mpletely. The use of satellite links in certain situations is one method for PSTN bypass. Another is to lease capacity from some other provider. Other provider could be a power company with excess capacity on its microwave or fiber-optic system. There are other examples such as a railroad with extensive rights-of-way which may be used for a fiber-optic network. Another possibility is to build a private network using any one or a combination of fiber optics, copper wire line, line-of-sight microwave, and satellite communications. Some private networks take on the appearance of a mini-PSTN. 1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS An overall telecommunications network (i.e., the PSTN) consists of local networks inter- connected by one or more long-distance networks. The concept is illustrated in Figure 1.1. This is the PSTN, which is open to public correspondence. It is usually regulated by a gov- ernment authority or may be a government monopoly, although there is a notable trend toward privatization. In the United States the PSTN has been a commercial enterprise since its inception. 1.3.1 End-Users, Nodes, and Connectivities End-users, as the term tells us, provide the inputs to the network and are recipients of network outputs. The end-user employs what is called an I/O, standing for input/output 1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS Local Network Local Network Long-Distance Network Local Network Local Network Figure 1.1 The PSTN consists of local networks interconnected by a long-distance network. (device). An I/O may be a PC, computer, telephone instrument, cellular/PCS telephone or combined device, facsimile, or conference TV equipment. It may also be some type of machine that provides a stimulus to a coder or receives stimulus from a decoder in say some sort of SCADA² system. End-users usually connect to nodes. We will call a node a point or junction in a transmission system where lines and trunks meet. A node usually carries out a switching function. In the case of the local area network (LAN),