telecommunications switching systems 9

7/29/2019 Telecommunications Switching Systems 9

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Chapter 2

Lecture 9

Advantages of Digital Voice Networks Ease of multiplexing

Ease of signaling

Use of modern technology

Integration of transmission and switching

Signal regeneration

Performance Monitorability

Accommodation of other services

Operability atlow signal-to-noise/Interferenceratios

Ease of encryption


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Digital Signal Processing

Operation on a signal to enhance ortransform its characteristics

Can be applied to both analog and digitalwaveforms

Common Examples: Amplification,equalization, modulation and filtering aresome signal processing functions.

DSP

Use of digital logic and circuits toimplement signal processing functions ondigitized signal waveforms.


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Some of the other applications of DSP are: audio signal processing,

audio compression, speech processing, speech recognition, digital image processing, video compression, image manipulation, computer-generated animations in movies, digital communications, RADAR, SONAR, weather forecasting, seismic data processing, analysis and control of industrial processes, medical imaging such as CAT scans and MRI.

DSP Features

Reproducibility

Programmability

Time Sharing

Automatic Test

Versatility

Refer Table 2.2, Pg 83, Text Book


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Reproducibility

The immunity of digital circuits to smallimperfections and parasitic elementsimplies that the circuits can be producedwith consistent operational characteristicswithout fine adjustments or agingtolerances.

Programmability

A single basic structure can be used for avariety of signal types and applications bymerely changing an algorithmic orparametric specification in a digitalmemory.


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Time Sharing

A single digital signal processing circuitcan be used for multiple signals by storingtemporary results of each process inrandom-access memory and processingeach signal in a cyclic (time-divided)fashion.

Automatic Test

Since the inputs and outputs of a digitalsignal processing circuit are digital data,tests can be performed routinely bycomparing test responses to data patternsstored in memory.


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Versatility

Because the decision-making capabilitiesof digital logic, digital signal processingcan perform many functions that areimpossible or impractical with analogimplementations.

DSP Applications

Echo Cancellers

Tone Receivers

High Speed Modems

Low-Bit-Rate Voice Encoding


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Disadvantages of Digital Voice Networks

Increased Bandwidth

Need for time synchronization

Topologically restricted multiplexing

Need for conference/extension bridges

Incompatibilities with analog facilities

Increased Bandwidth

The bandwidth penalty imposed by voicedigitization is directly dependent on theform of transmission coding or modulationused.


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Need for Time Synchronization

For optimum detection, the sample clockmust be synchronized to the pulse arrivaltimes.

As network becomes complex, syncbecomes an issue.

In SSB FDM, carrier sync is reqd.

Topologically Restricted Multiplexing

Since the time of arrival of data in a timeslot is dependent on the distance of travel,Guard times must be increased as thegeographic separation betweentransmitters increases.

Each TDM source must duplicate thesynchronization and time slot recognitionlogic needed to operate a TDM system.


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Need for Conference/Extension Bridges

Analog End

Digital End

Incompatibilities with Analog Facilities

Additional Costs for Analog Interfaces.

Artificial Delay inserted by digital matrix.


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Chapter 3

Lecture 9

Voice Digitization


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Digitization

First step

Sampling

Nyquist Criterion or Theorem (fs> 2B)

fs= Sampling frequency (Nyquist Rate)

B = Bandwidth of input signal


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PAM

Spectrum of PAM


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Fold-over Distortion (Aliasing)

Example Assume fs, the sampling frequency, is 100 Hz

and that the input signal contains the followingfrequencies: 25 Hz, 70 Hz, 160 Hz, and 510 Hz.These frequencies are shown in the followingfigure. Find aliases.


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Answer Frequencies below the Nyquist frequency (f

s/2 = 50 Hz)

are sampled correctly. Frequencies above the Nyquist frequency appear as

aliases.

Alias F2 = |100 70| = 30 HzAlias F3 = | (2)100 160| = 40 HzAlias F4 = | (5)100 510| = 10 Hz

End-to-end PAM system


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Filter Specs for PCM voice coders (ITU-T Recomm.)

PCM


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Quantization

3-bit vs.16-bit

telecommunications switching systems 9

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