The transport and application layers
The following image shows how openSAFETY exclusively specifies the higher application-oriented layers – the fundamental prerequisite for Black Channel operation. 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.
Structure of on openSAFETY frame
openSAFETY duplicates the frame to be transferred and conjoins the two identical frames into one openSAFETY frame. Hence, the openSAFETY frame consists of two subframes with identical content.
Each subframe is provided with an individual checksum as a safeguard. The recipient compares the content of the two subframes to ensure that it is identical. The special format of openSAFETY frames, i.e. the two subframes with their own individual checksums, also makes "masquerades" extremely unlikely to occur and precludes any erroneous processing of a masked standard message.