Copyright 2002 by
Prentice Hall, Inc.
6-6
OSI Reference Model
nAnalogy of mailing a letter: - see Figure 6-2
n Control information (address and type of delivery) is on the envelope - determines the services provided by the next lower layer and addressing information for next lower layer. When a layer receives a “message” from the next higher layer, it performs the requested services and “wraps” the message in its own layer of control information.  It passes the “bundle” to the layer directly below it. On the receiving end, a layer receiving a bundle from a lower layer unwraps the outermost layer of control information, interprets the information, and acts on it.