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© 2008 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialONT 1

ONT Module 2 Lesson 2 - Digitizing and PacketizingVoice

Cisco VoIP

Implementations



© 2008 Cisco Systems, Inc. All rights reserved.ONT 2

CCNP – ONT Bachkhoa Networking Academy

Học viện mạng Bách Khoa - www.bkacad.com

Objectives

Describe the process of analog to digital conversion.

Describe the process of digital to analog conversion. Explain how sampling rates are determined using the

Nyquist Theorem.

Explain how quantization can lead to noise.

Explain how MOS is used to judge voice quality.

Describe the purpose of DSPs.






Basic Voice Encoding: Converting Analog Signals to Digital Signals

VoIP systems rely on digital signal processors (DSPs). DSPs

convert analog voice signals into digital format and vice versa.They also provide functions such as voice compression,transcoding (changing between different formats of digitized voice),and conferencing. DSPs are hardware components often locatedon voice modules inside gateways.

Step 1: Sample the analog signal.

Step 2: Quantize sample into a binary expression.

Step 3: Compress the samples to reduce bandwidth.






Digitizing and Packetizing Voice

Basic Voice Encoding: Converting Digital Signals to

Analog Signals –Step 1 Decompression

–Step 2 Decoding

–Step 3 Reconstruction of the analog signal






Sampling –Sampling is the reduction of a continuous signal to a discretesignal

–Fs > 2(BW)

–Fs = Sampling frequency

–BW = Bandwidth of original analog voice signal






Quantization


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Digital Voice Encoding





Companding

Companding refers to the process of first compressing an analog

signal at the source and then expanding this signal back to itsoriginal size when it reaches its destination. The term companding comes from combining the two terms, compressing and expanding,into one word.





Common Voice Codec Characteristics





Selecting a Codec Using the Mean Opinion Score

The mean opinion score (MOS) is a system of grading the voice

quality of the telephone connection that uses codec. The MOS is astatistical measurement of voice quality that is based on the judgment of several subscribers.

CC O





A Closer Look at a DSP

CCNP ONT B hkh N t ki A d





Example: DSP Used for Confrencing






Example: DSP Used for Transcoding






Self Check

1. What sampling frequency is recommended by the

Nyquist Theorem for reconstruction of a signal?

2. What is the Hz range for traditional telephonesystems?

3. What is the implication of using 8 bits for quantization?

4. What is the purpose of logarithmic quantization?

5. What is MOS?






Q and A






Resources

Voice Codec Bandwidth Calculator (requires CCO

login) –http://tools.cisco.com/Support/VBC/do/CodecCalc1.do

DSP Calculator

–http://www.cisco.com/cgi-bin/Support/DSP/dsp-calc.pl

Free VoIP Quality Tester

–http://www.testyourvoip.com/


http://tools.cisco.com/Support/VBC/do/CodecCalc1.do

http://www.cisco.com/cgi-bin/Support/DSP/dsp-calc.pl

http://www.testyourvoip.com/

http://www.testyourvoip.com/






http://tools.cisco.com/Support/VBC/do/CodecCalc1.do





Summary

The benefits of a VoIP network include more efficient

use of network bandwidth and equipment, lower costand consolidated expenses.

Legacy analog and VoIP applications and devices cancoexist.

The 3 stages of a VoIP call include call setup, callmaintenance, and call teardown.

VoIP can be deployed in a centralized or distributedenvironment.

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