otion is that if the operational routine or software system is built in layers and each layer interacts or interoperates with the one above and below it, then the entire system is more likely to achieve its overall goals and effectiveness. Examples of layering include the ISO Open Systems Interconnect model, the four-layer Internet model, and the synchronous optical network/synchronous digital hierarchy (SONET/SDH) four-layer model. These models are three entirely separate models, and while they may not have been created without awareness of each other, taken literally, they don't appear to have any relationship. Figure 3-2 shows these three models side by side and how they relate to each other. Open Systems Interconnect Model Internet Model Application ASCII Presentation Binary Virtual Terminal Session TCP & UDP SONET/SDH Model Transport Network Data Link Section Physical Media FIGURE 3-2 Open Systems Interconnect Stack 3 CONTENT TRANSPORT NETWORK DEFINED It's almost impossible to design, build, and operate a network without some form of integration of all three. These models will be applied in detail in the Chapters 4 and 5, along with the symbols. For now, take notice of the matching shades of gray-dark, medium, and white. The intention is to use the darker levels to represent the bottom layers and the lighter layers to represent intermediate and top layers in each stack. DESIGN CONSIDERATIONS AND CRITERIA As with any project, content transport network design and construc- tion is not successful without some amount of performance criteria. Content transport networks are different than ordinary voice and data networks. Ordinary voice and data networks don't typically do content transport well. On the other hand, design and build a network capable of transporting valuable program content, and voice and data can come along for the ride. Below is a list of considerations and criteria that can be used when preparing to undertake a content transport network project. The following list is not to be taken liter- ally,