Data Encapsulation

          After reading about OSI and TCP/IP model, we need to know what this models do. For this, we first know about data encapsulation process.

          The term ENCAPSULATION describes a process of putting  headers and sometimes trailers around some data. Each layer adds its own header and Data Link Layer protocols also add a trailer to the data supplied by the higher layer.

          Consider the following example. Let's say that you are browsing a web site. The we server on which the web site is hosted will create and encapsulate the application data with the Application layer header. For example, the HTTP OK message will be stored in the header, followed by the requested content of the web page. The encapsulated data will then be forwarded to the next layer - the Network Layer. The Network layer will store the source and destination IP address in its header and forward the data to the Data Link layer.

          The data link layer is the only layer that uses both header and trailer. The data is then sent through a physical network link.

          Data Encapsulation illustration

          Each data packets (header + data + trailer) defined by a particular layer has a specific name:

• Frame : Encapsulated data defined by the Data Link layer. A frame can have both header and trailer.
• Packets : Encapsulated data defined by the Internet/Network layer. A header contains the source and destination IP addresses.
• Segment : Encapsulated data defined by the Transport Layer. The information such as the source and destination ports or sequence and acknowledgement numbers are included in the header.

          Note :-  The term decapsulation describes a process of removing header and trailers as data passes from a lower to higher layers. This process transpires on a computer that is receiving data.

Data Encapsulation in the OSI model

We read about data encapsulation in TCP/IP model, and now we will see how data encapsulation is done in OSI model. Like with the TCP/IP layer, each OSI layer ask for services from the next lower layer. The lower layer encapsulate the higher layer's data between header and trailer. While the TCP/IP model uses the terms like Segment, Packets and Frames to refer to a data packets defined by the particular layer, the OSI model uses the different term : Protocol Data Unit (PDU).

          A PDU represent a unit of data with headers and trailers for the particular layers, as well as theencapsulated data. Since the OSI model has 7 layers, PDUs are numbered from 1 to 7, with the physical layer being the first. For example, the term layer 3 PDU refers to the data encapsulated at the Network Layer of the OSI model. 

Graphical representation of the PDUs inthe OSI model.