The bottom three layers in the OSI communication model-Physical, Data Link and Network- are the Media layers. The top four-Transport, Session, Presentation and Application- are the Host layers. CAN by itself is only concerned with the lowest layers of the model. That is, ISO11898 defines the Data Link and Physical layers, which are shown here in red. Additionally, the CAN 2.0B specification covers a portion of the Transport layer: the retransmission/retry control function.