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communicate have changed. Whereas 25 years ago phone calls were the preferred
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way to converse across distances, the current trend is to send messages or conduct
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video-chat conference calls. The phone call is seen as a bit of a dead thing, especially
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by up-and-coming generations. We’re not quite ready for a funeral just yet.
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Asterisk remains a powerful technology, and we believe it is still one of the best hopes
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for any sort of sensible integration between telecom and all the other technologies
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businesses might want to interconnect with. It will need to find its place within a
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communications ecosystem that no longer places telephone calls in a place of impor‐
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tance. Our expectation is that WebRTC, which promises to commoditize web-based
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communications,1 will emerge as a replacement for all the copycat, closed, and pro‐
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prietary “collaboration” products currently flooding (and confusing) the market.
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Asterisk can play a role in this new future, and the Asterisk community has willingly
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and enthusiastically taken on this new concept. So, maybe you’re being told that voice
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1 And more, perhaps, given that WebRTC is revolutionizing native apps too!
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1
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is dead, but anyone who’s paid attention to any science fiction of any kind knows that
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being able to talk to each other across long distances is not going to be the sole
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domain of those who type on keyboards. Humans like to talk, and we’ll continue to
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find ways to do so.
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There also exists, it must be noted, a massive generation of people whose memories
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predate the internet, and for these folks the telephone is still a very useful technology.
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If one wishes to do business with them, one had better do a good job of handling tele‐
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phone calls. These folks are retiring from the workforce, but their wallets still carry a
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lot of clout. Perhaps the PBX is a dying thing, but its tail is very long.
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In this book, we’re going to explore the nuts and bolts of Asterisk. It is a flexible,
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open, standards-compliant toolkit, which we believe is still very relevant to businesses
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today, and will remain useful for many years to come. The power of Asterisk lies in its
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flexibility. It has proven to be very useful at tying various types of communications
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technologies together, and if it is to have any sort of future, it will need to continue to
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do so. Newer technologies such as WebRTC offer all sorts of possibilities for the
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future of communication, and the Asterisk community is very focused on this para‐
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digm shift.
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The remarkable flexibility of Asterisk comes with a price: it is not a simple system to
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learn or configure. This is not because it’s illogical, confusing, or cryptic; on the con‐
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trary, it is very sensible and practical. People’s eyes light up when they first see an
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Asterisk dialplan and begin to contemplate the possibilities. But when there are liter‐
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ally thousands of ways to achieve a result, the process naturally requires extra effort.
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Perhaps it can be compared to building a house: the components are relatively easy to
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understand, but a person contemplating such a task must either a) enlist competent
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help or b) develop the required skills through instruction, practice, and a good book
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on the subject.
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Asterisk and VoIP: Bridging the Gap Between Traditional
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and Network Telephony
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It sometimes seems that we’ve forgotten that the purpose of the telephone is to allow
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people to communicate. It is a simple goal, really, and it should be possible for us to
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make it happen in far more flexible and creative ways than are currently available to
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us. New technologies always seek to dominate the market with a proprietary offering.
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Few succeed. Communications technologies need to interoperate, and technologies
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such as Asterisk lower the barriers to entry for those wishing to innovate.
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It is for this reason—communication—that we believe a future still exists for open
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source telephony projects such as Asterisk. Yes, people might not want to make
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“phone calls” anymore, but we believe there will still be value found in conversations.
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2
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Chapter 1: A Telephony Revolution
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The technologies that can facilitate those conversations may evolve in seemingly radi‐
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cal ways, yet the underlying desire to communicate remains the same.
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Asterisk is plugged into the future, and it has a long track record of successfully inte‐
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grating communications technologies.
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The Zapata Telephony Project
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When the Asterisk project was started (in 1999), there were other open source
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telephony projects in existence. However, Asterisk, in combination with the Zapata
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Telephony Project, was able to provide public switched telephone network (PSTN)
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interfaces, which represented an important milestone in transitioning the software
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from something purely network-based to something more practical in the world of
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telecom at that time, which was PSTN-centric.
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The Zapata Telephony Project was conceived of by Jim Dixon, a telecommunications
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consulting engineer who was inspired by the incredible advances in CPU speeds that
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the computer industry has now come to take for granted. Dixon’s belief was that far
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more economical telephony systems could be created if a card existed that had noth‐
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ing more on it than the basic electronic components required to interface with a tele‐
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phone circuit. Rather than having expensive components on the card, digital signal
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processing (DSP)2 would be handled in the CPU by software. While this would
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impose a tremendous load on the CPU, Dixon was certain that the low cost of CPUs
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relative to their performance made them far more attractive than expensive DSPs,
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and, more importantly, that this price/performance ratio would continue to improve
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as CPUs continued to increase in power.
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Like so many visionaries, Dixon believed that many others would see this opportu‐
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nity, and that he merely had to wait for someone else to create what to him was an
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obvious improvement. After a few years, he noticed that not only had no one created
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these cards, but also it seemed unlikely that anyone was ever going to. At that point it
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was clear that if he wanted a revolution, he was going to have to start it himself. And
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so the Zapata Telephony Project was born:
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