Overview

Telephone communications systems, including PBXs, have always used circuit switched networks to establish connections between network endpoints the sender and receiver of a voice call. Circuit switching is simply defined as a type of communications in which a dedicated channel, referred to as a circuit, is established for the duration of a transmission between the originating and terminating endpoints. In communications parlance, a channel refers to a communications path between two connected endpoints, for example, two telephones. Circuit switched networks, also known as connection-oriented networks, are ideal for real-time voice communications requirements.

Until the development of digital communications, circuit switched networks used analog switching and transmission techniques to establish connections between calling and called parties. When the first digital PBXs were introduced in the mid-1970s the internal switching networks required a conversion of analog wave signals into a digital transmission format. Voice audio signals from the desktop telephone to the PBX common equipment hardware were transmitted over a 4-KHz communications channel, at which point a codec embedded on the port circuit card converted the analog signal into a digital signal for transmission across the internal circuit switched network. When digital telephones were introduced in 1980, the codec function resided in the desktop instrument itself, and transmission to the PBX common equipment was in digital format, ready for transmission across the internal circuit switched network. Digital switching, as opposed to analog switching, provides improved sound quality and more reliable transmission at a lower cost.

Digital PBXs from different manufacturers use different...