Functional vs Physical Design

In the System Engineering world, design tends to follow functional lines at the higher levels, and physical lines at the lower levels. This is a little different to the way most engineers are used the thinking, which puts physical at a higher precedence than functional.

In traditional physical design, each system component has a designer (or design team) primarily responsible for it. They go off and design their component to meet its requirements in isolation, and then everyone comes together to point fingers at each other when the components don't integrate properly. In a functional design approach, teams with representatives of each physical component or discipline are assembled to consider how each of the functions in the system are going to be partitioned across the physical components. Physical design tends to happen bottom-up, and functional design top-down.

The problem with physical design at the system level is that the partitioning of each system function is done by guess work in the design of each component. In simple systems this isn't a problem, but as complexity grows, it makes more sense to start thinking first about how functions are implemented, and then about how each component contributes to the function, rather than the other way around.

In software terms, you can iterate over everything a single-level system needs to do by saying:

for each function that the system must perform
{

design how that function is partitioned among the system components;
for each component of the system

design how this component makes its contribution to the system function;

}

In a more bottom-up environment, you would traditionally say:

for each component in the system
{

for each function that this component must contribute to
{

guess what the contribution of this component to the system function should be;
design how this component makes its contribution to the system function;

}

}

Both loops will traverse the entire space of system functions and components, so the amount of work involved is the same.

As the design process proceeds down the system heirarchy to detailed design and implementation, you start to focus increasingly on physical design once all the functional issues are resolved.

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