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information. This control information consists of headers and trailers that are
attached to the data as it moves through the layers. These headers and trailers contain
instructions and requests for the peer layers. Chapter Three: Advanced Computer Networks

As the data passes through the layers, each layer may add its own header and trailer
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to the existing data, which already includes headers, trailers, and data from the upper
layers. For example, the Transport Header (TH) is only processed by the Transport
layer, while lower layers treat it as part of the data, as shown in Figure 3.2. Similarly,
at the network layer, an information unit consists of a Layer 3 header (NH) and data.
The data link layer treats this as a single unit and adds its own header and trailer.

This process of adding headers and trailers from each layer to the data is known as
encapsulation.

Figure 3.2: Data encapsulation at each layer

For instance, when computer A needs to control information for its corresponding
send data from a software application to layer in computer B. When the message
computer B, the data is initially passed unit reaches the physical layer, it is
to the application layer. In computer transmitted over the network medium.
A, the application layer adds a header In computer B, the physical layer
to the data, which contains control receives the message unit and passes
information needed by the application it to the data link layer. The data link
layer in computer B. This combined layer in computer B reads the control
message unit, including the header, data, information from the header added by
and possibly a trailer, is then forwarded the data link layer in computer A. The
to the presentation layer. The size of the header and trailer are then removed, and
message unit increases as each layer the remaining part of the message unit is
adds its own header and trailer with forwarded to the network layer.

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Student’s Book Form Five

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