The common equipment base unit for the DSU II digital telephone system is a fully electronic device. It is essentially a special purpose computer system acting as a communications controller  between central office (CO), private branch exchange (PBX), or CENTREX supplied lines and the proprietary digital telephone stations. The software design of the common equipment provides complete system support and great flexibility of operation.  All DSU II systems have analog, loop-start line interfaces to the public switched network.  Special integrated circuits (COder/DECoder or CODEC chips) in the line circuits translate analog voice information to and from the digital domain. Internally, the system is fully digital and has Pulse Code Modulation (PCM) highways that are time-division multiplexed into PCM channels. Each digital station has two B-channels available for voice and/or data and one D-channel available for telephone control. This arrangement is known as 2B+D. The system maintains communication with the stations with digital loop transceiver circuits that are under system software control. A time switch integrated circuit, also under system software control, routes calls, creates conferences, and set padding levels as required.  The common equipment consists of a base unit, which provides complete feature support, and optional expansion modules for additional lines and stations. It is contained in a contemporary metal housing designed to be inconspicuous in a modern office environment. It is engineered to be wall or rack mounted.