Giao Trinh Ky Thuat Chuyen Mach Va Tong Dai_tc_ngan

7/30/2019 Giao Trinh Ky Thuat Chuyen Mach Va Tong Dai_tc_ngan

1/153

MC LC

CHNG I ......................................................................................................................................2M U .........................................................................................................................................21.1. Khi qut chung v mng vin thng .......................................................................................21.2. Cc cng c hoch nh mng ..................................................................................................61.3. S lc mng vin thng Vit Nam ....................................................................................... 14CHNG II .................................................................................................................................. 18CHUYN MCH KNH .............................................................................................................18

2.1. Phn loi chuyn mch knh .................................................................................................. 182.2. Chuyn mch khng gian tng t .........................................................................................192.3. K thut chuyn mch s ........................................................................................................222.4. Cc cu trc ca cc khi chuyn mch s dung lng ln ................................................... 312.5. iu khin cc khi chuyn mch s......................................................................................36CHNG III ................................................................................................................................. 49CHUYN MCH GI ..................................................................................................................493.1. Nhng khi nim chuyn mch gi ........................................................................................ 493.2. Phng thc hot ng c bn ca mng chuyn mch gi PSN .......................................... 523.3. ng gi thng tin .................................................................................................................. 593.4. nh tuyn trong mng PSN ...................................................................................................64CHNG IV .................................................................................................................................72CC CNG NGH CHUYN MCH TIN TIN ....................................................................724.1. Chuyn mch ATM ................................................................................................................ 724.2. Cng ngh chuyn mch MPLS ............................................................................................. 91CHNG V ................................................................................................................................106BO HIU ..................................................................................................................................1065.1. Phn tch mt cuc gi..........................................................................................................1065.2. Tng quan v bo hiu ..........................................................................................................111

5.3. Bo hiu s 7 .........................................................................................................................1175.4. V d v thit lp cuc gi n gin s dng h thng bo hiu s 7 ..................................1335.5. X l bo hiu trong tng i ...............................................................................................134CHNG VI ...............................................................................................................................144TNG I K THUT S SPC ...............................................................................................1446.1. Gii thiu v tng i SPC (Stored Program Control) .........................................................1446.2. S khi tng qut ca tng i SPC ................................................................................147

1


2/153

CHNG I

M U

1.1. Khi qut chung v mng vin thng

Mng vin thng l phng tin truyn a thng tin t u pht ti u thu.Truyn tin qua mng vin thng l hnh thc truyn thng tin t ni ny ti ni khc

bng cch s dng tn hiu in, in t, quang in thng qua cc thit b mng. Mng

c nhim v cung cp cc dch v cho khch hng.

Mng vin thng bao gm cc thnh phn chnh: thit b chuyn mch, thit b

truyn dn, mi trng truyn v thit b u cui.

1.1.1. Thit b u cui

Thit b u cui ca mng vin thng c th c hiu l thit b giao tip gia

ngi s dng vi mng

Chc nng ca thit b u cui:

- Bin i tin tc cn truyn nh ting ni, vn bn, s liu, hnh nh thnh

dng tn hiu in ph hp c th truyn qua mng vin thng

- Tham gia vo qu trnh thit lp, duy tr, gii phng tuyn truyn tin

Mi thit b u cui thng c ni vo mng bng mt i dy c gi l

ng dy thu bao. Tng ng mi ng dy thu bao s l mt mch in bn trong

mt thit b no ca mng. Mch in c gi l mch in giao tip ng dy

thu bao SLIC (Subscribers Line Interface Circuit). Mch in ny cho php thit b u

cui ca ngi s dng, qua ng dy thu bao giao tip c vi cc thit b bn trong

mng

Thit b u cui cho mng thoi truyn thng gm my in thoi, my Fax, my

tnh, tng i PABX (Private Automatic Branch Exchange).1.1.2. Trung tm chuyn mch

2

Thit bu cui

Trung tmchuyn mch Thit btruyn dn Trung tmchuyn mchThit btruyn dn

Thit bu cui

Mi

trngtruyndn

0

10

20

30

40

50

60

70

80

90

1st Qtr 2ndQ tr

3rdQtr

4thQ tr

East

West

NorthHnh 1.1. Cc thnh phn chnh ca mng vin thng


3/153

Trung tm chuyn mch hay tng i l thit b mng dng to ra tuyn thng tin

gia cc thit b u cui ca cng tng i vi nhau ( cuc gi ni ht) hoc gia cc

thit b u cui ca tng i ny vi thit b u cui ca tng i khc qua ng trung

k (cuc gi lin tng i, cuc gi ng di) hoc gia cc trung k vi nhau (cuc gi

qu giang)Chc nng ca trung tm chuyn mch:

- Thit lp, duy tr v gii phng tuyn ni gia 2 thu bao cn lin lc

- Cung cp v x l cc bo hiu thu bao v bo hiu trung k phc v chc

nng chuyn mch

- T chc, qun l mng (vai tr node mng, qun l trng thi ca cc u cui

v tuyn truyn dn), cung cp giao tip v cc dch v thu bao, phi hp

truyn dn (t chc ngn hng knh, dng, mc tn hiu) bin i v t chc

thng tin bn trong trung tm chuyn mch

1.1.3. Thit b truyn dn

Dng ni thit b u cui vi tng i, hay gia cc tng i thc hin vic

truyn a cc tn hiu in.

1.1.4. Mi trng truyn

Bao gm truyn hu tuyn v v tuyn. Truyn hu tuyn bao gm cp kim loi,

cp quang. Truyn v tuyn bao gm vi ba, v tinh.

1.1.5. T chc mng vin thng

- Mng hp ni

- Mng phn cp

3

Hnh 1.2. T chc mng kiu hp ni

Hng thayth

Trung tm quc t

Trung tm vng


4/153

Khi xy dng mng quc gia cn hoch nh cc cp chuyn mch. Nu mng

khng c phn cp th vic phc v cc cuc gi ng di s gp kh khn do tuyn

ng di khng c hoch nh trc, cuc ni s phi qua nhiu tng i, thi gian

thit lp lu, gi thnh cao, s dng cc thit b km hiu qu, cht lng phc v thp

Mng phn cp cho php khc phc c cc nhc im trn

1.1.6. Cc c im ca mng vin thng hin nay

Cc mng vin thng hin ti c c im chung l tn ti mt cch ring l, ng

vi mi loi dch v thng tin li c t nht mt loi mng vin thng ring bit phc

v dch v n.

Mng Telex: dng gi cc bc in di dng k t c m ho bng 5 bit(m Baudot). Tc truyn rt thp (t 75 ti 300 bit/s).

Mng in thoi cng cng, cn gi l mng POTS (Plain Old Telephone

Service): y thng tin ting ni c s ha v chuyn mch h thng

chuyn mch in thoi cng cng PSTN.

Mng truyn s liu: bao gm cc mng chuyn mch gi trao i s liu gia

cc my tnh da trn giao thc ca X.25 v h thng truyn s liu chuyn mch

knh da trn cc giao thc X.21.

Cc tn hiu truyn hnh c th c truyn theo ba cch: truyn bng sng vtuyn, truyn qua h thng mng truyn hnh cp CATV (Community Antenna

4

Hng trctip

Trung tm lin tnh

Tng i niht

Hnh 1.3. T chc mng kiu phn cp


5/153

Television) bng cp ng trc hoc truyn qua h thng v tinh hay cn gi l

truyn hnh trc tip DBS (Direct Broadcast System).

Trong phm vi c quan, s liu gia cc my tnh c trao i thng qua mng

cc b LAN (Local Area Network) m ni ting nht l mng Ethernet, Token

Bus v Token Ring.Mi mng c thit k cho cc dch v ring bit v khng th s dng cho cc

mc ch khc. V d ta khng th truyn ting ni qua mng chuyn mch gi X.25 v

tr qua mng ny qu ln. Ngi ta chia mng Vin thng theo cc kha cnh sau:

Xt v gc k thut bao gm cc mng chuyn mch, mng truyn dn, mng

truy nhp, mng bo hiu v mng ng b.

Xt v gc dch v th mng Vin thng gm cc mng sau: mng in thoi c

nh, mng in thoi di ng v mng truyn s liu:

PSTN (Public Switching Telephone Network)L mng chuyn mch thoi cng cng. PSTN phc v thoi v bao gm hai loi

tng i: tng i ni ht (cp 5), v tng i tandem(tng i qu giang ni ht, cp 4).

Tng i tandem c ni vo cc tng i Toll gim mc phn cp. Phng php

nng cp cc tandem l b sung cho mi nt mt ATM core. Cc ATM core s cung cp

dch v bng rng cho thu bao, ng thi hp nht cc mng s liu hin nay vo mng

chung ISDN. Cc tng i cp 4 v cp 5 l cc tng i loi ln. Cc tng i ny c

kin trc tp trung, cu trc phn mm v phn cng c quyn.

ISDN (Integrated Service Digital Network)L mng s tch hp dch v. ISDN cung cp nhiu loi ng dng thoi v phi thoi

trong cng mt mng v xy dng giao tip ngi s dng mng a dch v bng mt

s gii hn cc kt ni ISDN cung cp nhiu ng dng khc nhau bao gm cc kt ni

chuyn mch v khng chuyn mch. Cc kt ni chuyn mch ca ISDN bao gm

nhiu chuyn mch thc, chuyn mch gi v s kt hp ca chng. Cc dch v mi

phi tng hp vi cc kt ni chuyn mch s 64 kbit/s. ISDN phi cha s thng minh

cung cp cho cc dch v, bo dng v cc chc nng qun l mng, tuy nhin tnh

thng minh ny c th khng cho mt vi dch v mi v cn c tng cng t

mng hoc t s thng minh thch ng trong cc thit b u cui ca ngi s dng. S

dng kin trc phn lp lm c trng ca truy xut ISDN. Truy xut ca ngi s dng

n ngun ISDN c th khc nhau ty thuc vo dch v yu cu v tnh trng ISDN ca

tng quc gia.

PSDN (Public Switching Data Network)L mng chuyn mch s liu cng cng. PSDN ch yu cung cp cc dch v s

liu. Mng PSDN bao gm cc PoP (Point of Presence) v cc thit b truy nhp t xa.

Hin nay PSDN ang pht trin vi tc rt nhanh do s bng n ca dch v Internet

v cc mng ring o (Virtual Private Network).

5


6/153

Mng di ng GSM (Global System for Mobile Telecom)L mng cung cp dch v thoi tng t nh PSTN nhng qua ng truy nhp v

tuyn. Mng ny chuyn mch da trn cng ngh ghp knh phn thi gian v cng

ngh ghp knh phn tn s. Cc thnh phn c bn ca mng ny l: BSC (Base Station

Controller), BTS (Base Transfer Station), HLR (Home Location Register), VLR ( Visitor

Location Register) v MS ( Mobile Subscriber).

Hin nay cc nh cung cp dch v thu c li nhun phn ln t cc dch v nh

leased line, Frame Relay, ATM, v cc dch v kt ni c bn. Tuy nhin xu hng gim

li nhun t cc dch v ny bt buc cc nh khai thc phi tm dch v mi da trn IP

m bo li nhun lu di. VPN l mt hng i ca cc nh khai thc. Cc dch v

da trn nn IP cung cp kt ni gia mt nhm cc ngi dng xuyn qua mng h tng

cng cng. VPN c th p ng cc nhu cu ca khch hng bng cc kt ni dng mt

vi ti mt vi, cc lp a dch v, cc dch v gi thnh qun l thp, ring t, tch hpxuyn sut cng vi cc mng Intranet/Extranet. Mt nhm cc ngi dng trong

Intranet v Extranet c th hot ng thng qua mng c nh tuyn IP. Cc mng ring

o c chi ph vn hnh thp hn hn so vi mng ring trn phng tin qun l, bng

thng v dung lng. Hiu mt cch n gin, VPN l mt mng m rng t qun nh

mt s la chn c s h tng ca mng WAN. VPN c th lin kt cc ngi dng

thuc mt nhm kn hay gia cc nhm khc nhau. VPN c nh ngha bng mt ch

qun l. Cc thu bao VPN c th di chuyn n mt kt ni mm do tri di t

mng cc b n mng hon chnh. Cc thu bao ny c th dng trong cng (Intranet)hoc khc (Extranet) t chc. Tuy nhin cn lu rng hin nay mng PSTN/ISDN vn

ang l mng cung cp cc dch v d liu.

1.2. Cc cng c hoch nh mng

K hoch nh sTrong phn ny, chng ta s tm hiu v cc nh dng ca cc con s (thnh thong

gi l cc a ch) dng nhn dng cc thu bao ca cc mng Vin thng.

S thu bao (s th mc): Vng a l ca mt quc gia c chia thnh cc vngnh s ring r v cc s thu bao (SN Subscriber numbers) nhn dng cc

ng dy thu bao trong mt vng nh s c th. Mt SN bao gm mt m tng

i (EC Exchange Code) nhn dng mt tng i trong mt vng nh s,

c biu din bi mt s ng truyn (LN) nh sau: SN = EC + LN

S quc gia: Trong mt nc, mt thu bao c nhn dng bi mt s quc gia

(NN National Number), bao gm mt m vng (AC Area Code), m vng l

m dng nhn dng vng nh s, c biu din bi mt s thu bao nh sau:

NN = AC + SL = AC + EC + LN

6


7/153

S quc t: Trn th gii mt thu bao c nhn dng bi mt s quc t (IN

International Number). S ny bao gm mt m quc gia (CC Country Code),

c biu din theo mt s quc gia nh sau:

IN = CC + NN = CC + AC+ EC + LNKhi mt thu bao S1 gi mt thu bao c t cng mt vng nh s, th thu

bao S1 khng quay s thu bao SN. Nu thu bao c gi sng cng mt nc nhng

mt vng khc th S1 quay s NN v nu thu bao c gi sng mt nc khc th

S1 cn phi quay s IN

K hoch nh s quc gia th nh ngha cc nh dng ca thu bao v ca s quc

gia. Hu ht cc quc gia u c k hoch nh s ca ring mnh.

K hoch truyn dnKin trc thc t ca bt k mt mng u ph thuc vo mt s cc yu t, mt

trong nhng yu t quan trng nht l cc tiu chun truyn dn. Bt k mt tn hiu no

c truyn u mc phi hin tng suy gim, mc suy gim t l vi chiu di ca

ng truyn dn. Qu trnh chuyn mch trong tng i cng lm suy gim tn hiu.

tt c cc cuc gi c chp nhn cn phi gi s ng dng ca ting ni ngi

nghe hiu c, v vy mt k hoch truyn dn cho mng lun lun c yu cu. Mt

k hoch truyn dn tnh ton cc tht thot ti a cho php ca tt c cc loi ng

truyn, ng thi cng tnh ton cc tht thot ti thiu, bi v nhng ting lo xo do suy

gim trong tn hiu ting ni l khng th chp nhn c. Hnh 1.5 trnh by mt v d

ca mt k hoch truyn, trn ch ra cc tht thot thng qua i lng Decibels (dB).Cc tht thot ny c c bng nhiu phng php o c khc nhau trong nhiu mng

khc nhau.

7


8/153

Hnh 1.5. V d v mt k hoch truyn dnTrong cc mng ni ht, cc kt ni ca thu bao bao gm cc c p dy ng, mi

thu bao c cp mt c p. Chng c coi nh l phn u t quan trng v km hiu

qu v lng ti trung bnh hng ngy trn mi thu bao l rt thp. Gi thnh c gim

ti thiu bng cch dng cc dy c ch s gauge thp. Tuy nhin, cc dy mnh hn c

suy gim ln hn trn mt n v chiu di. V vy cn phi gii hn chiu di cc kt

ni thu bao. iu ny nh hng v tr ca cc tng i v hoch nh vng mng ni

ht.

Trong mng hp ni, cc tuyn gia cc tng i c dng ph hp vi yu cu

ca ti, v cng ti trn chng cao hn trong mng ni ht. V vy phi dng cc dy

c ch s gauge cao hn gim thiu mc suy gim tn hiu trn mt n v chiu di.

V suy gim khng n nh nn mt vi tuyn c qui m ln hn so vi khuch i t ra

khng kinh t v t c dng. Trong nhng nm gn y ngi ta dng truyn dn s

trong mng hp ni, dng k thut iu ch xung theo m (PCM) khc phc c vn

nu trn. Thuc tnh vn c ca PCM l dng cc ng dn ring bit cho mi hngtruyn, ti to tn hiu thay v khuch i em n cht lng truyn dn cao hn cng

nh n nh hn.

8


9/153

Cng ti trung bnh trn cc tuyn trung k ln hn hoc bng vi cng ti

trn mng hp ni. Ti trung k c t p trung t s ln cc thu bao, v cc tuyn c

h tr mt cch chnh xc ph hp vi nhu cu thc t (ngc li cc mch ni ht

phi c h tr mt cch ty khng ph thuc vo ti trn chng). Hn na, mng

trung k thc hin mt s lng ln c cc im chuyn mch v cc ng truyn dn.Do n tr thnh mt thnh phn cn lm vic khn trng v hiu qu cao trnh tnh

trng tht thot trong vic x l cc cuc gi. iu ny c th thc hin c bng cch

xy dng cc chin lc nh tuyn gii hn s lng cc lin kt trung k trong mi

cuc gi, bng cch khuch i trn cc tuyn analog v dng k thut truyn dn s.

V cc b khuych i l cc thit b khng nh hng nn cc mch 4 dy c

dng trn cc tuyn analog c khuch i. B chuyn i 2 dy sang 4 dy c dng

nhng ni mch trung k khuch i 4 dy c ni vi cc trung tm chuyn mch 2

dy. Do , mt khi s truyn 4 dy ang c s dng th cc trung tm chuyn mch 4dy tr nn c a chung hn. Mt chin lc nh tuyn thng c dng nht l

nu mt cuc gi yu cu nhiu hn hai lin kt trung k, chng s c nh tuyn qua

tng cao nht ca mng trung k trng vi cc tng i 4 dy v cc ng truyn dn

ring. S khuch i gim tht thot qua mng to iu kin mc tht thot c th bng

khng.

Vn suy gim c khc phc mt cch ng k trong cc mng truyn dn s v

c u th v chuyn mch. Bn cht t nhin ca truyn dn s c th t c s n

nh trong cng tc truyn dn, nh c cc b l p (repeater) ti sinh tn hiu s, hn hn

phng php khuych i trong truyn dn tng t v kh nng khng nhiu (noise).

Thc vy, trong mng s ha hon ton, s suy gim cn c xem nh mt phng

php nhn to to cm gic d chu cho ngi nghe. Do , trong mi trng s ha

tt c cc kt ni l rt tt. Hn na hin nay chuyn mch s r hn chuyn mch tng

t. Tt c h thng mng hin i u da trn c chuyn mch s v truyn dn s. Thc

t hin ti cp quang a c thay th cho cc mi trng truyn dn khc.

R rng trong tt c cc cuc gi quc t s dng mt s cc lin kt truyn dn tnht l ca hai quc gia, n i hi phi c khuch i v ti sinh tn hiu. Tt c cc

cuc gi quc t do s c h tr cc ng truyn 4 dy cng nh chuyn mch 4

dy ngay ti tng i chuyn mch quc t. Cc ng cp xuyn i dng v cc

ng viba c cung cp bi cc v tinh hnh thnh nn cc ng truyn quc t c

bn, v cc cu vi ba c dng ph kn trong cc mng chu lc. S phn x tn hiu

tng i lu c dng thng tin vi nhng vng nm bn kia chn tri. V d gia

mt quc gia trn t lin vi cc o xa hay cc tu du. Tt c cc ng truyn dn

quc t mi thng qua v tinh v ng cp xuyn bin u l ng truyn dn s, ngdng nhiu k thut mi nh cp quang lam gia tng cht lng ng truyn quc t.

9


10/153

K hoch nh tuynK hoch th 3 rt quan trng iu hnh mng, n quyt nh tnh hiu qu hot

ng ca mng, l k hoch nh tuyn. K hoch ny nh ra tt c cc tiu chun

nh tuyn cho cc cuc gi di mi tnh hung. N ch ra rng trong mt mng hp ni

mt cuc gi c th c nh tuyn gia hai tng i hoc qua mt lin kt trc tip hay

qua mt hay nhiu im trung gian. Lin kt trc tip c cung cp ty theo mt tiu

chun no , chng hn nh nu ti ln hn mt mc qui nh gia hai tng i v cc

qui nh ny l c th ha cc tiu chun, l mt phn ca k hoch nh tuyn.

Tng t, trong mng trung k, k hoch nh tuyn bao gm cc lut xc nh

nhim v cn thit ca cc tng i trung k, lm th no chng ni vi nhau, chng c

kin trc phn cp hay khng, hay tt c trn mt mng ngang hng. Trong cc mng

tng t, k hoch nh tuyn b nh hng bi k hoch truyn, n nh ra s ti a cc

lin kt khng cn khuych i c th c dng trn mt cuc gi, v do ch ra slin kt hp ni ti a, v tt c cc lin kt trung k u c khuych i, v cng ch

ra s ti a cc lin kt khuych i 4 dy khi chuyn mch 2 dy c dng. Bi v mi

lin kt phi c mt tht thot xc nh (tiu biu l 3 dB) m bo tnh n nh.

Trong mt mng s c nhiu iu lu khc trong k hoch nh tuyn.

C nhiu kha cnh v k hoch nh tuyn. V d cc mch trn bt k mt tuyn

no l mt hng hay hai hng; iu ny c ngha l chng c th tip nhn cuc

gi trn mt hng hay c hai hng. K hoch nh tuyn phi c cc lut cho cc quyt

nh ph hp vi tnh kinh t v k thut, v xem cc mch hai hng c hu ch trnmi tuyn hay khng.

Hnh 1.6. nh tuyn t ng c hai la chn

10


11/153


12/153

hin khng c lin h mt cch n gin vi s cuc gi cung cp. Thi gian chim

hu l mt thc th c th o lng v c xem nh l ti c chuyn. Tng thi gian

ca cc cuc gi chia cho khong thi gian gim st(vi cc n v tnh trc) gi l

cng ti. n v tnh l erlang(E).

Trong v d trn, mt mch c gn 60 pht chim hu mch trong khong thigian 1 gi, do cng ti l mt erlang.

Tng t, cng ti c th c tnh ton cho mt nhm mch. V d trn hnh

1.7trnh by mt nhm 5 mch, mi mch thc hin mt s cc cuc gi trong khong

thi gian 2 gi. Cc cuc gi b tht bi do tc nghn khng tnh n.

Trong nhm ny:

Tng thi gian gi= 349 pht

Cng ti= 349/(2x60) = 2,9 erlang(E)

Cng ti trn mt mch = 2,9/5 = 0,58ECng ti cng c th c tnh bng cch o lng ngay tc thi, trong trng

hp ny n bng s cuc gi trung bnh c x l.

A = Cxh/T

Trong : C l s cuc gi c x l trong thi gian cho trc;

h l thi gian gi trung bnh trn mt cuc gi;

T l thi gian xem xt.

xc nh mt cch chnh xc kh nng ca cc tng i v cc tuyn, ng thi

d on cng ti trong tng lai khi xt duyt cc k hoch mng, cn phi o lng

ti ti cc im khc nhau trong mng. Trong khi mong mun t c cc kt qu chnh

xc hon ho th vic gn cc ng h o ti vo mi mch u cui trn tng i l

khng kinh t. Mt phng php ly mu thun tin hn s c dng. Trong tng i

SPC vic ghi c thc hin qua phn mm, n c th thc hin gim st ton b. Tuy

nhin vic x l d liu c th rt nng n v t tin.

Cc ngha ch yu ca vic ly mu l kim tra cc mch trong khong thi gian

chim hu theo nh k.Tng s thi gian gi c pht hin c chia cho s ln kimth c c thi gian gi trung bnh. V d, nu kim th nhm ca cc mch nh

trong hnh 1.7 c thc hin mi 10 pht, nh trnh by bng cc ng dc, thi gian

cc mch bn l 36 pht trong khong 2 gi. V c 12 mu, ti trung bnh c thc hin

bi nhm c tnh bng 36/12 = 3,0E. iu ny rt ging vi gi tr trung bnh 349/120

= 2,9E t c bng cch chia tng thi gian bn thc t vi khong thi gian xem xt

tnh bng pht.

Ti thay i ty vo thi gian trong ngy, cc ngy trong tun, ma v v tr a l.

Cc thu bao c nhn thc hin cuc gi mt cch ngu nhin, mi tng i v mi tuyntri qua cc khong thi gian cao im s dng trong mi ngy. Trong cc tng i thuc

12


13/153

vng kinh t trng im, gi cao im thng l bui sng. Trong cc vng dn c c th

xy ra vo bui ti. Trong cc vng trng im kinh t, ti gim vo ngy ch nht v

thng cao im vo gia tun. Mt khc ti quc ni cao im vo cui tun khi cc gia

nh sum hp v gi cc gim. Ti quc t thng gia tng vo ma h.

Hnh 1.7.Lc biu din s chim gi ca cc cct trong mt nhm gm 5 mch c

gim st nh k 10 giy

Tng t, ti t cc thu bao c nhn th yu hn. Trong mt ngy ch vi cuc gi,ti trn cc thu bao ny ch c cng khong 0,33 erlang. Tuy nhin, v ti t nhiu

thu bao v mt tng i, mc trung bnh ti ln hn c th d on c trong bt c

thi gian cho trc no. Khi ti qua qu trnh x l ca tng i n tr nn trong sut

hn. Trn cc tuyn hp ni cng nh cc ng trung k ti tr nn thun thc v trong

sut. Cc tuyn ny c kh nng vn chuyn ln hn mc ti a c th. Cng tng t,

mt tng i c thit k vi cc thit b c kh nng thc thi cho lng ti d on

thay v cn c trn tng ti ca thu bao trong trng hp cng khi ng ng thi.

iu ny nhn ra rng s c trng hp mt cuc gi n tng i s khng c png.

Kh nng thc thi ti ln nht vp phi s gim st lin tc bi cc thit b s dng

theo ch ngn hn, v vy mc tht thot cho php c chn v kh nng ti t n

mt mc cho trc vi mc tht thot qui nh trong gi cao im. V d, nu mt

cuc gi tht bi trong mt trm cuc gi th hon ton c th chp nhn c.

Cc nghin cu ton hc v lu thoi hay l thuyt v lu lng trn mng Vin

thng c dng m bo kh nng tht thot cuc gi mt mc c th chp

nhn c i vi cc thu bao, ng thi c tnh kinh t i vi s gim st. Tuy nhin,

13


14/153

cn nh rng cc ng ni ht phi c cung cp trn mi thu bao v y l cc

ngun pht c bn ca tt c ti.

S lng chnh xc ca thit b, hay mch c cung cp bng cch tnh ton t cc

bng c dn xut t l thuyt lu lng. Nh trong tt c cc ng dng ton hc, tnh

cht thay i cn phi c xem xt cc iu kin bn trong tng i, ti ngu nhin, vdng php phn b xc sut xp x s lng ti.

k hoch y th cn o lng ti trong sut thi gian bn. Trong nhng nm

gn y, nhm gia tng vic dng in thoi, c trong phm vi quc gia cng nh cc

vng kinh t trng im, thi gian bn c tng thm mt s gi v khong thi gian o

lng ph hp khng phi lun lun trng mt cch chnh xc vi thi gian cao im.

i khi cc kt qu l khng y nhng hot ng ghi trong cc tng i SPC c th

hn ch c vn ny.

1.3. S lc mng vin thng Vit Nam

Cu trc mng phc v cho cc dch v thng tin nh thoi, s liu, fax, telex v cc dch v

khc nh in thoi di ng , nhn tin, nn nc ta hin nay ngoi mng chuyn mch

cng cng cn c cc mng ca mt s dch v khc. Ring mng Telex khng kt ni

vi mng thoi ca VNPT, cn cc mng khc u c kt ni vo mng ca VNPT

thng qua cc knh trung k hoc cc b MSU (Main Switch Unit), mt s khc li truy

nhp vo mng PSTN qua cc knh thu bao bnh thng, s dng k thut DLC (DigitalLoop Carrier), k thut truy nhp v tuyn,

V cu trc mng, mng vin thng ca VNPT hin nay chia thnh 3 cp: cp quc

t, cp quc gia, cp ni tnh/thnh ph.

Xt v kha cnh cc chc nng ca cc h thng thit b trn mng th mng vin

thng bao gm: mng chuyn mch, mng truy nhp, mng truyn dn v cc mng chc

nng.

Mng chuyn mch

Mng chuyn mch c 4 cp (da trn cc cp tng i chuyn mch): qu giangquc t, qu giang ng di, ni tnh v ni ht. Ring ti thnh ph H Ch Minh c

thm cp qu giang ni ht.

Hin nay mng VNPT c cc trung tm chuyn mch quc t v chuyn mch

quc gia H Ni, Nng, Thnh ph H Ch Minh. Mch ca cc bu in tnh cng

ang pht trin m rng. Nhiu tnh, thnh ph xut hin cc cu trc mng vi nhiu

tng i Host, cc thnh ph ln nh H Ni, Thnh ph H Ch Minh v ang trin

khai cc Tandem ni ht.

Mng vin thng ca VNPT hin ti c chia lm 5 cp, trong tng lai s cgim t 5 cp xung 4 cp.

14


15/153

Mng ny do cc thnh vin ca VNPT iu hnh: l VTI, VTN v cc bu in

tnh. VTI qun l cc tng i chuyn mch qu giang quc t, VTN qun l cc tng i

chuyn mch qu giang ng di ti 3 trung tm H Ni, Nng v TpHCM. Phn

cn li do cc bu in tnh qun l.

Cc loi tng i c trn mng vin thng Vit Nam: A1000E ca Alcatel,NEAX61 ca NEC, AXE10 ca Ericsson, EWSD ca Siemens.

Cc cng ngh chuyn mch c s dng: chuyn mch knh (PSTN), X.25 relay,

ATM (s liu).

Nhn chung mng chuyn mch ti Vit Nam cn nhiu cp v vic iu khin b

phn tn trong mng (iu khin nm ti cc tng i).

Mng truyn dnCc h thng thit b truyn dn trn mng vin thng VNPT hin nay ch yu s

dng hai loi cng ngh l: cp quang SDH v viba PDH. Mng truyn dn c 2 cp:mng truyn dn lin tnh v mng truyn dn ni tnh.

Mng truyn dn lin tnh: Bao gm cc h thng truyn dn bng cp quang,

bng v tuyn.

o Mng truyn dn lin tnh bng cp quang: Mng truyn dn ng trc

quc gia ni gia H Ni v TpHCM di 4000km, s dng STM-16/2F-BSHR,

c chia thnh 4 vng ring ti H Tnh, Nng, Qui Nhn v TpHCM.

Vng 1: H Ni H Tnh (884km)

Vng 2: H Tnh Nng (834km)Vng 3: Nng Qui Nhn (817km)

Vng 4: Qui Nhn TpHCM (1424km)

Cc ng truyn dn khc: H Ni Hi Phng, H Ni Ha Bnh, TpHCM

Vng Tu, H Ni Ph L Nam nh, Nng Tam K. Cc tuyn truyn dn lin

tnh ny dng STM-4. Ring tuyn H Ni Nam nh, Nng Tam K vn cn s

dng PDH, trong tng lai s thay th bng SDH.

o Mng truyn dn lin tnh bng v tuyn: Dng h thng vi ba SDH (STM-

1, dung lng 155Mbps), PDH (dung lng 4Mbps, 6Mbps, 140Mbps). Ch c

tuyn Bi Chy Hn Gai dng SDH, cc tuyn khc dng PDH.

Mng truyn dn ni tnh: Khong 88% cc tuyn truyn dn ni tnh s dng h

thng viba. Trong tng lai khi nhu cu ti tng th cc tuyn ny s c thay th

bi h thng truyn dn quang.

Mng bo hiuHin nay trn mng vin thng Vit Nam s dng c hai loi bo hiu R2 v SS7.

Mng bo hiu s 7 (SS7) c a vo khai thc ti Vit Nam theo chin lc trin khait trn xung di theo tiu chun ca ITU (khai thc th nghim t nm 1995 ti VTN

v VTI). Cho n nay, mng bo hiu s 7 hnh thnh vi mt cp STP (im chuyn

15


16/153

mch bo hiu) ti 3 trung tm (H Ni, Nng, H Ch Minh) ca 3 khu vc (Bc,

Trung, Nam) v phc v kh hiu qu.

Hnh 1.4. Mng bo hiu Vit Nam

Bo hiu cho PSTN ta c R2 v SS7, i vi mng truyn s liu qua IP c H.323,

i vi ISDN c bo hiu knh D, Q.931,

Mng ng bMng ng b ca VNPT thc hin xy dng giai on 1 v giai on 2 vi ba

ng h ch PRC ti H Ni, Nng, TP H Ch Minh v mt s ng h th cpSSU. Mng ng b Vit Nam hot ng theo nguyn tc ch t c d phng, bao gm 4

cp, hai loi giao din chuyn giao tn hiu ng b ch yu l 2 MHz v 2 Mb/s. Pha 3

ca qu trnh pht trin mng ng b ang c trin khai nhm nng cao hn na cht

lng mng v cht lng dch v.

Cc cp ca mng ng b c phn thnh 4 cp nh sau:

Cp 0: cp ng h ch.

Cp 1: cp nt quc t v nt quc gia.

Cp 2: cp nt ni ht.

Cp 3: cp nt ni ht.

Mng c phn thnh 3 vng c lp, mi vng c 2 ng h mu, mt ng h

chnh (Cesium) v mt ng h d phng (GSP). Cc ng h ny c t ti trung tm

ca 3 vng v c iu chnh theo phng thc cn ng b.

Cc tng i quc t v Toll trong vng c iu khin bi ng h ch theo

phng php ch t.

Cc tng i Tandem v Host ti cc tnh hot ng bm theo cc tng i Toll theophng php ch t. Cc tng i huyn (RSS) cng hot ng bm theo cc Host theo

phng php ch t.

16


17/153

Mng qun lD n xy dng trung tm qun l mng vin thng quc gia ang trong qu trnh

chun b tin ti trin khai.

Cc nh cung cp dch vuTi nc ta c 2 dng nh cung cp dch v: l cc nh cung cp dch v truyn

thng (ch yu l thoi) v nh cung cp dch v mi (cc dch v s liu, Internet, ).Cc nh khai thc dch v truyn thng bao gm tng cng ty bu chnh vin thng

Vit Nam (VNPT), cng ty vin thng qun i (Vietel), cng ty c phn vin thng Si

Gn (SPT), cng ty vin thng in lc (ETC).

Cc nh khai thc dch v mi bao gm FPT, SPT, Netnam,

17


18/153

.

CHNG II

CHUYN MCH KNH

2.1. Phn loi chuyn mch knh

Nh ta bit thng tin c trao i qua mng vin thng rt a dng. Mi loi c

yu cu ring i vi thit b truyn dn v chuyn mch. C loi thng tin rt nhy cm

vi tr nh thng tin thoi, thng tin hnh nh; v vy chng yu cu thit b dng

truyn dn v chuyn mch chng phi khng gy tr hoc c tr th phi rt nh

ngi dng khng cm nhn c v gi tr tr phi l hng s trong sut thi gian

ca cuc lin lc khng gy mo tn s tn hiu. i vi loi thng tin ny-thng tin

dch v thi gian thc th nn dng chuyn mch knh phc v.

Tn gi chuyn mch knh th hin nguyn l hot ng ca mng. Khi phc vmt cuc lin lc gia hai thit b u cui no th mt knh thng tin (n cng hoc

song cng ty thuc hnh thc lin lc) hoc mt tuyn ni c thit lp xuyn qua

mng t mch in ca thit b u cui ny ti mch in ca thit b u cui kia. Knh

thng tin hoc tuyn ni tn ti dnh ring cho cuc lin lc sut t khi bt u cho

ti khi kt thc cuc gi. Cc cuc gi c tin hnh mt cch ng thi qua mng th

phi c phc v trn cc knh c lp nhau. Trong lch s pht trin k thut chuyn

mch mt s phng thc chuyn mch knh c p dng trong cc h thng tng

i, l: Chuyn mch phn knh theo khng gian dng cho tn hiu lin tc, chuyn

mch phn knh theo thi gian i vi tn hiu ri rc ha ( tn hiu iu bin xung PAM

Pulse Amplitude Modulation), chuyn mch s PCM (Pulse Code Modulation).

Ngi ta thng k cc loi chuyn mch knh nh hnh di y

Vi chuyn mch khng gian tng t c th phc v cuc gi song cng trn 2

dy v c th cho qua c tn hiu analog, digital, thm ch c tn hiu ri rc PAM

Nguyn l chuyn mch PAM da trn vic phn chia cc khe thi gian. Mi phn

t chuyn mch c th phc v cho nhiu cuc gi c tin hnh ng thi theo nguyntc cho tn hiu ca cc cuc gi khc nhau qua n trong cc khe thi gian khc nhau

theo tng chu k. Tn hiu qua phn t chuyn mch l tn hiu c ri rc ha c

chu k lp ca cc xung iu bin bng vi chu k lp ca cc khe dnh cho tng mch

cho tn hiu ca mi cuc gi i qua. Tuy cc chuyn mch PAM khng c p dng

mt cch rng ri do nhiu hn ch ca chng, nhng chng l nn tng cho cng ngh

chuyn mch tin tin hn sau ny nh l chuyn mch s PCM

18


19/153

2.2. Chuyn mch khng gian tng t

2.2.1. Khi qut

Chuyn mch khng gian tng t to ra tuyn ni truyn thng tin hai hng qua

cc phn t chuyn mch ni gia mch in ca thu bao ny vi mch in ca thu

bao khc ang lin lc vi n. Gia cc tuyn ni ny khng c cc on mch in dngchung.

Trng chuyn mch khng gian tng t c th c to thnh t mt hoc nhiu

b chuyn mch khng gian c bn theo cc cu trc ghp khc nhau. S phn t chuyn

mch dng to ra mt tuyn ni xuyn qua trng chuyn mch ca tng i ph

thuc vo s cp chn, s khu trong mi cp chn v cch ghp ni cc b chuyn

mch. Mi b chuyn mch khng gian c bn c k hiu v c m t chc nng

nh hnh di y.

Hnh 2.1. M hnh chc nng chuyn mch khng gian

Chuyn mch knh

Chuyn mchkhng gian tng t Chuyn mch sChuyn mch PAM

Nhncng

Tthch

inchung

Tng

Nhync

Ta

Matrn

2dy

4dy

Tsw SswThp

Hnh 2.1. Phn loi chuyn mch knh

19


20/153

c im chung ca cc chuyn mch loi ny l:

- S u vo v s u ra c th ging hoc khc nhau

- Nu c nhiu cuc gi i qua b chuyn mch th phi m bo mt u vo ch

ni vi khng qu mt u ra

2.2.2. Cu to v nguyn l hot ng ca b chuyn mch khng gian c bnCu to

Nhn chung cc b chuyn mch khng gian tng t gm 2 phn: phn ma trn

chuyn mch v phn iu khin. Phn ma trn chuyn mch gm N u vo v M u

ra. Giao im gia mt u ra v mi u vo l mt phn t chuyn mch. Cc phn t

chuyn mch c th l r le in t, r le ng kn (Heccon), cc loi bn dn nh diode,

tranzitor. Phn iu khin chuyn mch thng l b nh v gii m s liu iu khin

a ra tn hiu iu khin tng phn t ca ma trn chuyn mch.

Yu cu chung i vi cc phn t chuyn mch ca b chuyn mch khng gian

tng t l khng lm suy hao tn hiu, khng gy xuyn m, khng to ra tp m nn.

iu ny c th hiu n gin l khi trng thi m (ni thng) th tr khng ca phn t

chuyn mch phi cc nh (l tng l =0) khng lm tn hao tn hiu truyn qua

n. Cn khi trng thi ng (ngt mch, h mch) phi c tr khng rt ln () tn

hiu t mt u vo khng b truyn ti cc u ra khng mong mun, d ch l rt nh,

nh th tn hiu knh thng tin ny khng b xuyn sang cuc ni khc. H s chuyn

mch H=tr khng h mch/tr khng kn mch (108). Ngoi ra, tp m nn (tp mni) to ra nhng nhiu ro trong tn hiu, cn tr vic truyn v thu cc tn hiu hu ch.

Cc tp m nn do tip xc ca cc tip im, cht lng linh kin gy ra.

Nguyn l hot ng

Theo nguyn l phn knh khng gian, khi tuyn ni cho mt cuc gi c thit

lp i qua mt b chuyn mch c bn no th phn t chuyn mch c dng

thit lp tuyn ni cho cuc gi s b chim gi t khi bt u thit lp tuyn ni

m thoi cho ti khi tuyn ni c gii phng v n c dnh ring cho cuc gi

trong sut thi gian tin hnh cuc gi. Vic truyn tn hiu cn c tin hnh trn 2

hoc nhiu dy v vy phn chuyn mch thng gm 2 hoc nhiu lp. Phn t chuyn

mch ti mi giao im phi c nhiu cp tip im, mi cp dng cho mt lp v c

chung iu khin hoc mi lp c phn t chuyn mch ti mi giao im nhng cc

phn t ca tng giao im cng tn c u iu khin chung.

Mt s k hiu v khi nim chung:

20


21/153

K hiu kiu ma trn tip im K hiu kiu ma trn giao im

K hiu kiu b chn

K hiu kiu quy c

- B chuyn mch ton thng: L b chuyn mch m mi u vo u c kh nng

ni ti u ra bt k. i vi loi b chuyn mch ny phi c quan h 1:1 gia u vo-

phn t chuyn mch-u ra, ngha l trn giao im gia mt u vo bt k vi mt

u ra bt k u phi c phn t chuyn mch tng ng.

- B chuyn mch khng ton thng: Loi ny khc loi trn l mt s giao im ca matrn chuyn khng c phn t chuyn mch. Nh th mt s u vo khng th ni ti u ra

no

21


22/153

2.3. K thut chuyn mch s

2.3.1. Gii thiu chung

Chuyn mch s l qu trnh lin kt cc khe thi gian gia mt s cc lin kt

truyn dn k thut s TDM. iu ny cho php cc tuyn s 2Mbps hay 1,5 Mbps t cc

tng i khc hay cc PABX k thut s c kt cui mt cch trc tip trn chuyn

mch s, khng cn chuyn i sang cc knh thoi thnh phn cho chuyn mch ging

nh trong mt tng i tng t. S b bt thit b nh th trn mi knh lm cho

chuyn mch s c xem l c u im v gi c v kch thc. D nhin, bt c mt

mch tng t no kt cui trn tng i chuyn mch s hoc l cc ng thu bao

hoc l cc mch trung k hay hp ni, u phi c chuyn sang dng PCM trc khivo cc chuyn mch s. Tng t cc mch ri khi tng i trn cc phng tin

truyn dn tng t cng phi c chuyn t s sang tng t ngay ti ngoi vi ca

khi chuyn mch. Cc chuyn i A/D v D/A ny, cng vi bt k s chuyn i bo

hiu cn thit no c m trch bi thit b lin mng

Vai tr ca thit b lin kt mng c m t trong mt kin trc tng qut hnh

3.1. Hnh ny trnh by cc lung s PCM nhp vo khi chuyn mch mt cch trc tip

ngay mc ghp knh, trong khi cc mch tng t kt cui ti mc mch ring trn thit

b lin kt mng. Do , thit b lin kt mng nh phi chu s tht thot u im v gi

c v kch thc so vi mt tng i chuyn mch s. i vi cc tng i trong mi

trng truyn dn tng t chim u th, iu ny c th l rt quan trng. Gi c lin

kt mng cao do cc b A/D v D/A t tin, khin cc ng dng thc tin u tin ca

chuyn mch s ch p dng trong cc tng i trung k v hp ni sut thi gian t cui

thp nin 60 n u thp nin 70. Cc tng i nh vy hot ng nh l cc mode

chuyn mch trung gian gia cc h thng ng truyn PCM, sau pht trin trong

cc mng trung k v hp ni. S pht trin ca cc tng i cc b k thut s gi cph hp phi i cho n khi cc h thng m ha ting ni r tin xut hin vo cui

thp nin 70, nh ti thiu c gi c lin mng trn mi ng dy thu bao.

22


23/153

Chng ny tp trung ch yu vo c cu ca chuyn mch s v cu trc ca cc

khi chuyn mch s thc t. Cc chc nng ngoi vi kt cui thu bao v cc ng

trung k s c xem xt chi tit trong chng k tip.

Trc khi xem xt chuyn mch s, cn cn nhc li mt s cc thut ng. H thng

chuyn mch trong mt tng i ty trng hp c gi vi tn khc nhau chuynmch, mng chuyn mch, mng chuyn mch trung tm hay khi chuyn mch.

trnh nhm ln vi cc thut ng c dng m t cc mng in thoi, trong ti

liu ny dng chuyn mch m t mt phn t chuyn mch, v khi chuyn mch

m t mt nhm cc chuyn mch. V d nh khi chuyn mch tp trung thu bao.

Khi chuynmch s

Thit b linm ng(A/D,signall ing

conversion,timedivis ionmultiplexing)

Thit b l inmn g(A/D,signall ing

conversion,timedivis ionmultiplexing)

H thng PCM

Tng i k thut s

A

B

C

D

E

F

G

HCc ng tngt(ana log) t c ctng i khc v

cc thu bao

Cc ng s tcc tng i khc

Cc ng s i cc tng i khc

Cc ng tngt(ana log) i n cctng i khc v c

thu bao khc

Hnh 3.1. Vai tr ca mt khi chuyn mchMt khi chuyn mch s cung cp cc kt ni gia mt s cc h thng PCM, mi

h thng thu bao gm 30 hay 24 knh trong mt khung TDM. Cc h thng PCM kt

thc ti khi chuyn mch trn cc bus tc cao. Trong mn K thut truyn dn

m t cc mu t mi knh hnh thnh nn cc t m PCM 8 bit nh th no, cc t m

ny c truyn trong cc khe thi gian trn mt bus ng nhp vo mt khe thi gian trn

mt bus li ra. Mc d thut ng knh v khe thi gian l ring bit, nhng chng

c dng ng ngha vi nhau trong ti liu. cho r rng, s m t ny xem chuynmch s n gin nh l cc lin kt khe thi gian, tm li pha sau cc m t cc dng

khc(nh d liu, bo hiu, kim tra, qun tr) c truyn qua khe thi gian ca knh.

Ngay lc ny rt hu ch khi xem xt mt v d n gin v kt ni xuyn qua mt

khi chuyn mch. Tham kho hnh 3.1, xem xt mt cuc gi c mang trong khe TS6

ca h thng PCM A, n yu cu kt ni n tng i qua tuyn trung k ca h thng

PCM F. Nu TS6 rnh trn h thng PCM F, th kt ni c th c thit lp bng cch

lin kt hai h thng PCM trong khong thi gian khi TS6 n mt cch ng thi trn c

hai h thng. Qu trnh ny l mt cuc ni khng gian n gin v c gi l chuyn

mch khng gian s.

23


24/153

Tuy nhin, ch da vo chuyn mch khng gian trong mt khi chuyn mch s s

lm xut hin cc vn tc nghn nghim trng do nhiu kh nng hai hay nhiu cuc

gi tranh chp cng mt khe thi gian li ra. V d tc nghn s xy ra nu TS6 ca h

thng PCM F ang bn(gi s ang kt ni vi TS6 ca h thng PCM C) v do

khng sn sng cho kt ni vi TS6 ca h thng PCM A. S tc nghn ny c th trnhc bng cch chn mt khe thi gian khc TS6 trn h thng PCM F cho cuc gi t

h thng A. Thng thng iu ny c th, bi v bt c khe thi gian t do no trn mt

tuyn n mt tng i u thch hp cho vic mang thng tin cuc gi. Kt ni gia khe

thi gian TS6 trn h thng PCM A vi mt vi khe thi gian khc trn h thng PCM F

lin h khng ch chuyn mch khng gian s gia hai h thng PCM m cn lin h vi

chuyn mch thi gian gia cc khe thi gian khc nhau ng nhp v ng xut. Cc

khi chuyn mch thc t thng thng dng mt t hp chuyn mch khng gian v

chuyn mch thi gian.Trong cc phn sau y, cc chuyn mch khng gian v chuyn mch thi gian

c m t mt cch ring bit trc khi s t hp chng vo cc khi chuyn mch

c xem xt. Cc m t ny gi s rng tt c cc h thng PCM kt thc khi chuyn

mch c ng b tt c cc khe thi gian tng ng n mt cch ng thi xuyn

qua cc chuyn mch. Do , trn hnh 3.1 TS1 ca h thng PCM A xy ra ng thi

vi cc TS1 ca cc h thng B, C, D, E, F, G, H v TS1 ca khi chuyn mch. Chc

chn hn, gi s rng cc kt ni xuyn qua cc chuyn mch c thit lp.

2.3.2. Chuyn mch khng gian k thut s

Tng chuyn mch khng gian s S(Space Switch Stage) cu to t mt ma trn

chuyn mch kch thc N u vo v M u ra vt l. Lu rng y l h thng TDM

s, do mi ng vt l cha n knh thi gian m chng mang cc tn hiu PCM.

Nh vy kt ni mt khe thi gian bt k no trong mt ng PCM bt k pha u

vo ca ma trn chuyn mch ti khe thi gian tng ng (ngha l c cng m s TS)

ca mt ng PCM bt k pha u ra ca ma trn th mt im chuyn mch thch hp

ca ma trn chuyn mch cn phi hot ng trong sut thi gian ca TS v lp li vi

chu k T = 125sec trong sut qu trnh to knh. Trong cc thi gian khc, vn im

chuyn mch c th s dng cho cc qu trnh ni khc. Tng t nh vy i vi tt

c cc im chuyn mch khc ca ma trn c th c s dng thit lp knh ni cho

cc cuc gi khc nhau.

Chuyn mch khng gian tn hiu TDM-s thng thit lp ng thi mt s lng

ln cc cuc ni qua ma trn vi tc tc th trong mt khung tn hiu 125 sec, trong

mi cuc ni qua ma trn tn ti trong thi gian ca mt khe thi gian TS. Mt cucgi in thoi c th ko di trong khong thi gian nhiu khung tn hiu PCM (thng

24


25/153

thng khong 1,2 2 triu khung v tng ng vi khong t 3-5 pht). Do vy mt

kiu iu khin theo chu k n gin cho mt mu ni l cn thit. iu ny d dng t

c nh mt b nh RAM iu khin cc b lin quan ti ma trn chuyn mch khng

gian.

Hnh 3.2 minh ha nguyn tc cu to v hot ng ca mt tng chuyn mchkhng gian S. Chuyn mch tng S cu to t 2 thnh phn c bn Ma trn chuyn

mch v khi iu khin chuyn mch cc b.

Ma trn chuyn mch vung kch thc NxN, trong hng dng cho cc ng

PCM pha u vo v ct dng cho cc ng PCM pha u ra. Ti giao im ca hng

v ct u ni im chuyn mch v thng thng l cng logic AND hay cng logic

3 trng thi. Ch rng AND hay cng logic ba trng thi l mch logic khng nh, do

vy chuyn mch cho cng mt khe thi gian gia u vo v u ra ca phn t chuyn

mch. Cc im chuyn mch trong mi ct c iu khin bi mt b nh iu khinC-Mem(Control Memory).

DEC

TScount

Selector

Add

C Mem

Clk

RData

W

012

WR /

Cc ng ra

SpaceMatrix

n

CC

u vo 1

u ra 1

u vo 2

u vo n

u ra n

Locall Controller

H

nh 3.2. Nguyn l chuyn mch tng S

Khi iu khin cc b bao gm b m khe thi thi gian TSCounter, b chna ch Selector v b nh iu khin C-Mem thc hin chc nng iu khin cc b

25


26/153

ma trn chuyn mch. B nh C-Mem lu tr cc s liu lin quan ti cc im chuyn

mch tng ng vi cc khe thi gian TS trong khung tn hiu cho.

M a ch nh phn c gn cho mi im chuyn mch trong mt ct. Mi a

ch thch hp sau s c s dng chn mt im chuyn mch yu cu thit lp

cuc ni gia mt u vo vi mt u ra ca ma trn chuyn mch. Cc a ch chnny c nh trong b nh iu khin C-Mem theo th t khe thi gian tng ng vi

vi biu thi gian kt ni hin thi. Nh vy i vi ct 1, a ch ca im chuyn

mch s c thng mch trong thi gian TS0 s c nh trong nh c a ch 0 ca

C-Mem cho ct, a ch ca im chuyn mch s thng mch trong khe thi gian TS1 s

c nh trong nh a ch 1. Tng t nh vy i vi tt c cc a ch khc trong

tng chuyn mch.

di ca cc nh C-Mem c xc nh trn c s a ch nh phn ca cc

im chuyn mch trong ct, ngha l c ldN(s nguyn ln hn nh nht) bits, cn slng nh ca C-Mem bng s lng khe thi gian TS c trong mt khung tn hiu ca

ng TDM s. Ngay sau khi b nh iu khin C-Mem c np s liu cc a ch ca

cc im chuyn mch trong ct, th qu trnh iu khin chuyn mch c th thc hin

bng cch c cc ni dung ca mi nh C-Mem trong thi gian thch hp tng ng

vi khe thi gian yu cu, s dng cc s liu a ch chn im chuyn mch cn

thit m n s thng mch trong thi gian ca TS nu trn. Qu trnh ny s c tip tc

lp li cho ti khi tt c cc nh ca C-Mem c c v cc im chuyn mch c

iu khin mt cch thch hp. Tip theo th tc ny s c lp li vi s chu k T =

125sec, bt u vi nh u tin ca C-Mem. Mi chu k l mt khung ca Format

tn hiu s s dng v trong thi gian t hp m tn hiu PCM t mi khe thi gian

u vo c th s c chuyn mch ti mt khe thi gian thch hp ti mt u ra xc

nh.

T hnh 3.2 ta c th nhn thy rng mi C-Mem ch iu khin mt ct ca ma trn

v do cch trang b ny gi l iu khin u ra. Tt nhin cng c th trang b iu

khin theo u vo.Kho st phn tch cu to v hot ng ca chuyn mch s tng S trn y ch

r rng chuyn mch tng S c vn nghim trng do hin tng blocking gy ra do

xc sut tranh chp ln khi c hai hay nhiu cuc gi cng xut hin cc u vo khc

nhau nhng cng mun chim cng mt khe thi gian trong lung PCM u ra ca ma

trn chuyn mch. Hin tng blocking c th c khc phc bng cch tm chn cc

khe thi gian ri khc nhau, iu ny c th thc hin c bi v bt k khe thi gian ri

no trong hng cho cng c th dng cho cuc gi xc nh. Ngoi ra dng kt hp

gia chuyn mch tng S vi chuyn mch tng T (Time Switch Stage) va c th phttrin dung lng khi chuyn mch va gim c hin tng blocking.

26


27/153

Sau y chng ta s kho st mt v d m t nguyn tc hot ng chuyn mch

to knh ca tng S. V d m t hot ng ca tng S phc v cho mt cuc ni gia

TS0 ca lung tn hiu PCM1 u vo vi TS0 ca lung tn hiu PCM1 pha u ra. Cn

c vo yu cu chuyn mch c th cho, trc ht h thng iu khin trung tm

CC(Central Control) ca tng i s to cc s liu iu khin np vo b nh C-Memca tng S. T hnh 3.2 ta thy im chuyn mch duy nht c th m bo cho yu cu

kt ni PCM1 pha u vo vi PCM1 pha u ra l AND11, do CC to m a ch nh

phn cho phn t AND11 ny. M theo yu cu phi thc hin chuyn mch cho khe thi

gian TS0 do vy CC s chim nh c a ch m nh phn 0 tng ng ca C-Mem.

Cc s liu c bn c CC np a ch nh phn AND11 vo nh 0 ca C-Mem tng S,

xong ri n giao quyn iu khin cho khi iu khin cc b(Locall Controller) iu

khin trc tip qu trnh tip theo.

m bo cho tng chuyn mch S hot ng chnh xc, yu cu tn hiu ng hphi hon ton ng b vi thi im bt u ca mi khe thi gian TS trong khung tn

hiu PCM c s dng.

Nh vy, khi bt u mt khung tn hiu PCM tn hiu ng h th nht tc ng

vo b m khe thi gian TS-Counter lm cho b m ny thit lp trng thi 0 c m

nh phn tng ng vi a ch nh 0 ca C-Mem, nh b chn a ch Selector m

trng thi ny c a ti BUS a ch ca b nh C-Mem. ng thi vi vic to m

a ch, Selector to ra tn hiu iu khin c a ti C-Mem, do ni dung cha trong

nh 0 c a ra thanh ghi gii m. V ni dung ny li chnh l a ch ca phn t

chuyn mch AND11, do to c tn hiu iu khin im chuyn mch ny, nh

tn hiu PCM cha trong khe thi gian TS0 ca PCM1 pha u vo c chuyn qua

phn t chuyn mch AND11 hng ti PCM1 pha u ra ca ma trn chuyn mch

S, tc l thc hin chc nng chuyn mch.

Kt thc thi gian ca TS0, xung ng h th 2 tc ng vo TS-Counter lm n

chuyn sang trng thi 1 c m nh phn tng ng vi a ch nh 1 ca C-Mem. Nh

vy kt thc vic to tn hiu iu khin cho AND 11 i vi qu trnh chuyn mch choTS0 theo yu cu. Tng t nh vy i vi cc khe thi gian tip theo v th tc c

lp li vi chu k T =125sec trong sut qu trnh thit lp ni cho cuc gi ang xt.

Khi cuc gi kt thc CC nhn bit v n s gii phng cuc ni mt cch n gin

bng hot ng xa s liu ghi vo C-Mem nh nu khi bt u cuc gi. Trong

cc tng chuyn mch S thc t, cc bits tn hiu PCM thng c ghp knh to lung

tc cao v bin i thnh dng song song trc khi qua tng S. V d nh lung tn

hiu s PCM 32 vi tc truyn bit ni ting l 2,048 Mbit/s c mang trong i dy

n a ti b bin i ni tip song song.

27


28/153

2.3.3. Chuyn mch thi gian s

Chng ta nhn thy rng cu to v hot ng ca chuyn mch tng S ch thc hin

cho cc qu trnh chuyn mch c cng ch s khe thi gian gia ng PCM vo v

ng PCM ra. Trong trng hp tng qut c yu cu trao i khe thi gian gia u

vo v u ra khc nhau th phi ng dng tng chuyn mch thi gian T(Time SwitchStage)

Trn hnh 3.3 di y minh ha qu trnh trao i khe thi gian gia TS2 v TS5

cho hai khung lin tip nhau gia ng PCM vo v PCM ra ca tng chuyn mch T.

0 1 2 3 4 5 6 n

T= 125s

0 1 2 3 4 5 6 n

T= 125s

TS#Cc ng SHW vo

0 1 2 3 4 5 6 n

T= 125s

0 1 2 3 4 5 6 n

T= 125s

TS#Cc ng SHW ra

0 1 2 3 4 5 6 n0 1 2 3 4 5 6 n

Tr 3TSSHW vo SHW ra

SHW: SubhighWay

Hnh 3.3. Trao i khe thi gianV cc khe thi gian TS c sp xp lin tip nhau theo th t tng dn, do vy

trao i thng tin gia cc khe thi gian TS2 v TS5, tn hiu PCM trong TS2 cn phic lu tm thi ti tng T trong khong thi gian 3 TS trong cng mt khung, sau

vo khe thi gian ca TS5, tn hiu PCM c a ra ng PCM pha u ra ca tng

chuyn mch.

Trong trng hp nu cn chuyn mch gia khe thi gian u ra vi khe thi

gian c ch s ln hn pha u vo, v d TS6 v TS2 nh minh ha trn hnh 3.4 th

tn hiu khng th tr trong cng mt khung m phi tr ti khung tip theo, c th l (n-

6) +2 khe thi gian.

28


29/153

0 1 2 3 4 5 6 n

T= 125s

0 1 2 3 4 5 6 n

T= 125s

TS#Cc ng SHW vo

0 1 2 3 4 5 6 n

T= 125s

0 1 2 3 4 5 6 n

T= 125s

TS#Cc ng SHW ra

0 1 2 3 4 5 6 n0 1 2 3 4 5 6 n

Tr (n- 6 +2)TSSHW vo SHW ra

SHW: SubhighWay

Hnh 3.4. Nguyn l chuyn mch thi gianNh vy, v nguyn tc i vi tn hiu s c nhiu c ch to tr thi gian

theo yu cu song vi nhng tnh nng u vit ca cng ngh vi mch hin i v tc v gi thnh, ngy nay b nh RAM c s dng trong tt c cc h thng chuyn mch

s DSS (Digital Switching System)

Nguyn l cu to ca chuyn mch tng T bao gm 02 thnh phn chnh l b nh tin S-

Mem (Speak Memory) v b nh iu khin C-Mem nh hnh 3.5 minh ha di y.

Chc nng c bn ca S-Mem l nh tm thi cc tn hiu PCM cha trong mi khe

thi gian pha u vo to tr thch hp theo yu cu m n c gi tr t nh nht l

1TS ti cc i l (n-1)TS.

Nu vic ghi cc tn hiu PCM cha trong cc khe thi gian TS pha u vo catng chuyn mch T vo S-Mem c thc hin mt cch tun t th c th s dng mt

b m nh phn Module(n) cng vi b chn rt n gin iu khin. Lu rng khi

tn hiu ng h phi hon ton ng b vi cc thi im u ca TS trong khung tn

hiu PCM c s dng trong h.

B nh C-Mem c chc nng dng iu khin qu trnh c thng tin lu

m ti S-Mem. Cng nh C-Mem trong chuyn mch tng S, b nh C-Mem ca tng T

cng c n nh bng s lng khe thi gian trong khung tn hiu PCM s dng. Trong

thi gian mi TS, C-Mem iu khin qu trnh c mt nh tng ng thch hp trong

T-Mem. Nh vy hiu qu tr ca tn hiu PCM ca T-Mem c xc nh mt cch r

rng rnh mch bi hiu s gia cc khe thi gian ghi v c tin PCM b nh S-Mem.

Tht l th v t c ch chuyn mch nu trn ta nhn thy rng tng chuyn T hot ng

khng bnh thng trong cch phn chia thi gian. Cng mt b nh C-Mem, cc nh

c s dng mt cch c quyn cho mt cuc gi xc nh trong sut thi gian ca

cuc ni. Nh vy chng ta c iu nghch l rng chuyn mch khng gian S c phn

chia thi gian trong khi chuyn mch thi gian T li c phn chia theo khng gian.

29


30/153

hiu nguyn l hot ng ca chuyn mch thi gian T, ta s xt v d sau y.

Gi s c yu cu chuyn mch phc v cho cuc ni gia TS5 ca lung tn hiu PCM

u vo vi TS9 ca lung tn hiu PCM u ra ca chuyn mch tng T nh minh ha

hnh 3.5.

TScount

Selector 2

Add

C Mem

ClkR

Data

W

012

WR /

n

Selector 1

CC

Locall Controller

WR

012

n

WR /

S Mem

Add

SHW vo

SHW ra

WriteRead

Chu trnh R /W

TS

Hnh 3.5. Cu to ca tng chuyn mch thi gian k thut sCn c yu cu chuyn mch, h thng iu khin trung tm CC ca tng i s to

cc s liu iu khin cho tng T. thc hin iu ny CC ca tng i s np s liu

v a ch nh phn nh s 5 ca T-Mem vo nh s 9 ca C-Mem, sau CC giao

quyn iu khin cc b cho chuyn mch tng T trc tip thc hin qu trnh trao ikhe thi gian theo yu cu chuyn mch.

Tip theo cho qu trnh m t c hon ton xc nh v d theo di, chng ta

kho st t thi im bt u TS0 ca khung tn hiu PCM. Qu trnh ghi thng tin PCM

cha trong cc khe thi gian pha u vo b nh S-Mem c thc hin mt cch ln

lt v ng b nh hot ng phi hp gia b m khe thi gian TS-Counter v b

chn a ch Selector1. C th l khi bt u khe thi gian TS0, tn hiu ng h tc ng

vo TS-Counter lm n thit lp trng thi 0 to t hp m nh phn tng ng vi a

ch m nh phn nh 0 ca S-Mem. B chn a ch Selector 1 c s dng iu

khin c hay ghi b nh S-Mem (RAM), trong trng hp ny n chuyn m a ch

30


31/153

ny vo Bus a ch Add ca S-Mem ng thi to tn hiu iu khin ghi W, do vy t

hp m tn hiu PCM cha trong khe thi gian TS0 ca lung s u vo c ghi vo

nh s 0 ca S-Mem. Kt thc thi gian TS0 cng l bt u TS1 song ng h li tc

ng vo TS- Counter lm cho n chuyn sang trng thi 1 to a ch nh phn cho

nh s 1 ca S-Mem. Selector1 chuyn s liu ny vo Bus a ch ca S-Mem, ng thito tn hiu iu khin ghi W do t hp m tn hiu PCM trong khe thi gian TS1 ca

lung s u vo c ghi vo nh 1 ca S-Mem. Qu trnh xy ra tng t i vi cc

khe thi gian TS2, TS3, TS4, TS5 v tip theo cho ti khe thi gian cui cng TSn ca

khung. Sau tip tc lp li cho cc khung tip theo trong sut thi gian thit lp cuc

ni yu cu.

Bt u khe thi gian TS9, tn hiu ng h tc ng vo TS-Counter lm n

chuyn trng thi to m nh phn tng ng a ch nh s 9 ca C-Mem. B chn a

ch Selector2 chuyn s liu ny vo Bus a ch ca C-Mem ng thi to tn hiu iukhin c R cho b nh C-Mem; kt qu l ni dung cha trong nh s 9 ca C-Mem

c a ra ngoi hng ti Bus a ch c pha u vo ca Selector1. V ni dung ca

nh s 9 C-Mem l a ch nh phn ca nh s 5 ca S-Mem do vy b chn a ch

Selector1 chuyn a ch ny vo Bus a ch ca S-Mem, ng thi n to c tn hiu

iu khin c R ca S-Mem. Kt qu l ni dung cha trong nh s 5 ca S-Mem

c a ra ngoi vo khong thi gian ca khe thi gian TS9, ngha l thc hin

ng chc nng chuyn mch yu cu cho trc. Qu trnh tip tc lp li nh trn vi

chu k 125s vi cc khung tip theo cho ti khi kt thc cuc ni.

C ch hot ng ca chuyn mch tng T nh trnh by trn y l qu trnh ghi

tn hiu PCM vo S-Mem c thc hin mt cch tun t, cn qu trnh ghi tn hiu

PCM vo S-Mem c thc hin theo yu cu theo cch ngu nhin. Ch lm vic

nh vy ca chuyn mch tng T gi l ghi tun t c ngu nhin vit tt l

SWRR(Sequencial Write Random Read). Ngoi ch SWSR trong thc tin cn phi

s dng ch ghi ngu nhin, c tun t RWSR(Random Write Sequencial Read)

m chng ta s kho st khi m t cu trc v hot ng ca tng chuyn mch s ghpkt hp T-S-T sau ny.

2.4. Cc cu trc ca cc khi chuyn mch s dung lng ln

2.4.1. Gii thiu chung

Trong cc ng dng thc t ca cc khi chuyn mch tn hiu s ta thng phi

gii quyt hai vn quan trng l cht lng dch v QoS(Quality of Service) v dung

lng cn thit ca khi chuyn mch yu cu. Cht lng phc v ch yu ph thuc

vo hin tin blocking v hin tin ny vi xc sut kh ln khi ch s dng cc chuyn

mch tng S. i vi tng T nh m t trn y n c th m bo chc nng chuynmch khng blocking cho tt c cc khe thi gian trong lung tn hiu tc cao PCM

31


32/153

m n m nhim phc v. V d vi h thng 32 lung PCM 30/32 c ghp knh s

thnh mt lung cao tc PCM 1024 TS hng ti chuyn mch tng T n l th tt c

1024 TS c th c kt ni mt cch t do m khng gy hin tng blocking. Nu mt

trng chuyn mch c xy dng bng mt tng T nh vy th dung lng thc t ca

n l 512 TS (1024/2) thc hin kt ni cc knh PCM theo c hai chiu thu/pht. Tuyvy, trong cc ng dng thc t tng i ni ht, trng chuyn mch ngoi vic to

knh cho knh thoi cn phi to knh cho bo hiu v iu khin. Do , vi mt tng T

n th trng chuyn mch ch bo m c khong 450 thu bao ngha l dung lng

tng i qu nh.

Ngoi ra i vi cng ngh ch to khi kch thc tng S tng ln th s lng chn

ra ca vi mch cng s rt ln gy kh khn ch to vi mch. Cn vic tng dung lng

ca chuyn mch tng T th b hn ch bi cng ngh ch to vi mch nh RAM v cc

mch logic iu khin lin quan. Nh vy vic tng dung lng trng chuyn mch s m bo cho s lng thu bao v trung k ln ty theo yu cu ch cn cch phi

xy dng trng chuyn mch s dng kt hp s dng kt hp cc chuyn mch tng T

v S tiu chun. C rt nhiu phng php ghp kt hp gia cc chuyn mch tng S v

T, v d nh T-S, S-T, S-T-S, T-S-T, T-S-S-T,

Do c kh nng tip thng hon ton v khng c hin tng blocking nn ngi ta

mong mun ch s dng mt tng T. Tuy vy mt tng T ch dng lm khi chuyn mch

khng blocking c dung lng ti a 1024TS. Vi cu trc hai tng T-S v S- T ch thch

hp cho cc tng chuyn mch dung lng nh v va. Nhng vi phng n ny xc

sut blocking s tng nhanh cng vi s gia tng dung lng ca tng chuyn mch S.

Do vy cc tng i dung lng va v ln nhm mc tiu gim blocking v tng dung

lng khi chuyn mch ngi ta thng dng cu trc ba tng.

Trc y, cu trc S-T-S c s dng nhng t cui thp nin 70 tr li y cu

trc T-S-T chim u th hn v ngy nay cu trc ny c s dng rng ri nht. S d

trc y ngi ta s dng S-T-S l v vi trnh cng ngh lc trnh chi ph ln

cho tc hot ng cao ca vi mch. Ngy nay cc u im v tc cao ca RAM b li c v chi ph gi thnh cho c hai cng ngh chuyn mch S v chuyn mch T

do m cu trc T-S-T c a chung hn.

Trong cc tng i dung lng cc ln, cc chuyn mch tng S c tc dng chia

nh trng chuyn mch thnh mt s tng thnh phn nhm hn ch kch thc ca

chng do cc cu trc 4 hoc 5 tng T-S-S-T hoc T-S-S-S-T c ng dng. Lu

rng vic s dng cu trc chuyn mch tng S a tng gim c tng chi ph gi

thnh nhng s tng chi ph gii quyt vn blocking.

Theo l thuyt cu trc chuyn mch T-S-T c h s tp trung l 1:1 c th bo mkhng xy ra blocking nu s lng khe thi gian ni b qua tng chuyn mch S l 2n-

32


33/153

1, trong n l s lng khe thi gian trong cc lung PCM vo v ra ca tng chuyn

mch T ngoi vi. Tuy vy thm ch l c khi m s lng khe thi gian trong v ngoi

bng nhau th cht lng phc v QoS vn rt tt (khong 3.10-17 cho knh c lu lng

0,7 Erl v s tng ln ti 4,7.10-8 khi lu lng knh l 0,8 Erl). Hn na, do khng phi

tt c cc khe thi gian ngoi lung PCM c s dng truyn ti lu lng m skhe thi gian ni thng lun lun c sn cho vic nh tuyn lu lng qua chuyn

mch tng S v ln hn so vi s lng khe thi gian TS ngoi, nh m thm ch c

vi nhng knh lu lng cao 0,8 Erl cht lng dch v QoS ca T-S-T vn c th c

gi tr t 10-8 n 10-10.

Tm li vic la chn cu trc c th ph thuc vo nhiu yu t nh phc tp,

kch thc trng chuyn mch, lu lng phc v, kch thc Module, kh nng kim

tra o th bo dng, m rng dung lng,Trong cc cu trc ghp cc tng chuyn

mch th cu trc T-S-T c s dng rng ri nht v n c thit k di dng ccModule c kch thc ph hp vi cng ngh, ng dng thc t v d pht trin, d vn

hnh v bo dng.

2.4.2. Khi chuyn mch T-S-T

Khi chuyn mch s cu trc T-S-T cu to t 3 tng chuyn mch T1, S v T2 kt

ni vi nhau nh minh ha trn hnh 3.6.

S-Mem

C-Mem

S-Mem

C-Mem

S-Mem

C-Mem

S-Mem

C-Mem

4 15

4

C-Mem

0-3 15

9 15

ICHW#0

OGHW#3ICHW#3

OGHW#0

9

Hnh 3.6. Trng chuyn mch s T-S-T

33


34/153

Tng chuyn mch thi gian T1 pha u vo kt ni khe thi gian vo vi mt khe

thi gian ri no trong ng Bus dn ti u vo ca tng chuyn mhc khng gian

S. Trong khi tng chuyn mch thi gian T2 pha u ra kt ni khe thi gian c

chn t chuyn mch tng S ti khe thi gian ra yu cu. Nh vy cuc gi c kt ni

qua trng chuyn mch c th c nh tuyn qua tng S vi bt k khe thi gian thchhp no.

Ph hp vi tnh cht ng dng ca cc lung ghp knh s cao tc PCM t bn

ngoi vo/ra khi chuyn mch T-S-T, cc chuyn mch thi gian tng T1 lm vic

theo ch SWRR cn cc chuyn mch thi gian tng T2 ngc li lm vic theo ch

RWSR. Ngoi ra u im ca ch hot ng c la chn trn y lm cho vic

iu khin ni b khi chuyn mch tr nn r rng, n gin v d thc hin hn.

Thng thng dung lng ca cc chuyn mch thi gian T khong 1024 TS, cn kch

thc ca ma trn chuyn mch S l 8x8, 16x16 v 64x64 ng cao tcHW(HighWay).

gii thch qu trnh thc hin nhim v chuyn mch ca khi chuyn mch s

cu trc T-S-T trn hnh 3.6 ch r cc s liu iu khin m CC to v np vo cc

C-Mem iu khin qu trnh chuyn mch phc v cho cuc ni gia khe thi gian

TS4 ca lung s cao tc u vo 0 ICHW#0 vi khe thi gian TS9 ca lung tn hiu

PCM cao tc u ra 2 OGHW#3 qua khe thi gian trung gian TS15 tng chuyn mch

khng gian S.

2.4.3. Khi chuyn mch knh 2 hng

ng dng cc tng chuyn mch nh trnh by trn y c th xy dng khi

chuyn mch n hng c kch thc bt k. Tuy nhin, trong thc t lun lun c nhu

cu phi thit lp knh song hng qua cc tng chuyn mch ch dn tn hiu mt chiu.

gii quyt yu cu ny ngy cng cn phi s dng hai ng dn mt chiu kt hp

to thnh mt ng dn hai chiu. C hai phng php c s dng thc hin

nhim v nu trn:

ng dn th hai c thit k mt cch hon ton c lp vi ng dn thnht

C hai ng dn c thit lp phi hp cng nhau.

34


35/153

Tng T1

IT#0

TS10TS9

ICSHW0

IT#1ICSHW1

ICSHW2IT#2

TS11TS19

ICSHW3IT#3

Tng T2

OT#0

TS11TS9

OGSHW0

OT#1OGSHW1

OGSHW2OT#2

TS19TS10

OGSHW3OT#3

Tng S

Hnh 3.7. Khi chuyn mch TST hai hngD thy rng phng php th nht s cho linh hot ca h thng cao hn,

trong khi phng php th hai li lm cho h thng c kh nng tit kim phn cng

hn nh tnh i xng ca thit b chuyn mch. Mt khc iu khin phng php th

hai cng n gin hn v vic tm chn ng ni cho c hai hng c thc hin trong

mt php tm chn trong khi vi phng php th nht yu cu hai php tm chn

ring bit cho mi hng truyn dn. Hnh 3.7 minh ha nguyn tc ca khi chuyn

mch theo phng n th nht.

S-Mem

S-Mem

C-Mem

S-Mem

S-Mem

2 7 + n/2

ICHW#0

OGHW#3ICHW#3

OGHW#0

2 7

Ti A

Ti B

n

n

0

9

n

0

2

9

n

0

C-Mem

T B

0

2

T A

C-Mem

n9 7 + n/2

9 7

0

0-3 7

n

0

3-07 + n/2

Hnh 3.8. Trng chuyn mch song hng kiu i pha

35


36/153

Gi s theo yu cu phi chuyn mch cho cuc ni gia TS9 ICHW#0 vi TS19

OGHW#3 (nh hnh 3.7). thc hin kt ni cho qu trnh trn trc ht CC tm chn

mt khe thi gian trung gian(ni b) ri kt ni chuyn mch thi gian pha u vo

IT#0 vi chuyn mch thi gian pha u ra OT#3 qua chuyn mch khng gian S. Gi

thit ti thi im trong Bus PCM ni b tng S cc khe thi gian TS10 v TS11 l rido vy CC c th chn cc khe thi gian ny lm khe trung gian cho cuc gi. CC s lnh

np cc s liu thch hp vo cc b nh C-Mem c th nh sau:

- Ti IT#0: nh 10 ca C-Mem a ch nh 9 ca S-Mem.

- Ti tng S: nh 10 ca C-Mem ct 1 a ch im chuyn mch 0-3

- Ti OT#3: nh 10 ca C-Mem a ch nh 19 ca S-Mem.

to hng ngc li CC lnh np cc s liu c th nh sau:

- Ti IT#3: nh 11 ca C-Mem a ch nh 19 ca S-Mem.

- Ti tng S: nh 11 ca C-Mem ct 1 a ch im chuyn mch3-0- Ti OT#0: nh 11 ca C-Mem a ch nh s 9 ca S-Mem.

Kt thc cuc ni CC s xa cc s liu trn y kt thc qu trnh trao i khe

thi gian v gii phng cuc ni, cc nh v cc khe thi gian tr v trng thi ri c

th phc v cho cc cuc gi ni tip theo.

Hnh 3.8 minh ha mt cch khc iu khin vic to knh hai chiu thc hin theo

phng php th 2. S ny cn c tn gi l phng php i pha. Theo phng php

ny nu tm c mt ng ni t A n B trong mt khe thi gian xc nh th ng

ngc li s lun lun c m bo na sau ca khung thi gian c s dng. Trong

v d minh ha trn hnh 3.8 hng thun c s dng khe thi gian trung gian s 7 v

tip theo hng ngc s chim dng khe thi gian (7+n/2). Phng php ny kt hp

vi vic gim c s lng b nh iu khin.

2.5. iu khin cc khi chuyn mch s

Trong cc mc trc y trnh by cc vn lin quan ti cc vn lin quan

ti cc phng php qun l khe thi gian qua cc trng chuyn mch s trng thi

bn vng xc lp, ngha l knh c chn v thit lp xong. Cn trong phn ny s cp n vn to cc s liu iu khin lin quan thch hp cho cc b nh iu

khin ca khi chuyn mch. Chc nng thit lp hay gii phng ng ni qua khi

chuyn mch thc cht l np vo hay xa b cc s liu a ch lin quan trong cc b

nh iu khin ca khi chuyn mch. Chc nng thit lp hay gii phng ng ni qua

khi chuyn mch thc cht l np vo hay xa b cc s liu a ch lin quan trong cc

b nh iu khin C-Mem. Hot ng ny l kt qu ca cc tc ng tng tc gia h

thng iu khin trung tm CC ca tng i v cc khi iu khin cc b chuyn dng

lin kt vi cc khi chuyn mch.

36


37/153

2.5.1. S khi v cc chc nng

Thnh phn iu khin ca khi chuyn mch s theo chc nng c th chia thnh

ba thnh phn chnh gm h thng iu khin trung tm CC, b iu khin khi chuyn

mch v iu khin qu trnh chuyn mch nh hnh 4.9 minh ha.

H thng iu khin trung tm CC m bo nhim v iu khin chung mc caocho tt c cc hot ng ca h thng chuyn mch bao gm cc chc nng x l cuc

gi. Trn hnh 4.9 trnh by mt h thng iu khin m nh mt thc th tp trung n

l nhng trong thc t th n thng c thc hin theo cu trc phn tn hn l tp

trung.

Trong mt h thng chuyn mch c th ch c mt hoc c nhiu khi chuyn

mch, v d nh trong tng i Transit c mt khi cn trong tng i ni ht li gm

mt khi chuyn mch trung tm v mt s khi chuyn mch tp trung thu bao. Mi

khi chuyn mch c b iu khin khi chuyn mch ring ca n v mi chuyn mchtng S/T trong khi chuyn mch li c n v iu khin ring cu thnh t cc b

nh iu khin lin kt vi mch logic iu khin cc b. Di y s trnh by cch

thc cc h thng iu khin thit lp ni qua cc khi chuyn mch s.

H thng iu khin trung tm CC

T

n vK

B iu khin khi CM

S T

n vK

B iu khin khi CM

S

n vK

T

n vK

n vK

Khi tp trung thu bao Khi chuyn mch trung tm

Hnh 3.9. iu khin khi chuyn mch sB iu khin khi chuyn mch c chc nng m bo vic qun l tt c cc khe

thi gian qua khi chuyn mch. Cc cng vic qun l in hnh bao gm:

Thit lp knh ni

Gii phng knh

Chun b kt ni Theo di knh ni

37


38/153


39/153

Mbntin

Trng tinM bntin tham

kho

Msali

b) Bn tin kiu 2(T b iu khin khi CM n CC

Mbntin

Mtontc

M tngchuyn mch

IT#

MlungPCM

M TS

Khe thi gian gi vo

M tngchuyn mch

IT#

MlungPCM

MTS

Khe thi gian gi raMsali

a) Bn tin kiu 1(T CC n b iu khin khi Chuyn mch

M tngchuyn mchIT#

NidungC-Mem

B

Tng chuyn mch IT

M tngchuyn mchOT#

M ct

Tng chuyn mch S

Ni dungC-Mem

P

Tng chuyn mch OT

NidungC-Mem

B P

a) Bn tin kiu 3(T b iu khin khi Chuyn mch n b iu khin khi CM

Hnh 3.10. Cc format bn tin iu khin khi chuyn mch

Cc lnh iu khin t h thng iu khin trung tm ti b iu khin khi chuyn

mch thng thng c truyn di dng bn tin mc cao sao cho t c hiu qu

iu khin cao v ti a s dng cc tnh nng ca cc b x l trong h thng iu khin

trung tm. V d v format bn tin ca lnh iu khin c minh ha trn hnh 3.10 (bntin loi 1). Cng ging nh bt k mt tn hiu da trn c s format bn tin no khc,

format bn tin hnh 3.10a s dng mt s trng s liu c bit. Mc d format bn tin

thc t kiu bn tin loi 1 s rt khc nhau ty thuc vo thit k ca cc nh cung cp

khc nhau, song cc trng s liu di y s lun lun cn c:

M ton tc: S liu ny ch th yu cu knh ni s c thit lp, chun b, gim

st hay gii phng,. m ha ton tc qun l t 1-6 nu trn y cho c hai

kiu knh n hng hay song hng cc trng s liu 4 bit c yu cu.

Nhm trng s liu khe thi gian u vo: Nhm trng s liu ny ch r ach knh vo di dng m s tng chuyn mch S/T, m s lung PCM v m s

khe thi gian trong lung tn hiu s PCM. Mc d l mt a ch n l nhng n

nhn dng c hai khe thi gian thu v pht ti u vo ca tng chuyn mch. Kch

thc ca hai trng s liu u tin ph thuc vo s lng tng chuyn mch

u vo v s lng lung PCM trong tng chuyn mch tng ng. Trng s

liu m s khe thi gian c dung lng 5 bit i vi cc lung PCM32 v PCM24

knh.

39


40/153

Nhm trng s liu khe thi gian ra: Nhm trng s liu ny ch r cc a ch

khe thi gian ra trong khun dng Format bn tin tng t nh format bn tin ca

nhm trng s liu khe thi gian u vo m t trn y.

M pht hin v sa li: Trng s liu ny ca format bn tin cho php b iu

khin khi chuyn mch pht hin bt k s sai li no xy ra trong bn tin m ngy ra trong qu trnh truyn tin t h thng iu khin trung tm n b iu

khin khi chuyn mch. Mt kiu m nh vy c th n gin l kim tra chn l

hay phc tp hn l m CRC(Cyclic Redundance Code).

M bn tin: Mi mt bn tin cn c gn mt nhn vi m s n gin c

trng cho vic xc nh chun chui lin tip cc bn tin pht v x l. Vic s

dng m bn tin nh trn cho php b iu khin khi chuyn mch c th thng

bo cho h thng iu khin trung tm bit c mt bn tin c th no nhn

c cha sai li v nh yu cu h thng iu khin trung tm pht li bn tin.Khi thu bn tin loi 1, b iu khin khi chuyn mch thc thi cc vi

lnh(instruction). Trong trng hp yu cu thit lp knh b iu khin khi

chuyn mch s s thc hin cc th tc tm ng v chn mt knh ni qua

trng chuyn mch. H thng iu khin trung tm sau s phi a ra thng

bo v vic ng ni tm c. Tng t nh vy, cc bn tin cn phi ch th

rng knh ni c gii phng hay chun b sn sng,Cc bn tin ngc li

t h thng iu khin trung tm ti b iu khin khi chuyn mch cn phi

cha cc trng s liu nh hnh 3.10b, m c m t nh trong cc phn sau.

M bn tin tham kho: Trng s liu ny cha m nhn dng ca bn tin t h

thng iu khin trung tm m bn tin ny s c quan h sau vi n.

Trng tin: Trng s liu ny cha cc thng tin s c gi ti h thng iu

khin trung tm. Bn tin c gi theo kiu nh l knh c thit lp,

Khng c knh sn sng, ng dn c chun b hoc Bn tin trc

thu sai,

M bn tin v m sa sai: Cc trng s liu ny c ngha tng t nh mt trong bn tin loi 1.

Trong trng hp thit lp knh ni phc v cuc gi, b iu khin khi chuyn

mch s cn phi xc nh c cc a ch cn thit m chng s c ghi vo trong

tng b nh iu khin C-Mem ca cc tng chuyn mch sao cho cc tng chuyn mch

S/T s m bo cung cp c cc knh theo yu cu, sau b iu khin khi chuyn

mch s np cc s liu yu cu c th vo cc i ch nh c th ca cc b nh iu

khin C-Mem.

n v iu khin ca chuyn mch thi gian tng T cu to t b nh iu khinC-Mem, mch Logic m khe thi gian TS-Counter v b chn a ch Selector cn n

40


41/153

v iu khin ca chuyn mch khng gian tng S cu to t b nh iu khin C-Mem

v mt s mch ph cn khc nh m t phn 3.2. trn y. Trc khi xem xt vn

thng tin cn thit c ghi vo cc b nh iu khin C-Mem nh th no, chng ta

hy kho st format ca bn tin cn gi t b iu khin khi chuyn mch ti mi mt

n v iu khin chuyn mch tng S hay tng T tng ng. Cc thng tin chnh baogm:

a ch ca b nh C-Mem

a ch ca nh C-Mem

Ni dung s liu trong nh C-Mem

Gi thit rng i vi khi chuyn mch cu trc 3 tng T-S-T th 3 trng s liu

c yu cu trong bn tin cho mi n v iu khin chuyn mch bao gm trong qu

trnh thit lp knh ni. Tuy nhin iu ny cng khng nht thit phi thc hin, v d

nh vi khi chuyn mch biu din trn hnh 4.6, knh ni c thc hin giaICHW#0/TS4 vi OCHW#0/TS6 s dng khe thi gian ni b l TS17 qua chuyn mch

tng S. iu cn lu rng cc nh ca tt c 3 b nh iu khin C-Mem u c a

ch nh phn l 0000010001(17). c im u vit ny c th m bo kh nng ch cn

cung cp mt trng s liu chung cho cc nh ca tt c 3 b nh iu khin C-Mem.

Tuy nhin, thm ch c th t c mt cch kinh t hn bng cch gi cc bn tin ti

cc b nh iu khin C-Mem trong khong thi gian ca khe thi gian TS17, nh vy

bn tin t b nh iu khin khi chuyn mch khng cn mt a ch no cho cc nh

C-Mem v do tit kim c 3 trng s liu a ch.

Mt format bn tin c th s dng cho bn tin loi III trao i thng tin gia iu

khin khi chuyn mch v cc n v iu khin chuyn mch ca cc tng S/T c

minh ha trn hnh 3.11. Cu trc bn tin gm 3 nhm trng s liu dng cho vic

iu khin cc tng chuyn mch thi gian pha u vo, chuyn mch khng gian S v

cc tng chuyn mch thi gian pha u ra tng ng.

Nhm trng s liu chuyn mch tng T u vo Trng s liu u tin trong

nhm ny xc nh m s mt chuyn mch thi gian tng T c th. Trng sliu th 2 cha ni dung cn phi ghi vo b nh iu khin C-Mem tc l a ch

nh hay ch s khe thi gian u vo TS#. C hai trng s liu 1 bit cn b

sung lin quan ti n v iu khin chuyn mch thi gian u vo l bit

Bn/Ri v bit Chn m trn s hnh 3.11 biu tng bng k t B v P

tng ng. Bit B b iu khin khi chuyn mch dng ch th trng thi

Bn/Ri ca cc khe thi gian TS# ra ca chuyn mch thi gian tng T pha u

vo, cn bit P dng cho vic pht hin sai li khi thu bn tin. Cc thng tin ny

s c s dng tm mt khe thi gian trung gian ri nh tuyn qua chuyn

mch tng S ti mt chuyn mch thi gian tng T pha u ra. Mc d s thit lp

41


42/153

mt cch tch bit cc b nh trong b iu khin khi chuyn mch b tr sp

xp trng thi ca cc khe thi gian ra, bit Bn/Ri c th d dng truy cp c

vo ni dung ca cc b nh iu khin khi chuyn mch b tr sp xp trng

thi ca cc khe thi gian ra, bit Bn/Ri c th d dng truy cp c vo ni

dung ca cc b nh iu khin C-Mem ca cc tng chuyn mch thi gian uvo. Do vy khi c np bi bn tin loi III, mi nh ca C-Mem trong mt

chuyn mch tng T u vo s cha cc a ch cc nh ca b nh T-Mem v

bit B s lp gi tr 0 hoc a ch Zero. Bit B s lp gi tr 1 tng ng vi vic

cuc gi c s dng khe thi gian ra ca n trong qu trnh hay khng?

IT# TS# B PCt#Ni dung C-

MemP OT# B P

a) Thit lp knh ni

TS#

IT# S# OT#

IT# 0 B P1 0P OT# B P

b) Gii phng knh

0

Hnh 3.11. V d bn tin loi III Nhm trng s liu chuyn mch khng gian tng S Trng s liu u tin

trong nhm ny xc nh (nhn dng) ct no ca chuyn mch tng S v b nhiu khin C-Mem tng ng lin quan s c nh a ch (Nu chuyn mch

khng gian tng S c iu khin theo hng th trng s liu u tin s cha

a ch hng). i vi khi chuyn mch s m c s tranh chp mt vi chuyn

mch tng S th phn u tin ca ch s m ct s cn phi c ch r tng

chuyn mch no c a ch ha. Bit P y cng c dng vi mc ch nh

trnh by trn y.

Nhm trng s liu chuyn mch thi gian T u ra Format bn tin nhm nycha 4 trng s liu tng t nh i vi n v iu khin chuyn mch tng T

u vo tho lun k trn y, ngoi tr iu rng bit B y c lin quan ti

trng thi Bn/Ri ca cc khe thi gian u vo ch khng phi u ra nh

ca chuyn mch tng T pha u vo ca trng chuyn mch s.

Mt v d minh ha cho vic s dng cc bn tin loi III gia b iu khin khi

chuyn mch v 3 n v iu khin tng chuyn mch biu din trn hnh 3.11. Tip

theo ta xt mt v d v vic xem xt yu cu cuc ni qua mng chuyn mch cu trc

T-S-T hnh 3.6, vi gi thit knh c thit lp l ICHW#0/TS4 v OGHW/TS5 qua khethi gian ni b l TS15 ca im chuyn mch hng 0 ct 1 ma trn chuyn mch S.

42


43/153

Nh trnh by trn y, cc thng tin thch hp lin quan cn phi c cp nht vo

nh s 15 ca cc b nh iu khin C-Mem tng ng ca cc tng chuyn mch S-T.

Nh vy format bn tin biu din trn hnh 3.10 s c gi ti 3 n v iu khin

chuyn mch trong thi gian xy ra tip theo ca khe thi gian TS15 thit lp knh ni

yu cu qua khi chuyn mch. Cc phn t cn li ca bn tin s c tch bi cc nv iu khin ca 3 tng chuyn mch nh c ch r trn hnh 3.11a.

EMBED Visio.Drawing.11


Logic chn ng

Thanh ghitrng thi

Thanh ghi BUS

B k khi C/m

n v K

k IT#2

SelectionTSCount

n v K

C-Mem

k IT#1

B

B

Set

add R W

W add

n IT#BUS a ch IT#

BUS ni dung C-Mem

n S#

BUS a ch S #

n OT#

BUS ni dung C -Mem

BUS a ch OT#

BUS ni dung C-Mem

Bn tin loi 1 v 2

Hnh 3.12. iu khin khi chuyn mch sBn tin cn gii phng knh ni qua khi chuyn mch c ch r trn hnh

3.11b. Bn tin ny c gi i trong thi gian din ra tip theo ca khe thi gian TS15

sao cho cc nh c a ch nh phn 0000001111(15) ca cc b nh iu khin C-Mem

43


44/153

ca chuyn mch thi gian pha u vo tng IT0, ct 1 ca chuyn mch khng gian

tng S v tn chuyn mch thi gian u ra OT#1 s c np vi cc a ch Zero. Bit B

ca cc nh s 15 ca c 3 b nh iu khin C-Mem u c lp gi tr 1 ch r

rng khe thi gian ni b TS15 b chim dng khng th s dng cho cc cuc ni

khc.Hnh 3.12. minh ha s dng cc Bus v cc dy iu khin kt ni cc thanh phn

iu khin chuyn mch khc nhau vi b iu khin khi chuyn mch. cho s

c n gin, r rng va d hiu, trn hnh 3.12 ch biu din n v iu khin chuyn

mch cho chuyn mch u vo IT#0. Vic lin lc gia cc thnh phn c m bo

bi 3 i Bus mt i cho vic lin kt tt c cc chuyn mch thi gian u vo IT#,

mt i cho cc tng chuyn mch khng gian S v mt i dng cho cc chuyn mch

thi gian u ra OT#. Ngoi ra cc dy iu khin n s kt ni cc trng s liu bit B

t tt c cc b nh iu khin C-Mem ca tt c cc IT# v OT# vi thanh ghi trng thitrong b iu khin khi chuyn mch s.

Bus a chi ca IT# c u ni ti tt c cc n v iu khin chuyn mch ca

chuyn mch thi gian u vo IT. Mi n v iu khin lin tc tm kim cc ni dung

ca Bus v a ch ca mt trong s cc chuyn mch tng T ca n. Khi mt a ch

thch hp tm c th a ch s c s dng cho vic nh tuyn s liu t Bus

ni dung b nh iu khin C-Mem ti nh ca mt b nh iu khin. Cc ng dng

tng t c sp t nh vy i vi cc Bus u ni chuyn mch khng gian tng S

vi chuyn mch thi gian u ra OT.

C 3 i Bus kt cui thanh ghi Bus trong b iu khin khi chuyn mch. iu

ny cho php cc trng s liu khc nhau ca cc bn tin loi III c m trong thanh

ghi Bus s c a ra ngoi ti cc Bus lin quan. Cc bn tin cn thit c to ra bi

b iu khin khi chuyn mch s da trn c s cc lnh iu khin t h thng iu

khin trung tm v i vi mt ng mi thit lp s dng kt qu x l ca mch

Logic chn ng.

2.5.2. Thut ton chn ng riTh tc chn knh ri cho khi chuyn mch s cu trc T-S-T bao gm vic tm

khe thi gian trung gian ri qua chuyn mch khng gian tng S. iu ny c ngha rng

khe thi gian phi c la chn sao cho n l ri c pha u ra ca tng chuyn

mch thi gian u vo IT# v c pha u vo ca tng chuyn mch thi gian u ra

OT#. Trng thi Bn/Ri ca mi trong s hai tp khe thi gian c biu th bng bit B

trogn cc nh tng ng ca cc b nh iu khin C-Mem. Phng php chn knh

ri s dng mt cch n gin l x l tm kim s trng khp cc khe thi gian ri nh

vic kim tra cc cp bit Bn/Ri t hai tp cc chuyn mch thi gian tng T.

44


45/153

C ch chn ng ni c thc hin trong phm vi b iu khin khi chuyn

mch m n thng c xy dng trn c s k thut vi x l v n thc hin vic tm

kim bng cch tin hnh mt s qu trnh x l logic s. Phng php ph bin nht l

s dng mt mt n nh trnh by di y:

Trng thi Bn/Ri ca mi nh ca b nh iu khin C-Mem ca chuyn mchtng T s c nh m trong khong mi khe thi gian trong thanh ghi trng thi ca b

iu khin khi chuyn mch. Thanh ghi ny cha hai trng s liu Mt trng gi

m trang thi cho cc OT#. Trong khong thi gian ca mi khe thi gian TS tt c cc

bit bn tng ng u c trnh din trong thanh ghi trng thi, v d nh trong thi

gian TS19 tt c cc bit bn t nh s 19 ca tt c cc b nh C-Mem u c trnh

din. Vic la chn ng tuy nhin c lin quan vi vic trng hp cc khe thi gian ri

ch xy ra trong tng chuyn mch IT# v OT# m chng c ch r bi h thng iu

khin trung tm. Nh vy b iu khin khi chuyn mch s to mt t Logic nh phnc di bng di ca thanh ghi trng thi. N cha cc gi tr Zero trong tt c cc v

tr bit, tr hai v tr tng ng vi cc b nh C-Mem ca IT# v OT# yu cu. T logic

ny gi l Mt n v n c lu m trong thanh ghi chn. Trong thi khong ca mi

khe thi gian cc ni dung ca thanh ghi trng thi c thc hin ton t logic AND

trn c s tng bit mt tng ng vi cc ni dung ca thanh ghi chn. Khi mt khe thi

gian ri(trng thi 1) xut hin trong hai b nh C-Mem theo kim tra, kt qu ca php

ton t AND s c t gi tr 1 trong c hai na ca thanh ghi kt qu. V d minh ha

hnh 3.13.

Nh vy s c sn cc knh thi gian ni b ri kh dng qua chuyn mch tng S

c xc nh bi vic kim tra kt qu khc 0 trong c hai trng s liu ca thanh ghi

kt qu trong thi gian ca mi khe thi gian hin ti.

Khe thi gian ni b c chn sau s c s dng to cc bn tin loi III

cn thit thit lp knh ni qua khi chuyn mch. B iu khin khi chuyn mch

c th yu cu chun b trc knh ni. Trong trng hp ny bn tin loi III s c to

ra m n cha cc gi tr 0 trong hai trng s liu B v n thun ni dung Zero trongba trng s liu b nh C-Mem.

45


46/153

1 0 1 1 1 0 0 0 1 1 0 0 0 0 1 0 1 1 1

0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0

0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0

1

0

0

Thanh ghi Trng thi

Mt n chn knh

Thanh ghi kt qu

Hnh 3.13. V d v chn knh ri

Cch thc hat ng m b iu khin khi chuyn mch s dng mt n tmkim mt knh ri c nh hng ti mc phc v ca h thng. C mt vi thut ton tm

kim knh ri c s dng trong cc h thng chuyn mch s hin i v mi thut

ton s cho cc c trng lu lng cng nh phc tp iu khin khc nhau. Sau y

gii thiu ba thut ton ng dng cho cu trc khi chuyn mch T-S-T:

Thut ton 1: Bt u tm lin tip ngu nhin. Vi thut ton ny vic tm kim

mt khe thi gian ni b ri qua tng S bt u mt cch ngu nhin v truy tm

lin tip qua cc khe thi gian. iu ny c hiu ng tri cc khe thi gian chim

dng thnh di, v d t TS0 ti TS512 trong mt khung tn hiu 1024 khe thigian. Mc phc v GOS(Grade Of Service) kt qu lm trn khong 2,5.10 -4 i

vi lu lng 0,825 Erl/mch.

Thut ton 2: Bt u tm lin tip c nh. Theo phng php ny vic tm knh

lun lun c bt u t mt khe thi gian ni b nh trc v sau truy tm

mt cch lin tip qua ton di. Thut ton 2 c hiu ng ng gi cc khe thi

gian c chim dng bi v slng cuc gi ln hn s c thit lp qua cc

khe thi gian c ch s thp hn so vi cc khe thi gian c ch s cao hn. Ktqu ny s lm tng GOS so vi phng php bt u ngu nhin. V d GOS c

gi tr 1,46.10-4 i vi lu lng 0,825 Erl/mch.

Thut ton 3. Th lp. Khi tm kim knh ri qua trng chuyn mch tng T, s

th lp li vic tm kim c khi u sau mt thi gian no c th s cho

php tm c mt khe thi gian ri. tr gia nhng ln tm kim s nh hng

ti GOS, c th thi gian ch cng lu th xc sut s cuc gi gii phng s cng

ln. Chng hn nh c 50% c hi tm kim knh sau 1 giy ch th lp li c th

s to thm tr nhn thy c i vi thi gian thit lp cuc gi m n bnhthng ch cn 0,5 n 1,0 giy qua h thng chuyn mch s.

46


47/153

2.5.3. tin cy v an ton khi chuyn mch

Cc khi chuyn mch s hin i c dung lng khng l, do bn thn chng

cng cc thit b iu khin lin quan cn phi c m bo an ton tin cy cao bi

v mt hng hc nh c th gy ra hu qu nghim trng cho ton b h thng. Cc h

thng chuyn mch u cui cng cng hin i yu cu thi gian hng bnh qun trongchu k 20-40 nm ph thuc vo cc tnh nng qun l. V rng ch c mt trong s cc

cu kin ca h thng, khi chuyn mch t n cn phi m bo ch mt phn nh trong

t sut hng hc ton b h thng. Hn na, hu ht cc doanh nghip in thoi qui nh

xc sut thp mt hng hc bt k no nh hng mnh hn mt phn nh mch in.

iu ny yu cu phi xem xt cn nhc thn trng vn m bo tin cy cao ca

cc khi chuyn mch s bi v dung lng knh ca chng khng l v khi lng cu

kin phn cng ln. V d nh nu hng hc phn cng mt Bus lung PCM ni b

gia tng T v tng S c th s lm tn tht 1024 knh truyn dn.Bo v an ton mt cch c hiu qu v n gin nht cho trng chuyn mch s

c th thc hin bng hai gii php: thit k ch to theo Module v trang b d phng

kp nh s minh ha trn hnh 3.14.

Theo phng n d phng kp ngha l khi chuyn mch c thit k ch to t

hai na khi hon ton nh nhau v mi na khi gi l mt mt(plane hoc side). Mi

cuc ni s c thit lp ng thi vi hai knh dn song song ng nht qua cc mt A

v B ca khi chuyn mch, trong mt mt lm vic thc s(mt tch cc) kt ni

knh vo/ ra phc v cuc gi, cn mt kia l d phng sao cho nu mt tch cc c

vn th n s t ng thay th.

gii quyt vn nh nu trn, b sung vo khi chuyn mch an ton cn phi

c c ch gim st, pht hin sai li v hng hc sao cho c th c lp

47


48/153

Tng S

TngT

TngTSW#


TngT

TngTSW#

Tng

T

Tng

TSW#

Hnh 3.14. Trang b d phng kp khi chuyn mch sc b phn thit b khi xy ra sai li. i vi phng php trang b kp i c th s

dng gii php kim tra tnh chn l n gin c hai knh song song tch cc/d phng

ch ra mt b sai li, c th l bit chn c b sung vo t hp m tn hiu PCM 8bittrong hng pht ti mi lung kt cui u vo ca khi chuyn mch. Lung tn hiu 9

bit sau c nhn i v a ti chuyn mch tng T u vo tng ng ca c hai

mt ca khi chuyn mch. Pha u thu mi t hp m PCM 9bit t c hai mt s c

kim tra tnh chn l pht hin c sai li hay khng. Nu pht hin thy li mt no

th lp tc mt s b c lp khi khi chuyn mch mt cch t ng hon ton.

Ngoi phng php trang b d phng kp c s dng rt ph bin nu trn c

th s dng phng php d phng phc tp hn v d nh d phng theo Module n+1

hay d phng n trong m,

48


49/153

CHNG III

CHUYN MCH GI

3.1. Nhng khi nim chuyn mch gi

3.1.1. Khi nim v chuyn mch gi (packet switching)

Trong chuyn mch gi thng tin c truyn i di dng cc gi, mi gi gm mt

khi thng tin iu khin, mt vng cha thng tin truyn hu ch v mt vng thng tin

kim tra sa li b sung. Nh vy, n v d liu (data unit) trong chuyn mch gi l

gi, trong ngoi d liu cn truyn ca ngun pht tin cn c mt s d liu do mng

thm vo. Mt bn tin trn vn ca ngun pht c chia thnh nhiu gi, chuyn i v

c ti thit ti ni nhn.

Head

u cui u cui

K Thng tinKT

K Thng tinKT K Thng tin

KT

Head

u cui u cui

Gi

Hnh 3.1. Chuyn mch trn BusMt t chc chuyn mch thi gian c th c dng cho chuyn mch gi. Thng

thng gm mt knh truyn di dng mt vng hay mt bus m t tn hiu c

a vo v ly ra ty thuc vo mt vi loi chun. Cc nhm tn hiu c th c tip

nhn v nh tuyn n cc li ra khc nhau ty thuc vo v tr ca chng trong mt tunt cc tn hiu. Trong cc tip cn khc, mi nhm tn hiu cha mt a ch ch ra ni

nh tuyn cho nhm. Trong hnh 3.1, cc tn hiu s (cc bit) ang chy trn mt bus

qua hai u cui s liu. Cc bit ny c nhm vo trong mt vng iu khin, thng

tin, v kim tra nh trnh by gia hnh. Thay

Documents

Giao Trinh Ky Thuat Chuyen Mach Va Tong Dai_tc_ngan