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 cus-tomer’s rather than the telephone company’s site.

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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 sur-vived 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 con-nect 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.

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In the early 1980s, Digital PBX was designed to provide all the data and voice networking in the industry. PBX was the multipurpose device through which all phones and computers were connected. Keeping in mind that in the 1980s the fastest modem capability was 9.6 Kbps and 10 Mbps and that Ethernet was yet to be created, so-called high-speed PBX data modules were introduced with rates of up to 57.6 Kbps. This new speed with digital signaling technology allowed PBX systems to interconnect computer systems at higher speeds, and introduced new advances in WAN connectivity with superior performance and flexibility of packet routing over fixed WAN circuits compared to circuit-switched alternatives. It appeared that Digital PBX had great potential and promise for its

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time in connecting local computers. This promise to use PBX as a hub for local computers, however, was short-lived as PBX was unable to compete after the introduction of Ethernet and cheap 10 Mbps capable hubs.

It is important to note that the three technologies described in the preceding paragraph are all used today and will be used for many years to come. Significant differences exist between analog and digital technologies. Although analog tech-nology is relatively simple and inexpensive, it fails to offer some of the complex functions of digital.

Compare the functions of your work phone to the one in your home. At home you might have speed dial, a mute button, and redial, versus at work where you have a button for voicemail that lights up when you have new messages and buttons for transferring calls, conferencing (to connect up to four other people), operator emer-gency, on-hold that plays music, and other functions.

Digital signaling offers increase of sophistication in the methods used for trans-porting audio signal (media) across time and space. Analog signaling takes video or voice and converts it into electronic impulses. Digital signaling takes video or voice and converts it into binary format of 0s and 1s. Binary format takes less line resource than electronic impulses in analog signaling. By using this increased complexity of signaling in digital lines, more functions can be passed down to end-users from the PBX system.

The third and final stage of PBX evolution, known as VoIP, produced increased user capability. An IP-enabled PBX system taps into WAN/LAN networks, allowing them to communicate with IP-enabled devices all over the world. Our phone sys-tems are no longer restrained to the cable distance and bandwidth limitations of tra-ditional PBX connectivity. IP-PBX systems truly deliver on-demand remote

workplace functionality, where your phone number is attached to your devices’

dynamically changing IP addresses anywhere in the world. We will concentrate on describing VoIP-enabled PBX and comparing them to traditional digital PBX sys-tems later in this chapter, and review details in Chapter 3.

Many new commercial and open source PBX systems have emerged with this new IP-based stage. One such IP-enabled PBX called the Asterisk™ has introduced this VoIP technology and graciously offers its services to the community free of charge. We will be reviewing some of Asterisk™ functionality within this chapter.

Asterisk is software running a PBX system that is freely available to anyone under GNU General Public License (GPL). Asterisk can be installed and run on a variety of different operating system (OS) platforms, such as Linux, BSD, and Mac OS X. Some of Asterisk’s features include Call conferencing, Call monitoring, Call

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forwarding, Call parking, Call routing, Caller ID, Caller ID blocking, Calling cards, E911, IVR, Music on hold, Voicemail, and many more. It supports codecs such as ADPCM, G.711, G.723, G.726, G.729, GSM, Speex, and others. Protocols supported include IAX, H.323, SIP, MGCP, and SCCP. Its support for traditional telephony interoperability includes E&M, FXS, FXO, Loopstart, Groundstart, DTMF, and others.To find out more about Asterisk™ open source PBX visit its Web site at www.asterisk.org.