HC VIN CNG NGH BU CHNH VIN THNG

K THUT CHUYN MCH 1

(Dng cho sinh vin h o to i hc t xa)

Lu hnh ni b

H NI - 2007

HC VIN CNG NGH BU CHNH VIN THNG

K THUT CHUYN MCH 1

Bin son: THS. HONG TRNG MINH

THS. NGUYN THANH TR

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LI NI U K thut chuyn mch l mt trong nhng k thut nn tng trong cc mng truyn thng. S pht trin ca k thut chuyn mch lun gn lin vi s pht trin ca h tng mng. p ng yu cu nhn thc v cc kha cnh k thut chuyn mch ca cc lp i hc t xa, nhm tc gi thc hin bin son bi ging C s k thut chuyn mch 1 da trn khung cng ca Hc vin Cng ngh Bu chnh vin thng ban hnh. Vi cch thc tip cn t cc vn mang tnh c s tin ti cc gii php k thut v gii php cng ngh, nhm bin son thc hin b cc ni dung bi ging thnh 4 chng. Cc chng ny cung cp cho ngi hc nhng kin thc c bn trong lnh vc chuyn mch gm cc c ch hot ng v k thut iu khin h thng chuyn mch, cc gii php k thut chuyn mch, gii php cng ngh c bn trong mng vin thng v mng my tnh. Tiu ca cc chng nh sau:

Chng 1. Gii thiu chung v k thut chuyn mch; Chng 2. K thut chuyn mch knh; Chng 3. K thut chuyn mch gi; Chng 4. K thut chuyn mch tin tin;

Cc vn c s ton lin quan ti lnh vc chuyn mch, s pht trin ca k thut mng v v tr chc nng cng nh tm quan trng ca k thut chuyn mch c trnh by trong chng 1. Chng 2 l cc kha cnh mu cht nht trong k thut chuyn mch knh bao gm cc dng tn hiu chuyn mch, cu trc ma trn chuyn mch v cc nguyn l c bn ca k thut chuyn mch knh. Cc nhn nhn v h thng chuyn mch gi trn phng din phn lp theo m hnh OSI, kin trc phn cng v cc c s k thut chuyn mch gi, nh tuyn v bo hiu ca h thng chuyn mch gi c trnh by trong chng 3. Chng 4 cp ti cc gii php k thut v gii php cng ngh chuyn mch tin tin ch yu hin nay trn c s ca mng IP v ATM, mng th h k tip, cng ngh chuyn mch mm v cc ng dng trong mng vin thng trong giai on hi t hin nay. Trong qu trnh thc hin bi ging nhm tc gi nhn c rt nhiu s gip v gp chn thnh t cc ging vin ca b mn Chuyn mch, khoa Vin thng 1, Hc vin cng ngh bu chnh vin thng. Nhm bin son chn thnh cm n v mong mun tip tc nhn c s gp ca cc thy c gio v c gi. Trong qu trnh bin son ti liu chc kh trnh khi mt s sai st, nhm tc gi rt mong nhn c s quan tm v gp ca c gi. Cc kin gp qua mail xin c gi v: Hoangtrongminh@yahoo.com; Thanhtraptit@yahoo.com. Nhm tc gi ThS. Hong Trng Minh ThS. Nguyn Thanh Tr

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Chng 1 GII THIU CHUNG V K THUT CHUYN MCH Chng m u ca ti liu k thut chuyn mch I nhm gii thiu cho hc vin tng quan v kin trc mng vin thng, xu hng pht trin cng ngh mng vin thng. Cc khi nim c s v cc m hnh ton a ra l c s ca cc vn s c a ra trong cc phn sau ca bi ging. Xu hng pht trin v cc cng ngh mng hin i c gii thiu trong chng ny nhm gip hc vin nhn nhn khi qut nhng hng tip cn mi trong lnh vc k thut chuyn mch.

1.1. NHP MN K THUT CHUYN MCH Vin thng l mt phn ca khi nim thng tin - mt dng thc chuyn giao thng tin. Mng vin thng (telecommunications network) c coi l h tng c s ca x hi s dng k thut in, in t v cc cng ngh khc chuyn giao thng tin. Mng vin thng gm tp hp cc nt mng, cc ng truyn dn kt ni gia hai hay nhiu im xc nh v cc thit b u cui thc hin trao i thng tin gia ngi s dng. Mt cch khi qut chng ta c th coi tt c cc trang thit b, phng tin c s dng cung cp dch v vin thng to thnh mng vin thng. Thit b u cui l cc trang thit b ca ngi s dng giao tip vi mng cung cp dch v. Thit b chuyn mch l cc nt ca mng vin thng c chc nng thit lp v gii phng ng truyn thng gia cc cc thit b u cui. Thit b truyn dn c s dng ni cc thit b u cui hay gia cc nt vi nhau thc hin truyn cc tn hiu mt cch nhanh chng v chnh xc. Cng tham gia xy dng mng vin thng c cc nh cung cp thit b, khai thc thit b v cc nh cung cp dch v, v..v.

Cng vi s pht trin ca cng ngh tin tin l xu hng hi t mng truyn thng gia mng c nh, mng di ng v mng internet sang mng th h k tip NGN (Next Generation Network). H tng mng vin thng thay i khng ngng nhm p ng cc yu cu ngy cng cao ca ngi s dng, s tc ng ny lin quan v nh hng ti rt nhiu lnh vc trn cc yu t khoa hc cng ngh v khoa hc k thut, trong bao gm k thut chuyn mch. Cun ti liu k thut chuyn mch ny tip cn t cc vn c bn v mu cht nht trong lnh vc chuyn mch ti cc xu hng v gii php chuyn mch tin tin nhm gip ngi c nhn thc cc kha cnh k thut lin quan ti lnh vc ny.

Trong cc phn u tin ca ti liu s gii thiu cc khi nim c s lin quan ti lnh vc chuyn mch, sau l cc k thut v nguyn tc hot ng ca cc mng chuyn mch knh v chuyn mch gi cng vi cc vn lin quan nh nh tuyn, nh s v cht lng dch v. Cc k thut chuyn mch mi trong mng tc cao c trnh by trong cc chng cui l s kt hp gia cc gii php cng ngh v gii php k thut, nhm th hin m hnh tng th ca cc cng ngh tin tin ang ng dng v trin khai trn mng vin thng hin nay.

1.2. CC KHI NIM V L THUYT C BN 1.2.1. Mt s khi nim c s. (i) nh ngha chuyn mch Chuyn mch l mt qu trnh thc hin u ni v chuyn thng tin cho ngi s dng thng qua h tng mng vin thng. Ni cch khc, chuyn mch trong mng vin thng bao gm chc nng nh tuyn cho thng tin v chc nng chuyn tip thng tin. Nh vy, theo kha cnh thng

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thng khi nim chuyn mch gn lin vi lp mng v lp lin kt d liu trong m hnh OSI ca T chc tiu chun quc t ISO. (ii) H thng chuyn mch Qu trnh chuyn mch c thc hin ti cc nt chuyn mch, trong mng chuyn mch knh thng gi l h thng chuyn mch (tng i) trong mng chuyn mch gi thng c gi l thit b nh tuyn (b nh tuyn). (iii) Phn loi chuyn mch Xt v mt cng ngh, chuyn mch chia thnh hai loi c bn: chuyn mch knh v chuyn mch gi. Mt khc, chuyn mch cn c chia thnh bn kiu: chuyn mch knh, chuyn mch bn tin, chuyn mch gi v chuyn mch t bo. Cc khi nim c s v cng ngh chuyn mch c th hin trong hnh 1.1 (a,b,c) di y.

Mng chuyn mch knh thit lp cc mch (knh) ch nh ring cho kt ni trc khi qu trnh truyn thng thc hin. Nh vy, qu trnh chuyn mch c chia thnh 3 giai on phn bit: thit lp, truyn v gii phng. thit lp, gii phng v iu khin kt ni (cuc gi) mng chuyn mch knh s dng cc k thut bo hiu thc hin. i ngc vi mng chuyn mch knh l mng chuyn mch gi, chia cc lu lng d liu thnh cc gi v truyn i trn mng chia s. Cc giai on thit lp, truyn v gii phng s c thc hin ng thi trong mt khong thi gian v thng c quyt nh bi tiu gi tin.

a, Chuyn mch knh; hai dng thng tin trn hai mch khc nhau.

b, Chuyn mch gi; cc tuyn ng c lp trn mng chia s ti nguyn

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c, Chuyn mch gi knh o; cc gi tin i trn knh o

Hnh 1.1: Cc kiu mng chuyn mch c bn (iv) K thut lu lng TE

K thut lu lng TE (Traffic Engineering) c coi l mt trong nhng vn quan trng nht trong khung lm vic ca h tng mng vin thng. Mc ch ca k thut lu lng l ci thin hiu nng v tin cy ca cc hot ng ca mng trong khi ti u cc ngun ti nguyn v lu lng. Ni cch khc, TE l cng c s dng ti u ti nguyn s dng ca mng bng phng php k thut nh hng cc lung lu lng ph hp vi cc tham s rng buc tnh hoc ng. Mc tiu c bn ca k thut lu lng l cn bng v ti u cc iu khin ca ti v ti nguyn mng thng qua cc thut ton v gii php k thut. (v) Bo hiu trong mng vin thng

Bo hiu s dng cc tn hiu iu khin truyn thng, trong mng vin thng bo hiu l s trao i thng tin lin quan ti iu khin , thit lp cc kt ni v thc hin qun l mng. Cc h thng bo hiu c th phn loi theo c tnh v nguyn tc hot ng gm: Bo hiu trong bng v bo hiu ngoi bng, bo hiu ng v bo hiu thanh ghi, bo hiu knh lin kt v bo hiu knh chung, bo hiu bt buc, v..v. Cc thng tin bo hiu c truyn di dng tn hiu in hoc bn tin. Cc h thng bo hiu trong mng chuyn mch in thoi cng cng PSTN (Public Switched Telephone Network) c nh s t No1-No7. (vi)Mng tch hp dch v s bng rng B-ISDN

Cung cp cc cuc ni thng qua chuyn mch, cc cuc ni c nh (Permanent) hoc bn c nh (Semi-Permanent), cc cuc ni t im ti im ti im hoc t im ti a im v cung cp cc dch v yu cu, cc dch v dnh trc hoc cc dch v yu cu c nh. Cuc ni trong B-ISDN phc v cho c cc dch v chuyn mch knh, chuyn mch gi theo kiu a phng tin (Multimedia), n phng tin (Monomedia), theo kiu hng lin kt (Connection-Oriented) hoc phi lin kt (Connectionless) v theo cu hnh n hng hoc a hng.

1.2.2. Cc m hnh ton hc ca lu lng L thuyt lu lng c nh ngha nh l ng dng ca l thuyt xc sut gii quyt cc

vn n lin quan ti k hoch, nh gi hiu nng, iu hnh v bo dng h thng vin thng. Ni mt cch tng qut hn, l thuyt lu lng c nhn nhn nh l quy tc lp k hoch mng, ni cc cng c (x l ngu nhin, hng i v m phng s) c a ra t cc nghin cu hot ng mng. Nhim v c bn ca l thuyt lu lng l s dng cc m hnh ton v a ra cc mi quan h gia cp dch v GoS (Grade of Service) v kh nng ca h thng thng qua cc cng c m hnh ho v m phng cho cc h thng thc t. [1]

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Lu lng trong k thut chuyn mch c m t qua cc s kin n ca cc thc th ri rc (yu cu chim knh, gi, t bo, v.v..), n c th m hnh ho bi tin trnh im. C hai dng tin trnh im l tin trnh m v tin trnh gia hai s kin n. Tin trnh m t=0{N(t)}

l mt

chui gi tr nguyn dng thi gian lin tc, vi N(t)= max{n:Tn t} l s s kin n trong thi gian (0:t]. Tin trnh gia hai s kin n l mt chui s thc ngu nhin n=1{An}

vi An=

Tn Tn-1 l thi gian gia hai s kin n th n v n-1. Lu lng c gi l lu lng t hp khi cc gi n theo tng nhm. c trng cho

nhm lu lng, k thut lu lng s dng tin trnh n theo nhm n=1{Bn} trong Bn l s

n v trong nhm. Tin trnh ti lm vic c m t qua chui n=1{Wn}

vi Wn l lng ti

phc v ca h thng ti s kin n th n. Bng 1.1 di y ch ra mt s ng dng c bn trong mng truyn thng c m t qua cc

m hnh lu lng vi cc hm phn b c bn.

ng dng M hnh Phn b

Thi gian tng tc phin Hm m

Thi gian phin Hm loga

Thi gian tng tc gi Hm Pareto

Telnet

Kch thc gi Kch thc nh

Thi gian tng tc phin Hm m FPT Kch thc Hm loga

Thi gian tng tc phin Hm m

Thi gian phin Hm loga

Thi gian tng tc gi Hng s

Thoi CBR

Kch thc gi C nh

Thi gian tng tc khung C nh Video VBR

Kch thc khung Phn b Gamma

Thi gian yu cu tng tc Hm m WWW

Kch thc gi Phn b Pareto

Bng 1.1: Mt s m hnh v hm phn b cho cc ng dng c bn a, Phn b Erlang

Phn b Erlang l mt phn b xc sut lin tc c gi tr dng cho tt c cc s thc ln hn zero v c a ra bi hai tham s: sc k (s t nhin;Int) v tham s t l (s thc; real). Khi tham s k =1 phn b Erlang tr thnh phn b m. Phn b Erlang l trng hp c bit ca phn b Gamma khi tham s k l s t nhin, cn trong phn b gamma k l s thc. Hm mt xc sut ca phn b Erlang c ch ra trn hnh 1.2 di y.

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Hnh 1.2: Hm mt xc sut ca phn b Erlang

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( ; , )( 1)!

k k kx ef x kk

= vi x>0

Nu s dng tham s nghch o = 1 / ta c:

1

( ; , )( 1)!

xk

kx ef x k

k

= vi x>0

Erlang l n v o lu lng ng dng rt nhiu trong k thut chuyn mch. Lu lng o bng Erlang tnh ton cp dch v GoS v cht lng dch v QoS (Quality of Service) trong GoS c coi l kha cnh v mt k thut ca cht lng dch v QoS. Hai cng thc tnh Erlang c s dng rng ri trong k thut lu lng tnh GoS l cng thc Erlang B v cng thc Erlang C, ngoi ra cn c cc cng thc nh Erlang B m rng v Engset.

Erlang B cho php tnh ton xc sut yu cu mt ngun ti nguyn s b t chi v l do thiu ti nguyn. M hnh lu lng Erlang B thng c s dng tnh ton trong bi ton thit k cc tuyn ni trong k thut chuyn mch knh, trn cng thc Erlang B (1.1) th hin xc sut mt ngun ti nguyn s b t chi. Cng thc tng qut c ch ra di y:

0

!

!

N

xN

x

ANP

Ax=

=

(1.1)

Trong :

N: s ti nguyn trong h thng A: Lu lng o bng Erlang

P: Xc sut b t chi Mc chim dng tuyn ni hoc thit b trong k thut chuyn mch knh thng c

o lng bng tc n cc cuc gi v thi gian chim gi th hin qua cng thc:

A s= (1.2) Trong :

A l lu lng tnh bng Erlang,

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l tc n ca cuc gi, s l thi gian chim gi trung bnh.

Thng thng, cc gi tr v s l cc gi tr trung bnh bi trong thc t cc cuc gi n v thi gian chim gi l ngu nhin, cc khong thi gian gia cc cuc gi n v phn b thi gian c th c xc nh qua phng php thng k, trn c s nhm xy dng mu m hnh lu lng (Bng tham chiu lu lng). Mt cch tip cn khc cng thng c s dng l da trn phng trnh trng thi cuc gi nhm tnh kh nng tc nghn ca thit b.

Erlang C cho php tnh ton thi gian i khi yu cu ti nguyn trong trng hp ti nguyn hn ch. M hnh lu lng Erlang C s dng rt nhiu trong k thut chuyn mch gi gn vi cc c ch hng i khc nhau, v d (M/M/1).

N

1N

0

A( 0)A !(1 )

!

xN

x

PA ANN x

=

> =+

(1.3)

Trong :

N: S ti nguyn trong h thng A: S lu lng yu cu P: Xc sut i ti thi im khi to t>0. b, Qu trnh Markov

Qu trnh Markov l mt qu trnh mang tnh ngu nhin (stochastic process) thng s dng m t cc h thng khng nh vi c tnh nh sau: trng thi ck ti thi im k l mt gi tr trong tp hu hn . Vi gi thit rng qu trnh ch din ra t thi im 0 n thi im N v rng trng thi u tin v cui cng l bit, chui trng thi s c biu din bi mt vect hu hn C = (c0,...,cN). Nu P(ck | c0,c1,...,c(k 1)) biu din xc sut (kh nng xy ra) ca trng thi ck ti thi im k khi tri qua mi trng thi cho n thi im k 1. Gi s trong qu trnh th ck ch ph thuc vo trng thi trc ck 1 v c lp vi mi trng thi trc khc. Qu trnh ny c gi l qu trnh Markov bc 1 (first-order Markov process). C ngha l xc sut trng thi ck xy ra ti thi im k, khi bit trc mi trng thi cho n thi im k 1 ch ph thuc vo trng thi trc, v d: trng thi ck1 ca thi im k 1. Khi ta c cng thc sau:

0 1 1 1( , ,..., ) ( )k k k kP c c c c P c c | = | (1.4) Ni tm li, mt h c thuc tnh Markov c nhn nhn l qu trnh Markov (bc 1). Nh vy, vi qu trnh Markov bc n, xc sut trng thi c th hin qua cng thc di y:

0 1 1 1( , ,..., ) ( ,..., )k k k k n kP c c c c P c c c | = | (1.5) Nu xc sut chuyn trng thi xy ra c cc gi tr nguyn (0, 1, 2..,n,,) th l chui

Markov ri rc v i ngc vi n l chui Markov lin tc. (i) Chui markov ri rc

Chui Markov thi gian ri rc bao gm mt tp hp cc trng thi v xc sut chuyn i gia chng ti nhng khong thi gian ri rc nhau. Vi yu cu xc sut chuyn i gia cc trng thi l mt hm ch ph thuc vo trng thi. S chuyn i ny khng cn xut hin ti nhng khong thi gian xc nh m ch tun theo mt quy lut thi gian no .

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Hnh 1.3: Chuyn tip hai trng thi chui Markov ri rc

Nu khng gian trng thi l hu hn, phn b xc sut chuyn trng thi c th c biu din di dng ma trn c tn gi l ma trn chuyn i trng thi. Ma trn chuyn i trng thi P c cu thnh t cc phn t (i,j) th hin qua cng thc pij = P(ck+1=jck=i), nu chui markov l chui markov thi gian thun nht th ma trn chuyn i trng thi P ch ph thuc vo k.

K thut chui Markov thi gian ri rc p dng cho cc s trng thi tu , m cc s ny lin kt cc i tng khc nhau vi mt s iu kin. N mt s trng thi khng th chuyn n trng thi khc th tin trnh mc li, bi v chui ny c rt gn thnh 2 hay nhiu hn 2 chui ring l. Vic tng s lng trng thi s m t h thng chnh xc hn nhng cng lm tng phc tp tnh ton. (ii) Chui Markov thi gian lin tc

Xt mt h thng a ngi s dng kt ni ti mt b nh tuyn hay truy nhp thit b chuyn mch. Mt kiu m hnh cho loi h thng ny l xem cc s kin xy ra ti cc khong thi gian rt nh. Khi gia s thi gian tin ti 0, gi tr gn ng l m hnh cho thi gian lin tc. Tuy nhin, by gi chng ta phi s dng cc php tnh vi phn ton hc thay cho nhng php nhn xc sut n gin c s dng phn tch chui Markov thi gian ri rc. Xt h thng c s trng thi c th xy ra l j=0,1,2,....n. Trc ht, ta xc nh xc sut chuyn i trng thi t trng thi i sang trng thi j vi thi gian t h thng ang trng thi i theo cng thc:

( ) tqtttq ijij =+ ., (1.6) Tham s qij l xc sut m h thng chuyn t trng thi i sang trng thi j trong khong thi gian v cng nh t. t qij nh l tc chuyn i trng thi. Ta quy v trng hp chung l tc chuyn i ca chng nhn cc gi tr khc nhau ph thuc vo trng thi ca h thng nh mt tin trnh MMPP (Markov modulate Poisson Process). Do c tnh ca h thng Markov ch ph thuc vo khong thi gian tng so vi trc (t), v vy kt qu trn ng vi tt c cc gi tr ca thi gian t. By gi, nh ngha trng thi h thng bi gi tr ngu nhin x(t)=j vi mt xc sut cho di y :

])(Pr[)( jtxtj == Chng ta c th a ra xc sut h thng trng thi j ti thi gian t nh sau (xc sut chuyn

trng thi c ch ra trn hnh 1.4):

( ) ( ) ( )[1 ]j j ij j iji j k j

t t t q t t q t

+ = + (1.7) Trong , phn u ca v phi phng trnh 1.7 trn l xc sut m h thng ang trng

thi khc v chuyn v trng thi j trong khong thi gian t. Phn sau ca v phi l xc sut m h thng vn trng thi j trong sut thi gian t.

P00=1-a P10=b

P11=1-b 0 1

P01=a

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Hnh 1. 4: S chuyn i trng thi trong chui Markov thi gian lin tc M hnh chui Markov c ng dng trong k thut chuyn mch trong cc bi ton m hnh

ho lu lng, tnh ton kh nng tc nghn, cp phc v GoS ca trng chuyn mch v mt s vn iu khin khc. Tuy nhin, vi mng a dch v mt s bi ton lu lng phi c xt m hnh thch hp hn, v d nh m hnh lu lng t tng ng. V d di y ch ra mt ng dng ca m hnh Markov trong tnh ton bi ton tc nghn.

Nh phn trn trnh by, trong thc t cc cuc gi n v thi gian chim gi l ngu nhin v vy s c rt nhiu dng m hnh mu lu lng khc nhau. Gi thit tng s lng thit b l N v i biu din s cc cuc gi ang trong qu trnh x l, i s tng ng vi s thit b bn v thng c gi l trng thi ca mng v h thng s tc nghn khi i=N. m t kh nng tc nghn ta gi thit: Cc cuc gi n c lp; Tc n ca cuc gi khi mng trong trng thi i l i; Tc i ca cuc gi khi trng thi i l i; Ch mt s kin (cuc gi n hoc i) xy ra trong mt thi im.

Lu trng thi c th hin trong hnh 1.5 di y:

Hnh1.5: Lu chuyn trng thi cuc gi

Trng thi (N+1) c coi l trng thi tc nghn, v vy ( 0 i N ) vi i: 1, 2, 3 i lin quan trc tip ti lu lng yu cu, ic xc nh bi c tnh ca t nhin lu lng. xc nh xc sut ca h thng trong trng thi i, ta phi tm cc xc sut trng thi. Xc sut trng thi h thng trong trng thi i ti thi gian t+dt c xc nh bng tng cc xc sut sau: Xc sut h thng trong trng thi i v thi im t vi iu kin khng c cuc gi n

hoc i. Xc sut h thng trong trng thi i -1 ti thi im t v mt cuc gi n trong thi gian

dt.

t t+t

n n-1 . . j . . 1

qnj

qoj

1-qjk.t

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Xc sut h thng trong trng thi i+1 ti thi im t v mt cuc gi gii phng trong thi gian dt.

Ta c biu thc biu din xc sut trng thi h thng [i] ti i nh sau:

[i]t + dt = [i]t (1-i dt - i dt) + [i +1]t (i +1 dt) +[i -1]t (i-1 dt) hay

( [i]t + dt - [i]t )/ dt = - (i+i) [i]t + i +1 [i +1]t + i-1[i -1]t ; (1.8) i= 1, 2, 3, ..

Ta xt i = 0 l trng hp c bit v 0 =0 (khng c cuc gi i t trng thi 0), -1 = 0 (khng tn ti trng thi -1). Phng trnh ti cng thc 1.8 trn tr thnh:

0 t 1 t

[ 0 ] [ 0 ] [ 0 ] [1 ]t d t ttd

+ = + ; Cho dt tin ti 0

t0 t 1 t

t

[ 0 ] [ 0 ] [ 1 ]dd = +

m rng ti trng thi i ta c:

( )t 1t 1 t tt

[i] [i] [i +1] [i -1]d i ii id += + + +

Nu ta gi thit s c lp ca cc xc sut trng thi l bng 0 th d[i]/ dt = 0; ta c:

[ ] [ ] [ ] [ ] [ ]0 1 01 1 20 1 ; ( ) 1 0 2 = + = + (1.9) dng tng qut nh sau:

( )[ ] [ ] [ ]1 11 1i ii ii i i ++ = + + Ta c: [ ] [ ] [ ] [ ]1 22 31 2 ; 2 3 ;.... = = (1.10) Thay cc gi tr t cng thc 1.10 vo cng thc 1.9 c cng thc tng qut sau:

[ ] [ ]1 1i ii i = (1.11) Cng thc 1.11 th hin khi nim cn bng trng thi trong h thng vi s lng cuc gi

n h thng cn bng vi s lng cuc gi i ra khi h thng khi h thng trong trng thi i. Cng thc trn thng c s dng trong tnh ton xc sut tc nghn v thi gian tc nghn ca cc cuc gi trong h thng chuyn mch knh. c, Phn b Poisson Phn b Poisson l phn b xc sut ri rc, n m t xc sut ca mt s cc s kin xy ra trong mt khong thi gian nu cc s kin ny xy ra vi mt tc trung bnh bit trc, v c lp vi thi gian xy ra s kin cui pha trc. Qu trnh Poisson l mt chui Markov vi thi gian lin tc. Hm mt xc sut ca phn b Poisson c ch ra trn hnh 1.6.

11

Hnh 1.6: Hm phn b mt xc sut ca phn b Poisson

( )( ) ( ; )!

t k

r te tP N k f k t

k

= = = (1.12) Trong :

: Tc n trung bnh Nt: S lng s kin trc thi im t k: S lng cc s kin

Nu thi gian i T ca bin c u tin l mt bin ngu nhin lin tc vi phn b hm m ( ) th cng thc 1.12 c th rt gn thnh cng thc 1.13 di y:

( ) ( 0) tr r tP T t P N e> = = = (1.13)

Mt qu trnh Poisson l mt qu trnh ngu nhin c nh ngha theo s xut hin ca cc bin c. Mt qu trnh ngu nhin N(t) l mt qu trnh Poisson (thi gian-thun nht, mt chiu) nu: S cc bin c xy ra trong hai khong con khng giao nhau l cc bin ngu nhin c

lp. Xc sut ca s bin c trong mt khong con [t,t + ] no c cho bi cng thc

1.13. Bin ngu nhin Nt m t s ln xut hin trong khong thi gian [t,t + ] tun theo mt phn b Poisson vi tham s .

Cc qu trnh Poisson thi gian thun nht (time-homogeneous) c xut pht t cc qu trnh Markov thi gian lin tc thi gian thun nht.

S cuc in thoi ti tng i trong mt khong thi gian xc nh c th c mt phn b Poisson, v s cuc in thoi ti trong cc khong thi gian khng giao nhau c th c lp thng k vi nhau. y l mt qu trnh Poisson mt chiu. Trong cc m hnh n gin, ta c th gi thit mt t l trung bnh l hng s, v d = 12.3 cuc gi mi pht. Trong trng hp , gi tr k vng ca s cuc gi trong mt khong thi gian bt k l t l trung bnh nhn vi khong thi gian, t. Trong cc bi ton thc t hn v phc tp hn, ngi ta s dng mt hm t l khng phi l hng s: (t). Khi , gi tr k vng ca s cuc in thoi trong khong gia thi im a v thi im b l ( )ba t d t .

12

1.2.3. L thuyt hng i L thuyt hng i l mt trong cc cng c ton hc mnh cho vic phn tch c lng trong

cc h thng vin thng v cc mng my tnh. L thuyt hng i thng thng c p dng cho cc h thng l tng a ra kt qu gn ng cho mt m hnh thc t. Tnh cht chung ca cc gii php ng dng l thuyt ny l lm r hn cc c trng lu lng, cung cp d bo nhng ranh gii ln hn trn nhng kt qu nghin cu nht nh v chng c th rt hu ch trong vic kim tra tnh chnh xc v hp l ca cc gi thit thng k. L thuyt hng i l mt hng pht trin ca l thuyt xc sut nghin cu cc qu trnh lin quan n hng i v cung cp cc phng php phn tch hoc dng thc ng (closed form) trong vi lnh vc nht nh.

L thuyt hng i khi pht mt cch t nhin trong vic nghin cu cc mng chuyn mch knh v chuyn mch gi. Trong mng chuyn mch knh cuc gi s n mt phng tin chuyn mch theo mt kiu ngu nhin, mi cuc gi s gi mt knh trong mt thi gian ngu nhin no v qu trnh ch c phc v ca cc cuc gi ng dng l thuyt hng i. Trong mng chuyn mch gi, cc bn tin c di bin i c chuyn qua mng, cc phng tin truyn dn (cc chuyn mch v kt ni) c chia s bi cc gi. Thi gian s dng trong b m l mt trong nhng tiu chun hot ng mng. D nhin thi gian ny ph thuc thi gian x l, di bn tin hoc mt ngi s dng n mt phng tin phc v (nt v tuyn lin kt) v b buc phi i khi phng tin ny bn. V vy, hng i l mt gii php khng th thiu trong cc k thut chuyn mch lu m v chuyn tip.

Trong ng dng tng tc v thi gian thc, thng th thi gian tr li trung bnh c xem nh mt tiu chun thc hin quan trng, trong khi cc ng dng khc, thng lng h thng l tin chun thc hin c bn. Cc m hnh phn tch da trn phn tch hng i c s dng d kin s thc hin ca nhiu vn thc t.

Cho d l thuyt hng i phc tp v mt ton hc th vic phn tch hot ng ca mt h thng mng s dng m hnh hng i c th n gin i rt nhiu. c th p dng vo thc tin, chng ta cn nhng kin thc v khi nim thng k c bn v hiu bit v vic p dng l thuyt hng i. M t hng i Kendall D.G.Kendall xut k hiu cho cc h thng hng i: A/B/X/Y/Z. Thng thng m hnh Kendall c xt theo nguyn tc n trc phc v trc FCFS, n sau phc v trc LCFS, phc v theo th t ngu nhin SIRO v chia s x l PS. Trong :

A: phn b thi gian tin trnh n B: phn b thi gian phc v X: s lng server Y: dung lng tng cng ca h thng Z: s lng cc khch hng

Cc h thng hin i ph thuc rt ln vo phn mm v x l i vi cc cng vic iu khin v qun l h thng. Trong cc h thng chuyn mch gi, cc b m c s dng cho cc nhim v xp hng i x l. Tt c cc hng i u c th phn tch bi cc l thuyt hng i, l mt tp cc m hnh ton nhm tm ra cc gi tr o lng ( v d: thi gian tr). Phn tch di y ch ra mt c ch hng i thng thng nht; hng i M/M/1:Thi gian ca tin

13

trnh n c phn b theo hm m m hay theo tin trnh Poisson ( thc cht l tin trnh khng nh hoc c tnh Markov) A := M.

Thi gian n ca cc s kin theo tin trnh Markov Thi gian phc v phn b theo hm m m B := M. H thng ch c 1 server. Hng i c khng gian m l v hn.

Ta c xc sut h thng trong trng thi i c xc nh trong cng thc sau:

[ ] [ ]1i

Ai is = (1.14)

V c 1 server nn l c nh v c lp vi i, thm vo ta c .A s= (1.2) nn: i= v .A s= thay vo cng thc 1.14 ta c. [ ] [ ]. 1

i

si is

= v [ ] [ ] [ ] ( ) [ ]21 0 ; 2 0 = = . (1.15) T ct 1.15 ta rt ra cng thc tng qut :

[ ] ( ) [ ]0ii = hay [ ]0

1;i

i

== ( ) [ ]

0

0 1;i

i

==

Hay:

[ ] ( )0

10 i

i

=

=

v [ ] ( )( )0

i

i

i

i

=

=

(1.16)

Gi thit tc n nh hn tc phc v trnh cho hng i tng ln v hn, gi hiu sut s dng l = / . T cng thc 1.16 ta c: [ ]

0

i

i

i

i == (1.17)

Mu s c th trin khai di dng (1- -1); nn ta vit li cng thc 1.17 nh sau: [ ] ( )1ii = (1.18) Gi N l gi tr trung bnh ca s khch hng n trong hng i c tnh bng tng cc xc sut trng thi h thng, ta c:

[ ] 20 0

(1 ). (1 )(1 ) 1

i

i iN i i i

= =

= = = = hoc N = (1.19) Thi gian i trung bnh (T) ca gi trong h thng l thi gian tng ca thi gian trong hng i cng vi thi gian phc v. T c th quan h vi N theo cng thc sau: T= .N , thay N t cng thc 1.19 ta c:

T= ( - ) -1 (1.20) T cng thc 1.20 ta c quan h thi gian i vi tc n v i ca cc gi c th hin qua hnh v 1.7 di y.

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Hnh 1.7: Thi gian tr ph thuc vo tc n v tc phc v ca hng i

Nh vy, tc n ca cc s kin n tng dn t 1/ v t ti trng thi bo ho khi tc n bng tc phc v . 1.3. QU TRNH PHT TRIN CA K THUT CHUYN MCH Vo khong thp nin 60 ca th k 20, xut hin sn phm tng i in t s l s kt hp gia cng ngh in t vi k thut my tnh. Tng i in t s cng cng u tin ra i c iu khin theo chng trnh ghi sn SPC (Stored Program Control),c gii thiu ti bang Succasunna, Newjersey, USA vo thng 5 nm 1965. Trong nhng nm 70 hng lot cc tng i thng mi in t s ra i. Mt trong nhng tng i l tng i E10 ca CIT Alcatel c s dng ti Lannion (Php). V thng 1 nm 1976 Bell gii thiu tng i in t s cng cng 4ESS. Hu ht cho n giai on ny cc tng i in t s u s dng h thng chuyn mch l s v cc mch giao tip thu bao thng l Analog, cc ng trung k l s. Mt trng hp ngoi l l tng i DMS100 ca Northern Telecom a vo nm 1980 dng ton b k thut s u tin trn th gii. H thng 5ESS ca hng AT&T c a vo nm 1982 ci tin rt nhiu t h thng chuyn mch 4ESS v c cc chc nng tng thch vi cc dch v mng s tch hp dch v ISDN (Integrated Service Digital Network). Sau hu ht cc h thng chuyn mch s u a ra cc cu hnh h tr cho cc dch v mi nh ISDN, dch v cho mng thng minh v cc tnh nng mi tng thch vi s pht trin ca mng li.

Khong nm 1996 khi mng Internet tr thnh bng n trong th gii cng ngh thng tin, n tc ng mnh m n cng nghip vin thng v xu hng hi t cc mng my tnh, truyn thng, iu khin. H tng mng vin thng tr thnh tm im quan tm trong vai tr h tng x hi. Mt mng c th truyn bng rng vi cc loi hnh dch v thoi v phi thoi, tc cao v m bo c cht lng dch v QoS (Quality Of Service) tr thnh cp thit trn nn tng ca mt k thut mi: K thut truyn ti khng ng b ATM (Asynchronous Transfer Mode). Cc h thng chuyn mch in t s cng phi dn thay i theo hng ny cng vi cc ch tiu k thut, giao thc mi. Mt v d in hnh l cc h thng chuyn mch knh khi cung cp cc dch v Internet s c tin cy khc so vi cc cuc gi thng thng vi thi gian chim dng cuc gi ln hn rt nhiu, v cng nh vy i vi cc bi ton lu lng. S thay i ca h tng mng chuyn i sang mng th h k tip NGN v ang tc ng rt ln ti cc h thng chuyn mch, di y trnh by mt s vn lin quan ti mng NGN v cc c im ca qu trnh hi t mng ca h tng mng cng cng. Mng chuyn mch knh cng cng PSTN v IP (Internet Protocol) ang dn hi t ti cng mt mc tiu nhm hng ti mt h tng mng tc cao c kh nng tng thch vi cc ng dng a phng tin tng tc v m bo cht lng dch v. Hnh 1.8 di y ch ra xu hng hi t trong h tng mng cng cng:

15

Hnh 1.8: Xu hng hi t cng ngh mng cng cng

S khc bit ny bt u t nhng nm 1980, PSTN chuyn hng tip cn sang phng thc truyn ti bt ng b ATM h tr a phng tin v QoS, sau chuyn hng sang cng ngh kt hp vi IP chuyn mch nhn a giao thc hin nay. Trong khi Internet a ra mt tip cn hi khc vi PSTN qua gii php trin khai kin trc phn lp dch v CoS (Class Of Service) v hng ti m bo cht lng dch v QoS thng qua m hnh tch hp dch v IntServ v phn bit dch v DiffServ, cc chin lc ca Internet theo hng tng thch vi IP, mng quang v hng ti mng chuyn mch nhn a giao thc tng qut GMPLS (Generalized MultiProtocol Label Switch). Cng ngh chuyn mch nhn a giao thc MPLS ra i vo nm 2001 l s n lc kt hp hai phng thc chuyn mch hng kt ni (ATM, FR) vi cng ngh chuyn mch phi kt ni (IP), cng ngh chuyn mch nhn a giao thc MPLS nh ngha khi nim nhn (Label) nm trn mt lp gia lp 2 v lp 3 trong m hnh OSI, vi mc tiu tn dng ti a cc u im ca chuyn mch phn cng (ATM, FR) v s mm do, linh hot ca cc phng php nh tuyn trong IP. Mt s quc gia c h tng truyn ti cp quang pht trin tt c xu hng s dng cc k thut chuyn mch quang v s dng cc cng ngh trn nn quang nh GMPLS, IP qua cng ngh ghp bc sng quang WDM (Wavelength Division Multiplexing), kin trc chuyn mch trong mng th h k tip NGN

Trong mi trng mng hin nay, s phn cp h thng thit b bin (ni ht), thit b qu giang v thit b li trong mng cung cp cc dch v PSTN vn ang tn ti. Cc mng bao trm nh FR, ATM v Internet ang c trin khai song song v to ra nhu cu kt ni lin mng. Cc truy nhp cng thm gm cp ng, cp quang v truy nhp khng dy ang c trin khai lm a dng v tng mt truy nhp t pha mng truy nhp.

S tng trng ca cc dch v truy nhp to nn sc p v t ra 3 vn chnh i vi h thng chuyn mch bng rng a dch v: Truy nhp bng thng rng, s thng minh ca thit b bin v truyn dn tc cao ti mng li. Cc thit b truy nhp bng thng rng bao gm cc thit b h tng mng truyn thng (tng i PSTN ni ht) v cc module truy nhp ng dy s DSLAM (Digital Subcriber Line Access Mutiplexer) phi truyn ti v nh tuyn mt s lng ln cc lu lng thoi v d liu ti thit b g mng. Cc thit b g mng hin c rt nhiu dng gm VoiP Cc cng truy nhp cho thit b VoiP (Voice Over IP), cng trung k, chuyn mch ATM, b nh tuyn IP v cc thit b mng quang. Cc thit b bin cn phi h tr cc chc nng nhn thc, cp quyn v ti khon AAA (Authentificaton, Authorization v Accounting) cng nh nhn dng cc lung lu lng t pha khch hng, v vy vic qun l v iu hnh thit b bin l mt vn rt phc tp.

16

Hnh 1.9: Cc thit b chuyn mch trong m hnh mng cng cng in hnh

Vi mi trng mng PSTN trc y, cc thit b li mng chu trch nhim chnh trong iu hnh v qun l v iu ny c thay i chc nng cho cc thit b g mng trong mi trng NGN. Cc h thng chuyn mch a dch v cn phi h tr cc chuyn mch lp 3 trong khi vn phi duy tr cc chuyn mch lp 2 nhm h tr cc dch v ATM v FR truyn thng, c tin cy cao v phi tch hp tt vi cc h tng c sn.

Hn na, cc h thng chuyn mch phi c mm do ln nhm tng thch v p ng cc yu cu tng trng lu lng t pha khch hng. V vy, c ch iu khin cc h thng chuyn mch c pht trin theo hng phn lp v module ho nhm nng cao hiu nng chuyn mch v m bo QoS t u cui ti u cui. Hng tip cn my ch cuc gi CS (Call Server) v hng trin khai phn h a dch v IP (IMS) c trnh by di y ch ra nhng s thay i ln trong lch s pht trin h thng chuyn mch.

1.3.1. Chuyn mch mm v hng tip cn my ch cuc gi CS. Hng tip cn my ch cuc gi CS c hnh thnh trong qu trnh chuyn i cc h tng

mng chuyn mch knh sang chuyn mch gi trong mng PSTN. thc hin qu trnh chuyn i v truyn thoi trn nn IP, mt gii php c th thc thi l to ra mt thit b lai c th chuyn mch thoi c dng knh v gi vi s tch hp ca phn mm x l cuc gi. iu ny c thc hin bng cch tch ring chc nng x l cuc gi khi chc nng chuyn mch vt l. Thit b B iu khin cng a phng tin MGC (Media Gateway Controller) c coi l thnh phn mu cht trong gii php k thut chuyn mch mm Softswitch.

Thc cht ca khi nim chuyn mch mm chnh l phn mm thc hin chc nng x l cuc gi trong h thng chuyn mch c kh nng chuyn ti nhiu loi thng tin vi cc giao thc khc nhau (chc nng x l cuc gi bao gm nh tuyn cuc gi v qun l, xc nh v thc thi cc c tnh cuc gi). Theo thut ng chuyn mch mm th chc nng chuyn mch vt l c thc hin bi cng a phng tin MG (Media Gateway), cn x l cuc gi l chc nng ca b iu khin cng a phng tin MGC. Mt s l do chnh cho thy vic tch 2 chc nng trn l mt gii php tt:

17

Cho php c mt gii php phn mm chung i vi vic x l cuc gi. Phn mm ny c ci t trn nhiu loi mng khc nhau, bao gm c mng chuyn mch knh v mng gi (p dng c vi cc dng gi v mi trng truyn dn khc nhau).

L ng lc cho cc h iu hnh, cc mi trng my tnh chun, tit kim ng k trong vic pht trin v ng dng cc phn mm x l cuc gi.

Cho php cc phn mm thng minh ca cc nh cung cp dch v iu khin t xa thit b chuyn mch t ti tr s ca khch hng, mt yu t quan trng trong vic khai thc tim nng ca mng trong tng lai.

Chuyn mch mm thc hin cc chc nng tng t chuyn mch knh truyn thng nhng vi nng lc mm do v cc tnh nng u vit hn. Cc u im ca chuyn mch mm mang li l do vic chuyn mch bng phn mm da trn cu trc phn tn v cc giao din lp trnh ng dng m.

Trong chuyn mch truyn thng, phn cng chuyn mch lun i km vi phn mm iu khin ca cng mt nh cung cp. iu ny lm tng tnh c quyn trong vic cung cp cc h thng chuyn mch, khng cung cp mt mi trng kin to dch v mi, lm gii hn kh nng pht trin cc dch v mi ca cc nh qun tr mng. Khc phc iu ny, chuyn mch mm a ra giao din lp trnh ng dng m API (Application Programable Interface), cho php tng thch phn mm iu khin v phn cng ca cc nh cung cp khc nhau. iu ny cho php cc nh cung cp phn mm v phn cng c c ting ni chung v tp trung vo lnh vc ca mnh. Vi cc giao din lp trnh m, chuyn mch mm c th d dng c nng cp, thay th v tng thch vi ng dng ca cc nh cung cp khc nhau.

Chuyn mch mm c xy dng trn c s mng IP, x l thng tin mt cch trong sut, cho php p ng nhiu loi lu lng khc nhau. c xy dng theo cu hnh phn tn, tch cc chc nng khc khi chc nng chuyn mch cng lm cho nhim v chuyn mch tr nn n gin hn v do nng lc x l mnh m hn. Cng ngh chuyn mch mm lm gim tnh c quyn ca cc nh cung cp, gp phn tng tnh cnh tranh v do gim gi thnh ca h thng chuyn mch mm.

1.3.2. Hng tip cn phn h a phng tin IP (IMS) thc hin hi t gia mng di ng vi mng c nh theo hng IP ho hon ton, mng th h k tip NGN ng dng ti mng 3G (Third Generation) trong nhiu cch. Vo nm 2000, 3GPP( 3rd Generation Partnership Project) thit lp cc c tnh ca WCDMA R4 (Wireless Code Division Multiple Access Release 4), l ln u tin a khi nim chuyn mch mm vo trong h thng mng li di ng. S thay i ny nh hng ti kin trc mng, cc giao din mng, s pht trin ca cc dch v trong h thng thng tin di ng hng s pht trin ca 3G ti NGN. Trong kin trc mng, NGN v 3G u nhm chuyn hng tch bit gia lp iu khin v lp knh mang trong cc giao thc giao tip. 3G v NGN a ra rt nhiu giao thc nh: Giao thc iu khin c lp knh mang BICC (Bearer Independent Call Control), Giao thc khi to phin SIP/SIP-T (Session Initiation Protocol), giao thc iu khin bo hiu H.248/Megaco, giao thc truyn ti bo hiu trong nn IP (SIGTRAN),v..v. 3G v NGN khng ch cung cp cc dch v nh thoi m cn l cc dch v a phng tin thng qua cc giao din dch v m. iu ny to kh nng kin to cc dch v mi qua cc nh cung cp th 3. Cc dch v c th kin to ng thi trn c vng mng c nh v v vng mng di ng. Cc phin bn R4/R5 u nh hng theo kin trc NGN nh vy, khi kin trc chuyn mch mm c ng dng trong vng NGN 3G, n c gi l chuyn mch mm di ng. Gii php tch hp v h

18

tr cc dch v IP trong di ng c thc hin qua phn h IMS (Internet Multimedia Subsystem) nm ti bin vng mng c nh v di ng. Tm tt chng 1 Ni dung c bn ca chng 1 gii thiu cc khi nim v thut ng c bn s dng trong k thut chuyn mch, mt s c s ton s c s dng trong tnh ton mt s tham s ca trng chuyn mch, lch s v kin trc mng chuyn mch hin nay v hng tip cn ca cc cng ngh chuyn mch hin i.

19

Chng 2 K THUT CHUYN MCH KNH Trong chng k thut chuyn mch knh s trnh by vi ngi c cc vn lin quan ti mng chuyn mch in thoi cng cng PSTN, gm cc khun dng tn hiu s, phng php chuyn i cc tn hiu thoi tng t sang s, cu trc khung truyn dn theo tiu chun Chu u v Bc m. Tm im quan trng nht ca k thut chuyn mch knh l cc h thng chuyn mch trong l cc trng chuyn mch, v vy cc c im mu cht ca k thut chuyn mch knh c trnh by qua cc nguyn l, nguyn tc cu to v phng thc hot ng ca cc trng chuyn mch gm c: chuyn mch thi gian, chuyn mch khng gian v chuyn mch ghp. Phng php ghp ni a tng chuyn mch v nguyn l ghp ni c bn ch ra cch thc tip cn v xy dng cc h thng chuyn mch thc t. Phn cui ca chng 2 trnh by hai trng chuyn mch trong h thng chuyn mch thc t ang khai thc ti Vit nam.

2.1. C S K THUT CHUYN MCH KNH K thut chuyn mch knh da trn nguyn tc thit lp knh ni dnh ring cho cc cuc ni phc v cho qu trnh truyn tin qua mng. K thut chuyn mch knh ng vai tr quan trng trong cc h thng mng vin thng k t mng chuyn mch in thoi cng cng truyn thng PSTN n cc mng quang hin i. Trong chng 2 ca cun ti liu ny s trnh by cc vn k thut chuyn mch knh c ng dng trong mng in thoi cng cng PSTN v tp trung vo cc nguyn l c bn ca chuyn mch knh tn hiu s. H tng mng PSTN truyn thng phc v cho truyn thng thoi thi gian thc v lin tc. Cc h thng chuyn mch kt ni vi nhau qua cc phng tin truyn dn to thnh mng chuyn mch, nhim v ca cc h thng chuyn mch l thc hin vic kt ni cho cc cuc gi t u cui ti u cui mng, bao gm c cc ng dn bo hiu v ng dn thng tin. Qu trnh kt ni thc hin theo yu cu ca thit b u cui v nh tuyn da trn a ch c nh s theo tiu chun E.164 ca ITU-T [2] Chuyn mch knh tn hiu s l qu trnh kt ni, trao i thng tin s trong cc khe thi gian c phn chia theo phng thc chia thi gian TDM. Vi mc tiu tm hiu v nguyn tc hot ng ca cc h thng chuyn mch knh, ta xem xt ln lt cc vn : Qu trnh chuyn i tn hiu tng t sang tn hiu s theo nguyn l PCM (Pulse Code Modulation), trao i ni dung thng tin trong cc khe thi gian, lu lng ca mng chuyn mch knh v nguyn l cu trc ca chuyn mch khng gian v thi gian trong h thng chuyn mch knh, k thut ghp ni trng chuyn mch v cc trng chuyn mch trong thc t.

2.1.1. K thut iu ch xung m PCM K thut iu ch xung m PCM l mt qu trnh gm nhiu bc nhm thc hin vic chuyn i tn hiu tng t thnh tn hiu s, mc d k thut ny c th ng dng vi mt s dng tn hiu tng t khc nhau nh: thoi, video. Nhng trn thc t k thut ny thng c ng dng cho tn hiu thoi trong mng PSTN. K thut iu ch xung m PCM ra i nm 1969, cc c tnh k thut c m t trong khuyn ngh G.711 ca ITU-T nm 1988 (sch xanh). Mt h thng PCM 30 c m t trong hnh 2.1 di y, h thng gm 30 knh thoi c gii hn tn s trong khong (0.3kHz 3.4kHz) v ly mu ti tn s 8 kHz. Cc tn hiu xung s ca tn hiu tng t sau qu trnh ly mu c m ho di dng nh phn

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v truyn i trong h thng truyn dn. Ti u cui thu tn hiu, mt qu trnh hon nguyn tn hiu c thc hin nhm khi phc li dng tn hiu pha pht.

Hnh 2.1: H thng PCM in hnh

Trc khi xem xt cc c tnh ca cc bc bin i ta xem xt th hin n gin qua hnh v 2.1 .

Hnh 2.2: Cc bc bin i trong nguyn l PCM

c trng ca k thut bin i tn hiu thoi tng t sang tn hiu s c th chia thnh cc bc c m t nh sau: (i) Ly mu tn hiu. Di ph ca tn hiu thoi bao gm nhiu loi tn s c khong tn s t 50Hz ti 20 kHz. Tuy nhin, ph tn s c vng nng lng tp trung ln vo khong 0.8kHz 1.2 kHz. gim thiu bng tn truyn dn, k thut PCM s dng di ph t 0.3kHz (fmin) 3.4 kHz (fmax) v chn tn s tn hiu ly mu 8kHz tun th theo nh l Niquist fs fmax [2]. Phng php ly mu trong k thut PCM l phng php ly mu theo bin tn hiu, cc xung ly mu c gi l xung PAM ( Pulse Amplitude Modulation). Cc xung tn hiu sau khi ly mu c a vo h thng ghp knh phn chia theo thi gian TDM (Time Division Multiplexer), nh vy khong thi gian gia cc tn hiu ly mu T s bng ng tn s ly mu v bng 125S. T = 1fs = 1/8.000Hz = 125S.

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(ii) Lng t ho. Lng t ho l mt qu trnh bin i cc mu tn hiu tng t thnh cc gi tr ri rc truyn trong cc h thng s. Cc mu tn hiu PAM c so snh vi mt tp hu hn cc mc lng t xc nh v gn cc mu xung PAM vo cc mc lng t tng ng. Gi thit khong cch gia hai mc lng t l V th khong cch im gia (Vj) ca hai mc lng t cng l V. Bt k mu tn hiu no trong vng Vj V2 u c nhn cng mt mc lng t. Qu trnh lm trn ny s gy ra cc sai s trong qu trnh bin i thng tin t tng t sang s. Cc li ny l cc li ngu nhin v c gi l nhiu lng t, tnh ton c nhiu lng t cn phi nm r hm mt xc sut bin ca tn hiu u vo ly mu. gim nhiu lng t c th s dng phng php tng s lng mc lng t, nhng iu ny s ko theo s lng cc thng tin cn phi truyn i, ng ngha vi vic tng bng thng cho tn hiu. Mt tip cn khc da theo c tnh ca dng sng tn hiu l phng php lng t ho khng u, cc tn hiu thoi c c tnh xc sut xut hin bin nh ln hn nhiu so vi xc sut xut hin bin ln. V vy, hm s loga c chn cho hm phn b mc lng t. Trong thc tin, ngi ta s dng k thut nn-gin tn hiu (COMPANDING) tc l thc hin vic nn mu tn hiu thc hin m ho v cui cng l gin tn hiu vi cc tham s ngc vi cc tham s nn hon nguyn tn hiu. Qu trnh nn c thc hin theo hm loga, cc mu tn hiu ln c nn nhiu hn cc mu tn hiu nh nhm a t s Tn hiu/nhiu (S/N) ca cc gi tr tn hiu u vo v gi tr khng i. Hai tiu chun ca ITU-T s dng cho k thut nn-dn l lut A v lut . Lc nn gin tn hiu theo lut A v lut c khuyn ngh trong tiu chun G.711 ca ITU-T nn cc mc tn hiu 16 bit theo phng php lng t ho tuyn tnh thnh 8 bit. Lut A c m t qua cng thc 2.1 di y.

A* |x| 1 + ln (A) 0 |x| < 1A (2.1) F (x) =

Sgn(x) * ( 1 + ln( A|x|) 1 + ln (A) 1A |x| 1 Trong : Sgn (x) l hm du A l hng s xc nh mc nn A = 87,6.

A|x| l hm nn tn hiu x gi tr mu a vo nn (nguyn) Cng thc 2.1 gm hai thnh phn cho mc tn hiu trong khong (0 1/A) c dng tuyn tnh, v mc tn hiu ln (1/A 1) c dng logarithm v s c ly xp x ho theo cc segments (on) tuyn tnh.Vng tuyn tnh m bo khi tn hiu vo x = 0 th tn hiu ra F(x) = 0; Vng phi tuyn m bo khi tn hiu vo x =1 th tn hiu ra F(x)=1. K thut PCM s dng 256 mc lng t cho tn hiu thoi v c chia thnh 2 cc tnh dng (+) v m (-) thng qua bit du. Nh vy, mi na ca tn hiu s gm 128 mc lng t. S lng bit s dng biu din cc mc lng t l 8 bit( xem v d trn hnh 2.3). c tnh nn theo lut A s dng 8 on

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cho cc tnh (+) v 8 on cho cc tnh (-), cc bc lng t c tng theo c s 2. Trong mi on s dng 4 bit biu din 16 mc lng t u.

Hnh 2.3: c tuyn nn lng t theo lut A

Lut c m t qua cng thc 2.2 di y: Sgn(x) * ln( 1 + |x|) ln (1 + ) 0 |x| 1 Trong : Sgn (x) l hm du (2.2)

l hng s xc nh mc nn = 255. x l hm nn tn hiu x gi tr mu a vo nn (nguyn) Cng thc 2.2 ch ra ton b di tn hiu u vo c nn theo dng logarithm, nh vy s lng on trong lut l 15 on v c chia thnh hai cc tnh (+) v m (-). Lut c s dng trong cc h thng PCM 24 knh. Cc c im tng t ca hai lut nn dn tn hiu: C hai lut u s dng phng php xp x tuyn tnh theo quan h loga. u s dng t m 8 bit m ho. Tc bit cho knh l 8 bit x 8 kHz = 64 kb/s. Tip cn phn on thc hin m ho tng t nhau.

Cc c im khc bit gia hai lut nn dn tn hiu : Phng php xp x tuyn tnh khc nhau cho cc ng cong khc nhau. Cc bit gn cho cc segment v cc mc lng t trong segment khc nhau. Lut A cung cp di ng ln hn lut . Lut cung cp t s S/N ti min tn hiu mc thp tt hn lut A.

(iii) M ho Qu trnh m ho tn hiu trong k thut PCM thc hin vic chuyn i cc mu tn hiu lng t ho thnh cc m nh phn 8 bit. Khun dng ca mt t m PCM nh sau: X = P ABC DEGH ; X th hin t m, P l bt du, ABC l bit ch th phn on, DEGH l bit ch th cc mc lu lng trong on. Trong thc t, qu trnh lng t v m ha c thc hin ng thi trong mt chip vi x l theo lut A hoc lut .

F(x) =

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2.1.2. Cu trc khung tn hiu PCM Tiu chun G.704 ITU-T m t cc dng cu trc khung tn hiu PCM cho cc phng tin truyn dn v chuyn mch. Trong mc ny chng ta xem xt cu trc khung v a khung ca h thng PCM 24 (T1) v h thng PCM 30/32 (E1). (i) Cu trc khung v a khung PCM 24 Cu trc khung PCM 24 c m ho theo lut v c mt s c tnh c bn sau: Tc truyn 1,544 Kb/s; Mt khung gm 24 DS0 (64 kb/s). di khung l 125 s c 193 bit c nh s t 1- 193; Bit u tin ca mi khung c s dng xp khung, gim st v cung cp lin kt

s liu.

Hnh 2.4: Cu trc a khung PCM 24

K thut m ho ng dy : AMI, B8ZS. Cu trc a khung gm hai khun dng: DS4 nhm 12 khung, v khung m rng EPS

nhm 24 khung (hnh 2.4 th hin cu trc a khung 24).

Bit bo hiu F s dng xp a khung, lin kt d liu, kim tra v bo hiu c ch nh trong tng khung th hin trong bng 2.1 di y.

Bit F

Cc bit gn

S bit trong khe thi gian

S th t khung

trong a khung

S bit trong a khung

FAS

DL

CRC

Cho th bo hiu

Bo hiu CAS

S dng cho bo

hiu

1 1 m 1-8

2 194 e1 1-8

3 387 m 1-8

4 580 0 1-8

5 773 m 1-8

6 966 e2 1-7 8 A

7 1159 m 1-8

8 1352 0 1-8

9 1545 m 1-8

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10 1738 e3 1-8

11 1931 m 1-8

12 2124 1 1-7 8 B

13 2317 m 1-8

14 2510 e4 1-8

15 2703 m 1-8

16 2896 0 1-8

17 3089 m 1-8

18 3282 e5 1-7 8 C

19 3475 m 1-8

20 3668 1 1-8

21 3861 m 1-8

22 4054 e6 1-8

23 4247 m 1-8

24 4440 1 1-7 8 D

FAS : Tn hiu xp khung ( ... 001011..)

DL: Lin kt s liu CRC: trng kim tra mch vng d theo chu k

Bng 2.1: Bit F trong cu trc a khung 24

Hnh 2.5: Cu trc khung v a khung PCM 30

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(ii) Cu trc khung v a khung PCM 30 Cu trc khung PCM 30 c m ho theo lut A v c mt s c tnh c bn sau: Tc truyn 2,048 Kb/s; Mt khung gm 32 TS/30 CH. di khung l 125 s cha 256 bit; nh s t 1 n 256. K thut m ho ng dy : AMI, HDB3. Cu trc a khung cha 16 khung. TS0 s dng xp khung v ng b, TS16 s

dng cho bo hiu. (cu trc khung v a khung ch ra trn hnh 2.5) Khi s dng m vng d theo chu k CRC-4 kim tra li tuyn PCM, cu trc a khung c chia thnh 2 phn nhm a khung nh SMF I v SMF II, mi nhm gm 8 khung. Vic phn nhm ny khng nh hng ti cc khe thi gian s dng cho bo hiu. B tr cc bit trong TSO khi s dng CRC-4 c ch ra trong bng 2.3 di y:

Cc bit trong TS0 SMF

S khung 1 2 3 4 5 6 7 8

c1 0 0 1 1 0 1 1

0 1 A Sa Sa Sa Sa Sa

c2 0 0 1 1 0 1 1

0 1 A Sa Sa Sa Sa Sa

c3 0 0 1 1 0 1 1

1 1 A Sa Sa Sa Sa Sa

c4 0 0 1 1 0 1 1

I

0

1 2 3 4 5 6 7

0 1 A Sa Sa Sa Sa Sa

c1 0 0 1 1 0 1 1

1 1 A Sa Sa Sa Sa Sa

c2 0 0 1 1 0 1 1

1 1 A Sa Sa Sa Sa Sa

c3 0 0 1 1 0 1 1

E 1 A Sa Sa Sa Sa Sa

c4 0 0 1 1 0 1 1

a khung

II

8 9

10 11 12 13 14 15

E 1 A Sa Sa Sa Sa Sa

Ghi ch: C1- c4 : Kim tra m vng d theo chu k; E: Ch th li CRC A: Cnh bo; Sa: Cc bit d phng.

Bng 2.2: Cu trc TSO ca a khung PCM30 s dng CRC-4.

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2.1.3. Trao i khe thi gian ni TSI Trong k thut chuyn mch knh, sau khi tn hiu thoi c m ho thnh cc t m nh phn 8 bit, cc knh thng tin c xc lp trn cc khe thi gian cch nhau 125s v c truyn i nh cc h thng truyn dn v chuyn mch. Trn nguyn tc s dng chung ti nguyn, cc thng tin ca ngi s dng c chuyn i trn cc knh c phn chia logic theo thi gian, s khc bit ca khe thi gian c n nh cho ngun tin pha pht v ngun tin pha thu l mt yu t yu cu c s chuyn i ni dung thng tin t khe thi gian ny sang khe thi gian khc trong cng mt khung, chnh l qu trnh trao i khe thi gian ni TSI. Mt c cu s dng chuyn i TSI c minh ho trn hnh 2.6 di y, cc khi thit b chnh gm c:

Hnh 2.6: Nguyn l trao i khe thi gian ni TSI

Cc tuyn PCM u vo v u ra c cu trc khung gm n khe thi gian, yu cu chuyn i ni dung thng tin ca mt khe thi gian bt k t u vo ti u ra.

B nh lu m tm thi hot ng theo nguyn tc truy xut ngu nhin c dung lng cha ton b thng tin d liu trong mt khung PCM, (S ngn nh: n, dung lng ngn nh: 8 bit).

Khi iu khin CM (Control Memory) s dng ghi cc thng tin iu khin chuyn i ni dung khe thi gian cho b nh lu m (S ngn nh: n, dung lng ngn nh: L= log2n).

Khi ng b cho qu trnh ghi c vo cc b nh c ng b thng qua mt b m khe thi gian TS.C.

Khi c yu cu chuyn i ni dung thng tin v tu thuc vo ngun ti nguyn ca h thng, khi x l trung tm s a cc d liu iu khin ti khi iu khin CM nhm sp xp v tr chuyn i ca cc khe thi gian. m bo tc lung thng tin u vo v u ra, trong cng mt khong thi gian b nh lu m phi thc hin ng thi hai tc v ghi thng tin vo v c thng tin ra. Theo nguyn tc trao i khe thi gian ni TSI, tr ti a ca thng tin trao i khng vt qu thi gian ca mt khung Td (max) = (n-1)TS < 125s.

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2.2. KIN TRC TRNG CHUYN MCH KNH Chuyn mch knh tn hiu s l qu trnh thc hin trao i ni dung thng tin s trong cc khe thi gian ca cc tuyn PCM u vo ti u ra. thc hin hiu qu qu trnh chuyn mch, cc tuyn PCM thng c ghp knh vi tc cao trc khi a ti trng chuyn mch. Vic b tr s dng cc trng chuyn mch trong h thng chuyn mch ph thuc ch yu vo kin trc iu khin ca h thng. Tuy nhin, kin trc trng chuyn mch knh c chia thnh hai dng phn chia theo nguyn tc hot ng: Trng chuyn mch khng gian (S) v trng chuyn mch thi gian (T). Di y s trnh by nguyn l cu trc ca trng chuyn mch khng gian S, thi gian T v kin trc ghp ni cc trng chuyn mch TST.

2.2.1 Trng chuyn mch khng gian s Trng chuyn mch khng gian s S thc hin qu trnh chuyn ni dung thng tin t cc tuyn PCM u vo ti cc tuyn PCM u ra m khng lm thay i v tr khe thi gian trn trc thi gian. to ra knh truyn thng cho cc cuc gi, cc thng tin c chuyn qua trng chuyn mch khng gian s c chuyn mch nh k vi khong thi gian 125s.

Hnh 2.7: Nguyn l chuyn mch khng gian S

Cc trng chuyn mch khng gian S c thit k h tr chuyn mch ng thi mt s lng ln cc cuc ni di s iu khin ca cc chng trnh ghi sn. Hnh 2.7 trn y ch

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ra s nguyn l cu trc ca trng chuyn mch khng gian S in hnh theo kiu iu khin u vo, kiu iu khin u ra c thc hin bng s hon i v tr gn cng u ra ca cc phn t kt ni. Trng chuyn mch khng gian S c cu to t hai khi chnh: Khi ma trn chuyn mch v khi iu khin cc b. (i) Khi ma trn chuyn mch Khi ma trn chuyn mch c cu trc di dng ma trn hai chiu gm cc cng u vo v cc cng u ra, trn cc cng l cc tuyn PCM c chu k khung 125s. Cc im ni trong ma trn l cc phn t logic khng nh ( thng thng l cc mch AND). Mt ma trn c (N) cng u vo v (M) cng u ra tr thnh ma trn vung khi N=M. (ii) Khi iu khin khu vc Khi iu khin khu vc gm mt s khi thit b nh: B nh iu khin kt ni CMEM (Control MEMory) lu tr cc thng tin iu khin

theo chng trnh ghi sn cho ma trn chuyn mch, ni dung thng tin trong CMEM s th hin v tr tng ng ca im kt ni cn chuyn mch (S ngn nh: n, dung lng ngn nh: L= log2N);

B gii m a ch DEC (DECode) chuyn cc tn hiu iu khin m nh phn thnh cc tn hiu iu khin cng cho phn t kt ni AND;

B m khe thi gian TS.C (Time Slot Counter) nhn tn hiu ng h t ng h h thng cp cc xung ng b cho b iu khin theo ng b ca cc tuyn PCM vo v ra.

TS.C a tn hiu ng b vo b chn (SEL) ng b qu trnh ghi dch a ch v tc v ghi c ca b nh CMEM.

Nguyn tc hot ng ca trng chuyn mch khng gian S gm mt s bc c bn sau:

Cc tuyn PCM trn cc cng u vo v u ra c ng b ho theo tn hiu ng b. Nh trn hnh 2.5 ch ra mi khi iu khin khu vc LOC m nhim mt cng u ra, v vy s b iu khin LOC s bng ng s cng u ra (M b iu khin). r hn nguyn tc iu khin v hot ng ca trng chuyn mch S, ta xt mt v d c th: Yu cu chuyn ni dung thng tin trn TS3 ti cng s 4 u vo ra TS3 trn cng s 1 u ra. Vi yu cu trn, khi iu khin khu vc LOC 1 s thc hin nhim v iu khin chuyn mch, thng tin iu khin c a trc vo CMEM thng qua ng d liu iu khin Data, thanh ghi a ch ca b chn SEL tr n ngn nh s 03: tng ng vi ch s TS 3 ca khung PCM, ni dung d liu iu khin th hin cng kt ni vo (cng 4). Qua s phi hp ng b ca TS.C, mt chu trnh iu khin tun t v ng b gia ch s khe thi gian v con tr a ch c d liu ra t CMEM. V vy, khi con tr ch n a ch 03, thi gian h thng s trng khp vi TS3 trn cc tuyn PCM, ng thi SEL ch th qu trnh c d liu v chuyn thng tin d liu qua b DEC gii m, thc hin ng tip im gia cng vo 4 v cng ra 1. Tip im c ng trong sut thi gian chuyn d liu v bng thi gian ca khe thi gian. Vi tnh cht logic ca tip im AND, khi hm iu khin c mc 1 th ton b s liu u vo s c chuyn ti u ra. i vi mi mt cuc ni thoi, chu k ng tip im c thc hin tun t theo chu k 125s, vic ngt cc kt ni c thc hin n gin thng qua qu trnh ghi li d liu trong b nh CMEM, cc khong thi gian cn li s c thc hin cho cc kt ni khc. Nu ma

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trn chuyn mch l ma trn vung th tng s knh ti a c th kt ni ng thi s l Ch= n x N (N: s khe thi gian trong mt khung PCM; N: s cng u vo chuyn mch S). Trng chuyn mch khng gian S mang tnh thi gian nu xt v tnh chu k ca qu trnh ng ngt tip im, tuy nhin chu k ny l c nh cho tt c cc cuc ni qua trng chuyn mch. Nhc im lun tn ti trong cc trng chuyn mch khng gian S l kh nng tc nghn khi c nhiu hn mt yu cu chuyn mch TS u vo cng mun ra mt cng u ra. Mt ma trn chuyn mch khng tc nghn hon ton c nh ngha l mt ma trn c kh nng p ng c cc kt ni t cc u vo bt k ti cc u ra bt k.

Hin tng tranh chp cng u ra trong ni b trng chuyn mch c gi l hin tng tc nghn ni. gii quyt vn trn, cc trng chuyn mch S thng c kt hp vi cc b m gy tr thi gian trnh tranh chp, gii php ghp ni vi trng chuyn mch thi gian T c s dng ph bin trong cc h thng chuyn mch hin nay.

2.2.2. Trng chuyn mch thi gian s Trng chuyn mch thi gian tn hiu s thc hin qu trnh chuyn i ni dung thng tin t mt khe thi gian ny sang khe thi gian khc, vi mc ch gy tr cho cc tn hiu. Qu trnh gy tr tn hiu c thc hin theo nguyn tc trao i khe thi gian ni TSI. Hnh 2.6 di y ch ra s nguyn l cu trc ca trng chuyn mch thi gian T. Trng chuyn mch thi gian T c hai kiu iu khin: iu khin u vo thc hin qu trnh ghi thng tin c iu khin v c ra tun t; iu khin u ra thc hin ghi thng tin tun t v c ra theo iu khin. Trong mc ny ta xem xt nguyn l hot ng ca trng chuyn mch T theo kiu iu khin u ra.

Hnh 2.8: Nguyn l chuyn mch thi gian T

Trng chuyn mch thi gian T c cu to t 2 khi chnh: Khi b nh thoi SMEM (Speech MEMory) v khi iu khin cc b LOC. Khi b nh thoi SMEM l mt thit b ghi nh truy xut ngu nhin RAM (S lng

ngn nh: n; dung lng ngn nh: 8 bit). Nh vy, b nh SMEM lu ton b thng tin trong mt khung tn hiu PCM. m bo tc lung thng tin qua trng

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chuyn mch, tc ghi c ca CMEM phi ln gp 2 ln tc lung trn tuyn PCM u vo hoc u ra.

Khi iu khin khu vc gm mt s khi nh: B nh iu khin CMEM lu tr cc thng tin iu khin SMEM, s th t ca ngn nh v ni dung d liu trong CMEM th hin cc ch s khe thi gian TS cn trao i ni dung tin. TS.C nhn tn hiu t ng h h thng iu khin cc b chn SEL1, SEL2 nhm ng b ho qu trnh ghi c thng tin d liu cho CMEM v SMEM.

Nguyn tc hot ng ca trng chuyn mch thi gian T trn c s ca nguyn tc trao i khe thi gian ni TSI. Hnh 2.8 th hin kiu iu khin ghi vo tun t c ra c iu khin SWRR (Sequence Write Random Read), r hn ta xem xt mt v d minh ho c yu cu chuyn i ni dung thng tin t khe thi gian TS3 sang TS8. Theo nguyn tc ghi vo tun t, ni dung thng tin trong TS3 s c lu vo ngn nh s 3. Vi yu cu chuyn i trn, h thng x l trung tm s chuyn thng tin ti CMEM thng qua ng d liu iu khin, b nh CMEM s lu ch s khe thi gian TS3 ti ngn nh s 8. Con tr a ch ca CMEM s qut ln lt, ng b vi cc khe thi gian trn tuyn PCM, nh vy khi con tr a ch ch n ngn nh s 8 trng vi thi im xut hin khe thi gian s 8 trong khung PCM. B chn SEL1 s chuyn tn hiu c ti CMEM CMEM chuyn d liu iu khin ti SMEM, thng tin cn chuyn t CMEM ti SMEM trong trng hp ny l 3 (0011) SEL2 a con tr a ch ti nh s 3, ng thi cp tn hiu iu khin c ra ni dung thng tin ti nh s 3 ca SMEM. Nh vy, ti thi im xut hin khe thi gian TS8, ton b ni dung thng tin ca TS3 c chuyn qua trng chuyn mch. Qu trnh chuyn mch c lp li theo chu k 125s hnh thnh nn knh thng tin qua trng chuyn mch. Nu cng vi yu cu chuyn mch t TS3 sang TS8 v trng chuyn mch hot ng theo kiu ghi vo c iu khin v c ra tun t RWSR (Random Write Sequence Read) th ti ngn nh s 3 ca CMEM s lu thng tin a ch TS8. Trng chuyn mch thi gian T mang tnh khng gian nu xt trn kha cnh v tr thng tin d liu trong cc ngn nh ca CMEM. Chuyn mch T lun gy tr tn hiu v tr ln nht khng vt qu mt khung PCM, Td max = (n-1)TS. Do tc ghi c ca b nh yu cu ln gp 2 tc lung PCM nn s lng khe thi gian trong mt khung khng vt qu 1024 TS do gii hn ca cng ngh vt liu in t. m rng dung lng, ngi ta la chn gii php ghp vi cc trng chuyn mch khng gian S.

2.2.3. Trng chuyn mch ghp TST Cc h thng chuyn mch s c nhn nhn nh l cc h thng tn tht, vn nng cao hiu nng chuyn mch lun l vn hng u trong thit k ch to trng chuyn mch. Trong k thut chuyn mch knh, cc hin tng tc nghn c coi l tham s ch yu nh gi cht lng dch v h thng v ch yu ri vo cc h thng chuyn mch s dng tng S. Trong khi vic s dng cc chuyn mch khng gian S trong h thng chuyn mch l mt trong nhng nhu cu bt buc p ng dung lng cn thit trong thc tin. Mc tiu kt ni a tng chuyn mch khng ch nhm tng dung lng h thng m cn lm gim bt phc tp v s lng thit b trong trng chuyn mch, kt ni a tng gm hai kiu: kiu kt ni y v kt ni tng phn u nhm mc tiu xy dng trng chuyn mch khng tc nghn (non-blocking).

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Trc khi tm hiu nguyn tc hot ng ca trng chuyn mch ghp kiu TST, ta xem xt m hnh ghp ni 3 tng chuyn mch khng gian theo nh l Clos. M hnh ghp ni c lin kt y 3 tng chuyn mch c th hin trn hnh 2.9 (a) di y. Ma trn chuyn mch khng gian c N u v M u ra (NxM) c kt ni bi r1 ma trn tng A (kch thc n x r2 ), r2 ma trn tng B (kch thc r1 x r3) v r3 ma trn tng C (kch thc r2 x m). Vi gi thit r2=1, r1=n v r3=m ta c m hnh kt ni trn hnh 2.9(b). nh l Clos pht biu nh sau: Ma trn chuyn mch kt ni 3 tng khng tc nghn khi v ch khi s kt ni trung gian r2 n + m -1 (2.3). Trng hp c bit khi n=m th r2 2n-1.

Hnh 2.9: Ma trn kt ni 3 tng chuyn mch

Chng minh: Mt ma trn chuyn mch khng tc nghn hon ton khi ton b cc yu cu u vo bt k c u ni ti cc u ra bt k. Gi thit c n-1 ng vo yu cu chim, vy c n-1 ng lin kt gia tng A v tng B b chim. Tng t nh vy, nu u ra c m-1 ng b chim th s c m-1 ng lin kt gia tng B v tng C b chim (hnh 2.9b). Trng hp xu nht xy ra khi n-1 ng lin kt A-B u ni ti cc khi chuyn mch tng B khc bit hon ton vi m-1 ng lin kt B-C. Vy tng s khi chuyn mch trong tng B bng (n-1) + (m-1) m bo khng tc nghn ngay c khi trng hp xu nht xy ra.

Ma trn chuyn mch khng tc nghn hon ton khi ng vo th n ca tng A kt ni c ng ra th m ca tng C, dn n s lng khi chuyn mch trong B ti thiu phi d 1 khi cho ng dn cui cng ny. Hay ni cch khc s lng lin kt ti thiu r2 (n-1) + (m-1) +1 = n + m -1.

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Nu ma trn chuyn mch l ma trn vung (N=M), (n = m) v (r1 = r3), ta c s lng im kt ni cho l: C = 2Nr2 + r12r2 = 2N(2n-1) + r12 (2n-1) = (2n-1) ( 2N + N2 n2 ) (2.3) Khi kch thc ca trng chuyn mch ln, n ln ta c th tnh s lng im kt ni cho C xp x theo cng thc (ct 2.4) sau.

C 2n (2N + N2n2 )= 4nN + 2N2n (2.4) ti u s im kt ni cho, ly vi phn C theo n (dC/dn) v cho kt qu tin ti 0 ta c n (N/2)1/2 (2.5). Thay n t cng thc 2.5 vo cng thc 2.4 ta c:

3/ 2 3/ 24 2. ( )C N O N= = (2.6) Nh trn cng thc 2.6 ch r, chuyn mch kt ni 3 tng Clos gim phc tp phn cng (s tip im u ni cho) xung cn N3/2 thay v N2 trong ma trn kt ni crossbar m vn m bo c mc tiu khng tc nghn. C rt nhiu m hnh phi kt hp ghp ni trng chuyn mch T v S nh: TS, ST, STS v TST,v..v [3]. Mi dng ghp ni u c nhng u nhc im nht nh v trong phn ny ta xem xt m hnh kt ni ph bin trong cc h thng chuyn mch knh hin nay, m hnh ghp ni TST. Trng chuyn mch ghp TST nhm gii quyt bi ton m rng dung lng v s dng cho cc kt ni hai hng trong trng chuyn mch thc t. Theo l thuyt, trng chuyn mch TST c h s tp trung l 1:1 v m bo khng tc nghn khi s lng khe thi gian trn lin kt trung gian tun th theo nh l Clos ( cng thc ct 2.3). Nh mc 2.2.2 trn y trnh by, trng chuyn mch thi gian T c th hot ng theo hai kiu iu khin u vo (RWSR) v iu khin u ra (SWRR), v vy khi ghp 2 tng T ta s c 4 phng n ghp ni. Trong v d trn hnh 2.10 di y ta chn chuyn mch thi gian tng T1 hot ng theo nguyn tc SWRR v chuyn mch thi gian tng T2 hot ng theo nguyn tc RWSR, A truyn v nhn thng tin d liu trn TS5, B truyn v nhn thng tin d liu trn TS10, khe thi gian trung gian gia T1-S v S-T2 c chn l TS15, thng tin iu khin ti cc CMEM tng T c vit tt di dng a(b) [a: ch s ngn nh, b: ni dung ngn nh] . Ni dung thng tin trn cc b nh CMEM c trnh by vn tt nh sau: Hng kt ni t A-B qua SMEM 1(T1) S SMEM N (T2), cc b nh CMEM 1

(T1) v CMEM N (T2) c ni dung tng ng 15(5) v 15(10). Hng kt ni t B-A qua SMEM N (T1) S SMEM 1 (T2), cc b nh CMEM

N(T1) v CMEM 1 (T2) c ni dung tng ng 15(10) v 15(5).

T ni dung cc khi iu khin chuyn mch T nh trn hnh v, ta nhn thy hai khi iu khin CMEM 1(T1) v CMEM 1(T2) hon ton ging nhau, cng nh vy i vi CMEM N (T1) v CMEM N (T2).

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Hnh 2.10: Trng chuyn mch ghp TST tit kim s b iu khin ta c th s dng 01 b iu khin s dng iu khin chung cho 2 b iu khin. V trng hp ang xt l trng hp c bit khi ta chn khe thi gian trung gian ging nhau (TS15), iu ny s khng ng vi trng hp tng qut khi A v B cng c kt ni ti cng mt khi chuyn mch thi gian trong tng T1, hin tng tranh chp s xy ra khi c hai yu cu u vo TS5 v TS10 cng mun ra u ra TS15. Mt gii php trnh trng hp ny l s dng kt ni th hai (B-A) qua tuyn trung gian c khe thi gian l TS15 + n/2. Lc ny ni dung b iu khin tng ng l: CMEM 1(T1) : 15(5); CMEM 1 (T2): 15+n/2 (5); CMEM N (T1): (15+n/2) (10); CMEM N (T2) : 15(10). Khi s dng mt b nh i ngu nh trn hnh 2.10 (A*) ta hon ton c th iu khin c hai b iu khin CMEM ti hai tng chuyn mch bng mt khi iu khin.

Trng hp s dng mt b nh dng chung nh trn hnh 2.10 (B*) khi ta chn chuyn mch tng T1 hot ng theo nguyn tc RWSR v chuyn mch tng T2 hot ng theo nguyn tc SWRR. Vn chn khe thi gian trung gian trong trng chuyn mch TST iu khin kt ni gia cc tng T v S l mt khu quan trng trong qu trnh x l chuyn mch. Phng php tm kim khe thi gian trung gian ri c thc hin mt cch n gin qua vic x l tm kim cc cp bit (bn/ ri) ti u ra tng T1 v u vo tng T2. Phng php c xut tm kim cc cp bit ri ti hai u l phng php tm kim kiu mt n chn knh. Cc bit trong thanh ghi ch th trng thi v thanh ghi mt n th hin r s bn/ri ca cc knh thng qua bn nh nh x trng thi. Hnh 2.11 di y ch ra nguyn tc hot ng ca phng php mt n chn knh.

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Hnh 2.11: Phng php chn knh ri theo mt n chn knh

Thanh ghi trng thi l s nh x trng thi hin thi ca cc knh c chn (bit 1 th hin trng thi ri ca knh, bit 0 th hin trng thi bn ca knh). Thanh ghi mt n l mt logic nh phn c di bng thanh ghi trng thi, trn tn ti 02 bit c gi tr 1 v cn li l cc bit c gi tr 0. Thanh ghi mt n c nhim v la chn 2 khe thi gian ri (u vo v u ra) cho kt ni, cc bit 1 s di chuyn trong thanh ghi mt n cho n khi trng khp v tm c khe thi gian ri tng ng trong thanh ghi trng thi, kt qu thu c thng qua mt php AND n gin gia thanh ghi trng thi v thanh ghi kt qu. 3 thut ton thng c s dng trong cch thc di chuyn mt n gm: Ngu nhin lin tip, c nh- lin tip v phng php th lp. (i) Phng php ngu nhin - lin tip: phng php ny da trn nguyn tc tm kim ngu nhin mt khe thi gian ri, nu khe thi gian u tin chn ngu nhin khng tho mn yu cu, h thng dch chuyn lin tip trong ton di nhm tm khe thi gian tho mn yu cu. Phng php ny to ra hiu ng chim dng cc b t cc im xc lp ngu nhin, thi gian tm kim s ko di khi s lng knh b chim tng ln. (ii) Phng php c nh lin tip: Phng php ny ch nh khe thi gian u tin sau tm lin tip trn ton di. Hiu ng tri di cc knh b chim dng bt u t knh c chn v xc sut chn knh trong phng php ny khng ging nhau.

(iii) Phng php th lp: Phng php ny da trn c tnh ca lu lng yu cu v s chim dng ngu nhin ca cc khe thi gian. Qu trnh th lp da trn theo khong thi gian chim dng khe thi gian. Phng php ny c bit hiu qu nu m hnh lu lng u vo c xc nh.

2.3. NH TUYN TRONG CHUYN MCH KNH Mng chuyn mch in thoi cng cng PSTN l h thng phn cp, qu trnh nh tuyn cho cc cuc gi trong mng da trn a ch ch, cc h thng nt mng thc hin qu trnh nh tuyn thng qua a ch v cc phng php bo hiu. Cc phng php nh tuyn phi thng nht vi k hoch nh s, tnh cc, truyn dn v bo hiu. Mc di y s trnh by v mt s c im c bn ca k hoch nh s trong mng PSTN.

2.3.1. K hoch nh s trong mng PSTN Nguyn tc c bn ca k hoch nh s trong mng PSTN phi m bo 3 yu t:

(i) Cung cp nhn dng duy nht trong phm vi quc gia v quc t, (ii) p ng c cc yu cu tng trng ca thit b u cui, (iii) di cc con s ph hp vi khuyn ngh ca t chc vin thng quc t.

Tiu chun nh s ITU-T E.164 ra i vo thng 5 nm 1997, thuc vo seri E: "Hot ng chung ca mng, dch v in thoi, hot ng dch v v cc tc nhn con ngi", quy nh

35

cc "Hot ng, nh s, nh tuyn v cc dch v di ng - Hot ng quc t - Quy hoch nh s cho dch v in thoi quc t". Trong E.164 quy nh "Quy hoch nh s vin thng cng cng quc t". E.164 tp trung quy nh cu trc s v chc nng ca 3 dng c bn ca s s dng trong vin thng cng cng quc t l theo vng, dch v chung v theo mng. E.164 quy nh c th mi loi tin t cn thit cho tng mc ch nhm thng nht vic nh s ton cu cho mi loi nhu cu dch v. N cng cung cp cc quy c dng nh tuyn cuc gi gia cc s v nhm s trn ton h thng.

Chun E.164 quy nh tng di cc con s khng vt qu 15 digits, c phn cp thnh cc vng m: m quc gia, m vng, m tng i v cc con s thu bao. Cc h thng m ny thng i km vi m trung k to ra cc tin t c nh (prefix), cc h thng chuyn mch da trn cc tin t phn bit cc cuc gi ra trong vng, trong nc hoc quc t. Cn c vo cc a ch tin t, h thng bng bin dch ca h thng chuyn mch s lu cc nh x a ch logic sang a ch vt l s dng trong qu trnh nh tuyn. Mt v d v h thng nh s theo vng a l nh sau: +84 4 556 9999; Trong : + l tin t cho bit s vin thng l y ; 84 l m quc gia ca Vit nam, 556 9999 l s thu bao gm 556 l m tng i v 9999 l mt phn khc thu bao.

2.3.2. Cc phng php nh tuyn trong mng chuyn mch knh nh tuyn trong mng chuyn mch knh l qu trnh xc nh ng i gia cc nt mng m bo ti u v kinh t v k thut ca mng, cc iu kin phi tun th trong qu trnh nh tuyn trong mng PSTN gm: Khng lp vng gia cc nt mng, th tc iu khin n gin, s dng v qun l thit b hiu qu v p ng c cc yu cu thay i trong tng lai. Hai phng php nh tuyn c bn thng s dng trong mng chuyn mch knh l nh tuyn c nh v nh tuyn lun phin.

nh tuyn c nh thng c s dng trong cc kt ni trc tip hoc cc mng cp thp. phng php ny n gin v nhanh chng khi ton b cc hng u c ngm nh. nh tuyn c nh b hn ch khi xy ra s c v khng linh hot la chn tuyn, dn ti kh nng tc nghn cao khi lu lng khng n nh.

Hnh 2.12: nh tuyn lun phin nh tuyn lun phin bao gm hai kiu lun phin c nh v lun phin ng, trong thc

t vic p dng nguyn tc nh tuyn lun phin c thc hin nh hnh 2.12. Trong , lu

nt A nt B

nt C

Trung k

Nt chuyn tip T

Tuyn trc tip

Tuyn d phng

36

lng gia hai nt mng A v C c th thc hin qua hai tuyn: tuyn trc tip A-C, tuyn trn A-T-C. Nguyn tc chung khi cc knh trn tuyn A-C b chim ht th lp tc lu lng trn s c chuyn sang tuyn A-T-C. Nu lc , tuyn A-T-C m cng b chim ht th lu lng trn ny s b tn tht. Trong nh tuyn nh hnh 2.12, c gi l nh tuyn lun phin c nh bi v mi mt tuyn s knh c khai bo s dng l c nh c tnh ton da trn kt qu d bo lu lng. Trong trng hp kt qu d bo lu lng sai th s xy ra hai trng hp: lu lng s b tn tht nhiu dn n cht lng dch v khng cao hoc cc knh b tha nhiu dn n hiu qu s dng knh khng cao. Trong nh tuyn lun phin t ng, nu mt cuc gi thnh cng trong mt tuyn cho th vic chn mch c lu li. Tri li, i vi la chn hin ti m cuc gi khng thnh cng th s thc hin mt la chn mi cho cuc gi tip theo. Do s dng bo hiu knh chung gia cc tng i kt hp vi cc tng i c cc tuyn ni n nhiu trung tm bc cao hn th c th to nn mt k hoch nh tuyn lun phin t ng phc tp. Nh vy, nu mt cuc gi gp tc ghn ti mc cao hn trong phn cp, n c th quay li tng i bc thp hn v chn mt tuyn i khc. nh tuyn lun phin t ng s nh tuyn li cc cuc gi ra t mt tuyn mc s dng cao bt c khi no m n khng th chuyn ti lu lng c. iu ny c th xy ra nu c s thay th tuyn lu lng cao hn hng hc. Nh th rt c li v lu lng vn n c ch ca n trong khi n khng th lm c nu ch c cc tuyn trc tip. Tuy vy, nu b sung thm mt lu lng ln vo tuyn trung k c th gy nn tc ghn cc cuc gi cho cc im thu m ch do tuyn ny phc v (n s n hu ht cc tng i khc trong mng). Mt gii php cho kh khn ny l dnh trc ng trung k. Mt phn cc mch trn tuyn cui cng c dnh ring cho cc cuc gi m ch xy ra trn tuyn . Nh vy, cc cuc gi ny vn t c mt mc dch v hp l khi lu lng trn qua mt tuyn tng bt thng.

khc phc nhc im trn kiu nh tuyn lun phin ng c a ra. N c chia thnh hai loi: nh tuyn ng theo trng thi trung k v nh tuyn ng theo thi gian. Nguyn tc nh tuyn ng theo trng thi trung k : truyn ti lu lng gia hai nt mng, s lng knh trn cc hng khng gn c nh. Trong trng hp c cc cuc gi xut hin gia hai nt trung tm qun l mng s xc nh c trng thi cc knh trn cc hng v iu khin chim vo mt knh ri. thc hin c nguyn tc ny cn phi xy dng c mt mng qun l tng th. Mt trung tm qun l mng s kt ni ti tt c cc phn t trn mng thng qua mt mng truyn s liu v x l cc s liu v tnh trng chim dng cc trung k. Nguyn tc nh tuyn ng theo thi gian: Trong thc t lu lng xut hin trong mt khu vc hay gia hai nt mng l thay i theo gi trong ngy. V d nu vng phc v ca hai nt mng l khu thng mi th lu lng vo bui sng hoc chiu l rt cao, trong khi lu lng vo bui ti thp. Ngc li, trong cc khu dn c, lu lng vo bui ti thng cao hn ban ngy. m bo p ng c s thay i ln v lu lng nh vy ti cc nt mng s lng knh cung cp cho cc tuyn s thay i theo nhu cu mt cch t ng (theo gi). nh tuyn lun phin t ng phc tp hn c th gy ra tc nghn bt ngun t mt phn ca mng dn n s bng n lu lng trn qua cc tuyn khc. R rng iu ny l khng mong mun, nn ngi ta da vo k thut qun l mng. Mt trung tm qun l tp trung c th gim st lu lng trn cc tuyn khc nhau v nu cn n c th gim hoc ct

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ton b lu lng bt ngun t cc trung tm chuyn mch ngn nga s qu ti. Theo kiu nh tuyn chuyn mch theo thi gian, cc thay i nh tuyn thay th lun ph hp vi iu kin lu lng trong mi mt chu k thi gian (tc l ngy/ m, ngy trong tun, cc dp c bit).

2.4. CC TRNG CHUYN MCH TRONG THC TIN Trong mc ny ta xem xt hai dng cu trc trng chuyn mch thc t ca h thng chuyn mch NEAX-61 v h thng chuyn mch A1000 E10. (i) Trng chuyn mch trong h thng NEAX-61 H thng chuyn mch NEAX-61 chia thnh cc phn h: Phn h ng dng, phn h chuyn mch, phn h x l v phn h vn hnh, bo dng. Phn h chuyn mch bao gm mng chuyn mch phn chia thi gian, b iu khin ng thoi. Mng chuyn mch phn chia thi gian l trng chuyn mch ghp TST. B iu khin ng thoi iu khin chuyn mch thi gian TSW v chuyn mch khng gian SSW theo cc bn tin iu khin t b x l cuc gi CLP thng qua HUB trong phn h x l. Cu trc ghp ni trng chuyn mch c th hin trong hnh 2.13 di y.

SSWJHWITSWKHWI

JHW

JHW

DTICLOC

RLUICDLTC

Phn h ngdng

KHW DN

KHW UP

TSC

HUBIU

HUB

HUBIU

SSC

Mng phnchia thi

gian

iu khinng

thoi dngcho c

imkhng tc

nghn

HUBCLP Phn h x l

Phn h chuyn mch

TSM SSM

Hnh 2.13: Cu trc phn h chuyn mch ca tng i NEAX-61

Cc khi chc nng c bn trong phn h chuyn mch gm: Giao tip KHW (KHWI) Card P-8A4W: Nhn cc loi tn hiu t TSC v tng hp cc mu, ng k knh trng thi ST ri gi chng ti khi x l bn tin giao thc PMH; la chn khi chuyn mch thi gian TSW lm vic, tm cc li v thng bo ti h thng; ghp tn hiu thoi t TSW, tn hiu trng thi t khi iu khin chuyn mch thi gian TSC v tn hiu bn tin t HUBI thnh tn hiu KHW. v gi tn hiu KHW ti phn h ng dng. Khi giao tip Hub (HUBIU) Card P-8A8J: Tp hp li cc tn hiu bn tin t d liu ph hp nm trong cc t bo iu khin nhn c thng qua Hub v gi cc tn hiu ti TSC v KHWI; tch cc tn hiu bn tin nhn c t TSC v KHWI, chn cc tn hiu vo t bo iu khin ti Hub.

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Khi chuyn mch thi gian TSW Card P-8A4X: Thc hin chuyn mch thi gian ca tn hiu thoi nhn c t KHWI ph hp vi tn hiu iu khin t TSC v gi tn hiu thoi thng qua JHW ti SSW; nhn cc thng tin li t HUBIU, TSW, KHWI v cc khi khc v gi cc thng tin ti b x l cuc gi CLP. Khi chuyn mch khng gian SSM Card P-8A4S:Thc hin chuyn mch khng gian v giao tip vi chuyn tip ng cao tc JHW; thc hin chuyn mch khng gian ca tn hiu thoi nhn c t JHWI ph hp vi tn hiu iu khin t SSC v gi chng ti JHWI. (ii) Trng chuyn mch trong h thng A1000 E10. H thng chuyn mch A1000 E10 c cu trc phn chia theo cc trm (phn h). Trng chuyn mch ca h thng A1000 E10 nm trong trm iu khin ma trn SMX. H thng ma trn chuyn mch thit lp tuyn ni gia cc khe thi gian cho cc n v truy nhp thu bao v cc trm SMT, SMA. c im chnh ca ma trn chuyn mch l: Cu trc kp hon ton (2 nhnh CCXA v CCXB) vi mt tng chuyn mch phn

chia theo thi gian (T).

M rng ti 2048 tuyn ni PCM trn mt nhnh m khng gy ra gin on. Chuyn mch 16 bit qua mi khe thi gian.

Ngoi 8 bit quy c ca knh chuyn mch phn chia theo thi gian, cc bit cn li c dng nh sau: 3 bit iu khin cung cp cc qu trnh kim tra chuyn mch v qun l vic lp li ca h thng ma trn chuyn mch; 5 bit thc hin chuyn mch ngoi bng, v d bng cch truyn cc tn hiu kt hp vi cc tuyn ni chuyn dng.

Hnh 2.14: S cu trc ca CCX H thng ma trn chuyn mch kp c cha cc thit b chn nhnh v khuych i (SAB), mt ma trn chuyn mch chnh (MCX) v cc ng ni ma trn (LR) vi tc 4Mb/s. Cc ng ni ma trn c chia thnh LRA (i vi nhnh A) v LRB (i vi nhnh B). Cu trc ca CCX c ch ra trn hnh 2.14 trn y.

Ma trn chuyn mch chnh MCX c chia thnh 2 nhnh A v B v ng trn gc phn cng, n c to thnh t cc trm iu khin u ni SMX. Mt nhnh ca ma trn chuyn mch chnh bao gm ti a 8 trm SMX nhn tn hiu ng b (8 Mhz) v tn hiu ng b khung t khi c s thi gian kp 3 STS.

mcxb

Ma trn chuyn mch chnh

MCXA

MCXA

Cc trm hoc CSNL

Cc trm hoc CSNL

LRBLRB

SMT SMA CSN

SMT SMA CSN

LRA

H thng ma trn chuyn mch CCX

LRA

LA LA

LA LA

SAB

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Hnh 2.15: Ma trn chuyn mch phn chia thi gian ca SMX Mi trm SMX x l 256 ng ni ma trn vo (LRE) v 256 ng ni ma trn ra (LRS). u vo, khi ra khi cc giao din ILR, cc ng ni LCXE (ng ni ni b ti ma trn chuyn mch chnh, xut pht t mt SMX ti mt SMX khc) cng s th t c a ti v tr ging nhau ca tt c cc trm SMX. Mi ma trn chuyn mch phn knh theo thi gian c kh nng chuyn mch bt k khe thi gian no trong s 2048 LRE ti bt k khe thi gian no trong s 256 LRS. Trong SMX, ma trn phn knh theo thi gian ca n bao gm cc khi c s chuyn mch phn knh theo thi gian 64x64 nh trn hnh 2.15. Cu trc gm 32 ct, mi ct c 4 khi c s to mt ma trn ca SMX vi dung lng ti a 2048 ng vo LRE v 256 ng ra LRS. Tt c hot ng kt ni khe thi gian u thng qua khi c s v thi gian truyn trung bnh l 1 khung (125 ms). SMX l ma trn vung vi mt tng T c cu trc kp hon ton, cho php pht trin u ni n 2048 ng mng (LR). Tm tt chng 2 Trong chng 2 trnh by v cc c im c bn ca k thut chuyn mch knh, tp trung vo gii quyt cc vn : nguyn l hot ng ca trng chuyn mch thi gian, trng chuyn mch khng gian, gii php nng cao dung lng h thng chuyn mch knh thng qua phng php ghp ni cc trng chuyn mch T v S, nguyn l c bn ca k thut nh tuyn trong mng chuyn mch knh v v d v cu trc trng chuyn mch trong thc tin.

Khi c s

1.32 Khi c s

1.2 Khi c s

1.1

Khi c s2.32

Khi c s2.2

Khi c s 2.1

Khi c s3.32

Khi c s3.2

Khi c s 3.1

Khi c s4.32

Khi c s4.2

Khi c s

4.1

256 LRS

64 64 64

64

64

64

64

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Chng 3 K THUT CHUYN MCH GI Trong s pht trin ca h tng vin thng, chuyn mch gi tr nn rt quan trng trong h thng truyn thng khi cc dch v s liu tng v vt s lng dch v thoi. Chng ny s trnh by cc c im quan trng trong k thut chuyn mch gi gm: c s k thut chuyn mch gi, cc nguyn tc, cu trc hot ng ca cc h thng chuyn mch gi, trng chuyn mch gi. Vi mc tiu hng ti cc gii php k thut chuyn mch v nh tuyn, hng tip cn trong chng ny s xoay quanh m hnh kt ni m OSI v gii thiu c th cc cu trc trng chuyn mch gi cng vi phng php lun nghin cu cc vn ny.

3.1. C S K THUT CHUYN MCH GI K thut chuyn mch gi da trn nguyn tc chuyn thng tin qua mng di dng gi. Gi tin l thc th truyn thng hon chnh gm hai phn: Tiu mang cc thng tin iu khin ca mng hoc ca ngi s dng v ti tin l d liu thc cn chuyn qua mng. Qu trnh chuyn thng tin qua mng chuyn mch gi c th khng cn xc lp ng dnh ring v cc mng chuyn mch gi c coi l mng chia s ti nguyn. Cc gi tin s c chuyn giao t cc nt mng ny ti nt mng khc trong mng chuyn mch gi theo nguyn tc lu m v chuyn tip, nn mng chuyn mch gi cn c coi l mng chuyn giao trong khi mng chuyn mch knh c coi l mng trong sut i vi d liu ngi s dng.

Trn hng tip cn tng i n gin t kha cnh dch v cung cp, cc dch v c cung cp trn mng vin thng chia thnh dch v thoi v dch v phi thoi, trong i din cho dch v phi thoi l dch v s liu. Chng ta hiu rng, s ho v gi ho thoi l hai vn hon ton khc nhau, trong mng chuyn mch in thoi cng cng PSTN hin nay tn hiu thoi c s ho, v k thut chuyn mch truyn thng c p dng l k thut chuyn mch knh. D liu thoi ch c gi l gi ho nu nhng gi ny c chuyn ti trn mng chuyn mch gi. Trong mc ny chng ta s xem xt nhng vn k thut c bn c ng dng trong h thng chuyn mch: K thut chuyn mch knh v k thut chuyn mch gi. Mng in thoi cng cng (PSTN) c pht trin trn mng chuyn mch knh cung cp cc dch v thoi truyn thng. Cc mng d liu nh cc mng cc b LAN (Local Area Network), mng Internet l mng chuyn mch gi rt thch hp trao i d liu. Trong Bng 3.1 ta thy s khc bit gia cc dch v thoi (chuyn mch knh) v dch v d liu (chuyn mch gi).

c im Dch v thoi Dch v d liu

Bng thng Bng pht bng thng Nhy cm vi li Pht li thng tin tr Kiu kt ni

C nh v thp (di 64kb/s) Khng m thoi li nu c li Khng th thc hin c Thp v n nh Hng kt ni

thay i (c th ln ti Gb/s) Ln (100/1000:1) Khng cho php li Thc hin d dng Ln v c th thay i C th l phi kt ni

Bng 3.1: So snh mt s c im ca dch v thoi v d liu Cc dch v thoi trong mng PSTN hin nay s dng k thut iu ch PCM v chim bng thng 64kb/s. Nu chng ta c th cung cp bng thng ln hn cho mi cuc gi th cht

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lng cuc gi thoi cng khng v th m tt hn. Tri li, i vi cc dch v d liu bng thng rt quan trng. Mt s ng dng i hi bng thng ti 1Gb/s hoc cao hn. S thay i v bng thng thng c gi l bng n bng thng. Trong khi dch v thoi c cung cp bi k thut chuyn mch knh lun i hi bng thng khng i, ngc li cc dch v d liu c th c nhu cu v bng thng thay i ti hng trm, thm ch hng ngn ln.

tr l tham s rt quan trng nh gi cht lng mng in thoi. Cc cuc gi thoi i hi thi gian tr thp v n nh. Nhiu mng d liu cng c yu cu tr tng i thp, tuy nhin khng i hi s n nh. Chng hn trong khi truyn file vic cc gi tin ca u hay cui file n trc khng c ngha g. m bo tr thp v n nh mng PSTN c thit k l mng nh tuyn theo hng kt ni. Mt s mng d liu cng l mng hng kt ni, tuy nhin mt khi yu cu v tr khng qu ngt ngho th mng d liu thng c xy dng theo m hnh phi kt ni.

3.1.1. M hnh kt ni h thng m OSI. Trong khong gia nhng nm 70, cng nghip my tnh bt u pht trin rt mnh, v nhu cu kt ni thng tin qua mng tng ln rt nhanh. Cc h thng my tnh cn trao i thng tin qua rt nhiu hnh thi khc nhau ca mng. H thng m ra i nhm tiu chun ho cho tt c cc u ni gi l m hnh kt ni h thng m OSI. Mc tiu ca m hnh OSI (Open System Interconnection) l m bo rng bt k mt x l ng dng no u khng nh hng ti trng thi nguyn thu ca dch v, hoc cc x l ng dng c th giao tip trc tip vi cc h thng my tnh khc trn cng lp (nu cc h thng cng c h tr theo tiu chun ca m hnh OSI). M hnh OSI cung cp mt khung lm vic tiu chun cho cc h thng. Cu trc phn lp c s dng trong m hnh v c 7 lp, c th phn loi thnh 2 vng chnh.

Hnh 3.1: M hnh phn lp OSI RM

Lp thp cung cp cc dch v u cui - ti - u cui p ng phng tin truyn s liu, cc chc nng hng v pha mng t lp 3 ti lp 1.

Lp cao cung cp cc dch v ng dng p ng truyn thng tin, cc chc nng hng v ngi s dng t lp 4 ti lp 7.

M hnh OSI c th chia thnh ba mi trng iu hnh: Mi trng mng: lin quan ti cc giao thc, trao i cc bn tin v cc tiu chun lin quan ti cc kiu mng truyn thng s liu khc nhau.

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Mi trng OSI: Cho php thm vo cc giao thc hng ng dng v cc tiu chun cho php cc h thng kt cui trao i thng tin ti h thng khc theo hng m. Mi trng h thng thc: Xy dng trn m hnh OSI v lin quan ti c tnh dch v v phn mm ca ngi sn xut, n c pht trin thc hin nhim v x l thng tin phn tn trong thc t.

Cc lp Chc nng V d ng dng

Lp ng dng Qun l truyn thng gia cc ng dng

Cc giao thc: Telnet, FTP

S dng cc bn tin

Lp trnh din Thm cc cu trc vo n v s liu trao i

Giao thc : Bin i, nn, gii nn.

S dng k thut thay i s liu; nn, v gii nn

Lp phin Qun l cc dch v v iu khin lung s liu

Cc giao thc: SIP, RTCP

S dng cc cuc gi th tc t xa

Lp truyn ti tin cy v ghp cc s liu truyn qua mng

Cc giao thc: TCP, UDP

S dng cc segments

Lp mng Truyn s liu qua mng Cc giao thc: IP,IPX

S dng tiu (datagram)

Lp lin kt s liu

Truyn dn, khung v iu khin li

Cc giao thc: PPP, HDLC, ATM, FR, MPLS

S dng khung, gi, t bo

Lp vt l Giao tip c, in ti phng tin truyn thng

Card giao tip mng

S dng bt

Bng 3.2: Tm tt chc nng cc lp ca m hnh OSI Nhng mi trng ny cung cp nhng c tnh sau: Giao tip gia cc lp. Chc nng ca cc lp, giao thc nh ngha tp hp ca nhng quy tc v nhng quy

c s dng bi lp giao tip vi mt lp tng ng tng t trong h thng t xa khc.

Mi lp cung cp mt tp nh ngha ca nhng dch v ti lp k cn. Mt thc th chuyn thng tin phi i qua tng lp.

Cc chc nng chi tit ca cc lp c nh ngha trong tiu chun ISO 7498 v chun X.200 ca ITU-T. Cc c tnh c bn c tm tt nh sau: (i). Lp ng dng

Cung cp cc dch v truyn thng ca ngi s dng vi cc dng thc s liu, bo hiu iu khin v cc p ng ca cc thit b u cui, cc h thng giao thc iu khin cc ng dng thng qua cc phn t dch v ng dng, qun l truyn thng gia cc ng dng.

(ii). Lp trnh din

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Lp trnh din chu trch nhim to ra cc khun dng d liu cho lp ng dng tng thch gia cc ng dng v h thng truyn thng. Cc giao thc lp trnh din a ra cc ngn ng, c php v tp c tnh ph hp cho truyn thng, ng thi thng nht cc m, d liu cho cc dch v lp ng dng. (iii). Lp phin

Lp phin qun l cc dch v v iu khin lung s liu gia cc ngi s dng tham gia vo phin truyn thng, cc giao thc lp phin ch ra cc lut v phng php thc hin phin truyn thng m khng can thip vo ni dung truyn thng.

(iv). Lp truyn ti Lp truyn ti cung cp cc dch v truyn ti d liu t u cui ti u cui, cung cp kh

nng truyn ti c tin cy gia cc thit b u cui m khng lin quan trc tip ti phn cng mng truyn thng. p ng cc yu cu ca lp phin qua cht lng dch v, kch thc n v d liu, iu khin lung v cc yu cu sa li. (v). Lp mng Lp mng cung cp chc nng nh tuyn, thit lp v qun l cc kt ni trong mng, cc giao thc lp mng cung cp thng tin v cu hnh logic ca mng, a ch v nh x cc kt ni ti cc thit b vt l trong mng. (vi). Lp lin kt d liu

Lp lin kt d liu hot ng trn cc lin kt d liu hoc mt phn mng ca kt ni, lp lin kt d liu cung cp cc chc nng lin quan ti h thng truyn dn nh ng b, iu khin lung d liu, pht hin v sa li truyn dn v ghp hp cc knh logic trn ng dn vt l.

(vii). Lp vt l Lp vt l cung cp mi trng truyn dn, tn hiu ng h v cch thc truyn bit trn phng tin truyn dn. Cc chun ca lp vt l cung cp cc c tnh v nguyn tc giao tip c, in, sng ti phng tin truyn thng.

3.1.2. Nguyn tc c bn ca chuyn mch gi Chng 2 xem xt cc c tnh c bn ca k thut chuyn mch knh, chng ta nhn

thy rng k thut chuyn mch knh thng c ng dng cho cc dch v thi gian thc, hng kt ni v lu lng khng bng pht. Trong khi mc tiu ca chuyn mch gi l s dng cho d liu nn lun phi sn sng chp nhn lu lng bng pht trong khi c th khng cn hng kt ni hoc thi gian thc.

c tnh hng kt ni yu cu cc giai on kt ni phn bit gm: thit lp kt ni, truyn thng tin v gii phng kt ni. Mt kiu kt ni khc i ngc vi kiu hng kt ni l kiu phi kt ni. Phi kt ni cho php cc thc th thng tin c truyn c lp vi cc c tnh kt ni c th hin trong cc tiu thc th thng tin. Cc giai on kt ni nh trong chuyn mch knh khng cn tn ti m thay vo l phng php chuyn theo mt giai on duy nht gm c ba giai on.

Hnh 3.2 di y ch ra s khc bit c bn xt theo phng din kt ni ca cc phng thc chuyn mch knh, chuyn mch bn tin v chuyn mch gi.

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Hnh 3.2: Cc phng php chuyn mch c bn

S khc bit gia chuyn mch bn tin v chuyn mch gi nm ti qu trnh x l bn tin, chuyn mch gi thc hin vic phn on bn tin thnh cc thc th ph hp vi ng truyn v cu hnh mng, cc gi c th c kch thc thay i hoc c nh, phng php chuyn mch vi cc gi c kch thc c nh c gi l chuyn mch t bo (cell). Nh vy, mt bn tin ngi dng c th phn thnh nhiu gi. Sau qu trnh chuyn mch cc gi s c ti hp hon nguyn li thng tin ca ngi s dng [7]

Qu trnh phn mnh v to gi c thc hin ti cc lp trong m hnh OSI th hin trong hnh 3.3 di y.

Bn tin nguyn thu ( di L bit)

Thng tin CRC

Tiu

Trng thng tin c di (M bit)

Ti tin (N bit)

Tiu CRC

Bt bt u khung Bt kt thc khung

Hnh 3.3: ng gi d liu theo m hnh OSI

K thut chuyn mch gi cho php kt ni thng tin t u cui ti u cui qua qu trnh chia s ti nguyn, s dng cc tp th tc v cc lin kt c tc khc nhau truyn cc gi tin v c th chuyn gi trn nhiu ng dn khc nhau. C hai kiu chuyn mch gi c bn: chuyn mch datagram DG (datagram) v chuyn mch knh o VC (Virtual Circuit). Trn hnh 3.4 m t s lc 2 kiu chuyn mch ny.

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Hnh 3.4: Chuyn mch Datagram v chuyn mch knh o

(i).Chuyn mch Datagram: Chuyn mch datagram cung cp cho cc dch v khng yu cu thi gian thc. Vic chuyn gi tin ph thuc vo cc giao thc lp cao hoc ng lin kt d liu. Chuyn mch kiu datagram khng cn giai on thit lp kt ni v rt thch hp i vi dng d liu c lu lng thp v thi gian tn ti ngn. Chuyn mch datagram l chuyn mch kiu n lc ti a (best effort), cc thng tin v tr s khng c m bo cng nh cc hin tng lp gi, mt gi cng d dng xy ra i vi kiu chuyn mch ny. Cc datagram phi cha ton b cc thng tin v a ch ch v cc yu cu ca lp dch v pha trn c th hin trong tiu , v vy tiu ca datagram l kh ln. Tuy nhin, chuyn mch datagram cho php la chn cc con ng ti ch nhanh nht p ng cc thay i nhanh ca mng. (ii) Chuyn mch knh o: Chuyn mch knh o VC (Virtual Channel) yu cu giai on thit lp tuyn gia thit b gi v thit b nhn thng tin, mt knh o c hnh thnh gia cc thit b trong ng dn chuyn mch; knh o l knh ch c xc nh khi c d liu truyn qua v khng ph thuc vo logic thi gian. Chuyn mch knh o yu cu mt tuyn hin ngay trong qu trnh nh tuyn v knh o c nhn dng thng qua trng nhn dng knh o VCI (Virtual Channel Identifier) nm ti tiu gi tin. Trong qu trnh thit lp knh o, nhn dng knh o VCI c to ra bi cc node chuyn mch ch nh cc ngun ti nguyn ca gi tin s chuyn qua (v d: b m, dung lng lin kt).

Mt khi knh o c thit lp dc theo tuyn ng t ngun ti ch qua cc lin kt v cc node th knh c s dng truyn cc gi tin. Cc gi c VCI trong tiu c th c s dng nh con tr truy nhp ti cc thng tin lu tr ti cc nt chuyn mch. Cc trng nhn dng knh o cn phi duy nht phn bit cc thng tin ngi s dng v ti s dng. Nu s dng cc VCI cho ton b mng th s lng VCI rt ln v khng ngng tng ln theo kch c mng. V vy, ngi ta s dng cc nhn dng knh o theo cc vng cc b, thm ch l trn tng lin kt. Vi cch ny, khi mt VC khi to mi mt nt chuyn mch dc tuyn ng s phi xc lp cc nhn dng knh o trn cc lin kt u vo v lin kt u ra ca nt chuyn mch . Cc nt phi tho thun vi nhau v nhn dng knh o duy nht trn lin kt gia hai nt cho mt knh o.

Nhn dng knh o trn cc lin kt u vo v u ra khng cn thit phi ging nhau, vic chuyn thng tin da trn tiu gi tin c cha VCI s thc hin vic chuyn i thng tin trong cc VCI u vo ti VCI u ra. Tt c cc gi c gi trn cng mt knh o VC s theo cng mt ng dn, v vy th t v thi gian tr lan truyn c khng ch. iu ny rt hu ch i vi cc lu lng thi gian thc v c thi gian tn ti di. Nu knh o li hoc hng, cc h thng nh tuyn s tm mt con ng khc thay th. Cc phng php tm

46

ng v thit lp ng dn s c trnh by trong mc nh tuyn. Khi mun gii phng knh o, gi tin iu khin ngt u ni c truyn ti tt c cc thit b m knh o i qua gii phng ti nguyn v knh o s c ti s dng cho cc kt ni khc.

3.2. CC KIN TRC CA TRNG CHUYN MCH GI Trc khi tm hiu cc vn lin quan ti kin trc trng chuyn mch gi, ta xem xt s

lc cc cu trc c bn ca h thng chuyn mch gi hin nay, cc h thng chuyn mch gi trong mng internet c cn c gi l cc b nh tuyn. c rt nhiu cu trc c ng dng cho cc b nh tuyn, cc cu trc c la chn a vo khai thc da trn rt nhiu yu t gm gi thnh, dung lng, cht lng yu cu v cng ngh hin thi. Th h th nht ca b nh tuyn tng i n gin nu chng ta nhn nhn t phng din cu trc (xem trn hnh 3.5).

Hnh 3.5: Kin trc b nh tuyn th h u tin

B nh tuyn s dng bus truyn thng gm mt b x l tp trung a chc nng, b m tp trung v mt BUS chung chia s d liu cho cc card ng truyn u vo v u ra. B nh tuyn nhn cc gi tin ti giao din v gi cc gi