The protocol data units are nested in each other by the layer formation in each node. In the data source, the protocol data unit is the pure user data. A header is added to these in the application layer of the OSI reference model. The new unit - user data plus layer 7 header - is added to the presentation layer and again called protocol data unit, namely application PDU. As each layer adds control information to the data sets passing through it, new PDUs are created in each layer, each supplemented by the headers of the layer above. The application layer is followed by the protocol data units of the presentation layer, communication control layer, transport layer, network layer, link layer, and physicallayer.
These are processed in the data sink in the corresponding layers. When passing through the individual layers from bottom to top, the header for layer 1 is removed from the protocol data unit in the physical layer and processed. The remaining protocol data unit goes to layer 2, and so on. A link layer protocol data unit consists of the Service Data Unit( SDU), which contains the user data, and a Protocol Control Information( PCI), which contains the control data and protocol data to control the link layer protocol.
Service data units are user data that pass through a service access point( SAP), i.e., that are passed from the network layer to the data link layer for secure transport over the corresponding communication system. In the case of large data sets, the data sets in a protocol layer may have to be broken down into smaller units because the system does not permit such large data sets.This is referred to as fragmentation in this context.