LAN-Legal Data: An Ethernet Frame
Before a NIC can send data over a LAN, all the details covered
in Chapter 4 must be complete. The
LAN must have the proper cabling installed. The NICs must know the electrical
encoding scheme used to signal a 0 or a 1. At that point, the computer could ask
the NIC to send data, and electrically, the bits could be sent over the LAN.
However, computers can't simply send end user data over the
LAN. Before a PC can send the end user data over a LAN, the PC must encapsulate
the end user data inside an Ethernet frame. Encapsulation
refers to the process of taking the data and putting it inside the Ethernet
header and trailer.
Encapsulation is similar in concept to putting a letter in an
envelope before sending the letter through the postal service. The postal
service requires that letters meet certain requirements, and the envelope must
have a properly formatted address. The postal service even defines what a
properly formatted address looks like. Similarly, Ethernet standards define the
headers and trailers so that a NIC can correctly encapsulate the data.
The resulting bunch of bits created by the encapsulation
process, including the Ethernet header and trailer, is called an Ethernet frame. Figure 5-1 shows an Ethernet frame, created by
encapsulating the data in a header and trailer, for frames sent to Larry from
Bob.
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