Practical VoIP Using VOCAL

An Introduction to VoIP and VOCAL

by Luan Dang, Cullen Jennings and David G. Kelly, authors of Practical VoIP Using VOCAL

For years, software has been available for making "free" long-distance calls between workstations over the Internet. The early versions of this software provided poor quality, but users were willing to suffer packet loss, jitter, and latency in return for bypassing normal long-distance toll charges. Today, users can choose from a large variety of Voice over IP (VoIP) software packages. Improvements in bandwidth and the processing speeds of home PCs have enabled practical conversations through VoIP devices.

In North America and Europe, the novelty of "free" long-distance calling has withered away with the reduction of toll charges from the major telecom carriers. For example, in the United States, cell phone providers are presently bundling free long-distance calling with their regular services. By itself, a perceived advantage in long-distance charges will not be enough to convince individuals and organizations in these countries to replace their traditional phone systems with IP-based systems. The current belief being expounded by the pundits is that before IP phone systems become widely accepted in developed markets, they are going to require advanced, integrated features of the type that are practically impossible to implement in traditional private branch exchanges (PBXs) and central offices (COs).

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Speaking About VoIP -- The authors of Practical VoIP Using VOCAL discuss why VoIP is on the verge of taking off, and how their book and VOCAL are helping the community to grow and build VoIP applications.

In other regions of the globe, the intersection of costs and features will play an important role in the adoption of VoIP. Some areas suffer from both crippling poverty and extravagant import duties imposed on communication equipment. In these countries, there are service providers who will do everything possible to bring low-cost systems into their market space. In others areas, landlines may be scarce, but capital and technical expertise are available to build new phone systems. Many of these new ventures are looking toward VoIP as a flexible solution that enables deploying phone services to millions of subscribers quickly.

The nature of VoIP, also known as packet telephony, permits the type of advanced features that will win over new users. As anyone who uses the World Wide Web knows, packets running over an IP network can deliver text, pictures, and audio and video content. The PC is becoming a redundant tool for Internet access with the advent of personal digital assistants (PDAs),cell phones, and other portable devices that provide access to email and other web content. Unlike the PSTN, the Internet is decentralized and permits smart endpoints. Someday, the concept of making a phone call may become obsolete by the concept of simply being in touch with people through a variety of smart IP-based devices.

Related Reading

Practical VoIP Using VOCAL
By David G. Kelly, Cullen Jennings, Luan Dang

Advances in packet telephony could also lead to new forms of virtual offices that would seem alien to our current telecommuting practices. Our descendants could know an enhanced mode of long-distance communication in which body language, along with the other 90% of communication that is lost over audio-only devices, is transmitted and received intact. This could make face-to-face meetings a rare novelty. What this might do to city planning, traffic jams, and, indeed, our lifestyles is a worthy subject for another book.


VOCAL (the Vovida Open Communication Application Library) is an open source software project that provides call control, routing, media, policy, billing information and provisioning on a system that can range from a single box in a lab with a few test phones to a large, multi-host carrier grade network supporting hundreds of thousands of users. VOCAL is freely available from the Cisco Systems-sponsored community Web site.

When we started designing VOCAL, we had three primary goals in mind:

  • Build a distributed architecture.
  • Build a system that was scalable.
  • Ensure no single point of failure.

System Architecture

A distributed architecture suited our aim to open source VOCAL as it provided components that developers from the community could build upon or build into their projects. Scaling the system meant assigning one type of server, which became known as the Marshal server, with the task of being a single point of contact for the subscribers and enabling duplicates of this server to be added to the system as the subscriber population grew. Our original idea was to achieve load balancing by assigning each additional Marshal server with a specific population of subscribers. Our original plan also called for a multi-host system with redundancy for all call control servers to avoid a single point of failure.

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