> Introduction: What Are We Trying to Accomplish?

Asterisk PBX (Private Branch Exchange) plays
an important role in our day-to-day lives of communication.We review some key
functionality of Asterisk PBX systems and how they are being used to benefit our
communication.We review some older PBX function technologies, such as analog
and digital, as well as the new integrated VoIP-enabled functions.We review the
benefits that IP-enabled PBX may have and its key differences from traditional voice
technology.
In addition to providing an overview of the Asterisk PBX and examining the
critical role of the PBX in our daily communications, we review the key functions
of PBX systems and the multiple ways the PBX contributes to modern information
exchange.The goal of this chapter is to offer solid instruction on the Asterisk PBX
and underscore its significance.
Understanding the function and process of the PBX is essential to securing it
successfully. After reading this chapter, you should be able to understand the Asterisk
PBX system and its place of functionality in today’s communication environment.


What Functions
Does a Typical PBX Perform?
One of the main functions and benefits of a PBX is to reduce the number of local
loops required from the PSTN central office switch. Further PBX core functions
include maintain the routing information for the customer’s telephone lines and to
route calls accordingly. As its name delineates, PBX is used primarily as a private
device owned by a solitary enterprise.The PBX system is positioned at the customer’s
rather than the telephone company’s site.
NOTE
Do not confuse PBX routing with data routing protocols such as RIP, EIGRP, BGP,
and such. PBX routes are sets of static or dynamic rules, which define routes
based on Least Cost (LC). LC rules generally are decided by the destination of
the number and the time of day it is dialed in order to determine the best call
path with the lowest per minute charge.
The functions of the PBX have evolved since its introduction.Through a process
similar to Darwin’s theory of natural selection, only those functions that have proven
to be the strongest and most adaptable still exist and define PBX as it is currently
understood. One of the many reasons for the endurance of PBX is its ability to adapt
to both emerging markets and the ever-changing needs of the consumer. It has survived
its manual switchboard installations in 1896 through its analog days of the 1970s
and remains relevant today as a partner to the new integrated VoIP technology.
The PBX has undergone four distinct stages of progression.The first stage was
introduced in 1896 when manual switchboard PBX was installed. Operators
switched calls manually by plugging wires into the PBX in order to route and connect
calls.The second stage has been termed the analog stage. PBX has used analog
signaling technology for communications between the PBX trunks and end-systems.
The third stage arrived around 1976 when digital signaling was introduced as the
newest communication method.The digital stage was accompanied by improvements
such as increased PBX functionality and better overall system performance.The final
and fourth and still evolving stage entered the picture in 1999 when the PBX began
using the Internet Protocol (IP) for its signaling communication methods. Internet
Protocol is the most common protocol used to transfer data in local area networks
(LAN) and wide area networks (WAN) such as the Internet.

PBX Administration
Programs used for management of calls, and overall functionality of the PBX system,
allows administrators to keep the PBX system healthy and running. Management
interfaces handle calls by adding and deleting new extensions, updating routing
plans, enabling Call parking feature, monitoring conference calls, and much more.
User-friendly administration programs for complex PBX systems allow better overall
support.

Asterisk Gateway Interface
Asterisk Gateway Interface (AGI) allows developers from all programming backgrounds
to create custom interfaces between Asterisk’s dial plan and outside programs.
Developers choose their own programming languages such as Perl, Pascal, C,
and PHP to manage the PBX system and its connected users. Unlike in proprietary
PBX systems such as Nortel,Avaya, or Cisco, this AGI functionality in Asterisk’s
open-sourced PBX system invites users who want to write their own custom-built
telephony management applications. For further information on AGI and available
applications visit www.voip-info.org and search for AGI.