The Top Down Approach to Network Design

Designing a large or even medium-sized network can be a complex project. Procedures have been developed to facilitate the design process by dividing it into smaller, more manageable steps. Identifying the separate steps or tasks ensures a smooth process and reduces potential risks.

A top-down design allows the designer to "see the big picture" before getting to the details. Top-down design clarifies the design goals and initiates the design from the perspective of the required applications. The top-down approach adapts the physical infrastructure to the needs of the applications. Network devices are chosen only after a thorough requirements analysis. Structured design practices should be integrated with the top-down approach, especially in very complex networks.

In contrast to top-down design, the network design approach in which network devices and technologies are selected first is called bottom-up, or connect-the-dots. This approach often results in an inappropriate network for the required services and is primarily used when a very quick response to the design request is needed. With a bottom-up approach, the risk of having to redesign the network is high.

Guidelines for producing a top-down design include the following:

■ Thoroughly analyze the customer's requirements.

■ Initiate the design from the top of the OSI model. In other words, define the upper OSI layers (application, presentation, and session) first, and then define the lower OSI layers (transport, network, data link, and physical)—the infrastructure (routers, switches, and media) that is required.

■ Gather additional data about the network (protocol behavior, scalability requirements, additional requirements from the customer, and so forth) that might influence the logical and physical design. Adapt the design to the new data, as required.

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