dinh tuyen va gan buoc song trong mang wdm

Lun vn

nh tuyn v gn bc sng trong mng WDM (Routing and Wavelength Assignment)

H thng thng tin quang ra i cng vi nhng u im vt tri ca n v

ang p dng rng ri trn mng li thng tin ton cu. Hin nay, cc h thng

thng tin quang truyn dn tt c cc tn hiu dch v bng hp, bng rng p ng

yu cu ca mng s tch hp dch v ISDN. V th, h thng thng tin quang s l

mi t ph v tc truyn dn v cu hnh linh hot cho cc dch v vin thng

cp cao.

i vi h thng thng tin quang, mi trng truyn dn chnh l si quang, n

thc hin truyn nh sng mang tn hiu thng tin t pha pht ti pha thu. nh

tuyn v gn bc sng tr thnh chc nng khng th thiu c trong mng

quang WDM. Vn t ra l nh tuyn ng i cho nh sng v gn bc sng

cho n trn mi tuyn nh th no t c mt mng ti u.

Trong n k thut thng tin ny, em xin trnh by v ti nh tuyn v gn

bc sng trong mng WDM (Routing and Wavelength Assignment). n c

chia thnh bn chng:

Chng 1: Tng quan v h thng thng tin quang.

Chng 2: Gii thiu v h thng WDM.

Chng 3: nh tuyn v gn bc sng trong mng WDM.

Chng 4: Thc hin m phng nh tuyn cho ng i nh sng lightpath.

ti nh tuyn v gn bc sng trong mng WDM ca n ny phn

tch s cn thit ca chc nng nh tuyn v gn bc sng trong mng quang

WDM, tr thnh chc nng khng th thiu trong vic iu hnh mng quang.

Phng php nghin cu ca ti: da vo chc nng ca nh tuyn v gn

bc sng trong WDM, thc hin m phng chc nng nh tuyn trong mng. nh

sng i trong si quang phi i qua nhiu node mng trung gian ti node ch,

tc l qua cc tuyn trung gian. Vic nh tuyn vi tiu ch ti u hm mc tiu l

cc tham s quen thuc nh bng thng, tr, chi ph tuyn,... V th dng thut

ton tm ng ngn nht Dijkstra thc hin m phng nh tuyn ti u mng.

Em xin chn thnh cm n thy gio Nguyn Vn Phng tn tnh hng dn,

cung cp ti liu, ng thi ng vin trong thi gian em nghin cu ti ny. Em

xin cm n tt c cc thy c gio trong khoa in T Vin Thng nhit tnh dy

d, cung cp trang b cho em nhng kin thc qu bu, cm n gia nh ng

vin em trong sut thi gian va qua, cm n cc bn gp nhng kin chn

thnh gp phn gip em hon thnh n.

Trong thi gian thc hin n kh ngn n, mc d c nhiu c gng nhng

n khng khi trnh nhng thiu st. Knh mong cc thy c gio trong khoa

cng cc bn tn tnh ch bo v gp kin n c hon thin hn. Em xin

chn thnh cm n.

Nng, ngy...thng...nm 2007

Chng 1: Tng quan v h thng thng tin quang

1

CHNG 1

TNG QUAN V H THNG THNG TIN QUANG. 1.1. Gii thiu chng

Lng thng tin trao i trong cc h thng thng tin ngy nay tng ln rt

nhanh. Bn cnh gia tng v s lng, dng lu lng truyn thng trn mng cng

thay i. Dng d liu ch yu l lu lng Internet. S ngi s dng truy cp

Internet ngy cng tng v thi gian mi ln truy cp thung ko di gp nhiu ln

cuc ni chuyn in thoi. Chng ta ang hng ti mt x hi m vic truy cp

thng tin c th c p ng mi lc, mi ni chng ta cn. Mng internet v

ATM ngy nay khng dung lng p ng cho nhu cu bng thng trong

tng lai.

Hnh 1.1: S gia tng lu lng d liu v ting ni qua cc nm.

K thut thng tin quang c th c xem l v cu tinh ca chng ta trong vic

gii quyt vn trn. Bi v h thng thng tin quang ra i vi nhng kh nng

vt tri ca n: bng thng khng l (gn 50Tbps), suy gim tn hiu thp (khong

0.2dB/km), mo tn hiu thp, i hi nng lng cung cp thp, khng b nh

hng ca nhiu in t, kh nng bo mt cao V vy thng tin quang c xem

l k thut cho h thng thng tin bng rng. Cc h thng thng tin quang khng

ch c bit ph hp vi cc tuyn thng tin ng di, trung k m cn c tim

nng to ln trong vic thc hin cc chc nng ca mng ni ht v p ng mi

loi hnh dch v hin ti v trong tng lai.


2

V vy vic pht trin v xy dng h thng thng tin si quang l cn thit cho

nhu cu pht trin thng tin trong tng lai. Trong chng ny s ni r v h thng

thng tin si quang v vic truyn nh sng trong si quang.

1.2. Gii thiu v thng tin quang

Khc vi thng tin hu tuyn hay v tuyn - cc loi thng tin s dng cc mi

trng truyn dn tng ng l dy dn v khng gian nh hnh 1.2 - th thng tin

quang l h thng truyn tin qua si quang nh hnh 1.3. iu c ngha l thng

tin c chuyn thnh nh sng v sau nh sng c truyn qua si quang. Ti

ni nhn, n li c bin i thnh thng tin ban u.

1.2.1. S pht trin ca thng tin quang

Cc phng tin s khai ca thng tin quang l kh nng nhn bit ca con

ngi v chuyn dng, hnh dng v mu sc s vt qua i mt. Tip mt h

thng thng tin iu ch n gin xut hin bng cch s dng cc n hi ng,

cc n hiu. Sau , nm 1791, VC. Chape pht minh mt my in bo quang.

Thit b ny s dng kh quyn nh l mt mi trng truyn dn, do chu nh

hng ca cc iu kin v thi tit. gii quyt hn ch ny, Marconi sng

ch ra my in bo v tuyn c kh nng thc hin thng tin gia nhng ngi gi

v ngi nhn xa nhau.

u nm 1880, A.G. Bell- ngi pht minh ra h thng in thoi ngh ra

mt thit b quang thoi c kh nng bin i dao ng my ht thnh nh sng.

Hnh 1.2: Thng tin hu tuyn

Hnh 1.3: Thng tin quang


3

Tuy nhin, s pht trin tip theo ca h thng ny b b b do s xut hin h

thng v tuyn.

S nghin cu hin i v thng tin quang c bt u bng s pht minh

thnh cng ca Laser nm 1960 v bng khuyn ngh ca Kao v Hockham nm

1966 v vic ch to si quang c tn tht thp. Bn nm sau, Kapron c th

ch to cc si quang trong sut c suy hao khong 20dB/km. c c v bi

thnh cng ny, cc nh khoa hc v k s trn khp th gii bt u tin hnh

cc hot ng nghin cu v pht trin v kt qu l cc cng ngh mi v gim suy

hao truyn dn, v tng di thng, v cc Laser bn dn c pht trin thnh

cng trong nhng nm 70, tn tht ca suy hao c gim n 0.18dB/km.

Hn na trong nhng nm 70, Laser bn dn c kh nng thc hin dao ng lin

tc c ch to, tui th ca n c lng khong 100 nm v cho php to ra

c ly truyn xa hn vi dung lng truyn ln hn m khng cn n cc b ti to.

Cng vi cng ngh ch to cc ngun pht v thu quang, si dn quang to ra

cc h thng thng tin quang vi nhiu u im vt tri hn hn so vi cc h

thng thng tin cp kim loi.

Hin nay cc h thng thng tin quang truyn dn tt c cc tn hiu dch v

bng hp, bng rng p ng yu cu ca mng s lin kt a dch v ISDN.

1.2.2. Nhng u im ca h thng thng tin quang

Thng tin si quang c nhng u im vt tri. Trong phn ny, em a nhng

u im th hin tnh vt tri ca n:

Bng thng khng l y tim nng: tn s sng mang quang trong

khong1013 n 1016 Hz (thng gn vng hng ngoi quanh gi tr 1014 Hz), cung

cp bng thng truyn ln hn nhiu so vi h thng cp kim loi (bng thng ca

cp ng trc khong 500Mhz). Hin ti, gi tr bng thng ca h thng si quang

cha s dng ht nhng vic mt vi GHz qua khong cch vi km v hng trm

Mhz qua khong cch hng chc Km m khng cn s can thip v in (dng b

lp) l c th. V th, dung lng mang thng tin ca h thng thng tin quang ln

hn nhiu so vi h thng cp ng tt nht. Do suy hao ln bng thng rng, h


4

thng cp ng trc gii hn khong cch truyn vi ch mt vi km bng thng

trn 100Mhz.

Si quang kch thc nh v nh: si quang c bn knh rt nh, thng

bn knh ny khng ln hn bn knh si tc con ngi. V th, thm ch khi si

quang c ph thm nhng lp bo v th chng vn nh v nh hn nhiu so vi

cp ng.

S cch li v in: si quang c ch to t thu tinh hoc i lc l cht

do, l nhng cht cch in, v th khng ging vi dy dn kim loi, n khng

cho thy nhng trc trc c bn. Hn na, c tnh ny lm cho vic truyn thng

tin ca si quang tr nn ph hp mt cch l tng cho s thng tin trong nhng

mi trng mo him v in.

Khng b nh hng bi nhiu v xuyn m: si quang c ch to t

cc cht in mi phi dn nn chng khng b nh hng bi nhiu in t, cc

xung in t, nhiu tn s v tuyn. V th hot ng ca h thng thng tin quang

khng b nh hng khi truyn qua mi trng nhiu in. iu c ngha l n

c th lp t cung ng vi cp in lc v c th s dng trong mi trng phn

ng ht nhn.

Bo mt thng tin: nh sng t si quang b bc x mt cch khng ng k

nn chng c tnh bo mt tn hiu cao. c tnh ny thu ht i vi qun i, ngn

hng v cc ng dng truyn d liu.

Suy hao thp: s pht trin ca si quang qua nhiu nm t c kt

qu trong vic ch to ra si quang c suy hao rt thp. Si quang c ch to

vi suy hao 0.2dB/km v c tnh ny tr thnh li th chnh ca thng tin

quang. iu ny thun li cho vic t b khuch i cho mi khong cch trn

ng truyn m khng cn chuyn sang tn hiu in bc trung gian, do

gim c c gi thnh v c phc tp ca h thng.

Tnh linh hot: mc d cc lp bo v l cn thit, si quang c ch to

vi sc cng cao, bn knh rt nh. Vi li th v kch thc v trng lng, si


5

quang ni chung l tt hn trong vic lu tr, chuyn ch, x l v lp t d hn h

thng cp ng.

tin cy ca h thng v d bo dng: do c tnh suy hao thp ca si

quang nn c th gim c yu cu s b lp trung gian hoc s b khuch i trn

ng truyn. V th, vi mt vi b lp th tin cy ca h thng c th c

nng cao hn hn h thng dn in. Hn na, tin cy ca cc thit b quang

khng cn l vn , cc thit b quang c tui th rt cao, khong 20-30 nm.

Gi thnh thp y tim nng: thy tinh cung cp cho thng tin quang

c ly t ct, khng phi l ngun ti nguyn khan him. V th, si quang em

li gi thnh thp.

Thng tin quang cng cho php truyn ng thi cc tn hiu c bc sng khc

nhau. c tnh ny cng vi kh nng truyn dn bng thng rng ca si quang

sn c lm cho dung lng truyn dn ca tuyn tr nn rt ln.

1.2.3. Cu trc v cc thnh phn chnh ca h thng thng tin quang

Cc thnh phn ca tuyn truyn dn quang bao gm: phn pht quang, cp si

quang v phn thu quang.

-Phn pht quang: c cu to t ngun pht tn hiu quang v cc mch iu

khin lin kt vi nhau. Phn t pht x nh sng c th l: Diod Laser (LD), Diod

pht quang (LED: Light Emitting Diode). LED dng ph hp cho h thng thng

tin quang c tc bit khng qu 200Mbps s dng si a mode. LED pht x t

pht, nh sng khng nh hng nn s dng LED tt trong h thng thng tin

quang th n phi c cng sut bc x cao, thi gian p ng nhanh. LD khc phc

nhc im ca LED, thng s dng LD cho truyn dn tc cao. LD c nhiu

u im hn so vi LED: ph pht x ca LD rt hp (khong t 1 n 4nm nn

M ho

Gii m

Pht Thu Si quang

Thit b pht

quang Si quang

B lp

Thit b thu quang

Hnh 1.4: Cu trc ca h thng thng tin quang


6

gim c tn sc cht liu), gc pht quang hp (5- 100), hiu sut ghp nh sng

vo si cao.

- Cp si quang: gm cc si dn quang v cc lp v bc xung quanh bo

v khi tc ng c hi t mi trng bn ngoi. C th chn cc loi si sau: si

quang a mode chit sut nhy bc, si quang a mode chit sut gim dn, si

quang n mode.

- Phn thu quang: do b tch sng quang v cc mch khuch i, ti to tn

hiu hp thnh. Trong h thng thng tin quang, ngi ta quan tm nht i vi cc

b tch sng quang l cc diod quang PIN v diod quang kiu thc APD c ch

to t cc bn dn c bn Si, Ge, InP.

Ngoi cc thnh phn ch yu ny, tuyn thng tin quang cn c cc b ni

quang, cc mi hn, cc b chia quang v cc trm lp. Tt c to nn mt tuyn

thng tin hon chnh.

Tng t nh cp ng, cp si quang c khai thc vi iu kin lp t khc

nhau, c th c treo ngoi tri, chn trc tip di t hoc t di bin,tu

thuc vo cc iu kin lp t khc nhau m ch to ca cp cng khc nhau v

cc mi hn s kt ni cc di cp thnh di tng cng ca tuyn c lp

t. Tham s quan trng nht ca cp si quang tham gia quyt nh di tuyn l

suy hao si quang theo bc sng.

Ngun pht quang thit b pht c th s dng LED hoc laser bn dn. C hai

ngun pht ny u ph hp cho cc h thng thng tin quang, vi tn hiu quang

u ra c tham s bin i tng ng vi s thay i ca dng iu bin. Bc

sng lm vic ca ngun pht quang c bn ph thuc vo vt liu ch to, on si

quang ra ca ngun pht quang phi ph hp vi si dn quang khai thc trn

tuyn.

Tn hiu nh sng c iu ch ti ngun pht quang s uc lan truyn dc

theo si quang ti phn thu quang. Khi truyn trn si dn quang, tn hiu

thng b suy hao v mo do cc yu t hp th, tn x, tn sc gy nn. B tch

sng quang phn thu thc hin tip nhn nh sng v tch ly tn hiu t hng


7

pht ti. Tn hiu quang c bin i tr li thnh tn hiu in. Cc Photodiod

PIN v Photodiod thc APD u c th s dng lm cc b tch sng quang trong

cc h thng thng tin quang. c tnh quan trng nht ca thit b thu quang l

nhy thu quang.

Khi khong cch truyn dn kh di, ti mt c ly no , tn hiu quang trong

si b suy hao kh nhiu th cn thit phi c cc trm lp quang t trn tuyn.

Nhng nm gn y, cc b khuch i quang c s dng thay th cho cc

thit b trm lp quang.

1.3. Si quang

1.3.1. Si dn quang

Si quang l nhng dy nh v do truyn cc nh sng nhn thy c v cc

tia hng ngoi. Chng c li gia v c phn bao bc xung quanh li. nh

sng c th phn x mt cch hon ton trong li th chit xut ca li phi ln hn

chit sut o mt cht.

V bc pha ngoi o bo v si quang khi b m v n mn, ng thi chng

xuyn m vi cc si i bn cnh v lm cho si quang d x l. bc ngoi ta

dng cc nguyn liu mm.

Li v o c lm bng thy tinh hay cht do (silicat, cht do, kim loi,

Flour, si quang kt tinh). Ngoi ra chng c phn loi thnh cc loi si quang

Hnh 1.5: Cu to si quang


8

n mode v a mode tng ng vi s lng mode ca nh sng truyn qua si

quang. Ngoi ra chng cn c phn loi thnh si quang c ch s bc sng v

ch s lp tu theo hnh dng v chit sut ca cc phn ca li si quang.

1.3.2. S truyn nh sng trong si quang.

Si quang l mi trng truyn thng c bit so vi cc mi trng khc nh

cp ng hay khng gian t do. Mt si quang cho suy hao tn hiu thp trn mt

phm vi tn s ln, c tnh ny cho php tn hiu c truyn qua cc khong cch

xa tc cao trc khi cn khuch i hoc ti lp li.

Mt si quang gm c mt li hnh tr c bao quanh bi lp v. C phn li

v phn v c lm ch yu t silica (SiO2), c ch s khc x (chit sut) xp x

1.45. Ch s khc x ca vt liu l t s vn tc nh sng trong chn khng so vi

tc nh sng trong vt liu .

n =vc

n: chit sut ca mi trng, khng c n v.

c: vn tc nh sng trong chn khng , n v : m/s

v : vn tc nh sng trong mi trng, n v : m/s.

V c v nn n 1

Trong qu trnh sn xut si, mt s tp cht no c a vo trong li hoc

v cho ch s khc x trong li ln hn mt t so vi v. Cc nguyn liu nh

Germani hoc Photpho lm tng chit sut silica v c dng thm vo phn li

ca si quang, trong khi cht Bo hay Flo lm gim chit sut ca Silica nn c

dng tp cht cho lp v.

nh sng c th c xem nh mt chm tia truyn theo nhng ng thng

trong mt mi trng v b phn x hoc khc x b mt gia hai vt liu khc

nhau. Mt tia sng t mi trng 1 n mt phn cch ca mi trng 2, gc ti l

gc gia tia ti v php tuyn vi b mt chung ca hai mi trng c biu th

bng 1 . Phn nng lng b phn x vo mi trng 1 l mt tia phn x, phn cn


9

li i xuyn qua mi trng 2 l tia khc x. Gc phn x r1 l gc gia tia phn x

v php tuyn, tng t gc khc x l gc gia tia khc x v php tuyn.

Ta c: 11 r

Theo nh lut Snell: 2211 sinsin nn

Khi gc ti 1 tng ln th gc khc x 2 cng tng theo. Nu 902 th

sin 1 =1

2

nn , lc ny gc 1 c gi l gc ti hn c gi tr

1

21sinnn

c , vi

21 nn .

Vi nhng gi tr c 1 , s khng c tia khc x v tt c nng lng t tia ti

c phn x ht. Hin tng ny c gi l hin tng phn x ton phn.

iu kin xy ra hin tng phn x ton phn:

Cc tia sng phi i t mi trng c chit sut ln sang mi trng c chit

sut nh hn.

Gc ti ca tia sng phi ln hn gc ti hn.

nh sng truyn trong si quang do hin tng phn x ton phn xy ra gia b

mt phn li v v.

Hnh 1.6: S phn x v khc x cc tia sng ti mt phn cch hai mi trng.


10

Hnh trn cho thy nh sng c ghp t mi trng bn ngoi (khng kh vi

chit sut n0) vo si.

1.3.3. Cc thng s ca si quang.

xc nh tc truyn dn v khong cch trm lp ca h thng thng tin

si quang, c hai tham s cn phi nghin cu l tn hao quang v rng bng

truyn dn. o tn hao quang xc nh tn hao cng sut nh sng lan truyn

trong si quang.

1.3.3.1. Suy hao ca si quang

1.3.3.1.1. nh ngha

Cng sut quang truyn ti si cng gim dn theo c ly vi quy lut hm s m

tng ng nh tn hiu in. Biu thc ca hm s truyn cng sut c dng:

P(Z)= P(0)x 10 10 z

Trong :

P(0): C cng sut u si.

P(z): cng sut c ly z tnh t u si.

: h s suy hao.

- H s suy hao ca si c tnh theo cng thc:

A(dB)= 2

1lg10PP

Trong :

P1: Cng sut a vo si.

Hnh 1.7: nh sng trong si quang


11

P2: Cng sut cui si.

- H s suy hao trung bnh:

kmLdBAkmdB /

Trong :

A: Suy hao ca si.

L: Chiu di ca si.

1.3.3.1.2. c tuyn suy hao

c tuyn suy hao ca si quang khc nhau tu thuc vo loi si. Hnh di

cho thy suy hao trong si quang nh mt hm theo bc sng. Ta thy rng suy

hao nh nht ba di bc sng dng trong thng tin quang: 0.8 m , 1.3 m v

1.55 m .

.

Hnh 1.8: c tuyn suy hao ca si quang


12

1.3.3.1.3. Cc nguyn nhn gy suy hao trn si quang

Suy hao do hp th: S hp th nh sng xy ra do cc nguyn nhn sau

gy ra: suy hao do s hp th ca cc tp cht kim loi, s hp th ca ion OH, s

hp thu bng cc tm v hng ngoi.

S hp th ca cc tp cht kim loi: cc tp cht kim loi trong thu tinh l mt

trong nhng ngun hp th nng lng nh sng, cc tp cht thng gp l st

(Fe), ng (Cu), mangan (Mn), choromium (Cr), cobar (Co), niken (Ni). Mc

hp th ca tng tp cht ph thuc vo nng tp cht v bc sng nh sng

truyn qua n. c si quang c d suy hao nh hn 1dB/km cn phi c thu

tinh tht tinh khit vi nng tp cht khng qu mt phn t (10-9).

S hp th ca ion OH: cc lin kt gia SiO2 v cc ion OH ca nc cn st

li trong vt liu khi ch to si quang cng to ra mt suy hao hp th ng k.

c bit hp th tng vt cc bc sng gn 950nm, 1240nm v 1400 nm.

S hp thu bng cc tm v hng ngoi: ngay c khi si quang c ch to t

thu tinh c tinh khit cao th s hp th vn xy ra. Bn thn thu tinh tinh

khit cng hp th nh sng vng cc tm v hng ngoi. S hp th trong vng

hng ngoi gy tr ngi cho khuynh hng s dng cc bc sng di trong thng

tin quang.

Suy hao do tn x: Suy hao do tn x bao gm tn x Rayleigh, tn x do

mt phn cch gia li v lp bc khng hon ho.

Tn x Rayleigh: khi sng in t truyn trong mi trng in mi gp nhng

ch khng ng nht trong si quang do cch sp xp cc phn t thu tinh, cc

khuyt tt nh bt khng kh, cc vt nt s xy ra hin tng tn x. Khi kch

thc ca vng khng ng nht vo khong mt phn mui bc sng th chng

tr thnh nhng ngun im tn x. Cc tia truyn qua nhng ch khng ng

nht ny s to ra nhiu hng, ch mt phn nng lng nh sng truyn theo

hng c, phn cn li truyn theo hng khc thm ch cn truyn ngc li

ngun quang. tiu hao do tn x Rayleigh t l nghch vi lu tha bc bn ca

bc sng.


13

Tn x do mt phn cch gia li v lp bc khng hon ho: khi tia sng truyn

n nhng ch khng hon ho gia li v lp bc tia sng b tn x. Lc 1 tia

ti c nhiu tia phn x vi nhiu gc phn x khc nhau.

Suy hao do b un cong: bao gm suy hao do vi un cong v do un cong.

Suy hao do vi un cong: si quang b chn p to nn nhng ch un cong nh

th suy hao ca si cng tng ln. Suy hao ny xut hin do tia sng b lch trc i

qua nhng ch vi un cong . Si n mode rt nhy vi nhng ch vi un cong

nht l v pha bc sng di.

Suy hao do un cong: khi b un cong vi bn knh cong cng nh th suy hao

cng tng.

1.3.3.2. Tn sc nh sng

Tng t nh tn hiu in, tn hiu quang truyn qua si quang cng b bin

dng. Hin tng ny c gi l tn sc. S tn sc mo dng tn hiu analog v

lm xung b chng lp trong tn hiu digital. S tn sc lm hn ch di thng ca

ng truyn dn quang.

.

Tn sc mode: trong si a mode, do xung nh sng vo mc d ch c mt

bc sng nhng lan truyn vi vi mode khc nhau vi tc truyn khc nhau,

n lm khong trng thi gian gia cc xung cnh nhau tr nn ngn hn v tng

theo khu s ca si. Hin tng ny gi l tn sc mode. Do , rng bng

truyn dn ca n b gii hn ch yu do tn sc mode.

Tn sc th: bao gm tn sc cht liu v tn sc dn sng.

Tn sc cht liu: nh sng s dng trong thng tin quang khng phi l nh

sng hon ton n sc. Chit sut ca thu tinh thay i theo bc sng nn vn

Hnh 1.9: Dng xung vo v ra do tn sc


14

tc truyn ca nh sng c bc sng khc nhau cng khc nhau. Chnh v th, nh

sng c phn b tc lan truyn khc nhau ca cc thnh phn bc sng nh

sng khc nhau. Hin tng ny c gi l tn sc cht liu.

Tn sc dn sng: s phn b nng lng nh sng trong si quang ph thuc

vo bc sng. S phn b ny gy nn tn sc ng dn sng.

1.3.4. nh hng ca tn sc n dung lung truyn dn trn si quang

Tn sc gy ra mo tn hiu v iu ny lm cho cc xung nh sng b gin rng

ra khi c truyn dc theo si dn quang. Khi xung b gin ra s dn ti chng lp

ln xung bn cnh. Nu vt qu mt gi tr no th thit b thu s khng cn

phn bit cc xung k nhau na v lc ny xut hin li. Nh vy, cc c tnh tn

sc s xc nh gii hn dung lng truyn dn ca si dn quang.

1.4. Kt lun chng

Qua chng ny, chng ta tm hiu tng quan v h thng thng tin quang

vi nhng u nhc im ca n. H thng thng tin quang da vo nhng u im

vt tri ca mnh ang pht trin mnh m p ng nhu cu thng tin bng rng

hin nay.

Hnh 1.10: nh hng ca tn sc.

Chng 2: Gii thiu mng WDM

15

CHNG 2

GII THIU MNG WDM. 2.1. Gii thiu chng

K thut mng ghp knh phn chia theo bc sng WDM (Wavelength

Division Mutiplexing) c coi l cuc cch mng v bng thng trong mng

xng sng Internet v hn th na. Nhu cu bng thng ang gia tng mt cch

nhanh chng vi nhiu ng dng mi phong ph, chng hn nh thng mi in

t, video theo yu cu, cc cng vic i hi hot ng ng b trn ton cu.

Mng quang WDM a ra ha hn ht sc ngha cho nhu cu bc thit trn.

Khi si quang c s dng truyn thng tin th thch thc c t ra i

vi chng ta trong giai on mi trc nhu cu thng tin ngy cng tng mnh m

ca con ngi. Khi m ngy cng c nhiu ngi bt u s dng cc mng d liu

v c mi ln s dng cng chim mt bng thng ng k trong cc ng

dng thng tin ca h chng hn nh c lt thng tin trn cc trang web, cc ng

dng s dng Java, hi ngh truyn hnh, T cho thy nhu cu thng tin bng

rng t ra ht sc bc thit, v nhu cu ny cn vt xa hn na trong tng lai.

Hnh 2.1 cho thy s gia tng bng thng ca cc mng khc nhau qua cc nm. S

pht trin mnh m ny ch yu l do s trin khai cc h thng thng tin quang.

Hnh 2.1: S gia tng bng thng ca cc mng khc nhau qua cc nm


16

thch ng vi s pht trin khng ngng v tho mn yu cu tnh linh

hot v thay i mng, cc cng ngh truyn dn khc nhau c nghin cu,

trin khai th nghim v a vo ng dng nh k thut TDM, CDM. Cng ngh

ghp knh phn chia theo bc sng WDM c a chung hn c. iu ny l do

cng ngh TDM c chi ph k thut v thit b lp t h thng tng i cao, c

bit trong TDM gy lng ph mt s knh thng tin khi mi khe thi gian c d

tr ngay c khi khng c d liu gi v pha thu kh khn khi phn bit cc khe

thi gian thuc v knh no gii ghp knh tn hiu. Bn cnh , ghp knh

phn chia theo m CDM cn tn ti nhng hn ch v k thut nh tc iu ch

v suy hao trong m ho cng nh gii m cao. WDM l tin b rt ln trong cng

ngh truyn thng quang, n cho php tng dung lng knh m khng cn tng tc

bit ng truyn cng nh khng cn dng thm si dn quang.

Vi WDM, mi knh vi mt bc sng khc nhau v cc bc sng nh sng

ny khng nh hng ln nhau bi v chu k dao ng ca cc cc knh khc nhau

l hon ton c lp nhau. Khc vi h thng TDM, mi phn t knh WDM c th

hot ng tc bt k v mi knh cng c th mang y dung lng ca mi

bc sng. Chng ny s trnh by r nguyn l hot ng ca h thng WDM v

cc thnh phn ca n.

Hnh 2.2: H thng TDM

Hnh 2.3: H thng WDM


17

2.2. Nguyn l hot ng ca h thng WDM

Ngy nay, nhu cu ca con ngi v cc dch v thng tin bng rng ngy mt

tng ln, th mng ghp knh a bc sng WDM tho mn c nhu cu .

Theo k thut ny, cc lung nh sng vi cc bc sng khc nhau c truyn

trn cng mt si quang. Mi bc sng mang mt dung lng in hnh, thung l

2.5Gbps.

Nguyn l c bn ca ghp knh theo bc sng l ghp tt c cc bc sng

khc nhau ca ngun pht quang vo cng mt si dn quang nh b ghp knh

MUX v truyn dn cc bc sng ny trn cng si quang. Khi n u thu, b

tch knh quang s phn tch thu nhn li cc bc sng .

Vi cng mt nguyn l hot ng c hai loi truyn dn trong WDM, l:

truyn dn mt chiu v truyn dn hai chiu mt si.

H thng WDM mt chiu: c ngha l tt c cc knh cng trn mt si

quang truyn dn theo cng mt chiu.

H thng WDM hai chiu: c ngha l knh quang trn mi si quang

truyn dn theo hai hng khc nhau, dng cc bc sng tch ri nhau thng

tin hai chiu.

Hnh 2.4: Nguyn l ghp knh phn chia theo bc sng


18

So vi h thng WDM mt chiu, h thng WDM hai chiu gim c s lng

b khuch i v ng dy. Tuy nhin, h thng WDM hai chiu thng b can

nhiu nhiu knh, nh hng phn x quang, vn cch li gia cc knh hai chiu,

tr s v loi hnh xuyn m, ng thi phi s dng b khuch i quang hai

chiu.

2.3. u im ca h thng WDM

C kh nng to dung lng ln ch trn mt si quang, v c th t

dung lng ln hn khi s dng k thut DWDM (Dense WDM: ghp knh phn

chia theo bc sng mt cao).

(b)

Ngun 1

Thu 2

Thit b WDM

Knh vo

Knh ra

Thit b WDM

Ngun 2

Knh vo

Knh ra Thu 1

Mt si quang

1 2

(a)

Ngun 1 Knh 1

Thit b WDM

mt si quang

quang n ,...,, 21

Knh 2 Ngun 2

Knh n Ngun n

Knh 1 Thu 1

Thu 2 Knh 2

Thu n Knh n

Thit b WDM

Hnh 2.5: H thng WDM theo mt hng (a) v hai hng (b)


19

H thng WDM thun tin khi cho php truyn dn ng thi tn hiu khng

ng nht.

C kh nng truyn dn tn hiu hai chiu.

2.4. Vn tn ti ca h thng WDM v hng gii quyt trong tng lai

Vi h thng WDM, si quang cung cp cho chng ta tc truyn mong mun

nhng bng thng mng li b gii hn bi tc x l cc nt, do tc x l

cc nt c thc hin bng in t, m tc in t li thp hn rt nhiu so vi

tc thng tin truyn trong si quang (khong vi Gbps). Nh vy, tn hiu quang

trn si khi n nt s c chuyn thnh tn hiu in thc hin x l in t (s

chuyn i quang- in O/E), sau c chuyn li thnh tn quang truyn i.

iu ny lm gim tc mng, gii php t ra l xy dng mng m trong

tn hiu c x l hon ton trong min quang, gi l mng ton quang.

Trong mng ton quang, d liu i t ngun n ch hon ton di dng

quang m khng cn bt c s chuyn i quang- in no trn ng i, vic iu

khin x l chuyn mch cng c thc hin di dng quang. Tuy nhin, mng

ton quang hin ti vn cha c tin hnh thnh cng bi nhng tn ti ca n.

Cc thit b logic hon ton trong min quang kh thc hin hn nhiu so vi cc

thit b logic in t. Bi v, khc vi cc electron th cc photon khng tng tc

nh hng ln nhau, thng th cc thit b logic phc tp u c to ra bng

cch s dng cng ngh in t. Bn cnh , cc trm lp bng quang cng rt kh

thc hin hn nhiu so vi cc trm lp in t mc d cc trm lp trong mng

ton quang c t nhng khong cch nh k rt xa nhau.

2.5. Chuyn mch quang

Hu ht cc thit b mng ngy nay u da trn tn hiu in, iu c ngha

tn hiu quang cn chuyn i sang tn hiu in c khuch i, ti to hoc

chuyn mch v sau c chuyn i tr li tn hiu quang. iu ny ni n s

chuyn i optical-to-electronic-to-optical (O-E-O) v l cng vic ct li ht sc

c ngha trong vic truyn tn hiu. S lng ln tn hiu i qua mng quang cn

c chuyn mch qua cc im khc nhau, c gi l cc node. Thng tin n


20

node s c chuyn v pha trc theo hng n ni m n c gi ti qua

ng tt nht c th, con ng ny c th xc nh bi cc yu t nh khong

cch, chi ph, tin cy,bng thng ca tuyn . Cch chuyn i tn hiu

thc hin chuyn mch l tch nh sng t nhng u vo si quang, chuyn i

n sang tn hiu in v sau chuyn i tr li tn hiu nh sng laser, tn hiu

ny c gi i trong si quang.

Vn c bn ca chuyn mch quang l thay th s tn ti ca chuyn mch

mng in bng mng ton quang, s cn thit ca vic chuyn i O-E-O c

loi b. Nhng thun li ca kh nng ny khi trnh c vic chuyn i O-E-O l

iu ht sc ngha. u tin chuyn mch quang c th r hn bi v khng cn

nhiu tn hiu in tc cao t tin.

Cc b chuyn mch quang cho nhiu ng dng trong mng quang. Mi ng

dng yu cu thi gian chuyn mch v s cng chuyn mch khc nhau. Mt ng

dng ca chuyn mch quang l cung cp cc lightpath. Vi ng dng ny, chuyn

mch c s dng bn trong b kt ni cho nhm cu hnh li chng cung cp

cc lightpath mi. Mt phn mm c thm vo qun l mng t u cui n

u cui. V th vi ng dng ny, cc b chuyn mch vi thi gian chuyn mch

ms c th chp nhn, nhng cc b chuyn mch y i hi phi c kch thc

ln.

Mt ng dng quan trng khc l chuyn mch bo v. y cc chuyn mch

c s dng chuyn cc lung lu lng t si chnh sang si khc trong

trng hp si chnh gp s c. Ton b hot ng nh thi gian tm ra li, thng

tin li n cc phn t mng iu khin vic chuyn mch v qu trnh chuyn

mch thc s i hi phi hon thnh trong thi gian rt ngn. C th c nhiu dng

chuyn mch bo v khc nhau, ph thuc vo phng php c s dng, s cc

cng chuyn mch cn thit c th thay i t hng trm n hng ngn cng khi s

dng trong cc b kt ni cho bc sng.


21

2.6. Cc thnh phn chnh ca h thng WDM

Cu trc ca mng WDM gm c cc thnh phn: thit b u cui OLT, cc b

ghp knh xen/rt quang OADM, cc b kt ni cho quang OXC lin kt vi nhau

qua cc kt ni si quang. Ngoi ra cn c b khuch i b suy hao trn ng

truyn.

2.6.1. Thit b u cui OLT

Thit b u cui OLT (Optical Line Terminator) l thit b c dng u

cui ca mt lin kt im ni im ghp v phn knh cc bc sng. Thit b

u cui gm c ba phn t: b tip sng (transponder), b ghp knh cc bc

sng (wavelength multiplexer) v b khuch i (optical amplifier).

B tip sng lm nhim v thch ng tn hiu i vo t mt ngi s dng mng

thnh mt tn hiu ph hp s dng trong mng. V hng ngc li n lm thch

ng tn hiu t mng quang thnh tn hiu ph hp vi ngi s dng. Giao din

gia ngi s dng v b tip sng c th thay i da vo ngi s dng, tc

bt v khong cch hoc suy hao gia ngi dng v b chuyn tip. Giao din ph

bin nht l giao din SONET/SDH.

S thch ng bao gm nhiu chc nng, tn hiu c th c chuyn i thnh

bc sng thch hp trong mng quang, n cng c th thm vo cc phn u

header nhm qun l mng. B tip sng cng c th gim st t l li bit ca tn

Hnh 2.6: OLT


22

hiu im i vo v i ra trong mng. V nhng l do ny nn b chuyn tip thc

hin chuyn i quang- in- quang.

hnh trn, s lm thch ng ch cho theo hng i vo v bc sng hng

ngc li c gi trc tip n hng ngi dng. Trong mt s trng hp, ta c

th trnh s dng b tip sng bng cch thc hin chc nng thch ng bn trong

thit b ngi dng, nh phn t mng SONET nh hnh trn, iu ny lm gim

c chi ph ng k.

Tn hiu ra khi b tip sng c ghp knh vi cc tn hiu khc cc bc

sng khc nhau s dng b ghp knh theo bc sng trn mt si quang. Thm

vo b khuch i c th c dng khuch i cng sut ln nu cn thit

trc khi chng c a n b phn knh. Nhng bc sng ny li c kt

thc trong mt b tip sng nu c hoc kt thc trc tip trong thit b ngi s

dng.

Cui cng OLT cng kt thc mt knh gim st quang (OSC). OSC c mang

bc sng ring l, khc vi cc bc sng mang lu lng thc s. N dng

gim st vic thc hin ca cc b khuch i dc theo lin kt cng nh cho cc

chc nng qun l khc.

2.6.2. B ghp knh xen/rt quang OADM

B ghp knh xen/rt quang cung cp mt phng tin iu khin lu lng

trong mng. OADM c th c dng cc v tr khuch i trong cc mng ng

di nhng cng c th s dng nhng phn t mng c lp. hiu c li ch

ca b xen/rt quang, ta xt mt mng gia ba node A, B v C nh hnh v di,

lu lng mng gia A v C i qua node B, gi thit cc tuyn lin kt hon ton

song cng.

Gi s yu cu lu lng nh sau: mt bc sng gia A v B, mt bc sng

gia B v C, ba bc sng gia A v C. By gi trin khai cc h thng WDM

im ni im cung cp nhu cu lu lng ny. Vi gii php trong hnh (a), hai

h thng im ni im c trin khai, mt gia A v B, mt gia B v C. Mi

lin kt im ni im s dng mt OLT cui lin kt. Node B c hai OLT, mi


23

OLT kt thc bn bc sng, v th cn yu cu bn b tip sng. Tuy nhin ch c

mt trong bn bc sng l dnh cho node B, cc b tip sng cn li dng cung

cp lu lng gia A v C. V th su trong tm b tip sng node B c dng

iu khin lu lng. y l vic rt tn km.

Vi gii php trong hnh (b), thay v s dng cc h thng WDM im ni im,

ta trin khai mt mng nh tuyn bc sng. Mng s dng mt OLT node A v

C, mt OADM node B. OADM rt mt trong bn bc sng, sau kt thc

cc transponder. Ba bc sng cn li i xuyn qua trong min quang m khng

cn kt thc trong cc transponder. iu ny thy c hiu qu l ch s dng hai

transponder thay v s dng n tm transponder nh gii php (a), do gim

c chi ph ng k.

Cu hi t ra l ti sao cc b tip sng cn thit gii php (a) iu khin

lu lng i qua. Ni cch khc l ti sao chng ta khng n gin loi b cc b

tip sng v thc hin kt ni trc tip cc b ghp knh v tch knh WDM gia

Hnh 2.7: Vai tr ca OADM trong mng


24

hai b tip sng node B nh trong hnh (b), hn l thit k mt OADM ring bit.

iu ny l c th, cc OLT c thit k h tr kh nng ny. Lp vt l c

xy dng trong cc mng phc tp hn nhiu cc h thng im ni im.

C nhiu kin trc xy dng nn OADM, cc kin trc ny in hnh s dng

cc b ghp/b lc. Ta xt OADM nh mt hp en c hai cng mang mt tp hp

cc bc sng v mt s cng ni b. Cc thuc tnh chnh ca OADM gm c:

Tng s bc sng c th cung cp c l bao nhiu.

S bc sng ln nht c th xen/ rt l bao nhiu.

C rng buc trn mt bc sng no c xen/rt. Mt kin trc ch cho

php mt s bc sng xc nh no c xen/ rt ch khng phi bt k bc

sng tu no cng c.

C d dng xen/ rt cc knh thm vo. C cn thit ph v mt knh ang

tn ti xen/ rt cc knh thm vo.

Tnh n chi ph.

Tnh phc tp ca vic thit k OADM lp vt l v khi thm vo cc knh

mi th nh hng n vic thit k ny nh th no.

Hnh di y cho ta thy cc kin trc ca OADM:


25

hnh 2.8(a), mt s knh c chn c th c tch ra v nhng knh khc

c i qua. V th khng c s rng buc trn cc knh c rt v xen. V vy

cu trc ny p t nhng rng buc nh nht trong vic thit lp cc lightpath

trong mng. Ngoi ra suy hao qua OADM c nh, c lp vi s knh c rt v

xen l bao nhiu. Tuy nhin kin trc ny li khng hiu qu v chi ph trong vic

iu khin mt s nh cc knh c rt, v bt k bao nhiu knh c rt, tt c

cc knh u cn phi c tch v ghp li vi nhau. Do ta phi tn chi ph cho

Hnh 2.8: Cc kin trc OADM


26

vic tch v ghp cho tt c nhng knh i vo. iu ny cng dn n suy hao cao

hn. Tuy nhin khi mt s lng ln s knh c rt v linh hot trong vic thm

vo hoc ly ra bt c knh no th cu trc ny cng cho ta hiu qu kinh t.

Hnh 2.8(b) l s ci tin ca hnh 2.8(a) nhm gim chi ph thit k trn, vic

ghp v tch knh c thc hin qua hai giai on. Giai on th nht tch ring

cc bc sng thnh nhng di (bands), giai on th hai tch nhng di thnh cc

bc sng ring l. V d nh h thng 16 knh, c th thc hin s dng bn di,

mi di gm bn knh. Nu ch c bn knh c rt mt v tr, th 12 knh c

th gi nguyn trong cc di, thay v phi tch xung thnh tng knh ring l. iu

ny cho thy ta tit kim c chi ph cho b MUX v DEMUX. Ngoi ra, vic

s dng cc di cho php tn hiu c i qua vi suy hao quang thp hn. Khi

mng c s knh ln th cu trc hnh 2.8(b) ghp knh nhiu giai on tr nn cn

thit.

Trong cu trc hnh 2.8(c), mt knh ring l c tch v ghp t mt tp cc

knh i vo. Ta gi thit b ny l b xen rt n knh (SC - OADM). tch v

ghp nhiu knh th cc SC - OADM c ni lin tip nhau. Kin trc ny b sung

cho kin trc ca hnh 2.8(a). Vic tch v ghp knh nh hung n cc knh ang

tn ti, nn nhm gim ti thiu nh hng ny th ln k hoch tp bc sng no

cn c ly ra tng v tr. Tuy nhin nu s knh cn c tch ra l ln th kin

trc ny khng cn ph hp na, do chng ta phi s dng nhiu thit b ring l

ni li vi nhau. iu cho thy n khng hiu qu v kinh t. Ngoi ra suy hao

cng gia tng theo.

2.6.3. B khuch i quang

Nhm b li s suy hao tn hiu trn ng truyn si quang cng nh ti cc

thit b (nh cc b ghp knh) th cc b khuch i c t gia cc kt ni si

quang nhng khong cch nh k. Trc khi cc b khuch i quang ra i th

la chn duy nht l ti to li tn hiu, ngha l nhn tn hiu v sau pht li n.

Qu trnh ny c thc hin bng cc b lp ti sinh. Mt b lp chuyn tn hiu


27

quang thnh tn hiu in, khi phc sau chuyn li thnh tn hiu quang

truyn tip. iu ny hn ch tnh trong sut v tng chi ph bo tr ca h thng.

K thut khuch i quang chim u th hn nhiu cc b lp. B khuch i

quang khng ph thuc vo tc bit v cc nh dng tn hiu. Mt h thng s

dng khuch i quang c th d nng cp hn nhiu, v d n mt tc bit cao

hn m khng cn phi thay th b khuch i. Hn na cc b khuch i quang

c bng thng ln nn c th c dng khuch i ng thi nhiu tn hiu

WDM. Nu khng vi mi bc sng ta phi s dng mt b lp.

Loi khuch i quang in hnh l b khuch i quang si EDFA (Erbium

Doped Fiber Amplifier - khuch i quang si c pha tp Erbium).

B EDFA thc cht l si quang c pha tp c chc nng khuch i c tn

hiu nh sng, chng c th thay i cc c tnh vt l ca si theo nhit , p sut

v chng c tnh cht bc x nh sng. c im ca si ny l chng c kh nng

t khuch i hoc ti to tn hiu khi c kch thch ph hp.

Theo hnh v th nh sng bm vo t laser c kt hp vi tn hiu vo nh

s dng b ghp WDM trn h thng s dng mt b ghp. nh sng bm ny

c truyn dc theo si c pha Eribium v tn hiu bm ny kch thch cc cc ion

Eribium ln mc nng lng cao hn. S dch chuyn mc nng lng ca in t

t cao xung thp s pht ra photon, c gi l bc x t pht nu khng c bt c

tc ng no t pha bn ngoi, cn gi l bc x kch thch khi do s c mt cc

Hnh 2.9: EDFA

u vo

B cch li

WDM EDF

B cch li

u ra


28

photon cha nng lng bng nng lng dch chuyn. Khi tn hiu d liu c

truyn n EDFA, tn hiu d liu ny n gp cc ion Er3+ c kch thch

mc nng lng cao. Qu trnh ny lm cho cc ion nhy t trng thi nng lng

cao xung mc trng thi nng lng thp nn pht ra photon, do s khuch i

cng sut tn hiu ln ri truyn i tip trong si quang.

Thng thng, mt b cch li c dng trc ng vo hoc ng ra ca b

khuch i tn hiu EDFA ngn s phn x vo trong b khuch i ny.

EDFA cho h s khuch i ln, cng sut ra ln v nhiu thp, n lm vic

bc sng 1550nm. Trong cc h thng thng tin quang, cho cc EDFA hot

ng th cn c ngun bm. Cc ngun bm thc t l cc diod laser bn dn cng

sut cao dng cung cp ngun nh sng cho EDFA.

EDFA c cc c im sau:

Khng c mch ti to thi gian, mch phc hi (b chuyn i O/E v

E/O).Do mch s tr nn linh hoc hn.

Cng sut ngun nui nh nn khi p dng cho cc tuyn thng tin vt

bin, cp s c cu trc nh v nh hn cp thng.

Gi thnh ca h thng thp do cu trc ca EDFA n gin, trng lng

nh, khong lp v dung lng truyn dn c nng cao.

Ngoi ra do EDFA c kh nng khuch i nhiu bc sng trong cng mt si

nn n c kh nng tng dung lng tc ln n 20Gbps hoc cao hn khi s

dng k thut WDM.

Ngoi loi khuch i EDFA cn c dng khuch i SOA (Semiconductor

Optical Amplifiers- b khuch i quang bn dn). V c bn, SOA l mt mi ni

P-N. Lp gia c hnh thnh mi ni hot ng nh l mt vng tch cc. nh

sng c khuch i do s pht x kch thch khi n lan truyn qua vng tch cc

ny. i vi mt b khuch i, hai u cui ca vng tch cc c ph mt lp

khng phn x loi b gn sng trong li b khuch i.


29

2.6.4. Gii thiu v b kt ni cho quang OXC

2.6.4.1. Chc nng OXC

Trong mng nh tuyn bc sng WDM, hnh trn gm c hai loi node l:

OXC v Edge node. OXC l node m ng vai tr kt ni cc si quang trong

mng. Edge node ng vai tr cung cp giao din gia nhng h thng kt cui phi

quang (nh l cc IP Router, chuyn mch ATM, hay cc siu my tnh) vi li

quang. Cc Edge node thng nm u cui ca h thng v cc lightpath c

thit lp gia hai edge node qua cc node trung gian nh hnh trn. y c mong

i mang li cu trc ca mng ton quang, thng tin truyn i trn lightpath khng

cn s chuyn i no t tn hiu in sang quang hoc ngc li t quang sang tn

hiu in.

OXC cung cp chc nng chuyn mch v nh tuyn h tr cc lin kt

logic gia hai Edge. Mt OXC lm nhim v truyn thng tin trn mi bc sng

mt u vo v n c th chuyn mch n mt cng ra ring bit. Mt OXC vi N

cng vo- N cng ra m cc cng ny c kh nng x l W bc sng trn mi cng

OXC ( optical cross connect) l thnh phn dng iu khin cc cu trc mt

li phc tp v mt s lng ln cc bc sng. OXC l thnh phn mng chnh

cho php cu hnh li mng quang, m cc lightpath c th thit lp v kt thc

Hnh 2.10: Mng WDM nh tuyn bc sng


30

khi cn thit m khng phi c cung cp c nh. OXC c cu trc vi mch

tch hp rt ln v kh nng ni kt hng ngn u vo vi hng ngn u ra to

nn chc nng chuyn mch v nh tuyn. Trong thng tin quang, bn mi knh

quang c th c truyn i trong mt si n, OXC l thit b cn thit c th

tip nhn nhiu bc sng khc nhau cc u vo v nh tuyn cc bc sng

ny n cc u ra thch hp trong mng. thc hin iu ny, OXC cn thit xy

dng cc khi chc nng:

Chuyn mch si: kh nng nh tuyn tt c cc bc sng trn mt si

quang u vo ti mt si quang khc ng ra.

Chuyn mch bc sng: kh nng chuyn mch cc bc sng c th t

mt si quang u vo ti nhiu si quang khc u ra.

Chuyn i bc sng: kh nng nhn cc bc sng u vo v chuyn i

chng thnh tn s quang khc ng ra, iu ny l cn thit tho mn cc kin trc

bt ng khi khi s dng chuyn mch bc sng.

Mt OXC c cc chc nng sau:

Cung cp dch v: Mt OXC c th dng cung cp cc lightpath trong

mt mng ln mt cch t ng, m khng phi thao tc bng tay. Kh nng ny tr

nn quan trng khi gii quyt s bc sng ln trong mt nt hoc vi s nt trong

Hnh 2.11: Cc khi chc nng ca OXC


31

mng ln. N cng quan trng khi cc lightpath trong mng cn cu hnh li p

ng vi s thay i lu lng ca mng.

Bo v: Chc nng quan trng ca b kt ni cho l bo v cc lightpath

khi si b t hoc thit b gp s c trong mng. B OXC l phn t mng thng

minh m n c th pht hin s c trong mng v nhanh chng nh tuyn li cc

lightpath.

Trong sut i vi tc bit: kh nng chuyn mch cc tn hiu vi tc

bit.

Gim st thc hin, nh v li: OXC cho thy tham s ca mt tn hiu

nhng nt trung gian, OXC cho php kim tra thit b v gim st cc tn hiu i

xuyn qua n.

Chuyn i bc sng: ngoi kh nng chuyn tn hiu t cng ny sang

cng khc, OXC cn kh nng c th chuyn i bc sng bn trong.

Ghp knh: cc OXC iu khin cc tn hiu ng vo v ng ra tc

ng dy quang, tuy nhin n c kh nng ghp knh chuyn mch lu lng

ni ti.

Mt OXC c phn theo chc nng thnh mt trung tm chuyn mch v mt

khu lin hp cng. Trung tm chuyn mch cha b chuyn mch m n thc hin

chc nng kt ni cho thc s. Khu lin hp cng cha cc card c dng nh

cc giao din lin lc vi cc thit b khc. Cc cng giao tip c th bao gm

cc b chuyn i quang- in, in- quang hoc khng.

Mt phn t kt ni cho c bn 2 x 2 gi cc tn hiu quang t hai ng vo n

hai ng ra v c hai trng thi, l: trng thi cross v trng thi bar. Trong trng

thi cross, tn hiu t cng vo pha trn c gi n cng ra pha di, v tn hiu

t cng vo pha di c gi n ng ra pha trn. Trong trng thi bar, tn hiu

t cng vo pha trn c gi n cng ra pha trn, v tn hiu t cng pha di

c gi ti cng ra bn di.


32

2.6.4.2. Phn loi OXC

OXC c chia lm hai loi:

- Hybrid OXC (hay OXC khng trong sut): hin ang rt ph bin, n thc hin

chuyn i tn hiu quang sang tn hiu in, thc hin kt ni bng cch s dng k

thut kt ni in t v sau li chuyn i tn hiu in sang tn hiu quang.

- All optical OXC (hay OXC trong sut): l cch kt ni trc tip cc knh quang

trong min photonic. Tn hiu dng photonic trong sut qu trnh chuyn mch

m khng cn thit qu trnh chuyn i O-E-O. OXC ny c th phn thnh cc

thnh phn thit b chuyn mch quang Free Space, thit b quang trng thi rn v

cc thit b gng c in. Trong s cc thit b chuyn mch ph bin nht kt ni

nhiu u vo vi nhiu u ra l WRG. Vi thit b ny, mt bc sng cho trc

Hnh 2.13: Hybrid OXC

Hnh 2.12: Trng thi ca OXC


33

cng vo bt k s xut hin mt cng ra xc nh nh hnh 2.13. Loi chuyn

mch quang Free Space ny c bit nh l chc nng nh tuyn bc sng.

Cc thit b chuyn mch quang Free Space: n c hiu l lm nhim v

nh tuyn bc sng, mt loi khc th chm laser c chiu mt cch c hc vo

mt trong nhng si quang. Trong trng hp ny, mt ma trn ca cc chm tia

trn n kt hp mt ma trn ca cc si quang, lc mt trong nhng chm tia

nng lng v mt si quang thu s c nh hng chng kt hp vi nhau

t c mt kt ni trong khng gian.

Cc thit b quang trng thi rn: l cc cp thit b bn dn nh hng,

cc thit b ny c th thay i mt trong nhng c tnh quang trn ng i da

vo cc ng dng iu khin tn hiu nh nhit , nh sng, dng in hay in p.

Cc c tnh quang bao gm s phn cc, s truyn nh sng, s hp th, ch s

khc x.

H thng vi c in: da vo s phn x nh sng trn mt b mt sng bng

lm thay i tnh nh hng ca nh sng. K thut ny da trn h thng gng

c in (MEMS Micro Electro Mechanical Systems).

Xt mt trung tm cung cp dch v ln, y c th kt thc nhiu kt ni,

mi kt ni mang nhiu bc sng. Mt s bc sng ny khng cn c kt thc

Hnh 2.14: OXC ton quang WGR


34

v tr m mun i n node khc. OXC thc hin chc nng ny, n lm vic

k bn cc phn t mng SONET/ SDH, b nh tuyn IP v cc chuyn mch

ATM, cc thit b u cui WDM v b ghp knh xen/ rt. Mt cch in hnh,

mt s cng OXC c kt ni n cc thit b WDM, cc cng khc c ni n

cc thit b kt cui. V th OXC cung cp dung lng hiu qu hn nhiu.

2.7. S chuyn i bc sng

Chuyn i bc sng l kh nng chuyn tn hiu t bc sng ny( 1 ) trn

mt ng vo sang bc sng khc ti ng ra ( 2 ). B chuyn i rt c ch trong

vic gim xc sut tc nghn mng. Nu cc b chuyn i c tch hp vo trong

b kt ni cho quang trong mng WDM, cc kt ni c th c thit lp gia

ngun v ch ngay c khi trn tt c cc tuyn ca ng i khng c sn cng mt

bc sng. Cc b chuyn i bc sng gip loi tr s bt buc tnh lin tc v

bc sng.

B chuyn i bc sng y gip cho vic gim xc sut tc nghn tt hn

nhng thc t b chuyn i ny rt kh thc hin bi cc l do v chi ph v gii

hn k thut. Trong mt mng c rt t node mng c trang b b chuyn i bc

sng, do cn phi c s la chn cc node t cc b chuyn i bc sng

cc v tr thch hp sao cho ti u mng, thng t cc b chuyn i bc sng

nhng node m lu lng mng xy ra cc i.

tr--

V d nh hnh trn, mt lightpath c thit lp gia Node A v Node B trn

bc sng 1 , v mt ng lightpath khc c thit lp gia Node B vi Node C

trn bc sng 2 . Nu c mt yu cu Node A n Node C, yu cu khng th

1

2

1

2

Node A Node B Node C

Hnh 2.15: S chuyn i bc sng


35

thit lp c v s bt buc tnh lin tc v bc sng. Nu c b chuyn i bc

sng c t Node B m n c kh nng chuyn i t bc sng 1 sang 2 , th

yu cu c th thc hin thnh cng. R rng cc b chuyn i bc sng c th

ci thin c hiu sut khi cc bc sng ri c sn trn cc tuyn, v mt bc

sng chung th khng c.

Chuyn i bc sng c chia ra lm hai loi:

Chuyn i bc sng quang - in: theo phng php ny, tn hiu trc

tin c chuyn sang tn hiu in s dng b tch sng. Lung bit c lu tr

trong b m. Sau tn hiu in c dng li ng ra ca mt tunable laser

to thnh mt bc sng mong mun ng ra. Phng php ny khng thch hp

cho tc bit cao hn 10Gbps, tiu hao cng sut ln v thc hin phc tp hn

cc phng php khc.

Chuyn i bc sng ton quang: qu trnh chuyn i bc sng c

thc hin hon ton trong min quang. Phng php ny da vo hiu ng trn

bc sng to ra mt bc sng khc.

Kh nng chuyn i bc sng c th thc hin qua nhiu mc khc nhau.

Hnh di y minh ho s khc nhau gia u vo v u ra, trng hp nhiu

cng th cng phc tp hn nhng cng tng t. Kh nng chuyn i bc sng

hon ton tc l c th chuyn i mt bc sng ng vo thnh mt bc sng

bt k ng ra. Kh nng chuyn i bc sng gii hn qui nh rng mi bc

sng u vo c th c chuyn i thnh mt s bc sng xc nh trc ng

ra. Trng hp c bit ca chuyn bc sng gii hn l chuyn i bc sng c

nh khi m mt bc sng u vo ch c th chuyn i thnh mt bc sng c

nh u ra. Nu mi bc sng c chuyn i thnh chnh n th chng ta

gi khng c s chuyn i no.


36

2.8. Kt lun chng.

Qua chng ny, ta thy c ng lc thc y mng WDM hin nay.

Nhng mng ny cung cp cc lightpath t u cui ny n u cui kia qua cc

node mng trung gian. Mt lightpath gm c mt knh thng tin quang, hoc bc

sng, gia hai node mng m c nh tuyn qua nhng node trung gian. Cc node

mng trung gian c th chuyn mch v chuyn i bc sng. V vy cc mng

ny c xem l cc mng nh tuyn bc sng.

Hnh 2.16: Cc kh nng chuyn i bc sng

Chng 3: nh tuyn v gn bc sng

37

CHNG 3

NH TUYN V GN BC SNG.

3.1. Gii thiu chng

Trong mng quang nh tuyn bc sng, ngi s dng lin lc vi nhau qua cc

knh thng tin quang c gi l cc lightpath. Lightpath l mt ng i ca tn

hiu nh sng t ngun n ch di dng quang thng qua cc kt ni trung gian.

Mt lightpath c th ko di qua nhiu tuyn truyn dn cung cp mt kt ni

chuyn mch mch gia hai node m c th cha mt lung lu lng ln gia

chng.

Khi cc lightpath thc hin vic mang thng tin t mt node ngun n mt node

ch no th n cn c nh tuyn v gn bc sng. nh tuyn v gn bc

sng cho lightpath l vn ht sc quan trng v xy ra thng xuyn trong mng.

Chng ny s ni r v vic nh tuyn v gn bc sng cho cc lightpath, cc

thut ton thc hin nh tuyn v cc phng php gn bc sng trong mng

WDM.

3.2. Gii thiu v nh tuyn v gn bc sng (Routing and Wavelength

Assignment - RWA).

Khi mt lightpath c chn v xc nh, mi lightpath cn c nh tuyn v

gn bc sng cho n. T t ra bi ton nh tuyn v gn bc sng.

nh tuyn l vn tm ng gia hai node bt k trong mng tho mn mt

mc ch no , thut ng gi l ti u hm mc tiu (cost function). Vn ny

rt quen thuc v rt quan trng trong mng. Thng thng nh tuyn trong IP s

dng thut ton tm ng Dijkstra, vi hm mc tiu l cc metric quen thuc nh

bng thng, tr, chi ph tuyn,

Trong mng quang, tm ng c hiu theo hai kha cnh, l tm ng vt

l mang c mu lu lng yu cu (Routing) v a ra bc sng ph hp mang

lu lng trn mi link dc path (Wavelength Assignment) trong s cc bc sng

cho php (bi mi path gm mt s fiber, m trn mi fiber ny, bn c th c W

sub-chanels, cng l W bc sng v W la chn cho yu cu kt ni hin ti). Vn


38

ny c vit tt l RWA. Khi tm c mt path vt l v nh du bc sng

trn cc link dc theo path , th chng ta c mt ng quang, cn gi l lightpath

(LP). Rc ri t ra i vi bi ton RWA l n a ra hai iu kin sau:

iu kin tnh lin tc bc sng: mt lightpath phi s dng chung mt

bc sng trn tt c cc link dc theo ng i ca n t ngun n ch. iu kin

ny c minh ho nh hnh di bng cch mi lightpath c th hin bng mt

mu nht nh trong sut ng i.

Hnh 3.1: iu kin tnh lin tc bc sng

iu kin tnh ring bit v bc sng: tt c cc lightpath s dng cng mt

link (fiber) phi c gn cc bc sng ring bit. iu kin c minh ho nh

(hnh 2.10) m n c tho mn khi hai lightpath cng chia s cng mt link c

th hin bng hai mu khc nhau (hai bc sng khc nhau).

Vn xy ra khi cc bc sng trn hai link k cn khc nhau, lc cn dng

n b chuyn i bc sng, l ti nguyn t ca mng. Cc gii thut lun tm

cch gim thiu chi ph ny.

Bi ton RWA c th a ra nh sau: cho mt s hu hn cc lightpath c thit

lp trn mng v mt s gii hn cc bc sng. Ta phi xc nh ng i cho mi

lightpath v xc nh s bc sng nn c gn cho cho cc lightpath ny t

c s lightpath c th thit lp l ln nht. Mc d nhng lightpath c ng i


39

ngn nht c v ti u hn, nhng i khi ta nh phi loi b s la chn ny

nhiu lightpath hn c th thit lp. V th cc gii thut thng cho php nhiu

ng i thay phin nhau i vi mi lightpath c thit lp.

Cc ng i nh sng (lightpath) m khng th c thit lp v nhng rng

buc v ng i v bc sng c gi l nghn, do vy vn ti u mng tng

ng hn ch n mc thp nht xc xut tc nghn ny.

Khi hai lightpath m chng c tuyn truyn dn trng nhau th chng s khng

c gn cng mt bc sng. Thng thng mt ng i nh sng (lightpath) hot

ng vi cng mt bc sng trn nhng si quang m n i qua. Trng hp ny ta

ni rng lightpath tho mn s rng buc v tnh lin tc bc sng. Tuy nhin nu

mt nt chuyn mch/nh tuyn c trang b vi mt b chuyn i bc sng th

iu kin rng buc v tnh lin tc bc sng khng cn na, lightpath ny c th

chuyn sang nhiu bc sng khc nhau trn ng i t ngun n ch ca n.

Mng li c m hnh bng Graph G(E,V) vi E (edge) l tp cc cnh v V l

tp cc nh (vertical). Vi mi cp node bt k S-D trong mng (v tng ng trong

Graph), tn ti mt tp cc ng i (path) vt l c th gia chng (mi path bao

gm mt s fiber hay link, edge trung gian), k hiu: R. Tp cc ng i ny c th

tm theo mt gii thut tm ng ph bin nh Dijkstra, Prim hay Mentor vi mt

hm mc tiu tu chn.

3.3. nh tuyn bc sng

Trong mt mng khng c b chuyn i bc sng, cc lightpath phi s dng

cng mt bc sng t ngun n ch. Khi c nhu cu cho cuc gi, b nh tuyn

bc sng WR phi s dng gii thut c thit lp t trc chn mt cng ra v

bc sng tng ng. S la chn bc sng ng vai tr quan trng i vi ton b

xc sut tc nghn. V vy mt WR phi tm ra ng i cho yu cu thit lp

lightpath v thc hin gn bc sng sao cho ti thiu ho xc sut tc nghn. Chc

nng ny c tm quan trng trong vic thit k cc mng ton quang.

Bi ton RWA c chia lm hai loi nh sau:


40

RWA dnh cho lu lng mng c nh (static traffic): vi loi ny th cc

yu cu v lightpath c bit trc, tt c mi ng i v bc sng gn cho cc

lightpath c thit lp c nh t trc ( v d nh yu cu truyn t Router ny

n Router l khng i, tnh theo n v LP, xt trn ton mng ta c ma trn hng

N*N ). Khi c yu cu i n, mt ng i v bc sng ch nh t trc

c gn cho yu cu tng ng . V vy, qui trnh nh tuyn v gn bc sng l

c nh, khng thay i theo thi gian. Vi loi ny, cng vic thc hin khng phc

tp, n n gin l gn mt ng i no cho lightpath. Mc ch ca phng

php ny l tng cc i ton b dung lng ca mng, tc l c th thit lp ng

thi s lightpath l ln nht. y l bi ton trong mng khng c s chuyn i bc

sng.

RWA dnh cho lu lng mng thay i (dynamic traffic): trong mng

quang nh tuyn bc sng, cc yu cu v lightpath i n theo mt qui trnh ring

bit v thi gian chim bi cc yu cu ny cng theo mt qui lut ring. Vi dng

lu lng mng thay i th cn c mt gii thut ng nh tuyn cc lightpath

qua nhng ng i khc nhau da vo s tc nghn trn cc tuyn truyn dn. T

gii thut cho bi ton RWA ng c a ra, n da vo trng thi hin thi ca

mng xc nh ng i cho mi yu cu thit lp lightpath. Mt kt ni b nghn

nu khng c ng i no c th dng mang n. Mt trong nhng thch thc

gii quyt bi ton nh tuyn v gn bc sng vi lu lng mng thay i l pht

trin cc gii thut v giao thc thit lp cc lightpath, nhm hn ch n mc thp

nht xc sut tc nghn trong mng (tc l s yu cu kt ni s b t chi/ tng s

yu cu), nng cao hiu sut s dng ti nguyn (cng mt lng fiber, node, b

chuyn i bc sng,c th to ra nhiu lightpath nht) v ci thin hiu nng

tng th ca mng (hiu nng = xc sut tc nghn ca mng + phc tp ca gii

thut) . Mt phng php n gin l da vo gii thut tm ng i b nghn t nht

thit lp cc lightpath ng. Trong gii thut ny, mt lightpath c thit lp trn

ng i t b nghn nht t tp cc lightpath khc nhau gia cp ngun - ch. Bc


41

sng c cp pht l bc sng u tin cn ri gia nhng tuyn lin kt trong

ng ny.

Bi ton RWA ( Routing and Wavelength Assignment) c chia lm hai phn:

nh tuyn v gn bc sng.

3.4. nh tuyn (Routing)

3.4.1. Gii thiu

nh tuyn c coi l thnh phn ct yu ca kin trc mng, thit k mng v

iu hnh mng ca mi mng thng tin, l thnh phn khng th thiu trong mng

vin thng. Cc yu t thc y cho qu trnh thay i v pht trin nh tuyn mng

ch yu do nhu cu ci thin hiu nng mng, cc dch v mi a vo khai thc v

s thay i cng ngh mng, v y cng l mt trong nhng thch thc khi xy dng

v khai thc mng. Hu ht cc mng vin thng truyn thng c xy dng theo

m hnh mng phn cp m hnh ny cho php s dng nh tuyn tnh trn qui m

ln.

Trong khi nh tuyn tnh vn cn tn ti th tnh cht c lp gia ngi s dng

v mng vn mc cao; nh tuyn tnh ch yu da trn mong mun ca ngi s

dng nhiu hn l tnh trng ca mng hin thi. Mng hin i hin nay c xu hng

hi t cc dch v mng, yu cu t ra t pha ngi s dng l rt a dng v phc

tp. Cc phng php nh tuyn ng c s dng nhm nng cao hiu nng mng

ca mng mi ny, tng thm tnh ch ng, mm do p ng tt hn yu cu ngi

s dng dch v.

nh tuyn ch s la chn ng i trn mt kt ni mng thc hin vic

gi d liu. nh tuyn ch ra hng, s dch chuyn ca cc gi (d liu) c nh

a ch t mng ngun n ch thng qua cc node trung gian; thit b chuyn dng

l b nh tuyn (router). Tin trnh nh tuyn thng ch hng i da vo bng

nh tuyn, l bng cha cc l trnh tt nht n cc ch khc nhau trn mng. V

vy vic xy dng bng inh tuyn, c t chc trong b nh ca router, tr nn v

cng quan trng cho vic nh tuyn hiu qu.


42

Khi c nhu cu cho cuc gi n, b nh tuyn xc nh ng i cho yu cu

thit lp lightpath. Nh vy bi ton nh tuyn l xc nh ng i cho mi yu cu

thit lp lightpath. Mi ng i l mt chui cc tuyn truyn dn t im ngun

n im ch. Nhm gim s phc tp trong tnh ton, ng thi bi ton n

gin hn, ta s xt ng i ngn nht gia hai im u cui ny. thc hin iu

ny, ta s dng mt gii thut tm ng i ngn nht da trn gii thut Dijkstra.

hiu r v thut ton dng trong nh tuyn, ta tm hiu v l thuyt th.

3.4.2. Phn loi nh tuyn

C nhiu cch phn loi nh tuyn, c th a ra mt s loi nh tuyn nh sau:

Da vo chc nng thch nghi vi trng thi hin thi ca mng phn loi

thnh: nh tuyn tnh v nh tuyn ng

nh tuyn tnh: vi nh tuyn tnh, ng dn c chn trc cho mi cp

ngun ch ca cc node trong mng. Cc gii thut nh tuyn chi ph ti thiu c

th c s dng. K hoch nh tuyn tnh c s dng hu ht cc mng truyn

thng, trong k hoch nh tuyn ny ch yu vi mc ch lm gim cc h thng

chuyn mch phi i qua vi yu cu kt ni ng di. K thut nh tuyn tnh bc

l mt s nhc im nh: quyt nh nh tuyn tnh khng da trn s nh gi lu

lng v topo mng hin thi. Cc b nh tuyn khng pht hin ra cc b nh

tuyn mi, chng ch c th chuyn thng tin n ti cc cc b nh tuyn c ch

nh trc ca nh qun l mng.

nh tuyn ng: nh tuyn ng la chn tuyn da trn thng tin trng thi

hin thi ca mng. Thng tin trng thi c th o hoc d on v tuyn ng c

th thay i khi topo mng thay i hoc lu lng mng thay i. nh tuyn ng

th hin tnh linh hot v d dng m rng mng.

Da vo phm vi nh tuyn, ta phn loi thnh: nh tuyn trong v nh

tuyn ngoi.

nh tuyn trong: nh tuyn xy ra bn trong mt h thng c lp (AS

Autonomous System), cc giao thc thng dng l RIP (Router Information


43

Protocol), IGRP (Interior Gateway Routing Protocol), OSPF (Open Shortest Path

First), EIGRP (Enhanced IGRP),

nh tuyn ngoi: nh tuyn xy ra gia cc h thng c lp (AS), lin quan ti

dch v ca nh cung cp mng s dng giao thc nh tuyn ngoi rng v phc tp.

Giao thc thng dng l BGP (Border Gateway Protocol).

3.4.3. L thuyt th

Trong ton hc v tin hc, th l i tng nghin cu c bn ca l thuyt

th. Mt cch khng chnh thc, th l mt tp cc i tng gi l nh ni vi

nhau bi cc cnh. Thng thng th thng c v di dng tp cc im

(nh, nt) ni vi nhau bi cc on thng (cnh). Tu theo ng dng m mt s

cnh c th c hng.

Hnh 3.3: L thuyt th

Hnh 3.2: nh tuyn trong v nh tuyn ngoi


44

C 3 loi th: th c hng, th v hng v th hn hp.

3.4.3.1. th v hng.

th v hng hoc th G l mt cp c th t (order pair) G=(V,E), trong

:

V l tp cc nh hoc nt.

E l tp cc cp khng th t cha cc nh phn bit, c gi l cnh. Hai

nh thuc mt cnh c gi l cc nh u cui ca cnh .

3.4.3.2. th c hng.

th c hng G l mt cp c th t G=(V,A), trong :

V l tp cc nt hoc nh.

A l tp cc cnh c th t cha cc nh, c gi l cc cnh c hng hoc

cung.

Hnh 3.5: th c hng

Hnh 3.4: th v hng


45

Mt cnh e=(x,y) c coi l c hng t x n y, x c gi l im u/gc v

y c coi l im cui/ngn ca cnh.

T ta phn loi ra: th n v a th.

th n: l th m gia hai nh ch c ti a mt cnh.

a th: l th m gia hai nh c th c nhiu hn mt cnh.

a th c hng l mt th c hng m trong nu x v y l hai nh th

th c php c c hai cung (x,y) v (y,x). th n c hng l mt th c

hng, trong , nu x v y l hai nh th th ch c php c ti a mt trong

hai cung (x,y) v (y,x).

3.4.3.3. th hn hp

th hn hp G l b ba c th t G=(V,E,A) vi V,E,A c nh ngha nh

trn.

3.4.3.4. V d

Vi hnh trn, ta c cc gi tr sau:

- V={1,2,3,4,5,6}

- E={{1,2},{1,5},{2,3},{2,5},{3,4},{4,5},{4,6}}

i khi thng tin ni t nh 1 n nh 2 c k hiu l 1~2.

Hnh 3.6: V d


46

Bi ton nh tuyn gn bc sng c lin h cht ch vi bi ton t mu cho cc

nt trong th. Bi ton ca chng ta l t mu cho cc nt thuc G sao cho hai

node k cn nhau phi mang mu khc nhau th hin mi trng thi ca node.

3.4.4. Cc thut ton c bn trong nh tuyn

Cc mng chuyn mch gi v internet da trn quyt nh nh tuyn ca n t

cc tiu ch ti thiu. y ta xt n chi ph tuyn c s dng nh tham s ng

vo ca thut ton nh tuyn chi ph ti thiu m c th pht biu n gin nh sau:

Cho mt mng gm cc node c ni bi cc tuyn song cng, trong , mi

tuyn c mt chi ph c gn cho mi hng, nh ngha chi ph ca ng dn gia

hai node l tng chi ph ca cc tuyn hp thnh ng dn. Vi mi cp node, tm

ng dn vi chi ph ti thiu.

Hu ht cc thut ton chi ph ti thiu ang s dng trong cc mng chuyn mch

gi v internet l Dijkstra hoc Bellman-Ford. Ta s xt hai thut ton ny di y.

3.4.4.1. Thut ton trng thi lin kt LSA

Trong thut ton trng thi lin kt, cc node mng qung b gi tr lin kt ca n

vi cc node xung quanh ti cc node khc. Sau khi qung b, tt c cc node u

bit r topo mng v thut ton s dng tnh ton con ng ngn nht ti node

ch l thut ton Dijkstra.

Thut ton Dijkstra, mang tn ca nh khoa hc my tnh ngi H Lan Edsger

Dijkstra, l mt thut ton gii quyt bi ton tm ng i ngn nht trong mt th

c hng khng c cnh mang trng s m.

3.4.4.1.1. Bi ton

Cho mt th c hng G=(V,E), mt hm trng s w: E [0, ) v mt nh

ngun s. Cn tnh ton c ng i ngn nht t nh ngun s n mi nh ca

th. V d: chng ta dng cc nh ca th m hnh cc thnh ph v cc cnh

m hnh cc ng ni gia chng. Khi trng s cc cnh c th xem nh di

ca cc con ng hay c th l chi ph (v do l khng m). Chng ta cn vn

chuyn t thnh ph s n thnh ph t. Thut ton Dijkstra s gip ch ra ng i

ngn nht chng ta c th i.


47

Trng s khng m ca cc cnh ca th mang tnh tng qut hn khong cch

hnh hc gia hai nh u mt ca chng. V d, vi 3 nh A, B, C ng i A-B-C

c th ngn hn so vi ng i trc tip A-C.

3.4.4.1.2. Thut ton

Thut ton Dijkstra c th m t nh sau:

Ta qun l mt tp hp ng S. Ban u S={s}.

Vi mi nh v, chng ta qun l mt nhn d[v] l di b nht trong cc ng

i t ngun s n mt nh u no thuc S, ri i theo cnh ni u-v.

Trong cc nh ngoi S, chng ta chn nh u c nhn d[u] b nht, b sung vo

tp S. Tp S c m rng thm mt nh, khi chng ta cn cp nht li cc nhn

d cho ph hp vi nh ngha.

Thut ton kt thc khi ton b cc nh nm trong tp S, hoc nu ch cn tm

ng i ngn nht n mt nh ch t, th chng ta dng li khi nh t c b sung

vo tp S.

Tnh cht khng m ca trng s cc cnh lin quan cht ch n tnh ng n

ca thut ton. Khi chng minh tnh ng n ca thut ton, chng ta phi dng n

tnh cht ny.

3.4.4.1.3. Chng minh

tng c chng minh nh sau:

Chng ta s ch ra, khi mt nh v c b sung vo tp S, th d[v] l gi tr ca

ng i ngn nht t ngun s n v.

Theo nh ngha nhn d, d[v] l gi tr ca ng i ngn nht trong cc ng i

t ngun s, qua cc nh trong S, ri theo mt cnh ni trc tip u-v n v.

Gi s tn ti mt ng i t s n v c gi tr b hn d[v]. Nh vy trong ng

i, tn ti nh gia s v v khng thuc S. Chn w l nh u tin nh vy.

ng i ca ta c dng s - ... - w - ... - v. Nhng do trng s cc cnh khng m

nn on s - ... - w c di khng ln hn hn ton b ng i, v do c gi tr

b hn d[v]. Mt khc, do cch chn w ca ta, nn di ca on s - ... - w chnh l


48

d[w]. Nh vy d[w] < d[v], tri vi cch chn nh v. y l iu mu thun. Vy

iu gi s ca ta l sai. Ta c iu phi chng minh.

3.4.4.1.4. Cc bc thc hin

Thut ton Dijkstra dng trong giao thc nh tuyn 0SPF i qua cc bc sau:

1. B nh tuyn xy dng th ca mng v xc nh cc node ngun ch, v d

nh V1 v V2. Sau n xy dng mt ma trn, c gi l ma trn lin k. Ma trn

ny th hin trng s ca cc cnh, v d nh [i,j] l trng s ca cnh ni Vi vi Vj.

Nu khng c kt ni trc tip gia Vi v Vj, trng s ny c xc nh l v cng.

2. B nh tuyn xy dng bng trng thi cho tt c cc node trong mng. Bng ny

gm cc phn:

Chiu di: th hin ln ca trng s t ngun n node .

Nhn ca node: th hin trng thi ca node, mi mt node c th c mt

trong hai trng thi l c nh hay tm thi.

3. B nh tuyn gn thng s ban u ca bng trng thi cho tt c cc node v thit

lp chiu di ca chng l v cng v nhn ca chng l tm thi.

4. B nh tuyn thit lp mt T-node. V d nh V1 l node ngun T-node, b nh

tuyn s chuyn nhn ca V1 sang c nh. Khi mt nhn chuyn sang c nh, n s

khng thay i na.

5. B nh tuyn s cp nht bng thi trng thi ca tt c cc node tm thi m cc

node ny lin kt vi node ngun T-node.

6. B nh tuyn nhn vo cc node tm thi v chn mt node duy nht m node ny

c trng s n V1 l nh nht. Node ny sau tr thn node ch T-node.

7. Nu node ny khng phi l V2 th b nh tuyn tr li bc 5.

8. Nu node ny l V2 th b nh tuyn tch node trc ca n khi bng trng

thi v c thc hin iu ny cho n khi n node V1. Mt lt cc node ch ra tuyn

ti u nht t V1 n V2.

3.4.4.1.5. V d v thut ton Dijkstra

Di y ta s tm ng ngn nht gia A v E.


49

Bc 1: Theo hnh sau, node A lm node ngun T-node, nhn ca n chuyn

sang c nh v c nh du bng

Bc 2: Trong bc ny, ta s thy c bng trng thi ca cc node ni trc

tip vi node A l cp node (B,C). ng t A n B l ngn nht (c trng s nh

nht), do n c chn lm T-node v sau nhn ca n chuyn sang c nh.

Bc 3: ging nh bc 2, da trn bng trng thi ca cc node kt ni trc tip

vi node B l cp node (D,E).Tng t nh th, node D kt ni vi node B l ng

ngn nht (mang trng s 2 nn nh hn trng s ca cnh BE), do node D c

lm T-node, v sau nhn ca n chuyn sang c nh.

Bc 4: trong bc ny chng ta khng c node tm thi no, v th ta ch c th

chn T-node tip theo. Node E c chn vo th, cnh DE c trng s nh nht.


50

Bc 5: Node E l node ch nn chng ta kt thc qu trnh nh tuyn ny.

3.4.4.2. Thut ton nh tuyn vect khong cch DVA

L mt thut ton nh tuyn tng thch nhm tnh ton con ng ngn nht

gia cc cp node trong mng, c bit n nh l thut ton Bellman-Ford. Cc

node mng thc hin qu trnh trao i thng tin trn c s ca a ch ch, node k

tip, v con ung ngn nht ti ch. Mi node trong mng c bng nh tuyn cho

thy ng tt nht n mi ch v mi node ch gi bng nh tuyn ca n n cc

node lng ging.

Vn tn ti ca thut ton DV l n thc hin m n v cng khi c mt kt

ni b hng. Vn ny c th thy r v d sau:

Vi hnh 3.8 cho thy c duy nht mt tuyn gia node A n nhng node khc.

Gi s trng s trn mi cnh u bng 1, mi node (Router) u cha bng nh

tuyn. By gi, nu ta ct kt ni gia A v B th node B s hiu chnh li bng nh

tuyn ca n. Sau khong thi gian, cc node trao i thng tin bng nh tuyn v B

nhn bng nh tuyn ca C. Khi C khng bit g xy ra vi kt ni gia kt ni gia

A v B, n s cho rng c mt tuyn kt ni vi trng s l 2 (1 cho kt ni C-B v 1

cho kt ni B-A), n khng bit rng kt ni A-B b ct. B nhn bng nh tuyn

ny v ngh rng c mt tuyn khc gia C v A, v th n sa li bng nh tuyn v

thay i gi tr trng s ca kt ni B-A v 3 (1 cho kt ni B-C, 2 cho kt C-A). Mt

Hnh 3.8: V d ca thut ton DVA


51

ln na cc node thay i bng nh tuyn ca n. Khi C nhn bng nh tuyn ca B,

n thy rng bng B thay i trng s ca tuyn B-A t 1 thnh 3, v th n cp nht

bng nh tuyn v thay i trng s ca tuyn C-A thnh 4 (1 cho kt ni C-B v 3

cho kt ni B-A). Qu trnh ny c xy ra mit cho n khi tt c cc node tm ra

trng s ca tuyn n A l v cng.

Thut ton Bellman-Ford l mt thut ton tnh cc ng i ngn nht trong mt

th c hng c trng s (trong mt s cung c th c trng s m).Thut ton

Dijksta i hi trng s ca cc cung phi c gi tr khng m. Do thut ton

Bellman-Ford thng dng khi c cc cung vi trng s m.

3.4.4.2.1. Thut ton

Gii thut Bellman-Ford c th pht biu: Tm cc ng dn ngn nht t node

ngun cho trc vi rng buc ch cha mt tuyn, sau tm ng dn ngn nht

vi rng buc ch cha ti a hai tuyn v c th tip tc. Nu ng dn trc l

ngn nht th li cn khng th cp nht ng dn mi. Thut ton c tin hnh

qua cc tng c biu din nh sau:

function BellmanFord (danh_sch _nh, danh_sch_cung, ngun)

// hm yu cu th a vo di dng mt danh sch nh, mt danh cung

// hm tnh cc gi tr khong_cch v nh_lin_trc ca cc nh, sao cho cc

//gi tr nh_lin_ trc s lu li cc ng i ngn nht.

// bc 1: khi to th

for each v in danh_sch_nh:

if v is ngun then khong_cch (v) := 0

else khong_cch (v) := infinity

nh_lin_trc (v) := null

// bc 2: kt np cnh

for i from 1 to size (danh_sch_nh) :

for each (u, v) in danh_sch_cung :


52

if khong_cch (v) > khong_cch (u) + trng_s (u, v) :

khong_cch (v) := khong_cch (u) + trng_s (u, v)

nh_lin_trc (v) := u

// bc 3: kim tra chu trnh m

for each (u, v) in danh_sch_cung :

if khong_cch (v) > khong_cch (u) + trng_s (u, v) :

error th cha chu trnh c trng s m

3.4.4.2.2.Chng minh

Tnh ng n ca thut ton c th chng minh bng qui np. Thut ton c th

pht biu chnh xc theo kiu qui np nh sau:

nh l: Sau i ln lp vng for:

1. Nu Khong_cch(u) khng c gi tr v cng ln, th n bng di ca mt

ng i no t s ti u;

2. Nu c mt ng i t s ti u qua nhiu nht i cung, th Khong_cch (u) c gi

tr khng vt qu di ca ng i ngn nht t s ti u qua ti a i cung.

Chng minh:

Trng hp c bn: Xt i =0 v thi im trc khi vng for c chy ln u tin. Khi , vi nh ngun khong_cch (ngun) := 0, iu ny ng. i vi cc

nh u khc, khong_cch (u) := infinity, iu ny cng ng v khng c ng i

no t ngun n u qua 0 cung.

Trng hp quy np:

Chng minh cu 1: Xt thi im khi khong cch ti mt nh c cp nht bi

cng thc khong_cch (v) := khong_cch (u) + trng_s (u,v). Theo gi thit quy

np, khong_cch (u) l di ca mt ng i no t ngun ti u. Do ,

khong_cch (u) + trng_s (u, v) l di ca ng i t ngun ti u ri ti v.

Chng minh cu 2: Xt ng i ngn nht t ngun ti u qua ti a i cung. Gi

s v l nh lin ngay trc u trn ng i ny. Khi , phn ng i t ngun ti

v l ng i ngn nht t ngun ti v qua ti a i-1 cung. Theo gi thuyt quy np,


53

khong_cch (v) sau i-1 vng lp khng vt qu di ng i ny. Do ,

trng_s (v, u) + khong_cch (v) c gi tr khng vt qu di ca ng i t s

ti u. Trong ln lp th i, khong_cch (u) c ly gi tr nh nht ca khong_cch

(v) + trng_s (v, u) vi mi v c th. Do , sau i ln lp, khong_cch (u) c gi tr

khng vt qu di ng i ngn nht t ngun ti u qua ti a i cung. Khi i

bng s nh ca th, mi ng i tm c s l ng i ngn nht ton cc, tr

khi th c chu trnh m. Nu tn ti chu trnh m m t nh ngun c th i n

c th s khng tn ti ng i nh nht (v mi ln i quanh chu trnh m l mt

ln gim trng s ca ng).

3.4.5. Kt lun

C hai thut ton ny u hot ng di iu kin tnh ca topo mng v chi ph

tuyn th c hai hi t v mt nghim. Khi mng c nhiu s thay i th thut ton s

c gng bm theo s thay i, tuy nhin, nu chi ph tuyn ph thuc vo lu lng,

tc l n li ph thuc vo ng dn c chn th vi p ng lm cho mng

khng n nh.

3.5. Gn bc sng

Vic gn bc sng l nhn t chnh nh hng n xc sut tc nghn v tnh

thc thi ca mng. Gn bc sng thch hp c th lm gim s bc sng s dng

hoc khng cn dng n b chuyn i bc sng, nn ta c th gim c chi ph

ca mng xung rt nhiu. Gn bc sng c chia lm hai loi cho lu lng mng

c nh v lu lng mng thay i. Khi lu lng mng c nh th php gn c

nh, cng mt bc sng c gn nu( nu c sn) cho mi yu cu c to ra

mt nt, nu khng th yu cu b chn. Khi lu lng mng thay i, lc c yu cu

n mt nt mng no th nt s dng mt gii thut chn mt bc sng

ring bit cn ri nt v gn cho lightpath nh tuyn n, nu khng th

yu cu khng c gii quyt. Gii thut cho phng php gn qun l mt danh

sch cc bc sng c s dng, cc bc sng cn ri mi nt.

Cc phng php gn bc sng c chia lm cc loi nh sau:


54

Kiu gn Random: khi c yu cu n mt nt, nt s xc nh nhng

bc sng cn hiu lc ( tc l cn ri) v chn ngu nhin mt i trong nhng bc

sng gn cho yu cu . Cc bc sng cn ri mi nt c xc nh bng

cch loi b bc sng i s dng ra khi danh sch bc sng cn ri; khi cuc

gi kt thc, i c loi ra khi danh sch bc sng b bn v c thm vo tr li

danh sch bc sng ri ban u. Phng php ny khng cn i hi nhng thng

tin v ton b trng thi ca mng khi thc hin gn bc sng. Php gn ny phn

phi lu lng mt cch tu , do vy s tn dng bc sng c cn bng v tranh

chp bc sng thp nn xc sut tc nghn cng thp hn.

Kiu gn First - Fit: php gn ny s tm v gn nhng bc sng theo mt

trnh t c nh. Tt c cc bc sng c nh s t thp n cao v cc bc sng

c chn gn cng theo ch s t thp n cao, tc l bc sng u tin c

chn l bc sng c ch s nh nht trong s bc sng ri v gn cho yu cu.

Cng tng t nh phng php gn Random, php gn ny khng cn bt k thng

tin no v thng tin trng thi mng. Hn ch ca phng php ny l cc bc sng

c ch s nh hn c dng nhiu, trong khi nhng bc sng c ch s ln hu nh

khng c s dng. Hn na s gia tng s bc sng trong si cng khng mang

li hiu qu no bi v nhng bc sng c ch s cao rt t khi c dng. Do s

tranh chp i vi nhng bc sng c ch s nh tng ln, lm xc sut tc nghn

cng tng ln. Php gn ny cho chi ph thp hn so vi php gn Random bi v n

khng cn phi kim tra tt c cc bc sng trong mi tuyn, v th n c a

chung hn.

Php gn Least - used: Php gn ny chn nhng bc sng m nhng bc

sng ny t c s dng nht trong mng. Mc ch ca php gn ny l cn bng ti

trn tt c nhng bc sng. Php gn ny i hi thng tin trng thi v mng tm

ra bc sng t c s dng nht. Tuy nhin phng php ny phi tn km cho chi

ph lu tr v tnh ton.

Php gn Most - used: n l php gn ch l ngc vi php gn Least-used,

n tm chn nhng bc sng c s dng nhiu nht trong mng. Php gn ny


55

phi i hi nhng thng tin v trng thi mng tm ra bc sng c s dng

nhiu nht. N cng tn nhng chi ph tng t nh trong php gn Least- used, tuy

nhin n thc hin tt hn so vi php gn Least- used.

Vi cc php gn bc sng k trn, phng php Random v First - Fit l thc t

hn v d thc hin. Khng ging nh hai phng php Least- used v Most- used

i hi phi c cc thng tin v mng. N n gin ch da vo trng thi nt lc

v chn mt bc sng t nhng bc sng ri kt ni ng ra . Mt cch tng

i, phng php ngu nhin Random cho hiu qu tt hn phng php First - Fit.

thc hin hai phng php gn Least - used v Most - used, mi nt cn trang

b thng tin ton b mng. Nn nhng phng php ny ph thuc vo s thng minh

v hiu bit chnh xc ca cc nt. V trng thi mng thay i mt cch nhanh chng

nn kh c th bit c mt cch chnh xc thng tin mng tt c cc thi im, do

vy nh hng n vic gn bc sng. Hn na cc nt trao i thng tin vi nhau

v mng sau mi khong thi gian c nh v nhng thng tin ny s tiu th mt

bng thng ng k, v th lm gim bng thng sn c truyn d liu.

3.6. S thit lp ng o (Virtual path)

Mt ng o c xem nh mt ng i ca nh sng t ngun n ch. Khi

c yu cu cuc gi c to ra nt, nt s dng gii thut nh tuyn v gn bc

sng tm ra mt ng i v mt bc sng cho cuc gi . Nt s gn bc

sng c chn cho cuc gi v nh tuyn n n nt k tip. mi nt trung

gian ca ng i, bc sng ca lightpath i ti c kim tra xem c sn c

gn v t c th i tip hay khng. Nu bc sng khng c sn, v nu nt

c b chuyn i bc sng, n c th chuyn sang bc sng khc nh tuyn

lightpath. ng i va thit lp c gi l ng o, c thit lp sn trc khi

bt k d liu no c truyn qua.

Mt ng vt l bao gm tt c cc tuyn truyn dn (link) hnh thnh trn l

trnh t ngun n ch, nhng ng o c th cha cc bc sng ging hoc khc

nhau t ngun n ch. Hai yu cu cho cuc gi c cng chung im u cui ch

v ngun c th c cng ng vt l nhng c cc ng o khc nhau. Hnh sau ch


56

ra s hnh thnh ca mt lightpath. y hai cuc gi c to ra t nt 1 v ng

o cho mi cuc gi to thnh c v ra. i vi cuc gi th nht, nt 1 gn bc

sng 1 v gi n n nt 2. Gi s nt 2 c mt b chuyn i bc sng nhng

khng c sn bc sng 1 , v th n chuyn sang bc sng 2 v gi n nt 3. Nt

3 gn tip 2 v n c sn v nh tuyn lightpath n ni. Bng cch ny ng o

th nht c thit lp. Nu cuc gi th hai c to ra nt 1 ngay sau , th mt

ng o th hai c to ra tng t. Ta thy rng ng vt l th ging nhau

nhng cc ng o th khc nhau. Tng s cc ng o c thit lp t ngun n

ch ph thuc vo s bc sng sn c trn si. S ng o c thit lp tht s

ph thuc vo tc cuc gi i n. Cc b chuyn i bc sng gip thit lp

c nhiu ng o hn.

3.7. Phn loi mng quang WDM

3.7.1. Mng single- hop

Trong mng quang WDM single- hop, mt khi lung d liu c pht i di

dng nh sng s n c ch trc tip m khng cn phi chuyn sang dng in

nhng node trung gian. truyn dn mt gi, mt trong nhng laser pht ca nt

gi v mt trong nhng b thu ca node nhn phi c chnh n cng mt bc

sng trong khong thi gian truyn dn gi.

Trong cc mng chuyn mch mch, tc iu chnh ca cc b thu pht thng

yu cu thp. Ngc li trong cc mng chuyn mch gi, cc b thu pht cc node

Hnh 3.9: S thit lp ng o


57

cn c chnh n cc bc sng khc nhau mt cch nhanh chng gi v nhn

cc gi tin khc tip theo. Bn cnh vn k thut ca vic chuyn i bc sng

nhanh, mt thch thc quan trng khc na l pht trin cc giao thc phi hp

hiu qu nhng kt ni cc bc sng khc nhau trong mng.

mt h thng single- hop hot ng hiu qu, bng thng c cp pht gia

cc node ang tranh chp phi c qun l linh ng. Cc h thng ny c th phn

thnh hai loi: c phi hp trc khi truyn dn v khng yu cu phi hp trc khi

truyn dn.

Cc loi phi hp dng mt knh iu khin n dng chung gia cc node v s

truyn d liu tht s xy ra thng qua mt s cc knh d liu. Cc node ri cn

gim st knh iu khin. Trc khi pht hoc thu gi d liu, mt gi chnh b pht

hay bt thu ca n n knh d liu thch hp. Ngc li trong h thng loi th hai,

khng c s tn ti ca knh iu khin v cc node pht hoc thu t cc knh c

nh trc.

3.7.2. Mng Multi- hop

Mng multi- hop khc phc c nhc im ny bng cch trnh s dng b thu

pht iu chnh bc sng. Mi node c trang b mt s cc b thu pht quang

c chnh c nh. Mi b pht trong mng c chnh n mt bc sng khc

nhau. Kt ni trc tip single- hop gia hai node ch c th xy ra khi nu nt n c

mt trong nhng b thu ca n c chnh n mt trong nhng bc sng ca node

gi. S kt ni gia mt cp node bt k trong mng t c bng cch nh tuyn

thng qua cc node trung gian. knh thng tin quang c chuyn thnh dng

in, a ch n ca gi c gii m, sau gi c chuyn mch in v c

pht li trn bc sng n node ch hoc n cc node trung gian khc m

qu trnh ny c lp li. V vy, mt gi s tri qua nhiu bc sng thng qua mt

s node trung gian trc khi n c node ch.


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3.8. Gii thut cho vn nh tuyn v gn bc sng vi lu lng mng thay

i DRWA

Bn c th hnh dung cc vn m mt gii php cho DRWA cn phi gii

quyt, mc ch ca n l ti thiu tc nghn ti node mng (tc l s yu cu kt ni

s b refuse/tng s yu cu), nng cao hiu sut s dng ti nguyn (cng mt lng

fiber, node, chuyn i bc sng,...c th to ra nhiu LP nht) v ci thin hiu

nng tng th ca mng (hiu nng = xc sut tc nghn + phc tp ca gii thut).

Gii thut c trnh by nh sau:

Gi s mi LP c ti a H hop (link). Trn mi link (fiber) s dng W bc sng

(sub-channel). Tp cc ng i c th gia hai node bt k l R*.

Trng thi ca mi bc sng trn link (fiber) c m ho bng hai bit b0b1. Khi

c yu cu LP, node ngun s gi bn tin cp nht trng thi dc theo cc path tim

nng tp hp thng tin trng thi ng truyn (bn tin c th nhng trong giao

thc bo hiu no )

Hai bit trng thi nh sau:

b0b1= 00: bc sng ang bn.

b0b1= 01: c th dng lin tc khng cn chuyn i bc sng.

b0b1= 10: mun dng phi chuyn i bc sng

b0b1= 11: c th dng c hai cch

Ti mi node trung gian thuc LP, 2*W bt trng thi bc sng c ghi (tagged)

vo sau bn tin ny, v gi n ch. Nu thi im node khng th thit lp

knh (do ht bc sng chng hn), n loi b (discard) gi tin bo hiu v gi bn

tin thng bo (notification) ti ngun hoc ch x l.

Ti ch, thng tin trong mi bn tin cp nht trng thi c a ra dng ma trn:

Ton b hnh nh v trng thi ti nguyn ng truyn t node 0 n node H-1

c phn nh trn ma trn ny. Gii thut nh du bc sng thc hin da trn

cc ma trn (thnh cng) t R* path tim nng ca mi cp node.

K hiu CS ca bc sng lamda(m) l bc lin tc ca bc sng, tc l c th

dng n lin tc trong dy lin tip cc node no dc theo path. Gii thut nh sau:


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1. Tm tp tt c cc t hp CS ca mi bc sng, trn mi path, k hiu CSij

2. Tm tp cc t hp CS* thuc {CSij} (i =1: W; j =1:R*) ph kn LP vi s phn t

ti thiu (tc l t on CS nht, iu ny tng ng t phi dng b chuyn i

bc sng nht)

3. p dng hm mc tiu (trong gii thut l tng chi ph) cho mi t hp CS tm

thy trong bc 2 chn ra t hp c tng chi ph ti thiu.

3.9. Kt lun chng

Qua chng ny, chng ta tm hiu v phng php nh tuyn v gn bc

sng trong mng WDM, khi c yu cu thit lp lightpath t node ngun n node

ch th b nh tuyn bc sng c nhim v xc nh ng i v gn bc sng

cho lightpath . Trong mng quang WDM, vic s dng thut ton nh tuyn bc

sng t c ti u mng l iu ht sc ngha.

Thut ton Dijkstra vi vic nh tuyn tm ng ngn nht c nhiu u im

trong mng tp trung nn em s s dng m phng vic nh tuyn trong mng

quang.

Chng 4: Thc hin m phng

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CHNG 4

THC HIN M PHNG 4.1. Gii thiu chng

nh tuyn l cng vic ht sc quan trng trong mng quang WDM, n thc

hin tm ng cho lightpath mang lu lng thng tin t ngun n ch vi mc

ch ti u mng. Trong chng ny, da trn phn mm Visual C++, em m

phng phn nh tuyn cho cc lightpath vi hm mc tiu chng ta c th tu chn

nh chi ph, tr, lng lu lng qua cc tuyn t ngun n ch. Thut ton

s dng thc hin nh tuyn l thut ton Dijkstra.

Cc trng s trn cc tuyn khng ch l di ng i ca tuyn m tu theo

mt tiu ch no ca mng nh chi ph tuyn, tr, bng thng, lu lng

thng tin... Nu ly theo tiu ch l chi ph thp nht th trng s trn cc tuyn

(cnh) l ch ph ca tuyn .

4.2. Gii thiu v ngn ng Visual C++

Visual C++ l ngn ng lp trnh da trn nn tng c bn ca C++, l lp

trnh hng i tng. Nu cc bn lp trnh trn C++ th vic xy dng cc ng

dng trn Visual C++ rt thun li.

Khi thc hin lp trnh C/C++, to cc giao din phc tp, trnh by p hon

ton khng n gin. Nhng i vi Visual C++ th vic kh n gin. Bn ch

cn s dng cc iu khin hay xy dng mt menu a vo ng dng ca mnh m

cc m lnh cn vit khng qu di dng v phc tp nh trong C/C++.

Trong chng trnh m phng ca em c th s dng bt k ngn ng lp trnh

no. Em chn ngn ng Visual C++ do kh nng ca n to giao din d dng hn

C/C++.

4.3. Lu thut ton

Gi s b nh tuyn m phng tm ng i vi ng i ngn nht qua cc

tuyn gia node ngun v node ch. Cc trng s trn cc cnh l di ca tuyn

thng tin t node ny n node kia.


61

Bt u

Xc nh node ngun v ch nh V1 v V2

Thit lp V1 l T-node

Thit lp nhn ca T-node sang c nh, sau cp nht bng trng

thi cc node ln cn.

Xc nh node tm thi ni vi V1 m c trng s nh nht v thit

lp thnh T-node

Da vo thng tin trong bng trng thi, lm nh th cho n khi ti

node V1, dy cc node l ng i ngn nht

Kt thc

NO

YES

T-node c phi l V2


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Thut ton s thc hin tm nh u trong tp hp Q m c gi tr d[u] nh nht.

nh ny c loi ra khi Q v c a vo tp S. Tp S cha mt bng cc nh

to thnh mt trong nhng ng i ngn nht t s n node ngun t no .

1 function Dijkstra(G, w, s) 2 for each vertex v in V[G] 3 d[v] := infinity // Gn cc gi tr ban u 4 previous[v] := undefined 5 d[s] := 0 // Khong cch t s n s bng 0 6 S := empty set // Thit lp S l tp hp rng 7 Q := V[G] // Tp Q cha tt c cc node ca th 8 while Q is not an empty set 9 u := Extract_Min(Q) 10 S := S union {u} 11 for each edge (u,v) outgoing from u 12 if d[u] + w(u,v) < d[v] 13 d[v] := d[u] + w(u,v) 14 previous[v] := u 4.4. Kt qu m phng

Thut ton Dijkstra tm ng i ngn nht t node ngun n node ch c

thc hin nh sau:

1.Click vo biu tng THEM NODE ly node ra nh sau:


63

2.Click vo biu tng THEM CANH ni cc cnh li vi nhau.


64

3.Click vo biu tng DUONG NGAN NHAT thc hin tm ng ngn nht

gia hai cp node bt k.


65

4.Click OK nhn c kt qu.


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4.5. Kt lun chng.

Ta thy c thut ton nh tuyn Dijkstra c ng dng hiu qu trong vic

nh tuyn cc lightpath trong mng WDM tm c ng i ti u vi cc

hm mc tiu (cost function) ca mng m ta c th p t cho n. Hm mc tiu

ny ta c th theo tiu ch no ca mng nh l chi ph tuyn, lng lu lng,

bng thng S p t ny thc hin bng cch t trng s trn cc tuyn l gi

tr ca cc hm mc tiu trn. Sau qu trnh nh tuyn n cc node mng, cc

node mng thc hin gn bc sng cho lightpath. Vic gn bc sng phi tho

mn iu kin lin tc bc sng nu khng node mng phi s dng b chuyn

i bc sng.


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ti nh tuyn v gn bc sng trong mng quang WDM cho thy

c vai tr quan tr

dinh tuyen va gan buoc song trong mang wdm

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