service data unit (OSI) (SDU)

Service data units (SDU) are data units that are passed from a higher- level layer of the OSI reference model to the next lower-level layer at the service access point( SAP).

The service data units are passed on in unchanged form to the corresponding higher-level layer. They are data whose identification is preserved from one end of a layer connection to the other end.

Service data units together with protocol data units( PDU) form the interface data units( IDU). In addition to the normal service data unit, there is a special service data unit, the expedited service unit. The layer to which these priority service data units are passed ensures that these special service data units are not passed to the corresponding higher layer after other subsequently passed service data units.