14
Chapter 1
The Campus Network
F I G U R E 1 . 5
Data encapsulation at each layer of the OSI reference model
Looking at Figure 1.5, you can follow the data down through the model
as it's encapsulated at each layer of the OSI reference model. Cisco courses
typically focus only on layers 24.
Each layer communicates only with its peer layer on the receiving host,
and they exchange Protocol Data Units (PDUs). The PDUs are attached to
the data at each layer as it traverses down the model and is read only by its
peer on the receiving side. Each layer has a specific name for the PDU, as
shown in Table 1.1.
T A B L E 1 . 1
OSI Encapsulation
OSI Layer
Name of Protocol Data Units (PDUs)
Transport
Segment
Network
Packet
Data Link
Frames
Physical
Bits
Application
Presentation
Session
Transport
Network
Data Link
Physical
Segment
PDU
Packet
Frame
Bits
Upper layer data
TCP header
Data
IP header
Data
LLC header
Data
MAC header
0101110101001000010
Upper layer data
FCS
FCS
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