viba số (tập 2)

T SCH KIN THC c s V IN T VIN THNG

Vbas

TP 2

TS. Bi Thin Minh

NH XUT BN THNG TIN V TRUYN THNG

Chng 7; Cc h thng viba s v cc ch tiu cht lng

CHNG 7

CC H THNG VIBA s V CC CH TIU CHT LNG

7.1. SO SNH VI CC H THNG FDM-FM

Mt s" u im ca h thng viba s" l:

1. Thit b s thng sut vi lu lng thng tin c truyn i, tc l lu'u lng thng tin c th xut pht t in thoi, my tnh, facsimile, telex... c tng hp li thnh mt lung bit truyn trn cng mt sng mang v tuyn (hoc trn mt h thng truyn dn khc), v c th c chuyn mch cng vi nhau. V lung s" liu lun c mt, cho nn d thng tin c hoc khng c trong , th ti ca h thng v tuyn l khng i, iu ny khng xy ra i vi cc h thng thng tin v tuyn FDM-FM tng t, ti ph thuc s lng v loi lu lng thng tin mang i Li mt thi diui no .

2. trnh tp m tch lu ca cc h thng FI)M-FM hn ch nghim trng n vic thit k thit b, trong h thng s ngi ta dng phng php ti sinh lung s' liu. Vic ti sinh ny c th c tin hnh tc bit cao nht ca bng tn gc m khng cn a xung tc bit ban u. Vic thit k thit b ti sinh phi m bo sao cho phc tp thp (lin quan n b trn a v' gc ban u).

3. Cc h thng v tuvn s" t thin v tp m v c th hot ng tt vi t s' sng mang trn tp m l 15-r30 dB. iu ny cho php cng tn s s dng li l vi phn cc trc giao, nh vy n duy tr rng bng tn v tng gp i mt cch hu hiu l vi ph hiu dng hoc tng gp i dung lng knh RF c th ti i. c im ny l mt trong nhng li th ln nht cua v tuyn s' so vi tng t, v

trong trng hp v tuyn tng t, yu cu t s sng mang trn tp m ln hn nhiu (Bo co ca CCIR).

4. Vi cht lng truyn dn cho, cng sut ra ca my pht v tuyn s c th b hn so vi cng sut ra ca my pht v tuyn tng t. iu ny lm gim chi ph thit b, tng tin cy, tit kim ngun v chi ph iu ho nhit . Cng sut ra nh hn cn c mt li th l to ra mc tp m nh hn, dn n kh nng s dng li tn s" trng knh cao hn trong mt vng a l cho v cng tn ti cc h thng s v cc h thng tng t hin c (Bo co 378 ca CCIR gii quyt vn ny) v kt lun rng i vi cc h thng dung lng trung bnh v nh, s dng cc h thng s" PSK a mc c th cng tn ti VI cc h thng tng t. i vi cc h thng dung lng cao (ln hn 100 MHz), trong cc h thng a mc c h thng cho php tng hp vi cc h thng FDM - FM, nu mc u ra RF ca h thng gi mt gi tr m bo ch tiu cht lng ca h thng c th chp nhn c. V hu ht cc thit b c trn th trng, ph pht x ca my pht s l lin tc khng c thnh phn ri rc hoc sng, mang, nn tp m m n gy ra cho cc h thng ln cn khc tng t nh tp m nhit.

5. Ph thuc vo h thng iu ch c s dng, cc h thng s c kh nng to ra mt knh ting ni chp nhn c vi t s" sng mang trn tp m nh hn 15 dB, trong khi mt h thng tng t t s' ny phi bng hoc ln hn 30 clB, tu thuc vo h thng.

Hnh 7.1 so snh t s tn hiu trn tp m l vi mc thu vo (ng cong tnh ca my thu) ca mt h thng v tuyn s" v mt h thng v tuyn tng t. Trong iu kin pha inh phng, h thng tng t c t s" tn hiu trn tp m tt xung dn dn theo tng dB, khi gim mc tn hiu thu. Trong lc th h thng thng tin s khng b nh hng cho n khi t n ngng. c tnh ny l do qu trnh ti sinh trong tn hiu s c ti sinh thnh dng ban u ca n, vi iu kin l h thng phi lm vic trn mc ngng xc nh trc (C/N xp x 15 dB). Nh vy c tnh ca ngng ny cho php t c mt cht lng truyn dn khng i, m iu ny ph thuc vo s bin i ca trng thu, di ca ng truyn v s" lng cc b lp. N cng cho php t c mt cht lng truyn dn khng i vt di ng ca tn hiu tin tc. Mt c im khc l kh nng ca mt sng mang cht lng thp nhng li to ra c mt

8__________________________________________________________ Vi ba s - Tp 2

mch c ch tiu cht lng cao v duy tr trong mt mi trng tp m cao. khng nh hng ti tn hiu ra.

Chng 7; Cc h thng viba s v cc ch tiu cht lng 9

Hnh 7.1- So snh tc ng ca s / N ra i vi h thng v tuyn tng tv s khi gim mc tn hiu thu.

a) H thng tng t (960 knh) b) H thng s (34 M bit/s, 4-PSK)

6. Ngy nay, s hD mng ca cc vng thnh ph" m l cc khi ngun hot ng truyn dn s, to ra mi lin kt s v gim chi ph ng trc trong thnh ph" bng cch s dng thit b thch hp trc tip vi truyn dn s" v cc dch v s' khc. Cc im ny cng vi thc t l s dng cc thp v cc anten hin c lm thu ht cc h thng s' to nn mng v tuyn s.

Vic gim chi ph cc mch tch hp s c ln (LSI) lm cho chi ph v thit b gim xung, gi thnh ca cc h thng s" cnh tranh c vi gi thnh ca cc h thng tng t. Thc t hin nay ni chung chi ph tng cng i vi mt h thng s" bao gm c tch, ghp s" v cc thit b v tuyn l thp hn so vi mt h thng tng t. iu ny c bit vi cc h thng ng ngn hoc trong cc h thng m cn mt s^ lng ng k cc mch xen r knh.

Mt s nhc im ca h thng v tuyn s l:

1. Cc h thng v tuyn s' v cc h thng s" phi c hp li thnh mt h thng thng nht trong mt phm vi gn vi mng tng t hin c. Lc xut hin mt kh khn l phi gii quyt cc bi ton c lin

quan n cc vn giao din, chi ph v tp m. Vi vic a vo s dng ISDN, do vic y nhanh ci tin cng ngh v gim thi gian m to thch nghi, vn khng lm gim nh c iu . Nhm gii quyt cc vn t ra, ngi ta s dng cc k thut mi, yu cu cn mt thi gian m. Thi gian m ny ang tr nn ngn hn, tuy vy cc vn cng tr nn ngy cng phc tp hn.

2. Cc h thVig v tviyn s hin nay hiu sut s dng bng tn RF khng cao nh i cc h thng tng t 600 hoc hn 600 knh thoi; tuy vy, ngi ta cm thy rng cc bi ton ny s c gii quyt trong mt thi gian khng xa hoc s khng cn l vn quan trng na, v cc tn s" sng mang cng tc c y ln cao hn hoc cc mng si quang mang mt khi dung lng ln ca cc mng ng trc. Ti thi im vit quyn sch ny cc si quang ang hot ng vi dung lng 565 Mbit/s, trong lc h thng v tuyn s" ch c th c dung lng 140 Mbit/s.

3. Dung lng thng tin l yu t^ lm cho cp si quang s thay th mt cch hin thc hn l vi v tuyn s" trong thi i hin nay, v chng c th mang lu lng m khng b suy yu do cc nguyn nhn t nhin (tt nhin khng xt n mt ngi no dng dao ct cp ra, theo ni dung ny, th cng l mt nguyn nhn t nhin). mang c mt lng thng tin nh si quang th phi yu cu v tuyn s" hot ng vi rng bng tn RF ln. rng bng tn ln ny lm cho ng v tuyn d b pha inh la chn to nn s tn tht t t yu cho tt c lu lng thng tin. Cng nh vy h tling tng t bng rng ch chu s tng tp m .

4. Vic a vo s dng cc k thut mi, khc phc cc yu cu trong khi thit k, o th, bo dng cc h thng tng t, song iu t ra mt vn v o to li con ngi v o to mt i ng k thut mi. Trong thc t c 3 hng ln. Hng u tin l u t v'n ln trong cc h thng tng t m ngi ta chng minh c th thc hin c. V nhn s con ngi vn b rng buc trong lnh vc ny v s cn phi ko di trong mt s" nm ti. Hng th hai l cc h thng s" v v tuyn s' i hi phi hc v t c cc trnh k nng mi. Hng th ba l cng ngh truyn dn mi c cc i th cnh tranh mnh m trong cc lnh vc cng ngh tin hc v cng ngh my tnh, c th hp dn i ng k thut mi trong tng lai.

10_________________________________________________________ Viba s - Tp 2

Cc u im quyt nh ca v tuyn s hn v tuyn tng t l; Tnh kinh t, chng tp m tt hn, v cc mch cht lng tt hn trn cc khong cch di. Cng vi vic tng s lng chuyn mch s" c lp t trn khp th gii, ngi ta t c s tit kim ln v chi ph v khng cn cc mch phc tp v cc mch giao din t tin.

7.2 CC CH TIU C TRNG l VI CC H THNG VIBA s

o cht lng truyn dn ca mt on s' l s lng xut hin cc k hiu khng ph hp hoc cc li. Cc li ny c th xut hin thng qua tp m nhit, nhiu gia cc k hiu hoc mt s nhiu bn ngoi to ra tp m xung v.v... D th no i na cc ngun phi c mt ch tiu c trng cho s li cho php xut hin vi mt h thng ang hot ng ng. Phn ny tp trung vo cc ch tiu ny v cc iu kin chng tho mn.

Cc li xut hin t ngt v c th khng phn b ngu nhin. Xut hin iu ny v trong cc mch bng gc ca my pht v my thu s dng cc b trn v gii trn t ng b.

Mt li xut hin c nhn ln do mch gii trn a tr li vo lung bit bng gc i vo cha trn khng b li. iu ny c bit nh l nh hng m rng ca li. Ngoi nh hng m rng li, cc t bin ca li c th xut hin do tp m xung, thng l ngun tp m t nhin. S xut hin cc t bin li, phn b" rng theo thi gian, c th c nh hng ng k ti s lng li xut hin trong mt chu k v nh vy nh hng n t s' li bit trung bnh, thc s khng nh hng trm Irng dn cc mch hot ng. V hu qu ca tnh trng ny nn cn phi thit lp mt bin php thay th xc nh c tnh li, trc tip lin quan n nh hng ca cc hng khc nhau trong xut hin cc li l vi cc dch v c ni n. V d, mt dch v c th khng nhy cm ti t s li nn thp c phn b" ngu nhin, nhng nhy vi cc t bin li nh trng hp cc dch v video. C th s dng cc m sa sai gii quyt cc t bin li n hoc nh.

V d tnh trng ngc li vi cc dch v s' liu, thng thng trong thng tin ti ngun c tp hp li v truyn i trong cc kho. Do c tnh cht h thng ca cc bit iu khin li trong mt khi nn u cui s liu thu kim tra tnh cht hp l ca khi s liu thu c v nu cc li xut hin, th n yu cu ngun pht li cho khi vo. Vi cch sp xp ny, s" li thc ra khng quan trng v do nu cc li xut hin th cc t bin li c thch

Chng 7: Cc h thng viba s v cc ch tiu cht lng 11

ng hn do s gim v s ln mt khi c pht li. D nhin iu ny lm tng hiu sut t u n cui.

C th phn cc ngun li thnh tin nh v khng tin nh. Cc ngun li tin nh c th l cc ngun to ra li xut pht t tp m nhit, cc nh hng xuyn m v hiu ng truyn lan, trong lc cc ngun li khng tin nh c th l do s khng hon thin ca thit b, tp m in v cc tp m do con ngi.

Cc ch tiu c trng cht lng ca CCIR (Bo co 930) xt trn c trng ca mt ng truyn s" chun gi nh l 2500 km (HDRP) hoc bt k mt on s" ngn hn c xc nh. iu ny c biu th trong cc thnh phn ca hai t s" li l ch tiu giy khng li (EPS) v ch tiu da trn t bin li. Trong nhiu h thng t s" li tng i cao, th EFS giai on ngn hoc ch tiu t bin li s c ngha nht v s xc nh khong cch ca b lp. Ch tiu t s li tng i thp s p ng c tnh iu khin trong phn ln thi gian. Ch tiu cht lng ch tr nn ng khi h thng c coi l c kh nng lm vic (nh s trnh by di y). HRDP c th to nn mt phn ca tuyn s" ni quc t tng l di, xc nh yu cu c tnh l VI mt tuyn n chun gi nh to nn mt phn mng s a dch v (ISDN). iu ny c a vo tnh ton cc ch tiu c trng t s li ci bin c trnh by mc 7.3.3.

7.2.1. T s li thp

CCITT ngh mt ch tiu thit k t s li 1 0 ' trn km l vi h thVig truyn dn ni s"chuan gia nh 25000 km. i vi HRDP 2500 km cho mt t s" li 2,5 X 10^ tr tc ng ca thit b ghp knh. Trong cc h thng viba s, t s li ny s ph hp vi tiu chun thit k hin nay, t c vi t nht l 99% thi gian. Do gi tr thp ca t l li khong 10^ l thch hp cho ng truyn dn s chun gi nh 2500 km. CCITT khuyn ngh l t l li bit l vi mt on bc cao 25000 km trong ng chun gi nh di nht khng ln hn 10 trong hn 4% ca tp m pht trong thng bt k.

7.2.2. T s li caoKhong thi gian m t s" li cao c th vt qu, lm nh hng r t ln

n thit k mt h thng. Mt yu cu l t s" li cao hn khng c vt qu 0,01% ca thng bt k i vi mt mch 2500 km, s c tc ng n tnh kinh t ca mt h thng thc. Tuv vy, mt vu cu l t s" li ny khng c vt hn 0,1% ca thng bt k s to nn mt h thng kinh t hn.

1^2_________________________________________________________Viba s - Tp 2

nhng n^i ta khng so snh mt cch tu tin vi cc h thng viba s FDM- FM hin c. Cc s' liu m n c chp nhn, cng l ph hp, mt ni no (t trong hai cc tr ny v l 0,054% ca thng bt k khi s dng thi gian hp thnh mt giy.

7.2.3. Ch t iu giy khng li (EFS) trong mt thi gian di

T l li bit (BER) khng cho mt nh gi chnh xc c tnh ca mch s liu tr khi bit phn b" ca li. Tiu chun t s" li ca khi nh gi tt hn. nhng li i hi v c ca khi s liu v tc s liu, m chng li thay i t ngi s dng ny sang ngi s dng khc.

Cc ch tiu EFS l mt phng php trong c th gii quyt c cc gii hn trn. CCITT cng b" rng i vi truyn a s" liu th s t ti mc tiu EPS 95% i vi on ni chuyn mch ni ht chun gi nh 25000 km, mt gi tr 99,5% EPS l vi HRDP 2500 km ph hp. Thc t y khng phi l mi tng quan n gin gia c tnh BER v c tnh EPS trong thi gian di i vi mt h thng v tuyn trong thi gian pha inh nhiu tia, v EFC l mt hm ca tc pha inh, trong lc BER khng ph thuc vo tc pha inh.

7.2.4. Ch t iu t bin li

Khi s dng vi cc thit b tch knh s th s xut hin ca cc t bin li trong cc h thng viba s' dung lng ln s gy ra cc nh hng khng mong mun vi mng li. Do , CCIR cng b" mt gii hn v kch thc cho php v khong thi gian ca cc t bin li ny, ch yu iu khin giai on ban u ca nhiu cuc gi c ly ra trong mng in thoi. Hnh 7.2 ch ra mc tiu t bin li cho php nh xut Bo co 930 ca CCIR.

7.3. CC CH TIU C TNH L l

7.3.1. ng truyn s chun gi nh (HRDP)(Khuyn ngh 556 ca CCIR)

' xc nh cc chun c tnh li ca mt h thng viba s, mt iu cn thit l phi hiu nh ngha ng truyn s" chun gi nh. iu ny s cho


mt s hng dn thit k thit b v cc h thng s dng trong cc mng thng tin quc t. di ca ng truyn i vi cc h thng c tc bit trn ghp s bc th cp (2048 kbit/s hoc 1554 kbit/s) l 2500 km. HRDP c ch ra trn hnh 7.3 v cng ch ra s cho php i VI mt hng truyn dn,9 b ghp knh s ti mc phn bc theo khuyn ngh ca CCITT v bao gm9 on v tuyn s^ ging nhau lin tip cng di.

7.3.2. Cc on v tuyn s (Khuyn ngh 556 ca CCIR)

Mt on s bao gm hai thit b u cui v tuyn lin tip v mi trng truyn dn lin kt gia chng, cung cp ton b cc phng thc pht v thu gia hai khung phn b s lin tip. Do vic b sung cc bit mo u k c trong thit b v tuyn, nn tc bit tng on v tuyn s' c th khc so vi mc phn c"p do CCITT khuyn ngh hoc khc so vi vic ghp tng hp ca n.

7.3.3 Cc t s li bit cho php

Do nh hng lan truyn v cc nh hng khc, nn c tnh t l li bit c biu th bng k hiu thng k l phn trm thi gian. Trong khi thit lp cc ch tiu BER, mt s vu t" khc c a vo xem xt, l o li bit yu cu trong mt khong thi gian nht nh ph thuc Vco ln t s li do hin tng t bin li v cn phi a trt vo tnh ton. iu c th ni rng, trong khi hot ng bnh thng, s xut hin cc khong chu k ty S li bit cao, chng s gy ra cc gin on ngn, ch yu do cc iu kin truyn lan bt li lm gim mc tn hiu mong mun hoc tng mc tn hiu tp m. C mt quan im l cc mc tiu cht lng l vi don nl s quc t to nn mt phn ca mng s lin kt a dch v (ISDN) c CCITT nh r (Khuyn ngh G.821). HRDP tng ng vi mch ch tiu cht lng bc cao c nh r trong Khuyn ngh G.821 ca CCITT, n cng to nn mt phn ca ISDN v l vi n CCITT xc lp mt quy lut chia cho php suy gim tng cng ca mch s chun gi nh. S chia ny l;

1. t hn 0,00016%/km tng ng VI 2000 km ca nhng khong thi gian 1 pht c t l li bit xu hn 10 (ch tiu cc pht suy gim*).

2. t hn 0,000128%/km, tng ng vi 25000 km ca nhng khong thi gian 1 giy c mt s" li b t k (tng ng VI (100 - 0,000128/kra)% cc giy li (ch tiu cc giy li trm trng*).

14_________________________________________________________Viba s - Tp 2


3, t hn 0.05% ca khong thi gian 1 giy c mt t s li xu hn10 i VI HRDP 2500 km (ch tiu cc giy li*).

* Xem muc 7.4.1.

Khong hi gian D (s)

Vng (1): Cc t bin li hn ch trong vng ny s c xc sut gy ra nhiu cuc gi rt ra c th b qua.

Vn^ (2); Cc t bin li m rng trong phm v ny s c xc sut xut hin nhiu cuc gi rt ra tng ln v kh nng khng chp nhn c.

Vng (3): Cc t bin li m rng trong phm vi ny thng c xc sut xut hin nhiu cuc gi r khng chp nhn c.

A I B Cl ' Cc ng bao lin quan ch tiu cht lng tn hiu gi

A2. B2, C2 -C L D '

C2. D2Cc ng bao in quan chi tiu cht lng ca ch th

Hnh 7.2- Mc tiu t bin li c th. (Theo Bo co 930-1, CCIR)

16 Vi ba s - Tp 2

V d vi HRDP 2500 km, s s dng nh sau:

1. t hn 0,4% cho cc khong thi gian 1 pht l c t l li bit xu hn 102. t hn 0,32% cho cc khong thi gian 1 giy c mt s' li no

(tng ng vi 99,68% ca cc giy khng li).

3. t hn 0,05% ca khong thi gian 1 giy c t l li bit xu hn l 10

2500 km&

HH --1 2 3 4

64 kbis 64 kbis

-Thil l) oh( ' | ) s bc nh;Vt

6 7

64 kbVs

9

64 kbis

> - Thi t b ghp s" bc khc ti cc nic p hn bc do C C IT T k h u y n ngh

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2. Cc giy li, trong BER ln hn 10^ s khng a vo tnh thi gian hp thnh.

3. S ng gp t thit b ghp s vo BER khng c trong cc ch tiu cng b" trn.

4. Khuyn ngh ch c ng dng khi h thng c coi l c kh nng lm vic. H thng c coi l khng c kh nng lm vic nu xut hin mt hoc c hai iu kin sau trong khong t nht l 10 giy lin tip: tn hiu b gin on (tc l m t ng chnh hoc nh thi) v t l li ln hn 10^.

Khuyn ngh ny bao gm cc thi k BER cao hn 10 '^ , v n vn cn dai dng trong cc khong thi gian t hn 10 giy lin tip.

7.3.3.2. Cc h thng thc

CCIR khng c mt ch tiu no cho cc mch thc, nhng cng nhn rng cc ng truyn s khc c v di ln kt cu so vi HRDP v to ra cc ch tiu k hoch i vi BER cho php ca cc ng truyn, mt iu thng xy ra thc t l cc ng truyn c di ngn hn so vi HRDP. Cng nhn rng vi cc ng truyn thc c hai vn :

1. Tm mt phng php chia cc ch tiu li ra tng phn i vi cc h thng d nh lin kt trong mt tandem to nn cc ng truyn tng t nh HRDP.

2. a vo tnh ton vic u ni chun gi nh CCITT bao gm ISDN.

vi bi ton th nht, gi thit ly phng php chia ra tng phn cc ch tiu ny da vo s ph thuc thng k trn on bt k ca HRDP khi phi chu pha inh, nh vy cho php xc nh ch tiu cht lng HRDP bng vic nhn chp cc hm mt xc sut ca t t c cc on. Trn c s gi thit rng HRDP c BER nn bng 10 th BER ca mt on ngn hn s tt hn 10 ^ Nu BER nn ca HRDP c th coi l tt hn 10^ iu c th tho mn xc nh c tnh cht lng ca mt on ngn hn bng cch chia ra tng phn t s" phn trm nh lm cho c th vt BER 10^. C th quyt nh cc ch tiu ny trn c s l gii tip theo do CCIR khi xng cc ch tiu c trng cht lng ca cc tuyn thc l vi in thoi FDM. Coi rng HRDP bao gm 9 on v tuyn mi on di 280 km c th thch hp i vi cc tuyn viba thc c di L:

1. i vi L < 280 km:a) BER < 10 trong hn (280/2500) X 0,4% ca thng bt k, thi gian hp thnh l mt pht.


Cn BER > 10 trong 0,045% ca thng bt k.b) BER < 10 '* trong hn (280/2500) X 0,054% ca thng bt k, thi gian hdp thnh l 1 giy.Cn BER > 10^ i vi 0,006% ca thng bt k.c) Tng cc giy li s khng vt (280/2500) X 0,32% = 0,036% ca thng bt k.

2. i vi 280 km < L < 2500 km

a) BER < 10 trong khong thi gian ln hn (L/250) X 0,4% ca thng bt k, thi gian hp thnh l mt pht,b) BER < 10 '* trong khong thi gian ln hn (L/2500) X 0,054% ca thng bt k, thi gian hp thnh l mt giy.c) Tng cc giy li s khng ln hn (LV200) X 0,32% ca thng b"t k.

7.4. C TNH LI CA MT ON N l s QUC T TO NN MT PHN CA ISDN (Khuyn ngh G.821 CCITT: Cc nh ngha)

Khuyn ngh G.821 ca CCITT gii quyt c tnh li ca mt on nl s' quc t to nn phn mng s" lin kt a dch v (ISDN). iu c ni y, nhm nh gi cc ch tiu HRDP v CCIR trong vin cnh ring ca chng l vi cc mch quc t. Khuyn ngh ny c xut v trong tng lai d tnh l cc dch v s da trn khi nin ISDN, yu cu lu thng hng lot cc dch v cng vi cc dch v thoi c coi l thch hp nh^t, ng vai tr chnh trong ISDN. Do khi nim ISDN v v cc li l nguyn nhn ch yu lm suv gim cc dch v thoi th hin ting ni b mo v nh hng ti cc loi hnh dch v s" liu th hin thng tm khng chnh xc hoc thng lng gim so vi cc ch tiu cht lng yu cu. Cc ch tiu ny c cng b' trong Khuyn ngh G.821 i vi mi hng ca chuyn mch knh 64 kbit/s s dng cho thng tin thoi hoc l mt "knh mang" cho cc dch v loi hnh s" liu. on ni chuyn mch knh 64 kbit/s gii thiu l mt on ni chun gi nh ton s^ (HRX) v c th hin trong hnh 7.1. N bao gm mt di tng cng 27500 km v bt ngun t on n chun gi nh (HRX) th hin trn hnh 7..

V cc ch tiu c hng vo ngi s dng mng li, c tnh ny c biu th bng cch a ra phn ln thng tin v cc nh hng suy gim

[^8_________________________________________________________Viba s - Tp 2

ch tiu cht lng trn cc dch v. v cc tham s" c tnh li, ch tiu c tnh cht lng c nh ngha nh sau;

"Phn trm mi mt khong thi gian T ca cc khong thi gian trung bnh trong tc li bit (BEF) ln hn gi tr ngng. Phn trm ny c nh gi trn mt thi gian kh di T|

T, c phn thnh hai thnh phn, l thi gian cho on ni c kh nng hot ng v thi gian khi on ni khng c kh nng hot ng.

km

1250 km 25000 km 1^50 Kma-[Z]OtH:TH:>O-0O0QO^

^ im chuP 1 iem chucin

Dng S n Chuyn mch sLE - Chuyn mch ni ht ng s' TC - Trung tm bc baPC - Trung tm chnh chuyn mch s ISC - Trung tm chuyn mch quc ts c - Trung tm bc haiC h : 'Khuyn ngh 1.411 (ch ng dng cho ISDN).

Hnh 7.5- on ni chun gi nh s'(ng di nht) (HRX)(Theo Khuyn ngh G.801, CCITT).

Cc ch tiu c tnh nhm p ng 2 chc nng chnh. Cc chc nng ny biu th c tnh li d tnh trong cc iu kin cng tc thc, cho khch hng ca cc mng s" quc gia v quc t tng lai do vy to thun li chovic t k hoch dch v v thit k thit b u cui, v th hai l to mtc s tfa ra cc tiu chun c tnh cho thit b v cc h thng trong ng ni ISDN. Mc d cc ch tiu c din t ph hp vi cc nhu cu ca cc

dch v khc nhau song trong cng thc nguyn thy th hai nhm khch hng ch yu c ng nht. Cc nhm ny l cc khch hng s dng in thoi v s dng s liu, cng thc trnh by v phn nh iu . Cc ch tiu c trng biu th s dung ho gia mong mun p ng cc nhu cu dch v v tnh kinh t k thut cn thit thc hin cc h thng truyn dn. Cc ch tiu c trng din t ph hp cc nhu cu ca cc dch v thch hp khc nhau, ng thi cng nhm biu th mc cht lng truyn dn n gin. c trng li c xc nh bng cc pht suy gim cht lng, cc giy li trm trng v cc giy li.

7.4.1. Cc ch tiu c trng

7.4.1.1. Cc p h t suy g im cht lng

Pht suy gim cht lng l khong thi gian 1 pht trong BER trung bnh xu hn 10 iu ny c ngha rng trong pht ny xut hin t nht l 5 li (gi s 64 kbit/s). Ch tiu c trng c cng b' di y, da trn c s khong thi gian trung bnh 1 pht. Khong thi gian trung bnh ny v loi tr cc li trong cc giy li nghim trng xut hin trong khong thi gian mt pht ny c th tha nhn cc quan h vi cc li t bin thng xuyn p ng phn ch tiu c bit ny, nhng cc hin tng nh vy s c iu khin cc b bng cc ch tiu giy li trm trng.

7.4.1.2. Cc g iy li trm trong

Giy li trm trng l khong thi gian mt giy, trong BER trung bnh xu hn 10 (xem ch di), sao cho vi tc bit 64 kbit/s, s c hn 64 li trong giy ny.

C h : Nhm nghin cu XVIII, vn 10 ca CCITT ngh 10^ l ngng BER cho s c i vi on ni ISDN.

7.4.1.3. Cc g iy li

Giy li l khong thi gian 1 giy trong xut hin t nht mt li bit.

Cc ch tiu c trng i vi on nl ISDN quc t c th hin trong bng 7.1. iu mun ni l cc on nl ISDN quc t s tho mn ng thi t t c cc yu cu ca bng 7.1. on ni khng p ng c yu cu ch tiu nu mt yu cu b"t k no khng tho mn. Mt th hin khc ca bng 7.1 c cho trong bng 7.2.

20________________________________________________________ Viba s - Tp 2


7.4.2. Phn chia ton b cc ch tiu

Cc ch tiu cho trong bng 7.1 v bng 7.2 lin quan n ton b on ni. Cn phi chia cc ch tiu ny cho cc on k tip nhau ca chng lm c s cho cc ch tiu c trng i V I thit b truyn dn v cc h thng c s dng h tr on nl. Vic phn chia ny s dng hai chin lc khc nhau mt t. Mt l c th s dng cho cc ch tiu cc pht suy gim cht lng v cho ch tiu cc giy li, hai l c th s dng cho cc ch tiu giy li trm trng.

Bng 7.1 Cc ch tiu dc trng ca li i vi oan ni ISDN

P h n loai c trng Ch tu

(a)(Cc pht suy gim cht lng)(ch 1 v 2)

t hn 10% cc khong 1 pht c t s" li xu hn 10' ^(ch 4).

(b)(Cc giy li nghim trng)(ch 1)

t hn 0,2% ca cc khong mt giy c t s^ li xu hn 10' .^ 1

(c)(Cc giy li)(ch 1)

t hn 8% cc khong mt giy c cc li bt k (tng ng vi 92% cc giy khng li).

Ch :1. Thut ng "cc pht suy gim cht lng", "cc giy li trm trng v "cc giy li

c s dng l nh tnh c trng thun li v sc tch ch tiu cht lng. Ni cch khc, cch dng ca n khng bao hm v nh lng mc c trng ny .

2. a cc khong thi gian mt pht bng cch loi tr thi gian khng c kh nng lm vic v cc giy li trm trng ra khi thi gian tng v sau nhm cc giy cn li lin nhau thnh cc khi 60. Cc khong mt giy cd bn c a ra t mt mu thi gian c nh.

3. Khong thi gian T| trong nh mc phn trm khng c xc nh, v khong thi gian c th ph thuc vo ng dng. Khong thi gian mt thng bt k c chp thun coi l mt chun .

4. Vi cc l do thc t 64 kbit/s, 1 pht c 4 li (tng ng vi t s' li 1,04 X 10 ) th khng c coi l suy gim cht lng. Tuy nhin iu ny khng ng gim nh ch tiu t s' li 1() \

7.4.2.1. Cc nguyn l chia thnh tng p h n v cc ch tiu li

Vic chia thnh tng phn da trn c s s dng gi thit cc h thng truyn dn c cht lng ri vo mt lp bt k trong ba lp: h thng ni ht, h thng bc trung bnh v h thng bc cao (ch khng ph thuc vo cc h thng truyn dn c s dng). Ni chung cch dng ca chng ph thuc

vo v tr ca chng trong mt mng. Hnh 7.4 ch ra s phn ranh gii cht lng mch ca HRX di nht. Khng th cho n mt s xc nh, tuy vy c th cho v tr bin gii gia cc on bc trung v bc cao ca HRX. Ch 4 ca bng 7.3 s lm r im ny.

Bng 7.2 Cc ch tiu biu din thay th

22________________________________________________________ Vi ba s - Tp 2

(a) Cc pht suy gim

(b) Cc giy li trm trng

(c) Cc giy li

t nht l 90% ca cc khong 1 pht cht Ing c < 4 li (ch 1).

It nht 99,8% cc khong thi gian trm trng mt giy c t hn 64 li (ch 2).

It nht 92% cc khong 1 giy c 0 li (ch 3).

Ch :

1. 4 li trong mt pht tng ng vi BER 5: 1 X 10 .2. 64 li trong mt giy tng ng vi BER a 1 X 10' .^3. Khong thi gian hp thnh 1 giy chn l n biu th mt khong thi gian

ngn ngi s dng s liu c th chn iu khin li.

C ba s phn loi cht lng mch l vi cc h thng truyn dn nh sau:

BC NI HT

Cc h thng gn vo y gi thit l hot ng gia cc thu bao v cc tng i ni ht (LE ) v c c trng bng cc h thng truyn dn tc s thp hot ng tc bit di cc mc s cp 2048 kbit/s hoc 1564 kbit/s, trn mt s loi cp v mi trng truyn dn khc.

BC TRUNG BNH

Gi thit cc loi h thng ny hot ng gia cc tn i ni ht (LE) v vt sang phn ng ni quc gia. Khong cch thc s m cc h thng ny bao ph c th coi l khc nhau gia nc ny v nc kia, nhng hu ht cctrng hp s khng vt qu 1250 km. cc nc ln th khong cch ny cth ch t trung tm chuyn mch s cp (PC), trong khi cc nc nh, th khong cch ny c th xa trung tm bc hai, bc ba hoc trung tm chuyn mch quc t. i vi hon cnh ny, cc h thng truyn dn quy cho lp ny s thay i v cht lng, ri vo gia lp bc ni ht v lp bc cao. Trong nhiu h thng s thay i v cht lng lin quan ti vic t k hoch v bo dng mng li nhiu hn so vi s khc nhau v bn cht trong thit

k h thng. Thng thng cc h thng ny lm vic vi cc tc bit thp, trung bnh hoc cao v s dng cc mi trng truyn dn khc nhau.

BC CAO

S dng cc h thVig ny thc hin truyn dn trn tuyn truyn ni quc gia ng di v tuyn nl quc t. Thng thng cc h thng nh vy hot ng cc tc bit trung bnh v cao s dng t t c cc loi mi trng truyn dn, nh l v tinh, h thng viba tm nhn thng, si quang... di tng cng ca HRX (27500 km) m ch cc h thng bc cao mi to nn on c ngha nh vy.

S dng cc gi thit tng qut sau y cho chin lc phn on:

1. Phn chia thnh tng phn tt c ch tiu cho cc phn k tip khc nhau ca mt on nl, iu c ngha l phn chia "phn trm thi gian" hoc "phn trm khong thi gian trung bnh" cho php khng vt cc gi tr ngng.

2. Vic phn chia u cc ch tiu c s dng cho c hai phn trm cc pht suy gim cht lng v phn trm cc giy li.

3. Khng phn chia ngng t l li, v d tnh rng c tnh ca cc mch thc to nn cc phn ca HRX bnh thng s quan trng hn so vi ngng pht suy gim cht lng.

4. S ng gp li do c hai thit b chuyn mch s" v ghp knh s" c gi thit l khng ng k so vi s ng gp ca cc h thng truyn dn.

i vi cc h thng viba tm nhn thng vic phn chia loi c tnh "pht suy gim cht lng" thnh nhng n v nh hn c th cn phn chia ch tiu t s li cng nh phn chia "phn trm thi gian" theo khong cch. Trong mc 7.3.3.2, ngi ta chp nhn l ch phn chia "phn trm thi gian". CCITT tuyn b' rng y l mt mc tiu nghin cu trong tng lai (Khuyn ngh G.821 ca CCITT).

7,4.2.2. Chin lc p h n oan i vi cc p h t suy g i m ch t lng v cc yu cu g iy b li

Trong bng 7.3 th hin vic phn thnh tng on s suy gim cht lng cho php, tc l 10% cc pht suy gim cht lng v 8% cc giy li. Bng 1.5 th hin cc ch tiu c trng ca mch xut pht.


7.4.2.3. Chin lc p h n chia thnh tng p h n i vi cc g iy li trm trong

Vic phn b" tng cng 0,2% cc giy li trm trng c phn chia cho mi loi mch nh sau:

a) 0,1% c phn chia cho 3 loi mch thu t l nh nhau, nh chp nhn v thc hin i vi 2 ch tiu khc. Bng 7.4 ch ra cc kt qu ca s phn nh ny.b) 0,1% cn li l hn nh khi cho cc lp bc trung bnh v bc cao ph hp vi s xut hin cc iu kin bt li ca mch th nh thong xy ra l vi cc h thng truyn dn ( mun ni thng xu nht trong nm). V ngi ta xc nh thng k s xut hin cc nh hng thng xu nht trong cc on nl khp th gii, ngi ta cho rng cc hn nh cho php sau y l ph hp vi tng s' 0,1% - 0,05% cho HRDP 2500 km l vi cc h thng c th s dng cc on ni bc trung hoc bc cao, v0,01% cho HRDP v tinh.

Ton b cc ch tiu bc cao c phn chia trn c s di, kt qu c th c s dng theo quan nim trn km phn nh a ra mt phn nh khi cho php i vi cc di ca h thng c bit. Da vo bng7.5, cc ch tiu c trng ca mng c a ra trong bng 7.6.

Bng 7.3 Phn b cc chi tiu pht suy gim cht lng v cc giy li cho 3 loai mach

24_________________________________________________________ Vi ba s - Tp 2

Loai mach Phn b cc ch t iu cc p h t suy g i m cht ng L cc g i y li i vi bng 7.2 1

Bc ni lil (2 u cui)

Bc trung bnh (2 u cl)

Bc cao

15% hn nh cho php ca khi ti mi u cu (ch 1, 4, 5).

15% hn nh cho php ca khi ti nii u cui (ch 2, 4, 5).

40% tng ng vi i lng chp thun 0,0016%/km i vi 25000 km (ch 3, 6 v 7).

Ch :1. S chia thnh tng phn bc ni ht c coi l s phn b^ khi, tc l s phn b

phn u n m khng k n di.2. S chia thnh tng phn bc trung bnh c coi l phn b^ khi, tc l s phn b'

cho mt phn m khng k n di ca on nl. di thc bao trm phn bc trung bnh ca on ni s c coi l thay i tu tng nc. Cc h thng truvn dn trong lp ny biu hin mt s bin i trong h thp cht lng gia cc lp khc.


3. S chia thnh tng phn bc cao trn c s chiu di, kt qu s dng quan nim trn km phn nh a ra s phn nh khi cho mt m hnh mng xc nh (v d tuyn s"chun gi nh).

4. Cc on bc ni h t v bc trung cho php bao trm n 1250 km kh u ca mch bt u t im chun T ko di Lrong mng li.

5. Mt s nh chc trch xc nh cc phn b^ khi cho cc on bc ni h t v bc trung v cn thit trong phn b^ phm vi tng cng 30% cho mt u nl bt k ca on ni. iu ny cng c s dng vi cc ch tiu cho cc bc ni h t v bc trung trong bng 7.4 .

6. Bit rng c tnh li ca thng tin v tinh khng ph thuc nhiu vo khong cch nn s phn b khi 20% ch tiu pht sviy gim cht lng cho php v giy li c phn phi cho HRDP v tmh n s dng trong on bc cao ca HRX .

7. Nu on bc cao ca ng ni bao gm mt h thng thng tin v tinh v khong cch cn li bao gm trong phm tr ny ln hn 1200 km hoc nu phn bc cao ca mt on ni khng qua v tinh ln hn 2000 km th c th vt qua cc mc liu ca Khuyn ngh G.821. Vic xut hin cc on ni nh vy tng i him. Cc nghin cu ang tip tc tm hiu su sc vn ny. Khi nim khong cch tng dng ca v tinh l b ch ( di ng truyn dn mt t tng ng). Trong mi quan h nv v ch rng d tnh gi tr nm trong khong 10000 - 13000 km.

B n g 7,4 P h n b c c g i y l i t r m t r n g

Loai rnach

Phn b cc ch t iu cc g i y li t rm trong

1 Bc ni ht 0,015% hn nh cho php ca khi ti mi u C U I (xem c h 5 VI bng 7.3)

Bc trung hnh 0,015% hn nh cho php ca khi ti mi u cui (ch 5 i VI bng 7.3)

Bc cao 0,04% (ch 1)

Ch :1. i V I cc loi h thng truyn dn bao trm loi bc cao, mi phn bao gm

2500 km c th phn b'khng ln hn 0,004%.2. 'i VI HRDP v tinh hot ng trong phn bc cao c mt phn b' khi

0,02% cc giy li trm trng (xem ch 6 i vi bng 7.3).

7.4.3 Din gii cc ch tiu c tnh li i vi m ch ni ISDN quc t

Bng 7.1 th hin cc ch tiu ny l vi HRX 27500 km. Khuyn ngh G.821 ca CCITT cung cp mt thut ton tr gip din gii cc ch tiu ca bng 7.1. Thay th cho thut ton ny l s gii thch nh sau:

Cc pht suy gim cht lng, cc giy li trm trng v cc giy li c

26 Viba s - Tp 2

gim st trn mt thi gian tng cng T ,. Nh ni trc y, gi tr ca Tj l khng nh r, nhng ng VI lhuyn ngh 594 ca CCIR n l mt thng (c tnh li ca HRDP). Nu l thi gian khng cho php tng cng, cng nh nh ngha trn. Ta c:

T. = T, - T (7- 1)

y c tnh bng giy. Nu s l s cc khong mt giy c tnh cha nhiu hn 64 li (cc gi}' li trm trng), s phi nh hn 0,2% ca T.J. Vy l:

lOOS/T,, < 0,2 (7-2)

Cc giy cn li T,, - s c nhm li thnh tng khi 60. S" cc khi M cha hn 4 li (cc pht suy gim cht lng), nhng nh hn 10%:

lOOM < 10 (7-3)

Trn khong T |, ta tnh s khong mt giy chng cha t nht l mt linhng nh hn 64 li (cc giy li). Tng s cc giy li ny E phi nh hn 8%ca T,,:

lOOE/T, < 8 (7-4)

Bng 7.5 Phn b cc chi tiu cc pht suy gim cht lngv cc giy li

Ch tiu c t inh 64 hbt /s

Loai mch % ca c khong 1 pht vi ln hn 4 li

( cc pht suy gim)

% ca cc giy c 1 hoc ln hn 1 li

(% cc giy li)

Bc ni ht hn nh cho )hp ca khi i vi mi u cui

< l. < 1,2

Bc trung bnh hn nh cho php ca khi cho mi au cui

< l. < 1.2

Bc cao vi 25000 km < 4.0 < 3,2

7.4.4. V d nghin cu t liu gii thiu trong muc 7.4

Xt mt mch chun gi nh 1200 km trong t t c cc on ng trc l bc cao. Cc mch c s dng trong h thng ny gm cc h thng phn b" ni ht, cc h thng truyn dn phn hi xut pht t mt nt phn

b^ (DN) s" liu ca khch hng n mt nt truy nhp trung k chnh (TAN), v cc tuvn trung k gia cc TAN. Xc nh cc ch tiu c tnh li i vi h thng ny trn c s Khuyn ngh G.821 ca CCITT v o 64 kbit/s. Cc ch tiu c nghin cu t mc 7.4.4.1 n 7.4.4.4.


7.4.4.1. Cc p h t suy g i m cht lng

T bng 7.5 tin hnh cc phn b" nh sau:

CC MCH NI HT

Cc mch ny bao gm cc mch phn b" ni ht.Hn nh cho php i vi mi u cui < 1,5%(Hn nh cho php ca khi).

CC MCH BC TRUNG BNH

Hn nh cho php i vi mi u cu"i < 1,5%(Hn nh cho php ca khi).

CC MCH BC CAO

Cc mch ny bao gm cc mch ng trc v cc mch v tuyn v hn nh cho php da trn c s di ca mch. Nh bng 7.5 ch ra, 4 hn nh l cho 25000 km (xem hnh 7.4). Nh vy i vi 1200 km, th hn nh cho php ny s t hn:

(1200/25000) X 4% = 0,192%

Bng 7.6 Pin b phn trm i vi cc ch tiu mt pht v cc giy di vi cc h thng bc cao

D d i h thng

bc caOy km '

Chi tiii c t nh ca m a n g li 64 kbt/s 1

% cc khong mt pht c > 4 li (% cc pht suy gim cht lng)

% cc giy c mt hoc hn mt li (% cc giy li)

280 < 0,0448 < 0,03584420 < 0,0672 < 0,053762500

28 Vib3 s - Tp 2

CC MCH BC TRUNG BNH

Hn nh cho php vi mi u cul < 1,2% phn b ca khi.

CC MCH BC CAO

Hn nh cho php tng on trong trng hp ny cho trong bng 7.5 l t hn 3,2%. Nh vy l vi 1200 kra, s phn chia thnh tng on tr nn t hn; (1200/25000) X 3,2% = 0,154%.

Bng 7.7 Cc chi tiu dc tinh li di vi v d trong mc 7.4

P h n b ni Hat

ng tr lai

Trung k s c p (cp)

Trung k 1 s cp 1

(Sng f)

di ng truyn dn mu (km)% cc pht suy gim % cc giy li % cc giy li trm trng

Hn nh cho php theo khi

< L< 1,2< 0,010

Hn nh cho php theo khi

< l. < 1,2

N(u tuyn trung k l h thng v tuyn chuyn tip, th s chia thnh tng on i vi 25000 km s l 0,05% v hn nh cho php s nh hn:

(1 200/25000) X 0,05 = 0,0024%

7.4.4.4^ Tm t t

Bng 7.7 ch ra mt s tm tt cc kt qu trn mt cch n gin. By gi cc mch bc cao c gi thit l 600 km. iu ny c ngha l i vi cc mch b c cao cc con s c tnh ton trn gim i mt na.

7.5. TN H KH DNG

T nh kh dng c th hin theo phn trm ca thi gian l kh nng ca mt thnh phn (thit b) thc hin mt chc nng yu cu, v phng din ' tm cy n l ch da ca kh nng bo dng, v bo dng/hu cn. Trong mng tng t, s dng cc ch tiu kh dng ch yu lm cc tham s" thit k h thng. Tri li, trong mt mng thng tin s, cc ch tiu kh dng c s dng l cc tham s c trng ca mng, v cng vi cc ch tiu cht lng trnh by trong mc 7.2 n 7.4 bao gm cc ch tiu c trng tng cng ca mt h thng. e thit lp cc ch tiu kh dng, cc iu kin to ra mt dch v cn phi xc nh kh nng khng th lin kt ca h thng. Mt dch v c coi l khng c kh nng cho ngi s dng khi cc mc c trng ca n khng truyn thng tin mt cch d hiu. Mc c trng ny s thay i tu thuc vo ngi s dng v do cn phi nh gi cc yu t" lm gim cht lng truyn dn s. C 6 tham s" truyn dn c th s dng nh gi c tnh cht lng. l BER, cc gin on ngn, thi gian tr, trt, tri v tp m lng t BER v cc s gin on ngn l cc ch s" chnh v khng kh dng. l v tri v trt s gy ra cc li v cc gin on ngn trong m ng li, cn thi gian tr v tp m lng t l cc i lng c nh lin quan trong mt on nl bt k. Hin nay cc dch v thoi chim s" phn trm ln nht trong tt c cc dch v thng tin. Chng c coi l mnh hn so vi cc dch v khc, khoan dung hn l vi cc li bit v cc gin on ngn. V l , cc dch v thoi c th c s dng biu th mc c trng t thiu i vi c tnh kh dng ca mt on ni. Tnh kh dng ca mt on ni thay i tu vo cc nh chc trch nhng nu chn ch tiu tnh kh dng ti thiu 99,8% thi gian trong khong 3 thng bt k c khng qu 30 ln gin on thng tin trong thi gian , th mng thng tin c coi l c mc dch v r t tt. lhi o c tnh li ca mng, gi thit rng tn hiu kim tra c mt trong ton b thi gian o T[. Trong cc thi k khi khng c

Chnig 7: Cc h thng viba s v cc ch tiu cht lng 29

tn hiu kim tra, hoc tn hiu kim tra suy gim n ni lm cho mch khng th thng tin c, php o b nh ch v c th ni mch khng th lm vic. Do cc khong thi gian khng kh dng khng c tnh trong cc php tnh phn trm.

C hai loi khng kh dng khc nhau. l:

1. Mch kim tra khng kh dng, ngay tc khc thit b pht hin li mt ng b vi bt c l do no v mch kim tra, kh dng ng b c khi phc li ngay.

2. Mt khong thi gian khng th lm vic c bt u khi BER trong mi mt giy l xu hn 10 trong khong thi gian 10 giy lin tip;10 giy ny c th coi l thi gian khng lm vic c. Khong thi gian khng lm vic c kt thc khi BER trong mi giv tt hn 10 * trong khong thi gian 10 giy lin tip; 10 giy ny c coi l thi gian c kh nng lm vic (Khuyn ngh G.821 ca CCITT). Hnh 7.6 gip gii thch nh ngha ny, vi mt s cch c th ghi khong thi gian khng lm vic. Ch rng cc pht suy gim cht lng bnh thng c mt s giy khng li.

Khi nim tnh khng kh dng ca CCIR i vi mt tuyn nl s chun gi nh (HRDP) a ra trong Khuyn ngh 557 ca CCIR. Khi t nht mt hng truyn dn xut hin hoc c hai iu kin sau trong t nht 10 giy lin tip: tn hiu s" b gin on (tc l m"t ng chnh hoc mt tn hiu nh thi gian); t s" li ln hn 10^. Khuyn ngh cng i n xc nhn rng, khi nh gi tnh khng kh dng tt c cc nguyn nhn bao gm trong l tin on thng k, khng tin nh v to nn do th i t b v tuyn, s cp ngun, truyn lan, thit b ph tr v hot ng ca con ngi. nh gi tnh khng kh dng bao gm c vic xt n thi gian hi phc trung bnh.

Gi tr ca ch tiu tnh kh dng vi HRDP 2500 km l 99,7% thi gian, s" phn trm vt mt khong thi gian di l c gi tr thng k; thi gian ny c th ln hn mt nm. C C IR (Khuyn ngh 557) chp nhn gi tr 99,7% l gi tr tm thi v trong thc t cc ch tiu chn c th ri vo trong khong 99,5 - 99,9%. Vic chn gi tr c trung trong khong ny ph thuc vo v tr ti u ca thi gian gin on thng tin s. Cc nguyn nhn khc nhau khng th cng ging nhau khi cc iu kin cc b nh truyn lan, quy m a hnh, phn b' dn c, t chc bo dng c a vo xem xt tnh ton. Tnh kh dng ca cc h thng v tuyn chuyn tip ch l mt trong nhiu kha cnh m bo cp chp nhn cc dch v thng tin thoi;

30__________________________________________ ____________ Viba s - Tp 2


c th tin hnh chn gi tr ti u i VI kha cnh c bit ny bng cch ch coi rng t t c cc h thng thng tin hin c hoc c thit k trong mng li ang nghin cu. V vy, cc nh chc trch c th chn cc gi tr ch tiu kh dng khc nhau cho cc c quan lp k hoch ca h s dng, trong thng trn 99,5 - 99,9%. Tnh kh dng ca cc thit b ghp knh khng c trong khong cc s" liu c tnh kh dng trch dn. S khc nhau gia cc nh ngha ca CCIR v CCITT ch l: Khuyn ngh 557 ca CCIR c c cc s gin on (bao gm mt ng chnh v thi gian nh thi) l cc s phn b" vi cc khong thi gian khng lm vic c. Cc khong thi gian bit lp ca BER khng chp nhn c ko di t nht lOs, CCITT coi l cc khong thi gian c trng h hng (cc giy li trm trng) trong thi gian c th cng tc, chng khng ng gp vo tng thi gian khng th cng tc. Nh vy cc gin on do chuyn mch bo v bnh thng s khng ng gp vo c tnh khng kh dng.

s

T h i Qcit; ^ ongk h n d u n g ^' \

hi gian kha dung ?s 3s 2s ^ 10s

1 ti yiniiT h i Qi an khiig kt in d n g k i d n g

Cric tjiay li Irani i rong

Cc phut ginm cap

Cnc g i jv li

Cac yiay kiQ li

Hinh 7.6- V d v cc dy cha thi g ian kh dng

Bo co 445-3 ca CCIR coi rng tnh kh dng ca mt h thng viba c th c m t bng cc thnh phn gin on. Mt gin on l mt thi k trong ton b hoc mt phn tn hiu b mt do tp m qu mc, hoc tn hiu b gin on hoc mo trm trng. C hai loi c th xut hin trong h thng viba. l cc s gin on di c th ko di t lOs cho n 1 gi, xut hin mt ln/mt tun, hoc ch mt ln/mt nm, v cc s gin on ngn l kh thng xuyn. Chng c th ko di t vi pht cho n mt phn rt nh ca giy, nhng chng c th xut hin vi ln mt gi i vi mt s trng hp. C hai loi gin on c tm quan trng khc nhau khi xt ti tnh kh dng ca h thng, l V I cc gin on di, cc tham s cn phi xc nh l thi gian trung bnh gia cc gin on, sao cho chng khng xut hin qu thng xuyn v tng s' t on thng tin trn mt chu k mt nm. iu ny m bo l tnh kh dng ca mt h thng l thch hp v khong trung

bnh ca mt gin on nu n xut hin s khng qu di. i vi cc gin on ngn cc tham s' yu cu phi xc nh l: t s" xut hin o trn c s mt gi, tng thi gian gin on thng tin t u n cui thng xu nht, v khong thi gian ca cc gin on ngn ny. Chng s c xc nh bng thng k. Trong mc 7.6.2 s xt tip mt s im c bit ca mt phn b" ch tiu.

7.5.1. Cc nguyn nhn khng kh dng

C th c hai l do to ra cc yu t" nh hng ti tnh kh dng ca ISDN v k c mch ni quc t trong HRX, iu tr li ti sao CCITT cho n nay vn cha cho mt ch tiu tnh kh dng tng qut. Khuyn ngh CCIR, khng ni n tnh kh dng ca thit b ghp s', hn ch i vi cc dch v viba c" nh v do c th cho mt gi tr d biu th hn i vi hin thc. Cc nguyn nhn khng kh dng trong cc h thng viba c th phn thnh ba loi. l:

1. Cc pha inh su hoc giao thoa khng chp nhn c do cc iu kin truyn lan bt li trn cc ng truyn v tuyn.

2. Thit b c s c^ hoc hng.3. S tc ng ca con ngi c th ln hn so vi hai yu t" trn cng li.

Cc pha inh su gy ra nhiu ln hn mt gii hn nht nh. iu ny c th do hin tng ng dn v thng xy ra trong mt thi gian qu di nh tiu hao ma qu mc ch yu l do ma ro v trong mt s trng hp l do tuyt ri dy. Pha inh gy ra cc gin on ngn. Cc gin on do cc s c thit b gy ra c th do s gim phm cht ca thit b v tuyn, bao gm c cc thit b iu ch v gii iu ch, cng nh cc thit b ph tr nh thit b chuyn i mch, thit b cp ngun s cp v s c ca anten hoc phi

7.5.2. Ch tiu kh dng t u cui ny ti u cui kia

Xt mt khong ch tiu kh dng 99,5 - 99,9% c khuyn ngh trong Khuyn ngh 557 ca CCIR, Nu chn 99,5% cho mt h thng v d tr 0,3% cho php cc t on thng tin xy ra do cc li ca con ngi th ch tiu kh dng i vi cc mc ch thit k h thimg l 99,8% t u cui ny ti u cui kia. Nu gim nh phm vi nh hcing ca con ngi bng cch trang b mng li t ng, khng cn bt c mt ,; hiu chnh no i vi cc thit b lp t hin c th c th b 0,3% d tr ny l vi cc nguyn nhn tc ng cha bit lm t on thng tin do nh hng ca con ngi, Bo co 445-3 ca CCIR cho tnh kh dng A% l:

32_________________________________________________________ Viba s - Tp 2

A = 100 - (2500 X 100/L) X [(T, + T 2 - T,,)/TJ (7-5)

y: A - Tnh kh dng tnh bng phn trm i vi 2500 km.L - di knh ca mt ng v tuyn ang xt.T, - Thi gian gin on tng cng t nht l trong 10 giy lin

tip i vi mt lung thng tin.T2- Thi gian gin on tng cng t nht l trong 10 giy lin tip

i vi hng t ru y n dn kia.T|,- Gin on song hng t nht l trong 10 giy lin tc

- Khong thi gian nghin cu.

i vi truyn dn khng c hng, T2 = 0 v T,, = 0. Tnh kh dng v khng kh dng c th c tnh trn c s ging nh trn. Ch rng i vi cc mch ngn, th tnh khng kh dng khng tun theo phng php cng tuyn tnh v cn phi xc nh mt gi tr tl thiu l vi tham s" L. i vi cc mch bc cao, th gi tr 280 km trch dn trong Bo co 930 ca CCIR c th ph hp.

Cc s' liu o c v tnh kh dng vi cc mch thc chng t chng c phn b" rng; s" liu ng tin cy v tnh kh dng thc t c nh gi c th ch l mt gi tr trung bnh ca mt s" ln cc s" liu thu thp c t nhiu tuyn v tuyn chuyn tip trong mt thi gian di.

7.5.3. S phn chia thnh tng phn khng kh dng

Mc 7.4 m t s phn chia thnh tng on cc t l li, iu cng c th s dng phn chia thnh tng phn khng kh dng cho cc phn t ca mng li ring bit, Chn 99,8% nh vi v d trong mc 7.5.2, gi tr cho php i vi tnh khng kh dng, gm c tc dng ca con ngi l 0,2% thi gian. Con s ny biu th tng tt c cc phn khng kh dng i vi cc mch phn b' ni ht, tuyn sau v cc dng khc nhau ca h thng trung k s cp. V cc tuyn trung k mang lng thng tin ln nht cho nn d tnh rng cc s liu khng kh dng i vi mch ny thp hn so vi cc tuyn sau hoc cc mch ni ht. Tuy vy v cc di mch ca chng thng thng di hn nhiu so vi cc tuyn sau hoc cc mch ni ht, nn gi tr khng kh dng c th ln hn hoc bng cc mng trn. Cc h thng si quang v cc h thng cp ng trc d tnh phi c khng kh dng thp hn so vi cc h thng viba tn s" cao do khng c nh hng truyn lan nh pha inh nhiu tia... Phn phi khng kh dng i vi cp si quang 600 km ng trc


chnh, c th chn l 0,030%, ,rong lc , l vi mt h thng viba tn s" cao 600 km ngi ta chn l 0,060%. D nhin 3 liu ny ph thuc vo cc yu t nh kh hu, a hnh, tn s"cng tc... C k h thng truyn dn pha sau d tnh c phn phi khng kh dng 0,C225% cho mi mt u cul v cc mng phn b' ni ht th phn phi tng l cao 0,0325%. Thng s c ch yu l do mng phn b" ni ht v do giao tip vi ngi s dng. Phn phi ca mng pha sau v mng ni ht ph thuc vo di ca chng hoc ph thuc vo cc yu t" nh s^ ngi s dng phc v cho mng ni ht. Mc ch ca phn phi khng kh dng l cho php n nh cc phn t ca mng sao cho ch tiu tnh kh dng t u cui ny n u cui kia s p ng cho ton b h thng. Ly mt v d i vi mt mch viba trc chnh 600 km, phn phi 0,060% c th phn tip lm hai phn. Mt phn l cho cc gin on thng tin do truyn lan v phn kia cho cc s c" thit b. T s" phn phn chia nh l mt s" tu hoc l da vo s liu tin cy d tnh l vi thit b. Phn tch tin cy c th chp thun l vn bo v d p ng lng phn phi tng cng cho h thng v tuyn. Phn phi khng kh dng do s c thit b l mt phn nh ca s phn phi khng kh dng ca v tuyn chuyn tip, bng cch b d tr tng l ln l vi s gin on thng tin do truyn lan. Mt khc, chp thun rng thit b cng tin cy th s phn phi ch yu cho cc nh hng truyn lan.

7.5.4. c tnh li v cc ch tiu kh dng i vi cc h thng viba s bc ni ht

Bo co 1053 ca GCIR cho rng: ni chung vic s dng cc thit b truyn dn mng ni h t v mt phn h thng v tuyn chuyn tip trong ISDN khuyn ngh c tnh li v cc ch tiu tnh kh dng vi mch HRX gia thu bao v chuyn mch ni ht. Mt phn cc khuyn ngh ny l:

1. BER < 10' ^ trong hn A% ca thng bt k vi thi gian hp thnh mt giy, gi tr ngh A nm trong khong 0,005 - 0,015.

2. BER < 10 trong hn B% ca thng bt k vi thi gian hp thnh mt pht, gi tr B ngh trong khong 0,5 - 1,5.

3. Tng cc giy li < c% ca thng bt k. Gi tr c ngh trong khong 0,4 - 1,2.

4. T l li bit d (RBER) l t l li bit ca h thng khi khng c pha inh v nhiu ngn, nhng li bao gm nhiu di xut hin do cc ngun trong cc tm nhn thng ca my thu b tp m v thng

34________________________________________________________ Viba s - Tp 2

thng l thp v mc v khng i v gi tr, s khng ln hn D. Gitr D c ngh trong khong 10^ n 10^.

. khng kh dng tng do tt c cc nguyn nhn ca mt h thngv tuyn chuyn tip s" song hng s dng trong cp ni h t ca ISDN s khng vt E% trung bnh trong F nm. Gi tr E c th nm trong khong 0,08 n 1 v F > 1.

BER d hoc RBER l t s li bit bit i vi t s^ li trung bnh khi khng c pha inh v gm c s phn phi vi cc li khng phi hp ca h thng, cc nh hng ca mi trng v s ho gi v cc nhiu thi gian di. Gi tr 5 X 10 i vi RBER c coi l ph hp vi Khuyn ngh 634 ca CCIR i vi mt mch bc cao 2500 km. Nh vy c th xut pht t cc ch tiu c trng da vo gi thit bit cc thng k ca li khc nhau mt gi tr ph hp vi gi tr v mt l thuyt.

VI khong cch L < 2500km, RBER < (5 X 10 '0 X L/2500 vi 280 < L < 200 km.

7.6. C TNH CC GIY KHNG L l

Khuyn ngh G.821 ca CCITT l vi 27500 km HRX ch r cc ch tiu c trng ca li t hn 8% ca cc khong mt giy l c cc li bt k, mi giy li l khong thi gian ca mt giy trong t nht mt li bit xut hin. iu ny dn n khi nim cc giy khng li (EPS), l tnh trng vi 98% thi gian khng c cc li hoc cc giy b li. Nu a ra HRDP tngng nh trnh by mc 7.3.3 th con s" ny s gim n 95%. Khuyn ngh 594 ca CCIR ch a vo tnh ton h thng v tuyn c s cc giy li < 0,32% hoc t hn tro n g thng bt k. iu ny c cp trong mc7.3.3.1, c ngha rng ch tiu cc giy khng li vi mt thng bt k t nht l 99,68% i vi HRDP 2500 km. Trong on ny chng ta s ch a ra mt b tng ng cc ch tiu BER u cui ny n u cui kia c c tnh khng li ny i vi mt mch ni hai ng 64 kbit/s. Cc ch tiu BER s ly dng:

Tt hn 10 ' l vi x% thi gianTt hn 10 i vi y% thi gianTt hn 10' ^i vi z% thi gian

S chuyn i gia BER v phn trm cc giy khng li (% EPS) i hi hi xt s phn b" cc li, l mt quan h ph thuc d dng. Khng c mt


cng thc chung, n gin no lun c gi tr. hiu qu trnh ny hy xt mt s phn b" li gi th it cng VI cc php ni suy khc nhau.

7.6.1. Phn b nh thc

Phn b" nh thc l mt phn b" m t mt trng thi m mt bit thng tin b li hoc khng li. Nu s' cc li c cho bng BER, hoc xc sut li p ,^ th xc sut khng xut hin mt li l (1 - Pg). Nu mt knh thng tin mang thng tin c tc bit bng T, th sau mt khong thi gian t, s bit thu c s l rjjt v xc sut c n bit b li l P(n) cho bi phn b" nh thc;

= (7-6)

y:

c . r - ( r t ) ! / [ n ! ( r t - n ) ! ]

Xc sut c t nht E li trong r,,t bit cho bi:

P { E ) = P { n ) (7-7)n = E

36_______________________________________________ Vi ba s - Tp 2

Xc sut khng li xut hin ti thi im t = 1 giy, n l tp khng li v phng trnh (7-6) tr nn:

P(0) = (1 - (7-8)

Nh vy phn trm cc giy khng li cho bi:

%EFS = 100 (1 - (7-9)

C th ni rng phn b" Poisson m t s xut hin cc bin c" bit lptrong mt min lin tc. Nh vy, nii r,, ln v Pg b, th c th coi cc li nhl cc bin c bit lp v phn b^ nh thc qui thnh phn b" Poisson, y:

P(n) = (r .P J" .e --Wn! (7-10)

Vi cc xc sut khng li:

P(0) = (7-11)

V khi l nh, th iu ny c th biu th gn ng bng phn trm cc giy khng li:

%EFS = 100 (1 - r,,.PJ (7-12)


Hnh 7,7- Hm phn b xc sut b F(E)'

c tnh li ca knh thng khng biu th bng cc thnh phn ca hm m t xc sut li, m tng qut hn l biu th bng cc thnh phn mc tiu BER thng gp, nh c trnh by trong mc 7.6. Cc mc tiu BER c th coi l cc im l vi hm phn b" tch lu F(E), n l xc sut Pp ln hn gi tr E. Hm phn b' tch lu b F(E) bng (1 - F (E)), n thng dng hn biu th cc mc tiu BER, v n xc nh xc sut Pp khng ln hn E, Xem hnh 7.7 l mt th ca F(E) i vi E. S dng cc php tnh scp, xc sut p,. nm trong khong (E - AE/2 ) n (E + AE/2) cho trong mt gii hn l vi AE/2, khi AE -> 0:

F(-AE / 2 < p , < AE / 2) = AE khi AE 0 (7-13)dE

T phng trnh (7-9) s phn b' gia s' giy khng li ca xc sut li trong khong gii hn AE cho bi:

cl(%EFS)/dE = 100.(1 - E) F(E)/dE.AE (7-14)

V phn b" tng cc giv khng li ca cc xc sut li E i vi trng hp ri rc cho bi:

%EFS = -100 1(1 - E) dF(E)/dE.AE (7-15)

V i vi trng hp lin tc cho bi:

%EFS = -100 (1 - E)\dF(E)/dE.dE (7-16)

38 Viba s - Tp 2

y:

dF(E)/dE = -d F(E)/dE (7-17)

7.6.2. Cc dng c a F(E)

Khuyn ngh 594 ca CCIR tuyn b rng t s" li bit i vi HRDP 64 kbit/s s khng vt cc gi tr tm thi di y, c tnh n pha inh, tp m v t t c cc ngun lm suy gim c tnh khc:

+ 10' ^trong hn 1% ca thng bt k; thi gian hp thnh ang nghin cu.+ 10' ^ trong hn 0,05% ca thng bt k: thi gian hp thnh mt giy.

S s dng ch tiu c trng li n}' chng minh lm th no c th suy ra c ch tiu phn trm EFS khc nhau i vi cc dng phn b' F(E) khc nhau.

7.6.2.1. M hnh ri rac

Xt hnh 7.8. Hnh ny c to nn bng cch coi rng 2 im cho trong Khuyn ngh CCIR trnh by mc 7.6.2. l cc im thuc gi tr ca F(E). Hnh 7.8 cng ch ra cc h s" o hm m.

T phng trnh (7-15) ta c:

%EFS = 100[(1 - 1 0 '^ )'>' .0,99 + (1 - .0,0095

Nu T, = 64 kbit/s th %EFS = 98,37

A) 1 01 ^ . ariEl

0 ,0 00 5 r

101____ I_____ L

10BE R

10*^ 10'^

C1F

oo 0 99

i(OOi i

1i1

0 ,0 09 5

\

10' 10B R

Hinh 7.8 M hnh ri rc ca cc ch tiu Khuyn ngh 594 ca CCITT

7.6.2.2. Ni suy tuyn tinh

Hnh 7.9 ch ra trng hp y cc ch tiu thuc Khuyn ngh 594 caCCITT c ni suy tuyn tnh gia cc im ch tiu. thu c ni suyng thng l vi ng 1, gi thit Ing BER thp nht t c l 10^. Gi tr clF(E)/dE l dc ca on ng thng, tc l:

dc ca on ng thng I = -9 X 10" d"c ca on ng thng II = -9,5.

S dng phng trnh (7-16), %EFS cho bi:

10' lO"''%EFS = 9 x lo ' j (l-E)*''dE + 9,0x10 |(l-E)" '>dE

io' 10''^

vi r,. = 64 kbit/s , %EFS = 98,69.

Chomg 7; Cc h thng viba s v cc ch tiu cht lng 39

i ' ' 10'' 10' 10 '^P-R E---

Hnh 7.9- Ni suy tuyn tnh ca Khuyn ngh 594 ca CCITT.

7.6.2.3. Ni suy logarit ca F(E)

Gi s rng F(E) v E c v bng cch s dng cc thang logarit, tc l lgF(E) l vi logE, v s dng ni dung logarit ny s to nn cc th ng thng c dng:

logF(E) = logK, + K^ . logE = Ig (K^.E' )^ (7-18)

hoc: F(E) = K,.E'= (7-19)

Nh vy:

-dF(E)/dE = (7-20)

Nu cc im cul (E,, F(E)|), (Eg, F(E)2) ca mt on thng c thay

vo phng trnh (7-18), Kj v K.2 c th c tnh:

Ki = [(logf2)(logE,) - (logf,)(logE,)] / [(logE,) - (logE,)] (7-21)

v:

K, = [(logf,) - (logQ] / [(logE,) - (logE,)] (7-22)

Do tch phn trong phng trnh (7-16) tr thnh:

%EFS = -100^2 (7-23)E ,;.

y E ,n, EgN v l cc gi tr ca E,v E,, K v K; gn ng vi on thng xp x th N. Bng 7.8 ch ra cc gi tr %EFS l vi cc gi tr cc tiu ca BER gi thit.

Bng 7.8- %EFS i vi Khuyn ngh 594 ca CCIR

40________________________________________________________ Viba s - Tp 2

BER cc t i u

gi th i t10 2 X 10 5 X 10 10 2 X 10 5 X l O **

%EFS 99,716 99,702 99,671 99,629 99,558 99,376

7.6.3. c tnh %EFS trong cc tuyn nhiu on

Xt mt tuyn N on bao gm cc on Sj, S2,... s^. Nu c tnh giy khng li mi on l EFSi, EPS.;,.-- EFSn v hng truyn lan bit l t S> S[>,, do cc giy li c truyn theo hng s, -> Sn, vi gi thit l li bit trong mt hng khng c hiu chnh trong on k tip. Hn na, v n ch l cc giy khng li, m cc giy d mc cc li, t s cc giy khng li ri on th 1 trn cc giy khng li i vo on th i l EFS,, v i vi cc tuyn nl vo nhau, th ton b %EFS cho bi:

N%EFS (nhiu on) = lo o p EFSj (7-24)

1 = 1

Nu cn mt 5 xp x hon ho hn th cn phi bit s phn b" xc sut li Pp trong mi mt on ca tuyn. Nu bit thng tin ny th s phn b tng th c th c xc nh bng cch nhn chp lin tip cc hm phn b^ ring r. Sau s dng phn b" tng th ny kt hp vi phng trnh (7-23) s nhn c c tnh % EPS ca tuyn.

7.7. O LI

Phn ny xem xt Bo co 613-3 ca CCIR i vi cc php o c tnh libit trong cc h thng viba s" trong c hai iu kin h thng lm vic bnhthng v ngng lm vic .

7.7.1 Tiu chun c bn nh gi c tnh li bit

Nh ni, tham s s dng m t c tnh ca mt h thng s' l xc sut li bit, tc l kh nng thu nhn khng ng mt bit duy nht. Trong thc t, hu ht tham s s dng l t s li bit BER. N c nh ngha nh sau:

T l li bit = NJN, = (7-25)

y: N^ : l s" cc li bit trong khong thi gian toN,; l s cc bit c pht i trong khong thi gian t1'!,: tc bit ca tn hiu nh phn ti thi im o

khong thi gian o hoc m li.

y qu t r n h smh li l ngu nhin, dng nh cc li c tnh trong khong thi gian di t, phng trnh (7-25) c th cho mt nh gi xc sut li. Nu tng Np th chnh xc ca nh gi tng ln. Nu trong khong thi gian ty xut hin li t, tc l Nj. b, nu mun Nj, ln nhn c nh gi tt th khong thi gian o t tr nn qu ln. Tuy vy, thc t yu cu khong thi gian o thng l hu hn, nhn c cc gi tr Ng. Gi tr chp nhn Np ti thiu c Ih khong 10 v do d xc sut li ng nm trong khong 50% ca N,,/Nt vi h s' tin cy 90%.

Cc s" liu yu cu trong phng trnh (7-25) c th thu c bng nhiu cch khc nhau. Trong thc t, php o c th to nn mt s" li trong khong thi gian c nh.

Phng php ny a ra mt u im i vi cc php o thng k bao qut ca cc b ch th c tnh v tnh kh dng. Cc php o 64 kbit/s tng ng vi cc tham s" cho trong sut chng ny s c to nn bng cch m s" li trong mi khong mt giy v sau s dng thut ton hnh 7.10 x l cc kt qu. Cc b ch th c tnh ca cc mch viba s" v cc on bnh thng o tc bit ca h thng. Tuy vy CCIR vn cha thit lp r rng mi tng quan gia cc kt qu o thu c t"c bit ca h thng v 64 kbit/s, nhng ngi ta ngh rng thi gian hp thnh cn t


42 Viba s - Tp 2

c cng mt mc tm cy s ph thuc vo dung lng ca h thng. Cc php o tc bit ca h thng chng t rng phn trm thi gian BER > 10 l thp hn 4 n 7 ln nu nh gi trong mt giy, thay cho thi gian hp thnh 1 pht (Bo co 930-1 ca CCIR) .

^tr.c; '^khng khci dung'khng kho dung ^tng

K ' ' " T s , h a d u n g - S ( > 1 ' ' )

Ket thuc

Co ioi

^li : h3 dung

Kel thcT' r S l> 1 0 '^

S(>10 J ^ kh : i d u n g

Co ^

kha dung

C)

c )

MiN.i.k

: Tng cc giy o = mt thng : s cc bt li trong khong 1 giy.: s'cc li bit trong mt giy gia giy th j v giy th k.

s (> 10 '*) ; Tng thi gian trong BER vt 10'^ trong mi giy, tinh bng giy.Thi gian khng kh dng, tnh bng giy.

Skhci dng T^ hi gian kh dng, tnh bng giy.S - : Tng thi gian trong s cc li > mt trong mi giy, tnh bng giy.^kh d n n '^hi gian kh dng, tnh bng ph t = kha d u H RM (>10'^) : Tng thi gian trong BER > 10 *^ o trong cc khi m.t giy k tip nhautrong BER > 10'^ , tnh bng pht.

Hnh 7.10- Thut ton o BER ca CCIR a) Kh dng b) Cc giy lic) Cc giy li trm trng d) Cc pht suy gim cht lng

Theo Bo co 61-3, CCIR)

C th s dng mt phng php khc to nn cc s" liu cho phng trnh (7-25) l o khong thi gian yu cu pht hin chnh xc Np li, hoc thun li hn, l s bit N| pht i trong mt khong thi gian nh vy. Phng php ny c mt s u im l chnh xc ca php o gn nh khng i i vi bt c gi tr t s^ li no v thi gian thc hin php o nh nht vi chnh xc theo yu cu. Trong mt s" trng hp nht nh th iu ny dng nh thun li hn o t s" li bit qua mt s khong thi gian mt giy ln k tip nhau. Cc php o trong lc h thng ang truyn tin th t s" li bit thc c th khc so vi gi tr c o v cn phi tin hnh phn phi kh nng ny trong tnh ton. Cng c th xut hin cc li t bin truyn dn, cn phi a chng vo tnh ton. Cc li t bin ny l cc nhm li thi gian ngn vi tn s" li tng i cao c khong thi gian di hn nhiu vi t l li thp hn nhiu. Xc sut xut hin hin tng nh vy, cc c tnh thng k ca n (khong t bin, mt li trong lc t bin, khong thi gian li thp,...) v nh hng ti cc dch v c truyn t r n tuyn s, cn nghin cu a n mt nh ngha tham s c tnh thch hp, nht l c th cn thit trong cc cu trc khung bt k.

Cng nh ni mc 6.3.3.2 i vi M-PSK, bin m Gray c s dng trong bn v cc trng thi ca b iu ch, cho php cc k hiu PSK k cn khc nhau ch mt v tr bit. Li ca mt v tr b"t k xut hin ch lin quan n mt li bit duy nht. T l li k hiu hoc xc sut li k hiu i vi cc s M-PSK c th lin quan n t l li bit (BER) bi:

BERpsK = (m Gray)log. M

Cng nh cho bi phng trnh (6-60), i vi cc tn hiu trc giao ng kh nng, nh M-PSK th t l li k hiu p,, c th lin quan n t l li bit bng:

MB E R p s K = - P e P S K

Mc 6.5.3 xt trng hp tp tn hiu khng c kh nng to m sao cho ch xut hin cc li bit n gia cc im ca tn hiu k cn, v BER bng li k hiu nhn vi mt h s" l/log2M. Tc l;

BER= (mNon-Gray)logsM

y K l h s" thit thi ca m Gray.


i vi M-QAM:p

BERq. ^ = ^QM loggVM

Hnh 6.5, 6.18 v 6.28: Tt c ch ra xc sut li k hiu p^ ., tr s nh phn.

7.7.1.1. T l li bit d (RBER)

RBER l t l li bit c hiu theo ngha t l li khi khng c pha inh, v n bao gm cc li khng phi hp ca h thng, cc nh hng ca mi trng, s lo ho v tp m thi gian di, nhng tr tp m thi gian ngn. Tp m thi gian ngn l tp m do cc iu kin truyn lan d thng v in hnh bao gm cc mc tp m r t cao m t khi chng ch xut hin, trong mt khong thi gian ngn. Tp m thi gian di l tp m xut hin do cc ngun trong ng nhn thng ca my thu b nh hng v in hnh l thp v mc v khng i v gi tr. i vi mch thc bc cao 2500 km, gi tr RBER c th t l tuyn tnh vi di ca mch L km, iu ny d tho Khuyn ngh 634 ca CCIR cho:

RBER (mch bc cao) < (L X 5,0 X 10 )/2500 (7-27)

C th thu c o RBER bng cch ly cc o RBER trong thi gian mt thng s dng, thi gian hp thnh 15 pht, b 50% ca cc khong 15 pht c cha cc gi tr BER xu nht v ly gi tr xu nht cn li. CCIR khng nh rng, hin nay, tm thi chp nhn phng php ny v cng cho rng cc khong thi gian hp thnh o kia cng c th thch hp thc t, c bit vi cc h thng dung lng nh. Mt kh khn xut hin khi xc nh phng php o RBER v khng d xc minh rng pha mh khng c trn tt c cc chng tip pht trong mt on trong thi k o. Trong phng php kim tra tm thi ca CCIR va trnh by, phi lu rng trong phng php ny cc khong thi gian cha cc li do cc nh hng khc nh pha inh cng c th b loi b, v trong mt s'^ trng hp c th bao gm c cc khong thi gian b nh hng bi pha inh. Khi xc nh s" phn trm ca cc khong thi gian b b, lng tnh ton cn thit phi m bo l cc li trong cc on s ni vi nhau khng tch lu to nn cc pht suy gim b sung trong t t c HRDP. Nu cc php o c tin hnh trong mt khong thi gian ngn hn mt thng, th c th thch hp loi b phn trm ca cc khong thi gian cao hn. i vi h thng viba s' bc ni ht c th to nn

44_________________________________________________________ Viba s - Tp 2

mt phn ca mng thng tin s" a dch v (ISDN), th t l li bit d (RBER) s khng ln hn D, y l 10" < D < 10

7.7.2. o c tnh li trong cc iu kin khng iu hnh nghip v

Kim tra khi khng iu hnh nghip v hoc o xm nhp c ngha l mt tuyn truyn dn tch ra khi dch v thng tin hoc nghip v gim st. Dng o ny l tng i thch hp vi thit k, kim tra sn xut, lp t v cc kim tra chy th. y l c th l vi cc h thng truyn dn ng di dung lng ln. Kim tra ny c thc hin bng cch a vo u vo ca tuyn mt mu th, bt chc dng s liu thng tin bnh thng. u ra ca tuyn c so snh tng bit mt vi mu chun khng li c to ra ti ch trong b pht hin li. Vic chn mu th thng c tin hnh trong cc dy nh phn gi ngu nhin (PRBS), i lc cn gi l dy "m", bt chc dng thng tin, hoc cc mu xc nh kim tra thit b truyn dn vi cc xu hng mu ph thuc hoc cc mu c hiu ng nh thi tiu chun. Thng thng mu c m ho thnh m giao tip ng dy chun no cho php cng n nl cc tn hiu s' liu gia cc u cui, thit b kim tra v cc thit b truyn dn. Mu th l mt dy ngu nhin c chu k lp li bng 2" -1. Cng nh mt b ghi dch to nn t mt b tr n bit, dy kim tra c th i t k to nn trong vng chu k lp gm mi t hp n bit hoc mt dy con ca n bit, t nht l mt ln. T hp gm n bit 0 b cm. Dy con c s" bt thp hn n cng s c lm cn xng trong s xut hin ca chng sut chu k lp li. c php o tin cy i vi cc nh hng nhiu gia cc k hiu th di ca b ghi dch phi ln hn hoc bng s xung b nh hng bi p ng xung i vi mt xung n. V yu cu ny ph thuc vo c tnh ca knh truyn dn, chng thng l khng c bit trc, nn thng chn gi tr n mt cch va phi. kim tra tnh thch hp ca dy th, c th so snh cc t l li quan st c vi cc di ca b ghi dch khc nhau. Mt phng din khc phi c a vo xem xt tnh ton khi nh gi chnh xc ca mt php o l s hot ng ca bin m. Cc hot ng bin m ging nh s ci bin dy k hiu trong tn hiu iu ch v c th lm tng s" li c pht hin khi gii m v cc hot ng t gii trn. Chu k ca dy th bng (2" - 1) c th p ng ph tn c cc thnh phn ri rc v c th c tn s" c bn l r^/(2"- 1), y l tc k hiu. Phi cn thn m bo rng cc dy th c p dng i vi u vo ca b iu ch s" khng gy ra ph RF khng mong mun, c bit khi tin hnh o tp m.


Bng 4.10 trong mc 4.4.3.1 ch ra di ca mu yu cu i vi tc bit tng bc khc nhau theo cc Khuyn ngh 0.151 v 0.152 ca C C IT T .

Bng 7.9 tng kt di mu kim tra i vi tc bit tng bc yu cu cho cc mc ch th .

Bng 7-9 Cc tc bit tng bc v cc di mu kim tra

46_________________________________________________________Viba s - Tp 2

Tc bit, kbit/s d i m u th 1

64 2 " - 1

2048, 8448, 1544, 6312, 32064, 447346 2' ^ - 1

34368 v 139264 223 _ ;

7.7.3. o c tnh li trong cc iu kin cng tc

Vn chnh trong loi o ny l pht hin s li bit trong lung xung s" cha bit. C th s dng cc phng php da vo nhng tin tc bit trc v mt s c tnh ca tn hiu thu c. Mt s" vn khc lin quan n loi o ny l khong thi gian o c gii hn bi nhng th cn thit cho hot ng trung gian v c tnh khng dng ca knh truyn dn n lm thay i cch din xut t s" li thu c t php o ny sang php o khc. Hu ht cc h thng truyn dn s u c gn cc phng tin d dng gim st li. Dng ca n da trn mt s" c tnh thng tin ang c truyn dn. i vi thit b u cui c th l tn hiu ng chnh khung, l vi thit b tuyn si quang c th l cc it pht sng biu th t s" li bit... Phng php chung nht gim st li trong lc lm vic l pht hin vic phm li m ng dy. Trong Khuyn ngh 0.161 ca CCITT ch r c im k thut l vi loi gim st ny cho thit b tc bit thp. iu ny cng c th c vi cc h thng cc tn hiu m giao tip i vi tuyn v tuyn c cng dng nh m ng dy, v d AMI, HDB3, B6ZS, hoc B8ZS. Mt phng php thng dng gim st li trong lc cng tc l kim tra mt s mu c" nh lun lun c trong tn hiu truyn dn. iu ny khc phc mt s" gii hn ca vic gim st phm lut m v c th to ra mt s" dng, nhng thng dng nht l tn hiu ng chnh (FAS) khung ca thit b ghp knh u cui ni vi v tuyn s". Dng nh c mt s" uu th trong phng php kim tra mu c" nh ti nhng im khc nhau dc tuyn, t t c s lin kt s' bao gm cc thit b ghp v tch knh v mt s" on ng dy s hot ng nhng tc bit khc nhau. Vic kim tra mu c" nh c th c lm bt c u c s xen, r s". Mt vn xut hin vi cc h thng ghp knh bc cao l

chuyn cc li xung cc mc 2 Mbit/s, 64 kbit/s. C th lm iu ny bng cch o cc li u ra ca knh nghip v 64 kbit/s hoc u ra ca lung 2048 kbit/s trong h thng bc cao. Ng y l bn thn b pht hin li phi c cc b tch knh sao cho c th tch c lung li u ra ca cc knh nghip v 64 kbit/s hoc 2 Mbit/s ra khi lu lng thng tin m h thng ang truyn ti cc im gim st ca h thng. Kh nng tch knh ny ca thit b o ang tr nn linh hot hn v gi thnh ca b phn cu thnh ang lin tc gim. Gi thnh ca mt cp mch 64 kbit/s u ra nghip v tng nu tng gi tr ca php o BER trong cc iu kin thng tin thc. Loi o ny c nh gi t l li trung bnh trong thng tin chnh xc hn, v c th da vo vic so snh tng bit mt ca tn hiu nh phn gii m bao gm c s m rng li bt k, iu s c nhn thy u CUI ca h thng.

Mc 7.7.3.1 n 7.7.3.3 cho mt s phng php m CCIR khuyn ngh (Bo co 613-3).


7.7.3.1. S xen vo gia cc dy kim tra

C th nh gi c tnh li bng cch a cc xung mu vo trong lung xung s tc ng h 1/N. my thu, cc xung ny c tch ra v m s li ca chng. c tnh li ca h thng c nh gi xut pht t vic m li ny trong phm vi cc xung mu. s dng phng php ny, thi gian nhn ra t s" li c trng l N ln thi gian yu cu m cc li ca tt c cc xung c truyn i. c tnh li ca mt mch viba c th c nh gi khcng ph thuc vo lng tn hiu giao din ni ng dy TT', nh ch ra trong hnh 5-1. Vic phn tch khiing ng b my thu khng phi bao gi cr.g cn thit tch cc xung mu. Cc xung c xen vo khng ph thuc cc khung ca b ghp s.

7.7.3.2. M k im tra t nh chn l

l vi phng php ny, b sung mt k hiu kim tra chn l cho mt nhem s k hiu u cui ca my pht. Gi tr ca k hiu b sung bng tn^ modulo M ca s k hiu, y M l s cc gi tr k hiu c th, tc l s" cc mc c pht i. l:

s

Gi tr ca k hiu chn - l = m un l M.n = l

y G l gi tr ca k hiu th n ca nhm s k hiu.

my thu, tng modulo M ca nhm s tn hiu thu c so snh vi gi tr ca k hiu kim tra chn l thu c. Gi thit rng nu c s ho hp th s khng c li xut hin v ngc li. i vi xc sut li thp (nh hn 10 '^ ) th s^ khng ho hp thu c t so snh kim tra chn l xp x bng tng cc li ca t t c cc xung trong tn hiu s. Phng php ny yu cu ng b khung.

7.7.3.3. Pht hin li m chu trnh

c tnh li ca mt h thng viba c th c l kim tra bng cch s dng mt k thut c gi l pht hin li m chu trnh. Khuyn ngh V.41 ca CCITT ch ra mt cp b m ho/gii m v m t s hot ng ca chng. K thut ny c s dng trong truyn a s" liu v kh nng ca n c th pht hin c cc li a hp. cn mt mu ng b 4 bit (0101) sao cho 16 bit kim tra c th kim tra trong cc khong bit lin tip. Khi mu c tha nhn th b ghi dch v b m bit trong b gii m tin n "0", vic gii m tip din bnh thng.

7.7.3.4. Tm t t

Lung xung s" t mt b ghp knh s" thng bao gm cc bit b sung v cc bit thng tin. V d chng c s dng to khung. Ni chung, c tnh li c th c nh gi khng ph thuc vo dng tn hiu thng tin. Trong mt h thng viba s" s dng nhng bit b sung nh vy o c tnh li bit c xem l khng thch hp.

7.7.4. Cc th it b gim st c tnh (PMU)

Trong mc 6.3.4 m t thit b gim st c tnh, s dng k thut ngoi suy t s' li bng cch ci bin cc ngng quyt nh kt hp vi mt my thu s" to ra t l li gi ln hn ca my thu. Phng php nh gi t l li gi tng ng vi hai hoc nhiu ngng quyt nh ci tin trnh by. Bng phng php ngoi suy, cc kt qu thu c t cc nh gi ca hai hay nhiu t l li gi cu thnh, ngi ta xc nh c mt nh gi t l li tng ng vi t l li c th thu c bng tiu chun quyt nh khng ci bin, tc l nh gi t l li thc ca my thu. u im ch yu ca vic s dng phng php ny so vi phng php m li trc tip l thi gian o nhn c mt s' lng thng k ng k ca cc li thc l nh hn, v t l li gi ln hn so vi t l li thc. CCIR trong Bo co 613-3 nu k thut

4^_______________________ _________ _______________________ Viba s - Tp 2

ny v cc li trch dn tham kho 10, 11 v 12 l mt phn ca cc php o t l li bit trong iu kin cng tc.

Mt phng php khc xc nh c tnh truyn dn trong khi lm vic l pht i mt s' cc k hiu thi gian c bit. Nu cc dy c bit ny khng thu c th truyn dn xem nh c vn . Do cc yu cu b sung ca cc h thng v tuyn s' nh l cc knh ph tr v vic ghp nhiu tn hiu s u vo, nn thng yu cu mt dng khung, thng xut hin cng vi bit chn l sinh ra t cc bit thng tin. T hp ny cho php xc nh chnh xc c tnh bng cch xc minh vic thu dy khung mong mun v cc bit chn l. Phng php ny c th pht hin c c hai s c trng thi logic v trng thi k hiu nhng s dng phng php t l li gi khng th pht hin cc s c logic. C mt s k thut gim st c tnh li ca h thng PSK. Phng php ny lm vic tt hn vi tp m Gauss, tp m xung, giao thoa gia cc knh hoc gia cc k hiu, hoc giao thoa vi cc h thng truyn dn khc cng nh khi h thng ang hot ng trn knh pha inh. Thi gian yu cu o cho cc nh gi ng tin cy ca t l li ph thuc vo cc thng k ca tp m, mc phn tp v t l li thc.

Cc thng tin thu c bng cch gim st lin tc c tnh h thng v tuyn s' trong lc lm vic v khng c s gin on thng tin, c a qua mt trung tm hot ng bo dng tp trung bng cch s dng mng o t xa hoc cc knh v tuyn ph tr. Thn tin chuyn n trung tm ny thng bao gm cc thng tin c tnh li, v tt c cc thng tin cnh bo.

7.7.5. Cc hot ng bo dng tp trung

t c hiu sut bo dng cc i cc hot ng bo dng tp trung, mt im quan trng l h thng truyn dn, v d h thng v tuyn s", phi trong hon cnh ch tn dng bo dSng sa cha, tc l cho php biu bo dng khng thng xuyn. Ngy nay iu c kh nng v tin cy ca thit b c nng cao, v cc thit k mch c ci tin. Khi xu't hin mt s c" cn phi lu , cu trc bn trong thit b th cn phi tin cy v biu th chnh xc s c n trung tm bo dng tp trung. Mt vn l phi nh gi cc nh hng t hp cn thit ch thc hin cc bo dng sa cha v cc yu cu i hi ca mt s dch v truyn dn s c th pht hin v thng bo tnh trng trc trc v cc s c' t l li rt thp. Cc gi tr t l li nh vy c th thp n 10 Nu xut hin mt li, th nhn vin vn hnh trung tm bo dng cn phi tin hnh nhng ng tc cn thit da vo cc thng tin c trong tay. Cc thng tin ny c th khng


ch xut hin t cc cnh bo ca b gim st c tnh, m cn t vic mt hon ton cc cnh bo truyn dn. iu ny t ra mt yu cu b sung cho h thng truyn dn l phn bit s mt hon ton cc cnh bo h thng so vi cc cnh bo c tnh v s^ liu.

C th phn bit cc yu t" lm tn hi truyn dn thnh cc loi sau:

T l li thp nhng khng chp nhn c.Cc t bin li gy ra trong thi gian ngn lm tn th t tc thi khung s v tuyn.Cc t bin li trong khong thi gian di gy ra t l li hn 10 .

Cc vn t l li thp nhng khng chp nhn c l:T l li trung bnh khng th xc nh c v cc li xut hin khng tng quan v thi gian.

- Khng c cc chu k c t l li cao hoc mt khung.

Cc t bin li thi gian ngn gy ra s mt khung tc thi c th coi l mt bi ton cc tr khc ca t l li thp. Phm tr ny ca bi ton c c trng bng:

Khong thi gian ngn c t l li cao c biu th bng mt ng b khung.

- Khong thi gian mt khung c th gia 5 v 25 ms. S tng quan thi gian c ngha ca cc hin tng, c ngha l cc li c t s li l 10 xut hin trong cc cm ko di vi giy v sau cc chu k vi gi khng c li xut hin.

Kh m phn bit c 3 loi suy gim cht lng truyn dn ny, v cc hin tng truyn lan nhiu tia v cc hin tng truyn lan khc cng gy ra li chn l tng t v mt khung tng t. in hnh, hin tng pha inh nhiu tia thng xut hin trong khong thi gian 1-2 gi 1 ngy, y t l li trn cc giy bin i chm. Ngoi ra trong lc pha inh c cc khong thi gian t l li cao xu hn 10 . C th ko di trong khong vi giy sau lc xut hin mt ng chnh khung.

Cc phng php phn tch s" liu c tnh k thu t hin trng l: biu th t cu trc ca thit b th; loi suy gim c pht hin c tnh t l li hin ti v biu th cc t bin li ngn hoc mt khung. Cch ny lm gim lng thit b cn phi chuyn n hin trng. Ngoi biu th cc b c tnh truyn dn, cc s" liu c tnh cng c phn tch bng cch s dng

50_________________________________________________________ Viba s - Tp 2

cc b vi x l, c thit k nhn ra bt k loi suy gim cht lng truyn dn no v to ra cnh bo c tnh n trung tm bo dng. Phng php ny tr gip phng nga cc cnh bo nhm xut pht t vic truyn s" liu sinh ra. V mi cp my pht v thu b gim st lin tc, nn cc biu th cnh bo trc tip c tnh khng chp nhn c th cung cp cho trung tm bo dng. iu ny gim thi gian s c cc b. Cc phng tin phn tch c tnh a ra cho trung tm a phng cng c th c to thnh trung tm bo dng bng cch truyn thng tin t cu trc bn trong thit b th hin trng hoc bng cch to ra mt tuyn t b vi x l ny sang b vi x l khc gia hin trng v trung tm bo dng.

7.8. CC CH TIU TIN CY

Mt s" nh chc trch coi l c th phn cc ch tiu khuyn ngh v kh nng khng kh dng thnh hai loi, mt lin quan n s c^ ca thit b, mt lin quan n pha inh (chng s dng ch yu cho cc h thng hot ng trn khong 10 GHz). iu c bn l ngi s dng thy c hai nguyn nhn kh nng khng kh dng ny hon ton khc nhau v t ra nhng yu cu hon ton khc nhau i vi ngi vn hnh.

c tnh khng kh dng cho s c" ca thit b xut hin trung bnh trong thi gian trung bnh gia cc s c (MTBF) vi tn sut tng i t (n s c trnh by phn sau trong mc ny), nhng kt qu gin on di hn. khng kh dng do thit b khng tin cy, cho bi biu thc;

khng kh dng = [1 - (MTBF)/(MTBF + MTTR)] X 100 %

= 100 X [(M T T R )/(M T B F + M T T R )] % (7-28)

V do :

kh dng = 100 X [(MTBF)/(MTBF + MTTR)] % (7-29)

y MTBF l thi gian trung bnh ca thit b gia cc s c" tnh bng gi, v MTTR l thi gian trung bnh khi phc li dch v tnh bng gi.Thi gian ny bao gm c thi gian p ng pht hin s c" v bt usa cha.

Hnh 7.11 ch ra khng kh dng tng cng l mt hm ca MTTR, cho rng MTBF = 25000 gi l vi mt tuyn gm hai my thu pht, mi ci c MTBF = 50000 gi. Do cc mi trng hot ng khc nhau, nn MTTR s bin i trn mt phm vi rng. Cc gi tr c chp nhn t 6 n 48 gi, tng ng vi cc khng kh dng nm gia 0,025 v 0,200%. Lu rng Khuyn


52 Viba s - Tp 2

ngh CCIR v kh dng (Khuyn ngh 557) hin nay s ng cho c hai hng truyn dn, ng thi, trong khi Khuyn ngh G.602 ca CCITT l cc khuyn ngh v c tnh li, s dng cho mt hng c lp nhau.

Trong lnh vc thng tin, tin cy gii quyt cc hin tng thch hp hoc khng thch hp m bo bi thit b vt l trong lc lm vic. V n khng th tin on c chc chn khi cc hin tng ny s xut hin, ngi ta s dng khi nim xc sut nh lng tin cy. Mt trong nhng khi nim xc sut thc hnh tng l quan trng l thi gian trung bnh gia cc s c (MTBF).

Cc nh sn xut thng nh gi MTBF i vi cc mu thit b khc nhau sao cho c th xc nh c kh dng c vi ton b h thng. Cn phi cp mt mu s chung l gii hn tin cy c nh gi i vi MTBF cho. iu ny khng phi bao gi cng t c v khi khng c, th hu ht cc tnh ton s c gi thit mt cch khng khoa hc l cc tnh ton kh dng c th hp l. Bng 7.10 lit k cc gi tr MTBF vi cc mu thit b khc nhau. D nhin cc gi tr ny s thay i tu thuc vo tng nh ch to, v y biu th mt quan nim v nhng vn d tnh v tr gip trong khi th i t k cc h thng.

1 10

hi gian trung binh khi phc (MTTR)

90

Hnh 7.11- tin cy ca thit b (Tuyn song hng bao gm hai my thu pht 50000 gi MTBF)

(Theo Bo co 1053 CCIR).


Tc s c x l t l ngc vi MTBF. Thng s dng tc s c khi tnh cc phn t ca h thng ni tng nhm xc nh ton b MTBF, Nu mt b ghp knh bc 4 c MTBF 5,8 nm v n c nl n mt h thng v tuyn ng ngn c MTBF = 5,7 nm, th tng MTBF ca tuyn c th tnh theo:

1

MTBF= 1/5,8+ 1/5,7+ 1/5,7+ 1/5,8

N cho tng MTBF vi tuyn xp x 1,44 nm hoc 12600 gi. Nu gi thit MTTR l 8 gi th kh dng tm c t phng trnh (7-29) l 99,937%. D nhin iu ny b qua s gin on thng tin do truyn lan v s ngng tr ca chuyn mch.

Bng 7.10 Cc gi tr c trng ca MTBF i vi mu thit b khc nhau

Thit b MTBF, Cc s Thit b MTBF, Cc s

Nm c ! nm N m c/nm

Thit b ghp knhGhp knh s cp 4,5 0,22 Mx bc 2 9.4 0,11MUX bc 3 8,2 ,12 Mx bc 4 5,8 0 J 7

My thu pht v tuynKhng bo v 2 Mbit/s 1,0 Bo v 34 Mbit/s 53,5 0,019Khng bo v 140 Mbit/s 5,7 0,18 Bo v 140 Mbit/s 540 0,002

Thit b ph trChuyn mch la chnNgun 250000

10"4 X 10

10'Chuyn rnch d phng nng

83333 1,2 X 1 0 ''

Thit b ng si quang(trn 100 km ng) 2,8 0,36

Mt biu din khc ca MTBF vi mt h thng N n v thit b chobi:

-1

i=l(7-30)

Trong mt h thng viba, cc s gin on thng tin ni chung do cc nh hng truyn lan tng hp khc nhau; s c thit b; cc hot ng bo dng; s c ngun; cc li ca con ngi; v s c thm ho (do cc yu t" nh bo,

chin tranh v cc thm ho thin nhin). S ng gp t nhng ngun ny s bin i r t rng i vi nhng thit b khc nhau. C cc phng php khc nhau gim ti thiu s gin on thng tin ca cc h thng viba tn s" cao. Chng bao gm vic thit k thit b c cht lng, k thut ng truyn tt, v mi trng hot ng ph hp, chng s m bo tin cy c bn ca cc h thng viba. Ch mt iu khng hu hiu cho cc h thng thng tin dung lng cao, c bit i vi cc thit b truyn lan, nu khng gn cc ch tiu tnh kh dng l vi cc h thng ny. Mt k thut no c s dng gim s gin on thng tin hoc thi gian khng kh dng, cn phi bo v dch v bo dng cho php m khng tng qu mc chi ph v phc tp ca h thng, v khng gim c tnh cht lng. Khng c mt phng php no hn bng vic d phng hon ton c th to ra kh nng chng li hu hiu cc s gin on nhiu knh do pha inh nhiu tia, hoc nhng ni c th s dng cc tn s" b pha inh do ma. Mt s" phng php bo v thng thng l phn tp theo tn s" v khng gian trong cc t hp khc nhau ca chng cng vi cc h thng d phng nng v phn tp phn cc. Trong chng 8 s trnh by k thut ny, ng thi trnh by chi tit hn v pha inh. iu ni rng i vi cc gin on gy ra do cc s c thit b, nu tp cc ch tiu kh dng i vi h thng khng c h thng bo v th cn phi c mt d no .

7.8.1. d n

Dng d n gin nht c th s dng l dng s dng h thng d phng hot ng song song vi h thng ang lm vic. H thng d phng ny a vo lm vic bt c lc no trn h thng chnh c s c. Nh vy mt s c" trn h thng chnh ch to nn s c ton b h thng nu h thng d phng cng khng lm vic c. Nu MTBF ca mt h thng chnh cho l T, th s" s c ca n s l 1/T. Nu h thng d phng cng ging nh h thng chnh, th n cng s c s" s c 1/T, nh vy xc sut mt trong hai knh khng lm trn nhim v trong mt n v thi gian t no bao gm MTTR l 2/(T + t). MTTR trong 1 n v thi gian c a vo tnh trng hp m mt trong hai h thng ny c th ang sa cha. iu ny cng c th ni rng, nu hai h thng ang cng hot ng ti mt thi im, th xc sut c hai h thng xy ra s c" ti cc thi im khc nhau (hoc cng) trong cng nm, c th ni l bng hai ln c s c ca h thng n. Cc s c ny s c ch khi xut hin s c" ton b h thng, l khi h thng d phng khng c kh nng lm vic v ang trong thi gian sa cha. Nu thi gian trung bnh

54________________________________________________________ Viba s - Tp 2

sa cha mt s c l t (tc l MTTR), th thi gian mt h thng khng c kh nng lm vic s l khi n ang c sa cha; o xc sut mi knh khng lm vic c ti mt thi im no l t s" ca MTTR trn tng thi gian (MTBF + MTTR), hoc t/(T + t), v nh vy tc s s c hu hiu ca cc h thng l xc sut s c ca mi mt h thng v h thng d phng ang sa cha.

Tc s c ca h thng i 1/T' = [2/(t + T) ].[t/(t + T)^

= 2t/(t + T)' (7-31)

Thi gian trung bnh ca h thng hu hiu gia cc s c" T' cho bi:

T' + t = (T + t f l2 t (7-32)

V kh dng c cho t cc phng trnh (7-29) v (7-32) l;

kh dng ca h thng = 100 X [((T + t f /2t) - t]/[(T + t/2t]%

= 100,x [(T + - 2t'/(T + t]% (7-33)

Phng trnh (7-33) s dng cho u cui v tuyn (MHSB) d phng nng c gim st bao gm hai my pht v hai my thu. Tuy nhin, ch c mt my pht l "lm vic". My pht kia cung cp mt ti gi qua n phn phi cng mt thng tin di c bn. Khi pht hin mt s c, chuyn mch RF t ng chuyn my pht d phng vo lm vic. Hai my thu ca u cu MHSB c ni n chuyn mch di c bn, n ct my thu trn ng c s c.

V d 1

Xc nh M TBF T' ca h thng v kh dng i vi mt h thng d phng nng trong mi mt h thng c MTBF l 10000 gi v gi thit rng M TTR l 12 gi nu mi mt h thng xy ra s c. Ngi ta gi thit rng mng o mch l c th thch hp v thi gian chuyn t h thng c s c sang h thng d phng hng 0.

Gii

S dng phng trnh (7-32) MTBF ca h thng hu hiu c tim thy l 4176661 gi (xp x 476 nm). H s'ci thin d l 4176661110000 = 417,66. S dng phng trinh (7-33) kh dng c xc nh l 99,99971%. Ch rng kh dng ca mt h thng khng c d l 99,88%.

to nn s phn tch tng th c th coi h thng l mt dy xch ca cc thnh phn to nn h thng . Nh vy c th tin hnh phn tch ni tip cc thnh phn ca nhiu ln s c, trong mi mt thnh phn ca h


thng c xc nh theo phng trnh (7-30). Sau , s dng phng trnh (7-29) xc nh kh dng.

Tuyn yu nh t trong mt dy chuyn vi h thng viba d phng nng l cc chuyn mch d phng nng (bng gc hoc RF). Nu cc chuyn mch ny khng lm vic th u im v'n c ca h thng d phng nng b mt.

7.9. CHUYN MCH BO V

7.9.1. Cc yu t nh hng n vic la chn tiu chun chuyn mch

iu cn thit l phi gim st lin tc h thng viba s' khi ng chuyn mch bo v khi c cc iu kin s c" knh. Tuy nhin vic chuyn mch ca b chuyn mch bo v trong h thng chuyn mch bo v nhiu ng khng phi lun c u im hoc thc t nu ch tn ti iu kin s c gin on tng lc, th n sinh ra t l li cao, hoc s mt khung vn cn dai dng trong vi |J.S. Tuy nhin mt iu cn thit l h thng phi c kh nng pht hin cc s gin on v bo cho ngi bo dng bit iu kin bt li , v iu ny c m t trong mc 7.7.4 v 7.7.5. Nu s dng chuyn mch bo v chng li s c" thit b, s nhn bit tiu chun chuyn mch tng i thp, th phi truyn cc ch th chuyn mch v chuyn mch thay i h thng. S chuyn i ny chm s gy ra s mt ng b u cu xa v c th cn d phng ti thiu ho s c" gin on do n gy ra. iu ny c ngha l thi gian chuyn mch b gii hn. Nu s dng chuyn mch nng cao c tnh cht lng do cc diu kin truyn lan xu, th tn ti mt tnh trng ngc li, v yu cu nhn bit tiu chun chuyn mch nhanh v s chuyn mch sang knh d phng khng mt ng b. S mt i hoc thm vo 1 bit sang knh u tin m khng c s sp xp trc i vi u cui xa m bo s trng khp, iu c th lm cho lung di ca chui truyn dn hon ton mt ng b. iu ny nh hng ti c tnh cht lng ca h thng v nh vy khng th coi l mt li c lp duy nht.

7.9.2. S b tr sp xp bo v

Loi thit b chuyn mch c s dng v c tnh ca n c xc nh bng dch v m n p ng, nh nng cao kh dng, tc chuyn mch nhanh..., im trong mch hoc trong tuyn truyn dn, b" tr chuyn mch v chuyn mch chun cng phng thc truyn dn ca c cu chuyn mch cng c m t trong mc 7.8; mc ch ca chuyn mch bo v l

56_________________________________________________________ Viba s - Tp 2

nng cao kh dng ca h thng bng cch chuyn sang knh d phng khi c hin tng s c" thit b. Ngoi ra cng c th t c li ch khc, l khi thit b bo v chng li s gin on thng tin do pha inh la chn tn s^ gy ra bng cch chuyn sang h thng khc trong h thng chuyn mch bo v nhiu ng. Cng c th s dng bo v phn tp khng gian nng cao c tnh ca h thng do iu kin truyn lan xu. iu ny c trnh by t m hn trong chng 8. nhng h thng c di chng tip pht tng ln, lm vic trong cc di tn m tiu hao do ma biu hin r nt th c th s dng cc bin php bo v phn tp tuyn p. Dng bo v ny gm vic s dng chuyn mch my thu u cul v v vy cho nn cn phi to s cn bng chnh lch v thi gian truyn dn ca cc ng truyn dn mi tuyn nhm cc tn hiu trn cc tuyn bt kp nhau ti thi im chuyn i. Cc h thng tuyn n c th c bo v bng cch s dng chuyn mch nhiu ng v chuyn mch n c knh bo v dnh ring hoc khng dnh ring, hoc bng hot ng phn tp khng gian, v l thuyt, hot ng chuyn mch c th hot ng : RF, IF hoc di c bn, nhng trong thc t, chuyn mch di c bn l thch hp, v n c th bo v ton b knh t u vo ti u ra vi vic gim mt na thit b ngoi ng truyn c chuyn mch.

BI TP

1. Trong m t m ng in thoi, mt t bin li c th ko di khong mt giy. Mt b o tc li kim tra knh 64 kbi t / s chng t rng t bin li ny to ra 7 li. Liu t bin li ny c ln gy ra m t nhiu cuc gi, nu n xu t hin i vi tt c cc knh 64 kb t / s trong m ng li'?

Tr li: Xem mc 7.2.4 ch ra rng xc sut c kh nng b qua ca s tn tht nhiu cuc m thoi to ra.

2. Hy xc nh ch tiu cc pht suy gim, ch tiu cc giy li, v ch tiu cc giy li trm trng i vi mt ng truyn dn s chun gi nh 1000 km s dng cc h thng viha.

Tr li: Cc p h t suy gim B E R khng vt qu 10 ^ trong hn 0,16% ca thng.

Cc giy li: BER s khng vt qu 10^ trong hn 0,128% ca thng bt k.

Cc giy li trm trng: t hn 0,054% ca khong mt giy l c BER xu hn 10'^ trong khong thi gian mt thng ht k.


3. Nu h thng xt trong bi tp 2 ghi cc giy trong BER ln hn 10' ^c th ly tnh ton xc nh ch tiu cc giy li c khng?

Tr li: Cc giy li trm trng cn phi c tch ra t gi tr ch tiu cc giy li.

4. Nu mt h thng xt trong bi tp 2 ghi BER ln hn 10' ^ trong khong thi gian lOs lin tip, cc giy li trm trng c th c chit khu t cc gi tr o cc giy li c khng?

Tr li: Vi iu kin ny, h thng c coi l khng c kh nng lm vic v khng c ba gi tr o hp l trong giai on ny.

5. Li pht biu: "t nht 99,944% ca cc khong mt giy l c khng li" thch hp vi ci gi?

Tr li: Tuyn truyn dn s 2500 km s dng cc h thng v tuyn chuyn tip s: ch tiu cc giy li.

58_________________________________________________________ Vi ba s - Tp 2

Chng 8: Truyn sng v cc knh pha dinh 59

CHNG 8

TRUYN SNG V CC KNH PHA INH

8.1. M U

Mt tham s^ quan trng nht nh gi cht lng ca h thng viba s" l thi gian ngng lm vic hoc xc sut xy ra li

p (Pg > BER) vt mt gi tr BER xc nh. Thi gian ngng lm vic c th c tnh ton bng cch xut pht t d tr pha inh, l hiu gia mc cng sut thu c RF v mc ngng ca my thu, vi BER cho trc.

T d tr pha inh c xc nh cn c vo cc s" liu ca h thng vi mi ng ta c th tnh c phn trm thi gian tiu chun cht lng v kh dng. Trong chng ny s a ra nhng thng tin v tnh ton cc tham s' pha inh do truyn lan nhiu tia, ma,... gip chng ta c th tnh c d tr pha inh. Cn c vo gi tr d tr pha inh tnh c, c th tnh ton thi gian ngng lm vic trung bnh, iu cho php tnh c phn trm v thi gian hot ng c hiu lc trung bnh cho tuyn hoc h thng. Qua y ta cng s xem xt cc bin php gim thi gian ngng lm vic. Sau thu c mt s th c th so snh vi cc vn cp chng 7 d quyt nh, nu cn phi thit k li tuyn hoc h thng tho mn cc vn . Nhng c s v truyn sng v tuyn bao gm nhng phng tr n h tng i quan trng s c biu th, trc ht ni rng nhng qu tr nh pha inh c th c nh gi bng mt ph cnh thch hp v t nn tng cho k thut tuyn viba chuyn tip.

8.2. C S V SNG V TUYN

C hai loi sng thng thy trong thc t l sng dc v sng ngang. Sng dc l sng truyn lan theo phng chuyn ng ca n. Mt v d n gin ca loi truyn sng ny l sng m thanh truyn trong khng kh. Cn pht x in t l cc sng ngang, tc l sng truyn lan theo hng vung gc vi phng chuyn ng ca n. S chuyn ng ca mt con rn cng tng

t nh vy, trong s chuyn ng c th ca n hu nh vung gc vi hng i. Mt sng in t trong khng gian ba chiu gm 2 thnh phn. l thnh phn in E v thnh phn t B hoc H (B = |aH). Cc thnh phn ny trc giao vi nhau. Sng v tuyn b phn cc trong hng thnh phn in ca n. B pht x lng cc c s dng c lp trong mt anten a hoc c s dng trong cc loi anten khc, bn thn xp thng hng vi trng E v nh vy biu th hng phn cc. Cn dipole nh hng thng ng biu th phn cc ng, tuy vy mt ng dn sng ch nht, trng E vung gc vi cnh di tit din ngang ca n. Cc sng v tuyn c th c truyn lan theo cc phng thc khc nhau trn mt t. Sng c th truyn t anten pht n anten thu bng 2 ng chnh: tng in ly (sng tri) hoc i st mt t (sng t). Sng t bn thn c phn thnh hai loi: sng b mt v sng khng gian. i vi sng khng gian c 3 ng i qua khong cch gia anten pht v anten thu. l sng trc tip, sng phn x t t v sng phn x t tng i lu.

8.2.1. Cc loi sng t

Sng t l sng khng b thng ging bi tng in ly, tc l sng b mt v sng khng gian.

8.2.1.1. Sng b m t

Khi truyn sng b mt, nng lng sng lt trn b m t t gn ging nh sng i dc theo ng dy. Phng php truyn sng ny ch s dng i vi cc tn s^ v tuyn tng i thp, di 30 MHz. Khi tng tn s th tiu hao o t tng nhanh, v tiu hao ph thuc vo hng s' in dn v in mi hiu dng ca t. Trn 30 MHz, t tc dng nh mt dy dn km v tiu hao l qu cao i vi s truyn lan sng VHP, HF, v SHF.

8.2.1.2 Sng khng g ian

Phng thc truyn sng ny hu nh c s dng cho t t c cc h thng c kho st trong quyn sch ny. Sng truyn trong tng l lu lan rng n 10 dm trn b mt t. Nng lng sng truyn t anten pht n anten thu theo mt ng thng hoc phn x mt t hoc phn x t tng l lu. Sng khng gian l mt loi sng quan trng trong thng tin VHP, UHF, v SHF.

60 Viba s - Tp 2

SNG TRC TIP

Sng ny pht x trc tip t anten pht n anten thu m khng b phn x u c. Trong cc iu kin truyn lan bnh thng, n c bin ln hn so vi mt sng bt k no n my thu.

SNG PHN X T

Sng ny n anten thu sau khi phn x mt vi ln t mt t hoc t cc vt th xung quanh. S phn x khng nhng ch xut hin trn mt phng ng, m c th xut hin trn mt phng ngang, nh vy sng b phn x t mt vt cn s lch so vi ng chnh. Sng phn x s c bin v pha khc vi bin v pha ca sng trc tip, nu khong cch truyn ln hn mt s' l bc sng th anten thu sng phn x lch pha vi sng trc tip mt gc 180 v kt qu l trit tiu tn hiu sng ti n mt mc no . Mc ph thuc vo bin ca sng phn x.

SNG PHN X TNG l Lu

Do thav i ch s" khc x ca khng kh theo cao so vi mt t, nn sng c th b tp m x, v tu theo gc sng ti c th xy ra phn x ton phn t tng i lu. Trong trng hp ny xut hin mt bin gii c tc dng ging nh mt

viba số (tập 2)

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