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VoIP (Voice over IP) is also known as packet telephony and enables you to make phone calls over the Internet. Initially this technology emerged to bypass expensive long-distance call charges. Since this technology has become more mature, VoIP is seen as a possibility to offer flexible new services to its users, without the slowness and the difficulty to change a Public Switched Telephone Network. VOCAL is open source software that enables a network to support VoIP. VOCAL is written by the start-up Vovida, now part of Cisco Systems. VOCAL is running on Linux, and everybody can set up a simple VoIP telephony network at home. The book describes version 1.3, but the latest release is already 1.5 at the time I was reading the book. The book describes how to install VOCAL, and perform your first tests and make simple phone calls. VOCAL is scaleable, and you can set up many services on numerous servers. You can do this without any coding, just by configuration. However if you want to offer new services, which are not provided in VOCAL, you would have to dive into the C++ code.

The authors do not assume that you have any telecom knowledge. All protocols, like SIP, RTP, MGCP, H.323, SNMP, which are used in VOCAL, are clearly explained in simple words and many message flow diagrams. Many chapters in this book describe the architecture and class structures of VOCAL, introducing you to the software (written in C++), in case you want to contribute to the open source project.

The screenshots of the provisioning server (responsible for maintaining and storing data and parameters needed by other servers) are described in too much detail; I assume that most people know that they need to click an OK button. The text is clearly written and is pleasant to read, there is no boasting about how perfect VOCAL is, and bugs or performance issues are honestly described.