In a certain community in Colorado, two individuals are named Jeff Doyle. One Jeff Doyle frequently
receives telephone calls for the person with whom he shares a name—so much so that his clever wife has
posted the correct number next to the phone to redirect errant callers to their desired destination. In other
words, because two individuals cannot be uniquely identified, data is occasionally delivered incorrectly
and a process must be implemented to correct the error.
Among family, friends, and associates, a given name is usually sufficient for accurately distinguishing
individuals. However, as this example shows, most names become inaccurate over a larger population. A
more unique identifier, such as a United States Social Security number, is needed to distinguish one
person from every other.
NOTE
Frame
Devices on a LAN must also be uniquely and individually identified or they, like humans sharing the
same name, will receive data not intended for them. When data is to be delivered on aLAN , it is
encapsulated within an entity called a frame, a kind of binary envelope. Think of data encapsulation as
being the digital equivalent of placing a letter inside an envelope, as in Figure 1.1[1] . A destination address
and a return (source) address are written on the outside of the envelope. Without a destination address, the
postal service would have no idea where to deliver the letter. Likewise, when a frame is placed on a data
link, all devices attached to the link "see" the frame; therefore, some mechanism must indicate which
device should pick up the frame and read the enclosed data.