LI CM NTrong qu trnh thc hin lm ti em gp rt nhiu kh khn. Tuy nhin em nhn c rt nhiu s gip ca mi ngi xung quanh. Chnh v vy m em c th hon thnh tt ti thc tp tt nghip. u tin em xin gi li cm n n c gio Trnh Th Dip, ngi trc tip hng dn em trong qu trnh lm ti. Trong thi gian va qua c lun tn tnh hng dn em vi tt c nhng g c th. Mt ln na em chn thnh cm n s gip ca c. Trong sut thi gian hc tp ti trng i hc Cng ngh thng tin v Truyn thng, c c nh ngy hm nay em lun ghi nh s tn tnh truyn th kin thc ca cc thy c, cng nh s ng vin gip ca bn b. V c bit em xin gi li cm n n cc thy, c trong B mn Cng ngh Truyn thng - i hc Cng ngh thng tin v Truyn thng trang b kin thc v to iu kin ht sc thun li em c th hon thnh ti thc tp ny mt cch tt nht.

MC LCLI NI U4CHNG 1: CC NH NGHA V JITTER V WANDER51.1nh ngha jitter v wander51.2Cc n v o jitter61.3Cc n v o wander81.3.1Sai s khong thi gian (Time Interval Error -TIE)91.3.2Sai s khong thi gian ln nht - MTIE101.3.3 lch thi gian (Time Deviation TDEV)13CHNG 2: CC NGUN CA JITTER V WANDER172.1 Jitter do cc phn t nh thi gy ra172.2 Wander do s thay i thi gian tr truyn dn.182.3 Jitter do giao thoa gia cc k t.182.4 Jitter do hiu chnh (justification).182.5 Jitter do hot ng ca b ti to xung ng h.242.6 Cc ngun ca jitter v wander trong mng SDH262.6.1 M hnh mng truyn dn bng rng.262.6.2 Cc ngun ca jitter trong mng hn hp SDH / PDH.272.6.3 Cc ngun ca wander352.6.4 Wander do kiu ghp knh ng b36CHNG 3: S TCH LY CA JITTER V WANDER39 3.1 Jitter trn mt trm lp ring l.393.2 S tch ly jitter trong mng gm cc trm lp ng nht ni tip403.2.1 Cc mu lp li423.2.2 Cc mu ngu nhin443.2.3 Jitter ng chnh45 3.3 Dung sai jitter trong mt trm lp (jitter ng chnh)453.4 Dung sai jitter trong b ghp/tch knh PDH48CHNG 4: NH HNG CA JITTER V WANDER494.1 Lm suy gim cht lng truyn dn v dch v494.1.1 Jitter lm tng xc xut li a vo trong tn hiu s494.1.2 Jitter a cc trt khng iu khin c vo tn hiu s534.1.3 Jitter lm suy gim cht lng tn hiu tng t c m ha544.2 nh hng ca jitter v wander trong mng SDH55CHNG 5: CC GII HN CA JITTER V WANDER58 CC GIAO DIN MNG585.1 Nguyn tc iu khin Jitter v Wander585.2 Cc tham s ca Jitter585.2.1Jitter Ng Ra ( Output jitter ) .595.2.2Dung sai jitter ng vo (Input jitter tolerance ).85.2.3Hm truyn t Jitter ( Jitter Transfer function - JTF ).95.3Cc tham s ca Wander115.3.1Wander ng ra (Output wander)115.3.2Dung sai wander ng vo (Input wander tolerance ):625.3.3Truyn t nhiu wander ( Wander noise transfer) :625.3.4p tuyn chuyn tip pha ( Phase transient response )625.3.5Cht lng Kim Gi ( Holdover - Performance ) :63 5.4 Cc khuyn ngh ca ITU v ETSI v Jitter v Wander635.5Gii hn ca Jitter & Wander cho cc giao din mng64 5.6 G.823, G.825: Gii hn Jitter ti phn cp s.66 5.7 ETS 300 462 - 3 : gii hn mng cho Wander665.7.1.Gii hn Jitter ng ra - Giao din ng b665.7.2Gii Hn Mng Cho Wander Ng Ra - MTIE v TDEV67KT LUN V HNG PHT TRIN73Kt lun73Hng pht trin ti73BNG CC CH VIT TT74TI LIU THAM KHO75

LI NI U nc ta hin nay, hu ht cc mng vin thng lin tnh, quc gia u s dng k thut truyn dn tn hiu thay v cc h thng tng t gi ci. Mt phn v d dng trong chuyn mch v nh tuyn; nh cng ngh vi mch tch hp qui m ln i vi thng tin, cc thit b x l r hn dn n vic h gi thnh. Mt phn do tnh u vit ca cng ngh truyn dn qua v tuyn, cp si quang c kh nng gim tp m trn mt an ng di nhiu chng nh nhng trm lp.Mt l do khc na i vi xu hng tin n truyn dn tn hiu l s tin b ang c thc hin trong vic t ng ha vn phng, nhu cu ny sinh v tng ln i vi truyn liu tc cao m thng bao gm c m thanh, hnh nh ng (video) v d liu. Hn na yu cu t c s tin b ngang bng v vin thng i vi cc nc trong khu vc cng l mt l do.Tuy nhin bn thn ca mng vin thng s cng c nhng vn c th nh hng xu n cht lng dch v, i hi phi nhanh chng khc phc. Mt trong nhng vn ny l tc nhn jitter v wander. Jitter v wander c th xut hin do nhiu nguyn nhn khc nhau nh iu kin ca mi trng truyn dn, qu trnh x l tn hiu ca h thng v cht lng ca trang thit b.Trong nhng nm gn y, cng ngh SDH c nhanh chng thay th dn cng ngh PDH trn mng vin thng lin tnh Vit Nam em li nhng tin b to ln trong vic nng cao cht lng truyn dn v dch v. Vic kt hp, vn hnh, khai thc mng SDH v PDH to ra nhng loi jitter v wander mi nh hng n cht lng dch v ca ton mng. V th nh hng ca rung pha v tri pha trong mng vin thng s l mt ti khoa hc cng ngh c tnh thi s ph hp vi yu cu thc tin.

CHNG 1: CC NH NGHA V JITTER V WANDER1.1 nh ngha jitter v wanderJitter v wander c nh ngha l s bin thin nhanh v chm ca cc thi im c ngha ca tn hiu s so vi cc thi im quy nh (thi im c ngha) ca chng trong h thng.Cc thi im c ngha c th ly im gia hoc im bt k c nh no c nhn bit mt cch r rng trn mi xung. Hnh 1.1 minh ha nh ngha ny, khi jitter xut hin thi im chuyn mc ca mt xung s sm hn hoc tr hn so vi tn hiu chun, cc sai lch ny l ngu nhin v lin tc.Tm li, jitter l bin ng chp chn ca tn hiu chung quanh cc thi im ng theo quy nh, bin ng ny dng thi gian hay pha

Hnh 1.1 Tn hiu c jitter v ng h chunS bin thin ca cc thi im c ngha nh hng n tn hiu ging nh trong trng hp tn hiu nh thi gc c s dng to ra dng bit iu pha vi mt tn hiu iu ch l dng sng jitter.Dng sng jitter c th c biu din bng 1 hm lin tc theo thi gian v c lp vi tn hiu b nh hng. Hnh 1.2 minh ha nh hng ca jitter vo mt tn hiu nh thi gian.

Hnh 1.2 Hm jitter ca mt tn hiu 1.2 Cc n v o jitterJitter c c trng bi bin v tn s. Bin jitter l gi tr nh - nh ca hm jitter theo thi gian cn tn s l s ln dao ng quanh thi im c trng trong mt giy.Bin ca jitter c o bng khong n v - UI (interval unit). Khong n v UI c th hiu n gin l rng ca mt bit (hnh 1.3). Do xung ng h ca tn hiu NRZ v RZ l nh nhau nn UI s khng ph thuc vo dng tn hiu. Ngoi ra, UI cn khng ph thuc vo tc bit v n c ly chun da trn chu k ca mt xung ng h chun.Nhng thun li ny lm cho UI tr thnh mt tham danh nh, cho php so snh bin jitter o cc tc bit khc nhau ca mt phn cp

Hnh 1.3 Khi nim khong n v

Khi tn jitter nh hn 10 Hz th s rung pha ny c gi l wander (bin ng chp chn chm) (hnh 1.4)

Hnh 1.4 Cc thnh phn jitter v wander ca s bin ng pha1.3 Cc n v o wanderVic o wander i hi phi c mt tn hiu ng h chun khng c wander. Wander ca mt tn hiu s o c so vi tn hiu tham chiu ny. Tn hiu chun c th l ng h tham chiu gc (primay reference clock - PRC). Mt PRC thng c lm trn ng h c s cesium ( chnh xc ca loi ng h ny khong 10-14). Tuy nhin PRC cn c th thc hin trn cng ngh V tinh nh v ton cu (Global Positioning Satellite - GPS), cc h thng ny s dng b dao ng ni rubidium hay quartz c iu khin bng thng tin ng b thu t GPS, cc ng h ny c kh nng duy tr chnh xc trong khong t vi 10-13 n vi 10-12.Wander cha cc thnh phn tn thp vi cc chu k di, do php o wander c th ko di nhiu gi c nhng thng tin pha cn thit. Tuy nhin cc qu trnh chuyn tip pha ht sc quan trng nn cng cn c nhng phn tch nhanh tm thi. V vy ba tham sau y thng c s dng nh gi chnh xc cht lng ng b.Sai s khong thi gian (time interval error -TIE)Sai s khong thi gian ln nht (maximum time interval error - MTIE) lch thi gian (time deviation -TDEV)Cc tham s ny v cc tham s khc c nh ngha trong khuyn ngh G.810.

1.3.1 Sai s khong thi gian (Time Interval Error -TIE)Sai thi gian hay sai khong thi gian, sai s ny tch ly khi mt ngun tn c s dng nh mt ng h. Tn s ti thi im bt k t l:

Vi: f0 tn s thi im ban u (t=0)

Pha tch ly theo thi gian ng T l:

ta c:

Sai khong thi gian (TIE) c nh ngha l lch pha gia tn hiu c o vi tn hiu ng h tham chiu (chun). TIE thng c tnh theo ns bng cch chia lch pha ny vi tn danh nh ca ng h chun (1.3.4). thun tin chn TIE = 0 ti thi im bt u ca chu k o T. Do TEE hin th s thay i pha ngay t khi php o bt u. Php o TIE l nn tng cho cc php tnh khc (MTIE, TDEV).Hnh 1.5 l mt v d v TIE, trong v d ny do s lch tn nn TIE c xu hng tng (vo khong 1ns trn ls).

Hnh 1.5 V d v o TIE1.3.2 Sai s khong thi gian ln nht - MTIEMTIE l mt tiu chun nh gi wander. Tiu chun ny c trng cho s lch tn v cc qu trnh chuyn tip pha. MTIE l mt hm ca khong thi gian quan st T.

Hnh 1.6 nh ngha MTIEMTIE () l gi tr TIE nh - nh ln nht (ngha l wander) trong bt k khong thi gian quan st c di trong chu k o T. tnh MTIE ti khong thi gian quan st nht nh t vic o TIE, cc ca s c di c dch chuyn qua khong thi gian o TIE v lu li gi tr nh. Gi tr nh l gi tr MTIE () trn khong thi gian ang quan st. Qu trnh ny c lp li cho mi ca s trn ton b thi gian o . MTJE () l gi tr ln nht ca cc gi tr ghi c.Php o trong trng hp n gin nht (s pht hin gi tr tc thi), im bt u cho mt khong l ph hp v khong c gia tng bng cch tng thi gian o. iu ny cho php lin h vi s lch tn s c pht hin v tnh xp x bng cch chia gi tr MTIE() vi () do :

V d:

Kt qu chnh xc hn nhn c nh dng mt thut ton y c ch ra trong ETSI 300 426. y, khong thi gian quan st chy qua ton b khong thi gian o T v lch cc i c ghi li. Hnh 1.7 minh ha xc nh gi tr MTIE Hnh 1.7 Php tnh gi tr MTIEThut ton MTIE n gin ha Quan st tt c cc khong 1s. Xc nh lch cc i trong mi khong s xem gi tr MTIE (1s). Nhp gi tr cao nht i vi 1s trong th MTIE. Quan st tt c cc khong 2s. Xc nh lch cc i trong mi khong 2s xem (gi tr MTIE (2s). Nhp gi tr cao nht i vi 2s trong th MTE. Lp li cho cc khong thi gian khc (3, 5, 8 ....)Php tnh MTIE l thch hp cho vic pht hin s dch tn s nhng khng cho bit bt k thng tin no v ph ca tn hiu li.Trn v d hnh 1.5 ca s thi gian quan st c di l 20s, gi tr TIE nh - nh (PPTIE) ti mt ca s l 4ns. Tuy nhin nu ca s ny trt qua ton b chu k o T th gi tr PPTIE ln nht l 11ns (ti thi im vo khong 30s trong chu k o). Do MTIE (20s) = 11ns.Hnh 1.8 l th MTIE () tng ng vi th TIE trn hnh 1.5. Qu trnh chuyn tip c dc 8ns ti = 30s th hin gi tr MTIE () = 8ns vi rt nh. Lu rng th MTIE tng n iu v qu trnh chuyn tip ln nht nh du cc s kin c bin nh hn.

Hnh 1.8 V d v o MTIE (tng ng hnh 1.3a).1.3.3 lch thi gian (Time Deviation TDEV) TDEV l mt tiu chun nh gi wander. Tiu chun ny c trng ph ca wander. TDEV cng l mt hm ca khong thi gian quan st k.C th nh ngha TDEV nh sau (hnh 1.9):

Hnh 1.9 nh ngha TDEVTDEV l gi tr rms ca TIE c lc bng b lc di thng(BPF) c tn s xung quanh tn s 0.42 / r .B lc di thng trong nh ngha trn c cc p ng xung h(t) v p ng tn H(f) nh hnh 1.10

Hnh 1.10 p ng xung v p ng tn ca b lc di thngGi tr TDEV l mt o ca s bin thin li pha theo thi gian trung binh ni cch khc l mt gi tr thng k. N c tnh t cc mu TIE. n gin, lch chun (i) c tnh cho mi im i trong thi gian o T ng vi khong i m n chy qua ton b thi gian o . Ly trung bnh cc gi tr tnh c trong ton b thi gian o th nhn c gi tr TDEV cho khong 1. K tng gi tr ca v lp li th tc tnh cho gi tr mi ca .Tng phn vi php tnh MTIE, phn tch TDEV cung cp thng tin v ph ca s bin thin pha (wander).1

Hnh 1.11 Xc nh gi tr TDEVHnh 1.12 trnh by hai th ca TDEV (). th th nht tng ng vi vic o TIE trn hnh 1.5, TDEV trong trng hp ny tng vi bi v trong chu k o ngn T = 100s hai qu trnh chuyn tip trn hnh 1.5 chim u thNu thi gian o TIE ko di n T = 250s v khng cn cc qu trnh chuyn tip no khc th tc ng ca hai qu trnh chuyn ln TDEV s t hn. Kt qu l th th hai c nh du bi T = 250s.Lu l TDEV khng nhy i vi dc pha khng i ( lch tn ). tnh TDEV theo mt gi tr cho trc th chu k o T phi t nht bng 3. o chnh xc TDEV, chu k o tng cng phi t nht bng 12, iu kin ny cn thit bi v khi tnh rms cn c thi gian t c gi tr trung bnh thng k tt.TDEV (ns)

Khong thi gian quan st T (s)Hnh 1.12 th ca TDEV tng ng vi TIE trong hnh 1.5

CHNG 2: CC NGUN CA JITTER V WANDERJitter (bin ng chp chn nhanh) c th xut hin t cc nguyn nhn khc nhau trong mt h thng truyn dn s, tuy vy ch yu cc nguyn nhn ny c th phn thnh cc loi sau:2.1 Jitter do cc phn t nh thi gy raJitter ny c bin v tn s tng i thp tc l wander, n xut hin do cc nguyn nhn sau: Tp m pha trong cc mch dao ng lm ng h trong ton h thng cng nh trong cc mch logic do s khng hon thin ca cc linh kin mch. nh hng ca nhit ln cc b ao ng ni dng thch anh lm thay i tn s ca n. S khng n nh do nhit thng rt ln, do cc b cng hng thng phi t trong cc b tn nhit m bo chnh xc ca nhit bn trong l 0. lc khi nhit phng my thay i t 0 ti 60c S bin i chm trong cc ngun cung cp, s bin i in p offset nh hng n mc ngng kch thch trong cc mch logic.Hnh 2.1 minh ha thnh phn jitter c to ra do s khng n nh ca vng chuyn tip trong mch logic. Thnh phn jitter ny t c nh hng ln cht lng truyn dn.

Hnh 2.1 Jitter do s khng n nh ca vng chuyn tip trong mch logic2.2 Wander do s thay i thi gian tr truyn dn.Bt c s thay i no trong thi gian tr ca tn hiu trn ng truyn dn u a n s bin ng v pha ca tn hiu tng ng. Thng th s thay i ny l do s bin i chm v nhit ni t cp gy ra. Jitter trong trng hp ny c tn thp v thng c coi l wander.2.3 Jitter do giao thoa gia cc k t.Knh truyn dn c c tnh bin v pha khng l tng. V th, chui tn hiu khi truyn qua knh truyn dn s b mt tnh trc giao. ng ra knh truyn dn, cc k t s b ko di v chng ln ln nhau to ra hin tng giao thoa gia cc tn hiu. Do , thi im ly mu ti u ti to li tn hiu ban u cng b lch i gy ra jitter (hnh 2.3). Jitetr ny gi l jitter mu ph thuc.

Hnh 2.2 Jitter do giao thoa gia cc k t2.4 Jitter do hiu chnh (justification).Trong k thut ghp knh cn ng b PDH, ng h ca cc tn hiu lung chy t do (free-running). iu ny c ngha l c s khc bit gia ng h ca cc tn hiu lung v ng h ghp knh so vi ng h danh nh. m bo s ng b trong qu trnh tch/ghp knh, cc b ghp knh s dng k thut thch nghi ng h nh sau: b ghp knh lun c ng h ln hn mt cht so vi cc ng h ca cc lung s v s khc bit gia hai ng h s c khc phc bng hiu chnh.Hiu chnh l mt qu trnh trong cc bit c u n chn vo cc lung bit n nng tc bit ln n tc m thit b ghp knh yu cu. u cui xa nh c tn hiu iu khin hiu chnh, cc bit c chn thm vo trc b ly ra khi phc tn hiu thng tin tr li gi tr danh nh ca n. Vic loi b bit hiu chnh u cui lm cho tn hiu c nhng khong trng (khe thi gian). Tn hiu bt thng ny ngoi vic i vo b lu tr n hi cn c a n u vo mch PLL (phase locked loop), mch ny tch c tn hiu nh thi t tc trang bnh ca tn hiu bt thng. Tn hiu nh thi tng hp c s dng c tn hiu n ra khi b nh n hi (n hp thu s bt thng ny). Cc qu trnh ny c minh ha trong cc hnh 2.3, 2.4, 2.5.B ghp knh pha pht c b nh m n hi (elastic store) bao gm cc t bo nh mt bit m c th c ghi vo, c ra mt cch c lp vi nhau (tc l ti mt thi im c th va ghi vo mt t bo, va c ra t mt t bo khc). Tn hiu i n c ghi vo cc t bo 1 . . .8 mt cch tun hon nh ng h c khi phc t tn hiu n vng kha pha TR. Cc bt nh c c ra nh xung nhp ca ng h ca b ghp knh c tn s cao hn nn a ch c ra tin dn n a ch ghi vo. S khc nhau gia a ch ghi vo v a ch c ra c gim st bi b so snh a ch (AC). Khi m s sai lch gia a ch c ra vi a ch ghi vo nh hn ba t bo b nh, b iu khin chn (logic) s kim ch c v qu trnh chn bt u: Nu v tr bit diu khin chn (justification service bit) trong khung xut hin, bit s c gi tr 1. Khi v tr bit iu khin chn xut hin th a ch c s c gi i mt chu k ng h v t bo nh hin ti s c c thm mt ln na, y l bit chn v bit ny s b loi ra u thu. Bi v a ch c c gi li 1 chu k ng h nn s khc nhau gia a ch c v a ch ghi tng ln.Chu trnh trn s c lp i lp li v th tn ng h ca cc lung cn ng b s tng thch vi tn ca ng h b ghp knh.Khong thi gian gia thi im bt u qu trnh chn v thi im thc hin c th ko di vi chu k khung. Trong thi gian ny a ch c cng tin gn a ch ghi hn na. V th qu trnh chn cn c khi ng ngay khi khong cch gia hai a ch nh hn 3 trnh tnh trng cn b nh.Do bit chn khng th a vo lp tc sau khi t ti ngng chn v ch c a vo v tr dnh cho n trong khung k tip. Nh vy c mt thnh phn jitter thi gian ch i (waiting time jitter) xut hin. Hnh 2.6 minh ha thnh phn ny. u thu cng c b nh n hi. Tn hiu iu khin chn s ngn cn vic ghi cc bit ti v tr chn hay iu khin chn vo b nh Vic xa chn gy ra gin on trong tn hiu ng h thu - tn hiu ng b khong h (gap timing) ti cc v tr chn. Tn hiu ng b khong h c a n ng vo mch PLL, mch ny tch c tn hiu nh thi (ng b lin tc) t tc trung bnh ca tn hiu ng b khong h. Tn hiu nh thi c s dng c tn hiu ra khi b nh n hi. Nh vy tn hiu c c ra vi s ng b lin tc, c ngha l tn hiu xut hin ng ra mt cch u n. lm c iu ny s t bin v pha ng vo b so pha (phase comparator) c bin i thnh s dch chuyn in p. S dch chuyn ny c lm phng bi b lc thng thp. Tn hiu ng ra ca b lc thng thp c dng iu khin b dao ng c iu khin bng in p (VCO) to ra tn hiu ng h lin tc t tc trung bnh ca tn hiu ng b khong h.Do tnh khng l tng ca b lc thng thp, s dch chuyn in p khng bao gi c lm phng mt cch hon ho. l l do khin tn s ca tn hiu nh thi c lm phng bin thin theo tn hiu ng ra ca b so pha, tc l jitter c pht sinh, jitter ny gi l jitter do chn (pulse stuffing jitter).Hnh 2.3 trnh by nguyn l ca mt b m do (elastic store) pha ghp knh.

Hnh 2.3 Nguyn l ca b m do (pha ghp knh) D1 d liu vo khng ng b vi ng h h thng TR: b ti to ng bAC: b so snh a ch TR: b ti to ng bJS: lng bit phc v chn AC: b so snh a chGAP: khe ng b cho th tc chn

Hnh 2.4 S khi ca m b m do (pha ghp knh)TR: B to ng b t tn hiu vo CTR: B m to a ch c Ghi ES: B m doAC: B so snh a ch JS: B lng bit phc v chn LOG: Mch logic - iu khin vic lng cc bit phc v chn, tch ra mt chu k ng h ty thuc vo tn hiu ra ca bTG: To cc tn hiu ng b cho pha ghp knh D1: Cc lung vo ta ng b D2: Tn hiu c x l chn, ng b vi ng h h thng T1: Xung ng h khi phc t lung vo dng ghi T2: Xung ng h c, c ly t ng h trung tm

So snh ca ia ch AC

Hnh 2.5 Nguyn l ca mt b m do (pha phn knh demultiplex side)ES: b m doCTR: b mVCO: b dao ng iu khin bng in pPHI / U: b so phaNi tm la jitter do hiu chnh gm hai thnh phn: Jitter theo thi gian ch i Jitter do chn

Hnh 2.6 Jitter do thi gian ch i vdo chn.Vng kha pha (PLL) pha phn knh c c tnh lc thng thp vi tn s ct 3dB (fc) danh nh l tn chn tht s.

p ng tn v p ng xung ca PLL l:

V thnh phn jitter thi gian ch i c tn thp hn fc nn lt vo bng thng ca PLL v xut hin ng ra (khng b suy gim) cn thnh phn jitter do chn c tn cao nn b suy gim ng k. Do , ng vo PLL ch cn ti thnh phn jitter do chn.

Hnh 2.7 nh hng ca vng lm phng ln jitter

2.5 Jitter do hot ng ca b ti to xung ng h.Cc thit b truyn dn cn ng b hin nay u s dng nguyn l t nh thi, tc l ng h cn thit cho cc mc ch ng b c khi phc t lung tn hiu thu c. Hnh 2.7 minh ha s khi phc xung ng h

Hnh 2.8 S khi ca mt b tch xung nh thiBi v m c s dng trn ng truyn dn khng cha cc thnh phn ri rc trong ph ca n cho nn thu c tn hiu tdn nh thi cn phi bin di phi tuyn tn hiu ng vo. V vy b to xung nh thi cha cc b nn v b hn ch pha di ca ph n hiu. ng ra ca b hn ch s cha thnh phn ri rc ca tn nh thi. Sau tn hiu ny c a n b lc bng thng. Tn hiu ra s l tn hiu cn hi vi tn xung nh thi. Tn hiu ny, qua b khuch i, i vo b to xung nh thi. B to xung ny s to ra dy xung nh thi, ng pha vi nhng tn hiu c ti to, ti nhng thi im m sng cn hi i qua zero.B lc bng thng c th l b lc thng hp c iu chnh tn nh thi hoc l khung cng hng vi tn ring bng tn nh thi. Ph ca tn hiu bao gm 2 thnh phn: phn ri rc cn thit cho vic to xung nh thi v phn lin lc mang thng tin nhng li l tp m cho b to xung. Bi v h phm cht Q ca b dao ng l gii hn nn ph ca tn hiu ra s bao gm vch ph ti tn nh thi v nhng on ph lin tc i lin vi n. Nhng on ph ny bao gm cc thnh phn ca tn hiu truyn i v cc thnh phn ngu nhin xut hin do tp m cng nh do nhiu xuyn knh. Hn na nhng thnh phn nhiu ny hon ton khng lin h vi cc b ti sinh trn ng truyn. S rung pha (jitter) do cc thnh phn ny sinh ra do cc trm lp trn ng dy l hon ton c lp v c xu hng tch ly theo bin (c th cng theo bin ). B to xung nh thi to xung ti nhng thi im m sng cn hi vi tn nh thi i qua zero. Trong trng hp l tng, nu ngng bt u lm vic ca b to xung nh thi ng bng zero th cc xung nh thi s c hnh thnh ng vo thi im i qua zero ca sng cn hi. Trn thc t b to xung nh thi c mt vng khng cm ng i vi s thay i ca tn hiu vo (hnh2.5c). Hu qu l cc xung nh thi c th c to ra vo cc thi im ngu nhin trong gii hn ca nhng vng khng cm ng ni trn. ln ca nhng vng ny (hu qu l bin jitter) ph thuc vo bin ca tn hiu. Nh vy s thng ging bin ca sng cn hi (hu qu s giao thoa gia cc tn hiu) bin i thnh s thng ging pha (jitter) ca cc xung nh thi.S giao thoa gia cc tn hiu cn a ti s thng ging v rng v v tr thi gian ca cc xung ng vo ca b khung dao ng. S thng ging ny s lm bin ng thi im chuyn qua zero ca cc tn hiu cn hi a n jitter ca cc xung nh thi tng ng. y mt ln na cn thy r rng do s iu hng khng chnh xc ca khung dao ng (gy ra bi cc nguyn nhn nu ra phn 2.1) tn ring ca khung s khc vi tn nh thi. iu ny dn ti s khng n nh ca nhng thi im i qua zero ca tn hiu cn hi v gy ra jitter.Hot ng ca b to xung nh thi ch c hiu qu khi mt ca chui tn hiu vo tng cao, tc l chui tn hiu vo cha nhiu xung kch thch cho khung cng hng. Trong trng hp tn hiu vo cha cc chui 0 (zero) di th khung cng hng s khng c kch thch mt cch hiu qu. S tt dn ca khung dao ng b ko di.Pha ca tn hiu nh thi v th s b bin ng. S rung pha ny c gi l "jitter mu ph thuc

Hnh 2.9 Sng cn hi v cc thi im hnh thnh xung nh thi.2.6 Cc ngun ca jitter v wander trong mng SDH2.6.1 M hnh mng truyn dn bng rng. Hnh 2.10 l m hnh ca mt mng PDH / SDH n gin. Tn hiu PDHc truyn qua tuyn SDH c th c ghp vi cc tn hiu PDH khc, kt

Hnh 2.10 Mng bng rng chunni cho vi cc ti SDH v ti to li. Mng ny c ng b t mt mng ng b ring bit mc d tn hiu ng b ch thc s truyn trn phn mng SDH.

2.6.2 Cc ngun ca jitter trong mng hn hp SDH / PDH.Mc d jitter l hin tng ring l, tuy nhin trong mng hn hp SDH / PDH cc nguyn nhn to ra, truyn t v bin i jitter rt khc nhau. Trn hnh 2.10 c th thy rng ngoi cc loi jitter tng t nh trong mng PDH nh jitter ni ti ca tn hiu PDH (ti ng vo mng PDH ca tn hiu PDH, gi l jitter ng PDH), jitter do hot ng ca cc trm lp (cn gi l jitter ng SDH)... cn c hai loi jitter mi, c nh hng rt mnh ln vic truyn tn hiu PDH qua mng SDH. l jitter nh x (mapping jitter) v jitter con tr (pointer jitter). iu ny i hi phi c cc k thut o v cc thit b o mi. Cc jitter ny c c tnh hon ton khc vi cc c tnh jitter trong qu trnh ng b tn hiu PDH. iu ny gy ra nhng vn mi trong vic truyn ti cc dch v qua mng SDH.2.6.2.1 Jitter nh X (mapping jiter). truyn cc tn hiu cn ng b trong khung SDH, trc tin cn phi nh x chng vo cc container (b cha) xem hnh 2.11. K thut chn c s dng cn bng tc gia cc lung s PDH v cc container tng ng. trm u cui xa, cc container c gii nh x tr li thnh tn hiu PDH bng cch xa chn.Do vic chn v xa chn cng nh vic lm phng pha trong qu trnh nh x / gii nh x m cc thnh phn jitter do chn v jitter thi gian ch i c sinh ra. Trong trng hp ny jitter c gi l jitter nh x/ gii nh x. Jitter ny tng ng vi jitter chn v jitter thi gian ch i trong mng PDHJitter nh x c bin thp v tn tng i cao. Bin jitter nh x thay i khi tc lung PDH dch chuyn so vi VC-N v t c gi tr nh ti dch chuyn nh so vi 0ppm.2.6.2.2 Jitter Con Tr (pointer jitter)Trong k thut truyn dn SDH, s khc nhau v ng h ca hai mng hay hai phn t ca mng ( im kt ni cho hay trm ghp knh u cui) c b tr bng cch hiu chnh con tr trong khung tn hiu ng b, tc l thay i v tr thi gian ca cc container ti thng tin trong khung tn hiu c truyn i. iu ny c thc hin bng cch tng hoc gim gi tr ca con i mt n v. Cc container o v th s dch i mt byte (trong SONET) hay ba byte (trong SDH), tng ng vi pha tn hiu b dch i 8 hay 24 UI.Do vic hiu chnh con tr ch c th thc hin t nht sau 3 khung tn hiu, u thu, tn hiu ng h thu cng phi c lm phng ging nh trong qu trnh chn bit. Cc thnh phn jitter tng t (jitter do chn v jitter thi gian ch i) c to ra. Trong trng hp ny cc bin ng pha c tn cao c loi tr, jitter cn li ch cha cc thnh phn tn s thp c bin cao.Nh vy, trong k thut truyn dn ng b, lung tn hiu c khi phc pha thu lun lun cha hai loi jitter l jitter nh x v jitter con tr. Hai thnh phn ny lun lun kt hp vi nhau thnh jitter kt hp.Ti cc im kt n cho trong mng SDH, s lch v tn /pha gia cc ti n v i c hiu chnh bng k thut con tr. Trong iu kin bnh tnng khong thi gian gia cc ln hiu chnh con tr c th t 30 giy n vi gi. Nu ng h ca phn t mng b mt ng b v chuyn qua trng thi hot ng t do vi lch tn theo qui nh l 4.6 ppm th mi ln hiu chnh con tr s xy ra sau 34ms. Tuy nhin trong cc mng thc t, cc chui 3 -7 ln hiu chnh con tr trong vi chc ms c th xy ra khi cu hnh li tn hiu ng b tham chiu.

Hnh 2.11 V d v chn dngKhi ghp tn hiu PDH vo VC-4 phi m bo c x dch tn sVpha cho php i vi giao din PDH. Theo khuyn ngh G.703 th x dch tn i vi tn hiu 139246 kbit/s l 1.5 ppm. Nh ni trn lch tn s cho php trong c ch hiu chnh con tr l 4.6 ppm. Nh vy vic nh x tn hiu 139246kbit/s vo VC-4 phi thc hin cht hn c ch x l con tr AU. iu ny cng ng cho cc tn hiu PDH khc.

Hnh 2.12 nh x ti hiu PDH vo khung STM-1C: containerSTM: Khi truyn dn ng bVC: container oSOH: Mo u onTUG: Nhm n v lung POH: Mo u ng truynAU: n v qun lAUG: Nhm n v qun lHai phng php c bn ng b vic nh x tn hiu PDH vo cc SDH VC: ng b s dng hiu chnh dng (positive justification)K thut ny s dng cho qu trinh ghp tn hiu 140 Mbit/s vo VC-4. Dung lng danh nh trong VC-4 lun lun ln hn vn tc ln nht ca tn hiu PDH (tc "c" lun lun ln hn tc "ghi") v vy b x l ghp tn hiu ch c gii hn mc cao v thc hin lm gim tc "c". K thut ny ging nh k thut chn dng trong PDH.

Hnh 2.13 Hiu chnh dng ng b s dng hiu chnh dng-zero-m (positive-zero-negative justification).K thut ny s dng cho qu trnh nh x tn hiu 2 Mbi/s v 34 Mbi/s vo cc VC-12 v VC-3 tng ng. Do dung lng danh nh trong vc tng ng vi vn tc bit ca tn hiu PDH , nn khi tc bit ca tn hiu PDH ln hn mc danh nh th lng bit ghi vo b m s ln hn mc ngng v khi tc bit ca tn hiu PDH nh hn mc danh nh th s lng bit ghi vo b m s nh hn mc ngng (xem hnh 2.15) Trong cc khuyn ngh v nh x (G.707) cha n nh r rng, c th v vic ng b tn hiu PDH nn trong cc loi thit b khc nhau s dng cc s ghp khc nhau. Hai s in hnh l: B x l ghp c hai mc ngng - trn v di, khong cch gia hai ngng l 1UI v vic hiu chnh c thc hin khi mc ngng b vt qua. a vo b x l mt tn hiu iu pha c s lng xung zero nhiu hn v c bin v tn thch hp. K thut ny tuy phc tp hn nhng c u im rt ln trong vic gim nh hng ca jitter.Mc jitter cho php ca qu trnh ghp PDH vo vcTOC BITGII HN

1554 kbis1.5 UI

2048 kbit/s0.3 UI

44736 kbisJ.5 UI

139264 kbit/s0.3 UI

Hnh 2.14 Hiu chnh dng - zero-m pha thu, tn hiu PDH trc khi c tch ra khi tn hiu STM-N phi tri qua cc qu trnh x l ti cc lp trung gian. Mi lp x l u thc hin qu trnh gin nhp (gin on) nhp ng h t nhp ng h ca lp cao hn (hnh 2.15).Cc qu trnh x l phi bo m sao cho mc jitter v wander ca tn hiu PDH phi nm trong gii hn cho php.

Nhp dng h STM-N

Hnh 2.15 ng b tch tn hiu PDH ra khi VC/SDH2 .6.2.3 Phn Tch Fourier Sai Pha Do Hiu Chnh Con Tr

Khi tn hiu ban u c tch ra, cc sai pha s to ra jitter trong tn hiu thu c. p dng chui Fourier ta thu c bin ca jitter nh sau:

Bi v 0 = 2 f /8T v lch tn s f rt nh, jitter s c cc thnh phn tn s thp, bin cao tc l to ra wander c bin cao. Do vng kha pha PLL trong b tch knh thc cht l b lc thng thp, wander ny chc chn s ri vo bng thng v do s xut hin trn lung ra.2.6.3 Cc ngun ca wanderTrong mng ng b SDH, tn hiu ng b cho lung d liu ri khi thit b khng nht thit phi tch t lung d liu vo.Trong hnh 2.10 b kt n cho (SDHXCONN) ly ra t tn hiu ng vo vi ti v to ra tn hiu tng ng ra mi. ng h nh thi cho tn hiu STM-N i l tn hiu ng b chun ly t mng ng b ring.Tn hiu STM-N n c th c cc chuyn dch pha chm so vi tn hiu chun, ngay c khi cc tn hiu ny xut pht t SDH MUX (ADMUX bn tri) c ng b t cng mt mng ng b.Cc nguyn nhn gy ra wander trong mt h thng ng b c th l: S thay i v tr ca cp do s bin thin v nhit . S dch chuyn ca DC offset trong cc PLL ca cc ng h S tri pha chm ngu nhin khi mng phn b ng b c cu hnh li do nhn cng hoc do chuyn mch bo v.S lch tn ti mt nt mng do mt ng b (vo khong 1 4,6 ppm ).Khc vi h thng PDH, h thng SDH rt nhy vi wander. C ch con tr c s dng bm theo wander ny. iu ny s gy ra jitter trong ti thng tin. Nu jitter ny qu ln, n s lm trn cc b m FIFO v gy ra li.Theo quan im trc y khi nghin cu v wander trn mng PDH, ngi ta thng cp ti khi nim l nh hng lm x dch v tr ca xung i vi tn rt thp (G.824, G.824 ca ITU ). Tuy nhin, hin nay khi nghin cu v jitter v wander n mng truyn dn s SDH v ATM, ngi ta lu nhiu ti nguyn nhn, phm vi nh hng v tng b nh hng trc tip ca wander.i vi mng truyn dn SDH, mt trong nhng dng wander c nh hng trc tip ti cht lng ca mng l loi wander bin i c chu k theo ngy (diurnal wander). Mi trng truyn dn vt l nh si quang, cp ng hoc khng kh s c nhng tc ng nh hng trc tip ti s lan truyn ca trng in t v thm ch s thay i di vt l ca ng truyn di tc ng ch yu ca s bin i nhit . Chu k ca tc ng ny l theo ngy hoc theo ma. nh hng trc tip ca tc ng ny l gy nn s sai lch v pha gia im u v im cui ca ng truyn.Chng ta c th xem xt c th hn bng minh ha sau: Gi s u pht ra c n xung ng h c pht i th lng xung Cp(n) ghi nhn ti u thu c xc nh bng cng thc: (2.6.4)Vi: br vntc bitL di ng truyn V vn tc lan truynS thay i lng xung p(n) do nh hng ca bin i nhit t c th xc nh nh sau:

Vi t(n) l s bin i nhit trong khong thi gian pht n xung.S thay i s lng xung ghi ti u thu tng ng vi s sai lch v pha gia u pht v u thu

Trn thc t, wander ny nh hng trc tip n chnh xc ca xung ng b nhn c ti cc im cch xa v tr ca ng h chun (PRC)2.6.4 Wander do kiu ghp knh ng bPhn ny m t mt i lng c gi l t s chn v mi lin h ca n vi ph thu. Sau s xem xt t chn hiu dng zero trong kiu ghp knh ng b SDH s to ra jitter tn s thp nh th no.Xem xt mt nt SDH c tn s c d liu vo l f0 v tn ghi ra khung l f1. Gi s lch pha gia ng h c v ng h ghi l (0(t) v fm l tn s ti a lm cho hiu chnh con tr xy ra.Tc chn trung bnh l fs = f1 f0 v do t s chn s l:

By gi c th xem p nh l mt phn ca chn v do =l/3 c ngha l trung bnh mi mt phn ba c hi chn c s dng.Trong SDH, thun tin hn ngi ta s dng t s chn hiu dng (effective stuff ratio). T s chn hiu dng c nh ngha l gi tri tuyt i ch i ca ln i pha k tip gia con tr t bo c v t bo ghi trong b nh m ca b ng b gia cc c hi chn (o bng UI). T s chn hiu dng c gi tr tng ng vi pnum nu mo u v ngng chn nh. Bi v SDH s dng ngng chn l 8UI v c mo u tng i ln nn hai i lng ny khng tng ng. S lch pha 0(t) phi c lm phng ti b ng b. Vng kha pha (PLL) trong b kh ng b c hm truyn t h(t) thc cht l b lc thng thp (ps(t). Do cn thit phi kim tra ph jitter c to ra bi . C th chng minh c rng ph cng sut ca s(t) l:

v p l jitter thi gian ch i hiu dng. Thnh phn quan trng trong Ss(f) l Q(f) bi v n to ra cc thnh phn tn s thp nht. S hng sinc2 l hm bao. Do s xut hin ca (fn) nhiu hn (fn) vi 0 cc thnh phn ph s xut hin ti tn thp hn v do s ri vo bng thng ca b kh ng b (desynchroniser). iu ny c ngha l jitter bin cao, tn s thp s truyn qua b kh ng b v xut hin trong tn hiu PDH truyn i. y l mt vn c hu to ra bi SDH.

Hnh 2.16 Ph jitter thi gian ch i

CHNG 3: S TCH LY CA JITTER V WANDERTrong chng ny s kho st cc m hnh ton hc v jitter, s tch ly ca chng trn mng s v t a ra mt s m hnh v gii hn ca jitter cho mng truyn dn.3.1 Jitter trn mt trm lp ring l.Gi s nguyn tc ti to ng h da trn mch cng hng vi h phm cht Q nh trong hnh 2.8, ph tn hiu ng vo khung cng hng cha mt vch ph mnh ti tn, ng h f0. Tn cng hng ca khung dao ng fj c iu chnh n gi tr f0 mong mun ni trn. Hm truyn t ca khung cng hng c biu din bng biu thc nh sau:

y R, L v C l tr khng, t cm v in dung tng ng ca khung (mch ni tip).Suy ra vic truyn tn hiu ng h qua khung cng hng c biu din bng:

Hiu ng ny thng khng nghim trng trong cc thit b thc t bi v lun c s cn chnh pha gia mch ti to ng h v cc phn t nh thi li.

3.2 S tch ly jitter trong mng gm cc trm lp ng nht ni tipNguyn l phn tch s tch ly jitter trong mt mng s gm mt chui cc trm lp ni tip c xy dng trn m hnh ton hc ca Byrne. M hnh c pht trin vi cc gi thit sau:1. Jitter a vo cc trm lp l nh nhau, cc trm lp l ng nht.2. Tt c cc ngun jitter c ngha u c th biu din bng jitter tng ng ti ng vo ca cc mch ti to ng h3. Jitter cng tuyn tnh t trm lp n m lp.4. Vic khi phc ng h c thc hin bng khung cng hng c iu hng ti tn ng h (tn s xung tip nhn).5. Tc thay i ca jitter nh hn tc ng h c khi phc, khung cng hng tng ng mt lc thp qua c mt cc n tng ng na bng thng ca khung cng hng. c biu din bng biu nh sau:

T cng thc 3.6 ta thy ch c jitter tn s thp mi c chuyn qua khung cng hng trong khi jitter tn cao b suy hao v dch pha.

Hnh 3.1 M hnh tch ly jitter dc theo chui trm lpJitter ti trm lp th N l tng jitter to ra trn mi trm lp v jitter tng hp t cc trm lp trc nn ph ca jitter tch ly sau N trm lp c th c biu din nh sau:

Vi Si(f) l mt ph cng sut jitter a vo mi trm. Hnh 3.2 minh ha ph mt cng sut jitter danh nh thu c t cng thc (3.9) i vi

Hnh 3.2 Ph cng sut danh nh ca Jitter sau N trm ni tip.chui gm N = 1, 10, 100 v 1000 trm lp. tn s rt thp, ph mt cng sut danh nh ca jitter t l vi cn bc hai ca N, tn s cao li cng sut suy gim nh l hm t l nghch vi bnh phng ca tn s.3.2.1 Cc mu lp liS dng hai mu kim tra A v B gm 32 bit. Gi s rng A cha ch yu 0 gia cc xung ng b ong khi B cha thnh phn ng b cao (s 1). Trong trng hp ny, nu chn mu A th tn ng h c khi phc s c xu hng tin ti tn s cng hng f1 ca mch cng hng. Trong trng hp ngc li nu chn mu B, tn ng h c khi phc s t gi tr tn tn hiu ng b f0.Gi s mu kim tra c chuyn t A sang B v ngc li. Trn mi trm lp phi c mt thi gian tr trc khi mch ng b kp p ng vi mu mi. tr ny s c tch ly dc theo tuyn v c th xem nh l mt trng hp gii hn ca jitter.Ni chung mi mu bit d liu m c th c trng bng mt c tnh jitter m(t) . Bt c s thay i no t mu m sang n u gy ra s nhy bin jitter t m(t) n n(t). S bin ng v jitter, ny l do p ng ca mch ti to ng h i vi mu mi. Tc thay i jitter t l vi bng thng ca mch ti to ng h v c th biu din bng B(m -n). Jitter tch ly trn chui N trm lp mt cch tuyn tnh v c th biu din bng N(m -n).Hnh 3.3 biu th s tch ly jitter sau N trm lp. Kt qu ny thu c da trn vic phn tch s chuyn mch mu, hot ng ca b lc thng thp, hot ng ca mch ti to ng h. C th rt ra vi kt lun nh sau:1. Bin ca jitter t l thun vi trm lp.2. Tc thay i pha t l vi rng bng tn hiu dng ca mch ti to ng h tc l t l nghch vi h phm cht Q.3. Tc thay i pha khng ph thuc vo lng trm lp N. Tuy nhin di ca n ph thuc vo N v tng ng vi vic dch chuyn tn tm thi ca tn ng b cc phn t tn hiu.Thng th do s dng b ngu nhin ha (scrambler) tn hiu truyn i c tnh ngu nhin hn l cc mu c nh nh trong trng hp trn. Tuy nhin cu trc nu trn rt c ch cho cc thit b o kim tra cng nh cho vic iu hng chnh xc tn cng hng ca cc mch ti to ng h.

Hnh 3.3 Jitter tch ly dc theo chui trm lp do s thay i mu d liu ti thi im t+t3.2.2 Cc mu ngu nhinTrong iu kin vn hnh bnh thng, d liu c truyn i c mu bit gn nh l ngu nhin. i vi trng hp ny, mt cng sut jitter tn s thp khng ph thuc vo tn s, do S1(f) trong cng thc (3.9) s l mt hngGi tr bnh phng trung bnh ca jitter thu c bng cch ly tch phn ph ca jitter (cng thc 3.8) trn ton b bng thng.

Vy gi tri trung bnh bnh phng ca jitter trong mt chui trm lp tng theo N v gi tr bin trung bnh (rms) tng theo gi tr cn bc hai ca N. hn na cng thc (3.11) cho thy cng sut jitter t l thun vi rng bng thng ca mch ti to ng h v t l nghch vi h s phm cht Q ca khung.Kt qu ny c th gii thch mt cch n gin l cc thnh phn jitter tn s thp c cng theo bin , trong khi cc thnh phn tn cao b lc v c cng theo vector. Kt qu ny ch ph hp khi cc ngun ca jitter l tng quan (correlated) do mu xung m l ngu nhin. Trong trng hp cc ngun jitter khng tng quan, jitter c cng li theo nguyn tc cng sut. S tch ly jitter trong trng hp ny t l vi cn bc bn ca N.3.2.3 Jitter ng chnhJitter ng chnh ti trm lp th N, N c nh ngha l hiu gia jitter ng vo v ng ra. Nh vy vi trm lp th N ta c:

Nh vy jitter ng chnh ti a khng tng theo di ca chui (trm lp) v e gii hn bi jitter ti a a vo mi trm lp. c tnh ny c th thy trn hnh 3.3. Trong hnh 3.3, dc c nh ca s thay i jitter cho thy jitter ng chnh l nh nhau gia hai trm lp k tip.3.3 Dung sai jitter trong mt trm lp (jitter ng chnh)R rng khi jitter xy ra nghim trng, t s li bit ca qu trnh ti to s suy gim. Vn c t ra y l qu trnh ti to c th chu ng (t s li bt khng vt qu mt gi tr ngng cho trc) mc jitter ti a no (gi tr ti a ca jitter)

Hnh 3.4 Con mt tn hiu nh phn v ng h tng ng (vi jitter)Trong trng hp l tng, vic nh gi thng tin c truyn i lun lun c thc hin im gia ca con mt. iu ny m bo vic truyn dn khng gy li. Tuy nhin n thc t tn hiu ng h c khi phc lun lun b trt (jitter) xung quanh tm im con mt, v do con mt thng tin cng b trt quanh v tr c ngha. vic nh gi thng tin c ng n (khng gy li), bin nh - nh ca jitter tng i gia con mt v ng h c khi phc phi nh hn rng hiu dng Wca con mt (hnh 3.4)D-d W (3.15) y:D: bin jitter ca con mt thng tin (nh - nh)d: bin ca jitter ng h c khi phc (nh - nh) W: rng con mt i vi jitter c dng sng hi, t phng trnh 3.15 ta c

Hnh 3.5 minh ha dung sai jitter theo tn thu c t phng trnh (3.19). th c hai c tnh: tn thp (< fN) v tn s cao. tn s thp mch khi phc

Hnh 3.5 Dung sai l tng ca bin jitterng h bm theo c s lch pha. Tuy nhin tn s cao, dung sai jitter khng cn ph thuc vo tn v ch c xc nh bng con mt W.im chuyn i gia hai c tnh ny ph thuc vo cc yu t sau:1. c tnh thng thp hay rng bng thng ca mch ti to ng h2. Vng kha pha trong b xa chn3. Qu trnh chn trong b ghp knh3.4 Dung sai jitter trong b ghp/tch knh PDHDung sai tiu biu cho mt h thng ghp knh cn ng b PDH c mt trn hnh 3.6. th do Standard Telephone Laboratories thit lp. Dung sai ti thiu ca mt b ghp knh chia lm 6 on gii hn theo tn s:1. Vng kha pha2. B nh xa chn b tch knh3. Qu trnh chn4. B nh chn b ghp knh5. Khi phc ng h cc mch giao tip ng dy6. rng con mt thng tin

Hnh 3.6 Gii hn dung sai ca h thng truyn dn PDH

CHNG 4: NH HNG CA JITTER V WANDERTi mt giao din lin kt, thit b tip theo phi c thit k chu ng c jitter do thit b pha c sinh ra. Ni cch khc, nu khng iu khin jitter mt cch thch hp th s tch ly jitter s lm cho cht lng truyn dn suy gim nghim trng.4.1 Lm suy gim cht lng truyn dn v dch v4.1.1 Jitter lm tng xc xut li a vo trong tn hiu sNh bit, jitter lm cho thi im ti to tn hiu (thi im ly mu tn hiu) b lch i so vi thi im l tng ca chng khi lch ny vt qu dung sai cho php th li bit xy ra.Tn hiu qua knh truyn dn thng b mo do giao thoa. Nu ng h ly mu khng c jitter th vic ly mu c thc hin ng thi im t u, do vic quyt nh tn hiu truyn i ( l 0 hay 1 ) vn c th thc hin chnh xc.Nu c tn hiu s v ng h c cng mt jitter ln, v tr chuyn tip ca ng h so vi v tr ti u ca tn hiu ( gia tn hiu) khng thay i. Do vic ly mu vn c thc hin chnh xc v li bit khng xy ra. y l trng hp i vi jitter tn thp m mch khi phc ng h dng PLL c th theo nhng bin thin pha ca tn hiu.Ti cc tn jitter cao, mch khi phc ng h khng th bm theo nhng bin i pha nhanh ca tn hiu. Khi ng h c khi phc dch pha hn 0,5 UI so vi tn hiu thu c th vic ly mu tn hiu s khng chnh xc v gy ra li bit. Trong thc t do tn hiu b mo, di quyt nh thng hp hn nhiu (H. 4.3)Trong truyn dn vi ba tn hiu ng ra knh truyn dn ti thi im ly mu [i, (i)] c th biu din bng:

y:y(i) tn hiu ng ra knh truyn dn.x(i) k t thng tin (i) Jitter ca ng h ly mui thi im ly mu l tng (i) lch pha sng mang (jitter pha sng mang) n(i) tp m Gausse Trong v phi ca phng trnh(4.1) thnh phn th nht trong {} l thnh phn mang thng tin, thnh phn cn li l nhng giao thoa gia cc k t. Trng hp knh truyn dn l tng v khng c jitter ca ng h ly mu v pha sng mang, th:c0 [i, (i)] = c0(i) = constcm [i, (i)] = cm(i)= 0; m # 0(4.2)Do thnh phn nhiu giao thoa gia k t bng 0 v :y(i) = x(i)+n(i)R rng khi c jitter pha sng mang v pha ng h k t, ngay c trong trng hp knh truyn dn l l tng vn xy ra giao thoa gia cc k t. Kt qu l t C/N (carrier/noise) gim v t s BER (bit error rate) tng.Cc tn hiu QAM (quadratureamplitude modulation) cng nhiu cng nhiu mc th cng d suy gim t s C/N do jitter pha sng mang v tn hiu nh thi gy ra.0

Hnh 4.1 Ly mu ng

Hnh 4.2 Ly mu ng ngay c khi c jitter bin ln

Hnh 4.3 Ly mu sai tn hiu do jitterHnh 4.4 biu din s suy gim t s C/N khi jitter pha sng mang tng cho cc mc QAM khc nhau v i vi BER = 10-6. Cc h thng 256 QAM b suy gim nhiu dB ngay c khi jitter pha sng mang ch t 0,5 n 1. Nhng h thng ny i hi iu khin jitter rt cht ch. Tng t, hnh 4.5 minh ha suy gim C/N khi jitter tn hiu nh thi tng. Mt ln na li thy r mc QAM cng cao th mc suy gim C/N cng ln i vi mt gi tr BER cho trc.

Hnh 4.4 th suy gim C/N theo pha jitter sng mang

Hnh 4.5 th suy gim C/N theo jitter pha tn hiu nh thii vi h thng 16 QAM cc ng cong trn c v cho b lc vi h roll - off khc nhau. C th nhn thy rng h s cng ln th c tnh jitter tng ng cng tt. Tuy nhin cn lu rng nu h qu ln th gii hn mt n ph pht c th b vt.4.1.2 Jitter a cc trt khng iu khin c vo tn hiu sKhi jitter qu ln, dung lng ca cc b nh m c s dng ht v th gy ra hin tng trn hoc cn b nh. Hiu ng ny gy ra trt, mt hoc nhn i (lp li) cc lung bt trong mt khung a n SES v cc s c khc.i vi cc dch v thoi, slip (trt) ny c th l nguyn nhn gy ra nhng ting ng nh c th nghe thy. Loi ting ng ny khng xut hin thng xuyn v khng nghim trng. Tuy vy cc dch v thoi vn ra nhng gii hn cho php vi slip.i vi cc dch v khc, slip c th gy ra nhng hu qu nghim trng. Trong truyn fax ch 1 slip cng gy nn nhiu hoc mt i 8 dng trong bn fax. iu ny tng ng vi vic mt 0,08 inch chiu ng (trc tung). Vic mt thng tin s dn n cn phi truyn li (fax li) bn tin cn fax. Vic uyn li ny phi do ngi s dng khng c thc hin t ng bi thit b.nh hng ca slip i vi truyn liu l nguyn nhn gy ra cc dy li. di ca cc li ny ph thuc vo tc c liu v loi modem t 10 mili giy n 1,5 giy.Trong thi gian b li, thit b thu u ni vi modem thu c liu t qung v kt qu l ngi s dng phi truyn li liu.Khi slip xy ra trong dch v hi ngh video, cng s dn n vic xy ra cc li v cn phi thit lp li qu trnh.Tc ng ca slip i vi vic truyn s liu ph thuc vo giao thc truyn dn. Nu s dng giao thc khng c kh nng truyn li s xy ra vic s liu b mt, b lp li, hoc b li. Giao thc truyn li c kh nng pht hin slip v do s khi ng vic truyn li.i vi vic truyn video s (hi ngh video), cc php kim a ch ra rng slip thng gy nn nhiu tng on ca nh. Mc nghim trng v chiu di ca nhiu ph thuc vo loi m v thit b nn tn hiu c s dng. S suy gim l nguy him nht i vi thit b gii m tc bit thp.Slip tc ng mnh ln cc dch v m ha (mt m). Slip dn n vic mt cha kha mt m. Vic ny lm cho vic truyn dn khng th hiu c v do cn phi truyn li cha kha v khi phc li lin lc. Tuy nhin, yu cu truyn li cha kha th khng ph hp vi tnh bo mt- vi cc ng dng bo mt khng th chp nhn c nhiu hn 1 slip trong 1 ngy.4.1.3 Jitter lm suy gim cht lng tn hiu tng t c m hai vi cc tn hiu tng t c m ha, jitter ong lung s ha s nh hng n tnh ng u gia cc mu ca tn hiu PAM c khi phc. Tn hiu ting ni c m ha PCM c th chu c nh hng ny, nhtig i vi tn hiu hinh v cc tn hiu khc th nhy hn nhiu.i vi cc tn hiu truyn hnh, khi truyn trong khong cch xa c th mo do jitter (dao ng v pha). Mo pha do thay i v tr xung ca cc mu gy ra mo ng bin, cc chi tit nh thay i lin tc theo cc khung. Trn cc hnh, mo th hin nh bin ca cc chi tit b chuyn ng gia tn s dao ng pha ff vi tn dng fH (bng 4.1; hnh 4.6) v tn s ff vi tn khung fv Bng 4.1 Cc loi mo ng bin do dao ng pha vi nhiu gi tr t s ff v fH. Nu ff = nfH th sai pha do mo bng 0 v th hin nh mo ng bin (conture) vung gc vi dng qut.Tn s dao ng pha ffVi sai pha (sau 1 dng hnh)Loi mo (hnh 4.8)Mc can nhim

NfH0ATrung bnh

NfH + BLn

n(+l/2)fHaCNh

(n+l)fH-DLn

(n+l)fH- 0ETrung bnh

n: s t nhin; 0 < < 0,5fH

Hnh 4.6 Cc loi mo pha do dao ng pha.Ngc li, nu ff = (n+1/2)fH th s sai pha trn cc dng lin tip l 180, do mo lm thay i pha theo tng dng, do vy t nhn thy trn nh trong cc trng hp cn li mo ng bin (conture) c dng sin v rt kh chu i vi mt.Trng hp ff = nfv th sai pha trn cc khung lin tc s bng 0 v mo nhn thy trn nh s c nh; do vy th hin khi ff = (n+l/2)fv th sai pha trn cc khung lin tc s bng 180 c ngha l mo lm thay i pha trn tng khung v bin cc chi tit nh s b nhp nhy vi tn 25Hz. Nu ff = n(l/4)fv hoc (n+3/4)fv th sai pha trn cc nh lin tip l 71, gy nhp nhy bin ca chi tit nh tn 12,5Hz (rt kh chu i vi mt).Dao ng pha ngu nhin gy ra iu ch v tr xung, to ra thnh phn ph tn hiu truyn, thnh phn ny lm tng nhiu th hin bin nh di dng nhiu ph gy mt nt, thay i mu v tn hiu mu.4.2 nh hng ca jitter v wander trong mng SDHTrong mng PDH, tn hiu ng b c ly t vng kha pha (PLL). Cc PLL c th bm theo cc bin thin pha chm (wander) v vy wander thng khng xut hin trong h thng. Cc thnh phn jitter ca pha m PLL khng theo c lm gim tin cy ca h thng. Do trong h thng PDH ngi ta thng quan tm n jitter.Trong h thng SDH cn quan tm n c jitter v wander. Cc vng kha pha rt nhy cm vi jitter vn c s dng trong mch khi phc ng h. Ngoi ra cn c cc b nh m FIFO (Frist In Frist Out) rt nhy cm vi wander. Cc FIFO hot ng vng m (open loop) ph thuc vo ng h c v ghi. Do mi bin ng pha t 10 Hz n DC (wander) tr thnh 1 tham ht sc quan trng.Trong mng hn hp PDH / SDH, khi c s khc nhau v pha gia tn hiu STM - N i v n ti im kt ni cho, khng b mt d liu, b m FIFO vi k thut hiu chnh con tr c s dng. Hiu chnh con tr khng lm thay i pha ca STM - N i nhng lm dch chuyn ti AU - 4 trong khung tn hiu STM - N.Qu trnh ny c minh ha trong hnh 4.7. Trong v d ny, lch tn c nh gia tn hiu STM - N / ti n v tn hiu STM / tn hiu ng b i l 10'11. iu ny c ngha l cc hiu chnh con tr phi xy ra thng xuyn; mi ln hiu chnh c bin pha l 154 ns (24 bit i vi tc 155 Mbit/s). Do , mi hiu chnh con tr c thc hin sau 1540 giy b tr s tng lch pha lin tc gia tn hiu i v n.Khi ti c tch ra (gii nh x), nhng bc dch pha nh vy s gy ra Jitter trong ti v phi c lm phng bng b kh ng b m bo rng jitter vn trong cc gii hn nht nh

Hnh 4.7 Pha ca ti d liu do hiu chnh con trNh vy, do hiu chnh con tr, wander tn s thp trong mng SDH c lu gi v sau c gii phng di dng chuyn tip pha tn SC) cao trong ti thng tin. S i pha ny c lng t ha - hon ton th.Nu jitter ca ti thng tin qu ln, n c th lm trn hoc cn cc b m FIFO. Hu qu cht lng dch v suy gim nghim trng a n mt mt v doanh thu. Ngay c khi jitter ti thng tin c mc thp khng th gy n cc b nh m, cht lng cc dch v video c th suy gim nghim trng.nh hung ny c th tm tt theo s sau:Wander - hiu chnh con tr - jitter ti thng tin - trn hoc cn FIPO -> SES -> $

CHNG 5: CC GII HN CA JITTER V WANDER CC GIAO DIN MNG5.1. Nguyn tc iu khin Jitter v WanderTrong mt mng truyn dn s, lun c nhiu loi thit b tch/ghp knh v thit b ng dy c kt ni vi nhau. Jitter v wander do cc ngun sinh ra v c tnh truyn t ca chng trong mi thit b c xu hng tch ly. Khi jitter v wander tch ly qu nhiu, chng c th nh hng nghim trng ln c tn hiu s to ra li bit, trt khng iu khin c (uncontrolled slip) ln tn hiu tng t (iu ch pha khng mong mun cc tn hiu truyn dn). Do , cn phi thit lp cc gii hn cho jitter v wander ti cc giao din mng m bo cht lng truyn dn . iu khin hiu qu tng jitter v wander ti cc giao din mng , cc khuyn ngh phi m bo rng bin jitter/wander cc i ti cc giao din mng phi nm trong cc gii hn nht nh. Mt khc ng vo cc thit b s phi chu ng c mt lng jitter/wander nht nh (- dung sai jitter/wander-jitter/wander tolerance). Jitter v wander tch ly trn mt tuyn truyn dn (mt chui cc trm lp ) phi c gii hn bng cc gi tr ca hm truyn t jitter (jitter /wander transfer function ).Cc khuyn ngh ca ITU ,ANSI, ETSI phi xc lp cc tiu chun ny cho tt c tham s jitter v wander ni trn.Lin quan n cht lng cho php ti cc giao din mng l cc gii hnsau: Jitter giao din ( cn gi l jitter ng ra ). Wander giao din ( cn gi l wander ng ra ).Lu rng cc tham s cho mng iu khin cc yu cu dung sai nhiung vo cho thit b.5.2 Cc tham s ca JitterC ba loi tham s v php o jitter c th s dng nh gi mt thnh phn hay c mng . l : Jitter ng ra . Dung sai jitter ng vo . Hm truyn t jitter .5.2.1 Jitter Ng Ra ( Output jitter ) .Jitter ng ra l jitter c t giao din mng (hay mt thit b). Jitter t mt giao din mng ch n t 2 ngun : Jitter ni ti c to ra t thit b trc n (jitter ny pht sinh t mt hay nhiu ngun ). Jitter c truyn t mt hoc nhiu ng vo ca thit b , tc l cc ng vo d liu hay ng b .Bin jitter tng cng ti mt giao din mng hot ng phi nm trong gii hn xc nh cho loi mng .Kim tra jitter ng ra l php kim tra c bn c thc hin trong qu trnh thit k, lp t, nghim thu v nh v h hng trn mng .Jitter ni ti (Intrinsic jitter) hay cn gi l S pht sinh jitter (Jitter generation) Tn hiu khng c jitter ng vo ca thit b s xut hin ng ra cng vi jitter . Jitter ny do chnh thit b to ra (jitter ni ti) l do cc nguyn nhn : Tp m nhit trong cc b dao ng ni. Cc hi ong cc b dao ng tinh th thch anh . Nhiu xuyn knh (lin h ngc t cc thnh phn h thng khcnhau n,b cung cp ng h ). Cc tn hiu k sinh do khuch i v ni vng iu kin (control loop) Tr cc mu ph thuc trong cc b ngu nhin ha v m ha . Cc cnh ca tn hiu khng sc.Jitter ni ti l gi tr o s iu pha k sinh do thit b sinh ra. V cc nguyn nhn ca jitter ni ti l ngu nhin ( stochastic) nn jitter ny khng lin h v khng tch ly dc tuyn . Cc gii hn ny phi nh hn rt nhiu so vi gii hn jitter ti giao din mng m bo c th ni mt chui cc thit b trong mng .Jitter ti giao din mng (Interface jitter) cn gi l jitter ng ra (Output jitter). trnh cc vn lin lc v kt ni, jitter ti ng ra ca cc phn t trn mt mng s khng c vt qu mt gi tr gii hn xc nh v cc gi tr ny cn c kim tra trong mt bng thng c th. Thng cc gii hn ny c thit lp da n cc iu kin vn hnh bnh thng cng nh tng s cc thit b c trc giao din. Ngoi ra cc gi tr ny cn phi tng thch vi vic bo m cc yu cu v dung sai jitter t thiu ti cc ng vo ca tt c cc thit b. Cc tiu chun cho jitter ti giao din mng tng ng vi cc phn cp s" khc nhau c th tm trong bng 5.1.Jitter con tr ( Pointer jitter).y l jitter to ra khi mt ti cn ng b PDH tch ra t ti SDH c cc hiu chnh con tr. Jitter con tr l ngun jitter ch yu trong mng hn hp SDH/PDH. Tham s' jitter con tr c xc nh bng cch s dng cc chui kim tra con tr SDH v qui nh gii hn ln jitter ng ra PDH thu c .Jitter nh x ( Mapping jitter)y l jitter c to ra khi lung tn hiu PDH c chn v nh x vo ti SDH. Jitter ny c o khi ti PDH c tch ra t mng. Jitter nh x c bin thp hn nhiu so vi jittter con tr .5.2.2 Dung sai jitter ng vo (Input jitter tolerance ).Ng vo cc giao din s phi chu ng c mt lng jitter nh't nh trc khi xy ra li bit hay li ng b . Cc tiu chun phi xc nh c mtn cho cc gi tr jitter ny; tc l cc gi tr ti a c php ca jitter cc tn s khc nhau (Maximun tolerable jitter MTJ).Khi tt c cc gi tr jitter o c u nm di mt n ny th c ngha l yu cu v dung sai ca jitter giao din c tha mn. Mt n ca jitter tolerance c dng nh hnh 5.1

Tn s jitter (log)Hnh 5.1 Dung sai jitterNh xem xt trong chng 3 , bin jitter m mt h thng chu ng c lun c gi tr ln tn s thp. Cc mch khi phc ng h trong cc h thng truyn dn s c th bm theo nhng bin ng v pha ca tn hiu s" tn s" thp, tc l ng h c khi phc s c cng mt lng jitter nh tn hiu s. Thi im ly mu trong qu trnh khi phc tn hiu s" s khng dch chuyn so vi tn hiu s ngha l vic ly mu c thc hin ng v tr ti u v th li bit khng xy ra. tn s cao, ng h khi phc khng bm theo kp vi s bin ng pha tng i nhanh. Do , c s lch pha gia tn hiu s v cc xung ng h ly mu, thi im ly mu khng ti u nn li bit tng v vy tn s cao dung sai jitter suy gim rt nhiu.Cc tiu chun cho dung sai jitter tng ng vi cc phn cp s khc nhau c tm thy trong bng 5.1.5.2.3 Hm truyn t Jitter ( Jitter Transfer function - JTF ).Nu ng vo ca mt phn t mng tn ti jitter th mt vi thnh phn ca jitter ny s xut hin ng ra. Hm truyn t ca jitter c nh ngha l t s gia jitter ng ra v jitter ng vo. Hm truyn t jitter l mt hm theo tn s v c o bng decibel Nh chng minh trong chng 3, cc thnh phn jitter tn s cao b ngn chn (suy hao ng k ), trong khi cc thnh phn jitter tn s" thp s xut hin ng ra m khng b suy hao. Thm ch trong mt s" trng hp cc thnh phn ny cn c th c khuch i (hnh 5.2). iu ny c th gy ra mt s vn nghim trng nu tuyn l mt chui cc trm lp ng nht c kt ni li vi nhau .Hnh 5.2 minh ha mt n c trng ca hm truyn t jitter . Trc tn s chia lm 3 di tn:Di thng ( pass range )Di chuyn tip ( transition range ). Di ct ( cut off range )

Hnh 5.2 Hm s truyn t c trng ca JitterTrong bng thng ( fj < fjC) jitter ng ra gn bng jitter ng vo. Jitter s b ngn chn trong vng chuyn tip, gim khong 20 dB/decade cho n khi t n vng ct. Jitter ng ra trong vng ny ch l jitter cn li ca jitter ng vo b suy hao cng vi jitter ni ti ca chnh phn t .Cc tiu chun cho hm truyn t jitter tng ng vi cc phn cp s khc nhau c th tm thy trong bng 5.1 Hnh 5.3 minh ha cc im o cc tiu chun jitter trn mt mng s .

Hnh 5.3 Cc tiu chun ca jitter .5.3 Cc tham s ca Wander m bo rng vic trt khung ( frame slip ) l chp nhn c tro cc h thng SDH/PDH rng ln, ITU - T v ETSI xc nh cc gii hn c cht lng wander ca thit b v wander ca tn hiu ti cc giao din .Ging nh jitter ch c ba loi tham s v php o c th s dng gi wander ca mt thnh phn, mt phn t, mt h thng hay c mng. l: Wander ng ra . Dung sai wander ng vo . Truyn t nhiu wander . Wander ng ra ( Output wander )

nn cua Jitter v Wander cc giao din mng

5.3.1 Wander ng ra (Output wander)

Wander ng ra l gi tr wander c ti thit b hay giao din mng, n t hai ngun : Wander to ra bi chnh thit b (c th pht sinh t mt hay nhiu ngun). Wander c truyn t ng ra ng b ca thit b.Bin wander tng cng ti giao din phi nm trong cc gii hn mng cho loi giao din .S pht sinh wander (Wander generation ):Wander tch ly trong mng ng b mt phn l do mi thit b to ra mt s wander. Wander ny l wander ng ra ca thit b khi khng c wander ti ng vo .Wander Giao din (Interface wander) hay cn gi l Wander ng raTham s wander ny c s dng ti cc giao din ng b v gii hn ny khng c vt qu. Tham s ny p dng khi qui nh rng s thit b trc giao din p ng m hnh tham chiu mng (network reference model) v hiu chnh hiu qu gii hn mng .5.3.2 Dung sai wander ng vo (Input wander tolerance ):Tham s ny cho bit mt giao din c th chp nhn c bao nhiu wander trc khi gii hn cht lng - hoc l li hoc l ng b - b ph v .Dung sai wander ng vo xc nh mt phn ca thit b c th chu ng c wander trong tn hiu n nh th no m khng gy ra li, mt bit hoc trt khung (thng l do cc b m ng vo FIFO b trn hoc cn). bo m s lin kt ca mng, tt c cc thit b phi c kh nng chu ng c cc mc wander, t nht l n gii hn ti a ca mng.5.3.3 Truyn t nhiu wander ( Wander noise transfer) :Tham s ny cho bit wander c khuch i hay suy hao i bao nhiu t ng vo n ng ra ca giao din ng b . Khi wander xut hin trn ngun ng b tham chiu (STM-N, 2Mbit/s hay 2MHz), mt phn wander ny c truyn n lu lng STM-N hay cc tn hiu 2Mbit/s nh thi c truyn i.S tch ly wander tr nn nghim trng khi n c khuch i bi thit b SDH. Do , li trong bng thng c gii hn 0.2 dB, v truyn t wander ca thit b phi c c tnh lc thng thp nh xc nh trong cc tiu chun tng ng .5.3.4 p tuyn chuyn tip pha ( Phase transient response )Chuyn tip pha c hnh thnh khi mt phn ca thit b cu hnh li ng b ca n, v d t ngun tham chiu 2MHz sang ngun tham chiu 2Mbit/s hoc tn hiu STM-N, n chuyn tip pha xy ra trn tn hiu STM-N i to ra wander tc ng ln gii hn cht lng ca mng. Chuyn tip pha c th nh hng nghim trng ln lu lng i tip trong mng hay khi ti mt ng b .

5.3.5 Cht lng Kim Gi ( Holdover - Performance ) :Cht lng kim gi dng nh gi kh nng duy tr chnh xc tn s ng ra ca ng h t (slave clock) khi tt c cc ngun tham chiu n b mt. Nhiu ng h t c trang b chc nng nh. Chc nng ny thu nhn cc c tnh ca tn hiu ng h tham chiu ng vo (tn s", tc tri dt,...). Cc c tnh c thu nhn ny sau c s dng iu khin ng h ni khi ng vo tham chiu b mt cho n khi ngun tham chiu c khi phc (tuy nhin thi gian kiu kim gi trong thit b SDH thng khng qu 48 gi, sau thit b chuyn sang ch chy t do).5.4 Cc khuyn ngh ca ITU v ETSI v Jitter v Wander Bng 5.1 tm tt cc khuyn ngh ca ITU-T v ETSI v jitter v wander cho cc phng php kim tra, cc thit b v giao din khc nhau. Trong bng ny tiu chun ca ESTI s b qua khi cc khuyn ngh ca ITU-T l tng ng . Cc khuyn ngh ny s c trnh by chi tit trong chng 6 v 7 .

.ln cua Jitter v Wander cc giao din mng

nn cua Jitter v VVander cc giao din mng

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625.5 Gii hn ca Jitter & Wander cho cc giao din mngBng 5.1 Cc tiu chun tham kho cho vic kim tra jitter/wander theo loi thit b v giao din mng

Quan im iu khin jitter phi m bo l cc mc tiu li v tri... s p ng i vi jitter xy ra trn phn t b't k ca mng u vo ca n, khng k n v tr ca n trong mng hoc tn hiu xut pht t u trong mng. Hn na cc mc tiu ny phi tip tc p ng d rng mng c m rng v cu hnh ca n thay i.

Hnh 5.4 minh ha cc giao din c th c trn mt phn t mng. Lu rng: Khng phi tt c cc giao din u phi c trn mt phn t . ng b c th truyn trn giao din truyn dn SDH.

Hnh 5.4 cc giao din ca mt SDH tng qutCc tham s cht lng chung cho giao din:Tt c cc giao din c th nh gi thng qua cc tham s sau: Tn s (frequency) S dch chuyn tn s (frequency offset) S tri dt tn s theo thi gian (frequency drift with time) Nhiu giao din (jitter) Nhiu giao din (wander)Giao din truyn dn SDHG.825 l mt tiu chun cho nhiu giao din truyn dn (xem phn 5.6 ). Nu giao din ng dy SDH cng c s dng truyn ng b, tiu chun ETS 300 426 - 3 s c p dng (xem phn 5.7).Vic phn tch hiu chnh con tr theo thi gian cho php nh gi cht lng ng b cho lu lng theo chiu ngc li v gip chn on cc vn ng b trong mng.Giao din dch v PDHG.823 l tiu chun quan trng cho nhiu giao din truyn dn PDH ( xem 5.7). iu khin wander trong cc dch v PDH ngy cng tr nn quan trng khi mng truyn dn SDH c a vo. Hiu chnh con tr c th gy ra wander qu ln v cc dch v ng b PDH mi ang c quy hoch cho tng lai. Lnh vc ny ang c ITU - T v ETSI nghin cu.Giao din ng b.ETS 300 462-3 l mt tiu chun quan trng cho nhiu giao din ng b (xem phn 5.7).5.6 G.823, G.825: Gii hn Jitter ti phn cp s.Bng 5.2 biu th cc gi tr jitter tia cho php ti cc giao din phn cp s tc l cc cng ng ra ti cc tc uyn dn d liu tiu chun. Cc gi tr ny khng tnh n tng s thit b c c giao din v vic hiu chnh c hiu qu gii hn mng.5.7 ETS 300 462 - 3 : gii hn mng cho WanderETSI xc nh gii hn wander STM - N v 2 Mbis tng ng ti cc im giao din trong mng ng b. kim a chnh xc tham s ny, mt ngun tham chiu c cht lng - PRC (thng l ng h cesium) s c s dng nh l ng h chun.Cc tham s jitter v cc mt n MTIE v TDEV sau y l cc gii hn jitter v wander ti cc im giao din mng STM N v 2 Mbis tng ng.

J1 nn cua Jiuer va vvaner cc giao din mng

.ln cua Jitter v Wander cc giao din mng

Cc ngun ng h c sp xp theo th t cht lng ng h gim dn nh sau: cc ng ra PRC, cc ng ra ssu, cc ng ra SEC, cc ng ra phn b PDH

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5.7.1. Gii hn Jitter ng ra - Giao din ng bCc tham s jitter b xung cho cc giao din ng b c tm tt trong bng 5.3. Cc gi tr UI c xc nh cho cc giao din 2 Mhz v 2 Mbit/s. i vi cc tc bit khc, trc tin cn bin i chng thnh cc n v thi gian ri mi p dng.

5.7.2 Gii Hn Mng Cho Wander Ng Ra - MTIE v TDEVHnh 5.5 tm tt tham s MTIE trn cc giao din ng b khc nhau. Trn hnh ny c th thy r rng mi quan h gia cc mc cht lng khc nhauCc gii hn mng TDEV cho wander ng raHnh 5.6 tm tt cc tham s TDEV trn cc giao in ng b khc nhau. Cng nh MTIE, trn hnh ny c th thy r rng mi quan h gia cc mc cht lng khc nhau.Cc hnh t 5.7 ti 5.14 tm tt cc ch tiu ca ETS 300 426 - 3 .

Hnh 5.5 300 462 3 cc mt n gii hn mng cho Wander

Hnh 5.6 ETS 300 462 3 Cc mt n gii hn mng cho Wander ng ra TDEV

Wander ng ra PRC cc gii hn MTIE v TDEV

Hnh 5.7 ETS 300 462 3 Ng ra PR gii hn MTIE

Hnh 5.8 ETS 300 462 3 Ng ra PR gii hn TDEV

Wander ng ra SSU cc gii hn MTIE v TDEV

Hnh 5.9 ETS 300 462 3 Ng ra SSU gii hn MTIE

Hnh 5.10 ETS 300 462 3 Ng ra SSU gii hn TDEV

Wander ng ra SEC cc gii hn MTIE v TDEV

Hnh 5.11 ETS 300 462 3 Ng ra SEC gii hn MTIE

Hnh 5.12 ETS 300 462 3 Ng ra SEC gii hn TDEV

Wander ng ra PDH cc gii hn MTIE v TDEV

Hnh 5.13 ETS 300 462 3 Ng ra PDH gii hn MTIE

Hnh 5.14 ETS 300 462 3 Ng ra PDH gii hn TDEV

KT LUN V HNG PHT TRINKt lun ti nh hng ca rung pha v tri pha trong mng vin thng s bc u thc hin c cc ni dung sau: Nghin cu, h thng ha l thuyt v jitter/wander trn mng. Trn c s nghin cu l thuyt, cc khuyn ngh ca ITU - T v cc tiu chun Chu u (ETSI) tng ng, phn tch cc kt qu o kim tra jitter/wander trn nhiu loi thit b SDH ca cc hng sn xut khc nhau (ALCATEL, SIEMEMS, AT&T, NORTEL, HP, ...), cng vi nhm thc hin ti, xy dng mt gii php hn ch jitter/wander nh sau: tiu chun la chn thit b SDH mi; yu cu v khai thc, vn hnh v bo dng thit b vin thng; hon thin v pht trin mng ng b Vit Nam.Hng pht trin tiCn tip tc nghin cu hn ch nh hng ca jitter/wander i vi cc dch v mi nh truyn d liu tc cao, hi ngh truyn hnh, kt ni gia cc tng i ATM...m sp ti y mng Vin Thng Vit Nam phi m ng.

BNG CC CH VIT TTADMAdd-Drop MultiplexerB ghp knh xenrtAUAdministrative Unitn v qun lAUGAdministrative Unit GroupNhm n v qun]ATM Asynchronous Transfer Mode Kiu truyn dn khng ng bESErrored SecondGiy c liETSI Euruopean Telecomunications Vin tiu chun vin thng chu uStandards InstituteFIFO FISRT IN FISRT OUT Vo truc ra trc (mt dng ca b m) GPSGlobal Positioning SatelliteV tinh nh v ton cuITU-TIntenational TelecomunicationHip hi vin thng quc tUnion (Telecommunication Sector) (lnh vc vin thng)MTIEMaximum Interval ErrorSai s khang thi gian ln nhtMUXMultiplexerB ghp knhPDHPlesiochronous Digital HierachyPhn cp truyn dn s" cn ng bPLLPhase Locked Loop Vngkha phaPRCPrimary Reference Clock ngh thitbSDHSDH Synchronous Digital Hierachy Phn cp truyn dn s ng bSONETSynchronous Otical Network Mng quang ng bSTMSynchronous Transport Module Khi truyn dn ng bUIUnit interval Khong n vUIppUnit interval Peak to Peak Khong n v nh n nhUIrmsUnit interval RMS Khong n v RMSTDEVTime Deviation S lch thi gianTIETime Interval Error Sai s khong thi gianTUGTributary Unit Group Nhm n v nhnhVCVirtual Container B (thng) cha oTI LIU THAM KHO1. ITU-T Recomemdations G.735, G.742, G.751. G.783, G.811, G.813, G.823, G.825, G.958, 0.71, 0.17S2. ETSI Standards ETS 300 417-1, ETS 300 462-1, ETS 300 462-2, ETS 300 462-3, ETS 300 462-4, ETS 300 462-6.3. Townsend A.A.R, Digital line of sight radio links,4. Robert G.Winch.Telecomunication transmission system, 1993.5. David R.Smith.Digital transmission system.6. Dan H.Wolaver, SONET JITTER AND WANDER - Their origin, effects, and Measurement - A guide to Bellcore and ANSI specification, Microwave logic, A Tektronics Company, Rev 3.3. May, 1995.7. Oskar Heilemann.The influence of jilter in digital transmission systems, Wandel & Goltermann - Electronic Measurement technology.8. Milton Gilmour, Davit A Lord, SDH network, Hewlett Parkard, 1977.9. Evaluating tributary jitter from the SDH network, Hewlett Parkard, Application Nole 1258.10. Frequency Agile Jitter Measurement System, Hewlett Packard, Application Note 1267.11. SDH basics, SIEMENT, 1996.12. PDH basics, SIEMENT, 1996.13. Nguyn Kim Sch, Digital Television and HDTV . 1995.14. Mike Sexlon, Andy Reid, Broadband Networking: ATM, SDH and SONET Artech House