SAE J1939-1 On-Highway Equipment Control and Communication Network specifies the minimum set of SAE J1939 documents that define the Truck and Bus Control and Communications Vehicle Network as it applies to on-highway equipment. Vehicles covered include all on-highway straight trucks and combination vehicles. A combination vehicle consists of one towing vehicle (tractor) and one or more towed vehicles (trailers and dollies). Dolly axles within the road train are considered to be towed vehicles.