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Example 1: These are two design approaches, which can be explained as:You can imagine a tree like structure , in which when you are following top-down approachyou move from root node to leaf node and when you follow bottom-up approach you followleaf node to root node.

Top-down approach:In this an overview of the system is first formulated, specifying but not detailing any first-level subsystems. Each subsystem is then refined in yet greater detail, sometimes in many

additional subsystem levels, until the entire specification is reduced to base elements.

Bottom-up approach:In this approach the individual base elements of the system are first specified in great detail.These elements are then linked together to form larger subsystems, which then in turn arelinked, sometimes in many levels, until a complete top-level system is formed.

Example2:

Top down design proceeds from the abstract entity to get to the concrete design. Bottom updesign proceeds from the concrete design to get to the abstract entity.

Top down design is most often used in designing brand new systems, while bottom up designis sometimes used when one is reverse engineering a design; i.e. when one is trying to figureout what somebody else designed in an existing system.

Bottom up design begins the design with the lowest level modules or subsystems, andprogresses upward to the main program, module, or subsystem. With bottom up design, astructure chart is necessary to determine the order of execution, and the development ofdrivers is necessary to complete the bottom up approach.

Top down design, on the other hand, begins the design with the main or top-level module,

and progresses downward to the lowest level modules or subsystems.

Top-Down assembly modeling

Top-down assembly modeling is an assembly-centric modeling method where the assemblydesign is started at the highest level possible and individual parts and subassemblies aredefined within the context of the overall assembly. With this approach, an assemblylayoutistypically created first, and this assembly layout is used to define individual part geometry andposition.

This approach is often used at companies where the product being designed is large enough

that it requires many people to complete the design. A senior-level designer might create theinitial assembly layout, then divide the assembly layout into logical subassemblies and partsfor the remainder of the organization to complete.

Bottom-Up assembly modeling

Bottom-up assembly modeling is a part-centric modeling method where the assembly designis started with a principal structural or functional element, and individual parts are designedin relative isolation from the overall assembly. Component parts and subassemblies aredefined as the process moves up towards the top-level assembly. With this approach, as thedesign of a key component is completed, its geometry may or may not be used to aid the

design of related mating components.

This approach is often used at companies where the product being designed is small enoughthat one or only a few people are needed to complete the design.

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