It must be understood from the outset that although this book focuses on packet-switched voice, we are not concerned here simply with the ability to transmit voice signals across a packet-switched network without unacceptable deterioration of voice quality. Since digital data sets transmitted across a packet-switched network might just as easily comprise digitized voice signals as anything else, there is no question that even very high fidelity digitized voice signals can be transmitted across a packet-switched transmission network with negligible loss of fidelity.

Rather, we are concerned with the ability to digitize and transmit voice signals across a packet-switched network and the ability to do this in a way that supports near-real-time, multidirectional voice exchanges. To distinguish this application, transmission capabilities designed to support such interactive exchanges of voice are referred to here as voice services. Under this convention, for example, the ability to transmit a digitized recording of a voice message via a streaming voice system does not constitute a voice service, because the transmission is not effected in near real time. Similarly, even the unbuffered, direct transmission of voice as part of a video clip fails to qualify as a voice service because no accommodations of the kind of interactive exchanges that would occur over a picture telephone are required.

Such voice services are often described in technical discourse as VoX, where Vo stands for voice over and X represents the transmission protocol used in the host packet-switched network. Thus, for example, an interactive voice exchange capability carried over packet-switched transport employing the Internet protocol (IP) is frequently described in the technical literature as VoIP. This nomenclature conveys information as to the type of network in which the voice service is to be implemented. However, it does not convey any information as to the kind of voice service involved.

Consequently, the use of the VoX (e.g., VoIP, VoFrame, VoATM) descriptors sometimes fosters the erroneous notion that there is a single voice service contemplated or implemented in each medium. In fact, in any particular packet-switched medium, such as the Internet, we may see the implementation of a wide variety of distinctly different voice services, each with its own requirements and functions. Where necessary to avoid confusion, lowercase letters will be added after the X to denote a specific voice service. Thus, for example, later in this book you will see VoIPtpt used to distinguish general-use voice transport via IP networks from the more special case of on-net telephony, denoted VoIPtel.