System Architecture Design

What

Documents and figures describing the static layout of the functional blocks, of the system to be.

Block or component diagrams, that shows the functional decomposition into HW and SW blocks respectively

How

General architechture design consists of several steps, that often requires several iterations, untill the optimal architechtre is crafted.

The system architechture and the technical platform, are highly dependant on each other, and again depends on the partition of functionality into HW and SW blocks. Therefore one should expect to iterate between the task in General Architechture design

Ingredients

• System constraints, see [Design Constraint Analysis]
• Availible requirements, in particular concerning
  • performance requirements
  • Interface compatability (i.e. must be able to connect on ethernet, communicate on CAN, have and SD card, run Android OS, etc...)

Process - An example

• Identify the system constraints
  • Technical constraints are evaluated, e.g.:
    • performance of the given technical platform
    • are there preselected BSP’s / Operating systems
    • Lifetime considerations
    • Target costs
    • compatibility issues (must be same CPU family as usual, same BSP as we use already, must be able to run older applications build for this architechture etc…)
• Asses the initial blockdiagram and proposed technical solution
  • attempt to map the system-to-be’s functionality onto whatever architechture the technical solution suggests e.g
    • does the required communication protocols match the available HW
    • estimate the system to be’s memory footprint, and evaluate the vailible meory and storage in the suggested technical platform (i.e. filesystems, graphical libraries, large databases, webservers etc often have quite high demands in terms of memory)
    • attempt to predict any bottlenecks, in terms of performance, when running the the system to be on the selected technical platform.
• Craft a detailed Block or Component [diagram] with:
  • hardware blocks sketched, based on what responsibility and function they are assigned
  • sw blocks based on functionality and responsibility. The 7 layer [OSI model] is good as a template for dividing functionality - link

• Based on the results above evaluate
  • if a slight [repartitioning] of functionality between HW and SW will improve performance
  • if the available memory is capable of accommodating system-to-be-needs (multiply your memory footprint estimations with 2-3, is a good rule of thumb…)
  • do I have the required HW support and available SW BSP’s, if not consult your system constraints, and evaluate if a better suited technical platform, BSP, development tool is available, and within budget reach.
  • do I have my design slip into enough functional blocks (HW & SW), and are the responsibilities within each block small enough

Partitioning

There is several ways to partition the general blocks from the architectural block diagram into software and hardware parts. In EUDP we recommend to follow the instructions given at Design Partitioning.

Having defined the most effective partitioning prepare the necessary class diagrams to describe the software partition and corresponding diagrams for the hardware.

Why

In order to find the most effective partitioning that at the same time respects the constraints and gives the best value for the customer it is essential to define the constraints for the project. Without proper definition of the constraints the design space is wide, but may not give the customer the best value.

Example

    Issue         Critical       Very Important    Important   Less Important  Notes

Performance                           X                                          1
Usage                X                                                           2
Reliability          X                                                           3
Easy serviceable                                       X                         4
Remote maintenance                    X                                          5
Cost effective                                         X                         6

Notes:

1. The performance is very important.

2. There must not be any difficulties using the system. All kinds of people must be able to use it, there must not be special training! The system-to-be must contain enough electronics to ease and secure the use. It is also preferable to have the possibility to collect different kinds of data from the system-to-be.

3. Errors must not occur. It will be critical if an error shuts down the heating of the house. If the temperature outside is below zero, the house and the system can be damaged if it is not heated. Therefore, the reliability must be high!

4. The system must be easy to repair, if errors occur.

5. All maintenance must be easy, with no or a minimum of disturbances for the user.

6. Many of the parts in the system may already have been developed by other companies. It must be considered how the lowest price is obtained. For all parts of the system-to-be it must be a choice between developing the different parts or buy something similar and redesign if necessary.