Abstract :
For the past two decades and more, private systems have played a central role in business communications. In the UK today, some form of private switching equipment is to be found operating across the entire spectrum of the business world. Since the early ´80s it has become the norm for private voice communications to be built around the concept of the digital PBX. These vary in their design concept and sometimes in the terminology used to describe them depending on the size of the system, but all share a basic common design which is of a system on the customer´s premises that provides communication (switching) between the end-users (the extensions) and communication with the outside world via “ trunk circuits” to public network exchanges. For almost as long as PBXs have been around there has been a school of thought that insists that the PBX is a historical quirk which sooner rather than later will be superseded by advances in public network technology that make them redundant. Leased lines, say the proponents of this theory, will give way to virtual private networks (VPNs), and the PBX itself will be replaced by the “Centrex” concept. With Centrex, instead of hosting end users off a PBX, users´ extensions would be connected directly to local public exchanges and software in these exchanges would then provide the users with the appearance of a private system by supporting all the features normally associated with PBXs
