During the process of peer-to-peer
communication, the protocols at each layer exchange packets of information
called protocol data units (PDUs) between peer layers.
The PDU should go through
OSI layers start from Application to Physical layer. When PDU goes to Transport layer , transport layer adds
its own header depending of protocol is in use ,and PDU is called SEGMENT .So segment is PDU that
transport layer adds its header to it . Then transport layer send the packet to
network layer , the netwotk adds its own header , now the PDU is called
packet . Then network layer sends packet to data link layer , data link layer
adds its own header and PDU at the data link layer is called FRAME .
if we review our video in OSI model again , we can get the concept of encapsulation and de-encapsulation
after review this video you can see that goods are encapsulated in containers and then put into truck . in the other side , reverse process occurs .
the same thing happen If one computer wants to send data to
another computer, the data must first be packaged by a process called encapsulation. Encapsulation wraps data
with the necessary protocol information before network transit. As the data
moves down through the layers of the OSI model, each OSI layer adds a header
(and a trailer, if applicable) to the data before passing it down to a lower
layer.
The following steps occur to encapsulate
data:
Step
1 The user data is sent from an application to the application
layer.
Step
2 The application layer adds the application layer header (Layer 7
header) to the user data. The Layer 7 header and the original user data become
the data that is passed down to the presentation layer.
Step
3 The presentation layer adds the presentation layer header (Layer
6 header) to the data. This then becomes the data that is passed down to the
session layer.
Step
4 The session layer adds the session layer header (Layer 5 header)
to the data. This then becomes the data that is passed down to the transport
layer.
Step
5 The transport layer adds the transport layer header (Layer 4
header) to the data. This then becomes the data that is passed down to the
network layer.
Step
6 The network layer adds the network layer header (Layer 3 header)
to the data. This then becomes the data that is passed down to the data link
layer.
Step
7 The data link layer adds the data link layer header and trailer
(Layer 2 header and trailer) to the data. A Layer 2 trailer is usually the
frame check sequence (FCS), which is used by the receiver to detect whether the
data is in error. This then becomes the data that is passed down to the
physical layer.
Step
8 The physical layer then transmits the bits onto the network
media.
De-Encapsulation
When the remote device receives a sequence
of bits, the physical layer at the remote device passes the bits to the data
link layer for manipulation. The data link layer performs the following steps:
Step
1 The data link layer checks the data-link trailer (the FCS) to
see if the data is in error.
Step
2 If the data is in error, it may be discarded, and the data link
layer may ask for the data to be retransmitted.
Step
3 If the data is not in error, the data link layer reads and
interprets the control information in the data-link header.
Step
4 The data link layer strips the data-link header and trailer, and
then passes the remaining data up to the network layer .
This process is referred to as
de-encapsulation. Each subsequent layer performs a similar deencapsulation process.
