640-407 | Cisco Certified Network Associate

Data Encapsulation

Each layer within the OSI model is primarily responsible for communicating with a peer layer on another machine. In other wordss, when two clients communicate, one layer, such as the Session Layer, on one client is primarily responsible with communicating with the exact same layer, in this case the Session Layer, on the other client.

Application
Presentation
Session
Transport
Network
Data Link
Physical
ó
ó
ó
ï Segments ð
ï Packets/Datagrams ð
ï Frames ð
ï Bits ð
Application
Presentation
Session
Transport
Network
Data Link
Physical

This communication between peers is done in "Protocol Data Units" (PDU). The actual terms for the PDU changes from layer to layer. For instance, as the above table depicts, the Transport layers communicate via Segments.

Even though this communication logically takes place between peers, each layer is actually dependant upon the layers below it for the actual delivery. Each layer passes it's PDU to the layer beneath. The underlying layer then adds a header, creating it's own PDU. In this manner, the "data payload" for lower layers is the PDU of the layer immediately above.

This process of taking one PDU and enveloping it within another is called data encapsulation. This process is often likened to that of a mail delivery system. Consider for a moment what it takes to to deliver a piece of mail from one building to another. At the top of the "mail delivery model" you have the composition of the letter itself. The letter is then placed in an "envolope" that gets address to the recipient. The envelope then gets placed into a mail carrier's bag. The bag is then placed into a delivery truck and driven to the recipient building. At the destination building the bag is opened and the letter given to the appropriate mail carrier. The letter reaches the recipient who then opens the letter and finally reads the contents.

The CCNA test objectives reference 5 steps for data encapsulation. They are:

1. The application message is converted into data for communication on the internetwork.
2. The data from step 1 is given a segment header by the "transport" system.
3. The segment from step 2 is given a network header that includes source and destination logical addresses.
4. The packet or datagram from step 3 is given a frame header, specific to the physical medium that is being used.
5. The frame from step 3 is converted into a series of bits and interpreted as electrical (usually) signal generated on the physical medium.

The following diagram depicts the 5 step encapsulation process:

Step 1

Application Data

Step 2

Segment Header |
Application Data

Step 3

Network Header |
Segment Header |
Application Data

Step 4

Frame Header |
Network Header |
Segment Header |
Application Data

Step 5
011111111100011100101010010011100011100011100011010010