The Interface Scheme

There are several schemes that can be used for the interface layer. We will consider a scheme in which each subsystem has a subsystem interface (SSI). The SSI has the following tasks:

• Transfer data to and from the subsystem.
• Send requests from the subsystem to the other interfaces through the central interface.
• Receive requests from other subsystem interfaces and translate them to the local language.

These subsystem interfaces can communicate in three different ways (see Figure 30):

1. Direct connection: which means that all interfaces can talk to each other. The advantage of this is that it has a high communication speed; however, it makes the design of such interfaces more difficult, and the addition or modification of one of the interfaces requires the modification of all other interfaces.
2. Message routing: in which any request or change in the data will generate a message on a common bus, and each SSI is responsible for taking the relevant messages and translating then to its subsystem. The problem with this scheme is that it makes the synchronization of the subsystems very difficult, and the design of the interface is more complicated.
3. Centralized control: in which all interfaces talk with one centralized interface that controls the data and controls flow in the environment. The advantage of this scheme is that it makes it much easier to synchronize between the subsystems, and the addition or modification of any of the SSIs will not affect the other SSIs.

 
Figure 30: Three different methods for subsystem interface communication.