According to Albert Einstein:
The world we've made as a result of the level of thinking we have done thus far creates problems that we cannot solve at the same level at which we created them.
To paraphrase Einstein, networking professionals have the ability to create networks that are so complex that when problems arise they can't be solved using the same sort of thinking that was used to create the networks. Add to this the fact that each upgrade, patch, and modification to a network can also be created using complex and sometimes convoluted thinking, and you realize that the result is networks that are hard to understand and troubleshoot. The networks created with this complexity often don't perform as well as expected, don't scale as the need for growth arises (as it almost always does), and don't match a customer's requirements. A solution to this problem is to use a streamlined, systematic methodology in which the network or upgrade is designed in a top-down fashion.
Many network design tools and methodologies in use today resemble the "connect-the-dots" game that some of us played as children. These tools let you place internetworking devices on a palette and connect them with local-area network (LAN) or wide-area network (WAN) media. The problem with this methodology is that it skips the steps of analyzing a customer's requirements and selecting devices and media based on those requirements.
Good network design must recognize that a customer's requirements embody many business and technical goals including requirements for availability, scalability, affordability, security, and manageability. Many customers also want to specify a required level of network performance, often called a service level. To meet these needs, difficult network design choices and tradeoffs must be made when designing the logical network before any physical devices or media are selected.
When a customer expects a quick response to a network design request, a bottom-up (connect-the-dots) network design methodology can be used, if the customer's applications and goals are well known. However, network designers often think they understand a customer's applications and requirements only to discover, after a network is installed, that they did not capture the customer's most important needs. Unexpected scalability and performance problems appear as the number of network users increases. These problems can be avoided if the network designer uses top-down methods that perform requirements analysis before technology selection.
Top-down network design is a methodology for designing networks that begins at the upper layers of the OSI reference model before moving to the lower layers. It focuses on applications, sessions, and data transport before the selection of routers, switches, and media that operate at the lower layers.
The top-down network design process includes exploring divisional and group structures to find the people for whom the network will provide services and from whom you should get valuable information to make the design succeed.
Top-down network design is also iterative. To avoid getting bogged down in details too quickly, it is important to first get an overall view of a customer's requirements. Later, more detail can be gathered on protocol behavior, scalability requirements, technology preferences, and so on. Top-down network design recognizes that the logical model and the physical design may change as more information is gathered.
Because top-down methodology is iterative, some topics are covered more than once in this book. For example, this chapter discusses network applications. Network applications are discussed again in Chapter 4, "Characterizing Network Traffic," which covers network traffic caused by application- and protocol-usage patterns. A top-down approach lets a network designer get "the big picture" first and then spiral downward into detailed technical requirements and specifications.
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