Practical VoIP Using VOCAL

Speaking About VoIP

by Bruce Stewart

Bruce Stewart recently sat down for a roundtable discussion with the authors of Practical VoIP Using VOCAL. In this interview Luan Dang, cofounder of Vovida Networks, Cullen Jennings, and David G. Kelly explain why VoIP is on the verge of taking off, and how their book and VOCAL, the open source software that enables a core network to support VoIP, are helping the community to grow and build practical VoIP applications.

Stewart: What is VoIP?

Dang: VoIP is a set of rules (also known as protocols) and devices that enable users to make phone calls over the Internet. VoIP systems transmit signals to set up and tear down calls and media to make it possible for users to hear each other talk. These signals and media are sent over networks as packets just like other forms of data. Another term for VoIP is "packet telephony."

More and more organizations are installing VoIP systems to make better use of their networks. If you are already using a large network for sharing text and images, it is not a large technical leap to deploy a VoIP system on the same network. Open source systems, such as VOCAL, help make setting up VoIP networks cost effective.

Kelly: Manufacturers of traditional phone equipment, also known as time division multiplexing (TDM) equipment, are offering VoIP solutions to help their customers switch from the older technology to the new Internet-based technology. Some VoIP architectures offer smart phones that are capable of doing much more than traditional home or office phones. We anticipate many new developments in the features and functionality available on IP phones in the coming years.

Stewart: What hurdles have to be overcome for VoIP to be widely used?

Dang: Compared to the traditional phone system, which is over 100 years old, VoIP is new and it takes time for many organizations to make large investments in new technologies. We are starting to see major banks, manufacturing companies, and other "old economy" organizations installing VoIP phone systems.

Kelly: There are several quality-of-service issues that still need to be addressed, although sometimes when I make a long-distance call over the public Internet, the quality is as good as what I would expect from a traditional land-line phone. Security is another concern. However, there are many people in the Internet Engineering Task Force (IETF) who are coming up with new proposals and standards to make VoIP secure enough to assure privacy and protection for end users.

VoIP equipment is still expensive, as much of it is in the early stages of product development. However, just as the cost of PCs and other electronic equipment has come down in price, soon VoIP equipment will be considered as a viable alternative to traditional systems by many more individuals and organizations.

Stewart: What is VOCAL?

Dang: VOCAL is the Vovida Open Communication Application Library, which is open source software that enables a core network to support VoIP. In 1999, Alan Knitowski and I founded a company called Vovida Networks (think about VOice, VIdeo and DAta) to kick-start VoIP application development by offering free open source protocol stacks to the public. Eventually, we sold Vovida Networks to Cisco Systems and made the VOCAL system open source and available from a community Web site called

VOCAL is governed by a BSD-style license that enables developers to download the code without paying any royalties or fees, make changes without having to send the changes back to our code repository, and use the code within new proprietary applications with our full permission. VOCAL was written primarily in C++ with a Java-based provisioning system. VOCAL runs on many flavors of Linux as well as Sun Solaris. VOCAL uses Session Initiation Protocol (SIP) to set up and tear down phone calls. You can download a copy of our SIP stack from It is also open source.

Kelly: Even though we don't require developers in the community to share their code changes with us, many people have sent us bug fixes, minor enhancements, and fully fledged open source applications. The community is growing and we are constantly surprised by the volume of email we receive and the diversity of the people contributing code and helpful information. The community has become truly international.

Stewart: Why did you decide to write Practical VoIP Using VOCAL?

Dang: VOCAL is cool and by providing useful documentation, we have helped our community grow and build practical VoIP applications.

Jennings: VOCAL empowers the end user with control over feature development and customized integration with legacy systems. Along with all other VoIP applications, VOCAL is actively inverting the way that telephony is deployed by allowing an Internet-style anarchy that was never possible in the traditional PSTN. It has been a fun area to work in.

Kelly: The book provides a parallel channel for documentation distribution and awareness of VOCAL. Writing an O'Reilly book has provided us with a valuable companion to our Web site,, our mailing lists, and our participation in different trade shows such as LinuxWorld, ISPCon, and ASPCon.

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