Open System Architecture. Client-Server Systems. Subsystems

Open System Architecture. Open system architecture is a design methodology that keeps options for updating systems open by providing liberal interfacing standards. Ralston and Reilly (22) state that open architectures pertain primarily to personal computers. An open architecture is one that allows the installation of additional logic cards in the computer chassis beyond those used with the most primitive configuration of the system. The cards are inserted into slots in the computer’s motherboard—the main logic board that holds its CPU and memory chips.

A computer vendor that adopts such a design knows that, because the characteristics of the motherboard will be public knowledge, other vendors that wish to do so can design and market customized logic cards. Open system architectures are increasingly important in modern aircraft applications because of the constant need to upgrade these systems and use the latest technical innovations. It is extremely difficult to predict interconnection and growth requirements for next-generation aircraft, which is exactly what an open architecture attempts to avoid the need for.

Client-Server Systems. A client-server system is one in which one computer provides services to another computer on a network. Ralston and Reilly (22) describe the fileserver approach as an example of client-server interaction. Clients executing on the local machine forward all file requests (e.g., open, close, read, write, and seek) to the remote file server.

The server accepts a client’s requests, performs its associated operation, and returns a response to the client. Indeed, if the client software is structured transparently, the client need not even be aware that files being accessed physically reside on machines located elsewhere on the network. Client-server systems are being applied on modern aircraft, where highly distributed resources and their aircrew and passenger services are networked to application computers.

Subsystems. The major subsystems of an aircraft are its airframe, power plant, avionics, landing gear, and controls. Landau (1) defines a subsystem as any system that is part of a larger system. Many of the subsystems on an aircraft have one or more processors associated with them. It is a complex task to isolate and test the assorted subsystems.

Another layer of testing below subsystem testing is unit testing. A unit of a subsystem performs a function for it. For example, in the radar subsystem, the units include its signal processor and its data processor. In order to test a system adequately, each of its lowest-level items (units) must be tested. As the units affect and depend on each other, another layer of testing addresses that layer of dependences. In the same fashion, subsystem testing is performed and integrated with associated subsystems.

It is important to test not only at the unit and the subsystem level, but at the system and operational level. The system level is where the subsystems are brought together to offer the system functionality. System integration is the process of connecting subsystem components into greater levels of system functionality until the complete system is realized. The operational level of testing is where the subsystem is exercised in its actual use.

Line Replaceable Units. LRUs are subsystems or subsystem components that are self-contained in durable boxes containing interface connections for data, control, and power. Many LRUs also contain built-in test (BIT) capabilities that notify air and maintenance crews when a failure occurs. A powerful feature of LRUs is that functionality can be compartmentalized. When a failure is detected, the LRU can easily be pulled and replaced, restoring the aircraft to service within moments of detection.

Graceful Degradation. All systems must have plans to address partial or catastrophic failure. System failure in flight controls is often catastrophic, whereas system failure in avionics can be recovered from. For this reason, most flight-critical systems have built-in redundant capabilities (sometimes multiple layers ofredundancy), which are automatically activated when the main system or subsystem fails.

Degraded system behavior occurs when the main system fails and backup systems are activated. The critical nature of system failure requires immediate activation of backup systems and recognition by all related subsystems of the new state of operation. Graceful degradation is the capability of aircraft computers to continue operating after incurring system failure.

Graceful degradation is less than optimal performance, and may activate several layers of decreasing performance before the system fails. The value of graceful degradation is that the aircrew has time to respond to the system failure before a catastrophic failure occurs.