In Open Systems Interconnection (OSI) terminology, a service data unit (SDU) is a unit of data that has been passed down from an OSI layer or sublayer to a lower layer.
The layer (n) may add headers or trailers, or both, to the SDU and may do other kinds of reformatting, recoding, splitting or transformations on the data, forming one or more layer (n) PDUs.
The added headers or trailers and other possible changes are part of the process that makes it possible to get data from a source to a destination.
MAC SDUS or MSDUS are data units transmitted between other Media access controllers on a lower OSI Layer.
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