The OMS Reference Architecture establishes the fundamental service-oriented design patterns and principles as well as key interfaces and modules (Figure 1). The functionality of avionicssystems is characterized as a set of services and a set of clients. In some instances, a program or system may be both a client and a service. The OMS standard defines the basic behavior of clients and services as well as the Avionics Service Bus (ASB) protocols for entering and exiting the system, supporting testing, fault tolerance, isolation, and authentication.