Typical communication model in motor vehicles

To implement a serial bus system in the automobile it is generally sufficient to implement the tasks defined of the two lowermost layers. Therefore, the use of a Layer 2 protocol (Layer 1/2 protocol) generally suffices for serial data exchange in the automobile. First, it satisfies the often challenging time conditions. Second, it also satisfies the requirement for a simple implementation of the communication layer.

If the available functionality proves to be inadequate, higher-level protocols must be defined and piggybacked. Reasons for this might include network management or diagnostic functionalities, but also objects and services needed to model distributed applications. Such additional functionalities are covered by the Application Layer and associated Layer 7 protocol. When an Application Layer is introduced, the interface between the application and communication of the Data Link Layer is shifted to the Application Layer.

Another higher-level protocol is necessary if the sizes of data blocks to be transmitted are greater than the defined Data Link Layer can handle. So-called transport protocols (Layer 4 protocols) are necessary for transmission of larger data blocks. Among other things, they provide for the necessary sender-side segmenting and sequencing as well as receiver-side assembly and flow control.