OSI Encapsulation Terminology
Like TCP/IP, each OSI layer asks for services from the next lower layer. To provide the services, each layer makes use of a header and possibly a trailer. The lower layer encapsulates the higher layer’s data behind a header. The final topic of this chapter explains some of the terminology and concepts related to OSI encapsulation.
The TCP/IP model uses terms such as segment, packet, and frame to refer to various layers and their respective encapsulated data (refer to Figure 1-13). OSI uses a more generic term: protocol data unit (PDU).
A PDU represents the bits that include the headers and trailers for that layer, as well as the encapsulated data. For example, an IP packet, as shown in Figure 1-14, using OSI terminology, ...
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