ABSTRACT
A telephone system is an analog telecommunication system that carries voice signals. The telephone set uses a transducer to convert human speech into electrical signals for transmission by a transmitter; the signals are reconverted into voice on the receiver side by another transducer. The traditional private automatic exchange (PBX) allows voice communication among different telephone sets within organizations and also provides interfaces with public switched telephone networks (PSTNs) and many other telecommunication networks. Digital transmission has opened a new era in accessing telecommunication services from the digital private branch exchange (DPABX), to which users can connect their computers and terminals along with their telephone sets to access services from existing PSTNs and also from new, emerging networks, such as integrated services digital networks (ISDNs). The services available on these networks include voice, data, text, images, video, motion pictures, etc.
