C bn v mngGPONThng Mi 27, 2014bynguyentiensu 1 Phn hiI. Khi qut
chungGPON hay Gigabit Passive Opticcal Networks c nh ngha bi ITU-T
trong chui cc ti liu t G.984.1 n G.984.6. GPON c th truyn ti d liu
khng ch Ethernet m cn ATM v TDM(PSTN, ISDN, E1 v E3). Mng GPON cha
hai thit b hot ng chnh l Optical Line Termination(OLT) v Optical
Network Unit(ONU). GPON h tr a dch v, tc cao v khong cch c th ln n
20km.
Hnh 1 ch ra kin trc mng FTTx:
Hnh 1 Kin trc mng FTTxMt si quang n ni t OLT n mt thit b quang
th ng Optical Splitter(OS)(th ng y hiu vi ngha l thit b khng cn
cung cp bt c ngun no hot ng), ci m c t gn khu vc ngi s dng. OSchia
tn hiu quang ra thnh N ng khc nhau n N ngi s dng. Gi tr ca N c th t
2 n 128. T OS, mt si quang n mode chy n mi user, nh trong hnh
2.
Hnh 2 Kin trc mng GPONGPON c hai c ch hot ng:- Hng
xung(downstream: t OLT n user): D liu c truyn broadcast nhng c m
ha(AES) trnh b nghe trm.- Hng ln(upstream: t user n OLT): D liu c
truyn trong mt khe thi gian TDMA.II. Cc khi nim c bn2.1 ONU
Identifier (ONU-ID)ONU-ID l mt chui c di 8 bits, dng nhn dng ONU. N
c OLT gn cho mt ONU trong qu trnh kch hot ONU thng qua bn tin
PLOAM.Mi ONU-ID c gi tr duy nht trong mng PON v c duy tr cho n tn
khi ONU tt ngun hoc khng cn lin h g vi OLT.2.2Allocation Identifier
(ALLOC_ID)ALLOC_ID l mt s 12 bit c OLT gn cho mt ONU nhn ra mt thc
th mang d liu theo hng upstream t ONU . Thc th mang d liu ny c gi l
T-CONT.Mi ONU c gn mt ALLOC_ID mc nh bng vi ONU_ID ca ONU
.2.3Transmission Containers (T-CONT)Mt T-CONT l mt i tng ONU i din
cho mt nhm cc kt ni logical ci m xut hin nh mt thc th n l theo hng
upstream trong PON. i vi mt ONU, s lng T-CONT h tr l c nh. ONU t ng
to ra tt c cc T-CONT c th trong qu trnh kch hot. OLT cng s nhn ra c
s lng T-CONT c h tr ca mi ONU. kch hot mt T-CONT mang d liu t user,
OLT phi thit lp mt nh x gia T-CONT v mt ALLOC_ID, ci m c gn cho ONU
trc thng qua bn tin PLOAM. Bt k ALLOC_ID no c gn cho ONU, bao gm
ALLOC_ID mc nh, u c th kt hp vi mt T-CONT.C 5 dng T-CONT: Dng 1:
T-CONT c bng thng c nh v ch yu c s dng cho cc dch v yu cu tr thp v
u tin cao, nh VOIP.- Dng 2 v dng 3: T-CONT c bng thng c m bo v ch
yu c s dng cho cc dch v video v d liu vi mc u tin cao hn.- Dng 4:
T-CONT c hiu qu nht v ch yu c s dng cho cc dch v d liu nh Internet
v cc dch v c u tin thp, khng yu cu bng thng ln.- Dng 5: T-CONT bao
gm tt c cc loi bng thng v c th mang tt c cc dch v.2.4Dynamic
Bandwidth Allocation (DBA)OLT chu trch nhim cp pht bng thng hng
upstream cho cc ONU. Bi v mng truy cp c chia s gia tt c cc ONU, cc
lung upstream t ONU c th xung t nu chng truyn trong nhng khong thi
gian ngu nhin. Do v tr t cc ONU so vi OLT trong GPON khc nhau nn tr
truyn t mi ONU n OLT l duy nht. OLT o c tr ny v thit lp mt thanh
ghi trong mi ONU thng qua bn tin PLOAM(Physical Layer Operations,
Administration and Maintenance) cn bng tr ca n vi tt c cc ONU khc
trn mng. iu ny c gi lRanging.Mt khi tr ca tt c cc ONU c thit lp,
OLT gi mtgrantsti tng ONU. Mt grants l s cho php s dng mt chu k thi
gian c nh ngha cho hng upstream. C sau vi milliseconds, bn grants
li c tnh ton li. Bn ny cp pht bng thng cho tt c cc ONU mi ONU nhn c
bng thng m n cn.DBA l mt phng php cho php p ng nhanh vic cp pht bng
thng cho mi user da trn yu cu v traffic hin ti.GPON s dng TDMA qun
l lung upstream truy cp bi cc ONU v ti bt k mt thi im no, TDMA cung
cp cc khe thi gian khng chia s ti mi ONU cho lung upstream.DBA cho
php cc khe thi gian upstream co li v ln thm da trn s phn phi ca ti
d liu upstream. Mt ONU phi c t nht mt T-CONT, nhng thng th s lng
T-CONT trn mi ONU ln hn 1. Mi T-CONT c mt mc u tin v lp traffic ca
n v tng ng vi mt khe thi gian upstream c th trong PON. Nu OLT khng
s dng DBA, bng thng upstream c gn tnh ti cc T-CONT v khng th chia s
vi cc T-CONT khc. Bng thng ca mi T-CONT ch c th thay i bi qun tr h
thng.C hai dng DBA Status Reporting DBA (SR-DBA) vNon-Status
Reporting DBA (NSR-DBA):- Trong NSR-DBA, mt OLT c cp pht c nh mt
lng nh bng thng m rng cho mi ONU. Nu ONU khng c data no gi, n s gi
cc idle frame. Nu OLT nhn thy rng mt ONU khng gi cc idle frame, n s
tng bng thng cp pht ti ONU . Mt khi ONU bt u gi cc idle frame, OLT
s gim bng thng cp pht xung. NSR-DBA c li ch l ONU khng cn bit DBA l
g. Tuy nhin, im bt li ca n l khng c cch no OLT bit c lm th no cp
pht bng thng cho vi ONU mt cc hiu qu nht.- SR-DBA bao gm cc trng
thi b m T-CONT c cung cp bi cc ONU khi OLT poll chng. Theo phng php
ny, OLT request trng thi ca b m T-CONT, v cc ONU respond li vi mt
bo co ring ca mi T-CONT. Bo co c cha d liu hin ti ang ch trong cc
T-CONT trong cc khe thi gian c th. OLT nhn c bo co ny s tnh ton li
bng thng cp pht(BW Map) thng qua thut ton DBA v gi BW Map mi ti cc
ONU trong lung downstream. ONU nhn c BW Map t OLT s gi li d liu
trong cc khe thi gian mi. Khi ONU khng cn d liu gi, da theo grants
nhn c t OLT, n gi cc idle cell n OLT thng bo rng b m ca n rng. OLT
nhn c thng tin ny s s cp BW Map ca T-CONT ny cho T-CONT khc.3.GPON
Transmission Convergence (TC) LayerITU-T recommentdation G.984.3
miu t lp GPON TC. Lp ny tng ng lp Data Link trong m hnh OSI. N c t
nh dng bn tin ca GPON, giao thc iu khin truy nhp, qu trnh x l OAM v
phng thc m ha thng tin. Hnh 3 miu t cu trc khung ca hai hng
download v upload. Khung GTC download cha cc khi iu khin vt l(PCBd)
v GTC payload. Khung GTC upstream cha cc cm truyn ti. Mi cm cha
overhead lp vt l v mt hoc nhiu chu k cp pht bng thng kt hp vi mt
ALLOC_ID c th.Khung downstream GTC cung cp thi gian tham chiu thng
thng cho PON v tn hiu iu khin thng thng cho lung upstream.
Hnh 3 GTC layer framing3.1Downstream GPON Frame FormatMt khung
GTC downstream c khong thi gian 125us v chiu di 38880 bytes, tng ng
vi tc d liu l 2.48832 Gbps. Hnh 4 ch ra chi tit nh dng khung GTC
downstream.
Hnh 4 Downstream GTC Frame FormatOLT gi PCBd theo kiu broadcast
v mi ONU u nhn c ton b PCBd. Cc ONU sau s hot ng da trn cc thng tin
lin quan cha trong . Trng Psync ch ra v tr bt u ca frame.. Trng
Ident cha mt trng8-KHz Superframe Counter ci m c s dng bi h thng m
ha v cng c th c s dng cung cp tc ng b thp gia cc tn hiu tham chiu.
Trng PLOAM thc hin chc nng nh bo hiu OAM hoc cnh bo ngng. Trng
BIP(Bit Interleaved Parity) c dng c lng t l li bit. Trng Plend cha
chiu di caupstream bandwidth (US BW) map. Plent c gi hai ln. Mi thc
th trong trngUpstream Bandwidth (US BW) map i din cho s mt cp pht
bng thng ti mt T-CONT c th. S lng thc th c ch ra trong trng
Plent.Trng ALLOC_ID ch ra cp pht bng thng nhn c cho mt T-CONT. Gi
tr ALLOC_ID thp nht l 254 thng s dng a ch ONU trc tip. Trong qu
trnh Ranging, ALLOC_ID u tin gn cho ONU phi l 254 v c coi l
ALLOC_ID mc nh. ALLOC_ID ny cng vi ONU-ID s dng trong bn tin PLOAM.
Nu nhiu ALLOC_ID c yu cu cho ONU, chng s c gi tr ln hn 255.
ALLOC_ID 254 l gi tr ALLOC_ID kch hot ONU, s dng pht hin nhng ONU
cha bit. Trng Slot Start v Stop ch ra bt u v kt thc ca s upstream.
Trng CRC s dng trong vic pht hin v sa li.Trng GTC payload cha mt
chui cc GEM((GPON Encapsulation Method) frame. Cc Downstream GEM
frame c lc ti ONU da vo trng Port ID trong header ca mi GEM frame.
Mi ONU c cu hnh c th nhn ra Port-IDs no thuc v n. Port-ID nhn dng
duy nht mt GEM frame.3.2Upstream GPON Frame FormatKhong thi
gianUpstream GTC frame cng l 125us v chiu di l 19400 bytes, do tc d
liu l 1.24416 Gbps. Mi upstream frame cha mt s cc burst ti t mt hoc
nhiu ONU. Mi upstream burst cha mt PLOu(upstream physical layer
overhead) v mt hoc nhiu on bng thng cp pht cho tng ALLOC_ID ring l.
BW Map ch ra vic sp xp cc burst trong frame v khong thi gian ch nh
cho mi burst. Mi khong ch nh c iu khin bi mt cu trc c bit ca BW
Map. Hnh 5 ch ra nh dng Upstream GTC frame.
Hnh 5 nh dng Upstream GTC FramePLOu ti v tr bt u ca ONU upstream
burst cha preamble ci m m bo chnh xc hot ng ca lp vt l ca burst-mod
upstream link. Trng PLOu cha trng ONU-ID nhn dng duy nht mt ONU. Lp
vt l upstream OAM(PLOAMu) chu trch nhim qun l chc nng Ranging, kch
hot ONU v pht thng bo cnh bo
