Based on OASIS’s model, there are several layers in an SOA. The layers which are commonly used are: the service layer, the business layer and data layer.
Service Layer: Services are exposed or resident at that level. This is the outsourcing company.
Business Layer: This layer defines the business processes of the company.
Data Layer: It is the ability to provide access to enterprise data that feed the layer or business processes.
Figure 1 is a representation of an SOA where you can see the different layers of this architecture.
SOA Architecture (adapted from EBSO Oasis, p. 28)