All communications on a network originate at a source and are sent to a destination. A networking protocol using all or some of the layers listed in the OSI reference model move data between devices. Recall that Layer 7 is the part of the protocol that communicates with the application, and Layer 1 is the part of a protocol that communicates with the media. A data frame is able to travel across a computer network because of the layers of the protocol. The process of moving data from one device in a network is accomplished by passing information from applications down the protocol stack, adding an appropriate header at each layer of the model. This method of passing data down the stack and adding headers and trailers is called encapsulation. After the data is encapsulated and passed across the network, the receiving device removes the information added, using the messages in the header as directions as to how to pass the data up the stack to the appropriate application.
Data encapsulation is an important concept to networks. It is the function of like layers on each device, called peer layers, to communicate critical parameters such as addressing and control information.
Although encapsulation seems like an abstract concept, it is actually quite simple. Imagine that you want to send a coffee mug to a friend in another city. How will the mug get there? Basically, it will be transported on the road or through the air. You can't go outside and set the mug on the road or throw it up in the air and expect it to get there. You need a service to pick it up and deliver it. So, you call your favorite parcel carrier and give them the mug. But, that's not all. Here's the complete process:
Step 1 Pack the mug in a box.
Step 2 Place an address label on the box so the carrier knows where to deliver it.
Step 3 Give the box to a parcel carrier.
Step 4 The carrier drives it down the road toward its final destination.
This process is similar to the encapsulation method that protocol stacks use to send data across networks. After the package arrives, your friend has to reverse the process. He takes the package from the carrier, reads the label to see who it's from, and finally opens the box and removes the mug. The reverse of the encapsulation process is known as de-encapsulation. The next sections describe the encapsulation and de-encapsulation processes.
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