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Asterisk Is a Private Branch Exchange (Pbx) for VoIP

|  4 min
In this article

What if there was a technology that allowed you to have a PBX without vendor lock-in? What if that technology could use the fastest and cheapest CPUs? What if it was open source, flexible enough to let your developers extend the system, with the backing of a community? 

Well there is. It’s called Asterisk and it’s an open source PBX, a perfect partner for VoIP. Unlike the technology from major telephony vendors, Asterisk is completely flexible. It allows competent developers to build and extend systems to behave exactly how their end-users expect. 

Being open source, it has a development community interested in its quality, security and performance. This allows Asterisk to evolve along with the underlying technologies. As Geoffrey A Moore said in Crossing the Chasm [1] “…product flexibility and adaptability, as well as on-going account service, should be critical components of any buyer’s evaluation checklist.” 

Here we take a brief look at why Asterisk came to be, particularly in comparison with the major telephony vendors, and where to get help and information. 

Vendor lock-in

Historically, telephony vendors had a perfect lock-in arrangement. Customers had to use their proprietary systems, with ageing hardware and monolithic code-bases. The systems were expensive and rigid, for example, application changes could only be made by the vendor’s own development team. It was next to impossible to have end-user functionality requests. 

With the increasing popularity of VoIP the vendors simply shifted their stance. For example, Nortel developed their Business Communications Manager product. It’s very comprehensive and the Operations Guide runs to some 999 pages, but tellingly it’s intended for the installer, operator and system administrator. 

By way of example, the setting Delayed Ring Transfer – which controls the number of rings before the call is forwarded to a prime telephone – can be set to Off, 1, 2, 3, 4, 6 or 10 rings. This is typical of a de-facto industry standard; it began as a gesture towards flexibility but never progressed and it’s now hard-baked into the software. 

If the lock-in was only limited to the number of rings, the situation would have been acceptable to all but the most demanding customers. However, this was not the case. 

A little earlier than the shift towards VoIP, computing technology vendors were beginning to exploit the capabilities of cheap high-power CPUs. Database vendors such as Oracle were competing with the likes of Microsoft by lashing together many CPUs to form parallel processing engines. By using commodity CPUs and the Linux operating system, they could deliver huge amounts of processing power at a much lower cost. 

The same shift failed to happen in the telecoms industry owing to the dominance of a few major players and their locked-in customers. 


The industry needed a paradigm shift and a telecoms consultant called Jim Dixon came up with one. A few manufacturers had already started to build proprietary cards to install in a computer to handle a few POTS lines. The cards required a 286 processor running under MS-DOS. However, these systems were still very expensive and Dixon was growing increasingly frustrated with the costs to his customers. 

Dixon’s breakthrough was in recognising that CPUs would get much faster and that the technology on the cards could be shifted into CPU processing. He devised a new Industry Standard Architecture (ISA) card with an accompanying driver written for BSD UNIX. He called the card “Tormenta”, Spanish for storm, and named the business after General Emiliano Zapata the Mexican revolutionary. 

A Linux specialist, Mark Spencer, re-formatted the software for Linux. He already had a concept – Asterisk – waiting for an enabler technology, and this was it. The cards evolved and were marketed under the Digium brand and Asterisk was made open source. 

Open Source

Asterisk now runs on Linux, BSD and OS X and handles VoIP in 4 protocols. A competent developer with telephony skills can use it to build and extend a PBX. It is standards-based so it can interoperate with any telephony equipment developed to industry standards. 

It is an ideal fit for VoIP, needing no additional hardware. Customers simply choose a VoIP DID provider such as IDT for incoming and outgoing calls. 

Asterisk community

There is an Asterisk community for exchanging ideas, business and support [3]. These are handled by a number of subscription-based mailing lists:

– asterisk-app-dev – Application development discussions
– asterisk-code-review – Peer code reviews
– asterisk-security
– asterisk-users – The main route for getting suppor 

Asterisk the perfect partner for VoIP

Asterisk fulfils the vision of Jim Dixon and Mark Spencer by allowing businesses to benefit from low cost, high-quality telephony with no vendor lock-in. By using commodity hardware with open source software, Asterisk is the perfect partner for VoIP. 

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