Gives the procedures used by data terminal equipment (DTE) at time of communication when transmitting or receiving under given conditions. Gives requirements to be met in each instance of the communication of a frame to and from a single media access control service access point (MAC-SAP), with conditions applying when multiplexing frames to and from one or more MAC-SAPs. Gives a range of additional functions for communication with the local node that are not required for the communication of frames, but which may be used for management purposes and to improve efficiency and error detection. Excludes requirements relating to the implementation of these procedures.