1. Physical layer: the physical layer refers to the hardware device layer, including servers, storage devices, network devices, and so on. Although the physical layer plays an important role in big data systems, it is considered an infrastructure layer and not part of the big data ecosystem.

2. Operating system layer: the operating system layer is the software layer that runs on top of the physical layer and is responsible for managing hardware resources and providing basic system functions. While the operating system is required in a big data system, it is not considered a separate layer of the big data ecosystem, but rather as part of the infrastructure.

3. Application Tier: The application tier refers to specific applications and solutions built on top of the big data ecosystem. These applications can be in the areas of data analytics, machine learning, artificial intelligence, etc. They rely on the underlying data processing and storage layers, but are not themselves part of the core layer of the big data ecosystem.

A big data ecosystem is a set of components, technologies, and tools that make up a set of systems for processing and analyzing big data.