Digital Communication Systems - NPTel

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Topics in Digital Communications. ○Digital communication system advantages and disadvantages. ○Digital communication system classification. ○Digitization  ...
Digital Communication Systems H. S. Jamadagni, CEDT, IISc, Bangalore

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Topics in Digital Communications zDigital communication system

advantages and disadvantages zDigital communication system classification zDigitization of analog signals zDigital transmission systems zData communication systems zIntegrated Services Digital Network and other advanced digital communication systems

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Digital Communication advantages zReliable communication; less sensitivity to changes in

environmental conditions (temperature, etc.) zEasy multiplexing zEasy signaling „ Hook status, address digits, call progress information zVoice and data integration zEasy processing like encryption and compression zEasy system performance monitoring „ QOS monitoring zIntegration of transmission and switching zSignal regeneration, operation at low SNR, superior performance zIntegration of services leading to ISDN

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Digital Communication System Disadvantages zIncreased bandwidth „ 64 KB for a 4 KHz channel, without compression (However,

less with compression) zNeed for precision timing „ Bit, character, frame synchronization needed zAnalogue to Digital and Digital to Analogue conversions „ Very often non-linear ADC and DAC used, some performance degradation zHigher complexity

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Types of Digital Communication Systems Signal Type

Transmission

Example

Analog

Analog

Classical telephony

Analog

Digital

PCM TDM

Digital

Analog

Modems

Digital

Digital

ISDN, LANs

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Digitization of analogue signals

Amplitude

zSignal sampling

Amplitude

Time

Time

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Nyquist Criterion, Aliasing Amplitude

Original signal

Time Amplitude

Aliased signal

Time Nyquist sampling rate fs > 2. fb Jamadagni H S

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Spectrum of baseband signals Amplitude

+B -B Frequency Spectrum of the properly sampled base band

Spectrum of the under sampled base band Jamadagni H S

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Speech signal digitisation

Input signal

PAM samples

Output signal

LPF

Pulse train Pulse amplitude modulation

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Pulse Code Modulation (PCM) PAM samples Analog input

Low pass filter

Sample clock

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D/A

A/D Digitally encoded samples

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Low pass filter

Analog Output

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Quantisation of speech signal samples 011

Quantisation errors

010 001 000 100 101 110 111 010

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010 000 011 001 001 Quantisation of analog samples

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000

101

11

Decoder output

Quantisation error in PCM

Quantized output signal Input signal

Input amplitude

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Quantisation error as a function of amplitude

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PCM system - Typical parameters 4 KHz Speech signal 8 KHz Sampling 8 bits / sample digitising per speech channel 8 x 8 bits = 64 kbps T1 carrier: 24 channels. 8 bits in 125 µs / channel 24 x 8 = 192 bits in 125 µs / frame, 1 bit per frame for sync 193 bits in 125 µs, Line rate 193/125 µsec = 1.544 Mbps ITU ( EUROPEAN) 32 Channels 8 bits/ 125 µss / channel 32 X 8 bits / 125 µs = 2.048 Mbps 30 channels info; 2 channels management

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Idle channel noise minimisation Idle channel noise: Caused by uncertainty in coding a sample near zero value

Mid-riser quantizer

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Mid-tread quantizer

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Signal to quantizing noise of uniform PCM number of bits / sample 13

80 60 SNR (dB)

11

40

9

20

-40

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-30

-20

A/Amax

(dB)

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-10

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Non-linear AD conversion

Compressed sample values

Uniform Quantisation

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Input sample values DC/V1/2004

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Compression law - µ law

Normalised input 0

0.2

0.4

0.6

0.8

1

0

tuptuo desilamroN

0.2

0.4

0.6

0.8

1

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Compression law - A law

Normalised input 0

0.2

0.4

0.6

0.8

1

tuptuo desilamroN

0

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0.2 0.4 0.6 0.8 1

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Non linear AD conversion laws used in PCM for speech

F z(x) = sgn(x)

ln(1 + z) ln(1 + z.|x|)

µ law F A (x) = sgn(x)

1 + ln A A|x|

A law

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Low bit rate coding: DPCM, Differential PCM

Differentiator

Speech signal

Band limiting filter

+ Σ

A/D

-

Previous input estimate Jamadagni H S

Sampler quantizer encoder

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Encoded difference samples

D/A Accumulator 20

DPCM implementation ADC

LPF

+ -

Σ S&H

S&H

+

Σ

+

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DAC

LPF

Σ + Decoder

+

Encoder DAC

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Delta modulation Input

+

LPF

Σ

-

+

"1"

LPF

-

Output

Integrator

Decoder 1 Integrator

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0 Pulse generator

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Slope overload distortion in Delta modulation

Slope over load

Granular noise

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Low Data Rate Modulation zCVSD (Continuous Variable Slope Delta Modulation) Receive clock

Sample clock + S -

+ -

1 0

FF

P G

P 1 G 0 C2 All 1s Step size control

P G

P G C1 Encoder

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All 0s

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Decoder

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Conceptual view of ISDN Telephone

Customer ISDN interface

Data terminal

"Digital pipe"

"Digital pipes"

Packet switched network Circuit switched network Other networks

PBX

Subscriber loop and ISDN channel structures

Alarm

Data bases

Other services

Local area network Jamadagni H S

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Block diagram of ISDN functions Integrated Digital Network Digital circuit switched backbone Common physical interface

ISDN central office

Packet-switched backbone

Subscriber loop to ISDN channel structures: Basic = 64 kbps + 64 kbps + 16 kbps Network-based processing services

Primary = multiplexed 64 kbps channels

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ISDN principles Ê ISDN

is based on concepts developed for telephony. Therefore, evolutionary changes

Ê Transition

from the present network to ISDN may require about one decade.

Ê End-to-end

digital connectivity to be obtained using digital transmission, TDM switching and or SDM switching.

Ê Present

ITU standards part of new standards

Ê In

early development of ISDN interim measures needed for interfacing with present networks

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Principles of ISDN (Cont.) Ê Supports

a wide range of voice and non-voice applications

Ê Switched

and non-switched connections Circuit switching and packet

switching Ê Based

on 64 Kbps channels

Ê Intelligence

for providing service features, maintenance and management integrated

Ê Layered

protocol used

Ê Flexibility

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for implementation at specific national situations

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ISDN evolution

z

Digital exchanges commissioned in late 60's and 70's

Integrated digital transmission and switching established (IDN) z

Integrating services in IDN is the latest step leading to ISDN INTEGRATED SERVICES DIGITAL NETWORK

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ISDN services: Definition of attributes

All services on the ISDN network are characterised by "attributes" defined in ITU 1.130 standards

z

z

Attributes have a definition and allowable values

z

Any service has a set of valid attributes

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ISDN services: Attributes

Attribute Name

Values

Info. transfer mode Info. transfer rate Info. transfer capability

Circuit, packet Bit rate Speech, 3.1 KHz audio 7 KHz audio 15 KHz audio Video Other values Bit error rate

Connection performance Jamadagni H S

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ISDN service classification

Services defined by attributes ÊBearer

services ÊTeleservices ÊSecondary services Bearer services provide capability to transfer information between ISDN access points and involve only low level layers (1,2 and 3)

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ISDN teleservices ÊLow

layer attributes

ÊHigh

layer attributes ÊType of user information ÊLayer 4 protocols ÊLayer 5 protocols ÊLayer 6 protocols ÊLayer 7 protocols ÊGeneral

attributes ÊQuality of service

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Customer access to services supported by ISDN 3

5

TEI

TE2

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4 R

2 S

1 T

NT2

NT1

TA S

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Functional grouping z

z

z

z

TE: Terminal equipment TE1: S interface terminal TE2: R interface terminal TA: Terminal adapter adapts TE2 to S interface NT: Network termination NT2: Optional, PBX applications NT1: S/T interface to U interface Interface structure 2B + D 192 Kbps line rate 23B + 4536 Kbps line rate

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Network functional principles Services to be internationally compatible zUNI standardised so that TE is portable zStandardise network capability z

High Layer Low Layer Operation & manage

Layer 1:

Physical layer connection activation deactivation, bit transmission channel structure mutiplex.

Layer 2:

Data link connection establishment, Data link congestion handling How control, error, sequence control, frame sync.

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Access channel and rate

Channel name

Bit rate

D

16 Kbps 64 Kbps

B H0 H1 H11 H12

64 Kbps 384 Kbps

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Bearer services

64 Kbps unrestricted, 8 KHz structured z 64 Kbps 8 KHz structured, speech z 64 Kbps 8 KHz structured, 3.1 KHz audio z 384 Kbps unrestricted z 1536 Kbps unrestricted z 1920 Kbps unrestricted z Packet - mode services z

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ISDN subscriber premises connections