Service data unit
Service Data Unit (SDU) is OSI (Open Systems Connection Interconnection) terminology used to describe a specific unit of data, that has been passed down from an OSI layer, to a lower layer, and has not been encapsulated into a PDU yet, 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.
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 it receives from layer n, making it into a PDU of layer n-1. The added data is part of process making it possible to get data from a source to a destination.