B GIAO THNG VN TI TRNG I HC HNG HI B MN: K THUT MY TNH KHOA: CNG NGH THNG TIN

BI GINGTRUYN D LIU

TN HC PHN M HC PHN TRNH O TO DNG CHO SV NGNH

: TRUYN D LIU : 17305 : I HC CHNH QUY : CNG NGH THNG TIN

HI PHNG - 2010

MC LCChng I: CC KHI NIM C BN ................................................................................. 4 1.1. Tin tc - d liu - tn hiu ........................................................................................... 4 1.2 M ha d liu ............................................................................................................. 5 1.3 Cc phng php truyn tin ...................................................................................... 17 Chng II: H THNG TRUYN THNG ........................................................................ 19 2.1. Gii thiu v h thng truyn thng .......................................................................... 19 2.2 H thng truyn s liu .............................................................................................. 21 2.3. Cc h thng truyn s liu thng gp..................................................................... 22 2.4. Mi trng truyn tin................................................................................................ 24 2.5. Cc chun giao tip truyn thng .............................................................................. 37 2.6. Mng truyn thng.................................................................................................... 49 Chng III: K THUT TRUYN S LIU ..................................................................... 50 3.1. Gii thiu v k thut truyn s liu.......................................................................... 50 3.2. K thut nh khung trong truyn s liu .................................................................. 50 3.3. K thut truyn ni tip khng ng b .................................................................... 51 3.4. K thut truyn ni tip ng b ............................................................................... 53 3.5. Cc k thut truy nhp ng truyn ........................................................................ 59 Chng IV: CC VN TRONG TRUYN S LIU .................................................. 63 4.1. Vn pht hin sai v sa sai.................................................................................. 63 ..................................................................................................... 74 ............................................................................................. 87 u l ng .................................................................................... 87 u khin khc phc li .............................................................................. 91 m bo cht lng dch v .......................................................................... 94 Chng V: MNG TRUYN S LIU ............................................................................ 102 5.1. Tng quan............................................................................................................... 102 5.2. Kin trc phn tng v m hnh OSI. TCP/IP .......................................................... 104 5.3. Phn loi mng theo k thut chuyn mch ............................................................. 106 5.4. K thut LAN ......................................................................................................... 109

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YU CU V NI DUNG CHI TITTn hc phn: K thut Truyn d liu B mn ph trch ging dy: K thut my tnh M hc phn: 17305 TS tit 45 L thuyt 45 Thc hnh/Xemina 0 T hc 0 Loi hc phn: 1 Khoa ph trch: CNTT Tng s TC: 2 Bi tp ln 0 n mn hc 0

iu kin tin quyt: Sinh vin phi hc xong cc hc phn sau mi c ng k hc phn ny: Kin trc my tnh, Mch v tn hiu, L thuyt truyn tin, K thut Vi x l, Nguyn l h iu hnh, Cu trc d liu Mc tiu ca hc phn: - Cung cp cho sinh vin nhng khi nim tng quan v K thut truyn s liu, Mng truyn thng... Ni dung ch yu - Chng I: Tng quan - Chng II: H thng truyn thng - Chng III: K thut truyn s liu - Chng IV: Cc vn c bn trong truyn thng - Chng V: Mng truyn s liu Ni dung chi tit ca hc phn: PHN PHI S TIT TS LT BT TH KT 9 9 2 3 2 2 6 5 1 1 3 1 1 8 8 1 7 9 8 1 1 4 1 3 13 12 1 -2-

TN CHNG MC Chng I: Tng quan 1.1. Mt s khi nim v thng tin 1.2. M ho d liu 1.3. Cch truyn thng tin trn ng truyn 1.4. Nhng vn c bn trong truyn thng Chng II: H thng truyn thng 2.1. H thng truyn thng 2.2. Phng tin truyn tin 2.3. Cc chun giao tip trong truyn thng Chng III: K thut truyn s liu 3.1. Tng quan 3.2. Cc k thut truyn s liu Chng IV: Cc vn c bn trong truyn thng 4.1. Tng quan 4.2. M ho pht hin sai, sa sai 4.3. Kim sot ng truyn Chng V: Mng truyn s liu

5.1. Tng quan 5.2. Kin trc phn tng v m hnh OSI, TCP/IP 5.3. Phn loi mng theo k thut chuyn mch 5.4. K thut mng cc b 5.5. Mng v tuyn v mng v tinh 5.6. Mng truyn d liu IDSN/ DSL

1 2 3 2 2 3

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Nhim v ca sinh vin: Tham d cc bui thuyt trnh ca gio vin, t hc, t lm bi tp do gio vin giao, tham d cc bi kim tra nh k v cui k. Ti liu hc tp: William Stalling, Data Computer and Communication ng Vn Chuyt, L thuyt truyn tin, NXB Gio dc Nguyn Vn Thng, C s k thut Truyn s liu, NXB Khoa hc k thut Quch Tun Ngc, X l tn hiu s, NXB Gio dc Nguyn Thc Hi, Mng my tnh v h thng m, NXB Gio dc Nguyn Hng Sn, K thut truyn s liu, NXB Lao ng X hi Hnh thc v tiu chun nh gi sinh vin: - nh gi da trn tnh hnh tham d bui hc trn lp, cc bui thc hnh, im kim tra thng xuyn v im kt thc hc phn. - Hnh thc thi cui k: thi vit rc phch, thi gian lm bi: 75 pht Thang im: Thang im ch A, B, C, D, F im nh gi hc phn Z = 0.2X + 0.8Y. Bi ging ny l ti liu chnh thc v thng nht ca B mn K thut my tnh, Khoa Cng ngh Thng tin v c dng ging dy cho sinh vin. Ngy ph duyt: 15 / 6 / 2010 Trng B mn: ThS. Ng Quc Vinh

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Chng I: CC KHI NIM C BN1.1. Tin tc - d liu - tn hiu D liu (Data): bao gm cc s kin, khi nim hay cc ch th c din t di mt hnh thc thch hp cho vic thng tin, thng dch hay x l bi con ngi hay my mc. Thng Tin (Information): ngha m con ngi qui cho d liu theo cc qui c c th. Tin tc c th biu th bi ting ni, hnh nh, cc vn bn, tp hp cc con s, cc k hiu, thng qua n con ngi hiu nhau . . .. Trong h thng truyn thng, thng ngi ta khng phn bit d liu v tin tc. Thng tin khi truyn: Theo cc dng nng lng khc nhau: m, in, sng quang, sng in Vt mang tin: Mi trng dng mang thng tin (L dng nng lng - C kh nng lu tr, truyn gi thng tin...) Tn hiu (Signal): l tin tc, d liu c chuyn i, x l (bi cc b phn m ha v /hoc chuyn i) cho ph hp vi mi trng truyn thng. Bn cht tn hiu vn l mt hm n tr bin thin theo thi gian hay tn s. C hai loi tn hiu: tn hiu tng t v tn hiu s. Tn hiu tng t (analog): Tn hiu c bt c gi tr no trong mt khong thi gian xc nh. Tn hiu tng t quen thuc c dng hnh sin. Mt tn hiu tng t c th c s ha tr thnh tn hiu s. Ba c im chnh ca tn hiu tng t bao gm: o Bin (Amplitute): o mnh ca tn hiu, n v: decibel (dB) hay volts. Bin cng ln th tn hiu cng mng. o Tn s (Frequency): Tn s (f) ca tn hiu l s dao ng ca tn hiu trong mt n v thi gian (thng tnh bng giy) hay cn gi l tc thay i ca tn hiu trong mt giy, n v Hz hay s chu k trong mt giy. Mt chu k l s di chuyn sng ca tn hiu t im ngun bt u cho n khi quay tr v li im ngun . o Pha (Phase): L n v o v tr tng i ti mt thi im trong mt chu k n ca tn hiu, n c trng cho tnh tr. Tc thay i quan h ca tn hiu i vi thi gian, c m t theo (degree). S dch pha xy ra khi chu k ca tn hiu cha kt thc, v mt chu k mi ca tn hiu bt u trc khi chu k trc cha hon tt. Tn hiu s: L tn hiu m bin ch c mt trong hai gi tr duy nht, tng ng vi hai trng thi logic c trng bi hai s 0 v 1 trong h nh phn. H thng truyn tn hiu ny l h thng truyn nh phn.

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Tn hiu s bao gm ch hai trng thi, c din t vi hai trng thi ON hay OFF hoc l 0 hay 1. Tn hiu s yu cu kh nng bng thng ln hn tn hiu tng t Cc vn khi truyn d liu: Thng dng tn hiu s cho d liu s v tn hiu tng t cho d liu tng t C th dng tn hiu tng t mang d liu s C th dng tn hiu s mang d liu tng t Bit Interval v Bit Rate: Hu ht cc tn hiu s l khng tun hon, chu k v tn s khng xc nh. Hai khi nim t ra y l Bit Interval v Bit Rate. Bit Interval l khong thi gian cn thit gi mt bit. Bit Rate l s lng Bit Interval trong 1 giy, theo cch ni khc, Bit Rate l s bit c gi i trong 1 giy (bps).

1.2 M ha d liu D liu lu tr trong mt my tnh l dng cc bt 0 v 1. c th mang c t ni ny sang ni khc ( trong hoc ngoi my tnh), th d liu thng c chuyn i sang dng tn hiu s. iu ny c gi l s chuyn i tn hiu s sang tn hiu s (digital to digital D/D) hoc m ho d liu s sang tn hiu s. i lc chng ta cn chuyn i mt tn hiu tng t (v d on ni chuyn trong in thoi) sang tn hiu s v mt vi l do no nh gim bt hiu ng ca ting n. iu ny c gi l s chuyn tn hiu tng t sang tn hiu s (A/D) hoc s ho mt tn hiu tng t. Vo lc khc chng ta li cn chuyn mt tn hiu s t u ra ca mt my tnh qua mt phng tin truyn thng c thit k cho dng tn hiu tng t. V d nh vic gi tn hiu t ni ny n ni khc qua mng in thoi cng cng, tn hiu s cung cp bi my tnh s c chuyn sang tn hiu tng t. iu ny c gi l bin i tng t sang s hay iu ch tn hiu s. Thng th mt tn hiu tng t c truyn qua mt khong cch di s dng phng tin truyn thng tng t. V d m thanh, m nhc t mt trm radio, bn thn n l mt tn hiu tng t c pht qua khng kh. Tuy nhin, tn s ca m thanh hoc m nhc khng thch hp cho vic truyn pht. Tn hiu pht i phi c mang bi mt tn hiu c tn s cao hn. iu ny c gi l bin i tng t sang tng t (A/A) hay iu ch tn hiu tng t.Cc phng thc chuyn i

S/s (D/D)

Tng t/s (A/D)

S/tng t (D/A)

Tng t/tng t (A/A)

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1.2.1 D liu s - Tn hiu s Chuyn i hoc m ho s / s l s miu t thng tin dng s sang mt tn hiu s. V d khi chng ta truyn tn hiu t my tnh n my in, c hai d liu gc v d liu c truyn u dng s. Trong kiu m ho ny cc s nh phn 0 v 1 pht ra bi my tnh c chuyn thnh cc xung in th, cc xung ny c th truyn c qua dy dn in.

01011101

M ho s /s Trong rt nhiu k thut m ho s / s, chng ta ch bn n nhng k thut hu dng nht cho vic truyn thng d liu. C 3 loi ph bin: n cc, cc v lng cc c ch ra nh hnh di. M ho s / s

n cc

Cc

Lng cc

M ho n cc: l dng n gin vi ch mt k thut c s dng. M ho cc: c 3 kiu con: NRZ, RZ, v biphase. Hai trong s chng c nhng s bit i phc tp. M ho lng cc c ba s bin i: AMI, B8ZS, v HDB3. 1.2.1.1. M ho n cc M ho n cc rt n gin v th s. Tnh n gin ca n cung cp s ch dn d dng lm c s pht trin cho cc h thng m ho phc tp hn v cho php chng ta nghin cu cc loi bi ton m bt k h thng truyn s no cng phi thc hin. H thng truyn s lm vic da trn xung in cng vi mt kt ni trung gian, thng l dy dn hoc cp. Trong hu ht cc kiu m ho, mc in p cao thp ng vi gi tr nh phn 1 hoc 0. Tnh c cc ca mt xung m ch vic la chn l cc dng hay cc m. M ho n cc c tn nh vy l bi v n ch s dng mt cc. Tnh c cc ny ch nh 1 trong 2 trng thi 0 hoc 1 (thng l 1). Trng thi cn li (thng l 0) c i din bi in p 0. M ho n cc ch s dng mt mc in p (mc in p dng hoc m). Bin

0

1

0

0

1

1

1

0

thi gian Trong v d ny, m nh phn 1 c m ho ng vi gi tr dng v m nh phn 0 c m ho ng vi gi tr 0. Hn na vic m ho n cc khng phc tp v d thc hin. Tuy nhin, m ho n cc c t nht 2 vn lm cho n t mong mun: thnh phn mt chiu v s ng b ho. Thnh phn mt chiu (DC): Bin trung bnh ca mt tn hiu m ho n cc l khc 0. iu ny to ra thnh phn dng mt chiu (DC) mt thnh phn c tn s bng 0. Khi mt tn hiu cha thnh phn DC, n khng th truyn i m khng x l. -6-

ng b ho: Khi mt tn hiu khng n nh, bn nhn khng th xc nh im u v im cui ca mi bit. V th vn ng b ho trong vic m ho n cc c th xy ra bt c khi no dng d liu gm mt lot cc ch s 0 hoc 1. Qu trnh s ho dng s thay i mc in p ch ra s thay i gi tr bit. S thay i tn hiu cng ch ra rng mt bit va kt thc v mt bit mi bt u. Tuy nhin trong m ho n cc mt lot cc bt cng gi tr, nh 7 s 1, tc l khng c s thay i in p, mc in p dng khng b ph v sau 7 ln min l nhn gi tr bit 1. Bt c khi no khng c tn hiu thay i ch ra im bt u ca bit tip theo trong chui, bn nhn phi da trn mt mc thi gian. Chng hn vi tc bit 1000 bps, nu bn nhn xc nh mt in p dng tr 0.005s, m tc c 1 bt l 0.001s, hay 5 bit. S thiu ng b gia ng h ca bn nhn v bn gi lm sai lch thi gian ca tn hiu, v d 5 bt 1 b ko di thnh 0.006s, v do bn nhn s hiu thnh 6 bt 1. Mt bit ph trong dng d liu gy ra mi th sau khi n c gii m nhm. Mt gii php c pht trin iu khin vic ng b ho trong truyn pht mt cc l s dng mt du tch, mc song song mt ng mang mt xung ng h v cho php bn nhn phn chia ng b ho li thi gian ca n. Nhng vic nhn i s ng s dng cho truyn pht ng ngha vi vic lm tng chi ph v v vy s khng kinh t. 1.2.1.2. M ho cc: M ho cc s dng 2 mc in th, mt in p dng v mt in p m. Bng vic s dng c 2 mc, trong phng php m ho cc, mc in th trung bnh trn ng truyn c gim xung v vn v thnh phn DC ca m ho n cc v th c gim nh. Trong m ho Manchester v Manchester vi sai (xem trang sau), mi bit gm c c hai in th dng v in th m, v vy thnh phn DC hon ton c th loi ra. M ho cc s dng 2 mc bin (mc dng v mc m) Trong s rt nhiu kiu m ho cc a dng, chng ta s ch kim tra 3 kiu thng dng nht: nonreturn to zero (NRZ), return to zero (RZ), v biphase. M ho NRZ bao gm 2 cch: nonreturn to zero, level (NRZ-L), v nonreturn to zero, invest (NRZ-I). Biphase cng c 2 phng php. u tin, Manchester l phng php c s dng bi mng LAN. K n, Manchester vi sai, l phng thc c s dng bi mng Token Ring LAN Polar

NRZ

RZ

Biphase

NRZ-L

NRZ-I

Manchester

Manchester vi sai

M ho Nonreturn to Zero (NRZ): Trong m ho NRZ, mc ca tn hiu lun l dng hoc m. Hai phng thc thng dng nht ca vic truyn pht NRZ c trnh by nh sau: M ho NRZ-L: Trong m ho NRZ-L, mc ca tn hiu ph thuc vo kiu ca bit m n trnh by. in th dng quy c l bit 0, tn hiu in th m quy c l bit 1; theo cch mc ca tn hiu ph thuc vo trng thi ca cc bit. Trong NRZ-L mc ca tn hiu ph thuc vo trng thi ca bit -7-

Mt vn c th ny sinh khi c mt dy di cc bit 0 v 1 trong d liu. Bn nhn nhn mt dng in th lin tc v c th xc nh c bao nhiu bit c gi da vo ng h ca chng, iu ny c th c ng b hoc khng c ng b vi ng h ngi gi. M ho NRZ-I: Trong NRZ-I . Mt s o ngc ca in th miu t mt bit 1. S chuyn i trng thi gia in th dng v in th m a ra mt bit 1. Mt bit 0 c miu t nh mt s khng thay i. NRZ-I tt hn NRZ-L v s ng b ho cung cp bi s thay i tn hiu trong mi thi im mt bit 1 gp phi. Hin trng ca chui bit 1 trong lung d liu cho php bn nhn ng b ho thi gian ca n n ni nhn thc s ca vic truyn. Mt chui bit 0 c th gy ra vn , tuy nhin do cc bt 0 khng hn nh vy, chng gim thiu vn xy ra. Trong NRZ-I cc tn hiu c o ngc nu mt bit 1 c gp. Bin 0 NRZ-L 1 0 0 1 1 1 0 thi gian

NRZ-I

thi gian

Trong chui NRZ-L, in th dng v m c ngha r rng; dng i vi 0 v m i vi 1. Trong chui NRZ-I , bn nhn tm kim s thay i t mt mc ny n mc khc nh l c s nhn ra bt 1. M ho Return to Zero (RZ) Nh chng ta c th thy, bt c thi im no th d liu gc cng cha ng cc s 1 v khng lin tip nhau. Bn nhn c th mt v tr ca n. V nh chng ta cp n trong phn tho lun v m ho n cc, mt cch m bo ng b ho l gi cc tn hiu thi gian phn tch trn mt knh phn tch. Tuy nhin gii php ny s lm tng chi ph ng thi d xy ra li ca bn thn chng. Mt gii php tt hn l bng cch no cha ng vic ng b ho trong tn hiu m ho. Mt vi th ging nh gii php c cung cp bi NRZ-I, nhng kh nng trnh by trnh by chui 0 v 1 l nh nhau. Gi tr 0 1 0 0 1 1 1 0 Thi gian

m bo vic ng b ho, cn phi c mt tn hiu thay i cho mi bit. Bn nhn c th s dng nhng thay i ny xy dng, cp nht v ng b ho ng h ca n. Nh chng ta bit trn, NRZ-I thc hin iu ny cho mt chui tun t cc bt 1. Nhng thay i vi mi bit, chng ta cn phi c nhiu hn 2 mc. Mt gii php l m ho theo kiu Return to Zero (RZ), bng vic s dng 3 gi tr: dng, m v khng. Trong RZ, nhng thay i tn hiu khng phi gia cc bt, nhng trong mi bit. Ging nh NRZ-8-

L, in th dng c ngha l 0, v in th m c ngha l 1. Trong khong thi gian ca mt na bit, mt na tn hiu cn li tr v 0. Mt bit 1 thc t c miu t l dng-0 v mt bit 0 c miu t l m -0 s tt hn ch c mt mnh dng v m. S bt li chnh ca m ho RZ l i hi 2 thay i tn hiu m ho 1 bit, v v vy n chim gi gii rng hn. Tuy nhin c ba kh nng chng ta kim tra tt hn, l hiu qu tt nht. M ha Biphase: C l gii php tt nht cho n nay gii quyt vn ng b ho l m ho Biphase. Trong phng php ny s thay i tn hiu trong khong thi gian ca mi bit nhng khng tr v 0. Thay vo n chuyn sang cc i din. Ging nh RZ, gia khong thi gian truyn cho php ng b ho. M ho Biphase c b sung l: Manchester v Manchester vi sai. Manchester: M ho Manchester s dng cch o ngc mi bt trong khong thi gian ca n ng b v miu t bit. Vic chuyn trng thi m-dng tng ng vi bt 1 v dng-m tng ng vi bt 0. y ta s dng vic mt chuyn trng thi n cho hai mc ch. M ho Manchester t c theo mc ca ng b ho nh RZ, nhng ch c 2 gi tr bin . Manchester vi sai: Trong Manchester vi sai vic o ngc trong khong thi gian ca mi bt c s dng cho vn ng b ho, nhng s c mt hoc thiu vng ca vic bin i c thm vo u trong khong thi gian tm ngng c s dng xc nh cho bit. Mt s bin i c ngha l bt 0 v s khng bin i c ngha l bt 1. Manchester vi sai yu cu 2 s thay i tn hiu trnh by bt 0 nhng ch c 1 trnh by bit 1. Bin Thi gian

Manchester

Manchester vi sai

Thi gian

1.2.1.3 M ho lng cc: M ho lng cc, ging nh RZ, s dng 3 mc in th: dng, m v 0. Tuy nhin khng ging nh RZ, mc 0 trong m ho lng cc c s dng miu t bit 0, cn bt 1 ng vi in th m hoc dng. Nu u tin mt bit c miu t bi bin dng, th bt 1 th hai s c miu t bin m, cn bt 1 th ba li c miu t bng bin dng... Vic lun phin ny xut hin c khi cc bit 1 ri rc nhau. Ba kiu ca m ho lng cc thng thng truyn thng d liu l: AMI, B8ZS, v HDB3 Lng cc

AMI

B8ZS

HDB3 -9-

M ho AMI (o du xen k lng cc): M ha AMI l kiu m ho lng cc n gin nht; trong tn gi ca n; t du xut pht t in tn v c ngha l 1. V vy AMI c ngha l o 1 xen k nhau. Mt v tr trung lp, in th 0 s trnh by bt 0. Nhng bt 1 c miu t bi cc in p dng m an xen nhau. Hnh 5.10 a ra v d ny.Bin

0

1

0

0

1

1

1

0Thi gian

Mt s bin i ca AMI c gi l gi ba bc vi bt 0 nm xen k gia in th dng v in th m. Vi vic o ln trong mi ln xut hin ca 1, AMI t c 2 mc ch: th nht cc thnh phn DC l 0, v th 2 mt dy di cc s 1 c ng b ho. Khng c k thut no chc chn vic ng b ho cho mt chui di cc s 0. Hai bin i ca AMI va c pht trin gii quyt vn ng b ho chui 0, c bit cho vic truyn pht khong cch ln. u tin, c s dng Bc M, c gi l B8ZS (lng cc thay th 8-zero). Th hai, c s dng Nht v Chu u, c gi l HDB3 (lng cc mt cao 3). C hai kiu ny u l s thch nghi vi AMI m ch thay i mu gc trong trng hp c nhiu chui 0 lin tip. B8ZS: B8ZS l mt s tho thun c chp nhn Bc M cung cp vic ng b ho cho chui 0. Trong tt c cc tnh hung, cc chc nng B8ZS tng t nh ca AMI. AMI thay i cc vi mi 1 gp phi. Nhng thay i ny cung cp s ng b ho cn thit bi bn nhn. Nhng tn hiu khng thay i trong sut chui 0, v vy vic ng b ho thng b mt. S khc nhau gia B8ZS v AMI xut hin bt c khi no c 8 hoc nhiu hn cc bt 0 lin tip gp phi trong dng d liu. Gii php cung cp bi B8ZS l p t s thay i tn hiu gi bn trong chui 0 (c gi l violation). mi thi im c 8 bit 0 xut hin lin tip, B8ZS a vo nhng thay i trong mu da trn s khc bit ca bit 1 trc (1 ch xut hin pha trc ca chui 0).

Nu bt 1 trc l dng, 8 bt 0 s c m ho l 0, 0, 0, +, -, 0, -, +. Hy nh rng bn nhn ang tm kim s thay i xc nh 1. Khi n thy c 2 in tch dng lin tip bao quanh 3 bt 0, n nhn ra mu, tnh ton v a vo violation khng gy ra li. Sau - 10 -

n tm kim phn cn li ca nhng violation trng i. Khi tm thy chng, bn nhn chuyn 8 bt thnh 0 v quay tr li ch AMI thng thng. Nu cc ca bt 1 trc l m, mu ca cc violation s l tng t nhng o ngc li cc. C mu dng v m c ch ra trong hnh 1.13. HDB3: Vn ng b ho chui lin tip cc s 0 c gii quyt Nht v Chu u khc vi M. Quy c ny, gi l HDB3, a s thay i vo mu AMI mi thi im 4 bt 0 lin tip thay cho 8 bit nh ca Bc M. Mc d tn gi ca n l HDB3, cc mu thay i bt c khi no c 4 bt 0 lin tip.

Nh trong B8ZS, mu ca cc violation trong HDB3 c da trn s phn cc ca bt 1 trc . Tuy nhin khc vi B8ZS, HDB3 cng nhn vo s cc bt 1 va xut hin trong dng bt k t ln thay th cui cng. Bt c khi no s ca bt 1 k t ln thay th cui cng l l, B8ZS y mt violation vo v tr th 4 ca cc bt 0 lin tip. Nu s phn cc ca bt trc l dng, violation l dng. Nu s phn cc ca bit trc l m, violation l m. Bt c khi no s ca bt 1 k t ln thay th cui cng l chn, B8ZS y violation vo v tr u tin v v tr th 4 ca cc bt 0 lin tip. Nu cc ca bt trc l dng, c hai violation l m. Nu cc ca bit trc l m, c hai violation l dng. V d 1: S dng B8ZS, m ho dng bt 10000000000100; p dng vi cc ca bt 1 u tin l 1. Bin 1 0 0 0 0 0 0 0 0 0 0 1 0 0 Thi gian

V d 2: S dng HDB3, m ho dng bt 10000000000100; p dng vi s cc bt 1 trc n l chn v bt 1 u tin l dng. Bin 1 0 0 0 0 0 0 0 0 0 0 1 0 0 Thi gian

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1.2.2. D liu tng t tn hiu s i khi chng ta cn s ho mt tn hiu tng t. V d gi ging ni ca con ngi trong mt khong cch xa. Chng ta cn s ho n gim nhiu v n. Vn ny c gi l chuyn i tn hiu tng t v tn hiu s hoc l s ho tn hiu tng t.

Analog/Digital conversion (codec)

Trong s chuyn i t tn hiu tng t v tn hiu s, chng ta a ra cc thng tin di dng sng lin tc nh l mt dy nhp ca cc tn hiu s (1s hoc 0s). Chuyn i tn hiu tng t v tn hiu s cn s dng mt vi tn hiu s c trnh by ti phn 5.1. Cu trc ca s chuyn i l khng c vn g. Vn l lm nh th no chuyn cc thng tin t s lng ln cc gi tr ti s ri rc ca cc gi tr m khng nh hng ti gic quan con ngi v cht lng ca tn hiu. 1.1.1.1.iu ch bin xung (Pulse Amplitude Modulation - PAM) Bc u tin trong qu trnh chuyn i tn hiu tng t v tn hiu s c gi l iu ch bin xung. K thut ny thao tc vi tn hiu thng t, n gin n v to ra nhng di ca xung c bn c kt qu mu tn hiu tng t. Thut ng mu c ngha l n v bin ca tn hiu trong 1 khong thi gian l bng nhau. L thuyt mu c dng trong iu ch bin xung l hiu qu hn trong cc vng khc ca nng lng, n l d liu truyn thng. Tuy nhin PAM l nn tng quan trng trong chuyn i tn hiu tng t v tn hiu s l thuyt ny c gi l iu ch xung theo m (pulse code modulation - PCM) Trong PAM, tn hiu ban du l mu ti thi gian ngh bng nhau Amplitude Amplitude

Time a. Anaglog signal b. PAM signal

Time

PAM gi l k thut mu v gi. hin ti, cp tn hiu l c, tip theo l tr gip tng hp. Nhng mu gi tr xy ra ch trong s xut hin ca sng, nhng nhn chung kt qu trong PAM vn cn ngn. L do PAM khng hu dng trong d liu truyn thng l, mc d n truyn i sng gc t mt di ca xung, nhng xung ny vn cn c bin (vn l tn hiu tng t, khng phi l tn hiu s). to ra tn hiu s chng ta phi sa i chng bng cch dng PCM. Lu : PAM c mt vi ng dng, nhng bn thn n khng dng trong d liu truyn thng. Tuy nhin y l bc quan trng ph bin u tin trong l thuyt chuyn i v c gi l PCM. 1.2.2.2. iu ch xung theo m (PCM) PCM sa i vn to ra xung bng PAM hon thnh mt tn hiu s. Lm nh vy, PCM u tin lng t ho nhng xung ca PAM. S lng t l l thuyt tha hng nhng gi tr trong mt dy bit ti mu c bit. - 12 -

Amplitude

+127

+125 +110 +90 +88 +77

+38 +24

+48 +39 +26

+52

-15

Time

-50

-80 Hnh 1.18 trnh by 1 mu phng thc ca tha k tn hiu v gi tr ln lng t ho mu. Mi gi tr chuyn i vo tng ng 7 bt nh phn. Bt th 8 l bt du (+ l 0 v l 1). +024 00011000 -015 10001111 +125 01111101 +038 +048 +039 +026 00100110 00110000 00100111 00011010 -080 -050 +052 +127 11010000 10110010 00110110 01111111 +110 +090 +088 +077 01101110 01011010 01011000 01001101

Tc ly mu (Sampling Rate). Nh chng ta thy t cc hnh v trc, s chnh xc ca mi tn hiu s c ti to li t cc tn hiu tng t ph thuc vo s mu em theo. S dng PAM v PCM chng ta ti to li sng chnh xc bng cch em theo s lng mu khng xc nh, hoc chng ta c th ti to s pht ra trng khng ca chnh n. R rng chng ta mun tm mt s u trn trc s. Nh vy, cu hi l C bao nhiu mu th ? Trn thc t, rt t thng tin ng ch cho vic gi v nhn to li cu trc ca tn hiu tng t. Theo nh l Nyquist m bo tnh chnh xc trong s ti to tn hiu tng t nguyn bn ta s dng PAM, s tc ly mu phi tin hnh hai ln tn s cao nht ca tn hiu gc. Nh vy, nu chng ta mun mu ging ni in thoi vi tn s ln nht 4000 Hz, chng ta cn tc ly mu ca 8000 mu trn 1 giy. Lu : Theo nh l Nyquist, s tc ly mu t nht phi tin hnh 2 ln tn s cao nht. Mt s tc ly mu hai ln tn s x Hz c ngha tn hiu phi c ly mu mi x giy. Dng ging ni qua ng dy in thoi trn l mt v d, iu ny c ngha l mt mu mi 1/8000 giy. hiu r hn v nh l xt v d sau: Tc ly mu l bao nhiu di bng rng 10000 Hz (1000 ti 11000)? - 13 -

Gii: Tc ly mu hai ln tn s cao nht ca tn hiu Tc ly mu = 2(11000) = 22000 mu/giy. Amplitude Tn s cao nht = x Hz Tc mu = 2x mu/giy

Time

Thi gian gia 2 mu = x

Bao nhiu bit cho mt mu? Sau khi chng ta tm c tc ly mu, chng ta cn xc nh s bit cn truyn cho mi mu. iu ny phc thuc vo mc ca chnh xc cn thit. S bit l nhng la chn cho tn hiu gc cn ti to vi chnh xc mong mun trong bin . V d: Mt tn hiu l mu, mi mu yu cu t nht 12 mc ca chnh xc (+0 n +5 v 0 n -5). Bao nhiu bit s c gi cho mi mu? Gii: Chng ta cn 4 bit: 1 bit cho du v 3 bit cho gi tr. 3 bit gi tr c th biu din bng 23 = 8 mc (000 n 111), iu l nhiu hn ci ta cn. Vi 2 bit gi tr l khng v 22 = 4. Vi 4 bit gi tr th qu ln v 2 4 = 16(tha qu nhiu). Tc Bit (Bit Rate). Sau khi tm c s bit trn mi mu, ta cn tnh ton Bit Rate theo cng thc: Bit Rate = Tc ly mu x S bit trn mi mu V d: Chng ta mun s ho ging ni con ngi. Bit Rate l g? vi gi thit 8 bit trn mt mu. Gii: Ging ni bnh thng ca ngi thng tn s t 0 n 4000 nh vy tc ly mu l: Tc ly mu = 2 x 4000 = 8000 mu/giy. Bit Rate c th tnh ton nh sau: Bit Rate = Tc ly mu x S bit trn mt mu = 8000x8 = 64000 bit/s = 64 Kbps. 1.2.3. D liu s - Tn hiu tng t Bin i D/A (hay cn gi l iu bin D/A) l qu trnh thay i mt trong nhng c trng ca tn hiu tng t da vo thng tin trong tn hiu s (0 v 1). Khi truyn d liu t mt my tnh n my tnh khc qua ng in thoi cng cng, chng hn nh: d liu gc l s, nhng v cc ng dy in thoi mang cc tn hiu tng t, nn d liu phi c chuyn i. D liu s phi c iu bin thnh tn hiu tng t, iu c thc hin trng nh hai gi tr phn bit tng ng vi s nh phn 0 v 1. - 14 -

C nhiu thit b bin i D/A, nhng chng ta s ch bn n nhng thit b c li nht cho vic truyn d liu. Mt tn hiu hnh sin c 3 c trng: bin , tn s v pha. Khi thay i mt trong 3 c trng ny, ta s to c phin bn mi ca tn hiu hnh sin . Chng hn tn hiu gc l mc 1, c th bin i thnh mc 0 hoc ngc li. V th, bng s thay i lin tc hnh dng ca mt tn hiu in n gin, ta c th dng n m t d liu s. Bt c ba c trng trn c th c thay i theo cch ny, a cho ta t nht 3 thit b bin i d liu s thnh tn hiu tng t: ASK (Amplitude Shift Keying - di bin ). FSK (Frequency Shift Keying - di tn s). PSK (Phase Shift Keying - di pha). Hn na, thit b th t tt hn c l thay i hn hp ca c bin , tn s v pha c gi l b iu ch bin cu phng QAM (Quadrature Amplitude Modulation). QAM l hiu qu nht so 3 thit b trc, v l thit b c dng trong tt c cc MODEM hin i (hnh 5.23).

Cc thnh phn ca b bin i D/A: Trc khi tho lun cc phng php c th bin i D/A, c hai vn c bn phi c nh ngha: tc truyn bit/baud v tn hiu mang. Bit Rate and Baud Rate: Hai thut ng c dng thng xuyn trong vic truyn d liu l bit rate v baud rate. Bit rate l s bit c truyn trong mt giy. Baud rate ch ra s n v tn hiu trn mt giy c yu cu m t nhng bit . Khi tho lun v hiu qu my tnh, th bit rate l quan trng hn, v ta mun bit thi gian x l tng mu tin.Tuy nhin, trong vic truyn d liu, th ch trng n tnh hiu qu ca vic chuyn d liu t ni ny n ni khc trong cc mu tin hay cc khi tin. Cc n v tn hiu t hn c yu cu l hiu qu hn cho h thng v bng thng hp hn c yu cu truyn cc bit; v vy, chng ta ch trng hn vo baud rate. Baud rate xc nh bng thng yu cu gi tn hiu. Baud rate = Bit rate / (s bt trn mt n v tn hiu) 1.2.3.1 iu ch bin (ASK) Cng tn hiu mang c bin i sang dng s nh phn 0 hoc 1. Trng hp ny tn s v pha l khng i trong khi bin thay i. in p biu din mc 1 v m t mc 0 l sang tri vi h thng. Khong thi gian 1 bit l l giai on c nh ngha l 1 bit. nh bin ca tn hiu trong sut khong thi gian mi bt l khng i v s thay i ca n ph thuc vo bit (0 hoc 1). Tc truyn dng ASK b gii hn bi cc tnh cht vt l ca mi trng truyn. - 15 -

Khng may, vic truyn ASK l nhy cm cao vi nhiu. Thut ng ting n (nhiu) m ch n in p khng nh trc can thip vo ng truyn bi cc hin tng khc nh l s nng ln hay in t c sinh ra bi cc ngun khc. Cc in p khng nh trc ny trn ln vi tn hiu lm thay i bin . Mc 0 c th b i thnh mc 1 hoc ngc li. R rng nhiu l vn kh gii quyt i vi ASK, do ch c da vo bin nhn bit. Nhiu thng tc ng ln bin ; v th ASK l phng php iu ch b nh hng nht bi nhiu. K thut ASK ph bin c gi l OOK (on-off-keying). Vi OOK, cc gi tr bit c m t khng theo in p. S tin b l gim bt trong nng lc yu cu truyn thng tin. 1.2.3.2 iu ch tn s (FSK) Tn s tn hiu mang c bin i m t cc ch s nh phn 0 hoc 1. Tn s tn hiu mang trong sut thi gian mi bit l khng i, gi tr ca ph thuc vo bit 0 hoc 1: cn li bin v pha l khng i. FSK trnh c hu ht cc vn nhiu ca ASK. Bi v thit b nhn coi tn s tiu biu thay i qua s chu k cho, n c th b qua cc nh in p. Cc yu t gii hn ca FSK l mi trng vt l ca sng mang. Bng thng FSK Mc d FSK dch gia 2 tn s mang, n d phn tch thnh 2 tn s trong cng thi im. Ta c th ni ph FSK l s hn hp ca 2 ph ASK tp trung quanh tn s f c0 v fc1. bng thng yu cu cho vic truyn FSK bng tc baud rate ca tn hiu cng vi tn s dch (khc nhau gia hai tn s mang): BW = (fc1-fc0) + Nbaud.

1.2.3.3 iu ch pha (PSK) Pha ca sng mang c bin i biu din sang s nh phn 0 hoc 1. C bin v tn s l khng i, cn pha th thay i. V d, nu ta bt u vi mt pha 0 o m t bt 0, th ta c th thay i pha sang 180 o gi s nh phn 1. Pha ca tn hiu trong sut thi gian mi bt l khng i v gi tr ca n ph thuc vo bit 0 hoc 1. - 16 -

Phng php trn thng c gi l 2-PSK hoc PSK nh phn, bi v 2 pha khc nhau (0o v 180o) c dng. Hnh 5.30 ch r iu ny bng mi quan h gia pha sang gi tr bit. S th hai, c gi l chm sao (constellation) hoc s trng thi pha, ch ra cng mi quan h c minh ho ch bng cc pha. 1.3 Cc phng php truyn tin 1.3.1 Cu trc knh truyn Truyn song song (Parallel) Mi bit dng mt ng truyn ring. Nu c 8 bits c truyn ng thi s yu cu 8 ng truyn c lp truyn d liu trn mt ng truyn song song, mt knh truyn ring c dng thng bo cho bn nhn bit khi no d liu c sn (clock signal) Cn thm mt knh truyn khc bn nhn bo cho bn gi bit l sn sng nhn d liu k tip

Truyn ni tip (Serial) Tt c cc bit u c truyn trn cng mt ng truyn, bit ny tip theo sau bit kia Khng cn cc ng truyn ring cho tn hiu ng b v tn hiu bt tay (cc tn hiu ny c m ha vo d liu truyn i)

- 17 -

C hai cch truyn ni tip: Bt ng b: Khi truyn bt ng b ngi ta pht tng k hiu ring r, cch bit nhau v phn bit cc k t ngi ta thm tn hiu u (start) v cui (stop) vo mi k t. ng b: Khi truyn ng b nhn bit gi tr cc thi im l 0 hay 1 cn phi c tn hiu xung clock gi l tn hiu ng b. Tn hiu ng b c chu k l T, ngha l mi giy 1 ngun s cung cp bits. T 1.3.2 Ch truyn tin Mch n cng (mt chiu: simplex): thng tin ch c th truyn t ngun sang thit b thu m chiu ngc li khng th thc hin c. V d: d liu c truyn t my tnh sang my in.Simplex operation

One-way only

Mch bn song cng (hai chiu ngt qung: half duplex). Hai thit b u cui c th truyn d liu cho nhau ti nhng thi im khc nhau.Half-duplex operation Two-way but not at the same time

Mch song cng (hai chiu ton phn: duplex). Hai thit b u cui c th truyn d liu cho nhau ng thi.Full-duplex operation Both-way at the same time

CU HI V BI TP 1.1. Anh ch hy trnh by v cc loi tn hiu c s dng trong truyn dn hin nay. Nu cc c trng, u nhc im ca chng 1.2. Trnh by hiu bit ca anh ch v cc i lng lin quan n tn hiu truyn dn. Nu cc khc phc cc nhc im (nu c) 1.3. Trnh by hiu bit ca anh ch v cc phng thc truyn dn hin nay. Cc u nhc im ca cc phng thc ny. 1.4. Cho chui bt cn truyn: 1001 0111 0011 1010 1101 1111 0101 1101. Anh ch hy m ha chui bt trn theo phng php m ha NRZ-I, 3 bc, Manchester, FSK. 1.6.Cho chui k t cn truyn theo m ASCII m rng l TEST. Anh ch hy m ha chui k t trn theo phng php m ha NRZ, AMI, Manchester, FSK. 1.7.Tn hiu xung PAM c ly mu vi tc 12 KHz t tn hiu tng t dng sng hnh sin 1KHz. Hy xc nh xung PAM t nhin, xung PAM nh phng, Xung DM. 1.8. Mt my thu DM thu c chui bt nh phn: 1001 0111 0011 1010 1101 1111 0101 1101. Anh ch hy v tn hiu tng t tm kim xut hin ti u ra ca b tch phn - 18 -

Chng II: H THNG TRUYN THNG2.1. Gii thiu v h thng truyn thng H thng truyn thng c hiu l mt h thng ton din gip truyn s liu t ni ny n ni khc. Xt m hnh mt h thng truyn thng tiu biu : Lut 1 Lut 2 Lut nD liu

Lut 1 Lut 2 Lut n

Sender Mi trng truyn

Receiver

Khi xt mt h thng truyn thng ta xt ti cc yu t sau: Sender: h thng pht L h thng pht d liu. Receiver: h thng thu L h thng thu d liu nhn c t h thng pht Hnh trng mng (Topology): cu hnh chi tit ca ng truyn v cc hthng Giao thc mng (Protocol): Tp hp cc qui tc, qui c v phng thc truyn, v khun dng d liu m khi tham gia truyn thng cc thit b trong h thng phi tun th. Mi trng truyn: cc vt dn cho php d liu c truyn ti trn truyn t ni ny sang ni khc. D liu: Thng tin truyn c nh dng cc dng khc nhau. Cc dng thng thy ca thng tin gm: Text: H thng m ha u tin lin quan n vn bn l h thng m Morse, c s dng rng ri trc khi c my tnh.

y l mt b m nh phn s dng 2 k t chm (.) v gch (-) s ha vn bn (c th xem tng ng vi cc bit 0 v 1). Tuy nhin n c im bt li l ngho nn: t cc k t c m ha. N s dng s phi hp ca cc du gch v du chm vi di khc nhau, iu ny khng c tin li c bit cho cc k t c tn sut xut hin ging nhau. Chnh v th n khng c dng s ha thng tin. Nu chng ta qui nh rng s bit dng m ha cho mt k t phi bng nhau th vi p bit ta c th m ha cho 2p k t. Ngy nay, text c tn ti di dng 1 chui bit (0 hoc 1). Chui c s dng th hin 1 k t gi l - 19 -

m (code). Cc m Unicode thng s dng 16 hoc 32 bit biu din v m ASCII s dng 1 byte biu din. S: Biu din di dng 1 chui bit c gi tr nh phn tng ng vi gi tr s (Khng biu din di dng m ASCII)vd: 8 1000 Hnh nh tnh:nh s tht s l mt nh c v nn t cc ng thng v mi ng thng c xy dng bng cc im. Mt nh theo chun VGA vi phn gii 640x480 c ngha l mt ma trn gm 480 ng ngang v mi ng gm 640 im nh (pixel).

Mt im nh c m ha ty thuc vo cht lng ca nh: nh en trng: s dng mt bit m ha mt im: gi tr 0 cho im nh mu en v 1 cho im nh mu trng. nh gm 256 mc xm: mi im c th hin bng mt byte (8 bits ; nh mu: ngi ta chng minh rng mt mu l s phi hp ca ba mu c bn l (Red), xanh l (Green) v xanh dng (Blue). V th mt mu bt k c th c biu bin bi biu thc: x = aR + bG +cB Trong a, b, c l cc lng ca cc mu c bn. Thng thng mt nh p s c lng mu vi gi tr t 0 n 255. V nh th, mt nh mu thuc loi ny c th hin bng 3 ma trn tng ng cho 3 loi mu c bn. Mi phn t ca mng c gi tr ca 8 bits. Chnh v th cn c 24 bit m ha cho mt im nh mu. Kch thc ca cc nh mu l ng k, v th ngi ta cn c phng php m ha gim kch thc ca cc nh. m thanh v video: D liu kiu m thanh v video thuc kiu tn hiu tun t. Cc tn hiu tun t c s ha theo cch thc sau y: 1 - Ly mu Tn hiu c ly mu: vi tn s f, ta o bin ca tn hiu, nh th ta c mt lot cc s o. 2 - Lng ha Ta xc nh mt thang o vi cc gi tr l ly tha ca 2 (2p) v thc hin vic ly tng ng cc s o vo gi tr thang o. 3- M ha Mi mt gi tr sau c m ha thnh cc gi tr nh phn v t vo trong cc tp tin. 111110101110110010...... - 20 -

Dung lng tp tin nhn c ph thuc hon ton vo tn s ly mu f v s lng bit dng m ha gi tr thang o p (chiu di m cho mi gi tr). 2.2 H thng truyn s liuNhiu

Mi trngG(t)

S(t) m(t)

G(t)

m(t)

S(t)

DTE

DCE

DCE

DTE

DTE (Data Terminal Equipment): thit b u cui d liu vi chc nng truyn cc d liu t ngi (ti ngi dng) s dng di dng s hoc tng t DCE (Data Circuit_Terminal Equipment): thit b chuyn i d liu vi chc nng chuyn i cc tn hiu t DTE sang dng tng thch vi mi trng truyn. Nhiu:

. .

.

. .

,m

. Trong h thng trn , cc tn hiu tng ng: S(t) S(t) M(t) M(t) G(t) G(t) Nu cc tn hiu tng ng cng khc xa nhau chng t s nh hng ca nhiu vi h thng cng ln. - 21 -

Vd: H thng internet ADSL s dng trn h thng thoi vn c:

Knh thoi

Model

Model

Phn loi h thng truyn s liu: A- Phn loi theo a l: 1) Mng cc b: 2) Mng din rng: 3) Mng ton cu: B- Phn loi theo tnh cht s dng 1) Mng truyn s liu k sinh: 2) Mng truyn s liu chuyn dng: C- Phn loi theo hnh trng mng 1) Mng tuyn tnh: 2) Mng hnh sao: 3) Mng vng: 4) Mng hn hp D- Phn loi theo k thut 1) Mng chuyn mch knh: 2) Mng chuyn mch thng bo: 3) Mng chuyn mch gi: E- Phn loi theo kin trc phn tng mng: 1) Mng OSI 2) Mng ring cng ty 3) Mng theo kin trc TCP/IP 4) . 2.3. Cc h thng truyn s liu thng gp :

- 22 -

: : : . : . 2.3.2. 2 bit 0-1) thp /D )

.

.

thp thp

/A

: . (baud rate). 1 giy. 2.3.3.

.

knh Ch ny hot ng theo m hnh ca h thng in thoi. c th giao tip vi my B, my A phi thc hin mt cuc gi (call). Nu my B chp nhn cuc gi, mt knh o c thit lp dnh ring cho thng tin trao i gia A v B. Tt c cc ti nguyn c cp cho cuc gi ny nh bng thng ng truyn, kh nng ca cc b hon chuyn thng tin u c dnh ring cho cuc gi, khng chia s cho cc cuc gi khc, mc d c nhng khong ln thi gian hai bn giao tip im lng. Ti nguyn (bng thng) s c chia thnh nhiu nhng phn bng nhau v s gn cho cc cuc gi. Khi cuc gi s hu mt phn ti nguyn no , mc d khng s dng n n cng khng chia s ti nguyn ny cho cc cuc gi khc. Vic phn chia bng thng ca knh truyn thnh nhng phn c th c thc hin bng mt trong hai k thut: Phn chia theo tn s (FDM Frequency Division Multi Access) hay phn chia theo thi gian (TDMA- Time Division Multi Access).

- 23 -

2.3.2 Trong phng php ny, thng tin trao i gia hai my tnh (end systems) c phn thnh nhng gi tin (packet) c kch thc ti a xc nh. Gi tin ca nhng ngi dng khc nhau (v d ca A v B) s chia s nhau bng thng ca knh tr hut lu v chuyn tip (store and forward), tc lu li cc gi tin cha gi gi chng i.

2.4. Mi trng truyn tin Trong h thng truyn d liu, mi trng truyn dn l ng truyn vt l gia ni gi v ni nhn. Phng tin truyn thng c phn lm hai loi l c dn v khng dn. Trong c loi trn, truyn thng thc cht l truyn sng in t. Vi phng tin truyn c dn, sng in t c truyn dc theo mt phng tin truyn dn nh cp xon i, cp ng trc hay cp quang. Kh quyn v khng gian xung quanh ta c xem nh l v d v phng tin truyn khng dn, iu c ngha l cc tn hiu in t c truyn i nhng khng theo mt hng nht nh v n thng c gi l mng khng dy. Cc c trng v cht lng ca vic truyn d liu c xc nh bng c cht lng ca phng tin truyn dn v ca tn hiu truyn dn. i vi mi trng truyn c dn, phng tin truyn dn l thnh phn quan trng xc nh gii hn truyn dn. Vi phng tin truyn khng dn, bng thng ca tn hiu dc cung cp bi ng ten quan trng hn mi trng truyn dn. c tnh ca tn hiu c truyn bi ng ten l nh hng hoc a hng. Thng thng th cc tn hiu c tn s thp thng l a hng, iu c ngha l cc tn hiu c pht theo mi hng t ng ten. Vi cc tn hiu c tn s cao, cc tn hiu c tp trung truyn theo mt ng thng. Trong khi thit k mt h thng truyn d liu th vn cn c quan tm l tc truyn d liu v khong cch truyn d liu - 24 -

2.4.1 Mt s khi nim Bng thng: Nu bng thng cng ln th tc truyn d liu t cng mt ngun cng cao. Cc h hi truyn dn: Cc h hi nh l s suy gim, gii hn khong cch, vi mi trng truyn c dy, thng thng th cp xon i c s h hi v tn hiu ln hn cp ng trc, v cp ng trc li b h hi tn hiu nhiu hn cp quang. Nhiu giao thoa: S xung t ca cc tn hiu trong cng mt di tn c th lam sai lch hoc lm mt cc tn hiu. Nhiu giao thoa l c trng ring ca cc phng tin truyn khng dy, tuy nhin n cng l mt vn i vi phng tin truyn c dy. Vi phng tin truyn c dy, nhiu giao thoa c th gy ra bi cc cp gn nhau.V d cp xon i thng b li vi nhau, v mt ng dn thng cha nhiu cp do s dn n nhiu. Nhiu giao thoa cng c xut hin trong truyn khng dn. Nu c bc tt th phng tin truyn c dn c th gim ti a vn ny. S thit cc b nhn: Mt phng tin truyn dn c th c s dng to ra kt ni im-im hoc c dng chia s ng truyn vi cc thit b khc. Trong trng hp phi chia s ng truyn vi cc thit b khc, cc thit b c chia s s gy ra nhiu cng nh s sai lch cc tn hiu trn ng truyn, v th s dn ti s gii hn khong cch cng nh tc truyn d liu.

2.4.2 Phng tin truyn c dn Vi phng tin truyn c dn, kh nng truyn c th hiu theo ngha tc d liu hoc bng thng, ph thuc ch yu vo khong cch v mi trng truyn thng trong kiu kt ni l im - im hay a im, nh trong mng LAN. Bng 2.1 di y cho ta bit cc c im chung ca phng tin truyn c dn thng dng vi ng dng truyn im - im ng di. Cc phng tin truyn c dn thng c s dng l cp xon i, cp ng trc v cp quang.

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2.4.2.1. Cp xon i Phng tin truyn c dn r nht v c s dng nhiu nht l cp xon i M t vt l: Mt cp cp xon i bao gm hai dy bc ng cch in c sp xp theo mt quy tc chun. Mi cp dy hot ng nh l mt kt ni truyn thng n. Mt s cp ny c b li vi nhau thnh mt cp c bc trong mt v bo v. Mt dy cp c th cha hng trm i dy dn. Vic xon cc dy li lm gim nhiu xuyn m, nhiu giao thoa gia cc cp gn nhau trong dy cp. Nhng cp dy gn nhau trong cng mt b c di khc nhau gp phn lm gim nhiu xuyn m, nhiu giao thoa. Trn mt kt ni ng di, di ca cc cp dy thng khc nhau khong t 5 ti 15 cm.Cc dy trong mt cp c ng knh t 0,4 n 0,9 mm. ng dng: Cp xon i c dng ph bin trong truyn dn c tn hiu s v tn hiu tng t. N thng c s dng trong mng in thoi v l phng tin truyn dn trong vic truyn thng ca ni b mt to nh. Trong mng in thoi, tp hp cc in thoi ca nhng gia nh ring l s c kt ni ti mt tng i cc b bi dy cp xon i v c gi l ng thu bao. Trong mt ta nh, mi in thoi s c kt ni bng cp xon i chy ti tng i con trong phng hoc ti tng i chnh ca ta nh. Cp xon i c thit k h tr truyn ting ni s dng tn hiu tng t. Tuy nhin, vi s h tr ca modem, cp xon i c th iu khin truyn d liu s vi tc va phi. Cp xon i cng l phng tin thng dng nht trong vic truyn tn hiu s. Tc truyn d liu s ca mng cc b trong mt to nh thng vo khong 64kbps. Chng ch c s dng rng ri trong mt to nh cho mng cc b kt ni cc my tnh c nhn. Tc truyn d liu ca mng ny thng l khong 10Mbps.Tuy nhin, mt s mng s dng cp xon i vi tc truyn t 100Mbps ti 1Gbps cng c thit k, mc d chng b hn ch v s lng cc thit b v phm vi ca mng. Vi cc mng xa nhau th tc truyn d liu trn cp xon i khong 4Mbps.

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Cp xon i r hn nhiu so vi cc thit b truyn dn khc nh cp ng trc, cp quang v chng d s dng hn. Tuy nhin chng li b hn ch hn v tc truyn d liu cng nh khong cch ca mng. Cc c im truyn dn: Cp xon i c th truyn c tn hiu s v tn hiu tng t. Vi tn hiu tng t, b khuych i tn hiu truyn c lp t cch nhau khong t 5 n 6 km. Vi d liu s (bng c tn hiu s v tn hiu tng t), cc b lp c t cch nhau 2-3 km.

So vi cc loi phng tin truyn dn khc(cp ng trc, cp quang), cp xon i b gii hn bi tc truyn d liu, bng thng v khong cch. Hnh 4.3 cho thy, s suy gim tn hiu ca cp xon i l mt hm tng nhanh theo tn s. Cp xon i cng b nh hng mnh bi cc h hi khc. Phng tin ny d bi nhiu giao thoa v nhiu tp bi v n d xy ra hin tng giao thoa gia cc trng in t . V d, mt dy dn chy song song vi mt dng in xoay chiu s tng thm 60Hz nng lng. Nhiu xung cng d nh hng n cp xon i. Cc dy c bc s lm gim s giao thoa. Cc dy c xon vo nhau lm gim giao thoa ca sng c tn s thp v vic s dng cc dy c di khc nhau ca cc cp lin k lm gim nhiu xuyn m. Vi tn hiu tng t trong kt ni im im, mt bng thng c th ln ti 1 MHz, n h tr mt s knh. i vi vic truyn tn hiu s im im ng di, d liu c th truyn vi tc ln ti vi Mbps, vi khong cch ngn, tc truyn d liu c th ln ti 100 Mbps thm ch ln ti 1 Gbps. Cp xon i c v v khng v bc Cp xon i c 2 loi: c v bc v khng c v bc. Cp xon i khng v bc (UTP) u tin c s dng lm dy in thoi. y l phng tin truyn dn r tin nht thng c s dng trong cc mng LAN. N rt d lp t v s dng. Cp xon i khng v bc ch yu b nh hng bi nhiu giao thoa sng in t, bao gm giao thoa t cc cp xon i gn nhau v t nhiu ca mi trng bn ngoi. Mt cch ci thin cc c trng ca cp xon i l ta bc si cp bng cc v bc li kim loi nhm lm gim s giao thoa. Cp xon i c v bc cho php truyn d liu vi hiu sut tt hn tc cao hn. Tuy nhin, n li t hn v kh lp t hn so vi cp khng bc. Cp UTP loi 3 v loi 5 Hu ht cc vn phng u c lp t sn loi cp xon i 100 ohm thng c gi l cp in thoi. V cp xon i in thoi thng c lp t sn, n cng thng c s dng cho cc ng dng LAN. Tuy nhin tc d liu v khong cch gia cc thit b ca cp xon i in thoi l kh gii hn. - 27 -

Chun EIA-568-A phn bit ba loi cp UTP: Loi 3: Cp UTP kt hp vi cc phn cng kt ni c cc c tnh truyn dn ln n 16 MHz. Loi 4: Cp UTP kt hp vi cc phn cng kt ni c cc c tnh truyn dn ln n 20 MHz. Loi 5: Cp UTP kt hp vi cc phn cng kt ni c cc c tnh truyn dn ln n 100 MHz. Cp loi 3 v cp loi 5 c s dng ch yu trong cc ng dng LAN. Loi 3 s dng ch yu cho cc ng in thoi trong cc vn phng. Loi 5 thng c s dng truyn d liu v ang tr nn thng dng v c ci t sn trong cc cao c. im khc nhau chnh gia cp loi 3 v loi 5 l s vng xon trn mi n v khong cch. Loi 5 xon cht hn vi bc xon thng thng khong 0.6 n 0.85 cm trong khi loi 3 l 7.5 n 10 cm. Loi 5 xon cht hn nn t hn v cung cp hiu sut cao hn loi 3.

Tham s u tin c s dng so snh s suy gim. Cng ca tn hiu suy gim theo khong cch trn mi phng tin truyn dn. Trn cc phng tin c dn, s suy gim thng theo hm logarit v thng c din t nh mt hng s theo decibel trn mi n v khong cch. Nhiu xuyn m gn cc thit b u cui trong cc h thng cp xon i l s kt hp cc tn hiu t mt cp dy sang mt cp khc ti cc cht kim loi. S kt hp din ra khi tn hiu truyn xm nhp vo cht ni ca ng thu tn hiu nhn ti cng mt u ca ng truyn. (v d tn hiu c truyn b thu bi mt cp dy nhn gn ).

Cp xon i c v bc chng nhiu

Cp xon i khng c v bc chng nhiu

Cp xon i UTP v STP s dng u ni RJ-11. RJ 45

2.4.2.2 Cp ng trc M t vt l: Cp ng trc, ging nh cp xon i bao gm hai ng dn in, nhng n c cu trc khc cho php n hot ng trong min tn s rng hn. N bao gm vng rng hnh tr dn in bn ngoi bc ly mt dy kim loi dn in n bn trong. Dy kim loi bn trong c gi bi mt lot cc vng cch in xp cch u nhau hoc c bc bi mt cht in mi. Vng dn in bn ngoi c bc bi mt v bc. cp ng trc n c ng knh - 28 -

vo khong 1 n 2.5 cm. Do c bc kn, c cu trc ng tm, cp ng trc chu nhiu v xuyn m tt hn cp xon i. ng dng: Cp ng trc c l l phng tin truyn thng a nng nht v c s dng rng ri trong nhiu ng dng khc nhau. Cc ng dng quan trng nht l: Phn phi tn hiu truyn hnh Truyn tn hiu in thoi ng di Kt ni cc h thng my tnh khong cch gn Mng ni b Cp ng trc nhanh chng c s dng rng ri phn phi tn hiu truyn hnh ti tng nh truyn hnh cp. Truyn hnh cp tr nn thng dng nh in thoi, s knh ln n hng trm v khong cch ln n vi chc kilomet. Trc y, cp ng trc c v tr quan trng trong cc mng in thoi ng di. Ngy nay, n ang phi i mt vi s cnh tranh ngy cng tng ca cp quang, sng viba mt t v v tinh. Bng cch s dng vic phn chia nhiu thnh phn tn s, cp ng trc c th mang ti 10000 knh ting ni cng mt lc. Cp ng trc cng c s dng nhiu trong cc kt ni khong cch ngn gia cc thit b. Bng cch s dng tn hiu s, cp ng trc c th c s dng cung cp cc knh vo ra tc cao trn cc h thng my tnh. Cc c tnh truyn dn: Cp ng trc c s dng truyn c tn hiu tng t v tn hiu s. Cp ng trc c cc c tnh tn s cao hn so vi cp xon i v v vy c th s dng hiu qu vi cc tn s v tc d liu cao hn. Do c v bc v cu trc ng tm, cp ng trc t chu nh hng bi nhiu v xuyn m hn cp xon i. Yu t nh hng ch yu n hiu sut l s suy gim, nhiu nhit v nhiu iu bin. Nhiu iu bin ch xut hin khi c mt vi knh hoc di tn s c dng chung trn mt ng cp. Vi cc ng truyn tn hiu tng t khong cch di, vic khuch i sau mt vi km l rt cn thit, tn s cng cao th khong cch cn khuch i tn hiu cng ngn. Ph c th s dng cho tn hiu tng t c th m rng n khong 500 MHz. i vi tn hiu s, cn s dng cc b lp sau 1km v nu tc d liu cao hn th khong cch cn lp li cng gn hn.

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2.4.2.3. Cp quang M t vt l: Cp quang l mt phng tin mm do, ng knh nh c kh nng truyn tia sng. Cc cht liu thy tinh hoc cht do c th c s dng lm nn cp quang. Cp quang c ch to bi silic yoxit nng chy tinh khit c kh nng truyn tt nht nhng rt kh ch to. Cp quang ch to bng si thy tinh nhiu thnh phn khng tt bng nhng kinh t hn v vn cho kt qu chp nhn c. Si cht do c gi r nht v c th s dng cho cc ng truyn ngn v chp nhn mt mt cao. Cp si quang c dng hnh tr v bao gm ba thnh phn ng tm: li, lp sn ph v v bc. Li l thnh phn trong cng v bao gm mt hoc nhiu si rt mnh lm bng thy tinh hoc nha. Li c ng knh khong 8 n 100 m . Mi si c bc mt loi v c bit lm bng thy tinh hoc cht do c c tnh quang hc khc vi li. B mt gia li v lp v bc c bit c tc dng to s khc x nh sng ton phn trong li. Lp ngoi cng bc ly mt hoc mt vi si cp ph sn l v bc. V bc oc lm bng cht liu nha tng hp v cc cht liu khc nhm bo v li chng li hi m, s n mn, va p v cc nguy hi t mi trng bn ngoi. ng dng: Mt trong cc cng ngh c ngha nht trong truyn d liu l s pht trin ca cc h thng truyn thng bng cp quang. Cp quang cn c s dng trong truyn thng ng di v cc ng dng ca n trong lnh vc qun s ang pht trin mnh. S ci thin lin tc trong hiu sut v gi thnh ngy cng gim cng vi cc u im vn c ca cp quang khin n c dng ngy cng nhiu kt ni mng ni b. Cc c tnh sau ca cp quang phn bit n vi cp xon i v cp ng trc: Dung lng ln hn: Tim nng v bng thng ca cp quang rt ln, v vy tc truyn d liu ca cp quang rt cao, ln ti hng trm Gbps trn qung ng hng chc km. Ta c th so snh vi tc thc t cc i hng trm Mbps trn qung ng 1 km ca cp ng trc v ch vi Mbps trn 1 km hay t 100Mbps n 1 Gbps trn vi chc mt ca cp xon i. Kch thc nh hn v nh hn: Mt si cp quang nh hn mt dy cp ng trc v mt b cp xon i. chy trong cc ta nh hay di lng t, u im v kch thc rt c li v kch thc nh hn cng gim cc yu cu v cu trc cng trnh. t suy gim hn: S suy gim trong cp quang nh hn nhiu so vi cp ng trc v cp xon i, v l mt hng s trn mt min rng. Khng b nh hng bi in t: Cc h thng cp quang khng b nh hng bi cc trng in t bn ngoi. V vy h thng khng b nh hng bi s nhiu giao thoa, nhiu xung v nhiu xuyn m. Si quang khng pht ra nng lng do t nh hng n cc thit b khc v c mc bo mt cao hn, trnh b nghe trm v kh b r r. Khong cch cn lp tn hiu ln hn: S b lp cn s dng t hn lm gim gi thnh v t cc ngun gy li. Hiu sut ca cc h thng cp quang theo quan im ny c s pht trin vng chc. Khong cch thng thng gia cc b lp vo khong 10 km v c cc ghi nhn khong cch ln ti hng trm km. Cc h thng cp ng trc v cp xon i thng phi dng b lp sau khong vi km. C nm loi ng dng chnh ca cp quang tr nn quan trng: Cc cp trc ng di Cc cp trc trong thnh ph Cc cp trc gia cc vng - 30 -

ng ni gia khch hng v tng i Cc mng ni b Vic s dng cp quang trong truyn tn hiu ng di ngy cng tr nn thng dng trong cc mng in thoi. Cc qung ng ln n 1500 km v dung lng rt cao (thng thng khong 20000 n 60000 knh ting ni). Cc h thng ny ang cnh tranh v mt kinh t vi sng viba v c gi thp hn nhiu so vi cp ng trc. Cc ng cp quang chy di bin cng ang c s dng nhiu hn. Cc ng trc trong thnh ph c di trung bnh khong 12 km v c th c khong 100000 knh ting ni trong mi nhm trc. Cc ng trc ny c lp t trong cc ng ng chn di t v khng c cc b lp tn hiu, c ni vi cc tng i in thoi. Cc ng trc gia cc vng c di khong 40 n 160 km kt ni cc thnh ph v cc vng qu hay l gia cc tng i in thoi ca cc cng ty khc nhau. Hu ht cc h thng ny c t hn 5000 knh ting ni. Cc k thut c s dng trong cc ng dng ny ang cnh tranh vi cc k thut s dng sng vi ba Cc ng ni gia khch hng v tng i l cc ng cp chy trc tip t tng i trung tm ti khch hng. Cc ng cp ny ang dn thay th cp xon i v cp ng trc tr thnh cc mng c y dch v khng ch x l ting ni v d liu m cn c hnh nh v video. Ban u ng dng ny ca cp quang l cho cc khch hng thng mi, nhng vic truyn dn n tng gia nh s sm xut hin. ng dng quan trng cui cng ca cp quang l cho mng ni b. Cc chun ang c pht trin v cc sn phm cho mng cp quang d c dung lng t 100 Mbps n 1 Gbps v c th h tr hng trm, thm ch hng nghn trm trong mt to nh ln hoc mt khu nhiu ta cao c. Cc c tnh truyn dn: Cp quang truyn tn hiu c m ha thnh cc chm tia sng phn x ton phn bn trong. iu ny c th xay ra trong bt c mi trng truyn dn no c ch s khc x cao hn mi trng bn ngoi. Trong thc t, cp quang hot ng nh mt sng truyn c hng vi tn s trong khong t 1014 n 1015 hertz, bao gm c nh sng hng ngoi v nh sng nhn thy c.

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Tia sng t mt ngun sng i vo trong li hnh tr bng thy tinh hoc cht do. Cc tia c gc rng b phn x v truyn dc theo si cp, cc tia khc b hp thu bi cht bao bc. Hnh thc truyn ny gi l truyn a cch, nhy bc, theo ngha c nhiu gc khc nhau s phn x. Khi truyn dn a cch, tn ti nhiu ng truyn khc nhau, mi ng truyn s c di khc nhau dn ti thi gian truyn ca mi ng cng khc nhau. iu ny khin cc thnh phn tn hiu (xung nh sng) tri ra theo thi gian v v vy gii hn tc truyn m d liu c th nhn mt cch chnh xc. Hay ni cch khc yu cu khong cch gia cc xung gii hn tc d liu. Loi cp ny ph hp cho vic truyn khong cch rt ngn. Khi bn knh ca li cp gim i, s gc phn x cng t i. Bng cch gim bn knh ca li theo yu cu ca bc sng.Vic truyn theo kiu n cch (single-mode) cung cp hiu sut cao hn v cc l do sau. V ch c mt ng truyn nn cc sai lch nh khi truyn theo kiu a phng thc khng th din ra. Kiu n cch thng c s dng cho cc ng dng ng di bao gm in thoi v truyn hnh cp. Cui cng, do cc ch s khc x ca li khc nhau nn ta c th c kiu truyn th ba l a cch bin i dn. y l kiu trung gian gia hai kiu trn v mt c tnh. Ch s khc x cao hn ti trung tm khin tia sng cng gn trc th cng chm hn cc tia gn lp v. Tia sng trong li i theo ng cong xon c v ch s khc x c phn loi v gim khong cch phi truyn ca n. Khong cch c thu ngn v tc cao hn cho php tia sng pha ngoi bin ti thit b nhn gn nh cng thi im vi cc tia truyn thng li. Cc cp c kiu a cch bin i dn thng c s dng trong cc mng ni b. C hai loi ngun sng khc nhau c s dng trong cc h thng cp quang: dioed pht quang (LED) v diode bc x laser (ILD). C hai u l cc thit b bn dn pht ra cc chm sng ti mt hiu in th no . n LED r hn hot ng trong min nhit rng hn v c thi gian s dng lu hn. Trong khi ILD hot ng theo nguyn l laser hiu qu hn v c tc truyn d liu ln hn. C mt mi lin h gia bc sng s dng, kiu truyn v tc d liu nhn c. C hai kiu truyn n cch v a cch u h tr mt vi bc sng nh sng v c th s dng c ngun nh sng laser hay n LED. Trong cp quang, nh sng c truyn tt nht trong ba ca s bc sng khc nhau, 850, 1300 v 1550 nanometer (nm). Tt c nhng bc sng ny u nm trong vng hng ngoi ca ph tn s, nm pha di vng nh sng nhn thy c (t 400 n 700 nm). S mt mt s gim i khi bc sng tng ln v cho php tc d liu cao hn trn cc khong cch di. Hu ht cc ng dng cc b hin nay u s dng ngun sng n LED. Mc d vic s dng n LED khng t nhng n gii hn tc d liu di 100 Mbps v khong cch l vi km. c th c tc d liu cao hn v khong cch xa hn phi s dng n LED pht nh sng c bc sng 100 nm hoc ngun sng laser. c tc d liu cao nht v khong cch truyn xa nht ta phi dng ngun sng laser c bc sng 1500 nm Dn thnh phn bc sng (Wavelength-Division Multiplexing): Tim nng ca cp quang s c s dng ton b khi nhiu chm sng vi cc tn s khc nhau c truyn trn cng mt si cp. y l cch truyn dn thnh phn tn s (FDM) nhng thng c gi l dn thnh phn bc sng (WDM). nh sng truyn trong si cp bao gm nhiu mu hay nhiu bc sng, mi bc sng mang cc knh d liu khc nhay. Nm 1997 phng th nghim Bell chng minh mt h thng WDM vi 100 chm sng s hot ng 10 Gbps vi tc d liu khong 1 t t bit mi giy. Hin nay cc h thng thng mi vi 80 knh v tc 10 Gbps i vo hot ng. 2.4.3. Truyn khng dy i vi cc phng tin truyn khng dy, vic truyn v nhn c thc hin bi mt ng ten. Khi truyn i, ng ten pht ra nng lng sng in t vo trong mi trng (thng l khng kh) v khi thu, ng ten thu sng t mi trng xung quanh. C hai loi hnh c bn trong h thng truyn dn khng dy: truyn dn c nh hng v truyn theo mi hng. - 32 -

Vi loi truyn c nh hng, ng ten s pht ra mt chm in t hi t, v th ng ten pht v ng ten nhn phi c chnh sao cho hng thng vo nhau. Khi truyn theo mi hng, tn hiu truyn pht ra theo mi hng v c th thu bi nhiu ng ten. Nhn chung, tn s ca tn hiu cng ln th kh nng tp trung chng vo mt tia c nh hng cng cao. C 3 loi tn s chnh c quan tm. Tn s trong khong 2GHz n 40GHz c gi l sng viba. tn s ny, c th to ra tia c nh hng tt v sng viba ph hp vi vic truyn im - im. Sng viba cng c s dng trong truyn thng v tinh. Tn s trong khong 30MHz n 1GHz ph hp vi truyn thng theo mi hng. Ta c th thy tn s ny ging nh tn s ca i pht thanh. Bng 2.3 tm tt cc c tnh ca truyn dn khng dy ti nhiu di tn. Sng viba l 1 phn ca di UHF v ton b di SHF, sng pht thanh gm di tn VHF v mt phn ca di UHF. Mt loi tn s quan trng khc, dnh cho cc ng dng cc b , l tia hng ngoi ca quang ph. Loi ny bao gm cc tn s xp x t 3.1011 ti 2.1014. Tia hng ngoi c s dng trong vic truyn thng im im v a im trong mt phm vi hp nh mt phng n.

2.4.3.1 Sng viba mt t M t vt l: Hu ht nhng loi ng ten pht sng viba l parabol (hay ng ten cho). Cc ng ten nay thng c ng knh khong 3m. ng ten c lp c nh v tp trung cc tia li trong khong hp truyn chng ti ng ten nhn. ng ten pht sng viba thng t ti im cao trn mt t c th m rng khong cch gia cc ng ten v c th truyn qua nhng vt cn gia. Nu nh khng c chng ngai vt no th khong cch ln nht gia hai ng ten: d=7.14. K.h Trong d: khong cch gia hai ng ten tnh bng km, h: cao ca ng ten tnh bng m, K: H s iu chnh tnh ton, trong thc t sng viba b un cong hoc b khc x do tri t cong v v th vic truyn i c xa hn vic truyn cc tia thng. Theo kinh nghim th K nn bng 4/3. V d hai ng ten sng viba cao 100m c th truyn i 7,14 133 =82 km truyn c vi khong cch xa, cn s dng mt lot cc trm chuyn tip v cc kt ni im im l mt chui cc trm lin tip c th vt qua c gii hn khong cch.. ng dng: Cc h thng s dng sng viba mt t u tin l cc dch v vin thng ng di bn cnh vic s dng cc cp ng trc hay cp quang. Sng viba yu cu cc b khuych - 33 -

i v b lp c t gn nhau hn cp ng trc vi cng mt ng truyn v i hi nm trong tm nhn. Sng viba s dng ph bin trong pht thanh v truyn hnh. Mt ng dng ph bin khc ca sng viba l kt ni im im vi khong cch ngn gia cc to nh. N cn c s dng trong cc mch nh ca ti vi hoc trong vic trao i d liu ca mt mng cc b. Sng viba khong cch ngn cng c th c s dng cho nhng ng dng ng vng.Mt doanh nghip cng c th s dng sng viba trong vic kt ni vi tng i xa trong cng thnh ph t tng i cc b ca cng ty. c tnh truyn dn: Truyn dn sng viba bao gm mt phn nh ca ph in t. Tn s ph bin c s dng truyn l t 2GHz n 40GHz. Tn s cng cao th bng thng ca v tc truyn cng cao. Nh vy vi bt k h thng truyn thng no, nguyn nhn chnh ca s mt mt l s suy yu cc tn hiu. Vi sng viba (v c cc tn s radio) s suy yu c tnh theo cng thc sau:

L 10 logtrong : d: khong cch truyn

4 d

2

dB

: di bc sng cc di trong cng mt n v

Do s suy yu gim theo bnh phng khong cch. V th, b lp khuych i c th lm cho h thng sng viba pht xa hn t 10 n 100 km. S suy yu c th tng ln do ma, s nh hng ca lng ma c bit ng lu . 2.4.3.2. Sng vi ba v tinh Cc c trng vt l: Trong thc t, v tinh truyn thng l mt trm chuyn tip sng vi ba. N c s dng kt ni hai hay nhiu hn cc trm pht v thu sng mt t. V tinh thu sng trn mt bng tn (uplink), khuch i hoc lp li tn hiu, sau truyn chng trn mt tn s khc (downlink). Mt v tinh trn qu o s iu khin mt s loi di tn v c gi l b tip v tch sng.

Trong hnh th nht, v tinh ang c s dng cho mt kt ni im im gia hai ng ten mt t. Trong hnh th hai, v tinh h tr truyn thng tin t mt trm pht trn mt t ti nhiu tram nhn. - 34 -

vic truyn thng c hiu qu th v tinh phi gi c v tr tnh so vi mt t. Tuy nhin s di chuyn ca v tinh s khin n khng nm trong tm nhn ca cc trm trn mt t ti mi thi im. gi c v tr tnh so vi mt t th v tinh phi quay cng qy o vi tri t v cn gi cao khong 35.784 km so vi mt t. Hai v tinh s dng cng mt di tn s, nu qu gn nhau th s gy ra nhiu. trnh s nh hng ln nhau, cc chun hin nay yu cu khong cch l 4 0 (gc gia hai v tinh c o t mt t) i vi di tn 4/6 GHz v khong cch l 3 0 i vi di tn 12/14 GHz. V th, s v tinh l c gii hn. ng dng: V tinh truyn thng l mt cuc cch mng cng ngh c tm quan trng nh cp quang. Mt s ng dng quan trng ca v tinh: Truyn hnh in thoi ng di Mng thng mi ring Do tnh cht truyn qung b t nhin ca n, v tinh rt ph hp cho truyn hnh nn n ang c s dng rng ri M v nhiu ni trn th gii.Theo cch truyn thng, chng trnh c pht t mt im trung tm sau s c truyn ti v tinh v s c pht qung b ti cc trm nhn. Cc trm ny li phn phi chng trnh n tng ngi dng ring l. Mt mng cung cp dch v qung b cng cng (PBS) phn phi cc chng trnh truyn hnh hu nh l thng qua cc knh v tinh. Cc mng thng mi khc cng mt phn s dng v tinh, cc h thng truyn hnh cp hin nay ang cn i vic tng cc chng trnh nhn t v tinh. ng dng mi nht ca cng ngh v tinh trong truyn hnh l v tinh qung b trc tip, tn hiu video t v tinh s c truyn trc tip n nh ca ngi s dng. Vic gim gi thnh v kch thc ca anten khin iu ny tr nn kh thi v mt kinh t, hin nay mt s knh i vo hot ng v mt s khc c ln k hoch Vic truyn v tinh cn c s dng kt ni im im cc ng trc gia cc tng i in thoi ca mng in thoi cng cng. y l phng tin truyn ti u cho cc ng trc quc t c mc s dng cao v ang cnh tranh vi cc h thng mt t trong vic kt ni quc t ng di. Ngoi ra v tinh cn mt s ng dng trong truyn d liu thng mi. V tinh c th chia tng dung lng ra thnh thnh mt s knh v cho nhng khch hng thng mi ring l thu cc knh ny. Mi khch hng c trang b mt angten ti mt s v tr v c th s dng cc knh ca v tinh cho mng ring ca mnh. Cc ng dng ny kh t v ch gii hn trong cc t chc ln c yu cu cao. Pht trin gn y v cc h thng u cui m rt nh (VSAT) cung cp mt s la chn gi thnh thp. Hnh 2.6 m t mt h thng VSAT thng thng. Mt s trm khch hng c trang b cc angten VSAT gi thnh thp. Theo mt s quy tc, nhng trm ny chia s mt dung lng truyn v tinh truyn ti mt trm trung tm. Trm trung tm c th trao i hay tip nhn thng bo gia cc khch hng. Cc c tnh truyn dn: Khong tn s ti u cho truyn v tinh l trong khong 1 n 10 GHz. Tn s di 1 GHz s b nh hng bi nhiu t cc ngun t nhin nh t ngn h, mt tri v kh quyn v c cc nh hng gy ra bi con ngi do cc loi thit b in khc nhau. Nu tn s trn 10 GHz, tn hiu s b nh hng d di bi sc ht v ma ca kh quyn. Hu ht cc v tinh cung cp cc dch v truyn im im ngy nay u s dng bng tn c rng trong khong t 5.925 n 6.425 GHz truyn t mt t ln v tinh (uplink) v bng thng trong khong 2.7 n 4.2 GHz truyn t v tinh v mt t (downlink). S kt hp ny c gi l di tn 4/6 GHz. Ch rng tn s uplink v downlink l khc nhau. hot ng lin tc m khng xy ra xung t, v tinh khng th nhn v truyn trn cng mt tn s. V vy tn hiu nhn t trm mt t trn mt tn s c truyn li trn mt tn s khc. - 35 -

Di tn 4/6 GHz nm trong vng ti u t 1 n 10 GHz nhng b s dng bo ha. Cc tn s khc trong vng ny li khng th s dng do cc ngun xung t vi cc tn s ny thng l cc sng vi ba trn mt t. V vy, di tn 12/14 GHz c pht trin (uplink: t 14 n 14.5 GHz; downlink: t 11.7 n 14.2 GHz). Trong di tn s ny, c th b qua vn nhiu. Tuy nhin, ta c th s dng cc trm thu mt t nh hn v r hn. C th thy rng di tn trn s b bo ha do phi ln k hoch cho vic s dng di tn 19/29 GHz. Khi s dng di tn ny, s suy gim s ln hn nhng li cho php bng thng rng hn v cc thit b thu nh v r hn. Ta cn cp n mt s tnh cht ca truyn thng v tinh. Th nht, do khong cch ln, tr do truyn vo khong mt phn t giy gia hai trm mt t. tr ny c th nhn thy khi ni chuyn in thoi. Ta cng phi quan tm n iu khin li v iu khin lu lng. Th hai, sng ngn v tinh rt hu hiu truyn qung b. Nhiu trm c th truyn tn hiu n v tinh v tn hiu t mt v tinh c th c nhn bi nhiu trm. 2.4.3.3. Sng v tuyn qung b M t vt l: S khc nhau c bn gia sng v tuyn qung b v sng ngn l sng v tuyn qung b s truyn theo mi hng cn truyn sng ngn th c nh hng. V vy sng v tuyn qung b khng yu cu phi s dng anten cho v anten cng khng cn phi quay theo mt hng chnh xc. Cc ng dng: Sng v tuyn l thut ng chung c s dng, bao gm cc tn s trong khong t 3 kHz n 300 GHz. Thut ng sng v tuyn qung b c s dng bao gm c bng tn VHF v mt phn ca UHF: 30 MHz n 1 GHz. Min tn s ny bao gm c bng tn FM ca radio v bng tn UHF v VHF ca tivi. Min tn s ny cng c s dng cho mt vi ng dng mng truyn d liu. Cc c tnh truyn dn: Min tn s trong khong t 30 MHz n 1 GHz ph hp cho vic sng v tuyn qung b. Khng ging nh trng hp ca cc sng in t tn s thp, tng in ly khng phn x cc sng v tuyn c tn s trn 30 MHz. V vy vic truyn b gii hn trong phm vi tm nhn v cc cc my pht s khng b cn tr bi cc my pht khc do s phn x t tng kh quyn nh khi truyn sng ngn. Khng ging nh cc tn s cao ca sng ngn, sng truyn qung b t b hng do nhiu t cc cn ma hn. Truyn sng v tuyn tun theo cng thc (2.1), tc l khong cch cc i gia my pht v my thu ln hn khng ng k so vi kh nng nhn ca mt ngi. Nh i vi sng ngn, suy gim theo khong cch tun theo cng thc (2.2). Bc sng cng di th sng v tuyn cng t chu nh hng ca s suy gim. - 36 -

Nguyn nhn chnh gy h hi cho sng v tuyn qung b l s giao thoa ca nhiu tn hiu. S phn x t mt t, nc, t t nhin hoc t cc i tng do con ngi to ra c th gy nn nhiu loi tn hiu gia cc anten. Mt v d l TV s thu v pht nhiu hnh nh khc nhau lc c my bay bay qua. 2.4.3.4 Tia hng ngoi Truyn thng bng tia hng ngoi s dng cc my thu pht c th iu chnh cc nh sng hng ngoi. Hai my thu pht phi nm trong tm nhn ca nhau trc tip hoc thng qua phn x t cc b mt c mu sng nh trn nh.

S khc nhau quan trng gia truyn sng hng ngoi v sng ngn l truyn sng hng ngoi khng th xuyn qua tng. V vy ta khng cn quan tm n vn bo mt v nhiu nh trong cc h thng truyn sng ngn. Hn na, khng c mt yu cu no trong vic s dng di tn s khi truyn sng hng ngoi. 2.5. Cc chun giao tip truyn thng 2.5.1. Cc chun chung Hu ht cc thit b x l tn hiu c kh nng truyn nhn tn hiu hn ch, thng thng cc thit b ny c gn trc tip vi cc thit b chuyn nhn tn hiu hoc qua mng, chng c gi l cc thit b truyn nhn d liu u cui (DTE, DCE). Mi thit b x l tn hiu (trm) thng c kt hp vi mt cp gm mt DTE v mt DCE. Hai trm truyn tn hiu cho nhau qua hai DCE ca mi bn c kt ni vi nhau. Hai DCE trao i tn hiu vi nhau trn mng hoc ng truyn phi tng t nhau, ngha l b phn nhn tn hiu bn ny phi tng ng vi b phn pht tn hiu ca bn kia. DTE v DCE truyn nhn tn hiu vi nhau do cng phi tng thch vi nhau v d liu v thng tin iu khin (cc chun) Cc chun v giao din ga DTE v DCE bao gm: Chun v cu trc: xc nh kt ni vt l gia DTE v DCE (tn hiu v mch iu khin thng qua cp ni v gic cm) Chun v tn hiu: xc nh mc hiu in th, thi gian bin i tn hiu Chun v chc nng ). Chun v th tc: xc nh th t thao tc trong truyn d liu da trn chun chc nng ca cc ng tn hiu. Chun EIA-RS 232 (Electronic Industry Association Recomand Standard): chun giao tip truyn thng cng nghip EIA cng b tiu chun RS-232C vi n lc nhm to ra kh nng ghp ni cc thit b do nhiu nh sn xut lm ra m khng i hi c mt tiu chun k thut c bit cho tng trng hp. tng xy dng tiu chun RS-232 l phi s dng cng loi ni dy, th d loi u ni 25 chn hoc 9 chn, c ni theo cng mt cch v s dng cng mc in p khi biu din cc s nh phn 1 v 0 tng ng. Vi tng ny, nu nh mi ngi u tham - 37 -

gia vo tiu chun theo cng mt cch th c th ni cc thit b vi cng RS-232 ca cc hng khc nhau, cc mu m khc nhau m khng cn c thm iu kin no. Cc mdem, cc my in v nhiu thit b khc c th c ni vo giao din RS-232. Ngy nay, hu ht cc my tnh u trang b mt hoc hai cng ni tip RS-232, v tt c u c kh nng s dng RS-232, t nht l nh mt kh nng tu chn t nh sn xut my tnh hoc t pha ngi s dng my tnh. Cc mch in tch hp c b pht v b nhn RS-232C c cc nh sn xut khc nhau thit k v ch to, th d Motorola, National, Semiconductors. Cc chip b pht/ b m RS-232 tip nhn mc in p TTL li vo v bin i chng thnh cc mc dnh ring cho RS-232C truyn. Cc b nhn RS-232 lm vic theo cch ngc li: tip nhn tn hiu li vo theo chun RS-232 v bin i cc tn hiu sang cc mc TTL tng ng. Cc b phn ny u nm trn bn mch chnh hoc trn mt Card vo/ ra, ngha l pha sau ca cng RS-232. Tn hiu truyn theo chun RS-232 ca EIA Chun RS-232 quy nh mc logic 1 ng vi in p t -3V n -25V (mark), mc logic 0 ng vi in p t 3V n 25V (space) v c kh nng cung cp dng t 10 mA n 20 mA. Ngoi ra, tt c cc ng ra u c c tnh chng chp mch. Chun RS-232 cho php truyn tn hiu vi tc n 20.000 bps nhng nu cp truyn ngn c th ln n 115.200 bps. Cc phng thc ni gia DTE v DCE: n cng (simplex connection): d liu ch c truyn theo 1 hng. Bn song cng (half-duplex): d liu truyn theo 2 hng, nhng mi thi im ch c truyn theo 1 hng. Song cng (full-duplex): s liu c truyn ng thi theo 2 hng.

Hnh 2.26: nh dng khung truyn d liu theo chun RS 232 Khi khng truyn d liu, ng truyn s trng thi mark (in p -10V). Khi bt u truyn, DTE s a ra xung Start (space: 10V) v sau ln lt truyn t D0 n D7 v Parity, cui cng l xung Stop (mark: -10V) khi phc trng thi ng truyn. Dng tn hiu truyn m t nh sau (truyn k t A):

- 38 -

Cc c tnh k thut ca chun RS-232 nh sau: Tc d liu cc i : 20 Kbps in p ng ra cc i : 25V in p ng ra c ti : 5V n 15V Tr khng ti : 3K n 7K in p ng vo : 15V nhy ng vo : 3V Tr khng ng vo : 3K n 7K Mc logic 1 (mc du) nm trong khong: -3 V n 12 V; trong khong t 5 V n 12 V l tin cy, mc logic 0 (mc trng) nm trong khong: +3 V n +12 V, khong t + 5 V n +12 V l tin cy. Tr khng ti v pha b phn ca mch phi ln hn 3.000 nhng khng c vt qu 7.000 . Cc li vo ca b nhn phi c in dung phi nh hn 2.500 pF. di ca cp ni gia my tnh v thit b ghp ni qua cng ni tip khng th vt qu 15m nu khng s dng mdem. Cc gi tr tc truyn d liu chun l 50, 75, 110, 150, 300, 600, 1200, 2400, 4800, 9.600, 19.200, 28.800,, 56.600 baud. Cc tc truyn d liu thng dng trong cng ni tip l: 1200 bps, 4800 bps, 9600 bps v 19200 bps.

Nh vic quy nh thng nht s dng mt u ni 25 chn v v sau b sung thm u ni 9 chn cho cng ni tip RS-232, c th hn l cm v pha dy cp cn cm v pha my tnh, m tt c cc sn phm u tng thch vi nhau. Quy nh ny cng p dng thng nht cho cc thit b ghp ni vi cng RS-232. Hnh trn ch ra cch sp xp chn ca u ni 25 chn v 9 chn dng cho RS-232C, cn vic nh ngha chc nng ca cc chn c lit k bng k tip. Tiu chun RS-232C quy nh r vic s dng u ni thng nht tt c cc sn phm u tng thch vi nhau. V vy th t v chc nng ca cc chn c quy nh rt c th v phi tun th mt cch nghim ngt. d dng nhn ra th t cc chn, bn cnh cc chn u c in r s th t trn phn nha ca phch cm cng nh cm. Nhn xt ny cn c lu khi kim tra cp ni hoc t hn mt cp mi. Cc chn v chc nng trn u ni 25 chn v 9 chn. 25 9 Tn Vit Chc nng chn chn tt Ch : =>: Li vo SG DCD => DTR : Li vo chung. Truyn thng gia hai nt: Cc s khi kt ni dng cng ni tip:

Khi thc hin kt ni nh trn, qu trnh truyn phi bo m tc u pht v thu ging nhau. Khi c d liu n DTE, d liu ny s c a vo b m v to ngt. Ngoi ra, khi thc hin kt ni gia hai DTE, ta cn dng s sau:

- 40 -

Khi DTE1 cn truyn d liu th cho DTR tch cc => tc ng ln DSR ca DTE2 cho bit sn sng nhn d liu v cho bit nhn c sng mang ca MODEM (o). Sau , DTE1 tch cc chn RTS tc ng n chn CTS ca DTE2 cho bit DTE1 c th nhn d liu. Khi thc hin kt ni gia DTE v DCE, do tc truyn khc nhau nn phi thc hin iu khin lu lng. Qu trinh iu khin ny c th thc hin bng phn mm hay phn cng. Qu trnh iu khin bng phn mm thc hin bng hai k t Xon v Xoff. K t Xon c DCE gi i khi rnh (c th nhn d liu). Nu DCE bn th s gi k t Xoff. Qu trnh iu khin bng phn cng dng hai chn RTS v CTS. Nu DTE mun truyn d liu th s gi RTS yu cu truyn, DCE nu c kh nng nhn d liu (ang rnh) th gi li C 2.5.2. thc hin cc phng php truyn mt cch c th, cc nh ch to cung cp mt lot cc IC chuyn dng, cc IC ny chnh l phn cng thuc lp vt l trong mt h thng thng tin,chng hot ng theo nguyn tc ca k thut s v v vy ch truyn ng b hay bt ng b ph thuc vo vic s dung ng h chung hay ring khi truyn tn hiu s i xa. Cc IC u l cc vi mch c th lp trnh. u tin lp trnh ch hot ng mong mun bng cch ghi mt byte c ngha vo thanh ghi ch mode register. Sau ghi tip byte iu khin vo thanh ghi lnh command register vi mch theo m hot ng. V cc giao tip truyn ni tip c dng kh rng ri trong cc thit b in t hin i, cc vi mch ngoi vi LSI c bit c pht trin cho php thc hin cc loi giao tip ny. Tn tng qut ca hu ht cc IC ny l: UART (Universal Asynchronous Receiver Transmitter) USRT (Universal Synchronous Receiver Transmitter): mch ng b thin hng k t. USART c th hot ng theo UART hay USART tu chn. BOPs (Bit-Oriented Protocol Circuits) mch ng b thin hng bit. UCCs (Universal Communication Control circuits) c th lp trnh cho c 3 loi trn. C UART v USART u c kh nng thc hin nhu cu chuyn i song song sang ni tip truyn s liu i xa v chuyn i ni tip sang song song khi tip nhn s liu. i vi s liu truyn bt ng b, chng cng c kh nng ng khung cho k t mt cch t ng vi START bit, PARITY bit v cc STOP bit thch hp. Cc a ch c s v ngt Cng a ch c bn IRQ COM1 3F8h IRQ4 COM2 2F8h IRQ3 COM3 3F8h (IRQ4) COM4 2F8h (IRQ3) Cc cng ni tip t th nht n th t u c phn bit qua cc v tr a ch trong vng vo/ ra ca my tnh v cc s ngt tng ng (IRQ). a ch u tin ca UART, c th l ca thanh ghi m truyn/ nhn, c tnh l a ch c s. Thng thng, a ch c s v IRQ c quy nh nh cc u ni (Jumper) trn Card vo/ ra hoc trn bn mch chnh. Mch iu khin truyn thng d b vn nng UART (VXL 8250A) Vi mch 8250A l mt UART c dng rng ri trong cc my IBM PC ti v phi ghp ni tip c u ni ra cng thng tin ni tip theo chun RS 232C

- 41 -

Cc thanh ghi c th chia lm 3 loi: Thanh ghi iu khin (Control Register): dng nhn v thc hin cc lnh t CPU. Thanh ghi trng thi (Status Register): dng thng bo cho CPU bit v trng thi ca UART hay UART ang lm g. Thanh ghi m (Buffer Register): dng gi k t trong lc truyn hoc x l. Cc thanh ghi ny cng gi cc k t nh phn c truyn v nhn. Vic truy nhp ln cc thanh ghi c thc hin thng qua a ch v khi iu khin. Mi thanh ghi c gn mt a ch tnh theo cch so snh tng i (Offset) vi a ch c s ca cng ni tip. Cc a ch ca hai cng ni tip u tin trong hu ht cc my tnh c tiu chun ho. vit phn mm ghp ni qua cng ni tip ta cn lu l: ton b hot ng ca giao din ni tip u c iu khin qua cc thanh ghi ca UART, trong thanh ghi m truyn/ nhn d liu thng c tnh l hai thanh ghi. Do ch c 8 a ch nn cn n s chuyn mch bn trong thng qua bit DLAB (Division Latch Access Bit, bit 7 ca thanh ghi iu khin ng truyn). Cc a ch ca tng thanh ghi u c tnh theo khong cch n a ch c s, khong cch ny thng c gi l Offset. Tu theo cc thanh ghi, Offset nhn gi tr c th trong khong t 0 n 7. DLAB A2 A1 A0 Thanh ghi a ch 0 0 0 0 B m c/ghi RBH 3F8 (2F8) 0 0 0 1 Cho php ngt - IER 3F9 (2F9) X 0 1 0 Nhn dng ngt (ch c) IIR 3FA (2FA) X 0 1 1 iu khin ng truyn LCR 3FB (2FB) X 1 0 0 iu khin modem MCR 3FC (2FC) X 1 0 1 Trn thi ng truyn LSR 3FD (2FD) X 1 1 0 Trng thi modem MSR 3FE (2FE) X 1 1 1 Khng dng 1 0 0 0 Cht s chia (LSB) 3F8 (2F8) 1 0 0 1 Cht s chia (MSB) 3F9 (2F9) - 42 -

Vi mch UART 8250 c tt c 10 thanh ghi, sau y ta s ln lt tm hiu cc thanh ghi ny: Cc thanh ghi lu tr: Nh thy r t tn gi, cc thanh ghi ny thc cht l cc b m c chuyn dng gi mt k t, k t ny hoc l c nhn nhng cha c c, hoc l c gi ti cng ni tip nhng cn cha c truyn i. Khi m t qu trnh truyn d liu qua cng ni tip, thanh ghi gi (Holding Register) thng c gi l b m nhn hoc b m truyn. Vic trang b cc b m nhn v truyn cng l mt c im ca vi mch 8250. c im ny cho php mt k t th hai c gi ti cng ni tip trc khi k t th nht c truyn hoc c c xong xui bi b x l. Trong thi gian ch k t th nht c truyn hoc c c, k t th hai c gi trong b m. Sau y ta s thy r hn l: trng thi ca b m truyn v b m nhn c quy nh bi thanh ghi trng thi ng truyn, c th hn l bit 7 ca thanh ghi iu khin ng truyn LCR (Line Control Register). Khi bit ny c t bng '0' th thao tc c t a ch c s s c t b m nhn RX v thao tc vit s vit vo b m truyn TX

Thanh ghi iu khin ng truyn: Mt thanh ghi khc trong vi mch 8250 c gi l thanh ghi iu khin ng truyn LCR (Line Control Register). Thanh ghi ny lu tr cc tham s c ngi lp trnh thit lp v xc nh khun mu khung truyn ca cuc trao i thng tin. Cc thng tin v: s cc bit d liu, s lng bit dng v kiu chn l c s dng trong khung truyn u c ct gi trn thanh ghi ny. D liu c th c vit vo thanh ghi ny v c c ra sau y. Chc nng cc bit ca thanh ghi LCR. Cc bit 0 v 1. Gi tr c ct gi trong hai bit nhi phn ny ch r s cc bit d liu trong tng k t c truyn. S cc bit trn mt k t c th nm trong khong t 5 n 8 bit, cho php xc nh di ca t (Word). Li gii thch cho bit 0 v 1 trn hnh v 12 lm sng t thm vai tr ca cc bit ny. Bit 2 ch r s cc bit dng trong mi khung truyn. Nu nh bit 2 c mt gi tr logic bng 0 th s bit dng s c vi mch 8250 to ra. Nu k t c truyn c su, by hoc tm bit d liu v bit 2 c t vo mt logic 1 th hai bit dng s c to ra v "nh km" vo tng t c truyn. Nu nh nm bit d liu c chn lm h thng m dng cho mt k t th cn n 1,5 bit dng chn vo trong t d liu. Yu cu ny cn thit thch ng vi cc thit b c trn s dng nm bit d liu. Bit 3. c quy nh l bit cho php chn l, ngha l c s dng bit chn l hay khng. Nu bit ny c gi tr logic 1 th bit chn l s c to ra v chn vo tng xu k t. Do tnh chn l c cho php nn bt k k t no nhn c cng u b kim tra v tnh chn l. Bit 4. Kiu chn l c chn, l hoc chn, c xc nh bng cch t bit 4. Khi ct gi mt trng thi logic 0 v tr ny c ngha l t tnh chn l l l v ngc li, ct gi mt trng thi logic 1 bit 4 c ngha l t tnh chn l l chn. Nu nh bit 3, tc l bit cho php chn l, b cm bng cch t mt gi tr logic 0 vo v tr ny th bt k l gi tr bit nh th no c t v tr bit 4 cng khng c tc dng. - 43 -

Bit 5. (Bit stick parity). Nu nh bit 3 v bit 5 c t gi tr logic 1 th khi b truyn xut ra mt k t, b nhn ti ch (local) s pht hin nh mt gi tr logic 3. Bit 6. c quy nh l bit BREAK (dng). Khi bit ny c t mt gi tr logic 1 th n bt buc SOUT (Serial out hay TxD) chuyn sang mc logic trng (mc LOW) cho n khi mt gi tr logic 0 c ct gi vo bit 6. Nh c bit ny m my tnh c th bo hiu cho thit b u cui bit l c ni nh mt phn ca h thng truyn thng. Bit 7. Phi c t mt gi tr logic 1 truy nhp cc cht s chia (divisor latches). Cc cht ny l nhng thanh ghi ct gi s chia i vi tn hiu gi nhp (ng h), s ny quy nh tc baud ca h thng truyn thng ni tip. Mi ln tc baud c t li th bit ny (bit 7) li c t v gi tr logic 0.

Thanh ghi tc baud: Tc baud c t bng cch np mt s chia chim 16 bit, trong 8 bit thp hn ca s chia c t trn a ch b m TX/ RX v 8 bit pha trn t a ch k tip sau b m TX/ RX. S tng gp i s cc thanh ghi l cn thit v khi bit 7 hoc thanh ghi LCR (thng vit tt l DLAB) c li v gi tr logic 0 hai a ch ny gn lin vi b m nhn v b m truyn. Khi bit DLAB c t vo mt gi tr logic 1 th hai a ch ny gn lin vi hai cht s chia. Cc cht s chia bao gm 16 bit hay hai byte, c sp xp thnh cc bit c gi tr thp LSB (Least Significant byte) v bit c gi tr cao hn MSB (Most Significant bit), c s dng trong vic t tc baud ca h thng truyn thng. Bi v cc cht s chia c rng l hai byte, gi tr 060 Hex cn c chia ra ct gi trn hai thanh ghi LSB v MSB. Vi gi tr tc baud bng 1200 trong th d ny, 60 Hex c ct gi trong LSB (bit c gi tr thp) v gi tr 0 c ct trong MSB (bit c gi tr cao hn).Tc baud mun c S chia c dng to ra: 16 x ng h Thp phn Hex Sai s theo phn trm (sai khc gia mong mun v thc t)

50 75 110 134,5 150 300 600 1200

2304 1536 1047 857 768 384 192 96

900 600 417 359 300 180 0C0 060

0,026 0,058 - 44 -

Tc baud mun c

S chia c dng to ra: 16 x ng h Thp phn Hex

Sai s theo phn trm (sai khc gia mong mun v thc t)

1800 64 040 2000 58 03A 0,69 2400 48 030 3600 32 020 4800 24 018 7200 16 010 9600 12 00C Mt s tc baud v cc gi tr s chia tng ng di c hai dng thp phn v thp lc phn (Hex). Gi tr ny ca s chia c np vo b m TX/ RX khi bit DLAB c mt gi tr logic 1 t vo. Thanh ghi trng thi ng truyn: Thanh ghi trng thi ng truyn (LSR: Line Status Register) thanh ghi 8 bit, cha thng tin v qu trnh d liu qua cng ni tip cn cung cp cho b vi x l.

Bit 0, c dng thng bo cho bit d liu nhn c (DR: Data Received). Khi bit 0 c gi tr logic 1 c ngha l d liu c nhn v sn sng b x l c. Bit 1: Mt gi tr logic 1 bit ny c ngha l k t nhn trc b mt v n khng c c trc khi mt k t mi c nhn nn k t mi ghi ln k t trc. Bit 2: Mt gi tr logic 1 bit li chn l c ngha l k t c nhn c tnh chn l sai. Khi thanh ghi trng thi ng truyn (LSR) c c th bit ny li c t v gi tr logic 0. Bit 3: y l bit li khung truyn. Nu k t nhn khng c mt bit dng hp l, ngha l c li khung truyn, th bit 3 trong thanh ghi LSR c t vo mt gi tr logic 1. Bit 4: c quy nh l bit gin on ngt (break interrupt bit). Bit ny c t ng t vo mt gi tr logic 1 khi d liu nhn c c gi mt mc trng trn ton b chiu di ca mt t d liu. Bit 5: c quy nh l bit bo hiu trng thi rng ca b m truyn (THRE: Transmit Holding Register Empty). Bit ny bo hiu l cng ni tip sn sng tip nhn k t khc c truyn ti. Bit 6: Bit ny l mt bit ch c. Khi bit ny c gi tr logic 1 th b m truyn ang cn trng. Bit 7: khng c s dng v lun c t gi tr logic 0. - 45 -

Khi vit phn mm truy nhp thanh ghi ln thanh ghi trng thi ng truyn ta cn lu ti mt s chc nng ca thanh ghi ny. Thanh ghi trng thi ng truyn (LSR: Line Status Resgister) xc nh trng thi ca b m truyn v b m nhn. Thanh ghi ny ch dng c ra, ni dung tt c cc bit c t ng t bng phn cng. Mt iu ri ro c th xy ra khi truyn d liu mt k t mi c th c vit vo b m truyn trc khi k t trc y c gi. Khi k t mi ny s vit ln ni dung ca k t ang c truyn. trnh tnh trng ri ro ny S5 c giao nhim v thng bo kt qu kim tra xc nh liu vn cn mt k t trong b nh. Nu c th n c t thnh '1', cn nu nh bit ny c gi tr bng 0 th c ngha l b m truyn ang trong trng thi trng rng. truyn mt k t: - Kim tra bit 6 cho n khi c t;(Test Bit 6 until set;) - Truyn k t; (Send character;) nhn k t: - Kim tra bit 0 cho n khi c t; (Test Bit 0 until set;) - c k t; (Read character;)

Thanh ghi cho php ngt: Vi mch 8250 c mt nhiu kh nng ngt. C hai thanh ghi c s dng iu khin v xc nh cc ngun ngt. Thanh ghi u tin trong hai thanh ghi l thanh ghi cho php ngt IER (Interrupt Enable Register) cn thanh ghi th hai l thanh ghi nhn dng ngt IIR (Interrupt Identification Register).

Bit 0: Mi ln nhn mt k t th mt ngt li c to ra. Bit ny c t li (Reset) sau khi k t c b x l c. Bit 1: Nu bit ny c t mt gi tr logic 1 th b m truyn (thanh ghi gi truyn) trng v mt ngt xut hin. - 46 -

Bit 2: cho php c s thay i trong trng thi ng truyn b nhn theo cch gy ra mt ngt. Bit 3: cho php c s thay i trong trng thi modem ngt b x l. Bit 4- 7: Cc bit ny lun c t gi tr logic 0. Nu nh kh nng ngt ca vi mch cho php v mt ngt xut hin th bit xut ra ngt t 8250 chim ly mc logic 1. Tn hiu ny c ni vi bus ngt cng ca my tnh. Logic 1 trn bus ny bo hiu cho b x l bit l cn phi ch ti cng ni tip. Hnh 15 minh ho s phn b ca cc bit trn thanh ghi IER. Thanh ghi nhn dng ngt

Nu nh mt ngt xut hin th phn mm chng trnh phi thc hin c chc nng kim tra thanh ghi xc nh xem s kin no ang gy ra ngt. Thanh ghi nhn dng ngt IIR cha ng m, nhn dng iu kin (ngt) no ang yu cu ch . Mt im cn ch l: gia cc ngt cng c mc u tin khc nhau, ni khc i l c mt vi ngt t ra l "quan trng" hn so vi cc ngt khc. V nguyn tc, ngt no quan trng hn s c u tin x l trc.Thanh ghi nhn dng ngt Bit Bit Bit Mc 2 1 0 u tin Kiu ngt Cc ngt v t li chc nng Ngun ngt iu khin t li ngt

0 1

0 1

1 0

Cao nht

c thanh ghi m b nhn 0 1 0 Th ba c thanh ghi IR (nu l ngun ngt) hoc ghi vo b m truyn 0 0 0 Th t Trng thi Xo gi hoc d c thanh ghi trng modem liu sn sng hoc bo thi modem chung hoc pht hin tn hiu ng nhn Bng trn lit k cc mc u tin ca tng ngt. Ct t li ngt lit k tc ng no l cn n t li ngt c cht. Mch iu khin truyn thng ng b - d b vn nng USART (VXL 8251A) Vi mch 8251A l mt USART c dng rng ri trong cc my IBM PC ti v phi ghp ni tip c u ni ra cng thng tin ni tip theo chun RS 232C 1 0 0 Th hai - 47 -

Khng dng Li trn hoc li chn l hoc li khung truyn hoc break interrupt C d liu C d liu nhn nhn B m B m truyn trng truyn trng

Khng dng Trng thi ng nhn

c thanh ghi trng thi ng truyn

S :m m d liu // nt iu khin pht Pht TxD

Reset Clk C/D RD WR

Logic iu khin ghi c

RS232 iu khin thu

DSR CS DTR CTR

iu khin mode m

m Thu RxD

Cc thanh ghi c th chia lm 3 loi: nt // RTS - Thanh ghi iu khin (Control Register): dng nhn v thc hin cc lnh t CPU. - Thanh ghi trng thi (Status Register): dng thng bo cho CPU bit v trng thi ca UART hay UART ang lm g. - Thanh ghi m (Buffer Register): dng gi k t trong lc truyn hoc nhn Thanh ghi t ch D6 D5 D4 D3 D2 D1 D0 D7 S2 S1 EP PEN L2 L1 B2 B1 Bit 0,1 c dng ng b v h s nhn tc 00 ng b 10 nhn 16 01 nhn 1 11 nhn 64 Bit 2,3: s bit m k t 00 5 10 7 01 6 11 8 Bit 4: cho php dng Parity hay khng Bit 5: Parity bit Bit 6,7: s bit STOP 00 khng hp l 01 1 10 11/2 11 2 Thanh ghi t lnh D6 D5 D4 D3 D2 D1 D0 D7 EH IR RTS ER SBRK RxE DTR TxEN Bit 0: cho php pht tn hiu Bit 1: DTE sn sng Bit 2: Cho php thu - 48 -

Bit 3: Gi k t gin on (k t vi tt c cc bit la 0) Bit 4: Xo c li Bit 5: Yu cu truyn Bit 6: Reset ni b Bit 7: tm kim k t ng b Thanh ghi trng thi: D6 D5 D4 D3 D2 D1 D0 D7 DSR SYNDET FE OE PE TxEMPTY RxRDY TxRDY Bit 0: bn pht sn sng Bit 1: bn thu sn sng Bit 2: m pht rng Bit 3: li Parity Bit 4: li thu Bit 5: li Frame Bit 6: k t ng b Bit 7: modem sn sng 2.6. Mng truyn thng Nhng h thng truyn thng c th m con ngi s dng v khai thc: H thng in tn dng nng lng in mt chiu H thng thng tin v tuyn in dng nng lng sng in t H thng thng tin quang nng H thng thng tin dng sng m H thng truyn s liu H thng truyn hnh H thng thng tin thoi Trong phm vi ca h thng truyn s liu Mng chuyn mch knh Mng chuyn mch gi Mng dch v tch hp s Mng cc b Mng v tuyn Mng v tinh CU HI V BI TP 2.1. Anh ch hy trnh by v thng truyn thng tng qut. 2.2. Anh ch hy trnh by v cc ng truyn hu tuyn c s dng hin nay. Nu c im v phm vi ng dng ca chng. 2.3. Trnh by hiu bit ca anh, ch v chun EIA-RS232C 2.4. Trnh by hiu bit ca anh, ch v mch iu khin truyn thng d b vn nng UART8250A 2.6. H thng truyn mt k t m ASCII chun l A thng qua mch iu khin truyn thng d b UART8250A. Anh ch hy cho bit gi tr cc bt ca thanh ghi iu khin ng truyn, thanh ghi tc Bound. Bit rng h thng c s dng 01 bt dng kim tra (parity) l bit, tc truyn l 9600bound 2.7. Gi s h thng nhn c mt k t m ASCII chun trong b m nhn ca mch iu khin ng b-d b nn nng USART 8251A. Anh ch cho bit gi tr cc bt trong thanh ghi t ch . Bit rng h thng s dng 01 bit Parity, v h s nhn tc c s dng l 16. - 49 -

Chng III: K THUT TRUYN S LIU3.1. Gii thiu v k thut truyn s liu C tn hiu tng t v tn hiu s u c th c truyn i bng cc thit b ph hp. Chng c x l nh mt hm ca h thng truyn. Xt m hnh truyn s liu n gin:

DTE (Data Terminal Equipment): Thit b u cui x l d liu thng gi tt l thit b u cui. DTE gm hai khi chc nng khc nhau: khi x l l ngun hoc b thu d liu, khi kim tra s lin lc l t hp cc b phn thc hin chc nng lin lc c hoc khng c b phn lc nhiu. DCE (Data Circuit Equipment): Thit b mch d liu. DCE c th c hai khi chc nng l m ha (hoc gii m) v iu ch (hoc gii iu ch). CODEC = Code + Decode: m ha v gii m. MODEM = Modulation + Demodulation: iu ch v gii iu ch. Cc mch truyn d liu thng truyn song cng (hai chiu) nn phi cn n mch CODEC v MODEM. ng dy: ng dy c hiu l mi trng mang tin ni gia h thng gi v h thng nhn, mi trng c th c phn lm 2 loi l mi trng hu tuyn v v tuyn. 3.2. K thut nh khung trong truyn s liu 3.2.1. K thut m k t K thut ny s dng mt trng trong phn tiu m t s lng cc k t c trong khung. Bt li ca k thut ny l nu mt k t m ca mt khung no b li s lm cho cc khung pha sau khng th xc nh c. K thut ny v th m t c s dng. Qu trnh c khung d liu: Khi c phn u tin h thng thu s xc nhn s k t ca frame d liu. c ht s k t qui nh khi c phn u tiu ca frame tip theo h thng thu tip tc xc nhn di ca frame d liu .

Qu trnh c khung d liu nu b li: Khi phn u khung d liu bt k b li, n s nhn sai khung d liu v ko theo l li ton b cc khung d liu tip theo.

- 50 -

3.2.2. K thut s dng byte lm c v cc byte n. K thut ny s dng mt byte c gi tr c bit lm c hiu (flag byte) nh du im bt u v kt thc ca khung. Mt vn pht sinh trong k thut ny l, trong d liu c th cha byte c gi tr ca c hiu. iu ny s lm gy khung. gii quyt vn ny, ngi ta a vo pha trc byte d liu c gi tr ca c hiu mt byte c bic gi l byte ESC. Bn nhn khi nhn c byte ESC theo sau l gi tr ca c hiu th s b i k t ESC ng thi bit y cha phi l im kt thc ca khung. Tng t, nu trong d liu c cha k t ESC th ta cng a thm vo pha trc n mt k t ESC. K thut ny c gi l k t n (character stuffing). Yu im ca K thut ny l n da trn k t dng 8 bits v th s khng s dng c trong cc h thng s dng chun m 16 bits hoc 32 bits nh Unicode chng hn.

3.2.3. K thut s dng bit n K thut ny s dng mu bit c bit, 01111110, lm c nh du im bt u v kt thc khung.Vn t ra nu trong khung d liu cng tn ti mu bit c bit, khi khung d liu s b gy. khc phc vn , khi bn gi pht hin c 5 bits 1 lin tip trong d liu gi i, n s thm vo bit 0. Ngc li, nu bn nhn pht hin 5 bits lin tip v theo sau bng mt bit 0, n s loi b bit 0 ra khi d liu. Nh th c s khng xut hin trong d liu gi.D liu gc D liu chuyn ln ng truyn,

D liu nhn sau khi loi b cc bit n

3.3. K thut truyn ni tip khng ng b 3.3.1. Mu tn hiu trong ch truyn bt ng b Trong ch truyn bt ng b thng tin c truyn i di dng tng k t v khong cch cc k t l ngu nhin. Tuy nhin to s ng b gia my pht v thu, giao thc tng 2 (Data link protocol) c qui nh c th v mu tn hiu trong h thng truyn bt ng b nh sau: Mi k t gm mt s bit gi l k t d liu, s ny c th l 5 i vi m Baudot, 7 nu l m ASCII (American Standard Code for Information Interchange) v 8 nu l m EBCDIC (Extended Binary-Coded Decimal Information Code, m BCD m rng) Ngoi ra, to s ng b, km theo cc bit m k t cn c cc bit Start trc mi k t v cc bit Stop sau mi k t. . Cc bit Start l cc bit 0 v cc bit Stop l bit 1. S bit Start lun lun l 1 bit cn s bit Stop c th l 1, 1,5 hoc 2 bit. Nu c thm bit kim sot chn l (parity bit) th bit ny nm trc bit Stop. - 51 -

trng thi ngh my pht lun pht i bit 1 gi l bit ngh (idle bit), nh vy my thu d ra bit Start khi c s bin i t 1 xung 0, sau l mt chui bit c s lng theo qui nh ca giao thc. 3.3.2. Nguyn tc

B ghi dch l thnh phn chnh ca phn cng mch truyn khng ng b. Bn pht: Khi LOAD = 1 d liu vo gm 8 bit (k c bit parity) c a ng thi qua 8 ng vo b ghi dch. Trong b ghi dch c sn mch to bit start (bit 0) v 2 bit stop (2 bit 1). Khi LOAD = 0 cc bit ca k t ln lt dch ra truyn trn ng dy. Vy b ghi dch hot ng nh mt mch bin i song song thnh ni tip. Bn thu: c mch pht hin bit vo (bit start) bo hiu bt u thu mt k t. READ = 0: cc bit trn ng dy c dch tun t vo b ghi dch (gm 11 bits: 1 bit start, 8 bits k t v 2 bits stop). READ = 1:cc bit trong b ghi dch (8 bits k t) c c mt cch song song u ra. Vy b ghi dch hot ng nh mt mch bin i ni tip thnh song song. Do bn pht khng pht xung clock pht: CLKT, nn bn thu phi c b to xung clock thu CLKR ring, sao cho CLKT = CLKR. Khi bn pht v bn thu c ng b vi nhau.Chng trnh con phat Nhan ky t Tnh toan bit chan le Tao bit chan le i=0 Tao bit start "0" Tre 1 T Bit phat TxD i=i+1 i=8 YES Tao bit stop TxD = 1 Tre 2 T Ve chng trnh chnh NO

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Hin nay do k thut ch to pht trin nhiu mch LSI trn th trng p ng cc chun truyn khng ng b nh UART 6402, 8251, 6850. Ta c th thc hin thu pht bt ng b bng phn mm thay cho phn cng, nhng b hn ch v tc (S th hin ti trang trc). 3.4. K thut truyn ni tip ng b 3.4.1. Khi qut Vic thm cc start bit v nhiu stop bit vo mi mt k t hay byte trong thng tin ni tip bt ng b lm cho hiu sut truyn gim xung, c bit l khi truyn mt thng ip gm mt khi k t. Mt khc phng php ng b bit c dng y tr ln thiu tin cy khi gia tng tc truyn. V l do ny ngi ta a ra phng php mi gi l truyn ng b, truyn ng b khc phc c nhng nhc im nh trn .Tuy nhin, cng ging nh truyn bt ng b chng ta ch cho php nhng phng php no cho php my thu t c s ng b bit , ng b k t v ng b frame. Trong thc t c hai lc truyn ni tip ng b: truyn ng b nh hng bit v truyn ng b nh hng k t. 3.4.2. Nguyn tc ng b bit. S khc nhau c bn ca truyn bt ng b v ng b l i vi truyn bt ng b ng h thu chy bt ng b vi tn hiu n, cn truyn ng b th ng h thu chy ng b vi tn hiu n, cc start bit v stop bit khng c dng, thay v vy mi frame c truyn nh l dng lin tc cc k s nh phn. My thu ng b bit trong hai cch .Hoc l thng tin nh thi c nhng vo trong tn hiu truyn v sau c tch ra bi my thu, hoc my thu c mt ng h cc b c gi ng b vi tn hiu thu nh mt thit b gi l DPLL (Digital Phase Lock-Loop). Nh chng ta s thy, DPLL li dng s chuyn trng thi t bit 1->0 hay t 0 ->1 trong tn hiu thu duy tr s ng b qua mt khong thi gian nh k no .Lc lai ghp l kt hp c hai cch Cc phng php ng b xung ng h M ho xung ng h

Dng DPLL

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Phi hp c hai cch

3.3.3 Truyn ng b nh hng k t. C hai kiu iu khin truyn ng b: ng b nh hng k t v ng b nh hng bit. C hai u dng cc nguyn tc ng b bit ging nhau. Khc nhau ch yu gia hai lc l phng php c dng t c s ng b k t v ng b frame. Truyn ng b nh hng k t c dng ch yu truyn cc khi k t, nh l cc tp tin dng t