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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.

OSI uses a more generic term to refer to messages, rather than frame, packet, and segment.
OSI uses the 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, is a PDU, more specifically a Layer 3
PDU (abbreviated L3PDU) because IP is a Layer 3 protocol. OSI simply refers to the Layer
x PDU (LxPDU), with x referring to the number of the layer being discussed, as shown in
Figure 1-16.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 1-16 OSI Encapsulation and Protocol Data Units

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