network layer creates the network header, which includes the
network layer address, and places the data (L4PDU) behind it. In
other words, the L3PDU is created here.
link layer creates the data link header, places the data (L3PDU)
behind it, and places the data link trailer at the end. In other words,
the L2PDU is created here.
medium to transmit the frame.
depicts the concept; the numbers shown represent each of the five steps.
needed to discuss the data that a particular layer is processing. Layer N PDU (protocol data
unit) is a term used to describe a set of bytes that includes the Layer N header and trailer, all
headers encapsulated, and the user data. From Layer N's perspective, the higher-layer headers
and the user data form one large data or information field. A few other terms also describe some
of these PDUs. The Layer 2 PDU (including the data link header and trailer) is called a frame.
Similarly, the Layer 3 PDU is called a packet, or sometimes a datagram. Finally, the Layer 4
PDU is called a segment. Figure 3-5 illustrates the construction of frames, packets, and
segments and the different layers' perspectives on what is considered to be data.
Interface