Two very general approaches exist to developing a network design, known as top-down and bottom-up. In this case, the “down” and “up” being discussed relate to a concept you are already familiar with, namely the OSI model.
The Top-Down Approach
In a top-down approach, a network designer looks at a project starting with the general applications required. In this sense, the term “application” does not necessarily mean things like a web server or Internet Explorer, although they may certainly be factors. Instead, this approach focuses on determining what the goals of the network are from an Application Layer perspective, namely the applications or services required. For example, an organization might want to implement or upgrade a network to support new applications like Voice over IP (VoIP), IP multicasting, and so forth. Along the same lines, the goal of the customer might be to interconnect to a partner network to enable an e-commerce platform. Notice that the top-down approach doesn’t begin by focusing on any particular technical elements. No discussion of Gigabit Ethernet, fiber optic cabling, or routing protocols occurs at this level. Instead, the top-down approach is solution-oriented, focusing on the specific business and technical goals of an organization. Of course technologies do need to be considered, but this typically happens later in the design process.
The Bottom-Up Approach
The alternate approach, known as bottom-up, is more commonly employed, but is far from optimal. Instead of focusing on the applications that drive the need for a new or redesigned network, this approach tends to start lower in the OSI model, worrying about issues like specific technologies, protocols, network media, and so forth. Generally speaking, this is the “stuff” that networking professionals are most familiar with. They have a tendency to begin the design process at this level, leaving applications and services as an afterthought to be considered later. After all, the network won’t do anything without the necessary equipment, or so popular thinking goes. In most cases, taking a bottom-up approach tends to require a less thorough initial analysis, and is easier to implement as a quick fix. Ultimately, however, the bottom-up approach is seldom truly successful, as it tends to rely on a number of fixes along the way in order to deal with issues that were not initially considered.