Each PDU contains a payload, called the service data unit ( SDU ), along with protocol-related headers and / or footers.
2.
Between the layers ( and between the application and the top-most layer ), the layers pass service data units across the interfaces.
3.
All this additional information, plus the original service data unit from the higher layer, constitutes the " protocol data unit " at this layer.
4.
In systems where the MPDU may be larger than the MAC service data unit ( MSDU ), the MPDU may include multiple MSDUs as a result of packet aggregation.
5.
Thus for multiple protocols to coexist within a layer, it is necessary to determine which protocol is to be invoked to process a service data unit delivered by the lower layer.
6.
The connectionless protocol classes provide the capabilities needed to transfer one Network Service Data Unit ( NSDU ) in the " data " field of an XUDT, LUDT or UDT message.
7.
The MSDU is the service data unit that is received from the logical link control ( LLC ) sub-layer which lies above the media access control ( MAC ) sub-layer in a protocol stack.
8.
DAPs may not trust the DIF they are using, therefore may decide to protect their data before writing it to the flow-for example using encryption-via the SDU ( Service Data Unit ) Protection module.
9.
When the PDU passes over the interface from the layer that constructed it to the layer that merely delivers it ( and therefore does not understand its internal structure ), it becomes a service data unit to that layer.
10.
In a communication system based on a layered OSI model, packet segmentation may be responsible for splitting one MPDU into multiple physical layer service data units so that reliable transmission ( and potential re-transmission via ARQ ) of each one can be performed individually.
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