regional, national, and international. Telecom networks are built using a layered architecture. This archi- tecture is based on international standards and consists of a physical layer, facilities, and service layers. Although this layering character- istic is related, it should not be confused with the seven-layer ISO model commonly used in data communications and information technology documentation. It is also related and should not be con- fused with the four-layer approach sometimes seen or referred to in Internet or Internet protocol (IP) documentation. PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS One of the main objectives of this book is to bring clarity to this picture for the reader. Clarity comes from understanding a greater level of detail of the telecom entity. For purposes here, the Internet is another facilities or service layer in the overall architecture. The Internet relies on classical telecom physical or transmission layer infrastructure. It is likely to remain that way for the foreseeable future. Many, if not most Internet people, netizens as they sometimes refer to themselves in a third-person way, lack appreciation and understand- ing of the physical layer or the entire classical telecom infrastructure. To many it's old, outdated, obsolete, and subject to complete disre- gard, yet it is critical to successful operation of the Internet. That's because the original Department of Defense Advanced Research Pro- jects Agency project included an assumption that the physical layer would always be 100% available. That general approach and attitude remains today and is reflected in such things as contemporary certifi- cation training and testing by router equipment manufacturers. There has always been a similar attitude and approach in classical data communications network design and operations. The simple message in this point is "don't forget the layer 1 and 2." Another area of misunderstanding and common misuse is the term private network. Networks are made up of individual e