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Introduction to Packet Voice Technologies

1© 2005 Cisco Systems, Inc. All rights reserved. Cisco PublicIP Telephony

Cisco Networking Academy Program

Traditional Telephony

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Basic Components of a Telephony Network

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Central Office Switches

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What Is a PBX?

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Basic Call Setup

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Supervisory Signaling

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Address Signaling

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Tone telephone

DTMF dialing• Rotary telephone

– Pulse dialing

Informational Signaling

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Digital vs. Analog Connections

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Time-Division Multiplexing

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Frequency-Division Multiplexing

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Packetized Telephony Networks

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Packet Telephony vs. Circuit-Switched Telephony

• More efficient use of bandwidth and equipment

• Lower transmission costs

• Consolidated network expenses

• Increased revenue from new services

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• Increased revenue from new services

• Service innovation

• Access to new communications devices

• Flexible new pricing structures

Call Control

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Distributed Call Control

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Centralized Call Control

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Packet Telephony Components

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Real-Time vs. Best-Effort Traffic

• Real-time traffic needs guaranteed delay and timing.

• IP networks are best-effort with no guarantees of delivery, delay, or timing.

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• Solution is quality of service end-to-end.

Foreign Exchange Station Interface

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Foreign Exchange Office Interface

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E&M Interface

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T1 Interface

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E1 Interface

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BRI

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Physical Connectivity Options

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Cisco IP Phone

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Analog Voice Basics

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Local Loops

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Types of Local-Loop Signaling

• Supervisory signaling

• Address signaling

• Informational Signaling

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On Hook

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Off Hook

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Ringing

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Ringing (Cont.)

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Pulse Dialing

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Dual Tone Multifrequency

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Informational Signaling with Call-Progress Indicators

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Trunks

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Foreign Exchange Trunks

• Foreign Exchange Office

Connects directly to office equipment

Used to extend connections to another location

• Foreign Exchange Station

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• Foreign Exchange Station

Connects directly to station equipment

Used to provision local service

Types of Trunk Signaling

• Loop start

• Ground start

• E&M Wink Start

• E&M immediate start

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• E&M immediate start

• E&M delay start

Loop-Start Signaling

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Ground-Start Signaling

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E&M Signaling

• Separate signaling leads for each direction

• E-lead (inbound direction)

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(inbound direction)

• M-lead (outbound direction)

• Allows independent signaling

E&M Type I

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E&M Type V

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E&M Type II

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E&M Type III

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E&M Type IV

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Trunk Supervisory Signaling—Wink Start

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Trunk Supervisory Signaling—Immediate Start

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Trunk Supervisory Signaling—Delay Start

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2-Wire to 4-Wire Conversion and Echo

• Echo is due to a reflection.

• Impedance

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• Impedance mismatch at the 2-wire to 4-wire hybrid is the most common reason for echo.

Echo Is Always Present

• Echo as a problem is a function of the

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function of the echo delay and the loudness of the echo.

Echo Suppression

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Echo Cancellation

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Analog-to-Digital Voice Encoding

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Digitizing Analog Signals

1. Sample the analog signal regularly.

2. Quantize the sample.

3. Encode the value into a binary expression.

4. Compress the samples to reduce bandwidth,

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4. Compress the samples to reduce bandwidth, optional step.

Basic Voice Encoding: Converting Digital to Analog

1. Decompress the samples, if compressed.

2. Decode the samples into voltage amplitudes, rebuilding the PAM signal.

3. Filter the signal to remove any noise.

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3. Filter the signal to remove any noise.

Nyquist Theorem

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Voice Compression Techniques

• Waveform algorithms

PCM

ADPCM

• Source algorithms

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• Source algorithms

LDCELP

CS-ACELP

Example: Waveform Compression

• PCM

Waveform coding scheme

• ADPCM

Waveform coding scheme

Adaptive: automatic companding

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Differential: encode changes between samples only

• ITU standards:

G.711 rate: 64 kbps = (2 * 4 kHz) * 8 bits/sample

G.726 rate: 32 kbps = (2 * 4 kHz) * 4 bits/sample

G.726 rate: 24 kbps = (2 * 4 kHz) * 3 bits/sample

G.726 rate: 16 kbps = (2 * 4 kHz) * 2 bits/sample

Compression Bandwidth Requirements

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Mean Opinion Score

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Perceptual Speech Quality Measurement

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Signaling Systems

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T1 Digital Signal Format

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Robbed-Bit Signaling

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Channel Associated Signaling—T1

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E1 Framing and Signaling

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Channel Associated Signaling—E1

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Common Channel Signaling

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ISDN

• ISDN

Part of network architecture

Definition for access to the network

Allows access to multiple services through a single access

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a single access

Used for data, voice, or video

• Standards-based

ITU recommendations

Proprietary implementations

ISDN Network Architecture

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Layer 3 (Q.930/931) Messages

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