Cisco Voice Gateways and Gatekeepers

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Overview

Cisco Voice Gateways and Gatekeepers

Understanding and configuring GW/GK in complex VoIP networks

Denise Donohue, CCIE® No. 9566

David Mallory, CCIE No. 1933

Ken Salhoff, CCIE No. 4915

Deployments of voice over IP (VoIP) networks continue at a rapid pace. Voice gateways are an essential part of VoIP networks, handling the many tasks involved in translating between transmission formats and protocols and acting as the interface between an IP telephony network and the PSTN or PBX. Gatekeepers and IP-to-IP gateways help these networks scale. Gatekeepers provide call admission control, call routing, address resolution, and bandwidth management between H.323 endpoints including Cisco IOS® voice gateways and Cisco® Unified CallManager clusters. IP-to-IP gateways allow VoIP calls to traverse disparate IP networks.

Cisco Voice Gateways and Gatekeepers provides detailed solutions to real-world problems encountered when implementing a VoIP network. This practical guide helps you understand Cisco gateways and gatekeepers and configure them properly. Gateway selection, design issues, feature configuration, and security and high-availability issues are all covered in depth. The abundant examples, screen shots, configuration snips, and case studies make this a truly practical and useful guide for anyone interested in the proper implementation of gateways and gatekeepers in a VoIP network. Emphasis is placed on the accepted best practices and common issues encountered in real-world deployments.

Cisco Voice Gateways and Gatekeepers is divided into four parts. Part I provides an overview of an IP voice network. Part II is dedicated to voice gateways, including discussions of Media Gateway Control Protocol (MGCP); H.323; Session Initiation Protocol (SIP); voice circuit options; connecting to the PSTN, PBX, and IP WAN; dial plans; digit manipulation; route selection; class of restriction; Survivable Remote Site Telephony (SRST) and MGCP fallback; digital signal processor (DSP) resources; and Tool Command Languaue (Tcl) scripts and Voice XML (VXML). Part III addresses voice gatekeepers, including detailed deployment and configuration. Part IV is dedicated to IP-to-IP gateways.

“With this book, the authors provide an in-depth look at the breadth of voice gateway features and capabilities, as well as providing voice gateway configuration guidance.”

–Christina Hattingh, Access Technology Group, Cisco Systems®

  • Understand the pros and cons of MGCP, H.323, and SIP, how they implement call signaling and describe call flow, and how to configure each protocol
  • Learn the various analog and digital voice circuit options used to connect a VoIP network to the PSTN
  • Configure and troubleshoot PSTN, PBX, and IP WAN connections
  • Build scalable dial plans and understand the different types of dial peers
  • Understand the various ways gateways control called and calling phone numbers
  • Examine call admission control (CAC) techniques
  • Configure Class of Restrictions (CoR) for both inbound and outbound calls
  • Deploy and troubleshoot SRST and MGCP fallback
  • Evaluate DSP considerations and resources
  • Support interactive voice response (IVR) and advanced call-handing applications using Tcl scripts and VXML
  • Deploy and configure basic and advanced gatekeeper functions
  • Configure and troubleshoot IP-to-IP gateways

This book is also recommended self-study training for the CCVP GWGK exam.

This IP communications book is part of the Cisco Press® Networking Technology Series. IP communications titles from Cisco Press help networking professionals understand voice and IP telephony technologies, plan and design converged networks, and implement network solutions for increased productivity.

Category: Cisco Press–IP Communications

Covers: VoIP gateways and gatekeepers

$65.00 USA / $81.00 CAN

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Product Details

  • ISBN-13: 9781587052583
  • Publisher: Cisco Press
  • Publication date: 8/31/2006
  • Series: Networking Technology Series
  • Edition description: New Edition
  • Pages: 619
  • Product dimensions: 7.50 (w) x 9.20 (h) x 1.60 (d)

Meet the Author

Denise Donohue, CCIE No. 9566, is a design engineer with AT&T. She is responsible for designing and implementing data and VoIP networks for SBC and AT&T customers. Prior to that, she was a Cisco instructor and course director for Global Knowledge. Her CCIE is in Routing and Switching.

David L. Mallory, CCIE No. 1933, is a technical education consultant with Cisco Systems, Inc. supporting Cisco voice certifications. Prior to this role, David was a systems engineer supporting several global enterprise customers. David has presented on voice gateways and gatekeepers at Networkers and has achieved four CCIE certifications: Routing and Switching, WAN Switching, Security, and Voice.

Ken Salhoff,CCIE No. 4915, is a systems engineer with Cisco Systems, Inc. Ken has been specializing in voice technologies with Cisco for the past six years. In the systems engineering role, Ken has supported several global enterprise customers using Cisco voice technologies. Ken has achieved two CCIE certifications: Routing and Switching, and Voice.

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Read an Excerpt

PrefaceForeword

Cisco IOS routers have shipped with voice interface cards since 1997, and after this capability was available the term voice gateway became part of the VoIP vernacular, seemingly overnight. The voice interfaces allowed routers to provide a critical interconnectivity link between the traditional data IP networks and the traditional voice (PSTN, PBXs, and key systems) networks. With this technology, the industry widely built toll bypass networks during the late 1990s: Enterprises connected their PBXs at different sites with VoIP "trunks" instead of with TIE lines or the PSTN, and service providers leveraged IP backbone networks to offer calling-card services and cut-rate long-distance and international calling.

VoIP did not fascinate the popular imagination of the likes of Jeff Pulver of Voice-on-the Net (VON) and other industry observers until end-user–visible devices, such as IP phones, and IP-based applications brought the technology to the forefront. Voice gateway technology is still the pale sibling of the IP telephony world that creates no buzz, and yet it is also the workhorse of every single VoIP network. Even as VoIP endpoints become ever more prevalent in businesses and residences, voice gateways still provide critical interconnectivity with billions of traditional PSTN and PBX voice endpoints, without which companies cannot operate their communications networks.

Although the idea of a voice gateway is conceptually simple enough—it’s a demarcation between two networks and translates the protocols from one (the TDM world) to the other (the IP world)—the technology has become increasingly sophisticated and thefeatures more intertwined over the years. Choosing the "right" voice gateway and configuring the "right" set of features for a particular network is no longer the task for the uninitiated. The question I hear most frequently is whether to deploy MGCP, H.323, or SIP gateways. Cisco gateways are protocol agnostic and support all of these protocols and several variations thereof, and the answer to the question posed is not a simple one: The optimal network design depends on a large number of considerations. Some protocols and designs are better suited to particular types of networks, partly owing to the architecture of the protocols themselves and partly due to the features that have been chosen for implementation over the years.

A Cisco Press book with comprehensive coverage focused entirely on voice gateway technology and features was a long time coming, and at last with this book, the authors provide an in-depth look at the breadth of voice gateway features and capabilities, as well as providing voice gateway configuration guidance. The book explains the major VoIP protocols, MGCP, H.323, and SIP, their structure and operation, and the considerations to choose among them. It discusses in detail the PSTN and PBX circuit connection technologies and choices. There are often multiple connection choices on the central office or PBX switch as well as on the voice gateway side of the circuit, and which of these would provide the features, cost points, and manageability that are optimal for your network might not be obvious at first glance.

The book goes on to provide insights into many other areas of gateway selection and deployment, including the myriad choices in carrying fax and modem traffic over IP, dial plan features and digit manipulation tools, call admission control capabilities to keep voice traffic off the IP network when it does not have the quality levels to carry it, a review of DSP technology and operation, and an examination of IP connectivity implications and QoS features required to carry voice traffic with decent quality. Later chapters in the book also include discussions on pure IP-oriented topics such as TCL and V

The book also covers key areas of interest in any network, including security measures and high availability. VoIP network security is a wide topic fully deserving of its own book-length treatment, but this book provides enough basic information to get your network deployed. It covers how voice gateway traffic passes through firewalls and NAT devices, how to encrypt voice signaling and media traffic to or from a voice gateway, as well as configuring class of service restrictions such that certain call patterns are allowed while others are blocked per the policy of your network. High availability is essential in all networks—a chapter in this book is dedicated to the discussion of how gateways fail over when other network components are out of contact, as well as how gateway features interoperate with IP Phone failover features such as SRST to maintain dial tone and PSTN network access for your end users at all times.

Throughout the book is a case study that solidifies the chapter discussions by providing practical, hands-on examples of how the configuration of the system implements the features. This, together with the detailed chapter-by-chapter coverage of crucial gateway topics, make this an invaluable book essential to the tool chest of anyone contemplating the implementation of a new network, actively designing a network, or evolving or optimizing the features in an existing network.

Christina Hattingh
Access Technology Group
Cisco Systems, Inc.

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Table of Contents

Foreword

Introduction

Part I Voice Gateways and Gatekeepers

Chapter 1 Gateways and Gatekeepers

The Role of Voice Gateways

Types of Voice Gateways

The Role of Voice Gatekeepers

The Role of IP-to-IP Gateways

Introduction to Voice Protocols

Media Gateway Control Protocol

H.323

Session Initiation Protocol

Skinny Client Control Protocol

Real-Time Transport Protocol

Call Control Agents

Cisco CallManager

Cisco CallManager Express

SIP Proxy Server

Cisco Enterprise Gateway

PBX with Toll Bypass

Deployment Scenarios

Single Site Deployment

Multisite with Centralized Call Control

Multisite Deployment with Distributed Call Control

Case Study: Introduction

Chapter Review Questions

Part II Gateways

Chapter 2 Media Gateway Control Protocol

Introduction to MGCP

Pros

Cons

MGCP Operation

MGCP Messages

Registering with CallManager

Call Flow with MGCP

Call Flow Between Analog Phones

ISDN Connections with Backhaul

MGCP Fallback

Dial Plan Considerations

Implementing MGCP Gateways

Basic MGCP Gateway Configuration

Configuring MGCP Fallback

Assigning an MGCP Source IP Address

Configuring MGCP PRI and BRI Backhaul

Enabling Multicast Music on Hold

Configuring Cisco CallManager

Configuring CallManager Redundancy

Configuring DTMF Relay

Securing MGCP Gateways

Troubleshooting Tools

Case Study: Configuring an MGCP Gateway

Review Questions

Chapter 3 H.323

H.323 Specifications

H.323 Network Components

H.323 Gateways

H.323 Gatekeepers

H.323 Terminals

Multipoint Control Units

H.323 Proxy Servers

Call Flow

H.323 Fast Start

H.323 Protocol Pros and Cons

Pros

Cons

When to Use H.323

Dial Plan Considerations

Implementing H.323 Gateways

Voice Class Configuration

Voice Service VoIP Configuration

Toll Bypass

Defining H.323 Gateways on CallManager

Redundancy

DTMF Relay

Securing H.323 Gateways

Troubleshooting Tools

Case Study: Configuring an H.323 Gateway

Review Questions

Chapter 4 Session Initiation Protocol

Description of SIP

SIP Functional Components

SIP Messages

SIP Call Flow

Call Flow Between Two SIP Gateways

Call Flow Using a Proxy Server

Call Flow Using Multiple Servers

Call Flow Using Cisco CallManager 5.x

SIP Pros and Cons

Pros

Cons

When to Use SIP

Dial Plan Considerations

Implementing SIP Gateways

SIP Dial Peer Configuration

SIP UA Configuration

SIP Voice Service Configuration

Toll Bypass

Registering with CallManager

DTMF Relay

Securing SIP Gateways

Allowing H.323 to SIP Connections

Troubleshooting Tools

Case Study: Configuring SIP Between a Gateway and CallManager 5.x

Review Questions

Chapter 5 Circuit Options

Circuit Signaling

Analog Circuits

FXS/FXO

E&M

Digital Circuits

T1

E1

E1 R2

ISDN

Echo Cancellation

Review Questions

Chapter 6 Connecting to the PSTN

PSTN Circuit Selection Overview

Supported Analog Connection Types

Supported Digital Connection Types

Analog Trunks

Configuring FXO Connections

Configuring DID Connections

Configuring Centralized Automated Message Accounting Connections

Caveats and Restrictions

Digital Trunks

Configuring E1/T1 Physical Layer Connections

Configuring ISDN PRI Trunks

Configuring E1 R2 Trunks

Configuring T1 CAS Trunks

Configuring ISDN BRI Trunks

Case Study: Add an E1 R2 Connection to the Leeds Gateway

Review Questions

Chapter 7 Connecting to PBXs

Analog Trunks

Configuring FXO/FXS Connections

Configuring E&M Trunks

Digital Trunks

Configuring E1/T1 Physical Layer Connections

Configuring ISDN PRI Trunks

Configuring E1 R2 or T1 CAS Trunks

Configuring Transparent Common Channel Signaling

Case Study: Implementing a Cisco Voice Gateway at the Shanghai Office

Review Questions

Chapter 8 Connecting to an IP WAN

Applications for Connecting to an IP WAN

Design Considerations

Quality of Service

Using Class Maps to Classify Traffic

Using Policy Maps

Mapping to MPLS Classes

Link Fragmentation and Interleave

Compression

AutoQos

Providing Fax and Modem Services

Providing Fax Services

Providing Modem Services

Security

Securing Voice Media and Signaling

V3PN

NAT and VoIP

Firewalls and VoIP

Case Study: Using a T1 Link as a Tie Line

Review Questions

Chapter 9 Dial Plans

Numbering Plans

Private Numbering Plans

PSTN Numbering Plans

Overlapping Numbering Plans

Building a Scalable Dial Plan

Dial Peers

Inbound Versus Outbound Dial Peers

Dial Peer Matching

Inbound Dial Peer Matching

Outbound Dial Peer Matching

Verifying Dial Peers

Outbound Dial Peer Targets

POTS Versus VoIP Outbound Dial Peers

Dial Peer Operational Status

Dial Peers Versus Cisco CallManager

Case Study: Configuring PSTN Access

Review Questions

Chapter 10 Digit Manipulation

Basic Digit Manipulation

Digit Stripping

Forward Digits

Prefix Digits

Number Expansion

Voice Translation Rules and Profiles

Creating Voice Translation Rules

Building Regular Expressions

Creating Voice Translation Profiles

Applying Voice Translation Profiles

Blocking Calls Using Voice Translation Rules and Profiles

Testing Voice Translation Rules

Manipulating Caller ID

CLID Commands

Station ID Commands

Order of Operation in Digit Manipulation

Troubleshooting Digit Manipulation

Case Study

Review Questions

Chapter 11 Influencing Path Selection

Hunt Groups

Using the preference Command

Using the huntstop Command

Using Digit Manipulation

Using Trunk Groups

Tail-End Hop-Off

Call Admission Control

Local CAC Mechanisms

Measurement-Based CAC Mechanisms

Resource-Based CAC Mechanisms

Resource Reservation Protocol

POTS-to-POTS Call Routing Considerations

Case Study: Implementing Gateway-Controlled RSVP

Review Questions

Chapter 12 Configuring Class of Restrictions

COR Overview

COR Operation

Implementing COR

Assigning COR Lists with SRST

Assigning COR Lists with Cisco CallManager Express

Assigning COR Lists to SIP Phones with CallManager Express

Restricting Inbound Calls

Case Study: Implementing COR for Miami

Review Questions

Chapter 13 SRST and MGCP Gateway Fallback

SRST Overview

Fallback Time

Restoral Time

Configuring SRST

Gateway Configuration

CallManager Configuration

Dial Plan Considerations

Planning

Configuring SRST Dial Plan Patterns

SRST Features

Auto Attendant

Maximum Line Appearances

Conferencing

Transferring Calls

Forwarding Calls

Voice-Mail Integration

Music on Hold

SIP SRST

Configuring SIP Registrar Server

Configuring a Voice Register Pool

Call Preservation

Secure SRST

Configuring Secure SRST

MGCP Gateway Fallback

Configuring MGCP Gateway Fallback

Verifying and Troubleshooting SRST

Verifying and Troubleshooting MGCP Gateway Fallback

Case Study: Integrating SRST with an Analog Voice-Mail System

Review Questions

Chapter 14 DSP Resources

Need for DSP Resources

Determining the DSP Resources Required

DSP Types

Voice Termination

DSP Sharing

Transcoding and MTP Resources

Conference Bridge Resources

Configuring DSP Resources

Configuring Transcoding and Conferencing (C549)

Configuring Enhanced Transcoding and Conferencing (C5510)

Transcoding for CallManager Express

Case Study: Add DSP Resources to the Miami Gateway

Review Questions

Chapter 15 Using Tcl Scripts and VoiceXML

Tcl IVR and VoiceXML Application Overview

Programming Resources

Sample Applications

Auto Attendant

Basic ACD

Fax Detect

T.37 Store and Forward Fax

Malicious Call ID

Cisco Voice Portal

Embedded Event Manager

Downloading Tcl Scripts from Cisco.com

Configuring the Gateway to Use a Tcl Script

Initializing Tcl Scripts and Specifying Parameters

Applying TclScripts

Tcl Packages and Parameter Namespaces

Tcl Parameters in Cisco IOS Release 12.3(14)T and Above

Implementing the AA Tcl Script

Creating Audio Files

Restrictions and Caveats

Case Study: Implementing ACD Application

Review Questions

Part III Gatekeepers

Chapter 16 Deploying Gatekeepers

Gatekeeper Functionality

Gatekeeper Signaling

RAS Signaling

Gatekeeper Update Protocol

Gatekeeper Transaction Message Protocol

E.164 Number Resolution

Zone Prefixes

Technology Prefixes

Gatekeeper Call Routing Process

Call Admission Control

Gatekeeper Deployment Models

Redundancy

Resource Availability Indicator

Directory Gatekeeper

Gatekeepers with CallManager

Security with Gatekeepers

Tokenless Call Authentication

Review Questions

Chapter 17 Gatekeeper Configuration

Configuring Basic Gatekeeper Functionality

Configuring Gatekeeper Zones

Configuring Gateways to Use H.323 Gatekeepers

Technology Prefixes

Configuring Zone Prefixes and Dial Peers

Dynamic Prefix Registration

Configuring Call Admission Control

Multiple Gatekeeper Configurations

Configuring Directory Gatekeepers

Troubleshooting Gatekeepers

Registration Issues

Call Routing Issues

CallManager and Gatekeepers

Configuring a CallManager Gatekeeper Trunk

Gatekeeper Redundancy

Hot Standby Routing Protocol

Gatekeeper Clustering

Load Balancing

Troubleshooting Gatekeeper Clustering

Configuring Resource Availability Indicator

Configuring Gatekeeper Security

Troubleshooting Gatekeeper Security

Case Study: Deploying Gatekeepers to Assist in Migration to VoIP

Review Questions

Part IV IP-to-IP Gateways

Chapter 18 Cisco Multiservice IP-to-IP Gateway

IP-to-IP Gateway Overview

Cisco Multiservice IP-to-IP Gateway

Architecture

Media-Handling Modes

Protocol Support

Basic Configuration

Via-Zones

IP-to-IP Gateway Features

Video Support

Address Hiding

Security

DTMF Interworking

Fax Support

Quality of Service

Call Admission Control

Transcoding

VXML and Tcl Scripts

Billing

show Commands

debug Commands

Case Study: Providing Enterprise VoIP Trunking to VoIP Service of the Service Provider

CallManager Configuration

Review Questions

Appendix A Answers to Chapter-Ending Review Questions

158705258x TOC 7/27/2006

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Preface

Foreword

Cisco IOS routers have shipped with voice interface cards since 1997, and after this capability was available the term voice gateway became part of the VoIP vernacular, seemingly overnight. The voice interfaces allowed routers to provide a critical interconnectivity link between the traditional data IP networks and the traditional voice (PSTN, PBXs, and key systems) networks. With this technology, the industry widely built toll bypass networks during the late 1990s: Enterprises connected their PBXs at different sites with VoIP "trunks" instead of with TIE lines or the PSTN, and service providers leveraged IP backbone networks to offer calling-card services and cut-rate long-distance and international calling.

VoIP did not fascinate the popular imagination of the likes of Jeff Pulver of Voice-on-the Net (VON) and other industry observers until end-user–visible devices, such as IP phones, and IP-based applications brought the technology to the forefront. Voice gateway technology is still the pale sibling of the IP telephony world that creates no buzz, and yet it is also the workhorse of every single VoIP network. Even as VoIP endpoints become ever more prevalent in businesses and residences, voice gateways still provide critical interconnectivity with billions of traditional PSTN and PBX voice endpoints, without which companies cannot operate their communications networks.

Although the idea of a voice gateway is conceptually simple enough—it’s a demarcation between two networks and translates the protocols from one (the TDM world) to the other (the IP world)—the technology has become increasingly sophisticated and the features more intertwined over the years. Choosing the "right" voice gateway and configuring the "right" set of features for a particular network is no longer the task for the uninitiated. The question I hear most frequently is whether to deploy MGCP, H.323, or SIP gateways. Cisco gateways are protocol agnostic and support all of these protocols and several variations thereof, and the answer to the question posed is not a simple one: The optimal network design depends on a large number of considerations. Some protocols and designs are better suited to particular types of networks, partly owing to the architecture of the protocols themselves and partly due to the features that have been chosen for implementation over the years.

A Cisco Press book with comprehensive coverage focused entirely on voice gateway technology and features was a long time coming, and at last with this book, the authors provide an in-depth look at the breadth of voice gateway features and capabilities, as well as providing voice gateway configuration guidance. The book explains the major VoIP protocols, MGCP, H.323, and SIP, their structure and operation, and the considerations to choose among them. It discusses in detail the PSTN and PBX circuit connection technologies and choices. There are often multiple connection choices on the central office or PBX switch as well as on the voice gateway side of the circuit, and which of these would provide the features, cost points, and manageability that are optimal for your network might not be obvious at first glance.

The book goes on to provide insights into many other areas of gateway selection and deployment, including the myriad choices in carrying fax and modem traffic over IP, dial plan features and digit manipulation tools, call admission control capabilities to keep voice traffic off the IP network when it does not have the quality levels to carry it, a review of DSP technology and operation, and an examination of IP connectivity implications and QoS features required to carry voice traffic with decent quality. Later chapters in the book also include discussions on pure IP-oriented topics such as TCL and VXML applications capabilities, conference mixing, transcoding, gatekeeper functions and connectivity, and IP-to-IP gateway (session border controller) services and features. All of these pure VoIP services are offered by the same platforms that are also voice gateways, even though TDM connectivity is not a necessary component for the IP-only services.

The book also covers key areas of interest in any network, including security measures and high availability. VoIP network security is a wide topic fully deserving of its own book-length treatment, but this book provides enough basic information to get your network deployed. It covers how voice gateway traffic passes through firewalls and NAT devices, how to encrypt voice signaling and media traffic to or from a voice gateway, as well as configuring class of service restrictions such that certain call patterns are allowed while others are blocked per the policy of your network. High availability is essential in all networks—a chapter in this book is dedicated to the discussion of how gateways fail over when other network components are out of contact, as well as how gateway features interoperate with IP Phone failover features such as SRST to maintain dial tone and PSTN network access for your end users at all times.

Throughout the book is a case study that solidifies the chapter discussions by providing practical, hands-on examples of how the configuration of the system implements the features. This, together with the detailed chapter-by-chapter coverage of crucial gateway topics, make this an invaluable book essential to the tool chest of anyone contemplating the implementation of a new network, actively designing a network, or evolving or optimizing the features in an existing network.

Christina Hattingh
Access Technology Group
Cisco Systems, Inc.

Read More Show Less

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  • Anonymous

    Posted November 18, 2006

    Must have for future Cisco VOIP engineers

    As the title states, Gateways and Gatekeepers, you are immedialtely immersed in the functions of cisco gateways and gatekeepers. This book is intended for those who have prior VOIP knowledge as far as core principles of protocols used in VOIP and basic understanding of dial peers. It is strongly recommended that the reader pick up the fundamentals of Cisco VOIP before delving into this book as it moves at fast pace explaining the configurations. The author goes into great detail about configuration and case scenarios of the various types of VOIP setup (Ex. FXS, FXO, T1, E1, E&M). There are several illustrations that help in understanding the roles of devices in the various configurations explained. Although, I felt the book needed more diagrams through its entirety. The chapter on Dial Plans is invaluable to engineers currently planning their VOIP infrastructure. From design to implementation it will be a handy reference during the process. There are also great example outputs of debug commands which I found missing in other books that fall in the same category. The troubleshooting sections help when your VOIP network is already implemented but not fine tuned. They are each covered at the end of each section as opposed to covering them at the end of the book. This was a good idea from my perspective. As I skimmed through the book the first time I read it, I found many commands I didn¿t use before due to the fact that it was not explained in a concise way until I read the sections in this book. I also found invaluable show command tables explaining the signs to look out for when using these show commands in certain configurations. The one negative comment I have about this book is the lack of review for those readers that are a bit rusty when understanding VOIP. I think there should be a chapter or appendix section that briefly covers the fundamentals of VOIP just as a refresher for readers. Also, this book was listed as the only Cisco Press book recommended in preparation for the CCVP 642-452 GWGK exam. There needs to be more test questions at the end of each chapter and unless the book under normal retail comes with a disc, it needs one with flash cards or sample tests. Overall, I would definitely recommend this book if the reader intends to implement a VOIP infrastructure in their company, but I would strongly suggest getting additional preparation material if the book was purchased in preparation for the CCVP 642-452 GWGK exam.

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