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The OSI, TCP/IP, and NetWare Protocol Architectures 83

Figure 3-3

Same-Layer Interactions on Different Computers

Data Encapsulation

The concept of placing data behind headers (and before trailers) for each layer is typically
called encapsulation by Cisco documentation. As seen previously in Figure 3-2, when each
layer creates its header, it places the data given to it by the next-higher layer behind its own
header, thereby encapsulating the higher layer's data. In the case of a data link (Layer 2)
protocol, the Layer 3 header and data are placed between the Layer 2 header and the Layer 2
trailer. The physical layer does not use encapsulation because it does not use headers or trailers.

Again referring to Figure 3-2, Step 1, the following list describes the encapsulation process
from user creation of the data until the physical signal is encoded at Step 2:

Step 1

The application has already created the data. The application layer
creates the application header and places the data behind it. This
data structure is passed to the presentation layer.

Step 2

The presentation layer creates the presentation header and places
the data behind it. This data structure is passed to the session layer.

Step 3

The session layer creates the session header and places the data
behind it. This data structure is passed to the transport layer.

Step 4

The transport layer creates the transport header and places the data
behind it. This data structure is passed to the network layer.

Step 5

The network layer creates the network header and places the data
behind it. This data structure is passed to the data link layer.

Host A

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network

Data Link

Physical

Router 1

Host B

Application

Presentation

Session

Transport

Network

Data Link

Physical

ch03.fm Page 83 Monday, March 20, 2000 4:58 PM