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