quipment. The use of the terms virtual private and software-defined came into vogue around the time of the divestiture of AT&T and the deregulation of the long distance industry. Early exam- ples of SDNs include AT&T and the Bell Operating Companies, who shared central office switching as an alternative to a private branch exchange and enhanced private switched communications services. VPN is often used to describe a data network, but it is nothing more than a combination of point-to-point private leased lines and shared core routers or, in the old days, multiplexers. Some will claim that VPNs include encryption, firewalls, and so-called tunneling and that they are capable of carrying voice traffic. These claims and attributes are all true; however, the concept of shared faculties is the same used in the SDNs that arose in the late 1970s and early 1980s to support multiple locations private dial plans for telephone service. UNDERSTANDING TELECOM INFRASTRUCTURE Much like broadcast systems and facilities, telecom systems and facilities have their unique symbols, labels, and documentation con- ventions. The key to understanding the technology and its applica- tion is getting a grip on the language. The keys to designing, specifying, and getting what you want include a basic knowledge of the subject matter and some design tools. The design tools include PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS a computer with word processor, spreadsheet, and drawing capabil- ity. For large complex systems and networks, a database manager becomes an important tool for thorough and accurate cost and oper- ations analysis. In addition to these tools, design reference material in the form of recommended practices, standards, product specifica- tions, and data sheets are a must. Another way of defining telecom infrastructure is a technique called layering. Layering, as used in computer and communications, is best explained as a technique whereby software and digital networks are designed and built in layers. The n