Reele digitalePrincipii generale

Material elaborat in colaborare cu As.ing. Kinga Marton

2Cuprins Introducere n transmisiunea digital

Bazele PCM

PCM Codec

Coduri de linie

Repetori

Sisteme de multiplexare PCM

Transmisiune PCM pe distane mari

Comutaie digital

Cerinele reelei digitale

3Introducere Transmisiune digital vs. analogic

Analogic Continuitate Informaia: magnitudinea unei caracteristici a semnalului (amplitudine,

frecven, etc.)

Extragerea informaiei: comparare cu un standard Zgomotul se acumuleaz

Digital Stri discrete Informaia: stri discrete ale semnalului (prez./abs. voltajului, contactul

este on/off, etc.)

Extragerea informaiei: asignarea de valori numerice la combinaiile posibile ale strilor discrete

Semnal binar (bit) -> circuite de decizie regeneratori Eliminarea zgomotelor

Semanul digital este mult mai tolerant la erori Compatibilitatea cu circuitele integrate lume digital

4Bazele PCM Reelele digitale folosesc modulaia n cod de

impulsuri - PCM (Pulse Code Modulation)

Reprezetarea digital a unui semnal analogic Magnitudinea semnalului este eantionata regulat la

intervale uniforme, cuantizata i codata (deob. binar)

La recepie semnalul se reface din eantioane

Dou standarde PCM

T1 (DS1): Standardul PCM Nord American Multiplexeaz 24 de canale PCM pe acelai fir de cupru

E1: Standardul European Multiplexeaz 30 de canale PCM pe acelai fir de cupru

PCM system -Typical parameters 4 KHz Speech signal

8 KHz Sampling8 bits / sample digitisingper speech channel 8 x 8 bits = 64 kbps

T1 carrier24 channels. 8 bits in 125s / channel24 x 8 = 192 bits in 125s / frame, 1 bit per frame for sync193 bits in 125s, Line rate 193/125 sec= 1.544 Mbps

ITU ( EUROPEAN)32 Channels 8 bits/ 125ss / channel32 X 8 bits / 125s = 2.048 Mbps30 channels info; 2 channels management

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6Bazele PCM - Exemplu Eantionarea i cuantizarea undei sinusoidale

pentru PCM pe 4 bii

Eantionare: La intervale regulate (axa X)momente: 0, 1,2,3,4, ...

Cuantizare Fiecrui eantion i se asociaz

o valoare de pe axa Y conf. Algoritmvalori: 7, 9, 11, 12, 13, 14, 14, 15, 15, 15, 14, ...

Codare Reprezentarea discret se traduce n date digitalecoduri: 0111, 1001, 1011, 1100, 1101, 1110, 1110, 1111, 1111, 1110, ...

7Bazele PCM Eantionare Teorema Nyquist Shannon

Un semnal analogic eantionat poate fi perfect reconstruit din eantioanele sale dac rata de eantionare este mai mare sau egal cu de dou ori frecvena maxim a semnalului original

Exemplu Canal de voce de 4-kHz: rata de eantionare >= 8000

eantioane pe secund, deci la fiecare 125 sec

PAM (Pulse Amplitude Modulation) Mai multe streamuri PCM pot fi multiplexate ntr-un

stream de date i trimis pe aceai fir folosind TDM (Time Division Multiplexing)

Eantionarea implic mai multe canale

8Bazele PCM Eantionare - PAM

9Bazele PCM - Cuantizare mprirea domeniului continuu de valori n

subdomenii contigue (nu neaparat egale) Asignarea de valori discrete i unice la fiecare subdomeniu La fiecare eantion se atribuie o valoarea discret coresp.

domeniul n care se ncadreaz

Distorsiune de cuantizare (funcie de rata de eantionare)

Compandare = Compresor + Expandor Previne suprancrcarea canalului de transmisie, reduce distorsiunea

de cuantizare, etc.

Compresie: Favorizarea vorbirii de

intensitate slab Mai multe segmente codate

se asigneaz eantioanelorde intensitate slabn mod progresiv

Fiecrui nivel are 8 segmentecodate

Valori rotunjite la cel mai apropiat ntreg

Linii de schimbare a valorii

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Bazele PCM - Cuantizare

Distorsiune de cuantizare

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Bazele PCM - Codare Cod de 8 bii Compandarea i

codarea se fac simultan Compresia i expandarea

au funcii logaritmice

Legea de codare A (E1) (legea de codare (T1)) Curba pseudologaritmic

este alctuit din segmente lineare

Granularitate fin pentru semnale de intesitate slab i granularitate mare pentru semnale de intensitate mare

Cuvinte PCM: 16 x 8 bii Ex: se recepioneaz

secvena 11010100

Dou elemente liniare

Voltaj negativ Identific segmentul

Poziia pe segment

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PCM Codec Codor

Accept 24 / 30 canale de voce Digitizeaz i multiplexeaz informaia Produce un stream serial de bii cu 1,544 Mbps sau 2, 048 Mbps

Decodor Accept un

stream serial de bii la una din ratele de modulare

Demultiplexeaz informaia digital

Efectueaz o conversie digital-analogic

Produce 24 30 canale de voce de 4 kHz

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Coduri de linie Semnalul digital PCM se convertete ntr-o

form de und Reprezint valorile de 1 i 0 al semnalului digital Pentru transmiterea prin canalul de transmisie Rezisten la anumite forme de pierdere de semnal

Diferitele coduri de linie au diferite atribute NRZ, AMI, CRI, etc.

AMI (Alternate Mark Inversion) sau Codare bipolar

0 binar : 0 V 1 binar : alternativ voltaj + sau Duty cycle pt. Biii de 1 = 1/2

Avantaj Media pulsurilor e zero (fara DC) -> distane mai mari ir de 1-uri produce tranziii detectarea erorilor

Problema ir de 0-uri nu produce tranziii

Timing signal sau substituie de bii (B8ZS, B6ZS, HDB3)

Taxonomy of coding

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Cryptography(Ciphering)

SourceCoding

CompressionCoding

Line CodingError Control Coding

Error CorrectionCoding

Error DetectionCoding

- Secrecy/ Security- Encryption (DES)

- Redundancy removal: - Destructive (jpeg, mpeg) - Non-destructive (zip)

- Makes bitsequal probable

- Strives toutilizechannelcapacity byadding extra bits

- for baseband communications- RX synchronization- Spectral shaping for BW requirements- error detection

- used in ARQ as in TCP/IP- feedback channel- retransmissions- quality paid by delay

= FEC- no feedback channel- quality paidby redundantbits

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FEC: Forward Error CorrectionARQ: Automatic Repeat RequestDES: Data Encryption Standard

Background

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Coding is used for error detection and/or error correction (channel

coding) ciphering (security) and compression (source coding)

In coding extra bits are added or removed in data transmission

Channel coding can be realized by two approaches FEC (forward error coding)

block coding, often realized by cyclic coding convolutional coding

ARQ (automatic repeat request) stop-and-wait go-back-N selective repeat etc.

Note: ARQ applies FEC for error detection

Source coding

Block-oriented Text: ASCII (7bits for each character) and EBCDIC;

extended ASCII uses 8 bits per character Compression techniques: "the" "e" occur a lot

Images: Fax of an 8" by 10" page with 400 by 400 pixels per

sq. inch results in 38.4Mbytes if three bytes are used, one each to represent R, G, and B.

GIF: lossless compression JPEG: lossy compression

Stream-oriented Voice: PCM (Pulse Code Modulation); 8000 samples/sec;

with 8 bits/sample, it results in 64Kbps.

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Compression techniques:

ADPCM - 32 Kbps

Residual excited linear predictive coding - 8-16 kbps

Audio (music): needs 32-384Kbps

Video:

H.261 coding: 176 by 144 or 352 by 258 frames at 10-30 frame/sec

Full motion MPEG-2

HDTV - 1920 by 1080 frames at 30 frames/sec (aspect ratio is important 16:9 vs. 4:3)

Requirements of different traffic types:

Text/data: sensitive to loss

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Source coding

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Audio: sensitive to delay and jitter (delay variation)

Transmission (emission) delay is L/R where L bits needs to be transferred over a channel operating at R bits/sec

Propagation delay is distance divided by speed of light in medium of the channel

Packetization delay: time to create an audio packet to send on a packet-switched network or to create a voice sample to send on a circuit-switched network; depends on the codec rate; for example, G.711 codecs operate at 64Kbps

For telephony traffic, the one-way delay should be

Less than 25ms for excellent quality voice without echo cancellers

Less than 150ms for excellent quality voice with echo cancellers

Less than 400ms for acceptable quality voice with echo cancellers

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Block coding: mapping of source bits of length k into (binary) channel input sequences n (>k) - realized by cyclic codes!

Binary coding produces 2k code words of length n. Extra bits in the code words are used for error detection/correction

(1) block, and (2) convolutional codes: (n,k) block codes: Encoder output of n bits depends only on the k input bits

(n,k,L) convolutional codes: each source bit influences n(L+1)

encoder output bits n(L+1) is the constraint length L is the memory depth

Essential difference of block and conv. coding is in simplicity of design of encoding and decoding circuits

Block and convolutional coding

k input bits

n output bits

n(L+1) output bits

input bit

Block coding - examples4B/5B encoding

- Each 4-bit 'nibble' of received data has an extra 5th bit added- there are 24 = 16 different bit patterns. - with 5-bit packets, there are 25 = 32 different bit patterns- as a result, the 5-bit patterns can always have two '1's in them even if the data is all '0's (at least two transitions)- enables clock synchronization required for reliable data transfer.

5B/6B Encoding - as 4B/5B but you can have DC balance (3 zero bits and 3 one bits in each group of 6) to prevent polarization. -5B/6B Encoding is the process of encoding the scrambled 5-bit data patterns into predetermined 6-bit symbols.- added error-checking capability: invalid symbols and invalid data patterns

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5B/6B

Same idea as 4B/5B but have DC balance (3 zero bits and 3 one bits in each group of 6) to prevent polarization-encoding the scrambled 5-bit data patterns into predetermined 6-bit symbols -equal numbers of 0's and 1's, to provide:

- guaranteed clock transitions synchronization - an even power value on the line - added error-checking capability

8B/6T8 data bits sent as six ternary (one of three voltage levels) signals - carrier: running at 3/4 of the speed of the data rate-there are 36 = 729 possible patterns (symbols) for 256 possible data values-rules for the symbols: there must be at least two voltage transitions (to maintain clock synchronization) and the average DC voltage must be zero (avoid polarization)

Other codes: 8B/10B, MLT-3, PAM 5 (similar but with 5 voltage) levels

Convolutional encoding

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Convolutional codes are applied in applications that require good performance with low implementation cost. They operate on code streams (not in blocks)

Convolution codes have memory that utilizes previous bits to encode or decode following bits (block codes are memoryless)

Convolutional codes achieve good performance by expanding their memory depth

Convolutional codes are denoted by (n,k,L), where L is code (or encoder) Memory depth (number of register stages)

Constraint length C=n(L+1) is defined as the number of encoded bits a message bit can influence to

n(L+1) output bits

input bit

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Repetori regenerativi Codul de linie al semnalului digital transmis prin canalul

de comunicaie este Atenuat Filtrat Corupt de zgomot

Pentru recuperarea semnalului se plaseaz pe linie i la recepie

Repetori regenerativi Amplific i reconstruiete semnalul digital distorsionat Zgomotul se oprete la repetori Sursa principal a jitterului de temporizare

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Sisteme de multiplexare PCM Multiplexare primar

Standardele T1 (DS1) i E1 sunt incompatibile

Multiplexare de nivel superior Alctuite din mai multe surse de multiplexare primar Problema:

Sincronizarea surselor deplasarea ceasurilor ntre diferitele surse

Soluia Stuffing

Se insereaz bii pentru compensarea diferenelor de temporizare; se adaug pulsuri la semnalul multiplexat, pn se potrivete cu ceasul local

Pulsurile se insereaz la poziii fixe i se elimin la demux Rata de transfer la transmisie e mai mare dect suma ratelor

de transfer la intrare (excepie biii de cadru)

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Transmisiune PCM pe distane mari Limitri

Fiecare mediu de transmisie are limitri Fiecare limitare este o funcie de lungimea liniei i rata de transmisie

Jitter Variaia brusc, aleatoare i nedorit a eantionului de la poziia

corect n timp sau faz (afect. durata i/sau poziia n timp) Repetorii regeneraitvi (inclusiv switchuri, receivere, radio digital)

introduc jitter care se acumuleaz Poate duce la crosstalk i distorsiune Reducerea repetorilor reduce jitterul (fibr optic 64-320 km)

Distorsiune Abaterea formei de und a semnalului de la forma ideal Introdus de caracteristicile legturii metalice

Pierdere, Distorsiunea de amplitudine, Distorsiunea de ateptare

Zgomot termal Considerate pe segmentele dintre repetori (nu se acumuleaz) Eroare de bii se acumuleaz -> rata de eroare de bii (BER = 10-3)

Crosstalk Semnalul de pe un canal produce efecte nedorite ntr-un alt canal

NEXT (Near-end Crosstalk): interferen msurat pe partea transmitorului FEXT (Far-end Crosstalk): interferen msurat pe partea opus transmitorului

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Comutaie digital (Digital Switching) Switch: se conpune din secvene T i S

T Time Division Switching S Space Division Switching

Time Switch (TSI - Time Slot Interchanger) Un time slot:

cuvnt PCM de 8 bii

Reprez. un canal de voce se repet de 8000 ori/sec. DS1 are 24 de time sloturi/cadru, E1 are 32 time sloturi/cadru

Mutarea datelor din fiecare time slot din streamul de intrare ntr-un stream de ieire n alt ordine

n funcie de destinaia fiecrui timeslot Blocuri funcionale:

Memorie pentru voce Memorie pentru control Numrtor i procesor de time sloturi

Moduri de lucru Scriere secvenial, citire aleatoare Citire secvenial, scriere aleatoare

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Comutaie digital (Digital Switching) Space Switch

Permite comutareasloturilor de timp ntr-un domeniu spaial

TST Switch STS Switch

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Reele digitale Cerine tehnice

Sincronizare la nivel de Bit: transmitorul i receptorul

s lucreze la aceai rat de bii Time slot: este asigurat de

sincronizarea cadrelor (frame)

Frame: DS1: bit de ncadrare E1: canalul 0 de sincronizare

Reea: sincronizarea ceasurilor master al fiecrui switch

Evitarea scprilor (slip)

Modaliti de sincronizareCeasuri identice de mare stabilitate, nesincronizate

Cerine de performan BER

raportul dintre biii eronai i numrul total de bii

Jitter funcie de numrul de

repetori n tandem