Page 39 - M. Abrahim Thesis

Computer Network 2021

2.8.2 Encapsulation

Encapsulation wraps data with the necessary protocol information before network
transit. Therefore, as the data moves down through the layers of the OSI model, each OSI layer
adds a header (and also a trailer at Layer 2) to the data before passing it down to a lower layer.

The headers and trailers contain control information for the network devices and
receiver, to ensure proper delivery of the data and to ensure that the receiver can properly
interpret the data. For example, think of a header as an address on an envelope. An address is
required on the envelope so that the letter inside the envelope can be delivered to the desired
recipient.

To see how encapsulation occurs, examine the manner in which data travels through the
layers, as illustrated in Figure 2-13. After the data is sent from the source, it travels through
the application layer down through the other layers. The packaging and flow of the data that is
exchanged go through changes as the layers perform their services for end users. The data, in
the form of electronic signals, must travel across a cable to the correct destination computer
and then be converted to its original form to be read by the recipient.

As you can imagine, several steps are involved in this process. For this reason, developers
of hardware, software, and protocols recognized that the most efficient way to implement
network communications would be as a layered process.

Figure 2-13 shown an Encapsulation process

As illustrated in Figure 2-13, networks must perform the following five conversion steps
to encapsulate data:

Step 1 Build the data

As a user sends an e-mail message, its alphanumeric characters are converted to data
that can travel across the internetwork.

43