Transport and Application Layers
The standardized OSI (“Open Systems Interconnection”) data communication model is the reference scheme for today’s most common non-proprietary data transfer protocols. This system is comprised of seven layers that enable the processing of data in a hierarchically structured way. Every layer represents a stage determined by protocols in which data transfer tasks are carried out according to specific rules. The two fundamental, low-level layers are the so-called Physical Layer and the Data Link Layer, which are also jointly referred to as physical layers. These layers define the physical interface to the transmission medium, and provide test functions to check whether an actual connection between a sending and a receiving device is established at all. Ethernet only specifies these two low-level physical layers. Also referred to as “transfer layers,” the third and fourth layer handle the timing and logical order of data transfer as well as data attribution to applications. Comprising all transport-oriented services, these four layers combined can be said to constitute no more than the traffic medium for application data, which, in the OSI model, is attributed to the upper layers.
These high-level layers include the Session Layer and Presentation Layer. These two are often grouped with the Application Layer, since all programs and applications directly access all three of them. The Session Layer administers the organization and synchronization of data exchange between applications. E.g. if a connection is interrupted, services on this layer ensure that communication is resumed from the point of interruption once the connection is back in operation. Layer 6, the Presentation Layer, translates system-specific representations into a format that higher-level applications can digest, and ensures syntactically correct data exchange. Other tasks carried out in the Presentation Layer include data compression and data encryption. The top layer in the OSI model is the Application Layer. For this layer, no strict definition of a task or range of tasks is applicable. It provides various services to actual applications that operate outside the scope of this model.
Inspired by the OSI model, the illustration below highlights that openSAFETY exclusively specifies the high-level, application-oriented layers of the protocol stack. The safety mechanisms implemented in this layer enable safety-oriented decoding and encoding of payload data pertaining to specific safety-sensitive applications. For the sake of simplicity, the blue area in the center of this illustration covers all the transport-oriented layers 2 through 6. The choice of transport medium, or, more precisely, of a specific data transfer protocol, is of marginal importance.