Capitolul 2. Nivelul fizic

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1 Capitolul 2 Nivelul fizic 1 1

2 Obiectivele capitolului 2 a) Bazele teoretice ale comicării de date b) Medii pentru transmisii de date c) Transmisii wireless d) Comunicaţii prin satelit e) Sistemul public de telefonie f) Telefonia mobilă g) Comunicaţii prin cablul pentru televiziune 2 2

3 a) Bazele teoretice ale comicării de date Analiza Fourier Semnale de bandă limitată Rata de transfer maximă a unui canal 3 3

4 a b k k 2 = T 2 = T 2 c = T T 1 0 T 1 T s s s a) Analiza Fourier Pentru o funcţie periodică s(t), de perioadă T1= 1/f1 seria Fourier este: Unde: s () t = c a sin( 2πkf t) + b cos( 2πkf t) () t sin( 2πkf t) () t cos( 2πkf t) () t 1 2 dt k = k dt, dt, Ak 1 k = 1 k În domeniul frecvenţă se reprezintă amplitudinele armonicelor de ordin k: A = a + b k 2 k 2 k , kt1 Frecvenţa f1 este numită frecvenţă fundamentală. Seria Fourier face trecerea din domeniul timp în domeniul frecventă. Dacă este cunoscută frecvenţa fundamentală şi amplitudinele armonicelor ak, bk şi c funcţia s(t) poate fi reconstituită.. Pentru un semnal binar spectrul este infinit. Deci, în cazul real, prin mediile şi circuitele implicate în transmitere nu se pot transmite toate componentele spectrale deoarece au banda limitată. Se pune problema câte armonici superioare trebuiesc transmise, ca la recepţie semnalul binar să poată fi reconstituit. 4

5 a) Spectrul unui semnal dreptunghiular 1.0 A Amplitudine T 1.0 T 1.5 T 2.0 T Timp -1.0A s 1 k () t A sin( 2πkf t), k = impar = k = 1 1 Spectrul conţine doar armonici impare a căror amplitudine scade cu ordinul armonicii 5 Să presupunem că se transmite un şir de date de forma unde 1 este codificat printr-o valoare pozitivă de tensiune, iar 0 prin valoarea negativă. Rezultă un semnal dreptunghilar simetric faţă de zero. Din spectrul semnalului se observă că partea mare a energiei (energia este poporşională cu amplitudinea la pătrat) este concentrată în primele armonici. Dacă la reconstrucţie se iau în considerare primele trei armonice, atunci banda necesară transmiterii semnalului este 3*f1- f1 = 2*f1. Dacă f1 = 1 MHz, atunci banda necesară este de 2MHz. Deoarece pe durata T1 = 1 microsecundă se transmite un 1 şi un 0 rezultă că în banda de 2MHz se transmie un semnal cu viteza de 2MBs. Dar nu orice receptor e capabil să reconstituie semnalul binar iniţial numai din primele 3 armonice. Dacă sunt necesare transmiterea primelor 5 armonice, conform raţionamentului de mai înainte, pentru a transmite cu o viteză de 2MBs este necesară o bandă de 4MHz, iar dacă trebuiesc primele 7 armonice, banda necesară este de 6 MHz. Dacă în raţionamentul nostru considerăm că receptorului îi sunt suficiente primele 5 armonice se observă că dublând viteza de transmisie şi banda necesară este dublă. Prin urmare un mediu de transmisie de bandă dată suportă viteze de transmisie diferite, funcţie de calităţile receptorului. 5

6 a) Semnale de bandă limitată Amplitudine Timpul Numărul de ordine k al armonicelor 1 ak = πk 1 bk = πk 3 c =, 8 [ cos( πk /4) cos3 ( πk /4) + cos6 ( πk /4 Prima ) amonica cos7 ( πk /4)] [ sin( πk /4) + sin3 ( πk /4) sin6 ( πk /4) + sin7 ( πk /4)] Amplitudine k = a 2 k + b Primele două armonici superioare Semnalul binar ce codifică caracterul b în cod ASCII ( ) şi valorile efective ale componentelor spectrale (Fourier). 2 k.,, 6 S-a luat în considerare transmiterea caracterului b codificat în codul ASCII pe un octet

7 a) Semnale de bandă limitată(2) Amplitudine Timpul Numărul de ordine al armonicelor Prima amonica Primele două armonici superioare (a) Semnalul binar iniţial. (b) (c) Aproximări ale semnalului iniţial folosind armonicele superioare. 7 S-a luat în considerare transmiterea caracterului b codificat în codul ASCII pe un octet Mediul de transmisie se caracterizează prin Banda de frecvenţe. Pentru semnalele a căror frecvenţă se situează în interiorul Benzii de frecvenţă atenuarea este mult mai mică decît pentru cele din afara benzii. În practică limitele benzii de frecvenţă nu sunt abrupte, ci trecerea de la atenuare mare la una mică se face treptat. De aceea frecvenţele limită între care se stabileşte banda sunt acelea pentru care puterea semnalului scade la jumătate. 7

8 a) Semnale de bandă limitată (3) Primele patru armonici Primele opt armonici Timpul Numărul armonicelor (d) (e) Aproximări ale semnalului iniţial folosind armonicele superioare. 8 Timpul T necesar pentru transmiterea caracterului depinde atât de metoda de codificare cât şi de câte ori pe secundă se schimbă valoarea (tensiunea) semnalului. Numărul de variaţii pe secundă se măsoară în baud. Pentru un semnal binar (codificat doar pe 2 niveluri de tensiune) numărul de biţi transferaţi pe secundă este egal cu viteza de transmisie. 8

9 a) Semnale de bandă limitată (4) Prima armonică (Hz) Nr. armonici trimise Relaţia dintre rata de transfer şi numărul de armonici ce pot fi transmise printr-o linie telefonică obişnuită. 9 Considerând viteza de transmisie egală cu n bit/sec şi că trebuiesc transmişi 8 biţi, atunci frecvenţa primei armonici este n /8. Linia telefonică obişnuită are banda de frecvenţă de 3000 Hz, ceea ce conduce la faptul că armonica superioară ce poate fi transmisă va avea cel mult ordinul 3000 /(n /8), adică /n. Observaţie: caracterul b din desenele anterioare poate fi reconstituit cu greu dacă se alege o viteză de transmisie de 9600 biţi /secundă. Există tehnici de codificare a datelor pe mai multe niveluri de tensiune şi acestea asigură rate mai mari de transmisie. 9

10 a) Rate maxime de transfer a) Pentru un canal fără zgomot de bandă H şi în cazul codificării semnalului pe V niveluri discrete Nyquist stbileşte: Rata maximă de transfer = 2*H*log2V [biţi/sec], b) În cazul existenţei unui zgomot N, prin canalul caracterizat prin raportul semnal/zgomot S / N, Shannon stabileşte: Numărul maxim de biţi/sec = H*log2(1+S/N) 10 Pentru cazul a) un canal fără zgomot, cu banda de 3 khz nu poate transmite semnale binare (cu 2 niveluri) la o viteză mai mare de 6000 bps În cazul în care canalul precedent prezintă un zgomot termic de 30 db (însemnând un raport semnal/zgomot = 1000), el nu va putea transmite mai mult de bps, indiferent câte niveluri are semnalul transmis sau cât de multe eşantioane se iau în considerare. 10

11 b) Medii pentru transmisii de date Medii inscriptibile Fire răsucite (Twisted Pair ) Cablu coaxial (Coaxial Cable ) Fibră optică (Fiber Optics ) 11 În categoria mediilor inscriptibile: benzi şi discuri magnetice, CD /DVD ROM, memorii electronice nevolatile portabile (sub formă de stick, card, etc.). Pe aceste medii datele pot fi transportate de la un calculator la altul. Viteza O bandă video standard de 8 mm poate înmagazina 7 gigaocteţi. Într-o cutie poştală standard, de carton încap 1000 benzi, adică 7000 gigaocteţi. Oriunde pe teritoriul SUA poşta transportă cutia în 24 ore (84600 secunde). Rezultă o viteză de transfer de 648 Mbps. Dacă cutia este transportată doar la o oră distanţă cu maşina rezultă o viteză de 15 Gbps. Costul 1000 benzi cumpărate en gros costă 5000 dolari. O bandă poate fi folosită de cel puţin 10 ori reducând costul benzilor transportate odată la 500 dolari. Considerând costurile poştale de transport de 200 dolari ajungem la costul transferului celor 7000 de gigaocteţi de date la 700 dolari. Rezultă un cost de 10 cenţi/gigaoctet. Este o performanţă greu de egalat. 11

12 b) Fire răsucite (a) Categoria 3 UTP (Unshielded Twisted Pair). (b) Categoria 5 UTP. 12 Firele răsucite reduc interferenţa electrică. Categoria 3 are mai puţine răsucire pe unitatea de lungime, au izolaţie de polietilenă (hârtie) şi 4 perechi sunt grupate şi izolate în palstic. Categoria 5 are mai multe răsuciri pe unitatea de lungime şi sunt izolate în teflon 12

13 b) Cablul coaxial Fir de cupru Material Izolator Tresă metalică Material plastic pentru protecţie Componentele unui cablu coaxial 13 Impedanţa caracteristică: 50 ohmi pentru transmisii de date şi 75 ohmi pentru transmisii analogice (tv analogic) Pentru lungimi de 1 km este posibilă o viteză de transfer de 1 până la 2 Gbps. Pentru a preveni reflexiile, cablurile trebuie să aibă la capete terminatori (rezistoare ce au valoarea egală cu impedanţa caracteristică). Pentru lungimi mai mari se intercalează amplificatoare (repetoare). 13

14 b) Fibra optică Suprafaţă de separare Aer/sticlă Aer Reflexie totală în sticlă Sticlă Sursă de lumină (a) (b) Reflexia şi refracţia la suprafaţa de separare aer/sticlă în cazul a trei unghiuri diferite de incidenţă a razei de lumină din interiorul fibrei optice. În cazul relexiei totale lumina rămâne în interiorul fibrei de sticlă. 14 Unghiul sub care se trimite lumina în fibră asigură reflexia totală. Dacă fibra este mai groasă se pot transmite mai multe raze de lumină, de diferite lungimi de undă şi sub diferite unghiuri. Avem o fibră multimod. Dacă fibra este subţire câteva lungimi de undă a luminii transmise atunci lumina se transmite practic fără reflexii şi fibra se cheamă monomod. Într-o astfel de fibră se pot atinge viteze de Gbps şi fară amplificatoare pâna la km. 14

15 b) Transmiterea luminii prin fibra optică Lungimea de undă în microni Fibra atenuează lumina în zona infraroşu. 15 ATENUAREA în decibeli = 10*lg(puterea_transmisă /puterea_recepţionată). În benzile de 1,3 sau 1,55 microni pierderile sunt de aprox 5 procente pe kilometru. Se preferă folosirea benzii de 0,85 deoarece aici laserul şi circuitele electronice pot fi făcute utilizând acelaşi material arseniura de galiu. 15

16 b) Cabluri cu fibre optice Miez (sticlă) Teacă Protecţie Îmbrăcăminte (sticlă) Protecţie (plastic) Miez Îmbrăcăminte (a) Componenţa unei fibre de sticlă. (b) Într-o teacă sunt trei fibre. 16 În fibrele monomod miezul are o grosime de până la 10 microni şi este făcut din sticlă foarte pură. Cele multimod au o grosime de până la 50 microni. Îmbrăcămintea este din sticlă cu un indice de refracţie mai mic decât cel al sticlei din miez. Astfel se păstrează lumina în miez. 16

17 b) Cabluri cu fibre optice (2) O comparare între două surse de lumină: dioda laser şi LED

18 b) Reţele cu fibră optică Semnal electric Semnal luminos Semnal luminos Fibră optică cu repetoare active

19 b) Reţele cu fibră optică (2) Conexiune pasivă de tip stea într-o reţea cu fibră optică

20 c) Transmisii fără fir (wireless) Spectrul electromagnetic Transmisii Radio Transmisii în domeniul microunde Infraroşu şi Unde milimetrice Transmisii în domeniul vizibil 20 20

21 c) Spectrul electromagnetic Utilizarea spectrului electromagnetic pentru comunicaţii

22 c) Transmisia radio (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere

23 c) Gestionarea spectrului electromagnetic The ISM bands in the United States

24 c) Transmisii utilizând spectrul vizibil Sistem bidirecţional (cu 2 lasere). Curenţii de convecţie datoraţi încălzirii pot interfera cu sistemul de comunicaţie cu laseri

25 d) Comunicaţii prin satelit Sateliţi geostaţionari (sincroni) Sateliţi situaţi pe orbite la medie altitudine Sateliţi de joasă altitudine Sateliţi versus fibre optice 25 25

26 d) Comunicaţii prin satelit Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage

27 d) Comunicaţii prin satelit (2) The principal satellite bands

28 d) Comunicaţii prin satelit (3) VSATs using a hub

29 d) Sateliţi pe orbite de joasă amplitudine: Iridium (a) (b) (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth

30 d) Globalstar (a) Relaying in space. (b) Relaying on the ground

31 e) Sistemul public de telefonie Structure of the Telephone System The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching 31 ADSL Asymetric Digital Subscriber Line 31

32 e) Structura sistemului telefonic (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy

33 e) Structura sistemului telefonic (2) Circuite uzuale implicate la stabilirea unei convorbiri pe o distanţă medie

34 e) Componentele principale ale sistemului telefonic Componente locale (Bucle locale) Analog twisted pairs going to houses and businesses Trunchiuri Digital fiber optics connecting the switching offices Oficii de comutare Where calls are moved from one trunk to another 34 34

35 e) Politici în domeniul telefonic The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it. 35 LATA Local Access and Transport Areas LEC Local Exchange Carrier, IXC InterXchange Carrier 35

36 e) Componente locale: Modem, ADSL, şi Wireless Calculatoarele comunică printr-o combinaţie de transmisii analogice şi numerice. Conversiile sunt făcute de modemuri şi codecuri

37 e) Modem (a) Un semnal binar (b) Modulare în amplitudine (c) Modulare în frecvenţă (d) Modulare în fază 37 37

38 e) Modem (2) (a) QPSK. Quadrature Phase Shift Keying (2 bits/symbol) (b) QAM-16.- Quadrature Amplitude Modulation (4b/sym) (c) QAM-64. (6bits/symbol) 38 Diagrame de constelatie deseneaza fazorial, în 4 cadrane modulaţia în fază şi amplitudine 38

39 e) Modem (3) (a) (b) (a) V.32 pentru 9600 bps. (b) V32 bis pentru 14,400 bps

40 e) Linii numerice pentru abonaţi Bandwidth versus distanced over category 3 UTP for DSL. 40 DSL Digital Subscriber Line 40

41 e) Linii numerice pentru abonaţi (2) Operation of ADSL using discrete multitone modulation. 41 ADSL Asymetric Digital Subscriber Line 41

42 e) Linii numerice pentru abonaţi (3) A typical ADSL equipment configuration. 42 DSLAM Digital Subscriber Line Access Multipexer NID = Network Interface Device 42

43 e) Componente locale Wireless Architecture of an LMDS system. 43 ISP Internet Service Provider LMDS Local Multipoint Distribution Service 43

44 e) Multiplexare prin divizarea în frecvenţă (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel

45 e) Multiplexare prin divizarea lungimii de undă Wavelength division multiplexing

46 e) Multiplexare prin divizarea timpului The T1 carrier (1.544 Mbps)

47 e) Multiplexare prin divizarea timpului (2) Delta modulation

48 e) Multiplexare prin divizarea timpului (3) Multiplexing T1 streams into higher carriers

49 e) Multiplexare prin divizarea timpului (4) Two back-to-back SONET frames

50 e) Multiplexare prin divizarea timpului (5) SONET and SDH multiplex rates

51 e) Comutare de circuit (a) Circuit switching. (b) Packet switching

52 e) Comutare de mesaje 52 (a) Circuit switching (b) Message switching (c) Packet switching 52

53 e) Packet Switching A comparison of circuit switched and packet-switched networks

54 f) The Mobile Telephone System First-Generation Mobile Phones: Analog Voice Second-Generation Mobile Phones: Digital Voice Third-Generation Mobile Phones: Digital Voice and Data 54 54

55 f) Advanced Mobile Phone System (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used

56 f) Channel Categories The 832 channels are divided into four categories: Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data 56 56

57 f) D-AMPS Digital Advanced Mobile Phone System (a) A D-AMPS channel with three users. (b) A D-AMPS channel with six users

58 f) GSM Global System for Mobile Communications GSM uses 124 frequency channels, each of which uses an eight-slot TDM system 58 58

59 f) GSM (2) A portion of the GSM framing structure

60 f) CDMA Code Division Multiple Access (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C s signal 60 60

61 f) Third-Generation Mobile Phones: Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace , fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.) 61 61

62 g) Cable Television Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable 62 62

63 g) Community Antenna Television An early cable television system

64 g) Internet over Cable Cable television 64 64

65 g) Internet over Cable (2) The fixed telephone system

66 g) Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access 66 66

67 g) Cable Modems Typical details of the upstream and downstream channels in North America

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