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.
That SDU is then encapsulated into the lower layer's PDU and the process continues until reaching the PHY, physical, or lowest layer of the OSI stack.
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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