Service data unit
Service Data Unit (SDU) is OSI (Open Systems Interconnection) terminology used to describe a specific unit of data, one that has been passed down from an OSI layer, to a lower layer, and has not yet been encapsulated into a PDU, by the lower layer. It is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user .
It differs from the PDU in that the PDU specifies the data that will be sent to the peer protocol layer at the receiving end, as opposed to being sent to a lower layer.
The PDU at any given layer, layer 'n', is the SDU of the layer below, layer 'n-1'. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes enapsulated within the PDU. The layer n-1 adds headers or footers, or both, to the SDU, transforming it it into a PDU of layer n-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.