System information

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
Voice over IP (VoIP) is often thought of as little more than a method of obtaining free
long-distance calling. The real value (and—let’s be honest—challenge as well) of VoIP
is that it allows voice to become nothing more than another application in the data
network.
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. Technologies such as Asterisk lower the barriers to entry.
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 interface (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 nothing
more on it than the basic electronic components required to interface with a telephone
circuit. Rather than having expensive components on the card, digital signal processing
(DSP)
*
would be handled in the CPU by software. While this would impose a tremen-
dous 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 impor-
tantly, 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 opportunity,
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,
* The term DSP also means digital signal processor, which is a device (usually a chip) that is capable of
interpreting and modifying signals of various sorts. In a voice network, DSPs are primarily responsible for
encoding, decoding, and transcoding audio information. This can require a lot of computational effort.
2 | Chapter 1: A Telephony Revolution