 we are stretching the definition. In this case, a network interface unit is used, through which one or more end-users may be connected. A connectivity links an end-user to a node, and from there possibly through other nodes to some final end-user destination with which the initiating end-user wants to communicate. Figure 1.2 illustrates this concept. The IEEE (Ref. 2) defines a connection as "an association of channels, switching systems, and other functional units set up to provide means for a transfer of information To/from other nodes or end-users End-user End-user End-user End-user Figure 1.2 Illustrating the functions of end-users, nodes, and connectivity. 2 SCADA stands for supervisory control and data acquisition. INTRODUCTORY CONCEPTS between two or more points in a telecommunications network." There would seem to be two interpretations of this definition. First, the equipment, both switching and transmission facilities, are available to set up a path from, say, Point A to Point B. Assume A and B to be user end-points. The second interpretation would be that not only are the circuits available, but also they are connected and ready to pass information or are in the information- passing mode. At this juncture, the end-users are assumed to be telephone users, and the path that is set up is a speech path (it could, of course, be a data or video path). There are three sequential stages to a telephone call. 1. Call setup 2. Information exchange 3. Call takedown Call setup is the stage where a circuit is established and activated. The setup is facilitated by signaling, 3 which is discussed in Chapter 7. It is initiated by the calling subscriber (user) going off-hook. This is a term that derives from the telephony of the early 1900s. It means "the action of taking the telephone instrument out of its cradle." Two little knobs in the cradle pop up, pushed by a spring action causing an electrical closure. If we turn a light on, we have an electrical closure allowing electrical current to pass. The same thing happe