tn 4 kHz pha DC dnh cho dch v thoi ring ra bng hai b tch dch v POTS splitter hai u ng dy thu bao.


22

Hnh 1.6 S khi modem pht tn hiu ADSL pha mng

Trong cu trc ca modem ADSL dng d liu tng cng t chiu downstream, cc knh song cng, cc knh bo dng c t chc thnh cc khi d liu v thc hin m sa sai trn tng khi d liu ny. My thu thc hin sa sai xy ra trong qu trnh truyn dn. Kh nng sa sai ph thuc vo m sa sai v chiu di khi d liu. Modem c th tng cng kh nng sa sai bng cc ghp xen d liu, sa c cc sai cm di hn v nng cao hiu qu truyn dn d liu cng nh video.

Tiu chun ho ADSL

Hnh 1.7 Tng quan gia cc c quan tiu chun ho

C bn c quan tiu chun ho quan trng nht thit lp cc tiu chun ADSL. Ngay t ban u, cc nh sn xut bn dn v cc nh sn xut thit b tin hnh cc cc th nghim ton cu. Nhng nh sn xut ni bt nht nhm vi nhau thnh lp din n ADSL gi l ADSL forum vi trang web chnh thc l: http://www.adsl.com. Trong s mt s nh sn xut li

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23

thnh lp mt phn nhm ca ADSL forum gi l Nhm cng tc ADSL ton cu (UAWG: Universal ADSL Working Group) vi tiu ch l n gin ho ADSL ADSL tr thnh mt sn phm thn thin vi ngi tiu dng hn. Cc nhm cng tc tiu chun ho tip theo l nhng nhm ng cp quc gia bao gm ANSI v ETSI. ANSI l c quan ban hnh tiu chun u tin nht trn th gii v ADSL. Cui cng c quan tiu chun ho ITU-T c gng kt hp tiu chun s dng ADSL trn ton th gii.

ADSL Forum

39355 California Street, Suite 307, Fremont, CA 94538, Tel: (510) 608-5905, Fax: (510) 608-5917.

Bng 1.3 Bn c quan tiu chun ho ADSL quan trng

C quan tiu chun ho

Nhm cng tc Tiu chun Mc tiu

ADSL Forum UAWG, SNAG TR-0xx Khuyn ngh cng nghip v ADSL

ANSI T1 T1E1.4 T1.413, Issue 2 Tiu chun ADSL c bn

ETSI TM6 Gn ging vi ANSI

ITU-T Study Group 15 G.992.1 G.992.2 G.992.3 G.992.4 G.992.5 G.994.1 G.995.1 G.996.1 G.997.1

Tiu chun quc t

ADSL Forum c thnh lp nm 1994, l t chc tin phong trong vic tiu chun ho ADSL v lm vic nh mt nhm cng tc kho st cc vn cha c cc c quan tiu chun ho cp ti. Hu ht cc vn k thut c cc thnh vin ca din n quan tm lin quan ti giao thc gia cc mng c k thut khc nhau. Din n c bn phn nhm l: UAWG ni trn, SNAG (System Network Architecture Group), MIB (Management Information Base) v nhm th nghim c trch nhim phi hp mi trng th nghim gia cc thnh vin trong din n v cc nh sn xut thit b khc. Nhiu vn k thut ca ADSL Forum cng l cc vn k thut ca ATM Forum (http://www.atmforum.com/).


24

ANSI

11 West 42nd Street, New York, N Y 10036, Tel: (212) 642-4900, Fax: (212) 398-0023.

Vin tiu chun ho quc gia Hoa K (ANSI: The American National Standards Institute) c website chnh thc ti http://web.ansi.org bao gm nhiu u ban hp thnh t nhng thnh vin k thut cng nghip cn bn nht. U ban T1 v c bit hn l tiu ban T1E1 ph trch cc tiu chun lin quan ti ADSL. Tiu ban T1E1 chu trch nhim v cc tiu chun v giao tip, cp ngun v bo v cho mng li. Mi tiu ban li c phn thnh nhiu nhm cng tc. Nhm cng tc T1E1.4 mi thc s lm vic vi cc tiu chun ca phng php truy xut xDSL bao gm ADSL. Truy xut DSL bao gm cc tiu chun lp vt l v k thut truyn dn qua cc giao din. Nhm cng tc T1E1.4 ban hnh tiu chun T1.413 dnh cho ADSL c vo nm 1995. Mt phin bn mi ca n l T1.413, release 2 ra i vi s hp tc ca cc tiu chun n gin ho UAWG.

ETSI

650 route des Lucioles, 06921 Sophia Antipolis Cedex, France, Tel: +33 (0) 4 92 94 43 95/43 64, Fax: +33 (0) 4 93 65 47 16.

ETSI c website chnh thc l http://www.etsi.fr cng c phn chia thnh cc nhm v phn nhm. Nhm cng tc truyn dn v ghp knh (TM: The Transmission and Multiplexing group) c phn nhm TM6 tng t nh phn nhm T1E1A. TM6 thng xuyn lm vic vi T1E1.4 bo m tnh tng hp v sau cho cc tiu chun quc t.

ITU-T

Place des Nations, CH-1211 Geneva 20, Switzerland, Tel: +41 22 730 5857 hay +41 22 730 5859, Fax: +41 22 730 5853.

Lin minh vin thng quc t (ITU: International Telecommunications Union) thng l t chc cui cng tiu chun ho mt k thut mi. Tuy nhin, s kin t chc ny tham gia cc n lc tiu chun ho ADSL ngay t nm 1998 cho thy mong mun hp tc quc t tng ln vi dch v ny. ITU-T ban hnh cc khuyn ngh lin quan ti ADSL trong lot G l lot khuyn ngh v loi nghi thc lp vt l. Cc khuyn ngh hin thi G.DMT l mt bn sao ca T1.413 trong khi G.lite li hp tc nhiu hn vi UAWG. G.test tp trung vo c tnh o th dnh cho xDSL cn G.OAM th ch trng vo cc lnh vc iu hnh, qun l v bo dng xDSL. G.HS tp trung vo cc nghi thc bt tay dng trong giai on m phn lc thit lp phin kt ni.

Chng 1: K thut ADSL

25

Cc website v ADSL

http://www.dsllife.com

http://www.dslforum.org

http://www.uawg.org

http://www.telechoice.com

http://www.xdsl.com

http://www.adsl.com

http://www.point-topic.co.uk

http://www.atis.org

http://www.webtorials.com

http://www.turnstonesystems.com

http://www.telechoice.com

http://www.generalbandwidth.com

http://www.genband.com

http://www.harris.com

http://www.acceleratednetworks.com

http://www.acti.com

http://www.gdc.com

http://www.paradyne.com

http://www.sageresearch.com

http://www.mitel.com

http://www.itracs.com


26

http://www.northpointdsl.com

http://www.efficient.com

http://www.virata.com

http://www.turnstone.com

http://www.telephonyworld.com

http://www.telogy.com

http://www.techweb.com

http://www.alcateldsl.com

http://www.aliant.ca

http://www.aware.com

http://www.videotele.com

Cu hnh mng ADSL

Cu hnh mng ADSL

ADSL l mt kin trc mng hon chnh. ADSL khng ch l mt cch truy xut nhanh cc trang web Internet m ADSL cn l mt phng tin h tr mnh m cc dch v s liu tc cao cho ngi s dng gia nh cng nh doanh nghip nh. Nhng dch v ny c cung cp di mt mi trng cnh tranh v rt phong ph t lnh vc gio dc cho ti lnh vc ti chnh.

Hnh v 1.9 m t chi tit hn mt b ADSL Terminal Unit-Remote (ADSL ATU-R). Thit b c th l mt hp giao tip TV hay mt my tnh c nhn m khng cn thm mt thit b no na. Vic i dy t ATU-R n n thit b u cui c th ch n gin nh i dy 10BASE-T LAN, cng c th phc tp nh mng ATM ring hay mi m nh Consumer Electronics Bus s dng dy in lc sn c gi thng tin. D sao th khi s dng cho truyn d liu tc cao th vic i dy cho dch v thoi vn khng thay i v c cc b splitter dng tch ring cc tn hiu tng t.

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Hnh 1.8 Cu hnh ton mng ADSL

tng i ni ht dch v thoi tng t c chuyn sang cho b chuyn mch thoi PSTN qua mt b tch dch v t ti tng i cn cc tn hiu ADSL c b DSLAM ghp li. D nhin l khc vi ISDN ta khng phi thc hin bt c mt thay i no trn phn mm chuyn mch ca tng i ni ht khi trin khai dch v ADSL. Hn na ADSL li lm gim lu thoi vn lm tc nghn cc thit b chuyn mch, truyn dn thoi do cc dch v d liu gy ra.

Hnh 1.9 Cc thnh phn thit yu ca mng ADSL

Nhng dch v c cng ngh ADSL em li c th t ti tng i ni ht hoc mt ni khc. Cc dch v ny c th do chnh tng i ni ht thc hin hay do cc nh cung cp dch v t nhn c giy php. Cc dch v

Access Node

M U X

ATU-C

ATU-C ATU-C

Telephone(s)

Premises Access

Set-top box PC 10BASE-T (LAN) ATM CEBus,...

Services: Internet Education Video Corporate Commercial Government

PSTN CO Switch

POTS Splitter


28

nh vy bao gm truy xut Internet, cung cp cc ti liu o to, gio dc, video (on-demand hay broadcast), corporate (telecommuting), thng mi (bn sch, xe hi, ...) v ngay c thng tin ca chnh ph (nh thng tin thu chng hn).

Lu rng cc ng lin kt ADSL vn phi s dng cc b DACS (Digital Access and Cross connect) gom lu lng a n cc nh cung cp dch v. D nhin cc nh cung cp dch v ny cng l cc nh cung cp cc ng lin kt ADSL nn tt c cc dch v c th t trc tip ti tng i ni ht nhng trn thc t c hai cch thc hin vic cung cp cc dch v ny. Theo cch th nht th cc ng lin kt ADSL c tp trung li ti DSLAM v chuyn sang cho thit b DACS. DACS a n h thng truyn dn tc cao nh ng truyn T3 khng phn knh chng hn c tc 45 Mbps dn n nh cung cp dch v Internet. Tt c cc lin kt u c kt thc ti b nh tuyn Internet v cc gi d liu c chuyn vn hai chiu nhanh chng vi Internet. Cc mng Intranet cng tc cng c cu hnh tng t. y l phng php n gin nht. Tuy nhin, tc tng cng ca cc lin kt ADSL theo phng php ny khng c qu 45 Mbps theo mi chiu.

Phng php th hai l im truy xut (access node) c lin kt trc tip ti mt b nh tuyn IP hay b chuyn mch ATM t gn im truy xut. B nh tuyn IP thng dng trong trng hp ny l B-RAS (Broadband Remote Access Server). phng php ny, vn s dng vic tp trung lu lng vo mt ng truyn vt l t im truy xut ti b nh tuyn IP hay b chuyn mch ATM. im truy xut l mt tiu im gy nhiu ch trong cng vic tiu chun ho ADSL. Hin nay, hu ht cc im truy xut ADSL u ch thc hin vic ghp lu lng n gin. iu ny c ngha l tt c cc bit d liu v gi d liu vo ra im truy xut u c truyn ti bng cc mch n gin. Chng hn, trong trng hp tng i in hnh nu c 10 khch hng ADSL nhn d liu theo chiu downstream vi tc 2 Mbps v gi d liu theo chiu upstream vi tc 64 Kbps th lin kt gia im truy xut v mng dch v (nh Internet chng hn) phi c dung lng ti thiu l 10 2Mbps = 20Mbps cho mi chiu trnh hin tng tc nghn hay b b bt gi s liu. Mc d tc d liu theo chiu xung l 64 Kbps 10 = 640 Kbps nh hn 20 Mbps rt nhiu nhng do bn cht truyn dn i xng ca cc ng truyn ghp knh s nn tc hai chiu phi nh nhau.

Vic ci tin cc h thng ADSL cn bn c th c l thc hin ghp knh thng k (statistical multiplexing) im truy xut ADSL hay cung cp cho b DSLAM ADSL mt vi kh nng chuyn mch gi trc tip. Nu thc hin ghp knh thng k th da trn bn cht xut hin tng cm ca s liu kiu gi b tr cc ng lin kt tc thp hn v khng phi lc no tt c cc khch hng u ang gi gi s liu. Trng hp nu im truy xut ADSL c sn b nh tuyn hay kh nng chuyn mch ATM th

Chng 1: K thut ADSL

29

dung lng ghp cng nh hn. D thc hin cch no th dung lng ghp cng gim hn 20 Mbps.

Kt ni ADSL G.dmt ti thu bao

Hnh 1.10 Kt ni ADSL G.dmt ti thu bao

B tch dch v POTS (POTS splitter) bao gm mt b lc thng thp cho tn hiu POTS i qua v mt b lc thng cao cho tn hiu ADSL i qua. Ngoi ra b tch dch v POTS cn c chc nng cch ly DC dng in mt chiu ca tng i in thoi khng chy qua modem ADSL. Tt c cc thit b trong mng thoi ca thu bao nh tng i in thoi ni b PABX (Private Automatic Branch Exchange), my in thoi, my tr li in thoi, my fax, modem quay s,... u phi nm ng ra PHONE ca b tch dch v POTS. Mng thit b ADSL c kt ni vi modem ADSL v ni vi ng dy qua ng ra MODEM ca POTS splitter.

Kt ni ADSL G.lite ti thu bao

Hnh 1.11 Kt ni ADSL G.lite ti thu bao


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Hnh 1.12 B In-line filter

Vi cu hnh G.lite ta khng cn s dng b POTS splitter m thay vo l cc b In-line filter hay cn gi l Microfilter l cc b lc thng thp c mc vo trc tt c cc thit b thoi trong mng hi ca tn hiu thoi t cc thit b ny khng gy nhiu cho tn hiu ADSL.

Cc k thut c trng dng trong ADSL

K thut chng nhiu xung

Nhiu xm nhp xut hin my thu khi tn hiu trong khng kh trong di tn trng lp tc ng ln ng dy in thoi. Nhiu xung l nhng xung nhiu c thi gian tn ti ngn nhng li c nng lng ln c kh nng nht thi nhn chm thng tin trn knh truyn. Nhiu xung trn dy xon i thng l do cc thit b in t v in c cng nh cc dng in trong gia nh khc. Thm vo s phng in ca ng dy in lc cng nh st cng l ngun gy ra nhiu xung.

Hnh 1.13 Nhiu xung trong min thi gian v trong min tn s

Trong min tn s sut khong thi gian tn ti, nhiu xung c c tnh ho bng mt ph nng lng phng v di rng nh minh ho hnh v 1.13. V s xut hin ca nhiu xung l khng th bit trc c nn nhiu xung c th ph hu nng nhng h thng khng c thit k tt.

Chng 1: K thut ADSL

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khc phc nhiu xung cc h thng c thit k tt s dng m sa sai FEC (Forward Error Correcting) kt hp vi ghp xen k d liu nh minh ho hnh v 1.14. M sa sai FEC thm thnh phn ph vo lung d liu truyn i sao cho khi c mt phn ca tn hiu b ph hu v nhiu xung th nhng phn cn li ca tn hiu cng cha thng tin c th phc hi li phn thng tin b mt. FEC c p dng theo tng byte ca khi d liu v c c tnh ho bi s byte n c th sa c trong mt khi d liu c di cho trc. Khi s byte sai trn thc t vt qu s byte b m c th sa c th dng d liu sau khi sa sai vn cn thng tin sai. ti u ho kh nng sa sai ca b m sa sai my pht s dng phng php ghp xen ln d liu. Sau khi p dng m sa sai FEC b ghp xen k (interleaver) sp xp li th t ca cc byte sao cho khi c tc ng ca nhiu xung ln dng d liu n s ph hu mt s byte m lc gii ghp xen k th cc byte sai s c tri ri rc ra. Nh vy ghp xen k lm gim thi gian nhiu xung ph hy d liu v to c hi cho m sa sai FEC pht huy tc dng.

Hnh 1.14 M hnh n gin ca vic tng cng hiu qu ca FEC bng tch ghp lung d liu

Bit swapping

Mt trong nhng c tnh quan trng v thit yu nht ca ADSL l k thut bit swapping. Bit swapping thc hin cc tnh nng ca ADSL bng cch:

Cho php p ng lin tc s thay i iu kin vt l ca ng dy gy ra bi tc ng ca mi trng.

p ng cc tnh hung xuyn knh thay i khi cc tc nhn xuyn knh mnh ln hay yu i.

Cho php tc d liu c th thay i m khng cn phi khi ng li kt ni.

Cho php nhiu ng dy ADSL c th tng hp ln nhau trong rng buc v mt ph cng sut.

Cho php thc hin tnh nng SRA.


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Hnh 1.15 C ch knh AOC

Hnh v 1.15 v 1.16 minh ho khi nim c bn v c ch ca hot ng bit swapping. Trong mi modem ADSL u c mt b iu khin iu ch DMT (DMT controller) B iu khin ny t s bit v mc tn hiu pht cho tng di tn con trong tn hiu ADSL. Trong knh AOC (ADSL Overhead Control hay Amati Overhead Control, Amati l cng ty u tin pht trin ADSL v s dng knh AOC) lin lc gia hai modem c mt nghi thc thc hin bit swapping.

Hnh 1.16 C ch knh AOC khi ACK b s c

C ba kiu lnh quan trng trn knh AOC: swap request (yu cu swap), swap acknowledge (xc nhn swap) v extended swap request (yu cu swap m rng). Cc thng ip swapping c nhn dng trn ton b knh truyn nh vo byte u tin l header ca thng ip.

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Bng 1.4 Cc loi thng ip trong knh AOC

Header di thng ip (byte) ngha

000011112 Khng xc nh Thng ip ti cu hnh 1100xxxx2 Khng xc nh Thng ip xc nh nh cung cp 111100002 1 Thng ip ch c header 111111002 13 Thng ip Extended bit swap request 111111112 9 Thng ip Bit swap request 111111112 3 Thng ip Bit swap acknowledge

Bit swap request Lnh swap request t my thu ti my pht trn knh AOC c thm 8 byte thng tin gm 4 trng 2 byte. Byte u xc nh s th t di tn con (t 0 ti 255) v byte th hai xc nh tc ng cn thit (tng/gim mt bit trong di tn con hay tng/gim cng sut pht trong di tn con 1, 2, 3 dB hoc chng thay i g).

Bng 1.5 Thng ip Bit swap request

Message header Message fields 1-4

{111111112} (8 bits)

Command (8 bits)

Subchannel index (8 bits)

Thng thng s th t di tn con c pht hai ln vi ln u iu chnh s bit v ln th hai iu chnh cng sut pht.

Bng 1.6 Cc loi thng ip trong bit swap request

Value ngha

000000002 Khng thay i

000000012 Tng s bit ln 1

000000102 Gim s bit i 1

000000112 Tng cng sut pht 1 dB



000001102 Gim cng sut pht 1 dB

000001112 Gim cng sut pht 2 dB

00001xxx2 Dnh cho nh sn xut


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Bit swap acknowledge Trong vng 400 ms sau khi nhn c thng ip yu cu bit swap, my pht s gi mt thng ip bit swap acknowledgment bao gm: message header v 2 byte ph, byte th nht l mu ton bit 1, byte th hai xc nh s th t ca superframe bt u iu chnh s bit/cng sut pht. Superframe c nh s th t t 0 ngay khi bt u truyn d liu khch hng theo module 256 trong vng 68 k hiu DMT (hay 69 nu tnh lun c k hiu ng b).

Bng 1.7 Cu trc thng ip bit swap acknowledge

Message header Acknowledge command

Bit swap superframe counter number

11111111 (8 bits)

11111111 (8 bits)

(8 bits)

Extended bit swap request Thng ip extended swap c thm 12 byte, cho php xc nh c ti 6 di tn cn iu chnh s bit/cng sut. Thng ip extended swap c th c dng iu chnh t 2 bit ti 0 bit v ngc li trn mt di tn con bng cch pht i lnh tng gim hai ln v iu chnh cng sut pht trn cc di tn con.

Bng 1.8 Cu trc thng ip Extended bit swap request

Message header Message fields 1-6

{111111002} (8 bits)

Command (8 bits)

Subchannel index (8 bits)

Sp xp tn s

Tn hiu DMT trn min thi gian c t s gi tr nh trn gi tr trung bnh rt cao. Khi nh ca cc tn hiu ln, nhiu bit b b bin i s sang tng t DAC xn u to ra nhiu cng m. T s SNR cng ln, cng m ho bng nhiu bit v cng b xn nhiu. Nu d liu qua b Interleaver th c th dng m sa sai FEC loi tr c nhiu cng m ny. V l do ny ngi ta gn 8 Nf bit cho cc tone t bit v 8 Ni bit cho cc tone nhiu bit.

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T1532470-99

SNR

3 bit

5 bit

7 bit

5 bit

3 bit

b0' = b5 = 0b1' = b0 = 3b2' = b4 = 3b3' = b1 = 5b4' = b3 = 5b5' = b2 = 7

Tone ordered

0 bittoneindex

0 1 2 3 4 5

Data frame buffer

1sbFast data buffer

msb 1sbInterleaved data buffer

msb

d0 d1 d2 d3 d4 d5 d6 d7 d0 d1 d2 d3 d4 d5 d6 d7

b0' + b1' - 1 b2' + b3' - 3 b4' + b5' - 3

tone 5 tone 0 tone 4 tone 1 tone 3 tone 20 bit 3 bit 3 bit 5 bit 5 bit 7 bit

Hnh 1.17 Tone ordering v tch bit (khng c m ho trellis)

Nghi thc kt ni ADSL

nh gi cc nghi thc mng dng cho ADSL

DSL Forum tho lun trong TR-43 v mt vi kin trc nghi thc mng c cc nh khai thc s dng cho ADSL. Ti liu ny lit k nhng tiu chun nh gi thch hp ca mt kin trc nghi thc i vi dch v ADSL.

Mc d TR-43 m t cc nghi thc hin ang s dng nhng tiu chun ny cng c th dng c cho bt c cc loi kin trc nghi thc no s c s dng cho ADSL trong tng lai.

Hiu qu truyn ti ca payload Hiu qu truyn ti payload ca mt kin trc xp chng nghi thc ph thuc vo lng overhead. Hiu qu ny thc ra ph thuc vo kch thc header ca nghi thc v t s gia phn header v d liu truyn c.

Kh nng d sai Khn nng d sai ca mt kin thc xp chng nghi thc l kh nng d ra cc sai do nhiu tc ng vo d liu trong qu trnh truyn dn trn knh


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truyn. Ngoi ra mt vi nghi thc c cho php sa c sai theo c ch FEC (Forward Error Correction).

Ghp phin Cc nghi thc c kh nng h tr a phin t pha ngi s dng ln pha mng hay khng. Nhng phin ring bit ny c th h tr nhiu ngi s dng hay nhiu dch v t mt ng dy. Cc phin dch v ring bit c th gip kt ni vi cc nh cung cp dch v khc nhau.

H tr a nghi thc L kh nng h tr nhiu nghi thc lp cao khc nhau qua nhng nghi thc lp di ging nhau.

T cu hnh v qun l T quan im ca thit b v phn mm pha khch hng th nhng thng tin no cn thit cu hnh thit v v lin lc trn mng nu dng kin trc xp chng nghi thc ang xt. Lng thng tin ny c truyn sn trong nghi thc l bao nhiu? Nhng thng tin no c th c cung cp cho thit b ti sn khch hng qua nhng nghi thc qun l c bit? Nhng nghi thc qun l ny l g? Cn phi trao i thng tin v hp tc qun l gia nh cung cp mng v nh cung cp dch v mc nh th no cu hnh c dch v? Khch hng c cn phi c thng tin c th cu hnh nhn cng thit b ca h vi mng khng?

La chn dch v Kin trc xp chng nghi thc c cho php ngi s dng chn la cc dch v qua mng hay khng? Kin trc xp chng nghi thc cung cp chc nng ny nh th no?

Bo mt Thng tin trn lin kt s dng kin trc xp chng nghi thc ang xt c bo mt nh th no? Nhng c tnh no l vn c ca nghi thc cung cp bo mt cho ngi s dng v nh cung cp mng? Nhng nhc im c trng ca kin trc xp chng nghi thc l g?

AAA (Authentication, authorization, and accounting) AAA l nhn thc, cp php v tnh cc. Kin trc xp chng nghi thc c xt h tr nhng chc nng ny nh th no? N tng tc vi cc cng c hin ti h tr chc nng ny nh th no?

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Tnh trng tiu chun ho Nhng phn no ca kin trc xp chng nghi thc c tiu chun ho? Vi nhng phn cha c tiu chun ho th hin trng tiu chun ho nh th no?

Mi trng ly ATM lm c s

Hu ht cc nh cung cp dch v ADSL trn th gii u s dng ATM cung cp kt ni hai u gia thit b ti sn ca khch hng (CPE) v nh cung cp dch v cung cp truy xut ti dch v c khch hng s dng.

Vic s dng ATM lm mng li cho mng truy xut nh hnh v 1.18 c mt s thun li cho nh cung cp truy xut.

Hnh 1.18 Tng quan v s dng ATM trong kin trc truy xut DSL

Cc cng ty in thoi ang s dng rng ri mng ATM cung cp cc dch v d liu khc cho khch hng ca h. H kh quen vi kin trc, k thut v qun l ATM. Bn cht nh hng kt ni ca ATM cung cp mt kin trc m cc cng ty in thoi thy thun tin v d qun l.

Khi s khch hng trong mng tng nhanh th mng li dng ATM vn hot ng tt. Mng c th c thit k sao cho vic thm khch hng mi vn khng nh hng ti cht lng dch v ca mi khch hng c.


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Cc mng v cc nghi thc ATM h tr nhiu loi cht lng dch v khc nhau cho cc khch hng khc nhau trn cng mt mng hay cung cp nhiu dch v cho cng mt khch hng. iu ny lm cho cc nh cung cp mng tng c kh nng gii thiu vi khch hng nhiu dch v.

Bng 1.9 Phm cht ca kin trc da trn ATM

Criteria ATM Qualities

Payload efficiency ATM s dng header 5 byte cho mi cell 53 byte lm cho phn header chim 10% vi tt c cc dch v truyn trn ATM.

Session multiplexing V bn cht ATM cung cp a phin qua vic s dng cc mch o (VC: virtual circuit) gia mng v thit b ti sn khch hng.

Multiprotocol support

Mt mch o ATM c th h tr nhiu kin trc xp chng nghi thc khc nhau. Nhiu mi trng da trn c s IP c th c chp nhn qua AAL5. AAL1 thch hp cho chuyn vn nhng dch v cn m phng mt mch im ni im tc khng i v AAL2 c pht trin truyn tn hiu s mt cch hiu qu.

Autoconfiguration and management

Giao tip qun l cc b tch hp (ILMI: Integrated Local Management Interface) cung cp mt phng php cu hnh dch v ATM pha ti sn khch hng. TR-37 ca DSL Forum m t vic s dng ILMI cu hnh cho cc dch v DSL.

Service selection Mc d vic s dng cc mch o c nh (PVC: Permanent Virtual Circuit) i hi phi ti cu hnh, vic dng cc mch o c chuyn mch (PVC: Permanent Virtual Circuit) cho php lin lc gia thit b khch hng v cc nh cung cp dch v c thit lp trong thi gian thc theo yu cu. Thit lp SVC c th c s dng nh mt phn ca qu trnh chn la dch v hay ngi s dng c th chn PVC c cu hnh trc thch hp cho dch v.

Security and AAA S h tr cc VC hai u gi lin lc gia nhng khch hnh khc nhau v cc dch v khc nhau cho cung cp thm c ch bo mt. Cc chc nng bo mt ca cc lp trn ca kin trc xp chng nghi thc khng b nh hng bi vic s dng ATM.

Error detection Nhng sai nhm header ca cc cell ATM trong qu trnh truyn dn s b d thy.

Standards status ATM l tiu chun ca ITU-T v ATM Forum.

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AAL5 Lp thch ng ATM 5 (AAL5) xc nh phng php truyn ti cc dch v gi (nh IP chng hn) qua cc mch o ATM. Mt PDU AAL5 ti trn nhiu cell ATM v c kh nng d sai trn d liu truyn dn.

Bng 1.10 lit k nhng phm cht ca AAL5 khi s dng cho DSL.

Bng 1.10 Phm cht ca AAL5

Criteria AAL5 Qualities

Payload efficiency AAL5 thm 8 byte vo ui mi PDU. Hiu qu truyn dn ca trng ti tin payload ph thuc vo di ca PDU (di ti a 65,535 byte).

Session multiplexing AAL5 khng h tr ghp a phin trong mt VC. Cc mc cao ca kin trc xp chng nghi thc c AAL5 h tr s phi h tr a phin.

Multiprotocol support RFC 2684 m t phng php ng gi (encapsulating) nhiu nghi thc khc nhau qua AAL5.


Vic s dng AAL5 trn cc mch o trn kt ni ADSL c th lin lc vi thit b ti sn ca khch hng qua giao din ILMI.

Service selection y l chc nng ca cc nghi thc lp cao. Security and AAA AAL5 khng m rng cng nh khng gim bt chc

nng bo mt ca cc lp trn hay di n trong kin trc xpc chng nghi thc.

Standards AAL5 c tiu chun ho trong khuyn ngh ITU-T I.363.5.

Kin trc xp chng nghi thc da trn AAL5 c s dng c bn cho hu ht cc nghi thc hai u DSL. Hnh v 1.19 minh ho kin trc xp chng nghi thc ny.


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Hnh 1.19 Bn kin trc xp chng nghi thc thng dng qua ATM/AAL5 giao din ngi s dng mng (giao din U)

Ghp cc nghi thc khc qua AAL5 RFC 2684 ca IETF xc nh mt vi phng php ng gi cc nghi th d liu qua cc mch o ATM. N nh ngha cc giao din cho php mch ATM c dng lm mt cu chuyn vn cc nghi thc mc lin kt s liu (nh Ethernet chng hn) n u bn kia. N cng xc nh cc giao din cho php cc nghi thc nh tuyn nh IP c chuyn vn trc tip qua cc mch o ATM. RFC 2684 h tr hai phng php khc nhau chuyn vn lu lng mng nh hng kt ni, cc PDU (Protocol Data Unit) qua mng ATM.

Phng php th nht cho php ghp cc nghi thc khc nhau qua mt VCC (virtual circuit connection: kt ni mch o). Nghi thc c PDU chuyn vn c xc nh bi phn u ca PDU l LLC header IEEE 802.2. Phng php ny gi l ng gi LLC (LLC encapsulation).

Phng php th hai thc hin ghp nghi thc lp cao hon ton bng VC v c gi l phng php ghp VC: VC multiplexing.

ng gi LLC c dng chuyn vn Ethernet qua VC ATM trong mi trng ADSL v VC multiplexing c dng chuyn vn trc tip IP qua lp thch ng AAL5.

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Hnh 1.20 Bridged Ethernet chuyn vn trc tip IP

Bng 1.11 Phm cht ca IP over Ethernet

Criteria IP over Ethernet Qualities

Payload efficiency Ethernet cn mt header 8 byte cho mi gi Ethernet 1.492 byte. Phng php ng gi LLC RFC 2684 cn thm mt trng header di 8 byte na.

Session multiplexing ng gi RFC 2684 khng h tr trc tip phin. Tuy nhin, cc nghi thc lp cao (nh PPPoE chng hn) c th h tr cc phin.

Multiprotocol support RFC 2684 m t cc phng php ng gi a nghi thc qua AAL5. Ethernet c th h tr a nghi thc.


ILMI ca ATM c th c s dng ch th rng ang dng ng gi LLC. Nu IP c truyn ti trc tip qua Ethernet th cc cng c IP tiu chun nh DHCP c th c dng cu hnh thit b khch hng qua mng.

Service selection y l chc nng ca cc nghi thc lp cao. Security and AAA y khng phi l chc nng c RFC 2684 cung cp.

Cc nghi thc lp cao c th c chuyn vn qua ng gi LLC v c th cung cp cc chc nng ny.

Standards IETF RFC 2684.

Hnh v 1.20 minh ho kin trc truy xut DSL dng IP qua bridged Ethernet. Dng phng php ng gi LLC RFC 2684 cho php cc gi Ethernet c chuyn vn qua cc VC ATM h tr thch ng AAL5. Nu cc PDU


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Ethernet chuyn vn cc gi IP th cc gi IP c thc hin trn mng LAN s c truyn ti qua Ethernet trn kt ni ADSL. PPP over Ethernet cng s dng Ethernet c ng gi LLC chuyn d liu mng LAN ti u kia ca mng.

Bng 1.12 Phm cht ca IP trn AAL5

Criteria IP on AAL5 Qualities

Payload efficiency Khng cn thm overhead ngoi lng overhead vn phi c ca cc thnh phn ATM, AAL5, and IP trong kin trc xp chng nghi thc.

Session multiplexing IP trc tip qua AAL5 khng h tr a phin Multiprotocol support H tr IP v cc nghi thc qua IP Autoconfiguration and management

Vic s dng IP qua AAL5 c th c lin lc bng giao din ILMI. Cc lp IP ca xp chng nghi thc c th c cu hnh bng cc cng c IP chun.

Service selection y khng phi l mt chc nng ca xp chng nghi th ny

Security and AAA Chc nng ny khng thuc v kin trc xp chng nghi thc ny. Tuy nhin cc cng c bo mt nh Ipsec c th c h tr qua IP.

Standards VC multiplexing c xc nh RFC 2684.

Hnh 1.21 IP trc tip trn AAL5

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IP c th c truyn ti trc tip trn AAL5 dng ghp knh VC RFC 2684. Trong trng hp ny mt b nh tuyn pha khch hng kt cui mch DSL v VC ATM AAL5. Hnh v 1.21 minh ho mt mng truy xut h tr IP trc tip qua AAL5.

S dng PPP Nghi thc im ni im (PPP: Point-to-Point Protocol) c xc nh l tiu chun Internet cho truyn dn cc gi IP qua cc ng truyn ni tip. PPP h tr vic gn v qun l cc a ch IP, ghp nghi thc mng, cu hnh lin kt, d sai v m phn nn d liu. Nghi thc ny c m t trong RFC 1661 ca IETF. Mc d ban u c xc nh cho kt ni im ni im nh kt ni qua modem quay s nghi thc ny c sa i cho cc kt ni tc cao nh DSL chng hn. C hai phin bn ca PPP c xy dng:

PPPoA (PPP over ATM)

PPPoA c m t trong RFC 2364 ca IETF v TR-17 ca DSL Forum. Hnh v sau minh ho kin trc xp chng nghi thc v kin trc mng ca PPPoA.

Hnh 1.22 PPP over ATM

PPP over ATM c thit k c bit h tr mi trng m ngi thit b khch hng h tr kt cui c phin ATM ln phin PPP. V d, mt my tnh c nhn h tr kin trc xp chng nghi thc ATM trc tip v c modem ADSL lp trong c th h tr nghi thc ny. Tng t nh vy, mt b nh tuyn s dng ADSL lm truy xut WAN c th kt cui mt phin PPP over ATM vi nh cung cp dch v trong khi nh tuyn cc b mng LAN.


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Bng sau cho thy phm cht ca PPP over ATM nh mt nghi thc mng cho truy xut ADSL.

Bng 1.13 Phm cht dch v PPP over ATM

Criteria PPP over ATM Qualities

Payload efficiency Khng cn thm lng overhead ngoi lng cn thit cho cc thnh phn ca xp chng nghi thc ATM, AAL5, PPP.

Session multiplexing Ch c phin PPP mi c th c h tr qua VC ATM. Multiprotocol support PPP over ATM tha k kh nng h tr PPP a nghi

thc. Autoconfiguration and management

PPP over ATM tha k cc cng c qun l c cc thnh phn ca kin trc xp chng nghi thc ATM v PPP cung cp.

Service selection C th dng qu trnh m phn PPP chn truy xut n mt dch v nht nh. Hn na, cc VC ATM c th c dng truy xut cc dch v khc nhau.

Security and AAA Chc nng c PPP cung cp cho php h tr bo mt v AAA cho PPP over ATM.

Standards Xc nh trong IETF RFC 2364.

Hnh 1.23 PPP over Ethernet

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Bng 1.14 Phm cht dch v PPP over Ethernet

Criteria PPPoE Qualities

Payload efficiency PPPoE thm 6 byte vo header ngoi nhng lng header tng thm do AAL5, Ethernet v PPP. V Ethernet c th c truyn ti qua mng ATM, overhead ca Ethernet c thm vo lng overhead ca lin lc hai u.

Session multiplexing a phin PPP c th c ghp li qua mng LAN Ethernet v qua mt VC ATM.

Multiprotocol support

PPP cho php h tr a nghi thc qua mi phin. Nu PDU cha mt trng nhn dng nghi thc nhn dng loi trng ti tin. Hn na, v Ethernet c m rng qua WAN (dng RFC 2684), cc nghi thc khc vi PPP c th c h tr trn cng mt VC ATM ng thi vi cc phin PPPoE.


Chc nng t cu hnh c ATM, PPP v IP h tr.

Service selection Kh nng m phn mt phin PPP ring t mi u cui ca mng LAN cho php mi ngi s dng truy xut mt dch v pha mng khc nhau.

Security and AAA Chc nng bo mt c PPP cung cp cng dng c vi PPPoE.

Standards Xc nh trong RFC 2516 ca IETF. y l mt RFC ca rt nhiu thc t cng nghip.

PPP over Ethernet c thit k gii quyt vn kt ni nhiu thit b qua truy xut ADSL t pha thit b khch hng. Nghi thc gii quyt vn ny bng cch cho php truyn ti nhiu phin PPP qua mt LAN Ethernet sau khi c ghp vo AAL5 trn kt ni giao din WAN ADSL. Cng nh vi PPP over AAL5, PPP over Ethernet c th cung cp cho mt ngi s dng PC mt mi trng truy xut tng t nh modem quay s. Tuy nhin, cc phin PPP c th c ghp qua mt VC ATM, cho php h tr tng i n gin nhiu thit b truy xut cng lc qua giao din ADSL v kh nng cho tng ngi s dng trong mng LAN c th chn nh cung cp dch v qua mt VC ATM duy nht. V vy, cu hnh ca mng ATM rt n gin khi h tr PPP over Ethernet, mt VC c th h tr cng lc nhiu ngi s dng hay nhiu dch v m khng cn phc tp, tn km khi mua sm mt router pha khch hng. Hnh v 1.23 minh ho kin trc ca mng PPP over Ethernet.

Ethernet trc tip qua ADSL

Kh nng h tr cc mng rng ca Gigabit Ethernet da trn cp quang l mt cng ngh ni tri cho mng truy xut ca mt cng ty khai thc. Nu nhng mng ny pht trin th hin nhin ADSL tr thnh chn la hin nhin cung cp cho truy xut khch hng ti cc mng mi ny. Hnh v


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1.24 cho mt cch nhn tng quan cc lp cao v vic s dng ADSL cung cp truy xut ti cc mng nh vy.

Hnh 1.24 Tng quan v s dng ADSL truy xut mng Ethernet dung lng cao

Gigabit Ethernet c sc hp dn tr thnh mt chuyn vn li cho cc mng truy xut dung lng ln:

Hu ht cc mng LAN pha ngi s dng u dng cng ngh Ethernet. V vy, m rng nhng mng LAN Ethernet ny trc tip qua mng truy xut tc cao cho mt kin trc mng n gin.

S pht trin ca cc router c kh nng nh tuyn gi v phn phi tc gigabit hay cao hn lm cho s pht trin ca nhng mng ny tng i vng chc.

Vn u t cho cc kin trc Ethernet dung lng ln nh hn khi so vi cc kin trc da trn ATM hay IP trn SONET.

Mt iu chc chn l vic qun l nhng mng ny s n gin hn so vi cc mng khc nhiu.

Khi nhng mng c pht trin, vic s dng ADSL truy xut thu bao cho nhiu khch hng tr nn khng cn thit.

Kh nng h tr Gigabit Ethernet trc tip ca si quang n ch thu bao ch kinh t vi cc thu bao ln ca mng Ethernet dung lng cao. Cc thu bao khc vn cn giao din ADSL c tc thp nhng r tin hn.

Chng 1: K thut ADSL

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Bng 1.15 Phm cht dch v Ethernet trc tip qua ADSL

Criteria Ethernet Directly over DSL

Payload efficiency Khng cn hay ch cn mt lng nh overhead ngoi lng overhead cn thit cho Ethernet v cc nghi thc trn n trong kin trc xp chng nghi thc.

Session multiplexing Ethernet c kh nng h tr cc phin qua vic s dng PPPoE.

Multiprotocol support V Ethernet c m rng qua WAN, bt c nghi thc no qua Ethernet u c th truyn ti c.


Ethernet l t cu hnh.

Service selection Cc chc nng chn la dch v ca cc nghi thc trn Ethernet vn dng c.

Security and AAA Bo mt l mt chc nng ca cc lp cao trong kin trc xp chng nghi thc.

Standards Ethernet c xc nh lot tiu chun IEEE 802. S iu chnh Ethernet s dng vi ADSL cn phi c tiu chun ho.

Ch c ADSL truyn trn dy ng xon i mi c th cung cp truy xut cho bt c thu bao no c yu cu.

Mc d c nhiu c gng c thc thi tiu chun ho mt lp vt l Ethernet qua dy in thoi xon i (phc tho Ethernet in the last mile ca IEEE), tiu chun v vic s dng ADSL vn l rng ri kh nng h tr Ethernet trc tip qua lp vt l ca ADSL. Cho d cn phi thm cc tiu chun cho Ethernet trc tip qua giao din ADSL, cng vic ny vn ang tin trin v da vo nn tng cc tiu chun lp vt l ca ADSL.


48

Cu hi t kim tra 1 Cho bit phng php iu ch b nh hng bi nhiu in lc

mc thp nht. A. 2B1Q B. HDB3 C. AMI D. DMT E. NRZ Hng dn: l loi tn hiu c ph khng cha tn s in lc 50 Hz. 2 Tnh nng no khng c trong b POTS splitter? A. Gi ho thoi B. Lc thng thp C. Ngn dng mt chiu D. Lc thng cao E. Tt c u sai Hng dn: POTS splitter ch tch thoi phn tn. 3 Thit b mng no thc hin qun l cc ca thu bao ADSL? A. B-RAS B. DSLAM C. MDF (Main Distribution Frame) D. CO Modem ADSL (ATU-C) E. CO POTS Splitter Hng dn: l thit b qun l lp cao nht trong cc thit b c lit k. 4 Yu t no ca DLC ngn cn vic trin khai ng dy DSL? A. ng dy thu bao di B. Ch h tr ng dy thu bao tng t C. Suy hao ln D. Khng c chuyn mch E. M ho PCM Hng dn: Tn hiu DSL khng th qua c b lc thng thp 4 KHz. 5 Trong k thut ADSL nhiu xung c khc phc bng: A. S dng m sa sai FEC B. S du ng nghi thc sa sai ARQ C. S dng m sa sai FEC kt hp m nn d liu D. S dng mch lc tn s thch hp E. S dng m sa sai kt hp tch ghp lung d liu Hng dn:

Chng 1: K thut ADSL

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Nhiu xung lm sai cm nn c trn khc phc sai cm bng m sa sai t bit l Reed-Solomon 6 Dch v truy xut Internet no khng lm pht sinh cc in thoi

quc t ngoi mun ca khch hng? A. MegaVNN B. VNN1260 C. VNN1267 D. VNN1268 E. VNN1269 Hng dn: Dch v khng lm pht sinh cc ph quc t, ng di l dch v khng qun l tn s thoi. 7 Nguyn nhn gy ra nhiu ISI? A. Xuyn knh B. c tnh tr ca knh truyn C. c tnh p ng tn s c hn ca knh truyn D. c tnh c nh ca knh truyn E. Tt c u ng Hng dn: Nhiu xung do ph tn ca tn hiu ban u khng c truyn y ti u thu. 8 Thoi phn tn trn ng dy ADSL s dng k thut chuyn mch

no? A. Chuyn mch s B. Chuyn mch knh C. Chuyn mch gi D. Chuyn mch ATM E. Tt c u ng Hng dn: Thoi phn tn l thoi POTS ca mng PSTN thng thng. 9 Modem ADSL c t trong thit b no? A. B-RAS B. MDF C. POST Splitter D. DSLAM E. DLC Hng dn: Trong cc thit b mng trn ngi ta thng t modem l mt ngn ca t my DSLAM. 10 Cng ngh ng dy thu bao s no c th c c ng dy

thu bao di nht vi cng mt c dy?


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A. ADSL2 B. ReachDSL C. M/SDSL D. ADSL2+ E. VDSL Hng dn: Cng ngh DSL dng iu ch tn s thp s c suy hao nh nht nn truyn ti c trn ng dy di nht. 11 Tng i no cn s dng c khi trin khai ADSL? A. Tng i in thoi ni b B. Tng i c in C. Tng i in t tng t D. Tng i in t s E. Tt c u ng Hng dn: ADSL khng dng tng i PSTN. 12 Tn hiu thoi v tn hiu ADSL ca mt thu bao c truyn dn

nh th no? A. Phn tn B. Trn 2 i dy ring l C. Phn thi gian D. Trn ng cp quang E. Trn ng cp ng trc Hng dn: ADSL ch c hai phng php song cng l theo tn s v trit tn hiu di. 13 Cho bit kin trc xp chng nghi thc nhiu tng nht. A. IP over AAL5 B. IP over Ethernet C. PPPoA D. PPPoE E. Tt c u nh nhau Hng dn: Ch c PPPoE l phi qua nhiu nghi thc nht. 14 Nghi thc no lp cao nht trong cc nghi thc sau? A. PPP B. IP C. ADSL D. ATM E. AAL5 Hng dn: IP l nghi thc lp cao nht.

Chng 1: K thut ADSL

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15 Cho bit mc ch ca k thut tone ordering. A. Nn d liu truyn c nhanh hn B. Gim sai bit C. Tng di tn truyn dn tn hiu ADSL D. Thc hin song cng E. Pht hin v sa sai d liu trn ng truyn ADSL Hng dn: Tone ordering phc v mc ch gim sai bit lung d liu fast.

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Chng 2: O TH NH GI MNG CP NG S DNG CHO ADSL

c tnh mng cp ng

o th cp ng

nh gi cp ng

Thit b o th ADSL

c tnh mng cp ng

Cp ng trong thi i mi

Hin trng cp ng

H tng ng dy thu bao thng c cc cng ty c quyn vin thng xy dng quy m, lu i. Nhng cng ty ny s hu v qun l ng dy thu bao v dng cung cp dch v in thoi cho cc thu bao. Cng ngh c cc cng ty in thoi ny s dng cng kh n gin v khng yu cu cht lng mng cao lm.

Thc t cc cng ty in thoi cung cp dch v thoi qua nhiu nm m khng gy s c no. Ngi ta c thc hin mt s k thut ci thin cht lng ng dy thu bao di nhng ch dng li cht lng thoi trn dy ng.

Mng cp ni ht khng c mt ln m pht trin lin tc qua mt thi gian di cng vi s thu bao tng dn. Kt qu l mng cp ni ht khng c ti u v k thut nn cn phi hp l ho v xy dng li sau tng giai on. Bn mng ngoi vi c xy dng v duy tr nhng thng chu mt thi gian di khng sa i, b sung cng nh sa cha. Thng thng cc bn ny khng ghi li c kp thi v y s xung cp ca mng.

Gn nh ton b vic lp t v bo dng c cng ty in thoi thc hin v cc cng ty lp t c lp hu nh khng c c hi tham gia.

Mi vic vn n cho n khi cuc cch mng cng ngh trn cp ng bng n m khi u l DSL.

Chng 2: o th, nh gi mng cp ng s dng cho ADSL

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Nhng thay i

C 2 yu t to ra s thay i trong mng cp ng.

Yu t th nht l s pht trin ca cng ngh trong vic li dng ng dy thu bao cp ng truyn ti d liu vi tc cao. Theo l s pht trin n tng nhu cu tc do s kin bng n Internet v s pht trin ca cc mng gia nh cng vi s thay i t ch tp trung sang phn tn ca cc cng ty, doanh nghip.

Yu t th hai l s thay i dn ca mi trng kinh doanh t c quyn chuyn sang cnh tranh. Vic m ca th trng cho cnh tranh bt u t t do ho v cho thu mng cp ni ht lm cho nhiu cng ty c th u t vo cung cp cc dch v truy xut. Cc cng ty cnh tranh pht trin tr thnh nhng thch thc cho cc cng ty c quyn v cc cng ty cnh tranh thng c cc hp ng lp t bo dng c lp. Cnh tranh t cu trc gi thnh di sc p ngy cng tng v lc lng lao ng lnh ngh trong cc cng ty cnh tranh ang dn c thay th bi cc i ng t chuyn nghip hn.

Nhng thay i ny to ra mt mi trng m nhu cu v s hiu bit v mng cp ng ngy cng tng nhng nhng kin thc nn tng li c chiu hng gim. Hin nay i ng k thut vin m trch vic trin khai cc dch v truy xut s liu tc cao i hi cht lng mng cp ng cao hn nhiu. Cng ngh truyn dn khng cn l chuyn mn ring ca tng nhm nh cc chuyn gia mng ngoi vi na.

Hng v tng lai

Nhng bc u tin trong vic s dng ng dy thu bao trong cc dch v ng dy thu bao s (DSL: Digital Subscriber Loop) tng i thnh cng khi cha k n cc nguy c tim n. Tuy nhin khi vic s dng cp cho DSL tng nhanh th cng vic tr nn kh khn v nhiu ri ro cho ti tnh trng bo ho khng th lp t thm DSL c na. Bng cch qun l nghim ngt vic lp t theo k hoch tn s chung v vic o th, nh gi mng cp ng im bo ho cho vic lp t DSL ny c th xa hn. Vic lp t cng nhiu th thu nhp cng cao v u t cng hiu qu.

Cng ngh khng bao gi dng li nn cc k thut tin tin lun lun y tc d liu ln cao. Tc d liu cng cao th nhu cu v cht lng cp ng cng tng ln v chin lc nh gi mng cp ng cng tr nn quan trng hn.

Khi mt cu hnh mng c a ra nh FTTKerb chng hn th mng cp ng c th khc phc c cc vn pht sinh khi dng cho truyn s liu tc cao. Tuy nhin, trong tng lai trc mt mng cp ni ht vn


54

gi v tr tip cn thu bao v nh vy vic nh gi chng vn l mt thch thc ngy cng tng.

c tnh ng dy cp ng

Mch tng ng ng dy thu bao

Ta hy xt mch tng ng ca ng dy hiu c c tnh truyn dn ca ng dy tc ng ln kh nng truyn ti dch v s nh th no. Mi ng dy u tng ng vi cc phn t mch phn b dc theo chiu di ca chng. V dng in chy xuyn qua dy dn nn ng dy tng ng vi chui cm khng ni tip nhau v v n lun c ng dy v nn li tng ng vi cc dung khng mc song song. Nhng dung khng ni tip v v cm khng song song l nhng yu t hin nhin trong mch tng ng ng dy v n cng c tnh n trong cc m hnh thun tu l thuyt. Tuy nhin trong trng hp ng dy thc t th cc cht dn in l khng l tng nn n c cc thnh phn in tr ni tip v in mi gia cc dy dn cng khng l tng nn n cng sinh ra cc in tr song song. Cc in tr ni tip v song song ni trn kh nh hn cm khng v dung khng tr phi ng dy b h hng nng.

Hnh 2.1 Mch tng ng ng dy thu bao xon i

Cn lu rng trong thc t cc phn t c phn b lin tc dc theo ng dy v dn cm nh trn hnh v.

Mi tng ca mch tng ng c dng nh mt b lc thng thp nn nh hng ca cm khng v dung khng l hn ch di tn truyn dn tn hiu truyn trn ng dy lm cho tn hiu tn s cao suy hao nhiu hn tn hiu tn s thp. Cc in tr ni tip v song song lm tng thm suy hao v gim kh nng truyn ti tn hiu ng dy mt cch hiu qu.

Tr khng c tnh

Ta c th v n gin li mch tng ng ng dy bng cch dng cc hp thay cho mi tng nh trong hnh v di y v nh rng cc phn t trn thc t phn tn ch khng dn cm li tng on.


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Hnh 2.2 Hnh v n gin ho mch tng ng ng dy thu bao

Ta c th o tr khng vo nhn t mt u ca ng dy v nu di ng dy l v hn th ta nh ngha tr khng ny l tr khng c tnh ca ng dy.

nh ngha: Tr khng c tnh Z0 ca mt ng dy l tr khng ng vo ca mt ng dy c di v hn.

Ta c th biu din ton b ng dy vi mt phn t c tr khng l Z0.

Hnh 2.3 Khi nim tr khng trn ng dy

V ng dy ca chng ta l tng i di nn ta c th theo tnh cht ca gi tr v hn thm c nhiu tng tr khng na m khng lm thay i c tnh ca ng dy. V di ng dy xem nh v hn nn tr khng vo vn l tr khng c tnh. V vy c th v li thnh:

Hnh 2.4 M hnh ho tr khng ti

Nh vy, mt on dy hu hn c kt thc bng tr khng c tnh ca n th tr khng ng vo s cng bng tr khng c tnh Z0.

Tuy nhin tt nht l khng nn dng mch tng ng vi nhng tn ring r m nn biu din nh sau:

Hnh 2.5 M hnh ho tr khng ti v tr khng ng dy


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Tr khng c tnh ca ng dy bao gm cm khng ni tip v dung khng song song gim theo tn s. V d nh vi cc h thng di tn thp nh in thoi (600 Ohm) chng hn th tr khng c tnh cao hn so vi ADSL (120 Ohm).

Hnh 2.6 Cc ng cong tr khng c tnh ca dy xon i

Hnh v 2.6 minh ho cc ng cong tr khng c tnh khc nhau ca ca cc ng dy xon i thc t khc nhau. 9/10 hay 4/10 chng hn l ng knh dy tng ng 0,9 mm v 0,4 mm.

Lu rng khng c ng dy in thoi xon i no c tr khng nh hn 120 Ohm bt c tn s no.

Phi hp tr khng

Cng vic ca ng truyn trong cc ng dng vin thng l chuyn vn lng cng sut ln nht c th c t u ny sang u kia.

Hnh 2.7 Phi hp tr khng

Hy xt mch mt chiu n gin bao gm ngun in mt chiu vi tr khng ra RO nui mt ti RL. D chng minh c bng php ton rng cng sut trn ti s t gi tr ln nht khi in tr ti bng in tr ngun.

Sau y th minh ho l quan h gia truyn ti cng sut v t s gia 2 in tr trn cng mt mch in.


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Hnh 2.8 c tnh truyn t cng sut

V d ny ch s dng ngun in mt chiu cho n gin, tuy nhin kt qu cng c th p dng c cho c ngun in xoay chiu.

Hnh 2.9 Phi hp tr khng ng dy

By gi nu ta xt ng vo ca ng dy truyn dn v biu din bng mt tr khng c nh Z0 th cho cng sut truyn t l cc i ngun tn hiu phi c tr khng bng Z0. Tng t nh vy, ng ra ta c th thay th ng dy bng mt ngun p c tr khng bng Z0 v cng sut s t cc i nu tr khng ti cng bng Z0.

Vy, cng sut cc i c truyn qua ng dy khi n c cung cp bi mt ngun c tr khng l Z0 v cp cho ti cng c tr khng Z0.

Mt cch khc hnh dung nh lut ny l hnh nh tng t ca mt si dy n hi.


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Hnh 2.10 Tng t vi si dy n hi

Nu ngi bn phi gi cht si dy v lm u dy dao ng ln xung th bt c chuyn ng no do ngi bn tri s lan truyn trn si dy v c tiu tn bi ngi bn phi. Lu rng iu ny ch xy ra nu tay ca hai ngi u nng v c kh nng n hi nh nhau v do do dai nh si dy. ng dy truyn dn cng cho thy t hiu sut ti a nu c hai u u c tr khng bng vi tr khng c tnh ca ng dy.

By gi li xt trng hp thay th ngi u dy bn phi bng mt vt c nh.

Hnh 2.11 Trng hp u dy bn kia c nh

Khi sng dao ng chy trn dy ti bc tng v c phn x tr li u dy to ra sng. iu ny tng ng vi trng hp ng dy b ngn mch u cui. Tt c cng sut u b phn x li v n khng b tiu tn ti.

D nhin l v si dy khng bao gi l n hi tuyt i nn sng phn x nhn c pha ngi pht dao ng nh hn sng do ngi truyn i. Tng t nh vy, cng sut in trn ng dy truyn dn b tiu tn bi chnh ng dy v sng phn x c bin nh hn sng truyn i.

Hin tng cng xy ra tng t khi u cui ca si dy c th cho chuyn ng t do d khng hon chnh lm do b ri xung sn. (Hy


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tng tng rng c th thay th bng si dy treo thng ng khng c treo vt g u t do).

Hnh 2.12 Trng hp u dy bn kia t do

Hin tng ny tng ng vi trng hp h mch u cui ca ng dy truyn dn, tt c cng sut u phn x li v khng c nng lng tiu tn ti ti.

S dng tr khng thch hp c hai u ca ng dy gi l phi hp tr khng.

Suy hao phn hi (Return Loss)

Lng cng sut phn hi tr li ngun t mt ng dy phi hp tr khng khng tt l mt tham s rt quan trng gi l suy hao phn hi, v tr s o c ca suy hao phn hi c th cho thy li phi hp tr khng ca ng dy.

Hnh 2.13 Khi nim suy hao phn hi

Nu cng sut pht ca ngun l PT v cng sut tn hiu phn x nhn c l PR th suy hao phn hi c tnh l PR chia cho PT.

nh ngha: Suy hao phn hi ca mt ng dy l t s cng sut phn hi t ng dy v cng sut pht vo ng dy.

Nng lng c th c phn hi t s phi hp tr khng khng ng u cui. Tuy nhin khi ng dy c di tng i di th s phi hp tr khng my pht tc ng ti suy hao phn hi nhiu hn s phi hp tr khng cui ng dy. iu ny l do nng lng phn x t u cui


60

ng dy b suy hao bi ng dy trc khi tr v ti u pht. Thng th s thay i tr khng c tnh ca cc mi ni cp gn u pht gy ra suy hao phn hi ln.

truyn t c cng sut ln nht th suy hao phn hi phi gi mc nh nht c th c. iu ny c ngha l t s PR/PT phi nh nht c th c. Chng hn, suy hao phn hi -40dB tt hn suy hao phn hi -20dB.

Suy hao chn (Insertion Loss)

Ta xt nh hng ca kt cui ng dy i vi s phn x cng sut. Tuy nhin r rng l khng phi ton b cng sut c a vo ng dy u ny u n c u bn kia. l do suy hao trn ng dy do cc phn t khc nhau trong mch tng ng ca n. Suy hao do chn ng dy truyn dn vo gia ngun pht v ti gi l suy hao chn ca ng dy.

Hnh 2.14 Khi nim suy hao chn

Nu cng sut pht ca ngun l PT v cng sut nhn c trn ti l PR th suy hao chn l t s gia PR v PT. R rng l t s ny s nh hn n v nn gi l li chn i khi li chnh xc hn l suy hao chn.

nh ngha: Suy hao chn ca ng dy l t s ca cng sut nhn c cui ng dy v cng sut pht vo ng dy.

c th truyn c ti a cng sut pht th suy hao chn phi gim mc nh nht c th c. Ni cch khc th t s PR/PT cng gn n v cng tt, hay cng gn bng 0dB cng tt.

Cn bng dc (Longitudinal Balance)

Hnh 2.15 Mch tng ng ng dy khi xt cn bng dc


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Tr li mch tng ng ng dy ta phi thm vo vi chi tit ch r ng dy khng cch in hon ton m c quan h vi t.

Ging nh cc phn t song song tn ti gia hai dy dn, mi dy dn s c mt in dung gia chng v t, v tr phi s cch in l hon ho vi mi dy dn ta vn c mt in tr so vi t. y mt ln na lu rng cc phn t in trn ng dy l lin tc ch khng dn cm nh hnh v m hnh ho.

Tr khng ca mi dy dn so vi t cng gn bng nhau cng quan trng cho truyn dn hiu qu c bit l vi cc tn hiu tn s cao nh xDSL. S cn bng ny gi l cn bng dc ca ng dy.

Hnh 2.16 M hnh ho khi xt cn bng vi t

Nu ta biu din tr khng vi t ca mi dy dn l Za v Zb nh hnh v th cn bng dc c nh ngha l:

Cn bng = 20 (log10 (Za - Zb) / (Za + Zb)) dB.

Cn bng dc c o bng cch p mt tn hiu im gia ca hai in tr bng nhau ni vi hai dy dn nh minh ho hnh v di y. Nu ng dy l cn bng th khng c chnh lch gia tn hiu trn hai dy dn.

Hnh 2.17 Nguyn l o cn bng dc

in p chnh lch (VDiff) gia cc dy dn v in p ng pha (VComm) im gia c o tnh cn bng dc nh sau:

Cn bng = 20 (log10 VDiff / VComm) dB.


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Xuyn knh (Crosstalk)

Dng in chy trong mt dy dn gy ra trng in t xung quanh dy dn theo chiu ca quy tc bn tay phi.

Hnh 2.18 T trng cm ng ca cc dy dn

Chng hn, dng in qua dy dn bn tri to ra mt t trng chy xung quanh dy dn theo chiu kim ng h. V tn hiu trn ng dy l dng in t trng lan truyn dc theo chiu di ca dy nn dng in chy trong mt dy dn ny s chy theo chiu ngc li dy dn kia. V vy cch xa i dy dn th hiu ng trng in t b kh ln nhau cn gn i dy dn th li mnh ln v cng chiu sut dc theo chiu di dy dn.

Nu by gi ta em thm mt i dy li gn i dy th nht th t trng ca i dy th nht ct mt phng ca i dy th hai v n to ra mt dng in cm ng trn i dy th hai.

Hnh 2.19 Dng in cm ng ca cc dy dn to ra trn cc dy dn ln cn

Nh vy ta to ra c mt bin p rt di v mng v to ra mt cch ghp gia hai ng dy gi l xuyn knh.


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By gi hy xt xem iu g s xy ra nu chng ta xon cc i dy li.

Hnh 2.20 Tc dng ca vic xon i dy dn

Trng in t vn xoay quanh cc dy dn theo cng mt hng v chng vn tc ng ti khng gian xung quanh gn . Tuy nhin, trong trng hp ny chng khng theo cng mt hng theo dc i dy. mi bc xon, hng trng in t li xoay c mt vng nn tc ng ln cc i dy k cn c kh i.

Nu i dy th hai cng c xon th xuyn knh c kh i nhiu hn, nu bc xon trong hai i dy khng ng b vi nhau. iu ny rt quan trng i vi vic thit k ch to cp.

Bng cch xon i cc i dy ta gim c xuyn knh.

Xuyn knh c o hai dng.

Hnh 2.21 Xuyn knh u gn

Xuyn kn u gn (NEXT: Near End Crosstalk) o c mt u cp bng cch pht tn hiu vo mt i dy v o kt qu xuyn knh trn i dy k cn cng mt u.

Xuyn knh u gn c tnh bi:

NEXT = POUT / PIN


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= 10 (log10 POUT / PIN) dB

bo m rng tn hiu o c l thc s do xuyn knh to ra v khng phi l ca bt c ngun nhiu no khc th my thu phi c chnh cng tn s ca my pht. Tn hiu ny phi l tn hiu mt tn s hay l cc tn s qut qua ph tn tn hiu.

Hnh 2.22 Xuyn knh u xa

Xuyn knh u xa (FEXT: Far End Crosstalk) c o t hai u cp bng cch pht tn hiu vo mt i dy mt u v o kt qu xuyn knh trn i dy k cn u bn kia ca si cp.

Xuyn knh u xa c tnh bi:

FEXT = POUT / PIN

= 10 (log10 POUT / PIN) dB

Bng cch o c NEXT v FEXT, v lu rng s khc nhau gia hai php o ta c th xc nh c xuyn knh gy ra bi li gn u dy hay do cc mi ghp dc theo ng dy.

Hnh 2.23 Hin tng nhiu trn ng dy

Trc khi ta kt thc vn xuyn knh cn lu rng tnh thun nghc cng ng vi mi hin tng xuyn knh ti ng dy truyn dn.


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iu ny c ngha l mt ng dy pht x xuyn knh t th chu tc ng ca xuyn knh t. Ngc li ng dy pht x xuyn knh nhiu cng s d b tc ng ca xuyn knh t bn ngoi. V vy xon i dy dn s em li ch li cho c chnh dy dn v cc dy dn bn cnh. Cc li lm cho i dy chu tc ng ca xuyn knh cng lm cho i dy pht x v gy ra xuyn knh.

Tc ng ca mi trng

Cc ng dy dng trong vin thng c th b tc ng ca nhiu tc nhn trong mi trng v hu ht cc tc nhn ny u lm gim cht lng truyn dn. iu ny c th ng lu v c bit nghim trng tu thuc vo bn cht ca xuyn nhiu v kh nng khng nhiu ca tn hiu truyn ti. Phn ny ta s xt mt vi loi xuyn nhiu v cch o c chng.

Hnh 2.24 Cc thnh phn ca tn hiu thc t

Cng sut tng cng trn ng truyn c c t hai thnh phn l tn hiu truyn dn v thnh phn khng mong mun gi l nhiu. Nhiu lun lun xut hin nhng v ta khng mong mun nhiu nn phi kim sot nhiu mc thp nht c th c so vi tn hiu. Quan h gia tn hiu v nhiu c biu din bng t s:

S/N = Signal Power / Noise Power

= 20 (log10 VSignal / VNoise) dB

Mt vi ngun nhiu tc ng ln ng dy.

Mi dy dn in pht ra nhiu trong chnh n. iu ny l do chuyn ng ngu nhin ca cc in t trong cht dn in v v chuyn ng ny tng ln theo nhit nn ngi ta gi l nhiu nhit (thermal noise).

Nh ta thy, nhiu c th cm ng ln ng dy bi tn hiu ca mt ng dy k cn gi l xuyn knh.


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Cc my pht sng v tuyn truyn hnh v pht thanh cng to ra trng in t tc ng vo cc ng dy gy ra nhiu.

H thng truyn ti v phn phi in to ra trng in t rt mnh cng c th ghp vo cc ng dy vin th

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Do thu lap dat ADSL - Ts Đặng Quốc Anh