The Protocol Tree and Application-Specific Choices

The communication protocols used for building DRTS applications follow a multilayered approach. The term protocol is actually a stack with multiple abstractions, as shown in Figure 3.5.

The lower-most layer represents the media access control and defines how the physical medium is handled as well as the electrical signals. The next layer defines the message formats and the flow control mechanism, and provides the interface for the various DRTS applications falling in different domains as shown in Figure 3.5.

FIGURE 3.5

(See color insert.) The protocol tree.

FIGURE 3.6

(See color insert.) The CIP in OSI context. (From "Networks built on a Common Industrial Protocol," http://www.odva.org/Portals/0/Library/Publications_Numbered/PUB00122R0_CIP_ Brochure_ENGLISH.pdf.)

Often an intermediate layer is introduced between the protocol layer and the application, and this can be viewed as a high-level abstraction, which allows additional advantages in terms of flexibility and configurability for DRTS applications. A typical example is the Common Industrial Protocol (CIP) [2], as shown in Figure 3.6 in the Opens Systems Interconnection (OSI) context.

CIP is object oriented. CIP-based implementations use the same application layer. The application data remains the same regardless of the type of network used.