What Is Design?

Before delving into the details of network design, consider what design is and, in particular, what you expect from a good design.

Dictionaries generally define design as planning how to create something, or the actual plans themselves. However, when you think of designing something, whether it is a product, an addition to a house, or a network, you likely contemplate a broader use of the word design.

For example, if you hire an architect to design an addition to your house, you expect her to produce detailed plans and engineering drawings that can be used to create the space that you want. What is the process that an architect uses to get to this final product, plans that can be used to create what you want and need? Crucial inputs are the dimensions, the state, and the use of the existing house, and your requirements for the addition (including your budget), as illustrated in Figure 1-1. The former are much easier to solidify than the latter. For the existing house, the architect must measure all the rooms and spaces, and take notes of the existing use and functionfor example, the layout of the existing kitchen, where the family usually eats, and so forth.

Figure 1-1. When Designing an Addition to a House, an Architect Needs to Have Knowledge of the Existing Structure and the Requirements for the AdditionAlong with Skills and Creativity.

Key Point

To determine the requirements for the addition, a good architect should ask probing questions. Determining the actual requirements, rather than your perceived solutions, is a key skill for a good architect.

For example, assume that you tell the architect that you want three skylights and a fireplace in the room. Rather than just including these items, a good architect should ask why you want them and what you see their function to be. You might want skylights to provide more light to the room and a fireplace to provide heat. The ability to determine the actual requirements (light and heat), rather than your perceived solutions (skylights and fireplace), is a critical skill for a good architect. The architect can then recommend different solutions to what you perceive the problems to belack of light and lack of heat. For example, a heated floor can provide warmth, while a wall of opaque glass blocks can provide light. (Of course, you might just want a fireplace and skylights, in which case, these would also be requirements.)

The architect then takes all the requirements together as inputs to the design. Using her creativity and training, the architect typically prepares several options, which are then presented to you. At this stage, the options are not fully developed, but will probably be at a conceptual level, and might include sketches and cost estimates. A meeting is normally held to review the options and to choose one option or a combination of them, to note changes and additions, and so forth. The architect then develops the final design, which must be approved by you before the engineering drawings are created, permits obtained, contractors selected, and so forth. The architect can also provide continuity and quality control for the project by reviewing the progress while the addition is being built.

Thus, as is true for any project, a good design requires good inputsclear requirements are critical. The design process must, of course, allow requirements to change; however, spending time up-front clarifying requirements goes a long way toward reducing major problems later. For example, many government projects include a mandatory requirements document that must be reviewed and signed off before any design or implementation work can be started. Finalizing this requirements document can be a lengthy processfor example, one large project had a requirements definition phase that was years long (thankfully, it has now been implemented successfully).

Understanding the existing structure, if one exists, is also important because it can introduce constraints (for example, in the house-addition project, the window area allowed on a side wall might be restricted by a building code), but it can also provide opportunity (for example, you might be able to reuse some doors that will be removed from the existing house in the new addition).

Good design also requires creativity and skills. For a residential architect, these traits come from both training and experience.

A network design is no different. Understanding the requirements for the network, as well as knowing how the existing network is structured and used, is key to understanding how the new or updated network should function and which features should be included. Understanding how the features operate, what they do, what their constraints are, and what alternative approaches are available comes from both training and experience. Part II of this book, "Technologies: What You Need to Know and Why You Need to Know It," introduces you to some of the fundamental networking technologies available today, while Part III, "Designing Your Network: How to Apply What You Know," explores the use of these technologies in the context of a case study. This chapter introduces you to network design principles and design models.