d applications that communicate with each other horizontally (peer to peer) via their interfaces. These applications provide specific functionality to satisfy end-users' requirements. For example, the GSOA used for the simpler service layer also contains client, server, and third-party applications. These distrib- uted applications may be used to provide telco, IT, or integrated services to end- users. The SDP framework also shows lower-layer intermediate and complex applications. These applications provide richer functionality by using intermediate or complex services. The layered GSOAs house SDP service enablers and manage access to their interfaces. Service enablers contained in a GSOA are accessed and used by multi- ple distributed applications via their interfaces. For example, simple service enablers are accessed via their interfaces by applications contained in the same GSOA. In Figure 2.5, we show service enablers that are self contained or invoke lower GSOA service enablers. For instance, some simple service enablers invoke interme- diate service enabler interfaces to fulfill application requests, while others provide functions local to their GSOA applications. In the figure, local service enablers are those that have one interface attached to a single middleware plane. We expand on these service enabler interactions in Section 2.6 by using standard-based APIs and protocols. The SDP framework GSOAs manage access to application and service inter- faces using their middleware plane. In Figure 2.5, the GSOAs middleware planes are distributed across the various domains. Communication across these domains and between the middleware parts are performed by standards-based mechanisms, such as software interfaces or protocols. However, these details are hidden from the service enablers and applications that use the middleware planes. All layered GSOAs, with the help of their middleware planes, hide lower tech- nology complexities. These complexities include the communication and comput- ing infrastructure that im