This part of ISO 11992 specifies the diagnostic communication over a CAN between the towing and towed vehicle(s) of a commercial vehicle and its trailer(s), according to ISO 11992-2 or ISO 11992-3, which allows a diagnostic tester (client) to control diagnostic functions in an on-vehicle ECU (server) embedded in a road vehicle using the communication gateways between the vehicles. It defines the data link layer’s specific implementation of the unified diagnostic communication requirements, mainly given in the ISO 14229 and ISO 15765 document series by additional requirements and restrictions specific to the implementation of UDS on an ISO 11992 network. This part of ISO 11992 does not apply to any non-diagnostic message transmission use of the communication data link between two ECUs.