leased line services, became the growth engine for the classical telephone companies, many of which were, and still are, prohibited from offering anything other than dial tone and relatively low-bitrate voice grade services. New services at layer 2, 3, and 4 of the OSI stack became "overlay" networks designed, built, and operated by organ- PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS izations of all kinds. Figure 4-8 shows how public network topology has changed. This is the infrastructure that will remain in place and continue to evolve as technology and methods of using it evolve in the next 10 to 20 years. The new public communications network includes more capacity and more capabilities. The shortcomings of TDM, or T-carrier, and circuit switching have been overcome with SONET/SDH transmis- sion, cell, and packet switching technology, except where analog or PDH runs alone in its own environment. The public network is the physical layer for the Internet. It will be a long time before that changes. Originally, the physical layer in the telephone network was passive wire conductors, later bundled into cable sheaths, and over time, evolving to other types of media, including radio and fiber. The architecture of the network's physical layer can be separated and classed into access and transport Packet Switching (IP) International Cell Switching (ATM) International Gateway Gateway Circuit Switching Inter-Exchange Carriers To Central & Latin America Circuit Switching Incumbent Local Exchange Carriers Competitive Local Exchange Carrier Circuit Switching Packet Switching Private Private Voice Internet Internet Service Provider Access Voice Internet Access FIGURE 4-8 US Public Communications Network C. 2003 4 NETWORK TECHNOLOGY AND METHODOLOGY facilities. The philosophy behind this architecture arises from the processes by which services are designed, used, and charged for. All end-to-end service originates via an access facility and terminates over a transport facility at the other end via an acce