Chuyển mạch burst quang - Nhóm 10- Hưng-Giang-Hoàn
Transcript of Chuyển mạch burst quang - Nhóm 10- Hưng-Giang-Hoàn
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Chuyn thng tin quang CHUYN MCH BURST QUANG
MC LC
MC LC .................................................................................................................1DANH MC THUT NG V T VIT TT .....................................................3
DANH MC HNH V ............................................................................................4
LI M U ...........................................................................................................5
Chng 1: .................................................................................................................7
Gii thiu v chuyn mch burst quang .....................................................................7
1.1 Chuyn mch knh quang ....................................................................................7
1.2 Chuyn mch gi quang ......................................................................................8
1.3 Chuyn mch burst quang .................................................................................10
1.4 So snh cc cng ngh chuyn mch quang ......................................................11
Chng 2 : ..............................................................................................................12
Cc kha cnh c bn ca chuyn mch burst quang ...............................................13
2.1 Kin trc mng OBS ..........................................................................................13
2.1.1 Cu to nt bin ...........................................................................................14
2.1.2 Cu to nt li .............................................................................................15
2.2 T hp burst ......................................................................................................172.2.1 T hp burst da trn b nh thi ...............................................................17
2.2.2 T hp burst da trn mc ngng ..............................................................18
2.3 Cc c ch bo hiu ...........................................................................................19
2.3.1 C ch bo hiu Just Enough Time (JET) ..............................................20
2.3.2 C ch bo hiu Just In Time (JIT) ........................................................23
2.3.3 C ch bo hiu Tell And Go (TAG) .....................................................25
2.3.4 C ch bo hiu Tell And Wait (TAW) ..................................................26
2.4 Cc thut ton sp xp knh ..............................................................................272.4.1 Knh ri ph hp u tin (FFUC) ...............................................................28
2.4.2 Knh ri vi LAUT gn nht (LAUC) .........................................................29
2.4.3 Knh ri ph hp u tin thc hin lp khong trng (FFUC-VF) ..........30
2.4.4 Knh ri vi LAUT gn nht - thc hin lp khong trng (LAUC-VF) . .31
2.4.5 Khong trng kt thc ti thiu (Min-EV) ...................................................31
2.5 Phn gii tranh chp ..........................................................................................31
2.5.1 B m quang ..............................................................................................31
2.5.2 Chuyn i bc sng ..................................................................................33
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Chuyn thng tin quang CHUYN MCH BURST QUANG
2.5.3 nh tuyn chuyn hng ............................................................................34
2.5.3.1 Gii thiu .................................................................................................34
2.5.3.2 Phng php nh tuyn chuyn hng ....................................................35
2.5.4 Phn on burst ............................................................................................36
KT LUN .............................................................................................................40
TI LIU THAM KHO .......................................................................................42
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Chuyn thng tin quang CHUYN MCH BURST QUANG
DANH MC THUT NG V T VIT TT
Vit tt Ting Anh Ting Vit
ACK Acknowledgement packet Gi tin bo nhn
BA Burst Assembler B t hp burst
ATM Asynchronous Transfer Mode Ch truyn ti khng ng b
BHP Burst Header Packet Gi tiu burst
FDL Fiber Delay Line ng dy tr quang
FFUC First Fit Unscheduled Channel Knh ri ph hp u tin
FFUC-VF First Fit Unscheduled Channel-
Void Filling
Knh ri ph hp u tin-thc
hin lp khong trng
FIFO First In First Out B m vo trc ra trc
IP Internet Protocol Giao thc Internet
JET Just Enough Time (Tn giao thc)
JIT Just In Time (Tn giao thc)
LAUC Latest Available Unscheduled
Channel
Knh ri vi LAUT gn nht
LAUC- VF Latest Available Unscheduled
Channel Void Filling
Knh ri vi LAUT gn nht-thc
hin lp khong trng
MEMS Microelectromechanical System H thng vi c in
Min EV Minimum End Void Khong trng kt thc ti thiu
NAK Negative Acknowledgment Bn tin bo nhn ph nh
OBS Optical Burst Switching Chuyn mch burst quang
OCS Optical Circuit Switching Chuyn mch knh quang
OPS Optical Packet Switching Chuyn mch gi quang
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Chuyn thng tin quang CHUYN MCH BURST QUANG
OXC Optical Cross Connect Thit b ni cho quang
REL Release packet Gi tin gii phng knh
RM Routing Module B nh tuyn
RWA Routing and Wavelength
Assignment
nh tuyn v gn bc sng
S Scheduler B lp lch
SCU Switching Control Unit n v iu khin chuyn mch
SONET Synchronous Optical Network Mng quang ng b
TAG Tell And Go (Tn giao thc)TAW Tell And Wait (Tn giao thc)
WDM Wavelength Division Multiplexing Ghp knh phn chia bc sng
DANH MC HNH V
Hnh 1.1: Mng nh tuyn bc sng......................................................................8
Hnh 1.2: Mng chuyn mch gi quang OPS...........................................................9
Hnh 1.3: Nt chuyn mch trong mng chuyn mch gi quang...........................10Hnh 1.4: S dng thi gian offset trong OBS.........................................................10
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Hnh 2.1: Kin trc mng OBS................................................................................13
Hnh 2.2: S khi chc nng ca mng OBS.....................................................14
Hnh 2.3: Cu to nt bin.......................................................................................15
Hnh 2.4: Cu to nt li.........................................................................................16
Hnh 2.5: T hp burst da trn b nh thi..........................................................18
Hnh 2.6: T hp burst da trn mc ngng.........................................................18
Hnh 2.7: C ch bo hiu Just Enough Time....................................................21
Hnh 2.8: Li ch ca DR........................................................................................23
Hnh 2.9: C ch bo hiu Just In Time.............................................................24
Hnh 2.10: So snh c ch bo hiu JET (trn) v JIT (di)..................................24
Hnh 2.11: C ch bo hiu Tell-And-Go................................................................26
Hnh 2.12: C ch bo hiu Tell And Wait.......................................................27Hnh 2.13: Thut ton FFUC v LAUC...................................................................29
Hnh 2.14: Thut ton FFUC-VF v LAUC-VF......................................................30
Hnh 2.15: M t gii quyt xung t bng b m.................................................32
Hnh 2.16: Dy tr FDL cng vi b khuch i v chuyn mch to thnh mt
vng lp tr..............................................................................................................32
Hnh 2.17: Gii quyt tranh chp bng b chuyn i bc sng...........................33
Hnh 2.18: Cu trc ca mng OBS vi k thut lm lch hng i.......................35
Hnh 2.19: Phng php nh tuyn chuyn hng.................................................36Hnh 2.20: M t gii quyt xung t bng phn on burst...................................37
Hnh 2.21: Cu trc ca burst c ng kiu phn on.......................................38
Hnh 2.22: Xung t lm chng ln cc on ln nhau...........................................39
LI M U
Nhng nm gn y din ra s bng n lu lng thng tin trn ton cu. Yu
cu v bng thng i vi cc dch v vin thng ngy cng gia tng. Mt trong
nhng xu hng pht trin ca mng vin thng hin nay l quang ha t mng li
cho n tn mng truy nhp ca khch hng. Cng ngh ghp knh phn chia theo
bc sng WDM v ang c trin khai trong cc h thng thng tin quang
hin ti cho php tc truyn dn cc ln v kh nng h tr cc lu lng khcnhau nh IP, Ethernet, SONET/SDH. Mt vn t ra cho mng quang WDM l
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Chuyn thng tin quang CHUYN MCH BURST QUANG
la chn c cng ngh chuyn mch thch hp c th s dng mt cch ti u
bng thng ca si quang v gim thiu tr x l ti cc thit b chuyn mch. Ba
cng ngh chuyn mch quang c nghin cu s dng trong mng WDM l:
chuyn mch knh quang, chuyn mch gi quang v chuyn mch burst quang.
Trong ba cng ngh ny, chuyn mch burst quang ra i nhm p ng s bng n
d liu, gii quyt c nhc im ca chuyn mch knh quang v l bc trung
gian trc khi tin ti chuyn mch gi quang trong khi cng ngh cha cho php
c mng truyn ti ton quang. Xut pht t thc t trn nhm chng em chn
hng nghin cu v chuyn mch burst quang. Chuyn Chuyn mch burst
quang trnh by nhng vn c bn nht v chuyn mch burst quang. Ni dung
chuyn bao gm:
Chng 1: Gii thiu v chuyn mch burst quang. Chng ny s gii thiuv cc cng ngh chuyn mch quang chnh l chuyn mch knh quang, chuyn
mch burst quang v chuyn mch gi quang.
Chng 2: Cc kha cnh c bn ca chuyn mch burst quang. Ni dung
chng 2 gm c:
+Kin trc mng OBS
+T hp burst: theo ngng v da trn b nh thi
+Cc c ch bo hiu: JET,JIT,TAG,TAW
+Cc thut ton sp xp knh: vi thut ton hng ngang (Hoziron) v lp
khong trng (Void Filling)
+Cc gii php gii quyt tranh chp: b m quang, chuyn i bc sng ,
nh tuyn chuyn hng, phn on burst.
Do gii hn v mt thi gian v kin thc nn chuyn khng trnh khi thiu
st. Rt mong nhn c nhng ng gp t thy c v cc bn chuyn canhm c hon thin hn.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Chng 1:Gii thiu v chuyn mch burst quang
Ni dung chnh ca chng s cp n ba loi chuyn mch quang chnh l :
chuyn mch knh quang, chuyn mch gi quang v chuyn mch burst quang.
1.1 Chuyn mch knh quang
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Mng WDM nh tuyn bc sng bao gm cc thit b ni cho quang OXC
(Optical Cross Connect) c kt ni vi nhau bng cc lin kt WDM trong mt
tp mng hnh li ty . Phng php chuyn mch trong mng nh tuyn bc
sng l chuyn mch knh quang. Chuyn mch knh quang l chuyn mch hng
kt ni (connection oriented). Kt ni t mt nt ngun gi thng tin n mt nt
ch nhn thng tin phi c thit lp trc khi thng tin c truyn i. Trong
mng nh tuyn bc sng th kt ni t ngun ti ch ny c gi l ng
quang (lightpath). ng quang tng ng vi mt tuyn v bc sng c gn
cho tuyn . S thit lp cc ng quang bao gm mt s bc thc hin. Nhng
bc ny bao gm tm ra ti nguyn v tp mng, nh tuyn, gn bc sng, bo
hiu v d tr ti nguyn.
Hnh 1.1: Mng nh tuyn bc sng
1.2 Chuyn mch gi quang
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Hnh 1.2: Mng chuyn mch gi quang OPS
Mng chuyn mch gi quang OPS bao gm cc OXC c ni vi nhau bng
cc lin kt WDM trong mt cu hnh mesh ty . Mng chuyn mch gi quang
bao gm phn li c kh nng truyn ti tc cao v phn bin giao din vi phn
t ca cc mng IP, SONET/SDH, Ethernet.
Cc gi tin truyn trong mng chuyn mch gi quang c phn tiu v phn
ti tin. Tiu c cha thng tin nh tuyn cng nh thng tin iu khin v c
truyn trong bng cng vi ti tin. Khi gi tin truyn ti OXC, tiu s c tch
ra v c x l trong min in (sau khi bin i quang in quang) cn ti tin
s c chuyn mch trong min quang. V phn tiu cn mt thi gian x l
nn phn ti tin c lm tr i bng cch lu m bi ng dy tr quang. Vnguyn l, chuyn mch gi quang mong mun truyn thng tin v x l thng tin
iu khin hon ton trong min quang. Nhng do hn ch v mt cng ngh hin
nay nn phn thng tin iu khin ch c th x l trong min in m thi. Trong
chuyn mch gi quang, tiu c so snh vi mt bng nh tuyn, ti tin s
c chuyn ra cng u ra tng ng trn mt si quang v mt bc sng mi.
Nu khng c bc sng mi no kh dng, gi tin s b hy hoc phi b tr i
ch bc sng kh dng mi.
Thnh phn chnh ca nt OXC l c cu chuyn mch quang v khi iu khin
chuyn mch. Khi iu khin chuyn mch duy tr thng tin v t p mng, duy tr
bng nh tuyn, x l tiu gi tin, iu khin vic lu m, lp lch v chuyn
tip cc gi tin, iu khin c cu chuyn mch chuyn mch gi tin ng thi gian
nh, pht hin tranh chp v phn gii khi tranh chp xy ra gia cc gi tin. C
cu chuyn mch thc hin to kt ni t cng u vo n cng u ra tng ng
theo yu cu ca khi iu khin chuyn mch.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Hnh 1.3: Nt chuyn mch trong mng chuyn mch gi quang
1.3 Chuyn mch burst quangChuyn mch burst quang ra i nhm t c s cn bng gia chuyn mch
knh quang v chuyn mch gi quang. Cc gi tin lp trn s c tp hp li
thnh cc burst truyn ti trong mng OBS . Cc burst c di khng c nh
gm c hai phn: gi tin iu khin (control packet) hay cn c gi l gi tiu
burst Burst Header Packet (BHP) v phn thng tin d liu cn c gi l data
burst. OBS thc hin vic truyn c lp gi tin iu khin v burst d liu trn
cc knh bc sng khc nhau. Thng tin trong gi tin iu khin gm c chiu diburst, thi im pht burst, cc thng tin nh tuyn. Gi tin iu khin c truyn
i trc burst d liu mt khong thi gian c gi l offset time cu hnh cc
chuyn mch trong sut ng i t ngun ti ch. Thi gian offset time ny bng
tr x l tng cng ca gi tin iu khin ti tt c cc nt trung gian.
Hnh 1.4: S dng thi gian offset trong OBS
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Chuyn thng tin quang CHUYN MCH BURST QUANG
y l mt trong nhng khc bit c bn gia chuyn mch burst quang so vi
chuyn mch gi quang. Khong thi gian ny cho php thng tin iu khin c
x l ti mi nt chuyn mch v cc nt sp xp ti nguyn knh bc sng cho
vic truyn burst d liu da trn thng tin trong gi tin iu khin. Vi OBS khng
yu cu phi x l gi tin iu khin trong min quang. OBS s dng cc m hnh
d tr knh v bo hiu d tr ti nguyn knh bc sng.
1.4 So snh cc cng ngh chuyn mch quang
Nh nu trn, ta c th thy chuyn mch knh quang ch chuyn mch cho
mt bc sng trn mt ng quang nn khng cn thch hp cho mng WDM
hin nay. Nhng n cng c nhng u im ring ca n, ni bt nht l tin
cy. Bn cnh nhc im chnh l tr ln v lng ph bng thng.
Chuyn mch gi quang l loi chuyn mch hng ti trong mng ton quang
vi tc x l nhanh nht trong cc loi nu. Nhng gii hn ca n l ch
s hn ch v cng ngh hin ti khng p ng c cc yu cu cho mt mng
ton quang.
Trong khi lu lng cng ngy cng bng n. Ta c th thy chuyn mch
burst quang p ng c s bng n nh th no. Vi vic t hp cc gi cng
ch n dng chung mt gi iu khin lm gim thiu ti a vic x l thng tin
iu khin. Cc burst d liu hon ton truyn i trn min quang. V tc v kh
nng s dng bng tn hn hn chuyn mch knh quang. Trong thi im hin ti
vi cng ngh nh hin nay th chuyn mch tng gi mt vi vic x l tng y
tiu trong chuyn mch gi quang s khng th p ng c lu lng nh
chuyn mch burst quang. Tuy nhin, ci g cng c hai mt ca n, chuyn mch
burst quang p ng c yu cu v bng n lu lng nhng n vn cha phi l
chuyn mch ton quang, tr t hp burst, vic thay th thit b hay ch l cn thay
th module. l vn trade off trong vin thng.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Cng ngh
chuyn mch
quang
Hiu qu s
dng bng
thng
Thi gian
chuyn mch
yu cu
X l headerKh nng thch
ng lu lng
Chuyn mchknh quang
Thp Chm Thp Thp
Chuyn mch
gi quangCao Nhanh Cao Cao
Chuyn mch
burst quangCao Trung bnh Thp Cao
Bng 1.1 So snh cc cng ngh chuyn mch quang khc nhau
Chng 2 :
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Cc kha cnh c bn ca chuyn mch burst
quang
2.1 Kin trc mng OBSNh cp chng 1, tng ca chuyn mch burst quang l phn chia
mt bng d liu v mt bng iu khin v thc hin bo hiu ngoi bng cho
php truyn ti d liu trong min quang mt cch hiu qu hn. n v d liu
truyn ti trong mng OBS l cc burst gm c gi tin iu khin v burst d liu.
Mng OBS thc hin vic bo hiu ngoi bng: gi tin iu khin c truyn trn
mt knh bc sng khc vi burst d liu cu hnh cc chuyn mch t ngun
ti ch. Gi tin iu khin mang thng tin v chiu di burst, thi im bursttruyn cng nh cc thng tin nh tuyn khc. Mt khi ti nguyn c d tr
cc burst s c pht i. lm c iu ngi ta xut xy dng mt
mng OBS vi kin trc nh hnh 2.1
Hnh 2.1: Kin trc mng OBS
Mng OBS v bn cht l mt mng WDM trn n thc hin cng ngh
chuyn mch OBS. Mng OBS bao gm cc nt bin (edge node) v cc nt li
(core node) c kt ni vi nhau bng cc lin kt WDM.
Nt bin mng OBS thc hin giao din vi mng khc nh mng IP,
SONET/SDH hay Ethernet. Nt bin v th c kh nng giao tip c trong min
in v min quang v c kh nng bin i in quang cng nh chuyn i bcsng tng thch vi tn hiu truyn trn cc lin kt quang WDM. Nt bin trn
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Chuyn thng tin quang CHUYN MCH BURST QUANG
c s truyn ti burst c th phn thnh nt bin u vo v nt bin u ra. Nt
u vo pha pht vo thc hin t hp cc gi tin t cc u cui thnh cc burst
v to cc gi tin iu khin, nh tuyn v sp xp bc sng truyn cc burst
d liu vo mng li OBS. Nt u ra pha thu thc hin gii t hp cc burst
thnh cc gi tin v gi ti cc mng ch. Nu mt nt bin thc hin thng tin hai
chiu th n s ng vai tr va l nt u vo va l nt u ra.
Nt li c nhim v c bn l chuyn tip burst t cc cng u vo ti cc
cng u ra tng ng, d tr cc knh bc sng cho cc burst d liu da trn
thng tin trong cc gi tin iu khin v gii quyt tranh chp. Hnh 2.2 m t cc
thnh phn ca mng OBS vi cc chc nng khc nhau.
nh tuyn
T hp Burst
Lp lch v spxp bc sng
Bo hiu
Lp lch
Gii quyttranh chp
Gii t hpBurst
Chuyn tipgi
Lu lng raLu lng vo
Nt bin u vo Node li Node bin u ra
Hnh 2.2: S khi chc nng ca mng OBS
2.1.1 Cu to nt bin
Cc nt bin l cc router bin c kh nng giao din in v quang, thc hin
chc nng phn loi gi tin, lu m cc gi tin, t hp cc gi tin thnh cc burst
v gii t hp burst thnh cc gi tin cu thnh. Cc phng php t hp burst khc
nhau nh da trn thi gian ca b nh thi hoc da trn kch thc cc gi tin c
th c s dng t hp cc gi tin d liu thnh cc burst v gi vo trong
mng li OBS. Cu to ca mt router bin bao gm mt b nh tuyn RM
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Chuyn thng tin quang CHUYN MCH BURST QUANG
(Routing Module), cc b t hp burst BA (Burst Assembler) v cc b lp lch
knh S (Scheduler).
B nhtuyn
Lp l chknh
C l a s s0
C l a s s m
C l a s s0
C l a s s m
L ulngv o
L ulng
rat i
nodel i
n node b in1
n node b in N
T h p burst N
T hp bu r s t1
Lp l chknh
Hnh 2.3: Cu to nt bin
B nh tuyn kim tra thng tin nh tuyn ca tng gi tin, chn la cc cng
ra thch hp cho tng gi tin v gi n n b t hp burst thch hp. Mi b t
hp burst to ra cc burst cha cc gi d liu n cng mt ch ti (cng mtrouter bin u ra). Trong mi b t hp burst cn c hng i khc nhau cho cc
loi gi tin ng vi cc dch v khc nhau. B lp lch knh d tr knh bc sng
cho cc burst d liu v chuyn cc burst d liu ti cc cng u ra tng ng.
nt bin u ra, b gii t hp burst s tin hnh tch cc gi tin t cc burst ny v
chuyn tip ln cc lp trn.
2.1.2 Cu to nt li
Nt li gm c OXC v mt n v iu khin chuyn mch SCU (SwitchingControl Unit), cc b chuyn i quang in quang, cc b ghp knh, phn
knh. Ta xt hai phn t chnh l OXC v SCU. SCU to v duy tr mt bng
chuyn tip v chu trch nhim cu hnh cho OXC. Khi gi tin iu khin ti nt
li n s c bin i t min quang vo min in v i n SCU. SCU c
thng tin trong gi xc nh ch n ca gi ny v burst d liu theo sau, k tra
cu thng tin trong bng chuyn tip a n quyt nh chuyn tip d liu n
cng ra no ca OXC. ng thi SCU cng chu trch nhim d tr knh bc
sng cho burst d liu u ra. Gi tin iu khin sau s c cp nht thmthng tin iu khin nu nh nt hin ti cha phi l ch cui cng ca n v
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Chuyn thng tin quang CHUYN MCH BURST QUANG
c bin i in quang v truyn ra knh bc sng u ra tng ng. Trc khi
burst d liu i n router li, SCU s iu khin OXC thit lp kt ni t cng u
vo n u ra tng ng cho burst d liu . Ti nt li c nhiu kch bn c th
xy ra. Nu gi tin iu khin khng thnh cng trong vic d tr ti nguyn cho
burst d liu th c gi tin iu khin v burst d liu s b hy b. Hoc khi cc
burst d liu ti cc u vo cng mun n mt cng u ra ca OXC, khi
tranh chp s xy ra v SCU s c nhim v pht hin v gii quyt tranh chp ny
theo cc chnh sch gii quyt tranh chp m mng s dng. C hai phng php
gii quyt tranh chp m phn 2.5 ca chuyn cp n l s dng cc ng
dy tr quang v chuyn i bc sng. thc hin c cc phng php ny
i hi nt li mng OBS phi trang b thm ng dy tr quang v cc b chuyn
i bc sng.
Hnh 2.4: Cu to nt liTa c th thy cc gi tin khi i vo cc node bin s c nh tuyn chuyn
ri mi chuyn n cc b t hp v sau c lp lch v sp xp trn bc sng
u ra tng ng. Ti cc node li s ch c trch nhim chuyn tip gi tin i nh
x l cc thng tin bo hiu v lp lch. Ti node bin u ra s burst s c gii t
hp v phn phi n a ch.Trong mng OBS x dng cc giao thc nh tuyn
OSPF v GMPLS. C th thy tuyn c la chn ti node bin, cc node ch
ch vic chuyn tip. Nu i thm v cc giao thc trn th chuyn s qu di v
khng tp trung vo c im ring chnh ca OBS nn nhm s khng trnh by c
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Chuyn thng tin quang CHUYN MCH BURST QUANG
th v cc giao thc nh tuyn trn. Mc tip theo s cp n qu trnh t hp
burst l mt trong nhng c im ni bt ca OBS.
2.2 T hp burst
T hp burst l tin trnh tp hp v ng cc gi router nt bin u vo tcc lp cao hn thnh cc burst truyn ti vo mng OBS. Khi cc gi tin i n
t lp cao hn, chng c lu m trong cc b nh m in v c phn loi
theo a ch v loi dch v. Vic t hp burst s quyt nh khi no to ra mt burst
v gi burst vo mng OBS. Hai phng php t hp burst ph bin nht l t
hp burst da trn b nh thi v t hp burst da trn mc ngng.
2.2.1 T hp burst da trn b nh thi
Nh trnh by trong phn 2.1.1, cc router bin c cu to gm c b nhtuyn, cc b t hp burst v cc b lp lch knh. Khi cc gi tin n router bin,
b nh tuyn s cn c vo a ch ch ca cc gi tin chuyn cc gi tin ny
n b t hp burst thch hp. Cc gi tin ny s c lu m tm thi trong cc
hng i khc nhau nm trong b t hp burst. Trong phng php t hp burst da
trn b nh thi, mi b t hp burt s tham chiu thi gian ca mt b nh thi
cc b nm trn mt hng i quyt nh vic t hp cc gi tin thnh cc burst.
Thi im b nh thi bt u m thi gian c th l ngay sau khi mt burst trc
c lp lch truyn i hoc ngay sau khi gi tin u tin n hng i sau
khi hng i trng. Sau mt khong thi gian Tc cu hnh t trc, cc gi tin
trong hng i s c t hp thnh mt burst v lp lch truyn i. Phng
php t hp burst ny s to ra cc burst c chiu di ngu nhin. Lu lng vo
mng thay i phn ln s quyt nh chiu di ca burst. Lu lng vo mng ln,
burst s c kch thc ln, lu lng vo mng nh, burst s c kch thc nh.
Tuy nhin, thi gian ca b nh thi cng l mt nhn t quyt nh kch thc cc
burst.
T T TTh i gian
L1 L2 L3
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Hnh 2.5: T hp burst da trn b nh thi
2.2.2 T hp burst da trn mc ngng
Trong phng php t hp burst da trn mc ngng, s lng cc burst b
gii hn hay chiu di ca cc burst l bng nhau. C th l khi khi kch thc cacc gi tin trong hng i t n mt gi tr ngng L, cc gi tin c t hp
thnh burst v lp lch truyn i. Phng php t hp burst ny khng m bo
v thi gian tr t hp burst.
X1 X2 X3Thi gian
L L L
Hnh 2.6: T hp burst da trn mc ngng
Mt vn t ra cho vic t hp burst l lm sao tm ra gi tr ca b nh thi
v kch thc ngng ti thiu ha xc sut mt gi trong mng OBS. Nu nh
mc ngng qu thp dn n kch thc burst nh, s lng burst truyn trong
mng s nhiu dn n xc sut xy ra xung t cc router li cao, nhng s
lng gi trung bnh b mt do xung t li nh. Tuy nhin, s lng burst nhiu slm tng p lc ln mt bng iu khin do phi x l nhiu cc gi tin iu khin
ca mi burst d liu. Nu thi gian cu hnh cho mi nt chuyn mch khng c
b qua, cc burst ngn s khin cho vic s dng ti nguyn mt cch km hiu qu
do phi mt nhiu thi gian chuyn mch. Ngc li, khi mc ngng ln dn n
kch thc burt ln, s lng burst vo mng s nh, do xc sut xy ra xung t
s nh nhng s lng gi trung bnh b mt do xung t s ln.
V th, cn c mt s cn bng gia s lng xung t v s lng gi mt
trung bnh ti mi ln xung t. Do , hot ng ca mng OBS s c ci thin
khi cc gi n c t hp thnh burst vi mt kch thc ti u. Tng t,
phng php t hp burst da trn b nh thi cng cn gi tr ti u v mt thi
gian.
Vic la chn phng php t hp burst ty thuc vo loi lu lng c
truyn i. Phng php t hp burst da trn b nh thi thch hp vi cc lu
lng b gii hn v mt thi gian nh cc dch v thi gian thc nh thoi, truyn
ti video v thi gian tr t hp burst b gii hn. Nu khng c gii hn v tr,phng php t hp burst da trn mc ngng ph hp cho cc dch v khng yu
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Chuyn thng tin quang CHUYN MCH BURST QUANG
cu thi gian thc nh truyn s liu, v cho php iu khin c s lng gi b
mt trong mi ln xung t.
Vic s dng c hai loi phng php da trn b nh thi v da trn mc
ngng l la chn tt nht v vic t hp burst s linh hot hn l ch dng mttrong hai phng php k trn. Bng cch tnh ton gi tr mc ngng ti u v s
dng gi tr ca b nh thi da trn tr gi cho php, ta c th chc rng s
lng mt gi l nh nht trong khi vn m bo tr cho php.
Sau khi mt burst c to ra s dng cc phng php c ni trn, burt
c lu m trong hng i trong mt khong thi gian trc khi truyn i sao
cho gi tin iu khin ca burst c thi gian d tr ti nguyn. Trong thi
gian ny, cc gi tin khc c th tip tc n router nt bin. Vic thm cc gi tin
ny vo burst l khng chp nhn c v ti nguyn cho burst lc u c d tr
cn c vo chiu di ca burst c trong gi tin iu khin. cc gi tin ny cho
cc burst ng sau c kh nng tng tr trung bnh trong trng hp lu lng
ln. Trong [4], theo Yang Chen, Chunming Quiao v Xiang Yu, mt cch gim
thiu tr l thc hin d on chiu di burst: Gi tin iu khin s mang thng tin
v chiu di burst l L + f(t) thay v l L vi L l chiu di chnh xc ca burst khi
gi tin iu khin c gi i, f(t) l chiu di d on ca cc gi tin i n trong
khong thi gian offset v c tnh ton da trn tc trung bnh ca lu lngti. Gi s chiu di thc s ca cc gi tin i n l l(t). Nu f(t) > l(t), chiu di
ca burst khi truyn vo nt li l L + l(t), ti nguyn d tr cho burst s b lng
ph. Nu f(t) < l(t), chiu di ca burst khi truyn vo nt li l L + f(t). Mt phn
gi tin c chiu di l(t) f(t) s c ghp vo truyn trn cc burst pha sau.
Nu f(t) = l(t) l trng hp l tng nht khi vic d tr ti nguyn cho burst s
l ti u v khng tng tr.
2.3 Cc c ch bo hiu
Khi burst c truyn vo mng li OBS, mt c ch bo hiu phi c thc
hin nhm mc ch phn b ti nguyn v cu hnh trng chuyn mch cho tng
burst ti mi nt thng qua cc gi tin tiu burst BHP hay gi tin iu khin.
OBS s dng bo hiu ngoi bng: Gi tin BHP c truyn trn mt bc sng
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Chuyn thng tin quang CHUYN MCH BURST QUANG
khc vi burst d liu. Tuy nhin, BHP c truyn trn cng mt ng i t
ngun ti ch nh burst d liu thng bo cho cc nt chuyn mch d tr ti
nguyn v cu hnh trng chuyn mch cho burst d liu tng ng.
Di y ta xt bn c ch bo hiu c bn trong OBS l Just Enough Time,Just In Time, Tell And Go v Tell And Wait. Trong ba c ch bo
hiu u l ba c ch bo hiu mt chiu: Nt ngun gi gi tin iu khin d
tr ti nguyn v thc hin truyn burst d liu m khng cn i nt ch thng
bo vic d tr ti nguyn t ngun ti ch c thnh cng hay khng. C ch bo
hiu Tell And Wait l c ch bo hiu hai chiu: nt ngun s ch truyn burst
d liu khi c xc nhn knh truyn c thit lp hon ton t ngun ti
ch.
2.3.1 C ch bo hiu Just Enough Time (JET)
Trong phng thc JET, c mt tr gia vic truyn dn gi tiu burst v
burst d liu. tr ny ln hn tng thi gian x l gi iu khin dc theo tuyn.
Mc ch l sao cho khi burst n mi nt chuyn mch trung gian th gi tiu
burst c x l xong v mt knh trn cng u ra c ch nh. Do
khng cn ng dy tr quang lm tr burst d liu mi nt. y l mt c
tnh quan trng ca JET v ng dy tr quang tn km v c nhiu hn ch (v d
nh ch cho tr c nh, chiu di ln). C ch bo hiu Just Enough Timec m t trn hnh 2.7, node ngun u tin gi mt gi tiu bust (Burst
Header Packet BHP) trn mt knh iu khin ti node ch. BHP c x l ti
mi node tip theo vi yu cu thit lp ng d liu ton quang cho burst d liu
tng ng. Nu qu trnh d tr ti nguyn thnh cng, chuyn mch s c cu
hnh cho burst d liu i qua. Trong lc , burst s i ti ngun trong min in.
Sau mt khong thi gian offsetime xc nh trc, burst c gi trong min
quang trn bc sng c chn.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
TRANSMISSION
&RELEASE PHASE
Time
Source Intermediate nodes Destination
BHP
SETUP &
CONFIRM
PHASE
ST
Burst
Hnh 2.7: C ch bo hiu Just Enough Time
Offset time c tnh ton c bn da trn s nt chuyn mch trung gian t
ngun ti ch v thi gian chuyn mch ca node ch. Offset time c tnh l[1]:
OT = h. + ST
y: h l s nt chuyn mch trung gian gia ngun v ch.
l thi gian x l tiu burst trn mt nt trung gian.
ST l thi gian cu hnh li chuyn mch nt ch.
Gi tiu burst cha thng tin v offset time v chiu di burst. V khi qua mtnt trung gian th gi tr offset time s phi c cp nht li v gi tr offset time s
gim i mt lng ng bng thi gian x l gi tin tiu burst. Khi qu trnh d
tr ti nguyn thnh cng, knh bc sng ti mt nt s ch c n nh hon
ton cho burst khi bit u tin ca burst d liu truyn ti nt. Tc l trong khong
thi gian offset time, knh khng b chim hay ri cho d ng k ti nguyn
thnh cng. Knh c th gn cho burst d liu khc min sao cho vic truyn cc
burst d liu khng b chng ln nhau. Kiu d tr ti nguyn ca JET gi l d tr
ti nguyn tr (Delayed Reservation). Sau khi truyn xong burst d liu knh bc
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Chuyn thng tin quang CHUYN MCH BURST QUANG
sng s t ng c gii phng m khng cn phi c bn tin gii phng knh
c gi t ngun ti ch. V th mt c im khc ca JET l gii phng ti
nguyn khng r rng (Implicit Release).
Nu ti nt trung gian no , qu trnh d tr khng thnh cng, burst s b loib.
Mt vn ny sinh trong vic tnh ton gi tr offset cho JET l phi xc nh
c s nt chuyn mch trung gian gia ngun v ch. Trong mng OBS, thng
tin v s lng cc nt chuyn mch trung gian trn mt ng i t ngun ti
ch thng thng l khng sn c. Thm ch khi nhng thng tin ny bng cch
no c bit th do nh hng ca l trnh thay i, n cng khng m bo
tnh hp l khi s dng [5].
Nh vy, cn mt gi tr offset time m khng ph thuc vo ng truyn s
dng v khng yu cu trao i thng tin gia cc node mng vi nhau. Hin nay
vi nhng tin b trong ch to phn cng cho cc giao thc truyn thng, tr x l
ti cc node trung gian l rt ngn trong hu ht cc chc nng chung ca giao thc
bo hiu. Trong trng hp ny, cc dy tr quang c th c s dng mt cch
hp l ti cc node trung gian lm tr mi burst u vo mt lng thi gian cn
bng vi tr x l tiu . Ta c th b qua cc tr ny trong tnh ton. S mi
ny c gi l giao thc ch c tr ch (Only Destination Delay ODD) v gi trtrong biu thc l [5]:
OT = + ST
,ST l cc gi tr tr ti ch.
*D tr tr (DR) trong vic s dng hiu qu bng thng:
D tr tr s em li hiu qu cao trong vic s dng bng thng. Hnh 2.8 minh
ho ti sao d tr tr to nn s dng hiu qu bng thng.
Gi s xt ti node X, mt gi iu khin ti v thc hin dnh trc ti nguyn
ti thi im t1' v thi im bt u tin ca burst th nht n l t1, vi t1> t1' .Ta
c khong thi gian offset time ca burst th nht l: offset time = t1 - t1'.
Cng gi s c mt gi iu khin khc (gi iu khin th 2) n node ang xt
thi im t2', tng t t2 l thi im bt u tin ca burst th 2 n. Ta c thi
gian tr ca burst ny l: offset = t2 - t2'.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Thi imgiiu khin th
nht ti Burst thnht
Thi imgiiu khin th
hai ti
t1'
t2'
t1
Burst thhai
t2
Trnghp 1
Trng hp 2
t2 + l2 t1+l1
Thi imgi
iu khin thnht ti Burst th 2
Thi imgiiu khin th
hai ti
t1'
t2'
t2
Burst th 1
t1 t1+l1 t2 + l2
Hnh 2.8: Li ch ca DR
xc nh thi im n ca Burst th nht (t1) , khi thi gian x l gi iu
khin c th thay i t node ny n node khc, trong gi iu khin s cho bit
gi tr thi gian tr c s dng ti node k tip. Gi tr ny c th c cp nht
da trn thi gian x l gi iu khin ti node hin ti.
Trong chuyn mch burst quang da trn giao thc JET , vic xc nh thi im
n ca burst t1 l rt quan trng. Trong giao thc JET rng bng c ng k
ti thi im t1 +l1, l1 l khong thi gian tn ti ca burst th 1 , thay v n v
hn. iu ny s lm tng hiu qu s dng bng thng v gim xc sut loi b
burst .
Nh hnh trn, trong c hai trng hp, chng ta s dng giao thc JET v d
tr tr th burst th 2 n vn c phc v nu t1' < t2' < t2+l2 < t1 (trng hp 1)
hay t1' < t2' < t1+l1 < t2 (trng hp 2).
DR lun s dng thi gian tr. tng hiu qu s dng thi gian tr trong JET
v gim tr khi phi truyn li burst, gi iu khin s c truyn i ti thi
im sm nht c th c bng cch c lng di burst. Nu di burst ln
hn di c lng th mt gi iu khin khc s c gi i xo rngbng ng k. Nu di burst nh hn di c lng th phn d liu cn
li c gi i nh mt hay nhiu burst b sung.
2.3.2 C ch bo hiu Just In Time (JIT)
C ch bo hiu Just-In-Time (JIT) tng t nh c ch bo hiu JET, nhng c
ch ny s dng phng thc d tr ti nguyn tc thi (Immediate Reservation) v
gii phng ti nguyn r rng (Explicit Release). Gia burst d liu v gi tiu
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Chuyn thng tin quang CHUYN MCH BURST QUANG
burst c khong thi gian offset time do JIT khng yu cu phi s dng ng
dy tr quang ti cc nt chuyn mch trung gian.
TRANSMISSION
&RELEASE PHASE
Time
Source Intermediate nodes Destination
BHP
SETUP &
CONFIRM
PHASE
ST
Burst
Hnh 2.9: C ch bo hiu Just In Time
Khi gi tiu burst c gi i d tr ti nguyn, ti mt nt chuyn mchtrung gian, mt bc sng kh dng s c n nh cho burst d liu ngay sau khi
gi tiu burst c x l. Bc sng ny s dnh ring cho burst d liu cho n
khi c mt bn tin gii phng c gi i t node ngun gii phng kt ni. Gi
tiu burst khng cn phi mang thng tin v thi im n ca burst d liu v
di ca burst [3]. Nu qu trnh d tr ti nguyn tht bi, burst d liu s b loi
b.
02 1
0
12
OT 2OT
1
OT 0
HeaderControl
Chanel
Data
chanel
2 1 00
Header Header HeaderREL
0
2 1
Hnh 2.10: So snh c ch bo hiu JET (trn) v JIT (di)
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Hnh 2.10, so snh c ch bo hiu JET v JIT. Trong c ch bo hiu JET s
dng phng thc d tr ti nguyn tr (Delayed Reservation), ngha l bc sng
kh dng ch c n nh khi burst d liu 0 ti c nt nn trong khong thi
gian offset time c th truyn burst d liu 1 trn cng mt bc sng m khng
xy ra tranh chp. Khi burst d liu 0 kt thc truyn, knh truyn s c gii
phng ngay v vy burst d liu 2 c th c gi vo mng OBS ngay khi burst
0 kt thc. Trong c ch bo hiu JIT, do s dng phng thc d tr ti nguyn
tc th nn trong khong thi gian offset time, burst d liu 1 khng th truyn
qua mng OBS trn cng bc sng vi burst d liu 0. V khi burst d liu 0
truyn kt thc th bn tin gii phng s c gi i t node ngun gii phng
knh, trong thi gian gii phng knh th burst d liu 2 khng th truyn qua
mng OBS c. Nh vy c ch bo hiu Just In Time s dng bng thngkhng hiu qu nh Just Enough Time. Trong [4], Jing Teng v George N.
Roukas tin hnh m phng v kt qu cho thy c ch bo hiu JIT cho t l
mt burst cao hn c ch bo hiu JET.
2.3.3 C ch bo hiu Tell And Go (TAG)
y l c ch bo hiu d tr ti nguyn tc th v gii phng ti nguyn r rng.
Trong TAG, gi iu khin c truyn dn trn mt knh iu khin c theo
sau bi mt burst, trn mt knh d liu vi lch bng khng hoc khng ngk. Burst c m bng cch s dng ng dy tr quang (Fiber Delay Line
FDL) trong khi gi tin iu khin c x l ti mi node trung gian. c im ny
ca Tell And Go ging vi chuyn mch gi quang. Nu vic chim dng bc
sng thnh cng th burst c truyn dn dc theo knh chim tri li burst s
b loi b v mt bn tin ph nh bo nhn (NAK Negative Acknowledgment)
c gi tr li ngun. Node ngun s gi mt gi iu khin sau khi truyn dn
burst gii phng ti nguyn b chim dc theo tuyn [2].
Nhc im c bn ca phng php ny l gii hn v mt cng ngh ca
ng dy tr quang FDL. FDL c th lm tr burst trong mt khong thi gian c
nh v khng th thch nghi vi kch thc burst d liu thay i. Cng nh c ch
bo hiu Just In Time, TAG khng tn dng ti nguyn hiu qu do s dng d
tr tc th v gii phng ti nguyn r rng. Khi cha c bn tin Release, knh s
khng c gii phng v trong khong thi gian gii phng knh, knh s khng
c gn cho bt c burst no khc.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Time
BHP
Burst
Source Intermediate nodes Destination
Gi tinNAK
Gi tinRELEASE
Hnh 2.11: C ch bo hiu Tell-And-Go
2.3.4 C ch bo hiu Tell And Wait (TAW)
C ch bo hiu Tell And Wait c m t trn hnh 2.12. Trong phng
thc TAW, node ngun s gi mt gi iu khin d tr ti nguyn cho burst d
liu dc theo tuyn. Sau nt ngun s i mt bn tin xc nhn ACK t nodech thng bo vic d tr ti nguyn thnh cng. Burst d liu s c gi i t
node ngun trn tuyn ng chn. V sau khi burst c truyn n ch s c
mt bn tin gii phng kt ni c gi i gii phng ti nguyn t ngun ti
ch.
Nu qu trnh d tr ti nguyn l khng thnh cng ti mt nt OBS trung gian
no n nt ch th ngay lp tc s c mt bn tin thng bo khng thnh cng
c gi v t nt gii phng ti nguyn c thit lp trn tuyn t nt
ngun n nt v burst s khng c pht i. Nt ngun s phi gi li gi iu
khin d tr ti nguyn t ngun ti ch mt ln na.
Nu chng ta so snh TAW v cc c ch bo hiu mt chiu th nhc im ca
TAW l thi gian cn thit cho thit lp knh; tuy nhin u im ca TAW l t l
mt d liu l rt thp. Do , TAW ph hp cho lu lng yu cu tin cy v t l
mt burst v cc burst c kch thc tng i ln so vi thi gian thit lp knh
truyn t ngun ti ch.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
Hnh 2.12: C ch bo hiu Tell And Wait
2.4 Cc thut ton sp xp knh
Khi mt gi tin tiu burst ti nt li, khi iu khin chuyn mch SCU x lgi tin ny v mt thut ton sp xp knh hay lp lch knh c thc hin gn
burst d liu cho mt knh bc sng thch hp u ra. Khi lp lch knh ly
thi im burst n v chiu di burst t gi tin iu khin. Thut ton c th cn
phi duy tr thi im knh truyn ri gn nht LAUT (Latest Available
Unscheduled Time) hay cn gi l horizon, cc tham s void v gap trn mi
knh d liu u ra. Theo l thuyt, LAUT ca mt knh d liu l thi im gn
nht mt knh d liu c th s dng cho mt burst d liu cha c lp lch hay
c th hiu l thi im kt thc ca mt burst c lp lch trong mt s trng
hp. Gap l khong thi gian c tnh gia thi im n ca mt burst cha
c lp lch v thi im kt thc ca mt burst c lp lch trc . Void
l khong thi gian gia hai burst d liu c lp lch trn mt knh d liu m
trong thi gian khng c burst d liu no khc c lp lch.
Nhng thng tin sau c s dng bi khi lp lch cho hu ht cc thut ton
lp lch knh:
- Lb l chiu di burst.
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Chuyn thng tin quang CHUYN MCH BURST QUANG
c sp t da trn ch s ca knh (D0, D1, D2, D3), burst d liu i n s c
sp xp trn knh D1 v theo th t knh D1 nm trc knh D2. u im ca thut
ton ny l n gin v tc nhanh. Nhc im ca thut ton ny l t l mt
burst cao v tn dng ti nguyn khng hiu qu.
Hnh 2.13: Thut ton FFUC v LAUC
2.4.2 Knh ri vi LAUT gn nht (LAUC)
Thut ton LAUC v c bn thc hin tng t nh thut ton FFUC, n cng
duy tr gi tr ca LAUT trn tt c cc knh d liu nhng trong qu trnh tm kim
knh d liu thch hp thay v sp xp burst d liu trn knh thch hp u tin tm
c, gii thut ny la chn knh c LAUT gn vi thi im n ca burst d
liu nht. Gii thut LAUC cng c m t trn hnh 2.13. Cc knh d liu D1 vD2 l thch hp v v thi im burst i n nm sau cc LAUT ca hai knh ny.
Nu nh thut ton FFUC c s dng th knh D1 c la chn. Tuy nhin, vi
thut ton LAUC cn quan tm n khong thi gian gia t v LAUT ca mi
knh, n s chn knh no c khong thi gian ny nh nht; iu ny lm tng
hiu qu s dng knh truyn so vi FFUC. Chng ta thy rng Gap 1 > Gap2 do
burst s c sp xp trn knh D2.
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2.4.3 Knh ri ph hp u tin thc hin lp khong trng (FFUC-
VF)
im khc bit ca cc thut ton lp khong trng ni chung l khng lu tr
gi tr LAUT m lu thi im bt u S v kt thc E ca mi burst c lplch trn mi knh d liu. u im ca thut ton ny l tn dng khong trng
gia hai burst d liu c sp xp lm tng hiu sut s dng knh truyn. FFUC-
VF cng ging nh FFUC, n la chn knh truyn theo th t. Knh truyn u
tin vi mt khong cch void thch hp c la chn.
Hnh 2.14: Thut ton FFUC-VF v LAUC-VF
Thut ton FFUC-VF c m t trn hnh 2.14. Da trn Si,j v Ei,j, tt c cc
knh d liu D0, D1, D2, D3 c kh nng p ng cho burst d liu n cha clp lch. Nu cc knh ny c sp t theo ch s ca tng knh, burst d liu mi
n s c lp lch ng ra D0 v n tha mn cc iu kin sau: l knh c s th
t u tin c E0,0 nm trc E0,a, khong trng void ca knh ny ln hn chiu
di ca burst mi n cha c lp lch.
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2.4.4 Knh ri vi LAUT gn nht - thc hin lp khong trng (LAUC-
VF)
Gii thut LAUC-VF cng lu nhng thi im bt u S v kt thc E cho mi
burst d liu c lp lch trn tt c cc knh d liu. u im ca gii thutny l tn dng c khong trng gia hai burst d liu c sp xp. Knh
truyn vi khong trng m khong cch nh nht s c chn. thit lp
LAUC VF, khi iu khin chuyn mch phi lu li thng tin s dng ca tt c
cc knh d liu. iu ny lm cho LAUC VF phc tp hn so vi FFUC v
LAUC. Nhng tn dng ti nguyn hiu qu hn 2 loi trn.
2.4.5 Khong trng kt thc ti thiu (Min-EV)
Mt dng khc ca LAUC VF l Minimum End Void (MinEV). MinEV s xts khc nhau gia thi im bt u ca burst d liu lp lch v thi im kt
thc ca burst d liu n mi trn knh. MinEV chn mt knh c gi tr trc n
nh nht. V vy gii thut ny cn lu thng tin ca cc knh d liu trc
2.5 Phn gii tranh chp
2.5.1 B m quang
Cn s dng b m khi c nhiu hn mt gi vo mun chuyn ti cng mt
u ra. Mt trong nhng c trng c bn ca chuyn mch OBS l khng cn b
m quang: nt trung gian trong mng quang khng yu cu phi c b m quang.
Cc burst i xuyn qua cc nt trung gian m khng c bt k s tr no.
Vic gii quyt xung t mng chuyn mch gi min in truyn thng s
dng cc b nh RAM lu gi tm thi cc gi tin khi xung t xy ra; tuy nhin
mt b m ging nh RAM trong min quang hin nay vn cha c hu hiu.
Trong mng quang, ngi ta xut dng dy tr FDL (Fiber Delay Line) lu
gi cc gi tin trong min quang mt khong thi gian c nh. Yu cu tr thay itrong trng hp nhiu gi cn lm tr v cn x l ti cng mt thi im. Ghp
theo bc sng v theo thi gian u c s dng trnh tranh chp. m quang
c thc hin khi s dng ng dy tr si quang (FDL). Chiu di ca FDL c
th tnh ton c. Chng hn m mt gi IP c 512 byte truyn vi tc 10
Gbit/s cn s dng 82 m si quang. FDL khng th a tn hiu ra ti thi im bt
k; nhng n l mt h thng vo trc ra trc (FIFO) nn cc gi i qua ton b
chiu di ca FDL l c m.
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Bng cch kt ni cc dy tr FDL theo tng hay kt ni song song, b m c
xut ny c th gi cc gi tin trong min quang hay burst d liu trong cc thi
gian khc nhau.
Hnh 2.15: M t gii quyt xung t bng b m
Cch ny c th gim mt d liu nhng li khng m bo gi n ng th
t. Trong mt s kin trc ca b m quang, kch thc ca nhng b m th b
gii hn khng ch xt n yu t v mt tn hiu m c yu t v kch thc vt l.
lm tr mt burst khong 1 ms th cn phi c mt on cp di hn 200 km.
Bi v nhng hn ch ca b m quang nn mt node mng c th khng th png c lu lng ti ln v trong iu kin ti t bin. Nhng b m quang
da trn nh tuyn bc sng v iu khin bc sng trong mt vng khp kn
cng l mt gii php cho b m quang
Hnh 2.16: Dy tr FDL cng vi b khuch i v chuyn mch to thnh
mt vng lp tr
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2.5.2 Chuyn i bc sng
m kt hp vi chuyn i bc sng gii quyt tranh chp cc gi trong
min thi gian. Tranh chp trong min khng gian phi s dng nh tuyn chch
hng. Tuy nhin gii php ny khng th s dng trong mng chuyn mch tonquang do tnh phc tp ca nh tuyn ng. Cn m khi hai gi quang c nh
tuyn ng thi ti cng mt cng ra. Nu c hai gi c cng bc sng th chng
khng th i ti u ra
Trong WDM, nhiu bc sng c ghp cng lc trn mt lin kt kt ni hai
chuyn mch quang. Nhiu bc sng c th gim ti thiu s lng xung t. Gi
s rng c hai burst u i n mt ch v ra cng ng ra ti cng thi im. C
hai burst c th vn cn truyn i tip nu trn hai bc sng khc nhau. Cch
thc ny rt c kh nng trong vic gim thiu xung t, c bit trong lc s
lng bc sng c th ghp trn si quang ngy cng tng ln. Ngi ta c on
trong tng lai c th ghp t 160 n 320 bc sng trn mt si quang. Chuyn
i bc sng l tin trnh chuyn i mt bc sng ng vo thnh mt bc
sng khc ng ra. B chuyn i bc sng l nhng thit b chuyn i bc
sng ng vo thnh mt bc sng khc ng ra, do vy tng kh nng s dng
li bc sng, ngha l cng mt bc sng c th c s dng li mang nhiu
kt ni trn nhng lin kt khc nhau trong mng. Do nhng b chuyn i bcsng cung cp kh nng nng cao hiu sut s dng bc sng
Hnh 2.17: Gii quyt tranh chp bng b chuyn i bc sng
Trong mng chuyn mch burst quang, s lng xung t c th gim xung
bng cch s dng thm kh nng s dng li bc sng cho php nhiu bc sng
cng hot ng trn mt lin kt t node ng vo n node ng ra. Mt burst
n xung t (contenting burst) c th c chuyn ti bt k bc sng no
ng ra ang ri. Hin nay cc b chuyn i bc sng ch c a ra v s dng
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trong cc phng th nghim, cng ngh ny cng cha hon thin v khong bc
sng c kh nng chuyn i cn hn ch. Sau y l mt s loi chuyn i bc
sng:
- Chuyn i ton b (Full conversion): Mt bc sng mi n c thchuyn n bt k bc sng no ng ra; do vy khng c mt bc sng no
xut hin lin tc trn mt kt ni t u cui n u cui.
- Chuyn i c gii hn (Limited conversion): Vic chuyn i bc sng b
gii hn khng phi tt c cc knh ng vo u c th kt ni n tt c cc
knh ng ra. Vic gii hn ny s lm gim chi ph ca chuyn mch trong khi
chp nhn mt s lng xung t.
- Chuyn i c nh (Fixed conversion): y cng l mt dng ca limitedconversion, trong mi knh ng vo c kt ni vi mt hay nhiu knh ng
ra c ch nh trc.
- Chuyn i mt phn (Sparse wavelength conversion): Trong mng c th bao
gm cc node c chuyn i ton b, c gii hn, c nh v khng c b
chuyn i bc sng.
2.5.3 nh tuyn chuyn hng
2.5.3.1 Gii thiu
Lm lch hng i l mt phng php gii quyt nghn bng vic nh tuyn
mt burst tranh chp n mt ng ra khc so vi ng ra theo d kin. Tuy nhin
burst lch hng c th n ch theo mt tuyn di hn. V vy c th xy ra tr
u cui u cui ca mt burst. Lm lch hng i khng c kh thi trong
mng chuyn mch in v kh nng lp v phn tn burst.
Trong khi x l gi iu khin truyn i burst trn tuyn chnh, nu burst
cm thy nghn th mt gi iu khin khc c bt u t nt nghn trung gian
v burst c truyn qua mt tuyn la chn t nt trung gian .C nhiu yu t
quyt nh nh tuyn, trc ht n xc nh c la chn tuyn cho mt burst
c hay khng hay loi b v thc hin gi li t nt ngun.
Trong nh tuyn chuyn hng, xung t c gii quyt bng cch nh
tuyn burst d liu n mt ng ra khc thay v l ng ra nh ban u; tc l k t
node i theo con ng khc n ch ch khng cn i theo con ng
ngn nht ban u.
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Hnh 2.18: Cu trc ca mng OBS vi k thut lm lch hng inh tuyn chuyn hng khng c quan tm i vi mng chuyn mch
gi trong min in; tuy nhin, n li thc s cn thit i vi mng ton quang
trong khi cc b m quang cn hn ch.
2.5.3.2 Phng php nh tuyn chuyn hng
Phng php nh tuyn lch hng i trong OBS bao gm cc bc c miu
t nh trong hnh 2.19
Bc 1: Nt ngun truyn i mt gi iu khin.
Bc 2: Nt trung gian x l gi iu khin v c gng dnh trc mt knh hot
ng cho burst.
Bc 3: Nt ngun truyn i burst sau mt khong offset.
Bc 4: Nu ti mt nt khng c knh ng ra no hiu lc cho burst, trc ht
n kim tra c phi nt hin ti l ngun hay khng. Nu nt hin ti l nt ngun
th nh tuyn chuyn hng khng thc hin. Thay v , sau khi i mt khongthi gian, ngun truyn li gi iu khin ri sau burst c truyn i. Nu nt
hin ti l nt trung gian th chuyn sang bc 5.
Bc 5: Nt hin ti c xem nh l mt nt trung gian. V vy nt hin ti
tnh ton thng s th hin v thc hin kim tra da trn nhng thng s . Do
vy n quyt nh c lm lch hng hay l loi b v thng bo cho ngun thc
hin truyn li. Nu quyt nh l lm lch hng i, th tuyn chn la c chn
trong DRT. Tuy nhin, nu khng c tuyn no c hiu lc trong DRT th nt hin
ti loi b burst v gi bn tin NACK n ngun truyn li t ngun.
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Hnh 2.19: Phng php nh tuyn chuyn hngThut ton a ra gii php iu khin nghn khc phc nhng s c xy ra i
vi mng, t hiu qu cao v chi ph mng tng i thp. Chng ta cng a ra
c mt s kt qu quan trng biu th bn cht ca node bin cng nh node
trung gian. Cc kt qu ny rt c ch trong vic nghin cu s bin thin ca xc
sut burst suy hao khi cc thng s h thng v lu lng thay i.
2.5.4 Phn on burst
Trong cc nghin cu v mng chuyn mch burst quang, khi xung t xy ragia hai burst m khng th gii quyt bng cc phng php trn th mt trong hai
burst b rt ton b, cho d cc phn chng ln ca cc burst ln nhau l rt nh.
Trong cc ng dng yu cu nghim ngt v tr nhng khng quan tm lm n
mt gi th ngi ta mong mun l rt cc gi chng ln hn l rt ton b burst.
ng V. M. Vokkarane a ra phng php cho vic gii quyt xung t bng
cch phn on burst (Burst Segment), lm cho s lng gi mt i gim xung
bng cch ch cho rt phn chng ln ln nhau ca mt trong hai burst. u
im ca phng php ny l c th xem xt n cc mc u tin ca tng burst t
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c th a ra quyt nh rt b mt phn ca burst n xung t (contenting
burst) hay burst b xung t (contented burst) (nh hnh 2.20)
Trong phn on burst, mt burst bao gm nhiu n v truyn ti nh hn gi
l on (segment). Mi on gm bao gm mt header v phn payload ca on.Header ca on bao gm cc trng cho bit ng b, thng tin sa li, thng
tin v ngun v ch n v chiu di ca on trong trng hp cc on c chiu
di khc nhau. Phn payload ca on c th mang nhiu loi d liu, nh gi IP, t
bo ATM hay cc frame Ethernet (Hnh 2.21). Khi hai burst xung t vi nhau
trong mng chuyn mch burst quang, ch c mt s on ca mt burst chng ln
ln burst kia mi b rt, nh hnh 2.22. Nu thi gian chuyn mch khng c b
qua th c thm mt s on c th b rt khi chuyn mch c cu hnh tr li.
Trong cc m t sau y, ta gi burst n trc hay burst b xung t gi lcontented burst v burst n sau hay burst n xung gi l contending burst.
t c tnh trong sut th lp quang khng cn phi nhn ra ranh gii thc
s ca cc on v khng cn bit nh dng phn payload ca mi on. Trong
trng hp ny, lp quang ch nhn ra thng tin nh cc node ngun v ch
ca burst, thi gian offset, thi gian ca burst v c th l u tin ca burst.
Tnh trong sut ny c th dn n vic quyt nh ti u nht cho vic gim thiu
mt gi nhng cng c th mt on c th b chia thnh hai phn tch bit do vyc th mt mt gi s tng ln trong trng hp on ny n ch khng c
nhn ra; tuy nhin, bng cch bo m tnh trong sut, lp li quang tr nn n
gin v khng cn thit phi tnh ton li cc gi tr trong gi iu khin cc node
li.
Hnh 2.20: M t gii quyt xung t bng phn on burst
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Nu nh ranh gii ca cc on hon ton trong sut trong mng li ton quang
th cc node bin phi chu trch nhim nh ngha v x l cc on trong min
in. Hn na, node nhn phi c kh nng nhn ra im bt u ca mi on v
xc nh xem th on c cn nguyn vn hay khng; do , mt s header dng
nhn ra li v sa li phi cha trong mi on. Thm vo thng tin v tn
hiu ng h c th cng cn phi c trong mi header ca mi on node nhn
ng ra c th xc nh v phc hi d liu trn mi on. Mt on c th mt
frame Ethernet. Nu mi on bao gm mt frame Ethernet, vic ng b v nhn
ra frame c thc hin bi trng preamble trong header ca frame Ethernet,
trong khi li v cc frame khng cn nguyn vn c th c nhn ra bng cch s
dng trng CRC trong frame Ethernet; do , khng cn thit phi c phn iu
khin cho mi on v mi th c sn trong mt frame Ethernet. Nu cc onkhng c nh ngha nh nhng frame Ethernet, vic chn la chiu di ca
mi on tr thnh mt thng s chnh ca h thng. Mt on c th c chiu
di c nh hay thay i. Khi cc on c chiu di khng i th vic ng b
my thu tr nn d dng; tuy nhin, nhng on c chiu di thay i li c kh
nng cha c nhng gi c chiu di khc nhau. Kch thc ca mi on cn
phi cn nhc gia mt mt trong mt ln xung t v s lng header trong mt
burst. on di s dn n mt nhiu d liu cho mi ln xung t; tuy nhin,
nhng on di cng dn n t overhead v t s gia chiu di header so vichiu di payload s nh theo. Trong chng ny, chng ta gi s rng mi on
l mt frame Ethernet c chiu di c nh v khng cn phi tm kch thc ti u
ca mi on.
Hnh 2.21: Cu trc ca burst c ng kiu phn on
Mt s vn khc trong phn on burst l quyt nh xem on no b rt khi
xung t xy ra gia hai burst. Chng ta t tn cho burst b xung t l
contented burst cn burst xung t l contenting burst. Ch rng chm c
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xem l contented hay contenting burst ph thuc vo th t ca n n chuyn
mch ch khng phi th t ca gi iu khin n trc hay n sau. C hai cch
xc nh xem nhng on no nn rt, c gi l tail-dropping (rt phn
ui) v head-dropping (rt phn u).
Hnh 2.22: Xung t lm chng ln cc on ln nhau
Trong cch rt phn ui tail-dropping th cc on chng ln ca contented
burst s b rt cn trong cch rt phn head-dropping u th cc on ca
contenting burst chng ln s b rt. u im ca vic tail-dropping hn head-
dropping trong vic thay i cc gi sai th t node ch vi gi thuyt rng cc
gi b rt c truyn li sau . Vic head-dropping lm cho cc gi n ch sai
th t; tuy nhin, u im ca head-dropping l n chc chn rng mt khi burst
n mt node khng bt gp mt xung t no v sau cc burst ny tip tc i
n ch m khng ph thuc vo cc burst i sau n c mc u tin no i chng
na.
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KT LUN
Sau mt thi gian su tp, tm hiu ti liu v thc hin chuyn "Chuyn
mch burst quang" gip nhm hiu c mt cch tng quan v nguyn l v
cc vn c bn ca chuyn mch burst quang. Chuyn trnh by nhng nidung:
- Gii thiu v so snh v cc cng ngh chuyn mch quang, nu bt c ngha
quan trng ca vic s dng chuyn mch burst quang trong giai on pht trin
hin ti ca mng truyn ti quang.
- Nhm i su phn tch kin trc mng OBS v hot ng ca mng OBS theo s
khi chc nng ca mng: v cc phng php t hp burst, cc c ch bo hiu
ch yu nh JET, JIT, TAG, TAW, cc thut ton sp xp knh, cc phng phpgii quyt tranh chp.
Qua cc ni dung trnh by nh trn, ta thy rng:
- Chuyn mch OBS c kh nng tn dng ti nguyn hiu qu, ph hp vi cc
cng ngh v phn cng v thit b trong mng WDM hin ti, v kh nng h tr
cho nhiu loi giao thc khc nhau.
- Cc c ch v phng php t hp burst bo hiu, sp xp knh v gii quyt
tranh chp ca OBS a dng v linh hot c th h tr cc lu lng loi dch v
yu cu QoS khc nhau.
- Tuy nhin, OBS l mt cng ngh cha c chun ha v cha c h thng
thng mi thc t no c a vo trin khai. Nhng vn c bn ca chuyn
mch OBS c trnh by trong chuyn ch yu da trn cc cng trnh nghin
cu, cc bi bo khoa hc v cc lun vn ca cc nh nghin cu v cc k s
nc ngoi.
- Vn nghin cu v OBS hin ti v tng lai l tm ra c cc giao thc ti
u cho hot ng ca mng v t c th thit k v ch to cc thit b mng ph
hp.
Do y l mt lnh vc cn kh mi, nn vic tm hiu v chuyn Chuyn
mch burst quang mi ch dng li mc l thuyt tng qut, cha i su vo
phn tch, nh gi c th ht cc vn ( nh vn nh tuyn, QoS..). Nhm rt
mong nhn c s nh gi, ng gp kin ca thy gio v cc bn chuyn
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c hon thin hn na, ng thi c th lm tin tip tc nghin cu,
pht trin cc cng ngh mi hn sau ny.
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TI LIU THAM KHO
[1] Jason P.Jue, Vinod M. Vokkarane, Optical Burst Switched Networks,2005.Chng 1 trang 1 9, chng 2 trang 11 15, chng 3 trang 23 26, chng
4 trang 37 53, chng 5 trang 57 66, chng 6 trang 81 98.
[2] Mrinal Nandi, A technique to minimize contention in Optical Burst
Switching Networks, 2007. Chng 1 trang 1 6, chng 2 trang 10 26,
chng 3 trang 32 - 39 .
[3] Jolyon Ambrose Scoreby White, Modelling and demensioning of Optical
Burst Switching Networks,2007. Chng 2, trang 11 34.[4] Jing Teng, George N.Rouskas, A comparison of the JIT, JET and Horizon
Wavelength Reservation Schemes on a single OBS node,2003.
[5] Lisong Xu, Harry G.Perros, George N.Rouskas, A simulation study of optical
switching and access protocols for WDM ring networks,2002. Trang 143 145
[6] Yang Chen, Chunming Qiao, Xiang Yu, Optical Burst Switching: A new area
in optical networking research,1998. Mc 1 n ht mc 6 trang 1 9.
[7] Mai Th Kim Lin, Nguyn ng Phc Lm, Gii php iu khin nghn
trong mng chuyn mch burst quang bng phng php lch hng i, Bo
co Hi tho sinh vin ngin cu khoa hc i hc Nng, 2010