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In a layered approach to protocol architecture, protocols have a relationship to one another such that a protocol at layer(n) used the services of the layer below it-the (n-1) services - which, in turn, are provided by a layer (n-1) protocol. One of the services used by a layer (n) protocol is the encapsulation of its (n) Protocol Data Units (PDUs) in a way which is transparent to it. Such encapsulation is realized by the carriage of the (n) PDUs as user data in an (n-1) Service Data Unit (SDU).
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