Book Review: Building Telephony Systems with Asterisk
- In providing an introduction, Chapter 1 enumerates what Asterisk is and isn’t. Asterisk is a PBX plus IVR, voicemail and VoIP system. Asterisk is not an off-the-shelf phone system, SIP proxy or multi-platform solution. This enumeration leads to a welcome discussion on Asterisk’s fit for your needs. The authors quantify this fit in terms of flexibility vs. ease-of use, configuration-management UI, TCO and ROI. Although the latter to topics are covered briefly, the authors’ coverage will certainly serve to stimulate the right kinds of discussions.
- Chapter 2 begins by enumerating the ways in which the Asterisk solution might connect to the PSTN. Next, in discussing the four types of terminal equipment (hard phones, soft phone, communications devices and PBXs), the major protocols supported by Asterisk are revealed – namely H.323, SIP and IAX. Whereas H.323 is well known to many of those who’ve delved in videoconferencing, and SIP to anyone who’s done any reading on VoIP, IAX is an interesting addition specific to Asterisk. The Inter-Asterisk eXchange (IAX) protocol attempts to address limitations inherent in H.323 and SIP relating to, e.g., NAT support, configurability, call trunking, sharing information amongst Asterisk servers plus codec support. IAX is not a standard and device support is somewhat limited but on the rise. As of this book’s writing, September 2005, IAX2 had deprecated IAX – and that still appears to be the case. Guidelines for device choice, compatibility with Asterisk, sound quality analysis and usability all receive attention in this chapter. The chapter closes with a useful discussion on the choice of extension length. Highly noteworthy, and already provided as a link above, is the voip-info.org wiki.
- The installation of Asterisk is the focus of Chapter 3. After reviewing the required prerequisites, none of which are especially obscure, attention shifts to the Asterisk-specific components. In turn Zaptel (device drivers), libpri (PRI libraries) and Asterisk are installed from source. (I expect packaged versions of these components are now available for various Linux distributions.) Asterisk includes a plethora of configuration files, and these are given an overview in this chapter. And although it’s not mentioned, disciplined use of a revision control system like RCS is strongly advised. The chapter concludes with sections on running Asterisk interacting with its CLI to ensure correct operation, start/stop the service and so on.
- With Asterisk installed, attention shifts to interface configuration in Chapter 4. In working through line and terminal configurations for Zaptel interfaces, one is humbled by the edifice that is the pre-IP world of voice. Our introduction to the intersection between the pre-IP and VoIP universes is completed by consideration of SIP and IAX configuration. Again humbling, the authors’ treatment affords us an appreciation of the application of acknowledged standards like SIP (which is itself based on RTP) through implementation. The final few sections of the chapter further emphasize the convergence capabilities of VoIP platforms by exposing us to voicemail, music-on-hold, message queues and conference rooms.
- Through the creation of a dialplan, Asterisk’s functionalities and features can be customized for use. Dialplans are illustrated in Chapter 5 by establishing contexts, incoming/outgoing-call extensions, call queues, call parking, direct inward dialing, voicemail, automated phone directory and conference rooms. Customization is involved, and it is in chapters such as this one that the authors deliver significant value in their ability to move us swiftly towards a dialplan solution. Also evident from this chapter, and to paraphrase the authors, is Asterisk’s power and flexibility as a feature/functionality-rich telephony solution.
- Under Asterisk, calls are tracked with Call Detail Records (CDRs). Data pertaining to each call can be logged locally to a flat file or to a database running on (preferably) a remote server. The database-oriented approach for managing CDR data is more flexible and powerful, even though it takes more effort to set up, as this solution is based on databases such as PostgreSQL or MySQL. CDR comprises the least-invasive approach for quality assurance. The remainder of the content in Chapter 6 focuses on more-invasive approaches such as monitoring and recording calls.
- Based only on the context provided by this review, it is likely apparent that an Asterisk deployment requires considerable effort. Thus in Chapter 7, the authors introduce us to the turnkey solution known as Asterisk@Home. Asterisk@Home favors convenience at the expense of flexibility – e.g., the flavor of Linux (CentOS) as well as support components such as the database (MySQL) are predetermined. The Asterisk Management Portal (AMP), a key addition in Asterisk@Home, Webifies access to a number of user and administrator features/functionalities – voicemail, CRM, Flash Operator Panel (FOP, a real-time activity monitor), MeetMe control plus AMP (portal and server Asterisk@Home management) itself. Before completing the chapter with an introduction to the powerful SugarCRM component bundled with Asterisk@Home, the authors detail required steps to complete the deployment of Asterisk@Home for a simple use case. It’s chapters like this, that allow us to all-at-once appreciate the potential for the Asterisk platform. (Packt has recently released a book on AsteriskNOW. AsteriskNOW is the new name for Asterisk@Home.)
- The SOHO, small business and hosted PBX are the three case studies that collectively comprise Chapter 8. Sequentially, the authors present the case-study scenario, some discussion, Asterisk configuration specifics, and conclusions. In taking this approach, the authors make clear the application of Asterisk to real-world scenarios of increasing complexity. In the SOHO case, the SIP shared object (chan_sip.so) is not loaded as this functionality is not required. This is but one example of how the authors attempt to convey best practices in the deployment of a production solution based on Asterisk.
- Maintenance and security are considered in the final chapter of the book (Chapter 9). The chapter begins with a useful discussion on automating backups and system maintenance plus time synchronization. Those familiar with systems administration can focus on the Asterisk-specific pieces that will require their attention. This focus naturally leads to a discussion of recovering the Asterisk deployment in the event of a disaster. Security gets well-deserved consideration in this chapter from both the server and network perspective. For example, there is very useful and interesting content on securing the protocols used by Asterisk with a firewall. Before closing the chapter by identifying both the Open Source and commercial support offerings for Asterisk, the scalability of Asterisk is given attention.
This book was first published in September 2005 and is based on version 1.2.1 of Asterisk. As of this writing, Asterisk’s production version is 1.4.x, and the version 1.6 beta release is also available (see http://www.asterisk.org/ for more). Even though the book is somewhat dated, it remains useful in acquainting readers with Asterisk, and I have no reservations in strongly recommending it.
Disclaimer: The author was kindly provided with a copy of this book for review by the publisher.