CNG NGH V CHUN HA MNG QUANG THU NG

Quang Minh 1. Gii thiu Nhu cu s dng h tng cp quang n h gia nh FTTH (Fiber to the Home) xut hin t nhng nm 1980 khi m cc cng ty in thoi nhn thy li ch mang li trong vic cung cp cc dch v bng rng ISDN ti cc thu bao. Nhng tin b nhanh chng trong k thut thu, pht v cp si quang m ra mt mt tim nng ln trong vic pht trin h tng FTTH. FTTH c xem nh l mt gii php hon ho thay th mng cp ng hin ti nhm cung cp cc dch v triple play (bao gm thoi, hnh nh, truy nhp d liu tc cao) v cc cc ng dng i hi nhiu bng thng (nh l truy cp Internet bng rng, chi game trc tuyn v phn tn cc on video). Tuy nhin nhc im chnh ca FTTH l chi ph cho cc linh kin v cp quang tng i cao dn ti gi thnh lp t nhng ng quang nh vy l rt ln. C nhiu gii php khc phc nhc im ny v mt trong s l trin khai FTTH trn nn mng quang th ng (Passive Optical Network - PON). Vy cng ngh PON l g v bao gm cc loi no. Bi bo ny gip cc bn c gii p nhng thc mc . 2. Cng ngh mng quang th ng Mng quang th ng (PON) c xy dng nhm gim s lng cc thit b thu, pht v si quang trong mng thng tin quang FTTH. PON l mt mng im ti a im, mt kin trc PON bao gm mt thit b u cui knh quang c t ti trm trung tm ca nh khai thc dch v v cc b kt cui mng cp quang ONU/ONT (Optical Network Unit/Optical Network Terminator) t ti gn hoc ti nh thu bao. Gia chng l h thng phn phi mng quang ODN (Optical Distribution Network) bao gm cp quang, cc thit b tch ghp th ng. Kin trc ca PON c m t nh trong Hnh 1.

Hnh 1: Cu trc mng quang th ng PON

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Trong h thng PON, kt ni mng quang (ONT) c kh nng h tr kt ni dch v in thoi truyn thng qua giao din POTS (Plain Old Telephone Service) v cc giao tip truyn d liu tc cao nh Ethernet v DSL. u cui ng dy quang (OLT) bao gm cc khi giao tip PON, mt kt cu chuyn mch d liu v cc phn t iu khin NE (Network Element). Ti hng xung, OLT pht qung b d liu ti tt c cc ONU. Tn hiu hng xung bao gm d liu cho cc ONT, t mo u Khai thc Qun l v Bo dng (Operations Administration and Maintenance - OAM) v cc tn hiu ng b cho cc ONT gi d liu hng ln. Da vo cc thng tin v khe thi gian (knh), a ch gi/t bo, bc sng, m CDMA m cc ONT tch d liu tng ng vi thu bao ca khch hng. Trong hng ln, mi mt ONU cn c giao thc iu khin truy nhp mi trng MAC (Medium Access Control) chia s PON. Giao thc MAC thng c s dng trong PON l a truy nhp phn chia theo thi gian TDMA (Time Division Multiple Access), khi mi ONT c cp mt khe thi gian (knh) gi d liu ca mnh ti OLT. Ngoi ra trong hng ln cn phi c khong thi gian bo v gia cc nhm gi d liu ca cc ONT, khong thi gian ny phi m bo sao cho ti b thu OLT d liu khng b trm ph ln nhau. Thng thng cc h thng TDMA PON gn trc mt t l phn chia c nh bng thng hng ln cho cc ONT m khng quan tm c bao nhiu d liu c gi i. Mt gii php phn b bng thng cho cc ONT l s dng giao thc phn b bng thng ng (Dynamic Bandwidth Allocation - DBA). DBA l giao thc cho php cc ONT gi yu cu v bng thng ti OLT nhm s dng hiu qu bng thng hng ln. Cc thng tin yu cu c th l cc mc y hng i u vo cho cc lp dch v khc nhau. OLT nh gi cc yu cu t cc ONT v gn bng thng cho gi d liu hng ln ln k tip theo. OLT cng c th tch hp chc nng tha thun mc dch v SLA (Service Level Agreement) kt hp vi DBA trong vic phn b bng thng. Thng thng cc h thng PON truyn d liu c hng xung v hng ln trong cng mt si quang. Trn mi si mc d cc b ni nh hng cho php s dng cng mt bc sng cho c 2 hng, tuy nhin i vi cc h thng truyn ti tc cao m bo cht lng th thng thng mi hng s dng mt bc sng ring. Trong cc mng PON cc bc sng c s dng l 1490nm hoc 1550nm cho hng xung v 1310nm cho tn hiu ng ln. 3. Cc chun mng PON: Cc chun mng PON c th chia thnh 2 nhm: nhm 1 bao gm cc chun theo phng thc truy nhp TDMA PON nh l B-PON (Broadband PON), E-PON (Ethernet PON), G-PON (Gigabit PON) (c tnh cc ca chun TDMA PON c so snh trong Bng 1); nhm 2 bao gm chun theo cc phng thc truy nhp khc nh WDM-PON (Wavelength Division Multiplexing PON) v CDMA-PON (Code Division Multiple Access PON). B-PON Mng quang th ng bng rng B-PON c chun ha trong chui cc khuyn ngh G.938 ca ITU-T. Cc khuyn ngh ny a ra cc tiu chun v cc khi chc nng ONT v OLT, khun dng v tc khung ca lung d liu hng ln v hng xung, giao thc truy nhp hng ln TDMA, cc giao tip vt l, cc giao tip qun l v iu khin ONT v DBA. Trong mng B-PON, d liu c ng khung theo cu trc ca cc t bo ATM. Mt khung hng xung c tc 155Mbit/s (56 t bo ATM c khch thc 53byte), hoc 622 Mbit/s (4*56 t bo ATM) v mt t bo qun l vn hnh bo dng lp vt l OAM (PLOAM Physical layer Operation Administration and Maintenance) c chn vo c mi 28 t bo trong knh. PLOAM c mt bt nhn dng cc t bo PLOAM. Ngoi ra cc t bo PLOAM c kh nng lp trnh c v cha thng tin nh l bng thng hng ln v cc bn tin OAM.

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Cn c vo cc thng tin v m s nhn dng knh o v nhn dng ng o (VPI/VCI) trong cu trc ATM, cc ONT nhn bit v tch d liu ng xung ca mnh. Cu trc khung hng ln bao gm 56 t bo ATM (53 byte). Mi mt knh (time slot) gm c mt t bo ATM/PLOAM v 24 bt t mo u. T mo u mang thng tin v thi gian bo v (guard time), mo u cho php ng b v khi phc tn hiu ti OLT, v thng tin nhn dng im kt thc ca t mo u. Chiu di ca t mo u v cc thng tin cha trong c lp trnh bi OLT. Cc ONT thc hin gi cc t bo PLOAM khi chng nhn c yu cu t OLT. B-PON s dng giao thc DBA cho php OLT nhn bit lng bng thng cn thit cp cho cc ONT. OLT c th gim hoc tng bng thng cho cc ONT da vo gi cc t bo ATM ri hoc lm y tt c hng ln bi d liu ca ONT. OLT dng nh k vic truyn hng ln do vy n c kh nng mi bt k ONT mi no tham gia vo hot ng h thng. Cc ONT mi pht mt bn tin phc hi trong ca s ny vi thi gian tr ngu nhin trnh xung t khi m c nhiu ONT mi mun tham gia. OLT xc nh khong cch ti mi ONT mi bng vic gi ti ONT mt bn tin o c ly v xc nh thi gian bao lu thu c bn tin phc hi. Sau OLT gi ti ONT mt gi tr tr, gi tr ny c s dng xc nh thi gian bo v ng vi cc ONT. E-PON v Gbit/s PON E-PON l giao thc mng truy nhp y dch v FSAN (Full Service Access Network) TDMA PON th nht c pht trin da trn khai thc cc u im ca cng ngh Ethernet ng dng trong thng tin quang. E-PON c chun ha bi IEEE 802.3. Trong E-PON d liu hng xung c ng khung theo khun dng Ethernet. Cc khung E-PON c cu trc tng t nh cc lin kt Gigabit Ethernet im ti im ngoi tr t mo u v thng tin xc nh im bt u ca khung c thay i mang trng nhn dng knh logic (LLID Link logic ID) nhm xc nh duy nht mt ONU MAC. Trong hng ln, cc ONU pht cc khung Ethernet trong cc khe thi gian c phn b. ONU s dng giao thc iu khin a im PDU (MPCPDU Multi Point Control Protocol Data Unit) gi cc bn tin Report yu cu bng thng, trong khi OLT gi bn tin Gate cp pht bng thng cho cc ONU. Cc bn tin Gate bao gm thng tin v thi gian bt u v khong thi gian cho php truyn d liu i vi ONU. OLT cng nh k gi cc bn tin Gate ti cc ONU hi xem chng c yu cu bng thng hay khng. Cc ONU cng c th gi Report cng vi d liu c pht trong hng ln. Ngoi ra, giao thc DBA cng c th c s dng trong E-PON thc hin c ch iu khin phn b bng thng. Do khng c cu trc khung thng nht i vi hng xung v hng ln, do vy trong cu trc ca E-PON, cc khe thi gian v giao thc xc nh c ly l khc so vi B-PON v G-PON. OLT v cc ONU duy tr cc b m cc b ring v tng thm 1 sau mi 16ns. Mi mt MPCPDU mang theo mt thi gian mu, mu ny l gi tr ca b m cc b ca ONU tng ng. Tc truyn d liu E-PON c th t ti 1Gbit/s. Mt chun khc cng cng h vi E-PON l chun Gbit/s Ethernet PON (IEEE 802.3av Gbit/s PON). Chun ny l pht trin ca E-PON ti tc 10Gbit/s v c ng dng ch yu trong cc mng qung b video s. Gbit/s PON cho php phn phi nhiu dch v i hi bng thng ln, phn gii cao, ng gi IP cc lung d liu video ngay c khi h s chia OLT/ONT l 1:64 hoc cao hn. Ti Trung Quc, E-PON v Gbit/s PON c pht trin kh mnh, theo d kin cui nm 2008 s lng E-PON c th t ti 20 triu thu bao.

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G-PON: G-PON l giao thc FSAN TDMA PON th 2 c nh ngha trong chui khuyn ngh G.984 ca ITU-T. G-PON c xy dng trn tri nghim ca B-PON v E-PON. Mc d G-PON h tr truyn ti tin ATM, nhng n cng a vo mt c ch thch nghi ti tin mi m c ti u ha cho truyn ti cc khung Ethernet c gi l phng thc ng gi G-PON (G-PON Encapsulation Method - GEM). GEM l phng thc da trn th tc ng khung chung trong khuyn ngh G.701 ngoi tr vic GEM ti u ha t mo u phc v cho ng dng ca PON, cho php sp xp cc d liu Ethernet vo ti tin GEM v h tr sp xp TDM. G-PON s dng cu trc khung GTC (G-PON Transmission Conversion) cho c hai hng xung v hng ln. Khung hng xung bt u vi mt t mo u PLOAM, tip sau l vng ti tin GEM v/hoc cc t bo ATM. PLOAM gm c thng tin cu trc khung v sp t bng thng cho ONT gi d liu trong khung hng ln tip theo. Khung hng ln bao gm cc nhm khung gi t cc ONT. Mi mt nhm c bt u vi t mo u lp vt l m c chc nng tng t trong B-PON, nhng cng bao hm tng hp cc yu cu bng thng ca cc ONT. Ngoi ra, cc trc PLOAM v cc yu cu bng thng chi tit hn c gi i km vi cc nhm hng ln khi c yu cu t OLT. OLT gn cc thi gian cho vic gi d liu hng ln t cho mi ONT . u nm 2008, Verizon bt u lp t cc thit b G-PON v ti gia nm h thit lp c hn 800.000 tuyn. British Telecom v AT&T cng ang th nghim cng ngh ny.

c tnh B-PON G-PON E-PON

T chc chun ha FSAN v ITU-T SG15 (G.983 series) FSANv ITU-T SG15 (G.984 series) IEEE 802.3 (802.3ah)

Tc d liu 155.52 Mbit/s hng ln. 155.52 hoc 622.08 Mbit/s hng xung

Ln ti 2.488 Gbit/s c 2 hng 1 Gbit/s c 2 hng

T l chia (ONUs/PON) 1:64 1:64 1:64 ** M ng truyn Scrambled NRZ Scrambled NRZ 8B/10B S lng si quang 1 hoc 2 1 hoc 2 1

Bc sng 1310nm c 2 hng hoc 1490nm xung & 1310nm ln

1310nm c 2 hng hoc 1490nm xung & 1310nm ln

1490nm xung & 1310nm ln

C ly ti a OLT-ONU 20 km (10 20) km (10 20) km Chuyn mch bo v C h tr C h tr Khng h tr

Khun dng d liu ATM GEM v/hoc ATM Khng (s dng trc tip cc khung Ethernet)

H tr TDM Qua ATM Trc tip (qua GEM hoc ATM) hoc CES CES

H tr thoi Qua ATM Qua TDM hoc VoIP VoIP QoS C (DBA) C (DBA) C (u tin 802.1Q) Sa li hng ti trc FEC (Forward Error Correction)

Khng RS(255, 239) RS(255, 239)

M ha bo mt AES 128 AES - 128, 192, 256 Khng

OAM PLOAM v ATM GTC v ATM/GEM OAM 802.3ah Ethernet OAM

Bng 1: So snh cc chun cng ngh TDMA PON

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WDM-PON WDM-PON l mng quang th ng s dng phng thc a ghp knh phn chia theo bc sng thay v theo thi gian nh trong phng thc TDMA. OLT s dng mt bc sng ring r thng tin vi mi ONT theo dng im im. Mi mt ONU c mt b lc quang la chn bc sng tng thch vi n, OLT cng c mt b lc cho mi ONU. Nhiu phng thc khc c tm hiu to ra cc bc sng ONU nh l:

- S dng cc khi quang c th lp t ti ch la chn cc bc sng ONU - Dng cc laser iu chnh c. - Ct ph tn hiu. - Cc phng thc th ng m theo OLT cung cp tn hiu sng mang ti cc ONU. - S dng tn hiu hng xung iu chnh bc sng u ra ca laser ONU.

Cu trc ca WDM-PON c m t nh trong Hnh 2. Trong WDM-PON c th c s dng cho nhiu ng dng khc nhau nh l FTTx, cc ng dng VDSL v cc im truy nhp v tuyn t xa. Cc b thu WDM-PON s dng k thut lc quang mng ng dn sng (Arrayed Waveguide Grating - AWG). Mt AWG c th c t mi trng trong nh hoc ngoi tri.

Hnh 2: Cu trc ca WDM-PON

u im chnh ca WDM-PON l n kh nng cung cp cc dch v d liu theo cc cu trc khc nhau (DS1/E1/DS3, 10/100/1000Base Ethernet) ty theo yu cu v bng thng ca khch hng. Tuy nhin, nhc im chnh ca WDM-PON l chi ph kh ln cho cc linh kin quang sn xut b lc nhng bc sng khc nhau. WDM-PON cng c trin khai kt hp vi cc giao thc TDMA PON ci thin bng thng truyn tin. WDM-PON c pht trin mnh Hn Quc vi nh cung cp in hnh l Novera Optics Inc. CDMA-PON Cng ngh a truy nhp phn chia theo m CDMA cng c th trin khai trong cc ng dng PON. Cng ging nh WDM-PON, CMDA-PON cho php mi ONU s dng khun dng v tc d liu khc nhau tng ng vi cc nhu cu ca khch hng. CDMA PON cng c th kt hp vi WDM tng dung lng bng thng. CDMA PON truyn ti cc tn hiu khch hng vi nhiu ph tn truyn dn tri trn cng mt knh thng tin. Cc k hiu t cc tn hiu khc nhau c m ha v nhn dng thng qua b

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gii m. Phn ln cng ngh ng dng trong CDMA PON tun theo phng thc tri ph chui trc tip. Trong phng thc ny mi k hiu 0, 1 (tng ng vi mi tn hiu) c m ha thnh chui k t di hn v c tc cao hn. Mi ONU s dng tr s chui khc nhau cho k t ca n. khi phc li d liu, OLT chia nh tn hiu quang thu c sau gi ti cc b lc nhiu x tch ly tn hiu ca mi OUN. u im chnh ca CDMA PON l cho php truyn ti lu lng cao v c tnh nng bo mt ni tri so cc chun PON khc. Tuy nhin, mt tr ngi ln trong CDMA-PON l cc b khuch i quang i hi phi c thit k sao cho m bo tng ng vi t s tn hiu/tp m (S/N). Vi h thng CDMA-PON khng c b khuch i quang th ty thuc vo tn hao b sung trong cc b chia, b xoay vng, cc b lc m h s t chia ONU/OLT ch l 1:2 hoc 1:8. Trong khi vi b khuych i quang h s ny c th t 1:32 hoc cao hn. Bn cnh cc b thu tn hiu trong CDMA-PON l kh phc tp v gi thnh tng i cao. Chnh v nhng nhc im ny nn hin ti CDMA-PON cha c pht trin rng ri. Kt lun Cng ngh PON ra i m ra mt tim nng ln cho trin khai cc dch v bng rng v thay th dn cc h thng mng truy nhp cp ng bng thng hp v cht lng thp. Cc mng PON s dng h thng thng tin quang c bng thng rng t l li bt thp (BER: 10-10 10-12) Tuy nhin, n cng c nhc im l gi thnh xy dng tng i cao v khng trin khai c ti nhng a hnh phc tp. C nhiu chun PON khc nhau, do vy trong thc t ty vo yu cu thc t m mt nh khai thc cn la chn gii php cho ph hp./.

Ti liu tham kho: [1] ITU-T G.984, Gigabit-capable Passive Optical Networks (GPON), 2003 [2] ITU-T G.983, Broadband optical access systems based on Passive Optical Networks (PON),

2005 [3] ITU-T G.983, ONT management and control interface specification for B-PON, 2005 [4] ITU-T G..983, A broadband optical access system with increased service capability using

dynamic bandwidth assignment (DBA), 2001 [5] Steven S. Gorshe, FTTH/FTTC Technologies and Standards, December 2006 [6] White Paper by Novera Optics Inc, WDM-PON for the Access Network, 2006 [7] Website: http://en.wikipedia.org/wiki/Pon

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