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Barries books

Asterisk Book

Building Telephony Systems with Asterisk

Asterisk is an open source PBX system which provides VoIP functionality as well as traditional telephony functions (SIP/IAX/POTS/ISDN/E1 etc..). This book covers how to set it up to be used as a VoIP and traditonal telephony server with a focus on getting a system up and running simply and quickly.
Published September 2005 - ISBN 1904811159
Buy from Packt or The Register. Also available in Spanish, Portuguese and Italian

IPCop Book

Configuring IPCop Firewalls: Closing Borders with Open Source

IPCop is a popular Linux based firewall solution with a web based interface and variety of features from firewalling of wired and wireless networks to VPN, proxy and application layer filtering. This book covers the installation, configuration and management of IPCop
Published October 2006 - ISBN 1904811361
Buy from Packt or The Register

TrixBox Book

TrixBox Made Easy

This book begins by introducing VoIP telephony concepts before detailing how to plan a telephone system and moving on to the installation, configuration, and management of a feature packed PBX. It provides examples of well laid out telephone systems with accompanying spreadsheets to aid the reader in building stable telephony infrastructure.
Published October 2006 - ISBN 1904811930
Buy from Packt or The Register