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
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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