Voice over Internet Protocol (IP)
The voice over Internet protocol (“VOIP”) is where conventional telecommunications meets the technologies used in the Internet. This chapter will begin by explaining communications theory and conventional telephony, which pre-date the popularized Internet.
They serve as a foundation for the VOIP, because almost all voice communications networks must interconnect, and therefore must be backwards compatible with conventional telephone networks. The chapter will conclude with specifics of VOIP design and deployment.
COMMUNICATIONS THEORY: BITS OF TRANSMITTED INFORMATION
Basic communications theory concepts form the building block of all communications applications, and their understanding. VOIP is no exception. “Communication” is simply the transmission of “information” from one place (or device or person) to another, over a “transmission network.” “Voice communications” refers to transmitting the information contained in a voice signal.
The distance traveled by a transmitted signal can be as short as from one chip to another chip on a circuit board, to across the street, to around the world, or even to the most distant galaxies in outer space.
Bits: The Most Basic Form of Information
In digital parlance, a “bit” is the smallest measure of information (Newton, 1998). It is the amount of information required to distinguish between two equally likely possibilities or choices. A bit can be used as a building block for all information or messages and/or as a measure of information.
Transmission of Voice Signals
Almost all voice communications signals begin as analog sound signals. An analog signal is one that is continuous in shape and typically analogous in shape to the original signal or physical phenomenon it represents. The physical signals that our bodies generate and receive are analog. This is very important in the design of telephony systems, because almost all information that we are concerned with in telephony is ultimately either originated and/or received by a human being.
When we speak or play an instrument, combinations of one or more waves of alternating compression and expansion of air are generated at the sound’s source, e.g., the vocal chords in our throats or the reeds of instruments. Because sound energy in air does not travel well over long distances, long-distance communications requires that these sound waves be converted to alternate forms of energy by a transducer, and then that signal energy is conveyed over a compatible transmission medium, such as an electronic transmission channel.
For voice communications, the microphone of a telephone device is typically used to transform analog sound wave energy into analog electrical wave energy to be transmitted over a caller’s local telephone wires.
The Characteristics of an Analog Voice Signal
Communications networks and their elements are designed to cost-effectively accommodate the signals they are intended to carry, with the least degradation. Telephony networks were optimized to carry analog voice signals over long distances using the most cost-effective technology available. However, the original designs were done from the late 1800s through the early 1900s. Those voice-centric design templates continue to influence telephony network design today due to the need for new networks to interoperate with older ones, and the need for those older networks to be backwards compatible with older equipment (which sometimes dates back to the early 1900s).
Most long distance transmission systems in use today (including those using VOIP) transmit voice signals digitally. To transmit an analog signal digitally, its analog version must be transformed from its electrical analog form to a digital form, thus changing its representation from a varying electrical voltage to an equivalent string of discrete 0s and 1s.