protocol running at the transport layer takes the data it
received from the application layer, adds its header
information, and passes the datagram PDU to the
Internet layer. Similarly, the Internet layer adds its own
header and passes the packet PDU to the data link layer.
The data link layer takes the data from the Internet layer
and builds a frame PDU that is ready to be sent on the
physical medium by adding its own protocol header and
a frame check sequence (FCS) footer. The role of the FCS
footer is to ensure that the data is transmitted without
errors. At each hop along the way to the destination, a
checksum of the whole frame is computed and added to
the FCS footer. If the computed checksum is different
from the checksum in the FCS footer, the receiving
device knows an error occurred on the transmission
path. At the data link layer, the user information with all
the header and footer information is finally transmitted
on the physical medium.
As the data is received at the destination host, it goes
through a similar process but in the reverse order. This
process, called de-encapsulation, consists of removing
the header and footer information at each layer and
passing the data to the layer above until it gets to the
application layer, where it is interpreted by the
application protocol. Figure 16-4 shows the processes of
encapsulation at the sender and de-encapsulation at the
receiver.
Figure 16-4 Encapsulation and De-encapsulation
SWITCHING CONCEPTS
Data frame switching is a critical process in moving data
traffic from a source network endpoint to a destination