The NMS ISDN supplementary services are modeled as a protocol element separate from the call control protocol element. The call control element of a message is used to manage the basic call state, while the supplementary services element of a primitive manages the supplementary service states.
In order for these two elements to work together but retain separation, the NMS ISDN basic call control buffer format allows for supplementary service information to be carried with the basic call control primitives in separate structures. In most cases, the structures are attached to ACU_FACILITY_RQ or ACU_FACILITY_IN messages as explicit control signaling for supplementary services. However, there are cases where extended data structures containing supplementary service information are attached to basic call control primitives such as ACU_CONN_RQ.
A single ACU message can carry multiple supplementary service structures. Each supplementary service structure contains information pertaining to a specific supplementary service.