# Evaluating the Effectiveness of Designs for Low-Cost Digital Manufacturing Systems
## 5 Evaluation of Designs
### Other Cases in Manufacturing System Designs
#### 5.2.3 Exceptions in Low-Cost Digital System Designs  
There are a handful of exceptional cases when designing low-cost digital systems, such as the addition of heterogeneous components, multiple components residing in the same location and the development of a large number of homogeneous systems. In what follows, we discuss how these cases impact the effectiveness of designs for each candidate reference architecture:  
* _Addition of heterogeneous components._ Low-cost systems are likely to consists of disparate components [16; 39]. Adding such heterogeneous components may yield different interface sizes between the architecture elements and components. For example, a sensor type other than the Raspberry Pi Sense HAT may require a more complex interface. While some sensors include an API, other types may require a listener to the port these types are connected to. However, these changes only have a small effect on the overall design effort required, as they need to be created for each system irregardless of the selected reference architecture.
* _Multiple components in the same location._ Applications for which multiple components reside in the same physical location are typically monitoring solutions. Whereas larger distributed monitoring applications are likely to consist of multiple sensing locations, this case considers all sensors to reside in only one location. There are two main effects on the required design effort: First, less hardware components are needed since all sensors can be attached to a single computational device. Second, the data aggregation can be implemented at a component level, thus reducing the network traffic. There are different impacts on design based on the three candidates: For Shoestring, instead of having a data collection service module for each location, all sensors can be represented by a single service module. Conversely, Wot and PROSA/Erlang imply wrapping each sensor into a separate architecture element. Therefore, the designs based on Shoestring are likely to be more effective than Wot and PROSA/Erlang since they require fewer architectural elements, system components and number connected elements. However, PROSA/Erlang enables the aggregation of multiple sensor holons, which results in a more streamlined design. As mentioned above, the size of the interfaces remains equal across the candidates since the interfaces have to be built for each sensor independent from the chosen reference architecture.  
* _A large number of homogeneous systems._ Given the limited resources of SMEs, low-cost digital systems may be developed successively [13; 16; 20]. Companies may start with an initial tracking or monitoring system for one workstation and proceed with additional systems once it has been successfully deployed. This case covers the successive design of a large number of homogeneous systems. For illustration, we consider the design of 30 monitoring solutions. Due to their homogeneity, the effort required to design subsequent solutions is vastly reduced since the design of the initial solution can simply be reused. In particular, the modularity of the candidate reference architecture is a key driver for reusability. However, as the candidates have similar levels of modularity, the effectiveness of the designs does not differ significantly between the reference architectures. Additionally, with an increasing number of systems, developers need to take the system performance into consideration, such as latency and throughput.  
In summary, although a low-cost design based on Shoestring is feasible for a typical WoT and PROSA/Erlang application, its design would be less effective. Specifically, the designs based on Shoestring would be larger and more complex to obtain the required functionalities for each case. However, due to its focus on reusability, a design based on Shoestring is more effective compared to WoT and PROSA/Erlang in cases where multiple similar low-cost digital systems need to be designed.