This book is a sequel to Building Telephony Systems with Asterisk, which started you on a journey to the summit of Asterisk knowledge, taking you from base camp to camp two, from being a complete Asterisk newbie to a competent telephony system builder and manager. Now it's time to push to the top, to take your telephony knowledge to a point where you can build high-performance, resilient, and professional PBXs using the most popular open source telephony software in the world—Asterisk.
In that book, the focus was very much on installing and configuring Asterisk for a number of common scenarios, including both home and office use. This it achieved admirably, so you may now wonder why another book is needed. Well, there are three main reasons for writing this book. Firstly, Asterisk is such a highly-capable and configurable telephony engine that the 150-odd pages in the book necessarily had to exclude discussion of some of the more advanced features, which we now have the opportunity to explore. Secondly, Asterisk is invariably implemented as part of an IP network, and further examination of network considerations is warranted. Finally, like all popular open source software, Asterisk is constantly being updated, and while this book still assumes the version 1.4 of Asterisk is in use, we do point out any differences in version 1.6 where relevant, such as the change from Zaptel to DAHDI.
Therefore, the goal of this book is to give you enough knowledge to build and install a telephony system with Asterisk at its core, which will stand comparison with the market-leading commercial IP-enabled systems. Whether you are building such a system as a result of an internal company requirement, or you plan to offer it as an element of a commercial package to customers, this book will take you through all the areas that require consideration. On reading this book you will also be in a position to understand the real-life issues you are likely to experience when deploying such a system, both technical and otherwise.
By its very nature, Asterisk demands that much of the focus of this book be on the technical aspects of building your professional system. However, as with most IT implementations, success will also rely on "soft" issues such as managing expectations, understanding and meeting the customer's particular needs, and ensuring delivery is on time and up to the budget. Hence, where appropriate, we make mention of the non-technical aspects that may make a difference to your deployment.
To achieve our goal, this book will build on knowledge already gained by reinforcing that learning and adding extra skills covering:
Security
Networks
Large-scale considerations
Resilience
Scalability
Integration with complementary products
Commercial aspects
If you have not already done so, it is recommended that you read Building Telephony Systems with Asterisk, or achieve a good degree of competence in building basic Asterisk PBXs through other means. These could include commercial training courses (see www.digium.com/en/training for further details) or openly available internet resources such as the excellent VoIP wiki at http://www.voip-info.org.
While most people with a day-to-day exposure to Asterisk systems should stand to gain much from this book, it has been written in the expectation that you will possess the following Asterisk skills and experience, ideally gained through text file configuration:
Connecting Asterisk to analogue and digital PSTN lines, and VoIP services
Configuring different types of terminal equipment (phones, communication devices, other PBXs)
Installing Asterisk, Zaptel and LibPRI
Configuring features (Voicemail, Music On Hold, Queues, Conference Rooms, and so on)
Creating a dialplan, including call distribution
CDRs, call monitoring and recording
Backups and restores
Basic security and load balancing
Once equipped with this knowledge you stand to gain the maximum from the topics covered in this book, enabling you to build professional Asterisk systems to be deployed internally, or to form the cornerstone of a commercial offering.
In this book you will, hopefully, learn many new things. At its conclusion you will have the knowledge to build and successfully implement systems that combine great performance, resilience and stability. In order to do so, we will mainly consider "pure" Asterisk systems that require a deep understanding of the dialplan and configuration files without the safety-net of a GUI in between. Think of it as learning to become a great car mechanic. You can certainly be a good mechanic earning a good living by learning how to use a laptop plugged into an engine management system. But if you want to take that extra step to being a true master of the trade then you need to understand at a very deep level just how the internal combustion engine works. So it is with Asterisk. It is perfectly feasible to put very good solutions together using GUI-based systems such as the Digium-owned Switchvox, Trixbox (formerly Asterisk@Home) or PBX in a Flash, but to construct the best systems you will need to understand what is happening "under the hood" so that you can tweak them appropriately to achieve or exceed the customers' expectations.
One advantage of eschewing the GUI approach is a potential increase in performance and scalability through the use of a highly-optimized dialplan and a reduction in applications running on the server. However, there are many situations where a GUI is at least as appropriate, particularly if the customer wishes to carry out day-to-day management tasks. Therefore, in Chapter 12 we look at the implications of choosing a GUI-based solution over a "vanilla" system.
Note
To follow the "trusted network" of Asterisk developers please visit: www.asteriskpro.co.uk
As a result of reading this book, you can expect to build on existing knowledge and gain new skills. Each chapter covers a particular topic, but throughout there is a focus on building an Asterisk system that can form the cornerstone of a serious commercial product, capable of matching or even exceeding the performance of well-known licensed products.
Chapter 1 talks about dialplan techniques including modular implementations by using macros, contexts, and so on to both refine the dialplan and improve the security of the system. It also discusses the use of the devstate() function.
Chapter 2 discusses customer network requirements and offers some good advice about potential issues within the customer network and how to resolve them, including the use of VLANs and Quality of Service.
Chapter 3 looks at routing in general, including Least Cost Routing (local, national, and international GSM gateways), fall-back routing, alternate routing, and so on. ENUM and DUNDi are also explained within this context.
Chapter 4 considers call center requirements, including queues, agents, call distribution strategies, performance monitoring and call recording issues. An Asterisk-based call center solution, VICIDIAL, is also discussed in some detail.
Chapter 5 introduces speech technology in the form of ASR, TTS, and SVI; followed by implementation advice and examples. Both Lumenvox and Cepstral packages are explored in detail.
Chapter 6 looks at methods that can be used to implement call accounting and billing solutions for Asterisk. In particular, Asterisk-stat and A2Billing are explored.
Chapter 7 discusses resilience and stability, giving you a guide to implementing highly-available Asterisk solutions for mission-critical applications. Use of failover and load-balancing techniques are explored.
Chapter 8 explores the comprehensive localization options within Asterisk, and also suggests some easily deployed security measures.
Chapter 9 considers interfaces with traditional analogue and digital telephony, giving more in-depth explanations of Libpri and DAHDI (formerly Zaptel), and discussing implementation considerations.
Chapter 10 tackles the good and bad points of using wireless technologies with Asterisk, covering Wi-Fi, dual-mode and DECT handsets. Some suggestions on routing via cell/mobile networks are also offered.
Chapter 11 looks at the good and bad points of Asterisk Graphical User Interfaces (GUIs), focusing on one of the most popular incarnations, FreePBX.
In Appendix A we also explore some of the softer skills required when selling Asterisk-based solutions, suggesting some sales strategies that can help you in a commercial environment.
In Appendix B you will find information you might want to include in sample emails when pitching.
In Appendix C you will find a sample appointment sheet which can be used as a template.
So now our campsite has been packed away and it is time for the next part of our journey to begin, for those first purposeful steps to be taken towards the summit. We will start in Chapter 1 by looking at the heart of any Asterisk system, the dialplan. You will already have significant knowledge in this area, but we are about to show you some of the techniques that are used in systems with thousands of extensions that handle many tens of thousands of calls per day. Without these techniques, a dialplan can become an unholy mess as system size increases. However, using these techniques will ensure that complexity is avoided and performance is maintained.
In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.
Code words in text are shown as follows: "We can include other contexts through the use of the include directive."
A block of code is set as follows:
exten => s,1,Dial(Zap/1,30)
exten => s,n,Goto(s-${DIALSTATUS},1)
exten => s,n,Hangup()
exten => s-NOANSWER,1,Voicemail(100,u)
exten => s-BUSY,1,Voicemail(100,b)
exten => i,1,Voicemail(0,s)
When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
[default]
exten => s,1,Dial(Zap/1|30)
exten => s,2,Voicemail(u100)
exten => s,102,Voicemail(b100)
exten => i,1,Voicemail(s0)
Any command-line input or output is written as follows:
# cp /usr/src/asterisk-addons/configs/cdr_mysql.conf.sample
/etc/asterisk/cdr_mysql.conf
New terms and important words are shown in bold. Words that you see on the screen, in menus or dialog boxes for example, appear in the text like this: "clicking the Next button moves you to the next screen".
Note
Tips and tricks appear like this.
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