I. CNG NGH XDSL1.1. Khi nimDSL (Digital Subscriber Line: ng thu bao s) l mt cng ngh s dng cc phng php iu bin phc tp, nhm mc ch bin i tn hiu c tn s cao u ra sao cho ph hp vi vic truyn trn ng dy in thoi nht.DSL tp trung vo cng ngh truyn ti trn ng dy in thoi nn cng ngh ny thng c p dng vo vic truyn dn gia khch hng v nh cung cp dch v hay ni cch khc l cng ngh phc v mng truy nhp.1.2. u, nhc imu im: Tn dng c c s h tng sn c ca mng in thoi sn c v rng khp. Kh nng truyn ti c nhiu ng dng khc nhauNhc im: Yu cu cht lng ca cp truyn dn tn hiu DSL cao hn nhiu so vi yu cu ca cp truyn dn thoi do DSL truyn thng tin c bng tn ln, tc cao nn nu ch c mt nhiu nh th lng thng tin b nh hng cng rt ln. Khong cch truyn ti tn hiu DSL khng c xa do tn hiu tn s cao c mc suy hao ln.1.3. Cc cng ngh xDSLDo c nhiu phng php bin i tn hiu tn s thp ln tn s cao truyn dn qua ng dy in thoi, mi phng php ny c c tnh, u im, nhc im v kh nng ng dng khc nhau.IDSL (ISDN Digital Subcriber Line): c pht trin t IDSN tc c bn, tc d liu ca n = 144kbps rt gn vi tc d liu ca IDSN 128 kbps.HDSL (High-bit-rate Digital Subcriber Line): Thc cht cc thit b thu pht HDSL l s k tha ca ISDN nhng mc phc tp hn. HDSL ra i da trn chun T1/E1 ca M/Chu u. L cng ngh c suy hao thp nn c th truyn xa hn m vn m bo c cht lng tn hiu.ADSL (Asymmetrical Digital Subcriber Line) chnh l mt nhnh ca cng ngh xDSL. ADSL cung cp mt bng thng bt i xng trn mt i dy. Thut ng bt i xng y ch s khng cn bng trong dng d liu ti xung v ti ln. Dng d liu ti xung c bng thng ln hn bng thng dng d liu ti ln. Cng ngh ADSL hin nay theo l thuyt c th cung cp cung cp 8 Mbps cho ng xung v 2 Mbps cho ng ln.ADSL 2+: Cng ngh ny tng di tn s hot ng ln ng k so vi ADSL, gim tnh trng nhiu xuyn m gia cc dch v, gim tiu hao nng lng v th ADSL 2+ c kh nng t c tc ti xung 24Mbps. ADSL 2+ vn s dng i cp ng truyn thoi sn c, cc nh cung cp dch v hon ton khng phi u t thm c s h tng ng truyn mi m vn c kh nng cung cp dch v vi bng thng ln hn, pht trin cc dch v tin tin nh video, hi ngh ... trn cng mt h tng truyn dn pha khch hng. RADSL (Rate-adaptive Digital Subscriber Line) l mt phin bn ca ADSL m cc modem c th kim tra ng truyn khi khi ng v p ng lc hot ng theo tc nhanh nht m ng truyn c th cung cp. RADSL cn c gi l ADSL c tc bin i.VDSL (Very-high-bit-rate Digital Subscriber Line) l mt cng ngh xDSL cung cp ng truyn i xng trn mt i dy ng. Dng bit ti xung ca VDSL l cao nht trong tt c cc cng ngh ca xDSL, t ti 52Mbps, dng ti ln c th t 2,3 Mbps. VDSL thng ch hot ng tt trong cc mng mch vng ngn. VDSL dng cp quang truyn dn l ch yu, v ch dng cp ng pha u cui.Bng tng kt tnh cht cc cong ngh xDSL:Cng nghLung xung ti aLung ln ti aKhong cch

IDSL144 kbps144 kbps5,5 km

HDSL2 Mbps2 Mbps3,5 km

ADSL8 Mbps2 Mbps2,5 5,5 km

ADSL 2+24 Mbps3.3 Mbps1,5 km

VDSL52 Mbps26 Mbps1,5 km

Cc dch v trin khai trn mng dng cng ngh DSL: Dch v Frame Relay, dch v Leased Line, dch v VPN, dch v Internet bng rng, game trc tuyn, truyn hnh Internet1.4. Tm hiu cng ngh ADSL1.4.1. Cc thnh phn thit b ca mng ADSL

Hnh 1.1. Cc thnh phn ca mng ADSLa) Cc thnh phn thit b pha khch hng ng dy thu bao: ng k s dng dch v My tnh c nhn: C h tr cng USB hoc card mng Ethernet Modem ADSL: Modem vit tt ca modulator-demodulator, lm nhim v chuyn tn hiu s t my tnh thnh tn hiu tng t truyn trn ng dy in thoi v ngc li.

Hnh 1.2. Thit b Modem ADSL B tch tn hiu Splitter: L thit b dng tch tn hiu thoi c tn s thp v tn hiu truy cp Internet c tn s cao. Hin nay, cc modem ADSL u c tch hp lun b Splitter.

Hnh 1.3. Thit b Splitter Card mng: Dng kt ni my tnh v modem ADSL Switch/hub (ty chn): Kt ni nhiu my tnh pht trin mng LAN cho nhiu ngi dng.b) Cc thnh phn thit b pha nh cung cp dch vDSLAM DSL Access Multiplexer: L thit b t pha tng i, l im cui ca kt ni ADSL. N cha v s cc modem ADSL b tr v mt hng ti cc mch vng v pha kia l kt ni cp quang. Mt thit b DSLAM c th tp hp nhiu kt ni thu bao ADSL (ln ti hng trm) v t li trn mt kt ni cp quang. Si quang ny thng c ni ti thit b BRAS.

Hnh 1.4. Thit b DSLAMBRAS Broadband Access Server: L thit b t gia DSLAM v POP ca ISP. Mt BRAS c th phc v cho nhiu DSLAM. Cc giao thc truyn thng c ng gi truyn d liu thng qau kt ni ADSL, mc ch ca BRAS l m gi hon tr li c giao thc trc khi i vo Internet. N cng m bo cho kt ni ca bn ti ISP c chnh xc.

Hnh 1.5. Thit b BRASSplitter B tch tnh hiu: L b lc c chc nng tch ring tn hiu thoi v d liu ti u nh cung cp dch v. Thng thng Splitter c th c tch hp cng vi DSLAM.1.4.2. Cc giao thc truyn thngKhi quay s PSTN/ISDN truy nhp vo Internet, chng ta s dng giao thc PPP vn chuyn d liu TCP/IP v kim tra cng nh xc thc tn v mt khu ngi truy nhp.Trong ADSL, PPP cng thng c s dng kim tra tn v mt khu truy nhp, v ATM th lun lun c s dng mc thp nht. Kt ni in hnh nh hnh di y.

Hnh 1.6. S kt ni ADSL in hnhVai tr ca ATM: ATM (Asynchronous Transfer Mode) c s dng nh l cng c chuyn ti cho ADSL mc thp. L do v l cch thun tin v mm do i vi cc cng tu thoi mun ko di khong cch kt ni t DSLAM ti BRAS gip h c th dt BRAS bt c u trn mng. C hai tham s cn phi thit lp cu hnh mt cch chnh xc trn modem ADSL m bo kt ni thnh cng ti mc ATM vi DSLAM:- VPI:Virtual Path Identifier: nh danh ng dn o- VCI: Virtual Channel Identifier: nh danh knh o Khc vi cng ngh chuyn mch knh trong mng in thoi, hay chuyn mch gi trong mng my tnh, chuyn mch ATM s dng VPI/VCI chuyn cc gi tin, khi mun truyn tin 2 my tnh trn mng ATM s thit lp cc ng dn o VPI, v cc knh o VCI ( trong mt knh o th c th bao gm nhiu ng dn o), cc gi tin s mang theo thng tin v VPI/VCI trong n, v khi chuyn mch ATM da vo cc thng s a gi tin (theo VPI/VCI ca n) ti ch hihi tm thi bn c th hiu ngun gc VPI/VCI...VPI-VCI mi nh cung cp dng v ADSL u c cc thng s khc nhau do quy nh cua nh cung cp.Vai tr ca PPP: PPP l giao thc dng vn chuyn lu lng Internet ti ISP dc theo cc kt ni modem v ISDN. PPP kt hp cht ch cc yu t xc thc v l l do ngi ta dng PPP vi ADSL. Mc d BRAS thc thi giao thc PPP v tin hnh xc thc, nhng thc ra vic c thc hin bng cch truy nhp vo cc c s d liu khc hng t ti ISP. Bng cch , ISP bit c rng cc kt ni do BRAS nh tuyn ti c xc thc thng qua gioa dch vi c s d liu ring ca ISP.Thng thng ADSL s dng hao giao thc chnh l:PPPoE PPP over EthernetPPPoA PPP over ATM