Data Encapsulation

The term encapsulation describes the process of putting headers and trailers around some data. A computer that needs to send data encapsulates the data in headers of the correct format so that the receiving computer will know how to interpret the received data.

You have seen several examples of encapsulation in this chapter already. The web server encapsulated the home page inside an HTTP header in Figure 2-2. The TCP layer encapsulated the HTTP headers and data inside a TCP header in Figure 2-3. IP encapsulated the TCP headers and the data inside an IP header in Figure 2-4. Finally, the network interface layer encapsulated the IP packets inside both a header and a trailer in Figure 2-5.

You can think about the complete process of data encapsulation with TCP/IP as a five-step process. In fact, previous CCNA exams referred to a specific five-step process for encapsulation. This included the typical encapsulation by the application, transport, network, and network interface (referred to as data link) layers as Steps 1 through 4 in the five-step process. The fifth step was the physical layer’s transmission of the bit stream. In case any questions remain in the CCNA question database referring to a five-step encapsulation process, the following list provides the details and explanation. Regardless, the ideas behind the process apply to any networking model and how it encapsulates data:

Step 1 Create the application data and headers–This simply means that the application has data to send.
Step 2 Package the data for transport–In other words, the transport layer (TCP or UDP) creates the transport header and places the data behind it.
Step 3 Add the destination and source network layer addresses to the data. — The network layer creates the network header, which includes the network layer addresses, and places the data behind it.
Step 4 Add the destination and source data link layer addresses to the data — The data link layer creates the data link header, places the data behind it, and places the data link trailer at the end.
Step 5 Transmit the bits–The physical layer encodes a signal onto the medium to transmit the frame.

This five-step process happens to match the TCP/IP network model very well. Figure 2-6 depicts the concept; the numbers shown represent each of the five steps.



Figure 2-6 Five Steps of Data Encapsulation–TCP/IP

* The letters LH and LT stand for link header and link trailer, respectively, and refer to the data link layer header and trailer.

When each layer encapsulates data given to it from the next higher layer, that layer does not really care about the details of the data. Figure 2-7 shows the encapsulated data from the perspective of the transport, internetwork, and data link (network interface) layers.



Figure 2-7 Perspectives on Encapsulation and “Data”

Each layer treats the data given to it by the next higher layer simply as “data.” For instance, IP just wants to transport what TCP gives it–IP does not really care what is inside the data. So, the IP packet shown in the figure shows the rest of the bits as data, meaning that IP does not care that the data field looks like the TCP segment above it in the figure.

Also notice the specific terms used for the framing as it exists at each layer, as shown in the figure. Throughout this book and on the CCNA exams, the term frame defines all the encapsulated data. The term packet includes the IP header but not any data link headers. Finally, the term segment includes the TCP or UDP header but not the IP header or data link header or trailer.