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Jared Smith, Jim Van Meggelen and Leif Madsen
Published by O'Reilly and Associates
404 pages
£ 28.50
Published: 23rd September 2005
reviewed by Greg Matthews
   in the December 2005 issue (pdf), (html)

Asterisk is an open source telephone exchange system or PBX. Unless you work in this field you can be forgiven for not knowing that this software is whipping up a storm of interest in the telecommunications community. For a long time, telephony has been a closed shop for independent developers, the market dominated by monopolistic players. But with the long touted, voice over IP (VoIP) finally finding a toehold and DSL delivering cheap bandwidth, the time is ripe for an open solution.

I have to admit that I only had the vaguest idea of what Asterisk was and what it could do before I received this book. In my line of work it pays to be conservative about technology that ``just works''. However, even I will admit that traditional telephony just doesn't cut it as a modern communication medium. The closed attitude of the big telecoms industries has stifled innovation and hindered equipment and standards compatibility, but all that is about to change. Asterisk, along with low cost interface cards, such as those from Digium, can do everything an expensive PBX system can do and more.

The authors are Asterisk users and all have been greatly involved in the documentation of the project at It's impossible to ignore the enthusiasm they have for this software which certainly helps to keep the subject matter interesting. This book is aimed at a technical audience, the early chapters walk through the installation and configuration of the software and the emphasis is on familiarising yourself with Asterisk on commodity hardware like an old PC. After that, the information starts to get more interesting and in-depth as the authors begin to explain the concepts of dialplans and telephony protocols.

The beauty of Asterisk is that it can be slotted into an existing PBX system in lots of different ways which makes it easy to add services to an old system and migrate users onto a new and featureful system one step at a time. This makes it extremely attractive to ICT staff wanting to upgrade an old private exchange but not wanting to risk downtime. Examples of this sort of configuration are given in the text.

So what exactly will Asterisk do? Well, anything you can think of really. It will provide VoIP, voice-mail, interactive services, you can get it to route calls to you wherever you are, read your e-mail to you, the possibilities are literally only limited by your imagination and technical ability. This could very possibly be the next big application for Linux and BSD. If its popularity continues unabated, it will be a major driver and enabler for standards compliance and interoperability between vendors.

It is difficult to criticise the technical merit of this book, though the authors do point out that they cover only a small percentage of the subject. It is written to quickly get you up to speed with the technology rather than provide an in depth reference guide to the software. The North American slant is a bit annoying, a little research would have provided equivalent information for readers on other continents.

A significant problem with this sort of book is that the technology is moving so fast, version 1.2 is already available for download, and the more popular it gets the faster it will develop. But the authors of this book are stout hearted -- they even devote a chapter to predicting the future, which as we all know, is a brave thing to do. That said, they are sensibly sceptical about technologies like voice recognition, and they are right to worry about a backlash from the telecoms industry (think deprioritisation of unapproved VoIP traffic). The book is littered with criticisms of telecoms providers which may alienate people already working in the industry but it is good to see such enthusiasm for the philosophy of free software and the empowerment that it will bring to ICT workers.

There is a strong business case for switching the company PBX to Asterisk and there are already commercial vendors and support for such systems in the UK. It should also appeal to hackers who want to replace their home phones with cool technology. This book will get you started, and I fear, once you've started you'll be hooked.

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