This SAE Recommended Practice details the Link Layer of the Intelligent Transportation Systems (ITS) Data Bus, which is generally intended for in-vehicle use. The ITS Data Bus (IDB) is a non-proprietary token passing bus, designed to allow disparate consumer, vehicle, and commercial electronic components to communicate and share information across a standard, open data bus. This first version of the IDB is called IDB-C. This Recommended Practice describes the Link Layer of the IDB-C, as shown in Section 1. Below the Link Layer of the IDB-C is the CAN 2.0B Link Layer. The Physical Layer of the IDB-C incorporates the Physical Layer as specified by SAE J2366-1 and J2590. The IDB-C Link Layer is logically divided into two functional sub-layers, viz., Logical Link Control (LLC) and Media Access Control (MAC). However, these are logical groupings that re-use some of the same bits within the CAN 2.0B frame for efficiency.